Yitshak Zohar

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Scholarly Contributions

Books

• Zohar, Y. (2003). Heat Convection in Micro Ducts. doi:10.1007/978-1-4757-3607-6
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  Foreword. Preface. Acknowledgements. 1. Introduction. 2. Fundamentals of Microscale Convective Heat Transfer. 3. Scaling, Similarity and Dimensionless Parameters in Convective Heat Transfer. 4. Fabrication of Thermal Microsystems. 5. Thermometry Techniques for Microscale Heat Convection Measurements. 6. Steady, Single-Phase Heat Convection in Micro Ducts. 7. Steady, Forced Convection Boiling in Micro Ducts. 8. Unsteady Convective Heat Transfer in Micro Ducts. 9. Micro Heat Pipes. References. Index.

Chapters

• Zohar, Y., Lee, L. M., & Cheung, L. S. (2006). Microfluidics: Device Science and Technology. In Microsystems Mechanical Design. Springer, Vienna. doi:10.1007/978-3-211-48549-1_8
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  This paper presents the basic concepts related to the newly emerging field of microfluidics. Following a brief introduction of the general conservation and particular laws, three size effects are introduced. The velocity slip boundary condition for gas flows as well as the electrokinetic and polar-mechanics effects for liquid flow in microdomains are introduced. Pressure-driven gas flows and electrokinetically-driven liquid flows in microchannels are analyzed in details. Finally, several flow diagnostic techniques and fabrication of microfluidic systems are described.
• Zohar, Y. (2001). Microchannel Heat Sinks. In The CRC Handbook of MEMS. doi:10.1201/9781420050905.ch32

Journals/Publications

• Jiang, L., Khawaja, H., Tahsin, S., Clarkson, T., Miranti, C., & Zohar, Y. (2024). Microfluidic-based human prostate cancer on chip.. Frontiers in Bioengineering and Biotechnology, 1-15.
• Tahsin, S. K., Jiang, L., Sane, N., Szewczyk, K., Khawaja, H., Zohar, Y., & Miranti, C. K. (2023).

  Abstract A073: Human prostate-on-chip models to define stromal and epithelial interactions in normal and cancerous prostate

  . Cancer Research, 83(11_Supplement), A073-A073. doi:10.1158/1538-7445.prca2023-a073
• Tello, J. A., Jiang, L., Zohar, Y., & Restifo, L. L. (2023).

  Drosophila CASK regulates brain size and neuronal morphogenesis, providing a genetic model of postnatal microcephaly suitable for drug discovery

  . Neural Dev. doi:10.1186/s13064-023-00174-y
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  Abstract Background CASK -related neurodevelopmental disorders are untreatable. Affected children show variable severity, with microcephaly, intellectual disability (ID), and short stature as common features. X-linked human CASK shows dosage sensitivity with haploinsufficiency in females. CASK protein has multiple domains, binding partners, and proposed functions at synapses and in the nucleus. Human and Drosophila CASK show high amino-acid-sequence similarity in all functional domains. Flies homozygous for a hypomorphic CASK mutation ( ∆18 ) have motor and cognitive deficits. A Drosophila genetic model of CASK-related disorders could have great scientific and translational value. Methods We assessed the effects of CASK loss of function on morphological phenotypes in Drosophila using established genetic, histological, and primary neuronal culture approaches. NeuronMetrics software was used to quantify neurite-arbor morphology. Standard nonparametric statistics methods were supplemented by linear mixed effects modeling in some cases. Microfluidic devices of varied dimensions were fabricated and numerous fluid-flow parameters were used to induce oscillatory stress fields on CNS tissue. Dissociation into viable neurons and neurite outgrowth in vitro were assessed. Results We demonstrated that ∆18 homozygous flies have small brains, small heads, and short bodies. When neurons from developing CASK -mutant CNS were cultured in vitro, they grew small neurite arbors with a distinctive, quantifiable “bushy” morphology that was significantly rescued by transgenic CASK + . As in humans, the bushy phenotype showed dosage-sensitive severity. To overcome the limitations of manual tissue trituration for neuronal culture, we optimized the design and operation of a microfluidic system for standardized, automated dissociation of CNS tissue into individual viable neurons. Neurons from CASK -mutant CNS dissociated in the microfluidic system recapitulate the bushy morphology. Moreover, for any given genotype, device-dissociated neurons grew larger arbors than did manually dissociated neurons. This automated dissociation method is also effective for rodent CNS. Conclusions These biological and engineering advances set the stage for drug discovery using the Drosophila model of CASK -related disorders. The bushy phenotype provides a cell-based assay for compound screening. Nearly a dozen genes encoding CASK-binding proteins or transcriptional targets also have brain-development mutant phenotypes, including ID. Hence, drugs that improve CASK phenotypes might also benefit children with disorders due to mutant CASK partners.
• Zohar, Y., Restifo, L. L., Jiang, L., & Tello, J. A. (2023). Drosophila CASK regulates brain size and neuronal morphogenesis, providing a genetic model of postnatal microcephaly suitable for drug discovery. Neural Development, 18. doi:10.1186/s13064-023-00174-y
• Zohar, Y., Schroeder, J. A., Jiang, L., & Frankman, Z. D. (2022). Application of Microfluidic Systems for Breast Cancer Research.. Micromachines, 13(2), 152. doi:10.3390/mi13020152
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  Cancer is a disease in which cells in the body grow out of control; breast cancer is the most common cancer in women in the United States. Due to early screening and advancements in therapeutic interventions, deaths from breast cancer have declined over time, although breast cancer remains the second leading cause of cancer death among women. Most deaths are due to metastasis, as cancer cells from the primary tumor in the breast form secondary tumors in remote sites in distant organs. Over many years, the basic biological mechanisms of breast cancer initiation and progression, as well as the subsequent metastatic cascade, have been studied using cell cultures and animal models. These models, although extremely useful for delineating cellular mechanisms, are poor predictors of physiological responses, primarily due to lack of proper microenvironments. In the last decade, microfluidics has emerged as a technology that could lead to a paradigm shift in breast cancer research. With the introduction of the organ-on-a-chip concept, microfluidic-based systems have been developed to reconstitute the dominant functions of several organs. These systems enable the construction of 3D cellular co-cultures mimicking in vivo tissue-level microenvironments, including that of breast cancer. Several reviews have been presented focusing on breast cancer formation, growth and metastasis, including invasion, intravasation, and extravasation. In this review, realizing that breast cancer can recur decades following post-treatment disease-free survival, we expand the discussion to account for microfluidic applications in the important areas of breast cancer detection, dormancy, and therapeutic development. It appears that, in the future, the role of microfluidics will only increase in the effort to eradicate breast cancer.
• Jiang, L., Liu, B. C., Wang, X., Xu, J., Yao, G., & Zohar, Y. (2021).

  Extracellular Fluid Flow Induces Shallow Quiescence through Physical and Biochemical Cues

  . Front. Cell Dev. Biol. doi:10.3389/fcell.2022.792719
• Zohar, Y. (2020). Pharmacokinetic analysis of epithelial/endothelial cell barriers in microfluidic bilayer devices with an air-liquid interface. Micromachines, 11(536). doi:10.3390/mi11050536
• Zohar, Y., Jiang, L., & Frost, T. S. (2020). Pharmacokinetic Analysis of Epithelial/Endothelial Cell Barriers in Microfluidic Bilayer Devices with an Air–Liquid Interface. Micromachines. doi:10.3390/mi11050536
• Luo, H., Zhao, C., Song, K., Liu, D., Ma, W., Yu, X., Su, H., Zhang, Z., Zohar, Y., & Lee, Y. (2019). A nonlinear two-degree-of-freedom mass-damper-spring model to predict the isolation of circulating tumor cells in microfluidic-elasto-filtration devices. MICROFLUIDICS AND NANOFLUIDICS, 23(5).
• Zohar, Y., Jiang, L., Estrada, V., & Frost, T. S. (2019). Convection–diffusion molecular transport in a microfluidic bilayer device with a porous membrane. Microfluidics and Nanofluidics. doi:10.1007/s10404-019-2283-1
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  The field of human cell research is rapidly changing due to the introduction of microphysiological systems, which commonly feature two stacked microchannels separated by a porous membrane for in vitro barrier modeling. An essential component to adequately representing a subset of human organ or tissue functions in these microfluidic systems is the concentration distribution of the biospecies involved. In particular, when different cell types are cultured, a delicate balance between media mixing and cellular signaling is required for long-term maintenance of the cellular co-culture. In this work, we experimentally measured the effects of various control parameters on the transient and steady average molecular concentration at the bilayer device outlet. Using these experimental results for validation, we then numerically investigated the concentration distributions due to the convection–diffusion mass transport in both microchannels. The effects of media flow rate, separation membrane porosity, molecular size, microchannel dimensions and flow direction have been systematically characterized. The transient response is found to be negligible for cell co-cultures lasting several days, while the steady-state concentration distribution is dominated by the media flow rate and separation membrane porosity. Numerically computed concentration profiles reveal self-similarity characteristics featuring a diffusive boundary layer, which can be manipulated for successful maintenance of cell co-culture with limited media mixing and enhanced cell signaling.
• Zohar, Y., Lee, Y. C., Zhang, Z., Su, H., Yu, X., Ma, W., Liu, D., Song, K., Zhao, C., & Luo, H. (2019). A nonlinear two-degree-of-freedom mass–damper–spring model to predict the isolation of circulating tumor cells in microfluidic-elasto-filtration devices. Microfluidics and Nanofluidics. doi:10.1007/s10404-019-2240-z
• Zohar, Y., Lynch, R. M., Jiang, L., & Frost, T. S. (2019). Permeability of Epithelial/Endothelial Barriers in Transwells and Microfluidic Bilayer Devices. Micromachines. doi:10.3390/mi10080533
• Zohar, Y., Miranti, C. K., Tran, M., Tahsin, S., Ivich, F., Jiang, L., & Frank, S. B. (2019). Human stroma and epithelium co-culture in a microfluidic model of a human prostate gland. Biomicrofluidics, 13(6), 064116. doi:10.1063/1.5126714
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  The prostate is a walnut-sized gland that surrounds the urethra of males at the base of the bladder comprising a muscular portion, which controls the release of urine, and a glandular portion, which secretes fluids that nourish and protect sperms. Here, we report the development of a microfluidic-based model of a human prostate gland. The polydimethylsiloxane (PDMS) microfluidic device, consisting of two stacked microchannels separated by a polyester porous membrane, enables long-term in vitro cocultivation of human epithelial and stromal cells. The porous separation membrane provides an anchoring scaffold for long-term culturing of the two cell types on its opposite surfaces allowing paracrine signaling but not cell crossing between the two channels. The microfluidic device is transparent enabling high resolution bright-field and fluorescence imaging. Within this coculture model of a human epithelium/stroma interface, we simulated the functional development of the in vivo human prostate gland. We observed the successful differentiation of basal epithelial cells into luminal secretory cells determined biochemically by immunostaining with known differentiation biomarkers, particularly androgen receptor expression. We also observed morphological changes where glandlike mounds appeared with relatively empty centers reminiscent of prostatic glandular acini structures. This prostate-on-a-chip will facilitate the direct evaluation of paracrine and endocrine cross talk between these two cell types as well as studies associated with normal vs disease-related events such as prostate cancer.
• Kim, S., Hinkel, C. J., Coffey, D. S., Zohar, Y., Lee, B., & Gretzer, M. (2018).

  PD21-06 FLUID DYNAMICS AND MORPHOLOGY IN THE OBSTRUCTED URETHRA

  . journal of Urology. doi:10.1016/j.juro.2018.02.1161
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  You have accessJournal of UrologyTrauma/Reconstruction/Diversion: Urethral Reconstruction (including Stricture, Diverticulum) I1 Apr 2018PD21-06 FLUID DYNAMICS AND MORPHOLOGY IN THE OBSTRUCTED URETHRA Sunchin Kim, Cameron Hinkel, Donald Coffey, Yitshak Zohar, Benjamin Lee, and Matthew Gretzer Sunchin KimSunchin Kim More articles by this author , Cameron HinkelCameron Hinkel More articles by this author , Donald CoffeyDonald Coffey More articles by this author , Yitshak ZoharYitshak Zohar More articles by this author , Benjamin LeeBenjamin Lee More articles by this author , and Matthew GretzerMatthew Gretzer More articles by this author View All Author Informationhttps://doi.org/10.1016/j.juro.2018.02.1161AboutPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookTwitterLinked InEmail INTRODUCTION AND OBJECTIVES Current objective methods to evaluate for urethral obstruction include uroflowmetry, cystoscopy, and retrograde urethrogram. We present a novel non-invasive method to identify the presence of urethral obstruction. Through evaluation of the fluid dynamics and morphological changes of a falling water stream, we have identified a characteristic measurement that may be used to indicate the presence of obstruction within the urethra. As fluid exits an opening, there is a breakpoint at which droplets form. Measuring the length to this point of droplet formation was found to relate to the presence of obstruction. METHODS High speed photography with a strobe light set to 3500 flashes per minute was used to capture the point of droplet formation. An 18 French, 20 cm segment of Penrose tubing was used as a model for the urethra, and a 3D printer was used to create models of strictures with diameters of 2 mm and 4 mm. These stricture models were placed midway at the 10 cm mark of the Penrose tubing. A saline bag was allowed to drain under gravity through the tubing, and pictures were obtained with and without the stricture models. RESULTS With the control tubing without any stricture device (Figure A), normal gravity flow resulted in a flow rate of 14.3 ml/sec with the natural breakpoint occurring at 12.5 cm. After the 4 mm stricture device was added to the tubing (Figure C), this resulted in a flow rate of 12.8 ml/sec, with the natural breakpoint occurring consistently at 11 cm. With a 2 mm stricture device added (Figure B), the flow rate was 6.89 ml/sec, with the natural breakpoint occurring at 6.5 cm. CONCLUSIONS While uroflowmetry is the current non-invasive objective method to identify possible urethral obstruction, identification and measure of the morphologic changes to urine flow represents a new way to identify the presence of obstruction. Given the results of our initial study that shows that looking at urine morphology is a consistent and accurate measurement of determining obstruction, the potential to incorporate this technique into consumer photographic applications may facilitate efficient long term follow up of men after urethroplasty. © 2018FiguresReferencesRelatedDetails Volume 199Issue 4SApril 2018Page: e469-e470 Advertisement Copyright & Permissions© 2018MetricsAuthor Information Sunchin Kim More articles by this author Cameron Hinkel More articles by this author Donald Coffey More articles by this author Yitshak Zohar More articles by this author Benjamin Lee More articles by this author Matthew Gretzer More articles by this author Expand All Advertisement Advertisement PDF downloadLoading ...
• Luo, H., Chao, Z., Cai, Y., Deng, Y., Zhang, R., Pan, Z., Zohar, Y., & Lee, Y. (2018).

  The Surface Hydrophobicity Effect on the Capture Efficiency of Cancer Cells in Microfluidic-Elasto-Filtration Chips

  . Microfluid Nanofluid. doi:10.1007/s10404-019-2240-z
• Zohar, Y., Jiang, L., Restifo, L. L., Restifo, L. L., Jiang, L., & Zohar, Y. (2015). Dissociation of brain tissue into viable single neurons in a microfluidic device.. IEEE Nano/Molecular Medicine and Engineering, 9, 29-32.
• Jiang, L., Zohar, Y., Stopeck, A., Schroeder, J. A., & Zheng, X. (2014). Isolation of viable cancer cells in antibody-functionalized microfluidic devices. Biomicrofluidics. doi:10.1063/1.4873956
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  Microfluidic devices functionalized with EpCAM antibodies were utilized for the capture of target cancer cells representing circulating tumor cells (CTCs). The fraction of cancer cells captured from homogeneous suspensions is mainly a function of flow shear rate, and can be described by an exponential function. A characteristic shear rate emerges as the most dominant parameter affecting the cell attachment ratio. Utilizing this characteristic shear rate as a scaling factor, all attachment ratio results for various combinations of receptor and ligand densities collapsed onto a single curve described by the empirical formula. The characteristic shear rate increases with both cell-receptor and surface-ligand densities, and empirical formulae featuring a product of two independent cumulative distributions described well these relationships. The minimum detection limit in isolation of target cancer cells from binary mixtures was experimentally explored utilizing microchannel arrays that allow high-throughput processing of suspensions about 0.5 ml in volume, which are clinically relevant, within a short time. Under a two-step attachment/detachment flow rate, both high sensitivity (almost 1.0) and high specificity (about 0.985) can be achieved in isolating target cancer cells from binary mixtures even for the lowest target/non-target cell concentration ratio of 1:100 000; this is a realistic ratio between CTCs and white blood cells in blood of cancer patients. Detection of CTCs from blood samples was also demonstrated using whole blood from healthy donors spiked with cancer cells. Finally, the viability of target cancer cells released after capture was confirmed by observing continuous cell growth in culture.
• Riahi, R., Yang, Y. L., Kim, H., Jiang, L., Wong, P. K., & Zohar, Y. (2014). A Microfluidic Model for Organ-specific Extravasation of Circulating Tumor Cells. Biomicrofluidics, 8(2), 024103.
• Stamm, M. T., Trickey-Glassman, A. S., Jiang, L., & Zohar, Y. (2014). Specific Interactions Between Functionalised Particles and Circulating Tumour Cells. IET Nanobiotechnology, 8(1), 18-23.
• Stamm, M. T., Trickey-Glassman, A. S., Jiang, L., & Zohar, Y. (2014). Specific interactions between functionalised particles and circulating tumour cells. IET Nanobiotechnology, 8(1), 18-23.
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  Abstract: Receptor-ligand binding has been one of the more popular approaches to specifically targeting tumour cells. In this work, targeting efficiency was quantitatively characterized using silica particles functionalized with EpCAM antibodies and EpCAM-expressing BT-20 breast cancer cells. The effects of incubation time and particle concentration on the number of functionalised particles bound to target cells were experimentally investigated. The number of bound particles was found to increase with particle concentration, but not necessarily with incubation time. Binding affinity loss because of cell-particle- cell interaction was identified as a limiting mechanism for the number of particles bound to target cells. While cell-surface coverage because of bound particles rises exponentially under low particle concentration, it features a peak value at high particle concentration. The current findings suggest that separation of a bound particle from a cell may be detrimental to cellular binding affinity. © The Institution of Engineering and Technology 2014.
• Zeng, L., Jiang, L., Teng, W., Cappello, J., & Zohar, Y. (2014). Engineering Aqueous Fiber Assembly Into Silk-elastin-like Protein Polymers. Macromolecular Rapid Communications, 35(14), 1273-1279.
• Zheng, X., Jiang, L., Schroeder, J., Stopeck, A., Zohar, Y., Zheng, X., Jiang, L., Schroeder, J., Stopeck, A., & Zohar, Y. (2014). Isolation of Viable Cancer Cells in Antibody-Functionalized Microfluidic Devices. Biomicrofluidics, 8(2), 024119.
• Zohar, Y., Jiang, L., Trickey-Glassman, A. S., & Stamm, M. (2014). Specific interactions between functionalised particles and circulating tumour cells. Iet Nanobiotechnology. doi:10.1049/iet-nbt.2013.0034
• Zohar, Y., Wong, P. K., Kim, H., Yang, Y. L., Riahi, R., & Jiang, L. (2014). A microfluidic model for organ-specific extravasation of circulating tumor cells. Biomicrofluidics, 8(2), 024103. doi:10.1063/1.4868301
• Zohar, Y., Wu, X., Cappello, J., Teng, W., Jiang, L., & Zeng, L. (2014). Engineering Aqueous Fiber Assembly into Silk-Elastin-Like Protein Polymers. Macromolecular Rapid Communications, 35(14), 1273-1279. doi:10.1002/marc.201400058
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  Self-assembled peptide/protein nanofibers are valuable 1D building blocks for creating complex structures with designed properties and functions. It is reported that the self-assembly of silk-elastin-like protein polymers into nanofibers or globular aggregates in aqueous solutions can be modulated by tuning the temperature of the protein solutions, the size of the silk blocks, and the charge of the elastin blocks. A core-sheath model is proposed for nanofiber formation, with the silk blocks in the cores and the hydrated elastin blocks in the sheaths. The folding of the silk blocks into stable cores--affected by the size of the silk blocks and the charge of the elastin blocks--plays a critical role in the assembly of silk-elastin nanofibers. Furthermore, enhanced hydrophobic interactions between the elastin blocks at elevated temperatures greatly influence the nanoscale features of silk-elastin nanofibers.
• Zohar, Y., Zhao, C., Xu, R., Piyawattanametha, W., Ma, W., Liu, D., Liang, C., & Lee, Y. (2014). Isolation of circulating tumor cells under hydrodynamic loading using microuidic technology. Advances in Mechanics, 44(1), 447-494. doi:10.6052/1000-0992-14-038
• Ma, W., Liu, D., Shagoshtasbi, H., Shukla, A., Nugroho, E. S., Zohar, Y., & Lee, Y. -. (2013). Experimental and theoretical study of hydrodynamic cell lysing of cancer cells in a high-throughput Circular Multi-Channel Microfiltration device. 8th Annual IEEE International Conference on Nano/Micro Engineered and Molecular Systems, IEEE NEMS 2013, 412-415.
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  Abstract: Microfiltration is an important microfluidic technique suitable for enrichment and isolation of cells. However, cell lysing could occur due to hydrodynamic damage that may be detrimental for medical diagnostics. Therefore, we conducted a systematic study of hydrodynamic cell lysing in a high-throughput Circular Multi-Channel Microfiltration (CMCM) device integrated with a polycarbonate membrane. HeLa cells (cervical cancer cells) were driven into the CMCM at different flow rates. The viability of the cells in the CMCM was examined by fluorescence microscopy using Acridine Orange (AO)/Ethidium Bromide (EB) as a marker for viable/dead cells. A simple analytical cell viability model was derived and a 3D numerical model was constructed to examine the correlation of between cell lysing and applied shear stress under varying flow rate and Reynolds number. The measured cell viability as a function of the shear stress was consistent with theoretical and numerical predictions when accounting for cell size distribution. © 2013 IEEE.
• Riahi, R., Yang, Y., Kim, H., Jiang, L., Wong, P. K., & Zohar, Y. (2013). A microfluidic-based platform for in vitro studies of cell signaling in blood vessels. 2013 Transducers and Eurosensors XXVII: The 17th International Conference on Solid-State Sensors, Actuators and Microsystems, TRANSDUCERS and EUROSENSORS 2013, 293-296.
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  Abstract: A microfluidic-based platform is developed for in vitro investigation of signaling within the blood-stream leading to preferential homing of specific cells such as white blood cells (WBCs) selectively to injured host organs. A microchannel has been coated with a monolayer of endothelial cells to mimic the microenvironment in blood vessels, while stable chemokine gradients have been generated locally at selected locations along the microchannel sidewalls. We demonstrate that both the chemokine gradients and the endothelial cells are simultaneously maintained during steady flow of cells as in the blood stream. © 2013 IEEE.
• Stamm, M. T., Trickey-Glassman, A., Jiang, L., & Zohar, Y. (2013). Quantitative characterization of specific targeting of tumor cells by antibody-functionalized particles. 8th Annual IEEE International Conference on Nano/Micro Engineered and Molecular Systems, IEEE NEMS 2013, 1108-1111.
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  Abstract: Receptor-ligand binding has been one of the more popular approaches to specifically targeting tumor cells. In this work, targeting efficiency was quantitatively characterized using silica particles functionalized with EpCAM antibodies and EpCAM-expressing BT-20 breast cancer cells. The effects of incubation time and particle concentration on the number of functionalized particles bound to target cells were experimentally investigated. The number of bound particles was found to increase with particle concentration, but not necessarily with incubation time. While particle desorption and cellular loss of binding affinity in time seem to be negligible, cell-particle-cell interaction was identified as the limiting mechanism for the number of particles bound to target cells. The current findings suggest that separation of a bound particle from a cell may be detrimental to cellular binding affinity. © 2013 IEEE.
• Jiang, L., Zohar, Y., Zha, Z., Stamm, M. T., Jiang, L., & Dai, Z. (2012). Functionalization of Ceramic Liposomal Nanoparticles, Cerasomes, with Antibodies. Journal of Physical Chemistry & Biophysics, 2(1), 1-5. doi:10.4172/2161-0398.1000105
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  Ceramic nanoparticles and silica microparticles are functionalized with antibodies on their surfaces that act as targeting ligands. For the ceramic liposomal nanoparticles, cerasomes, this is achieved by using the siloxane network present on the cerasome surface as the foundation for chemical treatment processes previously developed for silicon surfaces. The bio-functionality and physical integrity of the cerasomes are characterized, demonstrating successful immobilization of antibodies on the cerasome surface. The surface functionalization allows the cerasomes to deliver drugs to targeted cells expressing certain types of receptors with desired selectivity and specificity that are not possible using standard liposomes. The Silica microparticles are used to mimic cerasomes in experiments targeting cancer cells and the particle-cell specific binding due to the bio-functionalization process is demonstrated.
• Lee, M., Lee, Y., & Zohar, Y. (2012). Single-phase liquid flow forced convection under a nearly uniform heat flux boundary condition in microchannels. Journal of Micromechanics and Microengineering, 22(3).
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  Abstract: A microchannel heat sink, integrated with pressure and temperature microsensors, is utilized to study single-phase liquid flow forced convection under a uniform heat flux boundary condition. Utilizing a waferbond-and-etch- back technology, the heat source, temperature and pressure sensors are encapsulated in a thin composite membrane capping the microchannels, thus allowing experimentally good control of the thermal boundary conditions. A three-dimensional physical model has been constructed to facilitate numerical simulations of the heat flux distribution. The results indicate that upstream the cold working fluid absorbs heat, while, within the current operating conditions, downstream the warmer working fluid releases heat. The Nusselt number is computed numerically and compared with experimental and analytical results. The wall Nusselt number in a microchannel can be estimated using classical analytical solutions only over a limited range of the Reynolds number, Re: both the top and bottom Nusselt numbers approach 4 for Re < 1, while the top and bottom Nusselt numbers approach 0 and 5.3, respectively, for Re > 100. The experimentally estimated Nusselt number for forced convection is highly sensitive to the location of the temperature measurements used in calculating the Nusselt number. © 2012 IOP Publishing Ltd.
• Zheng, X., Jiang, L., Schroeder, J. A., Marron, M., Iannone, M., Stopeck, A. T., & Zohar, Y. (2012). Flow-rate dependent capture of circulating tumor cells in bio-functional microchannels. IEEE International Conference on Nano/Molecular Medicine and Engineering, NANOMED, 82-86.
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  Abstract: The capture of circulating tumor cells (CTCs) in shear flow is studied systematically by driving suspensions of breast cancer cells, with different EpCAM receptor expression levels, through microchannels functionalized with EpCAM antibody ligands at various densities. The dependence of cell capture efficiency on flow rate is established and can be described by an exponential function; a characteristic flow rate immerges as the most dominant parameter affecting the capture efficiency. Utilizing this characteristic flow rate as a scaling factor, all measured capture efficiencies for the tested receptor and ligand densities can be expressed using a single exponential formula. The effect of both ligand and receptor densities on the characteristic flow rate is found to be a product of two independent cumulative distributions. © 2012 IEEE.
• Zheng, X., Jiang, L., Schroeder, J. A., Marron, M., Iannone, M., Stopeck, A. T., & Zohar, Y. (2012). On the minimum detection limit of circulating tumor cells in an antibody-functionalized microchannel array. IEEE International Conference on Nano/Molecular Medicine and Engineering, NANOMED, 87-91.
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  Abstract: The minimum detection limit in isolation of circulating tumor cells (CTCs) from binary mixtures is experimentally explored utilizing microchannel arrays functionalized with EpCAM antibodies. Hence, the tested binary mixtures contained EpCAM-positive and EpCAM-negative cells serving as target and non-target cells, respectively. The ratio of target to non-target cells in the mixtures ranged from 1:1 to 1:100,000. The dependence of the device performance, particularly its sensitivity and specificity in isolating target cells, on the applied flow shear rate is characterized. The results demonstrate capture of circulating tumor cells from mixtures with high sensitivity and high specificity. The multichannel array devices allow testing of samples within a shorter time, which is critical for high throughput applications. © 2012 IEEE.
• Zohar, Y., Lee, Y. C., & Lee, H. M. (2012). Single-phase liquid flow forced convection under a nearly uniform heat flux boundary condition in microchannels. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/22/3/035015
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  A microchannel heat sink, integrated with pressure and temperature microsensors, is utilized to study single-phase liquid flow forced convection under a uniform heat flux boundary condition. Utilizing a wafer–bond-and-etch-back technology, the heat source, temperature and pressure sensors are encapsulated in a thin composite membrane capping the microchannels, thus allowing experimentally good control of the thermal boundary conditions. A three-dimensional physical model has been constructed to facilitate numerical simulations of the heat flux distribution. The results indicate that upstream the cold working fluid absorbs heat, while, within the current operating conditions, downstream the warmer working fluid releases heat. The Nusselt number is computed numerically and compared with experimental and analytical results. The wall Nusselt number in a microchannel can be estimated using classical analytical solutions only over a limited range of the Reynolds number, Re: both the top and bottom Nusselt numbers approach 4 for Re 100. The experimentally estimated Nusselt number for forced convection is highly sensitive to the location of the temperature measurements used in calculating the Nusselt number.
• Cheung, L. S., Zheng, X., Wang, L., Baygents, J. C., Guzman, R., Schroeder, J. A., Heimark, R. L., & Zohar, Y. (2011). Adhesion dynamics of circulating tumor cells under shear flow in a bio-functionalized microchannel. Journal of Micromechanics and Microengineering, 21(5).
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  Abstract: The adhesion dynamics of circulating tumor cells in a bio-functionalized microchannel under hydrodynamic loading is explored experimentally and analyzed theoretically. EpCAM antibodies are immobilized on the microchannel surface to specifically capture EpCAM-expressing target breast cancer cells MDA-MB-231 from a homogeneous cell suspension in shear flow. In the cross-stream direction, gravity is the dominant physical mechanism resulting in continuous interaction between the EpCAM cell receptors and the immobilized surface anti-EpCAM ligands. Depending on the applied shear rate, three dynamic states have been characterized: firm adhesion, rolling adhesion and free rolling. The steady-state velocity under adhesion- and free-rolling conditions as well as the time-dependent velocity in firm adhesion has been characterized experimentally, based on video recordings of target cell motion in functionalized microchannels. A previously reported theoretical model, utilizing a linear spring to represent the specific receptor-ligand bonds, has been adopted to analyze adhesion dynamics including features such as the cell-surface binding force and separation gap. By fitting theoretical predictions to experimental measurements, a unified exponential decay function is proposed to describe the target cell velocity evolution during capture; the fitting parameters, velocity and time scales, depend on the particular cell-surface system. © 2011 IOP Publishing Ltd.
• Gudipaty, T., Stamm, M. T., Cheung, L. S., Jiang, L., & Zohar, Y. (2011). Cluster formation and growth in microchannel flow of dilute particle suspensions. Microfluidics and Nanofluidics, 10(3), 661-669.
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  Abstract: The lifetime of microfluidic devices depends on their ability to maintain flow without interruption. Certain applications require microdevices for transport of liquids containing particles. However, microchannels are susceptible to blockage by solid particles. Therefore, in this study, the phenomenon of interest is the formation and growth of clusters on a microchannel surface in the flow of a dilute suspension of hard spheres. Based on the present experiments, aggregation of clusters was observed for particleladen flows in microchannels with particle void fraction as low as 0.001 and particle diameter to channel height ratio as low as 0.1. The incipience and growth of a single cluster is discussed, and the spatial distribution and time evolution of clusters along the microchannel are presented. Although the cluster size seems to be independent of location, more clusters are found at the inlet/outlet regions than in the microchannel center. Similarly as for an individual cluster, as long as particle-cluster interaction is the dominant mode, the total cluster area in the microchannel grows almost linearly in time. The effects of flow rate, particle size, and concentration are also reported. © Springer-Verlag 2010.
• Jiang, L. L., Zohar, Y., Palanivelu, R., Qin, Y., Cooper, J., & Yetisen, A. K. (2011). A microsystem-based assay for studying pollen tube guidance in plant reproduction. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/21/5/054018
• Lee, M., Lee, Y., & Zohar, Y. (2011). Design, fabrication and characterization of a thermal microsystem integrated with heaters, pressure and temperature microsensors. Journal of Micromechanics and Microengineering, 21(12).
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  Abstract: The design, fabrication and characterization of an intergrated thermal microsystem are presented. The system, consisting of thin-film heater elements, an array of microchannels, pressure and temperature microsensor arrays, is designed for studying forced convection heat transfer under well-controlled thermal boundary conditions. Utilizing a wafer bond and etch back technology, the heat source, pressure and temperature sensors are separated from the fluid flow by a membrane only 1.5 νm in thickness, thus allowing experimentally improved approximation of classical boundary conditions, especially the uniform heat flux at the solid/fluid interface. A three-dimensional simulation model is constructed for numerical analysis to complement the experimental characterization of the liquid single-phase flow in this microsystem. Pressure and temperature distributions, for various operating conditions, have been measured and compared with computed profiles. The agreement between the experimental and numerical results confirms that, though not ideal, the heat flux boundary condition is nearly uniform. © 2011 IOP Publishing Ltd.
• Stamm, M. T., Gudipaty, T., Rush, C., Jiang, L., & Zohar, Y. (2011). Particle aggregation rate in a microchannel due to a dilute suspension flow. Microfluidics and Nanofluidics, 1-9.
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  Abstract: Particle-laden flow in a microchannel results in cluster formation and growth on the channel surface and the cluster growth, due to aggregation of polystyrene microparticles, has been investigated in this study. In particular, the initial stage of cluster growth is examined, where particle-cluster interaction is the dominant growth mechanism. Both experimental measurements and theoretical considerations were utilized to explore the functional dependence of the cluster growth rate on the following parameters: suspension void fraction, flow shear strain rate, and channel-height to particle-diameter ratio. The growth rate of an average cluster is found to increase linearly with suspension void fraction which is consistent with previous reports. The growth rate coefficient is found to obey a power-law relationship with respect to the shear strain rate, and predictions based on the modernized flocculation theory agree well with the experimental results. Furthermore, the growth rate coefficient obeys a power-law relationship with respect to the channel-height to particle-diameter ratio as well, qualitatively similar to other reported studies. However, to our knowledge, the exponent value estimated in this study does not agree with any previously published values; this disagreement is likely due to differences in experimental conditions. © 2011 Springer-Verlag.
• Stamm, M. T., Gudipaty, T., Rush, C., Jiang, L., & Zohar, Y. (2011). Particle aggregation rate in a microchannel due to a dilute suspension flow. Microfluidics and Nanofluidics, 11(4), 395-403.
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  Abstract: Particle-laden flow in a microchannel results in cluster formation and growth on the channel surface and the cluster growth, due to aggregation of polystyrene microparticles, has been investigated in this study. In particular, the initial stage of cluster growth is examined, where particle- cluster interaction is the dominant growth mechanism. Both experimental measurements and theoretical considerations were utilized to explore the functional dependence of the cluster growth rate on the following parameters: suspension void fraction, flow shear strain rate, and channel-height to particle-diameter ratio. The growth rate of an average cluster is found to increase linearly with suspension void fraction which is consistent with previous reports. The growth rate coefficient is found to obey a power-law relationship with respect to the shear strain rate, and predictions based on the modernized flocculation theory agree well with the experimental results. Furthermore, the growth rate coefficient obeys a power-law relationship with respect to the channel-height to particle-diameter ratio as well, qualitatively similar to other reported studies. However, to our knowledge, the exponent value estimated in this study does not agree with any previously published values; this disagreement is likely due to differences in experimental conditions. © Springer-Verlag 2011.
• Stamm, M. T., Zha, Z., Jiang, L., Dai, Z., & Zohar, Y. (2011). Functionalization of ceramic liposomal nanoparticles, cerasomes, with antibodies. 2011 16th International Solid-State Sensors, Actuators and Microsystems Conference, TRANSDUCERS'11, 1614-1617.
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  Abstract: Ceramic liposomal nanoparticles, cerasomes, are functionalized with antibodies on their surfaces that act as targeting ligands. This is achieved by using the siloxane network present on the cerasome surface as the foundation for chemical treatment processes previously developed for silicon surfaces. The bio-functionality and physical integrity of the cerasomes are characterized demonstrating successful immobilization of antibodies on the cerasome surface. The surface functionalization allows the cerasomes to deliver drugs to targeted cells expressing certain types of receptors with desired selectivity and specificity, which are not possible using standard liposomes. © 2011 IEEE.
• Yetisen, A. K., Jiang, L., Cooper, J. R., Qin, Y., Palanivelu, R., & Zohar, Y. (2011). A microsystem-based assay for studying pollen tube guidance in plant reproduction. Journal of Micromechanics and Microengineering, 21(5).
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  Abstract: We present a novel microsystem-based assay to assess and quantify pollen tube behavior in response to pistil tissues. During plant reproduction, signals from female tissues (pistils) guide the sperm-carrying pollen tube to the egg cell to achieve fertilization and initiate seed development. Existing pollen tube guidance bioassays are performed in an isotropically diffusive environment (for example, a semi in vivo assay in petri dishes) instead of anisotropically diffusive conditions required to characterize guidance signal gradients. Lack of a sensitive pollen tube guidance bioassay has therefore compounded the difficulties of identifying and characterizing the guidance signals that are likely produced in minute quantities by the ovules. We therefore developed a novel microsystem-based assay that mimics the in vivo micro-environment of ovule fertilization by pollen tubes in the model research plant Arabidopsis thaliana. In this microdevice, the pollen tube growth rate, length and ovule targeting frequencies were similar to those obtained using a semi in vivo plate assay. As a direct measure of the microdevice's utility in monitoring pollen tube guidance, we demonstrated that in this device, pollen tubes preferentially enter chambers with unfertilized ovules, suggesting that the pollen tubes sense the concentration gradient and respond to the chemoattractants secreted by unfertilized ovules. © 2011 IOP Publishing Ltd.
• Yetisen, A., Jiang, L., Cooper, J., Qin, Y., Palanivelu, R. -., & Zohar, Y. -. (2011). A microsystem-based assay for studying pollen tube guidance in plant reproduction. Journal of Micromechanics and Microengineering, 21.
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  Correspondence: RP and YZ
• Zheng, X. J., Cheung, L. S., Jiang, L., Schroeder, J. A., Heimark, R. L., Baygents, J. C., Guzman, R., & Zohar, Y. (2011). Dynamic states of adhering cancer cells under shear flow in an antibody-functionalized microchannel. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 849-852.
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  Abstract: Dynamic states of cancer cells moving under shear flow in an antibody-functionalized microchannel are investigated experimentally and theoretically. A simplified physical model was adopted to analyze the cell motion; it features a rigid sphere, with receptors on its surface, moving above a solid surface with distributed ligands. The cell motion is described by the Langevin equation where the hydrodynamic interactions, gravitational drift force, receptor-ligand binding force, and thermal fluctuations are all taken into account. The receptor-ligand bonds are modeled as Hookean springs. In this study, three dynamic states of cell motion have been identified: (i) free motion, (ii) rolling adhesion, and (iii) firm adhesion depending on the flow shear rate. The numerical simulations allow exploring effects of numerous parameters such as cell-receptor and surface-ligand density.
• Zheng, X. J., Cheung, L. S., Schroeder, J. A., Jiang, L., & Zohar, Y. (2011). A high-performance microsystem for isolating viable circulating tumor cells. 2011 16th International Solid-State Sensors, Actuators and Microsystems Conference, TRANSDUCERS'11, 226-229.
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  Abstract: The attachment and detachment of target cancer cells from homogeneous and binary mixtures in antibody-functionalized microchannels have been studied experimentally. Under the same intermediate flow rate, the attachment rate was found to be higher, and detachment flow rate was lower, for cell lines expressing the target receptor at a higher level. For cells that do not express the target receptor, the attachment rate was much lower but did not diminish, due to non-specific binding, and the detachment rate was much higher. The bio-functional microfluidic system performance in selectively isolating target cells from binary mixtures is quantitatively characterized. While the system sensitivity is typically very high, almost 100%, the specificity is lower than 90%. Applying a unique flow scheme of a slow flow rate, for maximum capture of target cells, followed by a faster flow rate, for maximum removal of non-target cells, the specificity is enhanced to levels above 95%, even for mixtures with target cells present at 11,000 relative concentration ratio. © 2011 IEEE.
• Zheng, X., Cheung, L. S., Schroeder, J. A., Jiang, L., & Zohar, Y. (2011). A high-performance microsystem for isolating circulating tumor cells. Lab on a Chip - Miniaturisation for Chemistry and Biology, 11(19), 3269-3276.
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  PMID: 21837324;Abstract: A unique flow field pattern in a bio-functional microchannel is utilized to significantly enhance the performance of a microsystem developed for selectively isolating circulating tumor cells from cell suspensions. For high performance of such systems, disposal of maximum non-target species is just as important as retention of maximum target species; unfortunately, most studies ignore or fail to report this aspect. Therefore, sensitivity and specificity are introduced as quantitative criteria to evaluate the system performance enabling a direct comparison among systems employing different techniques. The newly proposed fluidic scheme combines a slow flow field, for maximum target-cell attachment, followed by a faster flow field, for maximum detachment of non-target cells. Suspensions of homogeneous or binary mixtures of circulating breast tumor cells, with varying relative concentrations, were driven through antibody-functionalized microchannels. Either EpCAM or cadherin-11 transmembrane receptors were targeted to selectively capture target cells from the suspensions. Cadherin-11-expressing MDA-MB-231 cancer cells were used as target cells, while BT-20 cells were used as non-target cells as they do not express cadherin-11. The attachment and detachment of these two cell lines are characterized, and a two-step attachment/detachment flow field pattern is implemented to enhance the system performance in capturing target cells from binary mixtures. While the system sensitivity remains high, above 0.95, the specificity increases from about 0.85 to 0.95 solely due to the second detachment step even for a 1:1000 relative concentration of the target cells. © 2011 The Royal Society of Chemistry.
• Zheng, X., Cheung, L. S., Schroeder, J. A., Jiang, L., & Zohar, Y. (2011). Cell receptor and surface ligand density effects on dynamic states of adhering circulating tumor cells. Lab on a Chip - Miniaturisation for Chemistry and Biology, 11(20), 3431-3439.
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  PMID: 21853194;Abstract: Dynamic states of cancer cells moving under shear flow in an antibody-functionalized microchannel are investigated experimentally and theoretically. The cell motion is analyzed with the aid of a simplified physical model featuring a receptor-coated rigid sphere moving above a solid surface with immobilized ligands. The motion of the sphere is described by the Langevin equation accounting for the hydrodynamic loadings, gravitational force, receptor-ligand bindings, and thermal fluctuations; the receptor-ligand bonds are modeled as linear springs. Depending on the applied shear flow rate, three dynamic states of cell motion have been identified: (i) free motion, (ii) rolling adhesion, and (iii) firm adhesion. Of particular interest is the fraction of captured circulating tumor cells, defined as the capture ratio, via specific receptor-ligand bonds. The cell capture ratio decreases with increasing shear flow rate with a characteristic rate. Based on both experimental and theoretical results, the characteristic flow rate increases monotonically with increasing either cell-receptor or surface-ligand density within certain ranges. Utilizing it as a scaling parameter, flow-rate dependent capture ratios for various cell-surface combinations collapse onto a single curve described by an exponential formula. This journal is © The Royal Society of Chemistry.
• Zohar, Y., Heimark, R. L., Schroeder, J. D., Guzman, R., Baygents, J. C., Wang, L., Zheng, X., & Cheung, L. S. (2011). Adhesion dynamics of circulating tumor cells under shear flow in a bio-functionalized microchannel. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/21/5/054033
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  The adhesion dynamics of circulating tumor cells in a bio-functionalized microchannel under hydrodynamic loading is explored experimentally and analyzed theoretically. EpCAM antibodies are immobilized on the microchannel surface to specifically capture EpCAM-expressing target breast cancer cells MDA-MB-231 from a homogeneous cell suspension in shear flow. In the cross-stream direction, gravity is the dominant physical mechanism resulting in continuous interaction between the EpCAM cell receptors and the immobilized surface anti-EpCAM ligands. Depending on the applied shear rate, three dynamic states have been characterized: firm adhesion, rolling adhesion and free rolling. The steady-state velocity under adhesion- and free-rolling conditions as well as the time-dependent velocity in firm adhesion has been characterized experimentally, based on video recordings of target cell motion in functionalized microchannels. A previously reported theoretical model, utilizing a linear spring to represent the specific receptor–ligand bonds, has been adopted to analyze adhesion dynamics including features such as the cell–surface binding force and separation gap. By fitting theoretical predictions to experimental measurements, a unified exponential decay function is proposed to describe the target cell velocity evolution during capture; the fitting parameters, velocity and time scales, depend on the particular cell–surface system.
• Zohar, Y., Jiang, L., Cheung, L. S., Stamm, M., & Gudipaty, T. (2011). Cluster formation and growth in microchannel flow of dilute particle suspensions. Microfluidics and Nanofluidics. doi:10.1007/s10404-010-0700-6
• Zohar, Y., Jiang, L., Schroeder, J. A., Cheung, L. S., & Zheng, X. (2011). A high-performance microsystem for isolating circulating tumor cells. Lab on a Chip. doi:10.1039/c1lc20331b
• Zohar, Y., Jiang, L., Schroeder, J. D., Cheung, L. S., & Zheng, X. (2011). Cell receptor and surface ligand density effects on dynamic states of adhering circulating tumor cells. Lab on a Chip. doi:10.1039/c1lc20455f
• Zohar, Y., Lee, Y. C., & Lee, H. M. (2011). Design, fabrication and characterization of a thermal microsystem integrated with heaters, pressure and temperature microsensors. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/21/12/125021
• Zohar, Y., Zheng, X., Cheung, L. S., Schroeder, J. A., Jiang, L., & Zohar, Y. -. (2011). A high-performance microsystem for isolating circulating tumor cells. Lab on a chip, 11(19).
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  A unique flow field pattern in a bio-functional microchannel is utilized to significantly enhance the performance of a microsystem developed for selectively isolating circulating tumor cells from cell suspensions. For high performance of such systems, disposal of maximum non-target species is just as important as retention of maximum target species; unfortunately, most studies ignore or fail to report this aspect. Therefore, sensitivity and specificity are introduced as quantitative criteria to evaluate the system performance enabling a direct comparison among systems employing different techniques. The newly proposed fluidic scheme combines a slow flow field, for maximum target-cell attachment, followed by a faster flow field, for maximum detachment of non-target cells. Suspensions of homogeneous or binary mixtures of circulating breast tumor cells, with varying relative concentrations, were driven through antibody-functionalized microchannels. Either EpCAM or cadherin-11 transmembrane receptors were targeted to selectively capture target cells from the suspensions. Cadherin-11-expressing MDA-MB-231 cancer cells were used as target cells, while BT-20 cells were used as non-target cells as they do not express cadherin-11. The attachment and detachment of these two cell lines are characterized, and a two-step attachment/detachment flow field pattern is implemented to enhance the system performance in capturing target cells from binary mixtures. While the system sensitivity remains high, above 0.95, the specificity increases from about 0.85 to 0.95 solely due to the second detachment step even for a 1 : 1000 relative concentration of the target cells.
• Zohar, Y., Zheng, X., Cheung, L. S., Schroeder, J. A., Jiang, L., & Zohar, Y. -. (2011). Cell receptor and surface ligand density effects on dynamic states of adhering circulating tumor cells. Lab on a chip, 11(20).
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  Dynamic states of cancer cells moving under shear flow in an antibody-functionalized microchannel are investigated experimentally and theoretically. The cell motion is analyzed with the aid of a simplified physical model featuring a receptor-coated rigid sphere moving above a solid surface with immobilized ligands. The motion of the sphere is described by the Langevin equation accounting for the hydrodynamic loadings, gravitational force, receptor-ligand bindings, and thermal fluctuations; the receptor-ligand bonds are modeled as linear springs. Depending on the applied shear flow rate, three dynamic states of cell motion have been identified: (i) free motion, (ii) rolling adhesion, and (iii) firm adhesion. Of particular interest is the fraction of captured circulating tumor cells, defined as the capture ratio, via specific receptor-ligand bonds. The cell capture ratio decreases with increasing shear flow rate with a characteristic rate. Based on both experimental and theoretical results, the characteristic flow rate increases monotonically with increasing either cell-receptor or surface-ligand density within certain ranges. Utilizing it as a scaling parameter, flow-rate dependent capture ratios for various cell-surface combinations collapse onto a single curve described by an exponential formula.
• Cheung, L. S., Zheng, X., Wang, L., Guzman, R., Schroeder, J. A., Heimark, R. L., Baygents, J. C., & Zohar, Y. (2010). Kinematics of specifically captured circulating tumor cells in bio-functionalized microchannels. Journal of Microelectromechanical Systems, 19(4), 752-763.
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  Abstract: The attachment kinematics of cancer cells under hydrodynamic loading in antibody-functionalized microchannels has been studied. Epithelial-cell- adhesion-molecule antibodies are immobilized on the microchannel surface for specific capture of the target cancer cells from homogeneous cell suspensions. The specific interaction between the cancer cell receptors and the immobilized antibodies under static conditions is demonstrated. The capture efficiency of the target cells from homogeneous suspensions under applied hydrodynamic flow field has been investigated, revealing a characteristic shear stress. Applying a lower stress allows the capture of most target cells, while the capture efficiency drops sharply with an increasing shear stress. The captured cells are spatially distributed along the microchannel; both the velocity and the distance travelled by cells prior to capture are measured. The characteristic time and length scales for cell capture are determined, and a log-normal statistical distribution is proposed to describe the observations. Furthermore, a first-order kinetic model for receptorligand bond formation provides a rough estimate of the cell adhesion rate constant. Under a low shear stress, the on-rate is much higher than the off-rate, allowing capture of most loaded cells. The off-rate constant increases exponentially with an increasing shear stress, such that above the characteristic stress level, most loaded cells avoid capture. © 2010 IEEE.
• Zheng, X. J., Cheung, L. S., Wang, L., Schroeder, J., Heimark, R. L., Baygents, J. C., Guzman, R., & Zohar, Y. (2010). Quantitative specific binding of breast cancer cells in an antibody-functionalized microchamber array. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 939-942.
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  Abstract: The system performance of an antibody-functionalized microchamber array, developed for selectively binding target cancer cells, is quantitatively characterized. The devices are designed to allow accurate counting of loaded and captured cells from heterogeneous suspensions. Specific interaction between cancer cell receptors and immobilized surface ligands has been demonstrated under static conditions. Performance measures such as specificity, sensitivity and accuracy are uniquely defined and experimentally evaluated. Several cell mixtures have been tested, and the quantitative system performance criteria are reported as a function of cell concentrations in the tested suspensions. These criteria provide a critical quantitative basis for a comparison among different systems aimed at selectively isolating target bio-species from complex mixtures. ©2010 IEEE.
• Zheng, X., Cheung, S. L., Wang, L., Schroeder, J. A., Heimark, R. L., Baygents, J. C., Guzman, R., & Zohar, Y. (2010). Specific binding of cancer cells using a microchamber array functionalized with antibodies. ASME International Mechanical Engineering Congress and Exposition, Proceedings, 12(PART B), 821-827.
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  Abstract: Specific binding of target suspended metastatic cancer cells to an antibody-functionalized surface utilizing a microfluidic device has experimentally been investigated under various conditions. The microfluidic devices, fabricated in silicon using DRIE process, consisted of a 5×5 micochamber array; each 1mm×1mm in area, and 50pm in depth. The oxide surface of the microchammber array was functionalized with various antibodies immobilized on a protein G layer. The microfluidic device design allows accurate counting of cells loaded into each microchamber and, thus, enabling a reliable counting of cells captured from homogeneous suspensions. The effects of cell suspension concentration, incubation times and ambient temperature on cell capture efficiency have been examined. Furthermore, to evaluate the specificity of the cell-surface interaction, several cell cancer types expressing different membrane receptors have been incubated on surfaces functionalized with various counter receptors. Specific binding of up to 100% of the suspended cells is observed when using surfaces functionalized with counter receptors matching the cell receptors; otherwise, non-specific binding of less than 15% of suspended cells is obtained if the functionalized counter receptors do not match the cell receptors. Copyright © 2010 by ASME.
• Chan, Y. C., Zohar, Y., & Lee, Y. (2009). Effects of embedded sub-micron pillar arrays in microfluidic channels on large DNA electrophoresis. Electrophoresis, 30(18), 3242-3249.
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  PMID: 19722207;Abstract: A study of the influences of embedding artificial structures in a microfluidic device for CE with a free buffer solution is presented. Compared with conventional slab-gel electrophoresis, three major additional effects on the overall system performance are identified when sub-micron pillar arrays are integrated into a standard CE microsystem. Since DNA molecules have to migrate in-between and interact with the pillars, pillar geometry is first demonstrated to have a direct impact on the DNA motion pattern. Electric field re-distribution is another inevitable outcome when features of sub-micron dimensions are placed inside a microchannel. This effect is verified by a numerical simulation tool. Furthermore, the integration of the closely packed sub-micron structures dramatically increases the surface to volume ratios in the microfluidic device and therefore generates a large EOF. The consequence of these additional influences implies a complexity in the measured DNA velocity and indicates that careful considerations have to be taken when these devices are used for DNA electrokinetics study or electrophoresis theory re-examination. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.
• Cooper, J. R., Qin, Y., Jiang, L., Palanivelu, R., & Zohar, Y. (2009). Microsystem-based study of pollen-tube attractants secreted by ovules. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 208-211.
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  Abstract: Microdevices are developed to resemble the in-vivo micro-environment of ovule fertilization by pollen tubes in model research plant Arabidopsis thaliana. The PDMSbased microdevices are filled with pollen germination medium (PGM) providing pollen tubes with a proper growth environment. Pollen tubes are found to grow within the microgrooves with an average rate of 130tm/hr, reaching a final tube length of about 450tm. Targeting ovules by pollen tubes has also been tested with an observed efficiency of about 67%. Both the pollen tube growth rate and the ovule targeting efficiency in microdevices are similar to those obtained using in-vitro plate experiments. Finally, initial results indicate that pollen tube bundles preferentially turn toward ovule containing chambers, suggesting that the pollen tubes respond to the attractants secreted by unfertilized ovules. ©2009 IEEE.
• Gudipaty, T., Cheung, L. S., Jiang, L., & Zohar, Y. (2009). Cluster formation and growth in flow of dilute particle suspension in microchannels. 2008 Proceedings of the 9th Biennial Conference on Engineering Systems Design and Analysis, 4, 357-362.
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  Abstract: Microchannels are susceptible to blockage by solid particles. The lifetime of microfluidic devices depends on their ability to maintain flow without interruption, while certain applications require microdevices for transport of liquids containing particles. In this work, the phenomenon of interest is the formation and growth of clusters in the flow of a dilute suspension of hard spheres. Based on the present experiments, aggregation of clusters was observed for particle-laden flow in a microchannel with particle void fraction as low as 0.001 and particle diameter to channel height ratio as low as 0.1. The incipience and growth of a single cluster is discussed, and the spatial distribution and time evolution of clusters along the microchannel is presented. Although the cluster size seems to be independent of location, higher number of clusters is found at the inlet/outlet regions than in the microchannel center. Similar to individual cluster, the total cluster area in the microchannel grows almost linearly in time. The effects of flow rate, particle size and concentration are also reported. Copyright © 2008 by ASME.
• Gudipaty, T., Stamm, M. T., Cheung, L. S., Zohar, Y., & Jiang, L. (2009). Cluster dynamics in flow of suspended particles in microchannels. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 379-382.
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  Abstract: Cluster dynamics in microchannels due to flow of dilute suspensions of polystyrene spherical particles has been studied experimentally. Cluster-cluster interaction as well as the functional dependence of cluster growth rate on several control parameters has been studied. Destructive, e.g. cluster collision, and constructive modes, e.g. cluster merging, of cluster-cluster interaction have been observed. Cluster growth rate with time is found to increase with either particle concentration or shear strain rate, and decrease with channel-height to particle-diameter ratio. ©2009 IEEE.
• Siu, L., Zheng, X., Stopa, A., Baygents, J. C., Guzman, R., Schroeder, J. A., Heimark, R. L., & Zohar, Y. (2009). Detachment of captured cancer cells under flow acceleration in a bio-functionalized microchannel. Lab on a Chip - Miniaturisation for Chemistry and Biology, 9(12), 1721-1731.
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  PMID: 19495456;Abstract: Attachment, deformation and detachment of N-cadherin expressing prostate and breast cancer cell lines in a functionalized microchannel under hydrodynamic loading have been studied. N-cadherin antibodies are immobilized on the microchannel surface to capture the target cancer cells, PC3N and MDA-MB-231-N, from a homogeneous cell suspension. Although difficult, a significant fraction of moving cells can be captured under a low flow rate. More than 90% of the target cells are captured after a certain incubation time under no flow condition. The mechanical response of a captured cancer cell to hydrodynamic flow field is investigated and, in particular, the effect of flow acceleration is examined. The observed cell deformation is dramatic under low acceleration, but is negligible under high acceleration. Consequently, the detachment of captured cells depends on both flow rate and flow acceleration. The flow rate required for cell detachment is a random variable that can be described by a log-normal distribution. Two flow acceleration limits have been identified for proper scaling of the flow rate required to detach captured cells. A time constant for the mechanical response of a captured cell, on the order of 1 min, has been identified for scaling the flow acceleration. Based on these acceleration limits and time constant, an exponential-like empirical model is proposed to predict the flow rate required for cell detachment as a function of flow acceleration. © 2009 The Royal Society of Chemistry.
• Zohar, Y., Cheung, L. S., Zheng, X., Stopa, A., Baygents, J. C., Guzman, R., Schroeder, J. A., Heimark, R. L., & Zohar, Y. -. (2009). Detachment of captured cancer cells under flow acceleration in a bio-functionalized microchannel. Lab on a chip, 9(12).
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  Attachment, deformation and detachment of N-cadherin expressing prostate and breast cancer cell lines in a functionalized microchannel under hydrodynamic loading have been studied. N-cadherin antibodies are immobilized on the microchannel surface to capture the target cancer cells, PC3N and MDA-MB-231-N, from a homogeneous cell suspension. Although difficult, a significant fraction of moving cells can be captured under a low flow rate. More than 90% of the target cells are captured after a certain incubation time under no flow condition. The mechanical response of a captured cancer cell to hydrodynamic flow field is investigated and, in particular, the effect of flow acceleration is examined. The observed cell deformation is dramatic under low acceleration, but is negligible under high acceleration. Consequently, the detachment of captured cells depends on both flow rate and flow acceleration. The flow rate required for cell detachment is a random variable that can be described by a log-normal distribution. Two flow acceleration limits have been identified for proper scaling of the flow rate required to detach captured cells. A time constant for the mechanical response of a captured cell, on the order of 1 min, has been identified for scaling the flow acceleration. Based on these acceleration limits and time constant, an exponential-like empirical model is proposed to predict the flow rate required for cell detachment as a function of flow acceleration.
• Zohar, Y., Heimark, R. L., Schroeder, J. A., Guzman, R., Baygents, J. C., Stopa, A., Zheng, X., & Cheung, L. S. (2009). Detachment of captured cancer cells under flow acceleration in a bio-functionalized microchannel. Lab on a Chip. doi:10.1039/b822172c
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  Attachment, deformation and detachment of N-cadherin expressing prostate and breast cancer cell lines in a functionalized microchannel under hydrodynamic loading have been studied. N-cadherin antibodies are immobilized on the microchannel surface to capture the target cancer cells, PC3N and MDA-MB-231-N, from a homogeneous cell suspension. Although difficult, a significant fraction of moving cells can be captured under a low flow rate. More than 90% of the target cells are captured after a certain incubation time under no flow condition. The mechanical response of a captured cancer cell to hydrodynamic flow field is investigated and, in particular, the effect of flow acceleration is examined. The observed cell deformation is dramatic under low acceleration, but is negligible under high acceleration. Consequently, the detachment of captured cells depends on both flow rate and flow acceleration. The flow rate required for cell detachment is a random variable that can be described by a log-normal distribution. Two flow acceleration limits have been identified for proper scaling of the flow rate required to detach captured cells. A time constant for the mechanical response of a captured cell, on the order of 1 min, has been identified for scaling the flow acceleration. Based on these acceleration limits and time constant, an exponential-like empirical model is proposed to predict the flow rate required for cell detachment as a function of flow acceleration.
• Zohar, Y., Lee, Y., & Chan, Y. C. (2009). Effects of embedded sub-micron pillar arrays in microfluidic channels on large DNA electrophoresis. Electrophoresis. doi:10.1002/elps.200900127
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  A study of the influences of embedding artificial structures in a microfluidic device for CE with a free buffer solution is presented. Compared with conventional slab-gel electrophoresis, three major additional effects on the overall system performance are identified when sub-micron pillar arrays are integrated into a standard CE microsystem. Since DNA molecules have to migrate in-between and interact with the pillars, pillar geometry is first demonstrated to have a direct impact on the DNA motion pattern. Electric field re-distribution is another inevitable outcome when features of sub-micron dimensions are placed inside a microchannel. This effect is verified by a numerical simulation tool. Furthermore, the integration of the closely packed sub-micron structures dramatically increases the surface to volume ratios in the microfluidic device and therefore generates a large EOF. The consequence of these additional influences implies a complexity in the measured DNA velocity and indicates that careful considerations have to be taken when these devices are used for DNA electrokinetics study or electrophoresis theory re-examination.
• Brand, O., & Zohar, Y. (2008). Welcome address. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), i.
• Jiang, L., Gerhardt, K. P., Myer, B., Zohar, Y., & Pau, S. (2008). An SU-8 based fluidic immuno-spectroscopic lab-on-a-chip for rapid quantitative detection of biomolecules. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 204-207.
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  Abstract: A 30mm-long multimode waveguide, 40μm wide and 40μm high, is fabricated on a silicon wafer using polymer SU-8 as the core and liquid buffer as the cladding. Antibodies are successfully immobilized on the SU-8 surface designated for binding target antigens dispersed in the buffer solution. Evanescent-wave spectroscopy is performed by exciting the fluorescently-labeled antigens, bound to the waveguide surface within its evanescence field, and measuring emission light intensity. This evanescent-wave biosensor detects specific molecular interaction; its output depends on antigen concentration with a sensitivity of 40 light counts per 1μg/ml. ©2008 IEEE.
• Jiang, L., Gerhardt, K. P., Myer, B., Zohar, Y., & Pau, S. (2008). Evanescent-wave spectroscopy using an SU-8 waveguide for rapid quantitative detection of biomolecules. Journal of Microelectromechanical Systems, 17(6), 1495-1500.
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  Abstract: A 30-mm-long multimode waveguide, 40 μm wide and 40 μm high, is fabricated on a silicon wafer using polymer SU-8 as the core and liquid buffer as the cladding. Antibodies are successfully immobilized on the SU-8 surface designated for binding target antigens dispersed in the buffer solution. Evanescent-wave spectroscopy is performed by exciting the fluorescently labeled antigens, bound to the waveguide surface within its evanescence field, and measuring the emission light intensity. This evanescent-wave biosensor detects specific molecular interaction. The optical output as a function of the antigen concentration can be described by Langmuir equation. Antigen concentration as low as 1.5 μg/mL is detected; concentrations higher than 100 μg/mL lead to sensor saturation. [2008-0058]. © 2008 IEEE.
• Chan, Y. C., Lee, Y., & Zohar, Y. (2007). Pillar size effect on DNA electrophoresis in microchips with sub-micron pillar arrays. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 413-416.
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  Abstract: This paper presents a systematic study of DNA electrophoresis in microchannels integrated with sub-micron pillar arrays. Electrophoretic mobility of different DNA fragments in channels having pillars of various dimensions is measured. In addition to confirming a previously observed nonlinear relationship with applied electric field following the Biased Reptation with Fluctuations Model, the mobility dependence on DNA size and pillar spacing is characterized and discussed. Similar to conventional slab-gel electrophoresis, short DNA molecules have higher migration mobility. However, a "band inversion"-like phenomenon is observed for larger DNA molecules. The pillar array significantly affects the electric field distribution in the separation channel and, consequently, the resulting mobility. Although this leads to a higher mobility in more closely packed pillars, excessive reduction of the pillar spacing generates a large retarding force. This counteracts the effect of electric field on the overall DNA mobility. © 2007 IEEE.
• Cheung, L. S., Quick, R., Singh, S. K., Weichsel, A., Montfort, W. R., & Zohar, Y. (2007). Microsystems for UV-visible and X-ray analysis of protein crystals. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 569-572.
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  Abstract: polydimethylsiloxane (PDMS) based microsystems have successfully been fabricated and characterized for studying protein crystals utilizing both UV-visible spectroscopy and X-ray crystallography. Transmittance tests have been conducted with PDMS and glass substrates; the measurements indicate that in PDMS, unlike glass, the emerging intensity is higher than 50% of the incident intensity as long as the total optical path is shorter than 100μm. Indeed, both the UV-visible spectrum and X-ray diffraction of a protein crystal enclosed in a PDMS device are almost identical to those of the crystal alone. Hence, PDMS is suitable as substrate material in device fabrication to study protein crystals. In glass, however, the UV-visible spectrum is significantly distorted and the X-ray diffraction pattern is rather weak resulting in poor signal to noise ratio. Furthermore, microsystems integrated with microchannels allowing continuous exchange of buffer solution around the protein crystals have been tested; this would greatly enhance the potential to induce, trap and characterize functional states in proteins. © 2007 IEEE.
• Iwama, R., Lee, L. M., Cho, E. S., & Zohar, Y. (2007). Fabrication of microchannels with patterned bio-active layers. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 333-336.
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  Abstract: Patterns of bio-active coatings on the inner surfaces of microchannels have been realized using a novel low-temperature, UV-epoxy glass-to-silicon bonding technique. Sucrose is applied as a protection layer for the immobilized bio-functional films during the bonding step. The bio-functional layer is composed of antibody patterns, for binding specific targets, next to polyethylene glycol (PEG) coated regions for preventing non-specific absorption. The activity of the patterned bio layer is tested, after the removal of the sucrose protection layer, utilizing fluorescent microscopy. A solution of fluorescent-labelled antigens is injected into the microchannels for incubation with the immobilized antibodies. Upon exposure to proper radiation, light is emitted only from the antibody patterns while the PEG regions remain dark. Hence, the sucrose-protection and UV-bonding techniques have not significantly compromised the functionality of the patterned antibodies, in binding to their counter receptors, and PEG molecules, in preventing non-specific adsorption, at the end of the fabrication process. ©2007 IEEE.
• Lee, M., Lee, Y., & Zohar, Y. (2007). The nusselt number in single-phase liquid flow forced convection in microchannels. Proceedings of the 2nd IEEE International Conference on Nano/Micro Engineered and Molecular Systems, IEEE NEMS 2007, 502-506.
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  Abstract: A thermal microsystem, integrated with pressure and temperature microsensors, is fabricated to study single-phase liquid flow forced convection under uniform heat flux boundary condition. Standard micromachining techniques were utilized in the fabrication of the integrated microsystem. Utilizing a waferbond-and-etch-back technology, the heat source, temperature and pressure sensors are separated from the fluid flow by a 1.5μm thick composite membrane; thus, allowing experimentally good control of the thermal boundary conditions. A threedimensional numerical simulation model has been constructed to investigate the heat flux distribution. The results show that upstream the cold working fluid absorbs heat, while downstream the warmer working fluid releases heat. The Nusselt number is calculated based on the computations, which are compared with analytical and experimental results. The wall Nusselt number in a microchannel can only be estimated by conventional analytical solution in a limited Reynolds number range. The estimated Nusselt number for forced convection is found to be highly dependent on the location of the temperature measurements. © 2007 IEEE.
• Lee, M., Lee, Y., & Zohar, Y. (2007). Two-phase flow oscillations in microchannel convective boiling. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 635-638.
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  Abstract: A thermal microsystem with integrated heaters, pressure and temperature microsensors, has been fabricated to study local temperature and pressure fluctuations occurring in forced convective boiling in microchannels. The observed two-phase flows can be classified into two patterns: oscillating liquid/vapour interface and liquid burst flow; both leading to unsteady temperature and pressure fields. FFT power spectra of the measured signals are correlated with flow visualizations to analyse the two-phase flow modes. The dominant fluctuation frequency of each flow mode increases with input power; and, under similar conditions, the frequency of the periodically oscillating liquid/vapour interface is higher than the dominant frequency of the liquid bursts. Dimensional analysis is performed to derive empirical correlations for the dimensionless fluctuation frequency, Strouhal number, for both flow patterns. © 2007 IEEE.
• Lee, Y., Lee, L. M., Lap, W., & Zohar, Y. (2007). Two-dimensional analysis of electrokinetically driven out-of-plane vortices in a microchannel liquid flow using patterned surface charge. Journal of Microelectromechanical Systems, 16(1), 58-67.
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  Abstract: The technology developed for photolithographically patterning the electric surface charge to be negative, positive, or neutral enables the realization of complex liquid flows even in straight and uniform microchannels with extremely small Reynolds number. A theoretical model to analyze a steady incompressible electrokinetically driven two-dimensional liquid flow in a microchannel with an inhomogeneous surface charge under externally applied electric field is derived. The flow field is obtained analytically by solving the biharmonic equation with the Helmholtz-Smoluchowski slip boundary condition using the Fourier series expansion method. The model has been applied to study three basic out-of-plane vortical flow fields: single vortex and a train of corotating and a series of counterrotating vortex pairs. For model verification, the solution for the single vortex has been tested against numerical computations based on the full Navier-Stokes equations revealing the dominant control parameters. Two interesting phenomena have been observed in out-of-plane multivortex dynamics: merging of corotating vortices and splitting of counterrotating vortices. The criteria for the onset of both phenomena are discussed. © 2007 IEEE.
• Wei, M. a., Zhu, R., Rufer, L., Zohar, Y., & Wong, M. (2007). An integrated floating-electrode electric microgenerator. Journal of Microelectromechanical Systems, 16(1), 29-37.
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  Abstract: Microfabricated electric generators, scavenging ambient mechanical energy, are potential power sources for autonomous systems. Described presently are the design, modeling, and implementation of a single-wafer floating-electrode electric microgenerator, integrating a micromechanical resonator and a number of electronic devices. Forming a plate of a variable capacitor, the resonator is responsible for converting mechanical vibration to electricity. A sense transistor and a diode bridge are integrated, respectively, for monitoring the "charging"of the floating electrode and for rectification. A lumped electromechanical model of the generator is developed and expressed in terms of a set of nonlinear coupled state equations that are numerically solved. For small-amplitude excitation, a circuit based on a set of linearized equations is developed. The generator is realized using a compatible combination of standard complementary metal-oxide-semiconductor (CMOS) "floating gate"process and a post-CMOS photoresist molded electroplating process. Adequate agreement between model predictions and measurement results was obtained. © 2007 IEEE.
• Zohar, Y., Hau, W. L., Lee, L. M., & Lee, Y. (2007). Two-Dimensional Analysis of Electrokinetically Driven Out-of-Plane Vortices in a Microchannel Liquid Flow Using Patterned Surface Charge. Journal of microelectromechanical systems. doi:10.1109/jmems.2006.885992
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  The technology developed for photolithographically patterning the electric surface charge to be negative, positive, or neutral enables the realization of complex liquid flows even in straight and uniform microchannels with extremely small Reynolds number. A theoretical model to analyze a steady incompressible electrokinetically driven two-dimensional liquid flow in a microchannel with an inhomogeneous surface charge under externally applied electric field is derived. The flow field is obtained analytically by solving the biharmonic equation with the Helmholtz-Smoluchowski slip boundary condition using the Fourier series expansion method. The model has been applied to study three basic out-of-plane vortical flow fields: single vortex and a train of corotating and a series of counterrotating vortex pairs. For model verification, the solution for the single vortex has been tested against numerical computations based on the full Navier-Stokes equations revealing the dominant control parameters. Two interesting phenomena have been observed in out-of-plane multivortex dynamics: merging of corotating vortices and splitting of counterrotating vortices. The criteria for the onset of both phenomena are discussed
• Zohar, Y., Wong, M., Rufer, L., Zhu, R., & Ma, W. (2007). An Integrated Floating-Electrode Electric Microgenerator. Journal of microelectromechanical systems. doi:10.1109/jmems.2006.885856
• Chan, Y. C., Lee, Y., & Zohar, Y. (2006). High-throughput design and fabrication of an integrated microsystem with high aspect-ratio sub-micron pillar arrays for free-solution micro capillary electrophoresis. Journal of Micromechanics and Microengineering, 16(4), 699-707.
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  Abstract: A new technology approach for the design, fabrication and application of an integrated free-solution capillary electrophoresis microsystem is presented. Combining the advantages of projection, contact photolithography and deep-reactive-ion-etching, this approach allows fast and flexible formation of micron-sized channels integrated with extremely high aspect-ratio (>50:1) sub-micron pillar arrays on a silicon substrate. Utilizing fluorescence video microscopy, free-solution DNA separation has been demonstrated. Furthermore, the detailed DNA molecular interaction with the pillars inside the microsystem can be analysed. In comparison with the previously reported fabrication technologies, such as electron beam lithography, the newly presented technology approach offers a significant improvement in fabrication time and design flexibility; both are highly desirable not only for potential commercialization of the free-solution electrophoresis microsystem in applications such as lab-on-a-chip but also for systematic studies of micro-scale DNA kinetics. © 2006 IOP Publishing Ltd.
• Lee, L. M., Heimark, R. L., Baygents, J. C., & Zohar, Y. (2006). Self-aligned immobilization of proteins utilizing PEG patterns. Nanotechnology, 17(4), S29-S33.
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  PMID: 21727351;Abstract: A novel self-aligned method to selectively immobilize proteins on a silicon dioxide surface is developed in conjunction with a standard lift-off patterning technique of a PEG layer. The approach is designed to photolithographically pattern regions that specifically bind target proteins and particles, surrounded by regions that suppress non-specific attachment of bio-species. The physical and biological properties of the derivatized surfaces at the end of the fabrication process are characterized. © 2006 IOP Publishing Ltd.
• Lee, L. M., Heimark, R. L., Guzman, R., Baygents, J. C., & Zohar, Y. (2006). Low melting point agarose as a protection layer in photolithographic patterning of aligned binary proteins. Lab on a Chip - Miniaturisation for Chemistry and Biology, 6(8), 1080-1085.
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  PMID: 16874382;Abstract: A novel photolithography method to build aligned patterns of two different proteins is presented. Chessboard patterns of 125 μm × 125 μm squares are constructed on a silicon dioxide substrate, using standard photoresist chemistries in combination with low-temperature oxygen plasma etching. Low-melting-point agarose (LMPA) is used to protect underlying protein layers and, at the appropriate stage, the digestive enzyme GELase (EPICENTRE) is used to selectively remove the prophylactic LMPA layers. Two antibodies, mouse-IgG and human-IgG, were immobilized and patterned by this procedure. The patterned antibodies maintained the specificity of their antigen-antibody binding, as demonstrated by fluorescence microscopy. In addition, normalized fluorescence intensity profiles illustrate that the patterned proteins layers are uniform (standard deviations below 0.05). Finally, a trypsin activity test was conducted to probe the effect of the patterning protocol on immobilized enzymes; the results imply that this photolithographic process using LMPA as a protection layer preserves 70% of immobilized enzyme activity. © The Royal Society of Chemistry 2006.
• Lee, L. M., Lap, W., Lee, Y., & Zohar, Y. (2006). In-plane vortex flow in microchannels generated by electroosmosis with patterned surface charge. Journal of Micromechanics and Microengineering, 16(1), 17-26.
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  Abstract: Electrokinetically driven in-plane vortex flows in a microchannel are studied utilizing a patterned surface charge technique requiring both positively and negatively charged regions on the same substrate. In the first part, a periodic flow pattern consisting of counter-rotating vortex pairs is analyzed experimentally and numerically; this is a relatively easy flow to experimentally realize in the lab since no charge-free region is necessary. The good agreement between the measured and computed flow fields demonstrates that: (i) the surface charge patterning technique can be used for driving electrokinetically complex vortex flow patterns in microchannels, and (ii) the applied CFD code can be used for calculating reliably such flow fields. In the second part, the numerical scheme is utilized to study a single, in-plane vortex in order to reveal the proper length and velocity scales as well as the dominant control parameters. This flow field, although simpler, is very difficult to realize experimentally due to the need for a large surface area carrying no charge. The resulting 3D flow field features a coherent vortex with its axis perpendicular to the symmetrically charged regions on the top and bottom surfaces of the microchannel. Three length scales, the active-region length and width as well as the channel height, and a velocity scale, the speed of the electroosmotic flow, have been identified as the relevant variables. The strength of the in-plane vortex along with several flow patterns has been characterized on the basis of these four independent variables.
• Lee, M., Siu, L., Lee, Y., & Zohar, Y. (2006). Boundary conditions and integrated sensors in microchannel convective heat transfer. Proceedings of the 4th International Conference on Nanochannels, Microchannels and Minichannels, ICNMM2006, 2006 B, 1365-1371.
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  Abstract: A microchannel heat sink, integrated with pressure and temperature microsensors, is fabricated to study convective boiling under uniform heat flux boundary condition. Utilizing a wafer bond and etch back technology, the heat source, temperature and pressure sensors are separated from the fluid flow by a membrane only 1.5μm in thickness; thus, allowing good control of the thermal boundary conditions. Temperature and pressure distributions for various power levels and flow rates are measured while, simultaneously, the flow patterns are recorded. Single-phase flow results, compared with numerical simulations, confirm that the heat flux boundary condition is indeed nearly uniform. The sensor arrays, particularly for two-phase flow, provide the spatial and temporal dependence of both the temperature and pressures fields. Copyright © 2006 by ASME.
• Lee, M., Siu, L., Lee, Y., & Zohar, Y. (2006). Two-phase flow in MicroChannel heat sink with nearly uniform heat flux boundary condition. Proceedings of 1st IEEE International Conference on Nano Micro Engineered and Molecular Systems, 1st IEEE-NEMS, 752-756.
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  Abstract: A thermal microsystem, integrated with pressure and temperature microsensors, is fabricated to study convective boiling under nearly uniform heat flux boundary condition. The temperature and pressure distribution along the microchannel is measured correspondingly. The pressure increases with input power when two phase flow develops. A pressure peak appears at the location of liquid-vapor interface region. The transient temperature and pressure fluctuation is also measured. The dominant frequencies of the temperature and pressure fluctuation are the same values at the liquid-vapor interface region and this dominant frequencies increase with input power. Simultaneously, the qualitative visualizations of the evolving flow patterns have been correlated with the quantitative temperature and pressure measurements. © 2006 IEEE.
• Zohar, Y., Baygents, J. C., Guzman, R., Heimark, R. L., & Lee, L. M. (2006). Low melting point agarose as a protection layer in photolithographic patterning of aligned binary proteins. Lab on a Chip. doi:10.1039/b603095e
• Zohar, Y., Baygents, J. C., Heimark, R. L., & Lee, L. M. (2006). Self-aligned immobilization of proteins utilizing PEG patterns. Nanotechnology. doi:10.1088/0957-4484/17/4/006
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  A novel self-aligned method to selectively immobilize proteins on a silicon dioxide surface is developed in conjunction with a standard lift-off patterning technique of a PEG layer. The approach is designed to photolithographically pattern regions that specifically bind target proteins and particles, surrounded by regions that suppress non-specific attachment of bio-species. The physical and biological properties of the derivatized surfaces at the end of the fabrication process are characterized.
• Zohar, Y., Lee, L. M., Heimark, R. L., Baygents, J. C., & Zohar, Y. -. (2006). Self-aligned immobilization of proteins utilizing PEG patterns. Nanotechnology, 17(4).
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  A novel self-aligned method to selectively immobilize proteins on a silicon dioxide surface is developed in conjunction with a standard lift-off patterning technique of a PEG layer. The approach is designed to photolithographically pattern regions that specifically bind target proteins and particles, surrounded by regions that suppress non-specific attachment of bio-species. The physical and biological properties of the derivatized surfaces at the end of the fabrication process are characterized.
• Zohar, Y., Lee, L. M., Heimark, R. L., Guzman, R., Baygents, J. C., & Zohar, Y. -. (2006). Low melting point agarose as a protection layer in photolithographic patterning of aligned binary proteins. Lab on a chip, 6(8).
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  A novel photolithography method to build aligned patterns of two different proteins is presented. Chessboard patterns of 125 microm x 125 microm squares are constructed on a silicon dioxide substrate, using standard photoresist chemistries in combination with low-temperature oxygen plasma etching. Low-melting-point agarose (LMPA) is used to protect underlying protein layers and, at the appropriate stage, the digestive enzyme GELase (EPICENTRE) is used to selectively remove the prophylactic LMPA layers. Two antibodies, mouse-IgG and human-IgG, were immobilized and patterned by this procedure. The patterned antibodies maintained the specificity of their antigen-antibody binding, as demonstrated by fluorescence microscopy. In addition, normalized fluorescence intensity profiles illustrate that the patterned proteins layers are uniform (standard deviations below 0.05). Finally, a trypsin activity test was conducted to probe the effect of the patterning protocol on immobilized enzymes; the results imply that this photolithographic process using LMPA as a protection layer preserves 70% of immobilized enzyme activity.
• Zohar, Y., Lee, Y. C., & Chan, Y. C. (2006). High-throughput design and fabrication of an integrated microsystem with high aspect-ratio sub-micron pillar arrays for free-solution micro capillary electrophoresis. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/16/4/005
• Zohar, Y., Lee, Y., Hau, W. L., & Lee, L. M. (2006). In-plane vortex flow in microchannels generated by electroosmosis with patterned surface charge. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/16/1/003
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  Electrokinetically driven in-plane vortex flows in a microchannel are studied utilizing a patterned surface charge technique requiring both positively and negatively charged regions on the same substrate. In the first part, a periodic flow pattern consisting of counter-rotating vortex pairs is analyzed experimentally and numerically; this is a relatively easy flow to experimentally realize in the lab since no charge-free region is necessary. The good agreement between the measured and computed flow fields demonstrates that: (i) the surface charge patterning technique can be used for driving electrokinetically complex vortex flow patterns in microchannels, and (ii) the applied CFD code can be used for calculating reliably such flow fields. In the second part, the numerical scheme is utilized to study a single, in-plane vortex in order to reveal the proper length and velocity scales as well as the dominant control parameters. This flow field, although simpler, is very difficult to realize experimentally due to the need for a large surface area carrying no charge. The resulting 3D flow field features a coherent vortex with its axis perpendicular to the symmetrically charged regions on the top and bottom surfaces of the microchannel. Three length scales, the active-region length and width as well as the channel height, and a velocity scale, the speed of the electroosmotic flow, have been identified as the relevant variables. The strength of the in-plane vortex along with several flow patterns has been characterized on the basis of these four independent variables.
• Chan, Y. C., Lee, Y., Wong, M., & Zohar, Y. (2005). High-throughput fabrication of sub-micron pillar arrays for free-solution DNA electrophoresis without e-beam lithography. 2005 IEEE International Conference on Robotics and Biomimetics, ROBIO, 2005, 101-104.
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  Abstract: A new technology approach for the design, fabrication and application of an integrated free-solution electrophoresis microsystem is presented. The technology approach, which utilizes the dual-photolithography provided from a stepper and a contact aligner, allows fast and flexible formation of sub-micron pillar array patterns on a silicon substrate. Combining with standard microfabrication technique and deep-reactive-ion-etching (DRIE), very high aspect-ratio sub-micron pillar arrays can be integrated in regular microfluidic system for free-solution DNA electrophoresis. DNA interaction with pillars inside the microsystem has been demonstrated, supporting the new technology approach a novel technique for Si-based free-solution electrophoresis microsystem fabrication. © 2005 IEEE.
• Lap, W., Leung, P. K., Lee, Y., Lee, L. M., & Zohar, Y. (2005). Electrokinetically-driven micro mixer with a novel surface-charge pattern. 2005 IEEE International Conference on Robotics and Biomimetics, ROBIO, 2005, 783-787.
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  Abstract: An electrokinetically-driven micro mixer with a special surface-charge pattern was designed, fabricated and characterized using fluorescence video microscopy. Zeta potential of the working fluids was measured to facilitate the numerical simulation and optimization of the proposed mixer. The mixing flow of the electrolyte with and without microparticle was digitally recorded and analyzed in terms of concentration profile and mixing index. The experimental mixing enhancement is consistent with the simulation result. ©2005 IEEE.
• Lee, L. M., Heimark, R. L., Baygents, J. C., & Zohar, Y. (2005). Self-aligned immobilization of proteins on silicon dioxide surfaces. AIChE Annual Meeting, Conference Proceedings, 14077-.
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  Abstract: In the last decade, the field of Bio-MEMS has emerged as an application using both microsystem technology and biotechnology. Novel surface nanotechnologies have been developed to selectively modify artificial surfaces with active bio-layers to be used for biosensors, cell studies and tissue engineering. These methods include local deposition of bio-molecules using ink-jet, laser ablation, vapor deposition, photochemical structuring and photolithography techniques. Currently, the most popular technique is contact printing, where the printing process allows the transfer of molecules to various surfaces in a wide concentration range with high efficiency. However, this may not be an attractive method if a precise alignment to prefabricated fine features is required or if precise immobilization of different proteins on the same chip is needed. Photolithography is a well-established technique in batch fabrication of ICs with high resolution and precise alignment. Patterns are transferred from a mask to a photoresist layer-and from the photoresist to a thin film of bio-molecules immobilized on a surface. However, the solutions used to develop and strip the photoresist layer lead to stability problems for functional proteins. In this work, we utilized standard photolithographic procedures to generate stable patterns consisting of a functional protein (protein A + IgG antibody) next to end-grafted PEG self-assembled monolayers on silicon dioxide surfaces. The PEG patterns were generated using the lift-off method, while the immobilized proteins were self-aligned on the surface not protected by the PEG layer. Both the PEG and the protein layers have been characterized by demonstrating their proper functionality at the end of the fabrication process. Physical characterization of the surface modifications included optical microscopy, contact angle measurements, and AFM profiling. Protein patterns were shown to be biologically active by the selective (antibody-antigen) binding of microparticles covered with IgG antigen; typically, selectivities on the order of 0.96 were obtained.
• Lee, L. M., Lap, W., Lee, Y., Wong, M., & Zohar, Y. (2005). Unsteady in-plane vortex motion in a microchannel liquid flow. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 734-737.
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  Abstract: An unsteady flow can dramatically enhance the mixing efficiency in a highly localized region, as the flow would become chaotic if time is an independent variable. In this work, the response of uniform electroosmotic flow to an oscillating electric field is first examined experimentally and numerically as a function of the driving frequency. Then a steady in-plane micro vortex flow pattern, traced by microparticles, is realized and compared to numerical simulations. Upon confirmation of the simulations for uniform but unsteady and steady but non-uniform flows, the CFD code has finally been applied to study unsteady non-uniform flow field, for which it is difficult to measure flow properties. The time evolution of liquid vortex motion in a microchannel, due to either sinusoidal or sudden electric field reversal, is numerically investigated revealing the relationship between length and time scales dominating momentum transfer in electrokinetically-driven unsteady liquid flow. © 2005 IEEE.
• Lee, M., Siu, L., Lee, Y., & Zohar, Y. (2005). Height effect on nucleation-site activity and size-dependent bubble dynamics in microchannel convective boiling. Journal of Micromechanics and Microengineering, 15(11), 2121-2129.
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  Abstract: Forced convection boiling in microchannels is studied experimentally under the uniform heat flux boundary condition. Several microchannel heat sinks with integrated temperature sensors, spanning two orders of magnitude in height 5-500 νm, have been fabricated with designed nucleation sites on the bottom surfaces. The microchannels are capped by a glass wafer to monitor bubble activity using video microscopy. Distributed micro heater elements on the device backside are used as the heat source, while the working liquid flow rate is adjusted using a syringe pump. The boiling curves of the device temperature as a function of the input power have been measured for various flow rates. The curves for increasing and decreasing heat flux exhibit a hysteresis loop, while the conditions corresponding to the onset of nucleate boiling and critical heat flux (CHF) are clearly distinguishable. The activity of nucleation sites as well as the ensuing bubble dynamics, from incipience to departure, is found to depend on the channel height. The critical size above which a nucleation site is active, along with three aspects of bubble dynamics, namely growth rate, departure size and release frequency, have been characterized experimentally and proper control parameters have been identified. © 2005 IOP Publishing Ltd.
• Lee, M., Siu, L., Lee, Y., Wong, M., & Zohar, Y. (2005). Fabrication and characterization of an integrated thermal microsystem. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 610-613.
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  Abstract: A thermal microsystem, integrated with pressure and temperature microsensors, is fabricated to study convective boiling under uniform heat flux boundary condition. Utilizing a wafer bond and etch back technology, the heat source, temperature and pressure sensors are separated from the fluid flow by a membrane only 1.5μm in thickness; thus, allowing experimentally good control of the thermal boundary conditions. Temperature distributions for various input power levels have been measured to obtain the boiling curves. The conditions corresponding to the onset of two phase flow, with boiling plateau, and the critical heat flux (CHF) are clearly distinguishable. Simultaneously, time-dependent pressure signals have also been recorded. In single liquid phase flow, the pressure is constant with time. However, in two phase flow, strong pressure fluctuations occur with certain frequencies. The qualitative visualizations of the evolving flow patterns have been correlated with the quantitative temperature and pressure measurements. © 2005 IEEE.
• Siu, L., Lee, M., Lee, Y., Wong, M., & Zohar, Y. (2005). Size-dependent bubble dynamics in a microchannel heat sink. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 670-673.
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  Abstract: The height effect on bubble dynamics in a microchannel is experimentally studied. We reported that the critical size for a nucleation site to be active increases linearly with the channel height. However, once a bubble is formed, its evolution from incipience to departure can also be channel-size dependent. Thus, various microchannel heat sinks have been fabricated, about 5-10μm in height, with integrated temperature sensors utilizing Si-to-glass anodic bonding technology. Nucleation sites have been formed on the microchannels bottom silicon surface in order to ensure regular bubble formation, while the sensors allow continuous monitoring of the wall temperature. The microchannels are capped by a glass wafer; hence, it is possible to record the bubble activity using video equipment. The three aspects of bubble dynamics: growth rate, departure size and release frequency have been characterized experimentally, and proper control parameters have been identified. © 2005 IEEE.
• Tak, Z., Lee, Y., Wong, M., & Zohar, Y. (2005). Fluid flows in microchannels with cavities. Journal of Microelectromechanical Systems, 14(6), 1386-1398.
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  Abstract: Pressure-driven gas and liquid flows through microchannels with cavities have been studied using both experimental measurements and numerical computations. Several microchannels with cavities varying in shape, number and dimensions have been fabricated. One set of microdevices, integrated with sensors on a silicon wafer, is used for flow rate and pressure distribution measurements in gas flows. Another set of microdevices, fabricated using glass-to-silicon wafer bonding, is utilized for visualization of liquid flow patterns. The cavity effect on the flow in the microchannel is found to be very small, with the mass flow rate increasing slightly with increasing number of cavities. The flow pattern in the cavity depends on two control parameters; it is fully attached only if both the reduced Reynolds number and the cavity number are small. A flow regime map has been constructed, where the critical values for the transition from attached to separated flow are determined. The numerical computations reveal another control parameter, the cavity aspect ratio. The now in the cavity is similar only if all three control parameters are the same. Finally, the vorticity distribution and related circulation in the cavity are analyzed. © 2005 IEEE.
• Wei, M. a., Rufer, L., Zohar, Y., & Wong, M. (2005). Design and implementation of an integrated floating-gate electrostatic power micro-generator. Digest of Technical Papers - International Conference on Solid State Sensors and Actuators and Microsystems, TRANSDUCERS '05, 1, 299-302.
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  Abstract: The design, modeling and implementation of a single-wafer integrated floating-gate electrostatic power micro-generator are described. Both linear and non-linear models are developed. Realized using a standard CMOS floating-gate process and a CMOS-compatible photo-resist molded electroplating process, the generator contains an electromechanical resonator for energy generation/conversion and electronic devices for "programming" and rectification. Good agreement between modeling and measurement results was obtained. © 2005 IEEE.
• Wei, M. a., Zohar, Y., & Wong, M. (2005). Fabrication and packaging of an inertia electrical micro-switch using low temperature metal-electroplating technology. Pan Tao Ti Hsueh Pao/Chinese Journal of Semiconductors, 26(6), 1239-1244.
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  Abstract: Presented in this paper is a separate and non-interfering photoresist-molded, metal-electroplating technology for the low-temperature fabrication and packaging of inertia electrical micro-switch. The low temperature process of the electroplating technology allows eventual modular integration and wafer-level packaging of the micro-switches on active substrates containing pre-fabricated electronic signal-processing circuits. The height of the inertia micro-switch and that of its cavity are independently controlled. Metal leads are provided for electrical access to the sealed cavities. The switches are designed using a simple but accurate lumped spring-mass model. Un-encapsulated switches making over 100 million contacts are demonstrated in room ambient. Both the strength and the hermeticity of the sealed cavity are tested and reported.
• Zohar, Y., Lee, Y. C., Cheung, L. S., & Lee, H. M. (2005). Height effect on nucleation-site activity and size-dependent bubble dynamics in microchannel convective boiling. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/15/11/018
• Zohar, Y., Wong, M., & Ma, W. (2005). Fabrication and packaging of an inertia electrical micro-switch using low temperature metal-electroplating technology. Chinese Journal of Semiconductors.
• Zohar, Y., Wong, M., Lee, Y., & Yu, Z. T. (2005). Fluid flows in microchannels with cavities. Journal of microelectromechanical systems. doi:10.1109/jmems.2005.859086
• Gang, L. i., Zohar, Y., & Wong, M. (2004). Piezoresistive microphone with integrated amplifier realized using metal-induced laterally crystallized polycrystalline silicon. Journal of Micromechanics and Microengineering, 14(10), 1352-1358.
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  Abstract: Compared to conventional polycrystalline silicon (poly-Si), high-temperature re-crystallized (RC) metal-induced laterally crystallized (MILC) poly-Si has significantly improved material, mechanical and electrical properties, thus potentially allowing sensor and electronic integration on non-silicon-based substrates. In this paper, a RC-MILC poly-Si piezoresistive microphone with integrated amplifier has been designed, fabricated and tested. Measured results indicate an airborne sound sensitivity of 50 μV Pa -1 and a flat frequency response within ±3 dB between 100 Hz and 8 kHz. Using RC-MILC poly-Si, piezoresistors with gauge factor improved about 25%, noise level reduced about 50% and transistors with performance approaching that of single-crystal silicon devices have been realized.
• Gang, L. i., Zohar, Y., & Wong, M. (2004). Plezoresistive microphone with integrated amplifier realized using metal-induced laterally crystallized polycrystalline silicon. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 548-551.
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  Abstract: Compared to conventional polycrystalline silicon (poly-Si), re-crystallized (RC) metal-induced laterally crystallized (MILC) poly-Si possesses improved mechanical and electrical properties. RC-MILC poly-Si piezoresistors exhibit ∼25% increase in gauge factor and ∼50% reduction in noise level compared to conventional poly-Si piezoresistors. RC-MILC poly-Si transistors exhibit performance approaching those built on single-crystal silicon. As a demonstration of the feasibility of using the RC-MILC poly-Si technology to realize non-trivial micro-systems, microphone integrating piezoresistive strain sensors and amplifiers has been designed, fabricated and characterized.
• Lee, M., Siu, L., Lee, Y., Wong, M., & Zohar, Y. (2004). Height effect on nucleation-site activity in microchannel convective boiling. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 300-303.
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  Abstract: Forced convection boiling in microchannels is studied experimentally under uniform heat flux boundary condition. Several microchannel heat sinks, ranging in height between 5 and 20μm, have been fabricated using standard micromachining techniques with nucleation sites varying in size etched at the channels bottom surface. The heat flux was provided by a heater integrated at the device back side, while constant water flow rate was supplied by a syringe pump. Boiling curves of device temperature as a function of the input power have been measured for a variety of conditions. The points corresponding to the onset of nucleate boiling and critical heat flux (CHF) are clearly distinguishable. Furthermore, the boiling curves for increasing and decreasing heat flux exhibit a hysteresis loop. The activity of the nucleation sites is found to depend on the channel height. The critical size, above which nucleation sites are active, increases exponentially with the microchannel height, asymptotically approaching a theoretical value.
• Siu, A., Lap, W., Lee, Y., & Zohar, Y. (2004). Electrokinetic generation of microvortex patterns in a microchannel liquid flow. Journal of Micromechanics and Microengineering, 14(2), 247-255.
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  Abstract: The technology developed for micropatterning the electric surface charge to be negative, positive or neutral enables the realization of complex liquid flows in simple microchannels. A commercial CFD code is utilized to numerically simulate a variety of electrokinetically-generated liquid flows in a straight and uniform microchannel due to non-uniform surface charge distribution under an externally applied, steady electric field. We present design methodologies to electrokinetically drive vortical flows in any desired direction. In particular, we investigate surface charge patterns required to generate single or multi, co-rotating or counter-rotating, in-plane or out-of-plane vortices. Finally, in view of its potential application to microscale mixing, we discuss a surface charge pattern that can give rise to streamwise vorticity.
• Tak, Z., Lee, Y., Wong, M., & Zohar, Y. (2004). Size-dependent flow separation in microchannels with cavities. Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 296-299.
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  Abstract: Pressure driven gas flow in microchannels with cavities is studied experimentally and numerically. Several microchannels, 1-2μm in height, have been fabricated using surface micromachining with varying number of cavities. Streamwise pressure distributions along microchannels with and without cavities are found to be the same within experimental error. The mass flow rate increases slightly with the number of cavities, less than 10% increase for the microchannel with the maximum number of cavities. Numerical simulations reveal the development of several flow modes depending on the channel height. The flow pattern inside the cavities is determined not only by the reduced Reynolds number, Re*, but also by the cavity aspect ratio, Ac. Fully attached flow in the cavity is obtained for Re*2/3. The normalized cavity circulation as a function of the reduced Reynolds number collapsed onto a single curve only for A c
• Wei, M. a., Gang, L. i., Zohar, Y., & Wong, M. (2004). Fabrication and packaging of inertia micro-switch using low-temperature photo-resist molded metal-electroplating technology. Sensors and Actuators, A: Physical, 111(1), 63-70.
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  Abstract: Separate and non-interfering photo-resist molded metal-electroplating processes have been applied to the fabrication of inertia micro-switches and their wafer-level packaging in sealed cavities. The height of a switch and that of its cavity are independently controlled. The low process temperature allows eventual modular integration of switches on active substrates containing pre-fabricated electronic signal-processing circuits. The scheme also provides leads for electrical access to the sealed devices. The switches were designed using a simple but accurate lumped spring-mass model. With the application of an anti-stiction hydrophobic coating, un-encapsulated switches making over 100 million contacts have been demonstrated in room ambient. © 2003 Elsevier B.V. All rights reserved.
• Zohar, Y., Lee, Y., Hau, W. L., & Ng, A. S. (2004). Electrokinetic generation of microvortex patterns in a microchannel liquid flow. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/14/2/012
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  The technology developed for micropatterning the electric surface charge to be negative, positive or neutral enables the realization of complex liquid flows in simple microchannels. A commercial CFD code is utilized to numerically simulate a variety of electrokinetically-generated liquid flows in a straight and uniform microchannel due to non-uniform surface charge distribution under an externally applied, steady electric field. We present design methodologies to electrokinetically drive vortical flows in any desired direction. In particular, we investigate surface charge patterns required to generate single or multi, co-rotating or counter-rotating, in-plane or out-of-plane vortices. Finally, in view of its potential application to microscale mixing, we discuss a surface charge pattern that can give rise to streamwise vorticity.
• Zohar, Y., Wong, M., & Li, G. (2004). Piezoresistive microphone with integrated amplifier realized using metal-induced laterally crystallized polycrystalline silicon. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/14/10/009
• Zohar, Y., Wong, M., Li, G., & Ma, W. (2004). Fabrication and packaging of inertia micro-switch using low-temperature photo-resist molded metal-electroplating technology. Sensors and Actuators A-physical. doi:10.1016/j.sna.2003.10.014
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  Separate and non-interfering photo-resist molded metal-electroplating processes have been applied to the fabrication of inertia micro-switches and their wafer-level packaging in sealed cavities. The height of a switch and that of its cavity are independently controlled. The low process temperature allows eventual modular integration of switches on active substrates containing pre-fabricated electronic signal-processing circuits. The scheme also provides leads for electrical access to the sealed devices. The switches were designed using a simple but accurate lumped spring-mass model. With the application of an anti-stiction hydrophobic coating, un-encapsulated switches making over 100 million contacts have been demonstrated in room ambient.
• Chan, V. C., Carles, M., Sucher, N. J., Wong, M., & Zohar, Y. (2003). Design and fabrication of an integrated microsystem for microcapillary electrophoresis. Journal of Micromechanics and Microengineering, 13(6), 914-921.
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  Abstract: A capillary electrophoresis microsystem integrated with feed-through platinum electrodes was designed and fabricated for the separation of DNA fragments. A novel glass-to-silicon bonding technology, which allows anodic bonding of a glass wafer to a silicon wafer coated with a thick dielectric film by the inclusion of a thin intermediate amorphous silicon layer, was developed and employed to construct the microsystem. Despite the existence of a thick insulating material and non-uniform topography, robust devices without fluid leakage were obtained. Electrophoretic manipulation and separation of DNA fragments after capillary pre-treatment have been demonstrated and several operational considerations are discussed. The system performance suggests that silicon-based microsystems can be advantageous and practical for the fabrication of integrated microcapillary electrophoresis devices.
• Cheng, K. B., Wong, M., & Zohar, Y. (2003). Parallel and series multiple microchannel systems. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 291-294.
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  Abstract: Microfluidic systems typically involve more than one microchannel. The simplest multiple microchannel systems can be classified into series or parallel flows. In this work, gas flow in complex microsystems of multiple channels, connected in parallel and in series, has been experimentally investigated and theoretically analysed for the first time. A set of microchannels connected in parallel and series have been successfully fabricated using standard micromachining techniques. Mass flow rate and pressure distribution measurements have been conducted using Argon gas and compared with calculations based on a simple two-dimensional model, taken into account both compressibility and slip flow effects. Pressure distributions along the microchannel systems have shown good agreement with the theoretical calculations.
• Hau, W. L., Trau, D. W., Sucher, N. J., Wong, M., & Zohar, Y. (2003). Surface-chemistry technology for microfluidics. Journal of Micromechanics and Microengineering, 13(2), 272-278.
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  Abstract: A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. A positively charged poly(allylamine hydrochloride) (PAH) layer is coated onto a negatively charged silicon oxide surface by electrostatic self-assembly (ESA). Combined with photolithography in a lift-off-based process, several different surface charge patterns were successfully produced. Due to definition of the pattern by photolithography, no limitations in the pattern geometry exist. Any surface charge pattern can be created to enable fine control of fluid motion in microfluidic devices. Physical properties of this PAH layer were characterized. The generation of a bi-directional shear flow was demonstrated by using alternating longitudinal surface charge pattern with a single driving force, i.e. an externally applied electric field inside a microchannel.
• Lee, M., Wong, M., & Zohar, Y. (2003). Characterization of an integrated micro heat pipe. Journal of Micromechanics and Microengineering, 13(1), 58-64.
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  Abstract: The characterization of a micro heat pipe system, integrated with a local heater, temperature and capacitive microsensors is presented. Two liquid charging schemes based on a single hole, requiring vacuum environment, and a pair of holes, utilizing capillary forces are compared. Taking advantage of the great disparity between the dielectric constants of liquids and gases, capacitance sensors are used for void fraction measurements. Since it is difficult to control the phase content of a liquid-gas mixture in a micro heat pipe, a calibration technique based on a traveling water-air interface due to evaporation is introduced. The integrated sensor capacitance for pure water is found to depend on measurement frequency, temperature and ion concentration, exhibiting trends that are different from previous reports. The measured temperature and void fraction distribution along the heat pipes are consistent with the two-phase flow patterns recorded during the microsystem operation.
• Lee, M., Wong, M., & Zohar, Y. (2003). Integrated micro-heat-pipe fabrication technology. Journal of Microelectromechanical Systems, 12(2), 138-146.
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  Abstract: This paper presents the design and fabrication of an integrated micro-heat-pipe system consisting of a heater, an array of heat pipes, temperature and capacitive sensors. Taking advantage of the large difference between the dielectric constants of liquid and vapor, the integrated capacitor can be used for void-fraction measurements in two-phase flows. Both CMOS-compatible and glass-based fabrication technologies are reported. In the CMOS-compatible technology, the heat pipes are capped by a thin nitride layer utilizing wafer bonding and etch back technique. In the glass-based technology, the heat pipes are covered by a glass substrate using die-by-die anodic bonding to allow visualization of the two-phase flow patterns. This approach also results in a significant reduction of the parasitic capacitance, thus enhancing the sensitivity of the capacitance sensor. A few particular problems related to this technology are discussed and proper solutions are proposed.
• Lee, M., Wong, Y. Y., Wong, M., & Zohar, Y. (2003). Size and shape effects on two-phase flow patterns in microchannel forced convection boiling. Journal of Micromechanics and Microengineering, 13(1), 155-164.
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  Abstract: An integrated microchannel heat sink consisting of shallow, nearly rectangular microchannels has been fabricated using standard micromachining techniques to highlight the effects of the micrometer sized channel shape on the evolving flow patterns and, consequently, on the thermal performance of the microsystem. An integrated heater serves as a local heat source, while an array of micro thermistors is used for temperature distribution measurements. The working fluid, DI water, is pressurized through the microchannels for forced convection heat transfer studies. Boiling curves for different flow rates have been recorded and analyzed based on the visualized flow patterns. Local nucleation, including bubble formation and bubble dynamics, is documented and found to be negligible. Although detected, in contrast with triangular microchannels, annular flow is observed to be unstable. Instead, the dominant flow pattern is an unsteady transition region connecting an upstream vapor zone to a downstream liquid zone with an average location depending on the input power. A physical mechanism based on the force balance across the vapor-liquid interface, and the development of a restoring force, is proposed to explain the flow visualization results.
• Wei, M. a., Gang, L. i., Zohar, Y., & Wong, M. (2003). Fabrication and packaging of inertia microswitch using metal-electroplating technology. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 610-613.
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  Abstract: Separate and non-interfering photoresist-molded metal-electroplating processes have been applied to the fabrication of inertia micro-switches and their wafer-level packaging in sealed cavities formed using eutectic bonding. The height of a switch and that of its cavity are independently controlled. The low process temperature allows eventual modular integration of switches on active substrates containing pre-fabricated electronic signal-processing circuits. The scheme also provides leads for electrical access to the sealed devices. The switches are based on a lateral mechanical resonator design responding to inertia change above a pre-determined threshold value. A simple but accurate lumped spring-mass model is developed based on analytical and numerical studies. Predictions of the behavior of switches with different geometric designs have been verified using both drop-hammer and shaker tests. Switches making over 100 million contacts have been demonstrated in room ambient.
• Wei, M. a., Zohar, Y., & Wong, M. (2003). Design and characterization of inertia-activated electrical micro-switches fabricated and packaged using low-temperature photoresist molded metal-electroplating technology. Journal of Micromechanics and Microengineering, 13(6), 892-899.
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  Abstract: Inertia-activated electrical switches have been designed and realized using a low-temperature photoresist molded metal-electroplating micro-fabrication technology compatible with processed substrates containing micro-electronic signal-processing circuits. Packaging of the switches has also been implemented using this low-temperature plating process. A simple but accurate lumped spring-mass model is developed based on analytical and numerical analyses. Predictions of the behavior of switches with a range of different designs have been verified using both drop-hammer and shaker tests. With the application of an anti-stiction hydrophobic coating, susceptibility to stiction-induced storage and operational failure has been reduced. Unencapsulated switches making over 50 million contacts have been demonstrated at room ambient.
• Wei, M. a., Zohar, Y., & Wong, M. (2003). Design and characterization of micro-inertia switches fabricated using low-temperature metal-electroplating technology. Proceedings of SPIE - The International Society for Optical Engineering, 4981, 214-221.
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  Abstract: Inertia micro-switches have been designed and realized using a low-temperature metal-electroplating technology compatible with processed substrates containing micro-electronic circuits. A simple but accurate lumped spring-mass model is developed based on analytical and numerical analyses. Predictions of the behavior of switches with different geometric designs have been verified using both drop hammer and shaker tests. With the application of an anti-stiction hydrophobic coating, improved storage time and un-encapsulated switches making over 50 million contacts have been demonstrated in room ambient.
• Zohar, Y., Wong, M., & Lee, M. (2003). Characterization of an integrated micro heat pipe. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/13/1/309
• Zohar, Y., Wong, M., & Lee, M. (2003). Integrated micro-heat-pipe fabrication technology. Journal of microelectromechanical systems. doi:10.1109/jmems.2003.809955
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  This paper presents the design and fabrication of an integrated micro-heat-pipe system consisting of a heater, an array of heat pipes, temperature and capacitive sensors. Taking advantage of the large difference between the dielectric constants of liquid and vapor, the integrated capacitor can be used for void-fraction measurements in two-phase flows. Both CMOS-compatible and glass-based fabrication technologies are reported. In the CMOS-compatible technology, the heat pipes are capped by a thin nitride layer utilizing wafer bonding and etch back technique. In the glass-based technology, the heat pipes are covered by a glass substrate using die-by-die anodic bonding to allow visualization of the two-phase flow patterns. This approach also results in a significant reduction of the parasitic capacitance, thus enhancing the sensitivity of the capacitance sensor. A few particular problems related to this technology are discussed and proper solutions are proposed.
• Zohar, Y., Wong, M., & Ma, W. (2003). Design and characterization of inertia-activated electrical micro-switches fabricated and packaged using low-temperature photoresist molded metal-electroplating technology. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/13/6/312
• Zohar, Y., Wong, M., Sucher, N. J., Carles, M. C., & Chan, Y. C. (2003). Design and fabrication of an integrated microsystem for microcapillary electrophoresis. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/13/6/314
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  A capillary electrophoresis microsystem integrated with feed-through platinum electrodes was designed and fabricated for the separation of DNA fragments. A novel glass-to-silicon bonding technology, which allows anodic bonding of a glass wafer to a silicon wafer coated with a thick dielectric film by the inclusion of a thin intermediate amorphous silicon layer, was developed and employed to construct the microsystem. Despite the existence of a thick insulating material and non-uniform topography, robust devices without fluid leakage were obtained. Electrophoretic manipulation and separation of DNA fragments after capillary pre-treatment have been demonstrated and several operational considerations are discussed. The system performance suggests that silicon-based microsystems can be advantageous and practical for the fabrication of integrated microcapillary electrophoresis devices.
• Zohar, Y., Wong, M., Sucher, N. J., Trau, D., & Hau, W. L. (2003). Surface-chemistry technology for microfluidics. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/13/2/315
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  A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. A positively charged poly(allylamine hydrochloride) (PAH) layer is coated onto a negatively charged silicon oxide surface by electrostatic self-assembly (ESA). Combined with photolithography in a lift-off-based process, several different surface charge patterns were successfully produced. Due to definition of the pattern by photolithography, no limitations in the pattern geometry exist. Any surface charge pattern can be created to enable fine control of fluid motion in microfluidic devices. Physical properties of this PAH layer were characterized. The generation of a bi-directional shear flow was demonstrated by using alternating longitudinal surface charge pattern with a single driving force, i.e. an externally applied electric field inside a microchannel.
• Zohar, Y., Wong, M., Wong, Y. Y., & Lee, M. (2003). Size and shape effects on two-phase flow patterns in microchannel forced convection boiling. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/13/1/322
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  An integrated microchannel heat sink consisting of shallow, nearly rectangular microchannels has been fabricated using standard micromachining techniques to highlight the effects of the micrometer sized channel shape on the evolving flow patterns and, consequently, on the thermal performance of the microsystem. An integrated heater serves as a local heat source, while an array of micro thermistors is used for temperature distribution measurements. The working fluid, DI water, is pressurized through the microchannels for forced convection heat transfer studies. Boiling curves for different flow rates have been recorded and analyzed based on the visualized flow patterns. Local nucleation, including bubble formation and bubble dynamics, is documented and found to be negligible. Although detected, in contrast with triangular microchannels, annular flow is observed to be unstable. Instead, the dominant flow pattern is an unsteady transition region connecting an upstream vapor zone to a downstream liquid zone with an average location depending on the input power. A physical mechanism based on the force balance across the vapor–liquid interface, and the development of a restoring force, is proposed to explain the flow visualization results.
• Chan, Y. C., Ming, R., Carles, M., Sucher, N. J., Wong, M., & Zohar, Y. (2002). DNA kinetics in microfabricated devices. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 60-63.
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  Abstract: The DNA kinetics in micro-capillary electrophoresis is presented. The mobility and diffusion coefficient of 14bp-DNA fragments as a function of concentration in two types of separation sieving matrices, hydroxyethylcellulose (HEC) polymer solution and agarose gel, are extracted through a series of experiments performed in microfabricated devices. In addition, the motion of a DNA plug through a miter bend and splitting a plug in a branch are quantitatively characterized. The concept of equivalent length is introduced to quantify the effect of a bend on the DNA plug motion. In a branching system, a simple kinematic relationship was discovered relating the quantity of DNA in each downstream branch to its relative channel cross-sectional area.
• Lap, W., Trau, D. W., Sucher, N. J., Wong, M., & Zohar, Y. (2002). Micro flow patterns on demand using surface-chemistry technology. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 475-478.
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  Abstract: A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. Unlimited flow patterns can be generated under an externally applied electric field by electro-osmotic and electrophoretic driving forces to enable fine control of fluid motion in microfluidic devices. Two basic flows, shear and vortical, have been realized experimentally to demonstrate the tremendous potential of this technology, especially in analytical microsystems for genomics or cell biology.
• Lee, M., Wong, M., & Zohar, Y. (2002). Design, fabrication and characterization of an integrated micro heat pipe. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 85-88.
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  Abstract: The design, fabrication and characterization of an integrated microsystem consisting of micro heat pipes, a micro heater, temperature and capacitive microsensors are presented. CMOS-compatible micromachining techniques are utilized to fabricate the micro heat pipe device capped by a nitride layer. In order to allow clear visualizations of the flow patterns during operation, the process has been modified using a glass wafer to cap the heat pipes. Temperature distributions along the micro heat pipes have been measured using the microsensors located next to the heat pipes. The capacitive microsensors have been used to measure the void-fraction, taking advantage of the large difference between the dielectric constants of the liquid and vapor phases.
• Lee, M., Wong, Y. Y., Wong, M., & Zohar, Y. (2002). Size and shape effects on two-phase flow instabilities in microchannels. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 28-31.
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  Abstract: An integrated microchannel heat sink consisting of shallow, trapezoidal microchannels has been fabricated using standard micromachining techniques to highlight the effects of the micrometer sized channel shape on the evolving flow patterns and, consequently, on the thermal response of the system. An integrated heater provides the local heat source, while an array of temperature microsensors is used for temperature distribution measurements. DI water, serving as the working fluid, is pressurized through the microchannels for forced heat convection. Temperature plateaus are observed in the boiling curves, corresponding to the latent heat of phase change of the working fluid from liquid to vapor phase. The evolving two-phase flow patterns have been recorded and analyzed using high-speed camera. Bubble formation, growth and detachment at specific nucleation sites have been observed. Annular flow mode has been found to be unstable in trapezoidal channels. Instead, a highly unsteady transition region from upstream vapor phase to downstream liquid phase flow is developed, and the average location of this region depends on the input power.
• Lee, W. Y., Wong, M., & Zohar, Y. (2002). Pressure loss in constriction microchannels. Journal of Microelectromechanical Systems, 11(3), 236-244.
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  Abstract: Constriction devices contain elements inserted into the fluid stream, which change the local streamwise flow area. One such element is the orifice-like obstruction with sharp corners, a back-to-back abrupt contraction and expansion, which could trigger flow separation. A series of microchannels, 40 μm × 1 μm × 4000 μm in nominal dimensions, with constriction elements at the centers of the channels has been fabricated using standard micromachining techniques. The channel widths at the constriction sections varied from 10 μm to 34 μm, with pressure sensors integrated in each channel. Nitrogen gas was passed through the microdevices under inlet pressure up to 50 psi. The mass flow rates were measured for all the devices as a function of the pressure drop. A monotonic decrease of the flow rate with decreasing constriction-gap width was observed. The pressure distribution along the microchannel with the smallest constriction gap showed a pressure drop across the constriction element. Both mass flow rate and pressure measurements indicate that flow separation from the constriction sharp corners could occur.
• Lee, Y. W., Wong, M. A., & Zohar, Y. (2002). Microchannels in series connected via a contraction/expansion section. Journal of Fluid Mechanics, 459, 187-206.
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  Abstract: Fluid flow in microdevices consisting of pairs of microchannels in series was studied. The dimensions of the channels are about 40 μm × 1 μm × 2000 μm for the wide and about 20 μm × 1 μm × 2000 μm for the narrow channels. Pairs of wide and narrow channels, with integrated pressure sensors, are connected via transition sections with included angles varying from 5° to 180°. Minor pressure losses (not due to friction) were studied by passing nitrogen through the channels under inlet pressures up to 60 p.s.i. Each device was tested in the contraction mode, flow from wide to narrow channel, and in the opposite expansion mode, flow from narrow to wide channel. Mass flow rate was first measured as a function of the overall pressure drop. The detailed pressure distribution along the straight segments and around the transition section was then measured in order to understand the flow pattern. The Reynolds number for these flows is less than 1, suggesting the flow to be of the Hele-Shaw type with no separation such that the results for all the devices should be similar. However, the flow rate was found to decrease and the pressure loss to increase significantly with increasing included angle of the transition section, regardless of the flow direction. Flow separation due to the transition sections, if indeed there is any, cannot explain the large pressure drop since the kinetic energy is negligible.
• Tengge, M. a., Man, T. Y., Chan, Y. C., Zohar, Y., & Wong, M. (2002). Design and fabrication of an integrated programmable floating-gate microphone. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 288-291.
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  Abstract: An integrated, programmable floating-gate capacitive microphone has been designed and fabricated. The conducting floating-gate is electrically insulated and is "programmed" by injecting electrons into it using Flower-Nordheim tunneling through a thin silicon dioxide film, thus capable of simulating an electret and generating a permanent electric field. A current-driving buffer based on metal-oxide-semiconductor field-effect transistors is integrated to reduce the capacitive loading of the microphone. The fabrication process is MOS compatible and promises the potential of integrating a variety of signal processing electronic circuits.
• Wang, R. X., Zohar, Y., & Wong, M. (2002). Residual stress-loaded titanium-nickel shape-memory alloy thin-film micro-actuators. Journal of Micromechanics and Microengineering, 12(3), 323-327.
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  Abstract: Residual stress in titanium-nickel thin films has been exploited as a force load for thin-film shape-memory alloy micro-actuators, thus eliminating the need for providing 'external' force load for device training and operation. Thermal cycling during device operation is accomplished using electrical current-induced Joule's heating for temperature ramp-up and thermal conduction/convection-induced cooling for temperature ramp-down. In response to thermal cycling, hysteresis loops in both the displacement and electrical resistance of the micro-actuator have been observed - thus demonstrating the existence of shape-memory effect in the micro-actuators. Elimination of manual training of individual devices makes it possible to operate large arrays of shape-memory alloy micro-actuators realizable using micro-fabrication techniques.
• Yuk, S., Tak, Z., Wong, M., & Zohar, Y. (2002). Gas flow in a microdevice with a mixing layer configuration. Journal of Micromechanics and Microengineering, 12(1), 96-102.
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  Abstract: We have designed an integrated microsystem with a mixing layer configuration, fabricated using standard micromachining techniques, in order to study fluid flows, in complex microfluidic systems, that are certain to find numerous important applications, especially in biomedical or chemical analysis. The device features two narrow and parallel channels merging smoothly into a wide channel downstream of a splitter plate, all 1 μm in height, integrated with distributed pressure sensors. The characterization of the device included measurements of flow rate and pressure distribution for single-phase gas flow. Argon gas was passed either through one of the inlet channels, while the other was blocked, or through both inlet channels. Simple flow models of either a single straight microchannel or a pair of microchannels with different widths, connected in series, have been found to provide reasonable predictions of the evolving flow fields.
• Zohar, Y., Tong, P., Jiang, L., Lee, W. Y., & Lee, S. Y. (2002). Subsonic gas flow in a straight and uniform microchannel. Journal of Fluid Mechanics. doi:10.1017/s0022112002002203
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  A nonlinear equation based on the hydrodynamic equations is solved analytically using perturbation expansions to calculate the flow field of a steady isothermal, compressible and laminar gas flow in either a circular or a planar microchannel. The solution takes into account slip-flow effects explicitly by utilizing the classical velocity-slip boundary condition, assuming the gas properties are known. Consistent expansions provide not only the cross-stream but also the streamwise evolution of the various flow parameters of interest, such as pressure, density and Mach number. The slip-flow effect enters the solution explicitly as a zero-order correction comparable to, though smaller than, the compressible effect. The theoretical calculations are verified in an experimental study of pressure-driven gas flow in a long microchannel of sub-micron height. Standard micromachining techniques were utilized to fabricate the microchannel, with integral pressure microsensors based on the piezoresistivity principle of operation. The integrated microsystem allows accurate measurements of mass flow rates and pressure distributions along the microchannel. Nitrogen, helium and argon were used as the working fluids forced through the microchannel. The experimental results support the theoretical calculations in finding that acceleration and non-parabolic velocity profile effects were found to be negligible. A detailed error analysis is also carried out in an attempt to expose the challenges in conducting accurate measurements in microsystems.
• Zohar, Y., Wong, M. H., & Lee, Y. (2002). Microchannels in series connected via a contraction/expansion section. Journal of Fluid Mechanics. doi:10.1017/s0022112002008030
• Zohar, Y., Wong, M., & Lee, Y. T. (2002). Pressure loss in constriction microchannels. Journal of microelectromechanical systems. doi:10.1109/jmems.2002.1007402
• Zohar, Y., Wong, M., & Wang, R. (2002). Residual stress-loaded titanium$ndash$nickel shape-memory alloy thin-film micro-actuators. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/12/3/319
• Zohar, Y., Wong, M., Yu, Z. T., & Lee, S. Y. (2002). Gas flow in a microdevice with a mixing layer configuration. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/12/1/315
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  We have designed an integrated microsystem with a mixing layer configuration, fabricated using standard micromachining techniques, in order to study fluid flows, in complex microfluidic systems, that are certain to find numerous important applications, especially in biomedical or chemical analysis. The device features two narrow and parallel channels merging smoothly into a wide channel downstream of a splitter plate, all 1 μm in height, integrated with distributed pressure sensors. The characterization of the device included measurements of flow rate and pressure distribution for single-phase gas flow. Argon gas was passed either through one of the inlet channels, while the other was blocked, or through both inlet channels. Simple flow models of either a single straight microchannel or a pair of microchannels with different widths, connected in series, have been found to provide reasonable predictions of the evolving flow fields.
• Zohar, Y., Yuk, S., Lee, W. Y., Jiang, L., & Tong, P. (2002). Subsonic gas flow in a straight and uniform microchannel. Journal of Fluid Mechanics, 125-151.
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  Abstract: A nonlinear equation based on the hydrodynamic equations is solved analytically using perturbation expansions to calculate the flow field of a steady isothermal, compressible and laminar gas flow in either a circular or a planar microchannel. The solution takes into account slip-flow effects explicitly by utilizing the classical velocity-slip boundary condition, assuming the gas properties are known. Consistent expansions provide not only the cross-stream but also the streamwise evolution of the various flow parameters of interest, such as pressure, density and Mach number. The slip-flow effect enters the solution explicity as a zero-order correction comparable to, though smaller than, the compressible effect. The theoretical calculations are verified in an experimental study of pressure-driven gas flow in a long microchannel of sub-micron height. Standard micromachining techniques were utilized to fabricate the microchannel, with integral pressure microsensors based on the piezoresistivity principle of operation. The integrated microsystem allows accurate measurements of mass flow rates and pressure distributions along the microchannel. Nitrogen, helium and argon were used as the working fluids forced through the microchannel. The experimental results support the theoretical calculations in finding that acceleration and non-parabolic velocity profile effects were found to be negligible. A detailed error analysis is also carried out in an attempt to expose the challenges in conducting accurate measurements in microsystems.
• Chu, R. K., & Zohar, Y. (2001). Non-equilibrium temperature and velocity fields in a microchannel flow using discrete kinetic approach. Journal of Non-Equilibrium Thermodynamics, 26(1), 15-29.
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  Abstract: Discrete kinetic theory approach has been used to study dilute, monatomic gas flow between two walls in microdomains. A four-velocity coplanar model has been adopted, where the microscopic velocity-orientation angle is a function of the Knudsen number. Diffusive reflection boundary condition has been incorporated to obtain the solution. Subsequently, the macroscopic velocity slip at the wall, the velocity profile across the walls and the volume flow rate are calculated for the transition flow regime, where the Knudsen number varies between 0.1 and 10. Furthermore, the 'kinetic' temperature for this non-equilibrium system has been calculated based on the velocity profiles. The temperature jump at the wall and the Nusselt number are estimated and compared with the continuum-limit approximations.
• Jiang, L., Wong, M., & Zohar, Y. (2001). Forced convection boiling in a microchannel heat sink. Journal of Microelectromechanical Systems, 10(1), 80-87.
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  Abstract: Micromachining technology was utilized to fabricate a transparent microchannel heat-sink system by bonding glass to a silicon wafer. The micro heat sink consisted of a microchannel array, a heater, and a temperature sensor array. This integrated microsystem allowed simultaneous qualitative visualizations of the flow pattern within the microchannels and quantitative measurements of temperature distributions, flow rates, and input power levels. Boiling curves of temperature as a function of the input power were established. No boiling plateau was observed in the boiling curves, consistent with our previously reported data but different from results reported for macrochannel heat sinks. Three stable boiling modes, depending on the input power level, have been distinguished from the flow patterns. Local nucleation boiling was observed in microchannels with a hydraulic diameter as small as 26 μm at the lower input power range. At the higher input power range, a stable annular flow was the dominant boiling mode. Bubbly flow, commonly observed in macrochannels, could not be developed in the present microchannels. Consequently, no boiling plateau was detected in the boiling curves.
• Lee, M., Wong, Y. Y., Woag, M., & Zohar, Y. (2001). Two-phase flow boiling in a microchannel heat sink. ASME International Mechanical Engineering Congress and Exposition, Proceedings, 2, 3139-3145.
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  Abstract: A micro heat sink comprising 10 microchannels integrated with a local heater and a temperature microsensor array has been fabricated on a silicon wafer using standard micromachining techniques. A glass wafer was anodically bonded to the silicon wafer in order to cap the trapezoidal microchannels, about 14μm in depth and 120μm in average width. DI water was pressurized through the heat sink serving as the working fluid. Boiling curves of device temperature, at a few locations along the centerline, were measured as a function of the input power. In contrast to previously reported results, the boiling plateau associated with latent heat of phase change from liquid to vapor was detected. The transparent glass ceiling allowed the visualizations of flow phenomena dominated by surface effects. The classical bubble dynamics of bubble formation, growth, and detachment was observed at relatively low input power. However, this mode was completely suppressed at moderate power levels. Further increase of the input power resulted in a clear separation between the upstream vapor and the downstream liquid. The average location of the vapor/liquid interface shifted downstream with the input power, and near the critical heat flux condition the interface was located at the channel outlet. Thus, the added heat resulted in increased quality of the two-phase flow rather than increasing the mixture temperature.
• Lee, M., Wong, Y. Y., Wong, M., & Zohar, Y. (2001). Two-phase flow boiling in a microchannel heat sink. American Society of Mechanical Engineers, Micro-Electromechanical Systems Division Publication (MEMS), 3, 567-573.
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  Abstract: A micro heat sink comprising 10 microchannels integrated with a local heater and a temperature miciosensor array has been fabricated on a silicon wafer using standard micromachining techniques. A glass wafer was anodically bonded to the silicon wafer in order to cap the trapezoidal microchannels, about 14μm in depth and 120μm in average width. DI water was pressurized through the heat sink serving as the working fluid. Boiling curves of device temperature, at a few locations along the centerline, were measured as a function of the input power. In contrast to previously reported results, the boiling plateau associated with latent heat of phase change from liquid to vapor was detected. The transparent glass ceiling allowed the visualizations of flow phenomena dominated by surface effects. The classical bubble dynamics of bubble formation, growth, and detachment was observed at relatively low input power. However, this mode was completely suppressed at moderate power levels. Further increase of the input power resulted in a clear separation between the upstream vapor and the downstream liquid. The average location of the vapor/liquid interface shifted downstream with the input power, and near the critical heat flux condition the interface was located at the channel outlet. Thus, the added heat resulted in increased quality of the two-phase flow rather than increasing the mixture temperature.
• Lee, S. Y., Wong, M., & Zohar, Y. (2001). Pressure losses in microchannels with bends. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 491-494.
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  Abstract: Bends in fluidic systems are necessary features due to design or technology constraints. Bends or curves in a duct always induce flow separation at the walls, which results in additional pressure loss. In this work, this phenomenon is investigated on a microscale. A set of microchannels, 20μm×1.1μm×5810μm in dimensions, with a 90°-bend at the channel center has been fabricated using standard micromachining techniques. Three bend configurations have been tested: miter, curved, and double-turn. All the microchannels were integrated with pressure microsensors. Argon gas was passed through the microdevices under inlet pressure up to 50 psi, and the mass flow rate was measured for all the devices as a function of the pressure drop. The flow rate through the channel with the miter bend, a single sharp turn at a right angle, was found to be the lowest. Pressure distributions along this microdevice were recorded, showing a pressure jump across the miter bend. The mass flow rate and pressure measurements indicate the existence of flow separation in the microchannels.
• Lee, W. Y., Wong, M., & Zohar, Y. (2001). Flow separation in constriction microchannels. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 495-498.
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  Abstract: Constriction devices contain an element inserted into the fluid stream, which changes the local streamwise distribution of the flow area. One such element is the orifice-like obstruction with sharp corners, basically a back-to-back abrupt contraction and expansion, which could trigger flow separation. A series of microchannels, 40μm×1μm×4000μm in dimensions, with orifice element at the centers of the channels has been fabricated using standard micromachining techniques. The channel width at the orifice section varied from 10μm to 34μm, and each channel was integrated with a set of pressure sensors. Nitrogen gas was passed through the microdevices under inlet pressure up to 50psi. Mass flow rate was first measured for all the devices as a function of the pressure drop, showing a monotonic decrease of the flow rate with decreasing orifice width. Then, the pressure distributions along the microchannel with the narrowest orifice were recorded, showing a pressure jump across the 10μm-wide orifice. The mass flow rate and pressure measurements indicate the existence of flow separation in the microchannels.
• Wang, M., Meng, Z., Zohar, Y., & Wong, M. (2001). Metal-induced laterally crystallized polycrystalline silicon for integrated sensor applications. IEEE Transactions on Electron Devices, 48(4), 794-800.
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  Abstract: A novel metal-induced lateral crystallization (MILC) technique, involving a low temperature crystallization step followed by a high temperature recrystallization step, has been applied to the formation of polycrystalline silicon (poly-Si) with enhanced material characteristics. A range of devices, including piezo-resistors, thermisters, resistors and thin- film transistors, has been fabricated both on MILC and regular low-pressure chemical vapor deposited (LPCVD) poly-Si. Compared to the latter, MILC poly-Si leads to much improved device performance. The piezo-resistive gauge factor of MILC poly-Si is at least double that of LPCVD poly-Si, with a maximum value of 60 measured. Higher mobility, steeper subthreshold slope, lower threshold voltage, and higher on-off current ratio have been obtained for thin-film transistors realized on MILC poly-Si that those realized on LPCVD poly-Si. A variety of sensing and electronic devices based on MILC poly-Si can be simultaneously realized, thus making MILC an enabling technology for integrated silicon sensor applications.
• Wang, M., Yeung, W. L., Zohar, Y., & Wong, M. (2001). Metal-induced laterally crystallized polycrystalline silicon for micro-systems: Mechanical and etching characteristics. ASME International Mechanical Engineering Congress and Exposition, Proceedings, 2, 2919-2923.
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  Abstract: It is determined that for metal-induced laterally crystallized (MILC) polycrystalline silicon (poly-Si), the mechanical and etch properties compare favorably with and the sensing and electrical properties are better than those of conventional low-pressure chemical vapor deposited (LPCVD) poly-Si. The etching characteristics of MILC poly-Si in tetra-methyl ammonium hydroxide (TMAH) solution have been studied, A unified model based on preferential grain boundary (GB) etching has been proposed to explain the etching behavior of poly-Si in TMAH.
• Wang, M., Yeung, W. L., Zohar, Y., & Wong, M. (2001). Metal-induced laterally crystallized polycrystalline silicon for micro-systems: Mechanical and etching characteristics. American Society of Mechanical Engineers, Micro-Electromechanical Systems Division Publication (MEMS), 3, 345-349.
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  Abstract: It is determined that for metal-induced laterally crystallized (MILC) polycrystalline silicon (poly-Si), the mechanical and etch properties compare favorably with and the sensing and electrical properties are better than those of conventional low-pressure chemical vapor deposited (LPCVD) poly-Si. The etching characteristics of MILC poly-Si in tetra-methyl ammonium hydroxide (TMAH) solution have been studied. A unified model based on preferential grain boundary (GB) etching has been proposed to explain the etching behavior of poly-Si in TMAH.
• Wang, R. X., Zohar, Y., & Wong, M. (2001). Effects of Stress Induced by the Underlying Layers on the Micro-Structures and Characteristic Phase Transformation Temperatures of Sputtered TiNi Thin Films. ASME International Mechanical Engineering Congress and Exposition, Proceedings, 2, 2951-2955.
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  Abstract: The micro-structures and the phase transformation temperatures of sputtered titanium-nickel (TiNi) thin films, both free-standing and attached on different underlying multilayer substrates have been studied. Differences in the micro-structures, such as the lattice constants and the relative concentrations of TiNi, Ti2Ni and TiNi3 phases, were observed (1) among the free-standing and the attached films, (2) among the films attached on different underlying multi-layers and (3) among films with different relative orders of aging and release. Not surprisingly, the corresponding phase transformation temperatures are also different. It is proposed that both substrate- and process-induced stress significantly affect the micro-structures, hence the phase transformation characteristics, of the resulting shape-memory alloy thin films.
• Wang, R. X., Zohar, Y., & Wong, M. (2001). Effects of Stress Induced by the Underlying Layers on the Micro-Structures and Characteristic Phase Transformation Temperatures of Sputtered TiNi Thin Films. American Society of Mechanical Engineers, Micro-Electromechanical Systems Division Publication (MEMS), 3, 377-381.
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  Abstract: The micro-structures and the phase transformation temperatures of sputtered titanium-nickel (TiNi) thin films, both free-standing and attached on different underlying multilayer substrates have been studied. Differences in the micro-structures, such as the lattice constants and the relative concentrations of TiNi, Ti2Ni and TiNi3 phases, were observed (1) among the free-standing and the attached films, (2) among the films attached on different underlying multi-layers and (3) among films with different relative orders of aging and release. Not surprisingly, the corresponding phase transformation temperatures are also different. It is proposed that both substrate- and process-induced stress significantly affect the micro-structures, hence the phase transformation characteristics, of the resulting shape-memory alloy thin films.
• Wang, R. X., Zohar, Y., & Wong, M. (2001). The effects of process-induced stress on the microstructures and the phase transformation characteristics of sputtered titanium-nickel thin-film shape-memory alloys. Journal of Micromechanics and Microengineering, 11(6), 686-691.
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  Abstract: The microstructures and the phase transformation temperatures of sputtered titanium-nickel (TiNi) thin films, both free-standing and attached on different underlying multi-layer substrates, have been studied. Differences in the microstructures, such as the lattice constants and relative concentrations of TiNi, Ti 2Ni and TiNi 3 phases, have been observed among the free-standing and the attached films, among the films attached on different underlying multi-layers and among the films with different relative orders of ageing and release. Not surprisingly, the corresponding phase transformation temperatures are also different. It is proposed that both process- and substrate-induced stresses affect the microstructures, hence the phase transformation characteristics, of the resulting shape-memory thin films.
• Yu, T. F., Yuk, S., Wong, M., & Zohar, Y. (2001). Instability modes of two-phase flows in a mixing-layer microdevice. ASME International Mechanical Engineering Congress and Exposition, Proceedings, 2, 3153-3158.
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  Abstract: Extensive development of biomedical and chemical analytic microdevices involves microscale fluid flows. Merging of fluid streams is expected to be a key feature in such devices. An integrated microsystem consisting of merging microchannels and distributed pressure microsensors has been designed and characterized to study this phenomenon on a microscale. The two narrow, uniform and identical channels merged smoothly into a wide, straight and uniform channel downstream of a splitter plate. All of the devices were fabricated using standard micromachining techniques. Mass flow rates and pressure distributions were measured for single-phase gas flow in order to characterize the device. The experimental results indicated that the flow developed when both inlets were connected together to the gas source could be modeled as gas flow through a straight and uniform microchannel. The flow through a single branch while the other was blocked, however, could be modeled as gas flow through a pair of microchannels in series. Flow visualizations of two-phase flows have been conducted when driving liquid and gas through the inlet channels. Several instability modes of the gas/liquid interface have been observed as a function of the pressure difference between the two streams at the merging location.
• Yu, T. F., Yuk, S., Wong, M., & Zohar, Y. (2001). Instability modes of two-phase flows in a mixing-layer microdevice. American Society of Mechanical Engineers, Micro-Electromechanical Systems Division Publication (MEMS), 3, 581-586.
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  Abstract: Extensive development of biomedical and chemical analytic microdevices involves microscale fluid flows. Merging of fluid streams is expected to be a key feature in such devices. An integrated microsystem consisting of merging microchannels and distributed pressure microsensors has been designed and characterized to study this phenomenon on a microscale. The two narrow, uniform and identical channels merged smoothly into a wide, straight and uniform channel downstream of a splitter plate. All of the devices were fabricated using standard micromachining techniques. Mass flow rates and pressure distributions were measured for single-phase gas flow in order to characterize the device. The experimental results indicated that the flow developed when both inlets were connected together to the gas source could be modeled as gas flow through a straight and uniform microchannel. The flow through a single branch while the other was blocked, however, could be modeled as gas flow through a pair of microchannels in series. Flow visualizations of two-phase flows have been conducted when driving liquid and gas through the inlet channels. Several instability modes of the gas/liquid interface have been observed as a function of the pressure difference between the two streams at the merging location.
• Yuk, S., Wong, M., & Zohar, Y. (2001). Gas flow in microchannels with bends. Journal of Micromechanics and Microengineering, 11(6), 635-644.
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  Abstract: Bends or curves are unavoidable features in fluidic systems due to design or technology constraints. These fluidic elements in a macrochannel always induce secondary flows, which result in pressure loss in addition to frictional loss. In this paper, this phenomenon is investigated on a microscale where flow separation is not expected to develop. A set of microchannels, with the dimensions 20 × 1 × 5810 μm3, with a 90° turn at the channel centre, has been fabricated using standard micromachining techniques. Three bend configurations have been tested: miter, curved and double-turn. All the microchannels were integrated with pressure microsensors. Argon gas was passed through the microdevices under an inlet pressure of up to 50 psi, and the mass flow rate was measured for all the devices as a function of the driving pressure drop. The flow rate through the channel with the miter bend, a single sharp turn at a right angle, was found to be the lowest. Pressure distributions along the microchannels were recorded, showing an additional pressure drop across the bends. The largest drop was found in the miter bend with the lowest flow rate. The mass flow rate and pressure measurements indicate that secondary flow could develop in microchannels also due to a bend, contrary to expectations.
• Zohar, Y., & Chu, R. K. (2001). Non-Equilibrium Temperature and Velocity Fields in a Microchannel Flow using Discrete Kinetic Approach. Journal of Non-Equilibrium Thermodynamics. doi:10.1515/jnetdy.2001.002
• Zohar, Y., & Jiang, L. (2001). Forced Convection Boiling in a Microchannel Heat Sing. JMEMS, 10, 80-87.
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  Micromachining technology was utilized to fabricate a transparent microchannel heat-sink system by bonding glass to a silicon wafer. The micro heat sink consisted of a microchannel array, a heater, and a temperature sensor array. This integrated microsystem allowed simultaneous qualitative visualizations of the flow pattern within the microchannels and quantitative measure- ments of temperature distributions, flow rates, and input power levels. Boiling curves of temperature as a function of the input power were established. No boiling plateau was observed in the boiling curves, consistent with our previously reported data but different from results reported for macrochannel heat sinks. Three stable boiling modes, depending on the input power level, have been distinguished from the flow patterns. Local nucleation boiling was observed in microchannels with a hydraulic diameter as small as 26 m at the lower input power range. At the higher input power range, a stable annular flow was the dominant boiling mode. Bubbly flow, commonly observed in macrochannels, could not be developed in the present microchannels. Consequently, no boiling plateau was detected in the boiling curves. (535)
• Zohar, Y., Wong, M., & Jiang, L. (2001). Forced convection boiling in a microchannel heat sink. Journal of microelectromechanical systems. doi:10.1109/84.911095
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  Micromachining technology was utilized to fabricate a transparent microchannel heat-sink system by bonding glass to a silicon wafer. The micro heat sink consisted of a microchannel array, a heater, and a temperature sensor array. This integrated microsystem allowed simultaneous qualitative visualizations of the flow pattern within the microchannels and quantitative measurements of temperature distributions, flow rates, and input power levels. Boiling curves of temperature as a function of the input power were established. No boiling plateau was observed in the boiling curves, consistent with our previously reported data but different from results reported for macrochannel heat sinks. Three stable boiling modes, depending on the input power level, have been distinguished from the flow patterns. Local nucleation boiling was observed in microchannels with a hydraulic diameter as small as 26 /spl mu/m at the lower input power range. At the higher input power range, a stable annular flow was the dominant boiling mode. Bubbly flow, commonly observed in macrochannels, could not be developed in the present microchannels. Consequently, no boiling plateau was detected in the boiling curves.
• Zohar, Y., Wong, M., & Lee, S. Y. (2001). Gas flow in microchannels with bends. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/11/6/304
• Zohar, Y., Wong, M., & Wang, R. (2001). The effects of process-induced stress on the microstructures and the phase transformation characteristics of sputtered titanium-nickel thin-film shape-memory alloys. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/11/6/309
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  The microstructures and the phase transformation temperatures of sputtered titanium-nickel (TiNi) thin films, both free-standing and attached on different underlying multi-layer substrates, have been studied. Differences in the microstructures, such as the lattice constants and relative concentrations of TiNi, Ti2Ni and TiNi3 phases, have been observed among the free-standing and the attached films, among the films attached on different underlying multi-layers and among the films with different relative orders of ageing and release. Not surprisingly, the corresponding phase transformation temperatures are also different. It is proposed that both process- and substrate-induced stresses affect the microstructures, hence the phase transformation characteristics, of the resulting shape-memory thin films.
• Zohar, Y., Wong, M., Meng, Z., & Wang, M. (2001). Metal-induced laterally crystallized polycrystalline silicon for integrated sensor applications. IEEE Transactions on Electron Devices. doi:10.1109/16.915728
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  A novel metal-induced lateral crystallization (MILC) technique, involving a low temperature crystallization step followed by a high temperature recrystallization step, has been applied to the formation of polycrystalline silicon (poly-Si) with enhanced material characteristics. A range of devices, including piezo-resistors, thermistors, resistors and thin-film transistors, has been fabricated both on MILC and regular low-pressure chemical vapor deposited (LPCVD) poly-Si. Compared to the latter, MILC poly-Si leads to much improved device performance. The piezo-resistive gauge factor of MILC poly-Si is at least double that of LPCVD poly-Si, with a maximum value of 60 measured, higher mobility, steeper subthreshold slope, lower threshold voltage, and higher on-off current ratio have been obtained for thin-film transistors realized on MILC poly-Si that those realized on LPCVD poly-Si. A variety of sensing and electronic devices based on MILC poly-Si can be simultaneously realized, thus making MILC an enabling technology for integrated silicon sensor applications.
• Chu, R. K., & Zohar, Y. (2000). Class of discrete kinetic solutions for non-boundary-driven gas flow. Journal of Non-Equilibrium Thermodynamics, 25(1), 49-62.
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  Abstract: Discrete kinetic theory approach has been used to study dilute, monatomic, non-equilibrium gas flow between two walls in microdomains. A four-velocity coplanar model has been adopted, where the microscopic velocity-orientation angle and the Knudsen number are free parameters, which have to be prescribed. Diffusive reflection boundary condition has been incorporated to obtain the solution. A bounded range for the admissible orientation angle of the discrete velocity vectors for any given Knudsen number is identified. Consequently, the macroscopic velocity slip at the wall, the velocity profile across the walls and the volume flow rate is calculated as a function of the free parameters. The calculations based on a single model, 4-velocity, cover the transition flow regime between the continuum and the free-molecular flow. The calculated volume flow rate is compared with experimental data as well as with other theoretical models.
• Jiang, L., Wong, M., & Zohar, Y. (2000). Phase change in microchannel heat sink under forced convection boiling. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 397-402.
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  Abstract: A microchannel heat sink system, consisting of parallel microchannels, distributed temperature micro-sensors and a local heater, has been fabricated and characterized. V-grooves with hydraulic diameter of either 40 μm or 80 μm were formed by bulk silicon etching. The heater and temperature microsensor array were fabricated using surface micromachining. Microchannels were realized by bonding a glass wafer to the silicon substrate, resulting in a transparent cover for flow visualization. Phase change during the boiling process was studied under forced convection conditions, where DI water was used as the working fluid. No boiling plateau, associated with latent heat, has been observed in the boiling curves of microchannel heat sinks. Flow visualization was carried out to understand the boiling mechanism in such a system. Three phase-change modes were observed depending on the input power level. Local nucleation boiling within the microchannels occurred at low power level. At moderate levels, large bubbles developed at the inlet/outlet regions, and the upstream bubbles were forced through the channels and out of the system. At higher input power levels, a stable annular flow mode was observed, where a thin liquid film coated each channel wall until critical heat flux conditions developed with a dryout of the system.
• Jiang, L., Wong, M., & Zohar, Y. (2000). Transient temperature performance of an integrated micro-thermal system. Journal of Micromechanics and Microengineering, 10(3), 466-476.
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  Abstract: A novel integrated thermal microsystem was designed and fabricated with a heater, microchannels and distributed temperature sensors. This device allows, for the first time, an experimental study of the transient behavior of a thermal microsystem. The transient temperature behavior of the device was studied for a variety of heater power levels and forced convection flow rates, where DI water was used as the working fluid. Both heating-up rise time and cooling-down fall time due to a step current input were determined for natural and forced convection heat transfer. The transient temperature response to a sinusoidal power input was also investigated. The resulting temperature distribution was measured as a function of the input signal and the flow rate. The step response under natural convection is exponential for both heating and cooling processes. However, under forced convection, the heating-up time response exhibits a clear overshoot. The response time for both heating and cooling process is about four times faster than that for the natural convection case. Furthermore, under certain conditions, the periodic temperature response can exhibit a large peak-peak temperature without the occurrence of dry-out phenomenon.
• Jiang, L., Wong, M., & Zohar, Y. (2000). Unsteady characteristics of a thermal microsystem. Sensors and Actuators, A: Physical, 82(1), 108-113.
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  Abstract: A novel integrated microsystem consisting of a heater, microchannels and distributed temperature sensors was successfully fabricated. The transient temperature behavior of the device was experimentally studied for a variety of power dissipation levels and forced convection flow rates, where DI water was used as the working fluid. Both heating-up and cooling-down time constants were determined for a pulsed current input. The device frequency response to a sinusoidal input voltage, with the device operating either in a single or in a two-phase mode, was characterized. The dependence of the resulting temperature average and amplitude on the input signal was studied for a variety of cycle conditions. It was found that, contrary to expectations, the heating-up time constant was larger than the cooling-down time constant. Furthermore, the periodic temperature field stabilized the system to avoid the occurrence of the dryout phenomenon.
• Ma, C. C., Wang, R., Sun, Q. P., Zohar, Y., & Wong, M. (2000). Frequency response of TiNi shape memory alloy thin film micro-actuators. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 370-374.
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  Abstract: Relatively independent of the physical dimensions of micro-actuators based on shape memory alloys (SMA), the reported frequency response typically is capped at a few tens of Hz. The slow response agrees well with that of the rotating micro-actuators fabricated in this work. On the other hand, based on heat transfer analyses, a theoretical response time on the order of a few milli-seconds should be possible for scaled micro-actuators, thus implying a frequency performance of at least a few hundred Hz. Therefore it is concluded that the response of SMA micro-actuators may not be limited by heat transfer, but by the slow rate of phase transformation between the austenitic and the martensitic phases. This is consistent with the slow phase growth rate of about 0.3 μm/s observed using in-situ transmission electron microscopy.
• Wang, M., Meng, Z., Zohar, Y., & Wong, M. (2000). New polycrystalline silicon technology for integrated sensor applications. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 114-119.
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  Abstract: A novel metal-induced lateral crystallization (MILC) technique has been applied to the formation of polycrystalline silicon (poly-Si) with greatly enhanced material characteristics. Compared to conventional poly-Si, MILC poly-Si gives rise to much improved performance not only in sensors but also in thin film transistors. A variety of sensing and electronic devices can be realized simultaneously on MILC poly-Si, thus making MILC an enabling technology for integrated silicon micro-machining.
• Xinxin, L. i., Lee, W. Y., Wong, M., & Zohar, Y. (2000). Gas flow in constriction microdevices. Sensors and Actuators, A: Physical, 83(1), 277-283.
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  Abstract: Constriction devices contain an element inserted into the fluid stream, which either changes the local streamwise distribution of flow area or applied frictional resistance to the flow. Two such devices: a micro orifice plate and a micro Venturi tube have been successfully fabricated for the study of fundamental flow phenomena in microdomains. Each device was integrated with a set of pressure sensors, and fabricated using standard micromachining techniques. Nitrogen gas was passed through the microdevices under inlet pressure up to 50 psi. Mass flow rate was first measured as a function of the overall pressure drop, and compared with results for a straight microchannel. Then, the detailed pressure distribution along each device was measured to understand the flow pattern around the flow obstruction. The results demonstrate that flow separation may occur in a microchannel flow with a very small Reynolds number. It is very possible that a micro vortex, on the order of 10 μm in size, can develop upstream and downstream of the obstruction.
• Xinxin, L. i., Zohar, Y., & Wong, M. (2000). Fabrication and characterization of nickel-induced laterally crystallized polycrystalline silicon piezo-resistive sensors. Sensors and Actuators, A: Physical, 82(1), 281-285.
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  Abstract: A novel metal-induced lateral crystallization (MILC) technology has been applied to the formation of improved polycrystalline silicon (poly-Si) piezo-resistors. Nickel has been used for the MILC of amorphous silicon formed by low-pressure chemical vapor deposition. Independent of the physical layout of the resistors, the MILC poly-Si is found to consist of elongated grains with grain lengths comparable to the physical lengths of the resistors. Surface micro-machined micro-channels with integrated pressure sensors using both MILC and conventional low-pressure chemical vapor deposited (LPCVD) poly-Si piezo-resistors have been fabricated and characterized. Compared to the sensors with the LPCVD piezo-resistors, those with the MILC piezo-resistors show 40% higher pressure sensitivity as well as lower temperature-induced drift in both the zero-pressure offset and the pressure sensitivity. Stability in terms of small temporal drift in the zero point offset has been obtained for the MILC pressure sensors.
• Zohar, Y., & Chu, R. K. (2000). A Class of Discrete Kinetic Solutions for Non-Boundary-Driven Gas Flow. Journal of Non-Equilibrium Thermodynamics. doi:10.1515/jnetdy.2000.004
• Zohar, Y., Wong, M., & Jiang, L. (2000). Transient temperature performance of an integrated micro-thermal system. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/10/3/324
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  A novel integrated thermal microsystem was designed and fabricated with a heater, microchannels and distributed temperature sensors. This device allows, for the first time, an experimental study of the transient behavior of a thermal microsystem. The transient temperature behavior of the device was studied for a variety of heater power levels and forced convection flow rates, where DI water was used as the working fluid. Both heating-up rise time and cooling-down fall time due to a step current input were determined for natural and forced convection heat transfer. The transient temperature response to a sinusoidal power input was also investigated. The resulting temperature distribution was measured as a function of the input signal and the flow rate. The step response under natural convection is exponential for both heating and cooling processes. However, under forced convection, the heating-up time response exhibits a clear overshoot. The response time for both heating and cooling process is about four times faster than that for the natural convection case. Furthermore, under certain conditions, the periodic temperature response can exhibit a large peak-peak temperature without the occurrence of dry-out phenomenon.
• Zohar, Y., Wong, M., & Jiang, L. (2000). Unsteady characteristics of a thermal microsystem. Sensors and Actuators A-physical. doi:10.1016/s0924-4247(99)00317-9
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  A novel integrated microsystem consisting of a heater, microchannels and distributed temperature sensors was successfully fabricated. The transient temperature behavior of the device was experimentally studied for a variety of power dissipation levels and forced convection flow rates, where DI water was used as the working fluid. Both heating-up and cooling-down time constants were determined for a pulsed current input. The device frequency response to a sinusoidal input voltage, with the device operating either in a single or in a two-phase mode, was characterized. The dependence of the resulting temperature average and amplitude on the input signal was studied for a variety of cycle conditions. It was found that, contrary to expectations, the heating-up time constant was larger than the cooling-down time constant. Furthermore, the periodic temperature field stabilized the system to avoid the occurrence of the dryout phenomenon.
• Zohar, Y., Wong, M., & Li, X. (2000). Fabrication and characterization of nickel-induced laterally crystallized polycrystalline silicon piezo-resistive sensors. Sensors and Actuators A-physical. doi:10.1016/s0924-4247(99)00361-1
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  A novel metal-induced lateral crystallization (MILC) technology has been applied to the formation of improved polycrystalline silicon (poly-Si) piezo-resistors. Nickel has been used for the MILC of amorphous silicon formed by low-pressure chemical vapor deposition. Independent of the physical layout of the resistors, the MILC poly-Si is found to consist of elongated grains with grain lengths comparable to the physical lengths of the resistors. Surface micro-machined micro-channels with integrated pressure sensors using both MILC and conventional low-pressure chemical vapor deposited (LPCVD) poly-Si piezo-resistors have been fabricated and characterized. Compared to the sensors with the LPCVD piezo-resistors, those with the MILC piezo-resistors show 40% higher pressure sensitivity as well as lower temperature-induced drift in both the zero-pressure offset and the pressure sensitivity. Stability in terms of small temporal drift in the zero point offset has been obtained for the MILC pressure sensors.
• Zohar, Y., Wong, M., Lee, Y. T., & Li, X. (2000). Gas flow in constriction microdevices. Sensors and Actuators A-physical. doi:10.1016/s0924-4247(99)00308-8
• Biao, L. i., Xiong, B., Jiang, L., Zohar, Y., & Wong, M. (1999). Applications of germanium to low temperature micro-machining. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 638-643.
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  Abstract: Though germanium (Ge) shares similar physical properties with silicon (Si), it also possesses unique characteristics that are complementary to those of Si. The advantages of Ge include its compatibility with Si micro-fabrication, its excellent gas and liquid phase etch selectivity to other materials commonly used in Si micro-machining, and particularly its low deposition temperature (
• Biao, L. i., Xiong, B., Jiang, L., Zohar, Y., & Wong, M. (1999). Germanium as a versatile material for low-temperature micromachining. Journal of Microelectromechanical Systems, 8(4), 366-372.
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  Abstract: Though germanium (Ge) shares many similar physical properties with silicon (Si), it also possesses unique characteristics that are complementary to those of Si. The advantages of Ge include its compatibility with Si microfabrication, its excellent gas and liquid phase etch selectivity to other materials commonly used in Si micromachining, and its low deposition temperature (
• Chai, H., & Zohar, Y. (1999). Unsteady wire sweep due to transfer molding in a 160L QFP package. Journal of Electronic Packaging, Transactions of the ASME, 121(2), 127-134.
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  Abstract: Wire sweep has been recognized as one of the major defects in encapsulation of electronic components by transfer molding. The phenomenon is very complicated as it is sensitive to a large number of parameters. In this experimental work, where a 160L QFP package used as the test vehicle, the detailed time-dependent wire displacement is measured for the following two different flow initial conditions: (i) the wire is immersed in the liquid and is displaced due to the acceleration of the flow from rest to the steady-state velocity, and (ii) the wire is surrounded by the ambient air and is displaced first due to the passage of the liquid front and then due to the hydrodynamic load. Significant differences have been observed between the two cases, with important implications for analytical and numerical studies of wire sweep.
• Chai, H., & Zohar, Y. (1999). Wire Sweep Due to Transfer Molding in a 160L QFP Package under Steady-State Conditions. Journal of Electronic Packaging, Transactions of the ASME, 121(3), 137-142.
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  Abstract: Wire sweep has been recognized as a critical process defect which can result in device failure. The phenomenon is very complicated as it is sensitive to a large number of parameters. In this experimental work, the influence of some of the more important factors is investigated. These parameters include fluid-flow speed, wire-bond density, mold cavity height, and mold vent size. A 160L QFP package bonded with gold wires in a transparent mold is used as the test vehicle.
• Jiang, L., Wang, Y., Wong, M., & Zohar, Y. (1999). Fabrication and characterization of a microsystem for a micro-scale heat transfer study. Journal of Micromechanics and Microengineering, 9(4), 422-428.
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  Abstract: A micro-system consisting of micro-channels with integrated temperature sensors was successfully designed and fabricated for the study of the heat-transfer properties of fluid flow in micro-domains. Surface micro-machining technology was used to construct the micro-channels with dimensions of about 40 μm×1.4 μm×4000 μm. Polysilicon thermistors, 4 μm×4 μm×0.4 μm in size were suspended across the channels and directly exposed to the fluid for local temperature measurements. The micro-channels and the micro-sensors were calibrated and the micro-system was characterized. The integrated micro-system performance was theoretically analyzed to provide a framework for the interpretation of the experimental data, and the various heat-transfer mechanisms are subsequently discussed.
• Jiang, L., Wang, Y., Wong, M., & Zohar, Y. (1999). Integrated micro thermal sensors for local temperature measurements in microsystems. Proceedings of the 1999 3rd ASME/JSME Joint Fluids Engineering Conference, FEDSM'99, San Francisco, California, USA, 18-23 July 1999 (CD-ROM), 1-.
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  Abstract: An array of integrated thermal micro sensors was successfully designed, fabricated and characterized for the measurement of temperature distributions in forced convection microchannel flows. Polycrystalline microsensors with the size of 4μmx4μmx0.4μm were suspended across the micro-channels, with hydraulic diameters of less than 3μm, directly exposed to the fluid for local temperature measurements. The fabrication of the integrated microsystem was accomplished using surface micromachining techniques. The sensor sensitivity, time response, accuracy and long term stability are documented. A theoretical analysis of the sensor performance, as part of an integrated microsystem, is presented to provide a framework for the interpretation of the experimental results.
• Jiang, L., Wong, M., & Zohar, Y. (1999). Micro-channel heat sink with integrated temperature sensors for phase transition study. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 159-164.
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  Abstract: A unique technique of maskless and self-aligned Si etch between bonded wafers was developed and applied to fabricate a micro-channel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed sensors, allowed direct temperature measurements to evaluate local heat transfer rates under forced convection boiling conditions. Temperature distributions along the heat sink were measured for different levels of power dissipation. The onset of critical heat flux (CHF) condition was investigated as a function of channel size and liquid flow rate. The results suggest that the bubble dynamic mechanism in micro-channel maybe different compared with conventional channels.
• Jiang, L., Wong, M., & Zohar, Y. (1999). Micromachined polycrystalline thin film temperature sensors. Measurement Science and Technology, 10(8), 653-664.
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  Abstract: Polycrystalline thin films based on two elemental semiconducting materials, Si and Ge, have been utilized to fabricate microthermistors. The thermistors are designed in a heavy-light-heavy doping concentration arrangement. The design, fabrication, analysis and characterization of a variety of thermistors under different doping schemes is described. Finally, the operation of the thermistors in self-heating operation is discussed. The results provide a systematic framework for the application of semiconducting microthermoresistors.
• Jiang, L., Wong, M., & Zohar, Y. (1999). Phase change in microchannel heat sinks with integrated temperature sensors. Journal of Microelectromechanical Systems, 8(4), 358-365.
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  Abstract: A unique technique of mask-less and self-aligned silicon etch between bonded wafers was developed and applied to fabricate a microchannel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed temperature microsensors, allowed direct temperature measurements for different levels of power dissipation under forced convection using either nitrogen or water as working fluids. The measured temperature distributions are used to characterize the micro heat sink performance under forced convection boiling conditions. The onset of critical heat flux (CHF) condition was investigated for different channel size and liquid flow-rate. The results suggest that the bubble dynamic mechanism in microchannel might be different compared with conventional channels.
• Ma, C. C., Wu, Z. J., Sun, Q. P., Zohar, Y., & Wong, M. (1999). TiNi shape memory alloy thin film rotating actuator topic D: sensors and actuators. Proceedings of the IEEE Hong Kong Electron Devices Meeting, 84-87.
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  Abstract: The operation of an SMA actuator at temperatures higher than the transformation temperatures of a stress-free film was demonstrated. Flat frequency response up to 2Hz and maximum amplitude of 4μm was possible.
• Wong, M., Ho, W., Yeung, M., Chin, G., C., P., & Zohar, Y. (1999). Pregate Oxidation Treatment using Radio Frequency Activated Nitrogen in a Rapid Thermal Reactor. Journal of the Electrochemical Society, 146(2), 707-709.
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  Abstract: A rapid thermal treatment in nitrogen could be inserted immediately before the gate oxidation step in a typical process flow for making metal-oxide-semiconductor field effect transistors. The treatment, performed in a radio frequency induction heated singlewafer rapid thermal reactor, is used to reduce the subsequent oxidation rate and to incorporate nitrogen in the resulting gate dielectrics. It is believed that activated molecular or atomic nitrogen is generated in the rapid thermal reactor and is responsible for the slight nitridalion of the exposed silicon area during the rapid thermal treatment. Potential radiation damage to the silicon area is minimized by activating the nitrogen remotely from the wafer chamber. The effects of the nitrogen flow rate and the temperature during the treatment process on the kinetics of the subsequent oxidation in a regular furnace and the chemical composition of the resulting gate dielectrics are studied. Potential use of the proposed treatment in making scaled devices is discussed. © 1999 The Electrochemical Society. All rights reserved.
• Zhang, X., Zhang, T., & Zohar, Y. (1999). Rapid thermal annealing for residual-stress relaxation in undoped or doped polysilicon thin films. Materials Research Society Symposium - Proceedings, 546, 27-32.
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  Abstract: The residual stress in doped and undoped polysilicon films, before and after rapid thermal annealing (RTA), is investigated using both wafer-curvature and micro-rotating structures techniques. Microstructure characterization has been conducted as well to understand the mechanism of the stress evolution. The results show that the compressive residual stresses in undoped polysilicon films can be reduced or eliminated within a few seconds RTA. Surface nitridation and grain growth are identified as the mechanisms responsible for the stress evolution.
• Zohar, Y., & Chai, H. (1999). Unsteady Wire Sweep Due to Transfer Molding in a 160L QFP Package. Journal of Electronic Packaging. doi:10.1115/1.2792668
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  Wire sweep has been recognized as a critical process defect which can result in device failure. The phenomenon is very complicated as it is sensitive to a large number of parameters. In this experimental work, the influence of some of the more important factors is investigated. These parameters include fluid-flow speed, wire-bond density, mold cavity height, and mold vent size. A 160L QFP package bonded with gold wires in a transparent mold is used as the test vehicle.
• Zohar, Y., Chan, P. C., Chin, G., Yeung, M., Ho, W. H., & Wong, M. (1999). Pregate Oxidation Treatment using Radio Frequency Activated Nitrogen in a Rapid Thermal Reactor. Journal of The Electrochemical Society. doi:10.1149/1.1391667
• Zohar, Y., Wong, M., & Jiang, L. (1999). Micromachined polycrystalline thin film temperature sensors. Measurement Science and Technology. doi:10.1088/0957-0233/10/8/301
• Zohar, Y., Wong, M., & Jiang, L. (1999). Phase change in microchannel heat sinks with integrated temperature sensors. Journal of microelectromechanical systems. doi:10.1109/84.809049
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  A unique technique of mask-less and self-aligned silicon etch between bonded wafers was developed and applied to fabricate a microchannel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed temperature microsensors, allowed direct temperature measurements for different levels of power dissipation under forced convection using either nitrogen or water as working fluids. The measured temperature distributions are used to characterize the micro heat sink performance under forced convection boiling conditions. The onset of critical heat flux (CHF) condition was investigated for different channel sizes and liquid flow-rates. The results suggest that the bubble dynamic mechanism in the microchannel might be different compared with conventional channels.
• Zohar, Y., Wong, M., Jiang, L., Xiong, B., & Li, B. (1999). Germanium as a versatile material for low-temperature micromachining. Journal of microelectromechanical systems. doi:10.1109/84.809050
• Zohar, Y., Wong, M., Wang, Y., & Jiang, L. (1999). Fabrication and characterization of a microsystem for a micro-scale heat transfer study. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/9/4/320
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  A micro-system consisting of micro-channels with integrated temperature sensors was successfully designed and fabricated for the study of the heat-transfer properties of fluid flow in micro-domains. Surface micro-machining technology was used to construct the micro-channels with dimensions of about 40 µm × 1.4 µm × 4000 µm. Polysilicon thermistors, 4 µm × 4 µm × 0.4 µm in size were suspended across the channels and directly exposed to the fluid for local temperature measurements. The micro-channels and the micro-sensors were calibrated and the micro-system was characterized. The integrated micro-system performance was theoretically analysed to provide a framework for the interpretation of the experimental data, and the various heat-transfer mechanisms are subsequently discussed.
• Chu, R. K., & Zohar, Y. (1998). Non-Equilibrium Sound Propagation in Discrete Velocity Models. Journal of Non-Equilibrium Thermodynamics, 23(3), 195-202.
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  Abstract: Discrete velocity models developed by Gatignol are used to study the propagation properties of one-dimensional, plane sound waves in monatomic gases, which is fundamental in Rarefied Gases Dynamics and Nonequilibrium Thermodynamics. The governing equations for 4-, 6- and 8-velocity models are first linearized around a uniform state. Then, plane-wave solutions are found where the sound dispersion and attenuation are determined. The results show that only the 4-velocity model properly predicts the propagation of sound waves in the high Knudsen number regime in comparison with experimental data.
• Wu, S., Mai, J., Zohar, Y., Tai, Y. C., & Ho, C. M. (1998). Study of flow in a micro channel suspended in vacuum. Microscale Thermophysical Engineering, 2(4), 232-234.
• Wu, S., Mai, J., Zohar, Y., Tai, Y. C., & Ho, C. M. (1998). Suspended microchannel with integrated temperature sensors for high-pressure flow studies. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 87-92.
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  Abstract: A freestanding microchannel, with integrated temperature sensors, has been developed for high-pressure flow studies. These microchannels are approximately 20 μm×21 μm×4400 μm, and are suspended above 80 μm deep cavities, bulk micromachined using BrF3 dry etch. The calibration of the lightly boron-doped thermistor-type sensors shows that the resistance sensitivity of these integrated sensors is parabolic with respect to temperature and linear with respect to pressure. Volumetric flow rates of N2 in the microchannel were measured at inlet pressures up to 578 psig. The discrepancy between the data and theory results from the flow acceleration in a channel, the non-parabolic velocity profile, and the bulging of the channel. Bulging effects were evaluated by using incompressible water flow measurements, which also measures 1.045×10-3N-s/m2 for the viscosity of DI water. The temperature data from sensors on the channel shows the heating of the channel due to the friction generated by the high-pressure flow inside.
• Zhang, T., Zhang, X., & Zohar, Y. (1998). Buckling of polysilicon microbeams during sacrificial layer removal. Journal of Micromechanics and Microengineering, 8(3), 243-249.
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  Abstract: In situ observations of buckling evolution of polysilicon microbeams during etch of the underneath sacrificial layer were carried out under an optical microscope. The surface geometry was obtained by AFM measurements. As the etching progressed, three buckling patterns were identified. Closed formulas were derived from theoretical analysis based on both boundary conditions: simply supported and clamped. The theory predicts either the buckling pattern for a given residual stress or the compressive stress level for a given buckling pattern. The residual stress evaluated from the buckling pattern agrees with that measured by the curvature method.
• Zhang, X., Zhang, T., & Zohar, Y. (1998). Buckling evolution of microelectromechanical structures. Materials Research Society Symposium - Proceedings, 518, 155-160.
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  Abstract: This study reports in-situ observations of the buckling evolution of microelectromechanical structures during etching of their underneath sacrificial layers. As the etching went on, the buckling pattern evolved from mode I, the sinusoidal half-waves, to mode II, the constrained sinusoidal half-waves, to mode III, the conventional mode, and finally to mode IV, the blisterlike local buckling. Closed formulae were derived from theoretical analysis, and the experimental results agreed well with the theoretical ones.
• Zhang, X., Zhang, T., & Zohar, Y. (1998). FEM simulation of micro-rotating-structures and their applications in measurement of residual stresses in thin films. Materials Research Society Symposium - Proceedings, 505, 21-26.
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  Abstract: FEM simulation of micro-rotating-structures was performed for local measurement of residual stresses in thin films. A sensitivity factor is introduced, studied and tabulated from the simulation results. The residual stress can be evaluated from the rotating deflection, the lengths of rotating and fixed beams, and the sensitivity factor. The micro-structure technique was applied to measure residual stresses in both silicon nitride and polysilicon thin films, before and after rapid thermal annealing (RTA), and further confirmed by wafer curvature method. Residual stresses in polysilicon films at different RTA stages were also characterized by micro-Raman spectroscopy (MRS). The experimental results indicate that micro-rotating-structures indeed have the ability to measure spatially and locally residual stresses in MEMS thin films with appropriate sensitivities.
• Zhang, X., Zhang, T., & Zohar, Y. (1998). Measurements of residual stresses in thin films using micro-rotating-structures. Thin Solid Films, 335(1-2), 97-105.
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  Abstract: In the present study, micro-rotating-structures for local measurements of residual stresses in a thin film were simulated by the finite element method (FEM). A sensitivity factor - the ratio of the deflection of the micro-structure to the normalized residual stress is introduced and tabulated from the FEM results. Thereafter, a formula to calculate the residual stress is given so that the residual stress can be easily evaluated from the deflection of the rotating beam. A variety of optimized micro-rotating-structures were then designed and fabricated to verify the FEM results. Residual stresses in both silicon nitride and polysilicon thin films were determined by this technique and compared with measurements by the wafer-curvature method. The two methods lead to comparable results. In addition, the micro-rotating-structures have the ability to measure spatially and locally a large range of residual tensile or compressive stresses. © 1998 Elsevier Science S.A. All rights reserved.
• Zhang, X., Zhang, T., Wong, M., & Zohar, Y. (1998). Rapid thermal annealing of polysilicon thin films. Journal of Microelectromechanical Systems, 7(4), 356-363.
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  Abstract: In comparison with conventional heat treatment, high-temperature rapid thermal annealing (RTA) in a radio frequency (RF) induction-heated system can reduce or eliminate residual stresses in thin films in a few seconds. In this work, changes in the stress level due to the RTA of polycrystalline silicon thin films were studied as a function of annealing time and temperature. The corresponding variations in the microstructure and surface layer of the thin films were experimentally investigated by a variety of analytical tools. The results suggest that the residual stress evolution during annealing is dominated by two mechanisms: 1) microstructure variations of the polysilicon thin film and 2) effects of a surface layer formed during the heat treatment. The fact that the microstructure changes are more pronounced in samples after conventional heat treatment implies that the effects of the formed surface layer may dominate the final state of the residual stress in the thin film. [337].
• Zhang, X., Zhang, T., Wong, M., & Zohar, Y. (1998). Residual-stress relaxation in polysilicon thin films by high-temperature rapid thermal annealing. Sensors and Actuators, A: Physical, 64(1), 109-115.
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  Abstract: Rapid thermal annealing reduces stress in a very short time, compared to regular furnace annealing, and can be an effective method for relaxing residual stress in polysilicon thin films. In this work, the effects of regular furnace and high-temperature rapid thermal annealing (RTA) on the residual stress of LPCVD polysilicon thin films have been investigated. The as-deposited 0.5 μm thick polysilicon films have an initial compressive stress of about 340 MPa, and the residual stress is relaxed quickly after a few cycles of RTA at higher temperatures. The stress dependence on annealing time at temperatures of 900-1150°C has been analysed. Using X-ray diffraction (XRD), micro-Raman spectroscopy and transmission electron microscopy (TEM), we have studied the changes in the microstructure of the thin films induced by the RTA during the stress relaxation. Furthermore, variations in the composition of the surface layer due to annealing have been characterized by X-ray photoelectron spectroscopy (XPS). © 1998 Elsevier Science S.A.
• Zhang, X., Zohar, Y., & Zhang, T. (1998). Blister-like local buckling of polysilicon microbeams during wet-release. American Society of Mechanical Engineers, Dynamic Systems and Control Division (Publication) DSC, 66, 379-384.
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  Abstract: A blister-like local buckling of polysilicon microbeams is observed at the end of the fabrication process. This buckling mode is a result of the capillary forces, developed during the drying stage, and the compressive residual stress in the polysilicon beams. A variety of microbeams were fabricated to study this phenomenon. The major parameters controlling the blister-like buckling have been identified experimentally. Furthermore, a model equation is derived to predict the blister properties as a function of the initial conditions. The agreement between the predictions and the measurements is found to be satisfactory.
• Zohar, Y., & Chu, R. K. (1998). Non-Equilibrium Sound Propagation in Discrete Velocity Models. Journal of Non-Equilibrium Thermodynamics. doi:10.1515/jnet.1998.23.3.195
• Zohar, Y., Ho, C., Tai, Y., Mai, J. D., & Wu, S. (1998). Study of flow in a micro channel suspended in vacuum. Microscale Thermophysical Engineering.
• Zohar, Y., Wong, M., Zhang, T., & Zhang, X. (1998). Rapid thermal annealing of polysilicon thin films. Journal of microelectromechanical systems. doi:10.1109/84.735342
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  In comparison with conventional heat treatment, high-temperature rapid thermal annealing (RTA) in a radio frequency (RF) induction-heated system can reduce or eliminate residual stresses in thin films in a few seconds. In this work, changes in the stress level due to the RTA of polycrystalline silicon thin films were studied as a function of annealing time and temperature. The corresponding variations in the microstructure and surface layer of the thin films were experimentally investigated by a variety of analytical tools. The results suggest that the residual stress evolution during annealing is dominated by two mechanisms: 1) microstructure variations of the polysilicon thin film and 2) effects of a surface layer formed during the heat treatment. The fact that the microstructure changes are more pronounced in samples after conventional heat treatment implies that the effects of the formed surface layer may dominate the final state of the residual stress in the thin film.
• Zohar, Y., Wong, M., Zhang, T., & Zhang, X. (1998). Residual-stress relaxation in polysilicon thin films by high-temperature rapid thermal annealing. Sensors and Actuators A-physical. doi:10.1016/s0924-4247(97)01661-0
• Zohar, Y., Zhang, T., & Zhang, X. (1998). Buckling Evolution of Microelectromechanical Structures. MRS Proceedings. doi:10.1557/proc-518-155
• Zohar, Y., Zhang, T., & Zhang, X. (1998). Measurements of residual stresses in thin films using micro-rotating-structures. Thin Solid Films. doi:10.1016/s0040-6090(98)00952-3
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  In the present study, micro-rotating-structures for local measurements of residual stresses in a thin film were simulated by the finite element method (FEM). A sensitivity factor – the ratio of the deflection of the micro-structure to the normalized residual stress is introduced and tabulated from the FEM results. Thereafter, a formula to calculate the residual stress is given so that the residual stress can be easily evaluated from the deflection of the rotating beam. A variety of optimized micro-rotating-structures were then designed and fabricated to verify the FEM results. Residual stresses in both silicon nitride and polysilicon thin films were determined by this technique and compared with measurements by the wafer-curvature method. The two methods lead to comparable results. In addition, the micro-rotating-structures have the ability to measure spatially and locally a large range of residual tensile or compressive stresses.
• Zohar, Y., Zhang, X., & Zhang, T. (1998). Buckling of polysilicon microbeams during sacrificial layer removal. Journal of Micromechanics and Microengineering. doi:10.1088/0960-1317/8/3/011
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  In situ observations of buckling evolution of polysilicon microbeams during etch of the underneath sacrificial layer were carried out under an optical microscope. The surface geometry was obtained by AFM measurements. As the etching progressed, three buckling patterns were identified. Closed formulas were derived from theoretical analysis based on both boundary conditions: simply supported and clamped. The theory predicts either the buckling pattern for a given residual stress or the compressive stress level for a given buckling pattern. The residual stress evaluated from the buckling pattern agrees with that measured by the curvature method.
• Zohar, Y., Zhang, X., & Zhang, T. Y. (1998). Rapid Thermal Annealing for Residual-Stress Relaxation in Undoped or Doped Polysilicon Thin Films. MRS Proceedings, 546, 27-32. doi:10.1557/proc-546-27
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  The residual stress in doped and undoped polysilicon films, before and after rapid thermal annealing (RTA), is investigated using both wafer-curvature and micro-rotating structures techniques. Microstructure characterization has been conducted as well to understand the mechanism of the stress evolution. The results show that the compressive residual stresses in undoped polysilicon films can be reduced or eliminated within a few seconds RTA. Surface nitridation and grain growth are identified as the mechanisms responsible for the stress evolution.
• Ho, C., & Zohar, Y. (1997). The PVC technique - A method to estimate the dissipation length scale in turbulent flows. Journal of Fluid Mechanics, 352, 135-159.
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  Abstract: A time-averaged length scale can be defined by a pair of successive turbulent-velocity derivatives, i.e. [dnu(x)/dxn]′/[dn+1u(x)/dx n+1]′. The length scale associated with the zeroth- and the first-order derivatives, u′/u′x, is the Taylor microscale. In isotropic turbulence, this scale is the average length between zero crossings of the velocity signal. The average length between zero crossings of the first velocity derivative, i.e. u′x/u′xx, can be reliably obtained by using the peak-valley-counting (PVC) technique. We have found that the most probable scale, rather than the average, equals the wavelength at the peak of the dissipation spectrum in a plane mixing layer (Zohar & Ho 1996). In this study, we experimentally investigate the generality of applying the PVC technique to estimate the dissipation scale in three basic turbulent shear flows: a flat-plate boundary layer, a wake behind a two-dimensional cylinder and a plane mixing layer. We also analytically explore the quantitative relationships among this length scale and the Kolmogorov and Taylor microscales.
• Zhang, X., Zhang, T., Wong, M., & Zohar, Y. (1997). Effects of high-temperature rapid thermal annealing on the residual stress of LPCVD-polysilicon thin films. Proceedings of the IEEE Micro Electro Mechanical Systems (MEMS), 535-540.
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  Abstract: The effects of regular furnace and high-temperature rapid thermal annealing (RTA) on the residual stress of LPCVD-polysilicon thin films were investigated. The as-deposited 0.5 μm thick polysilicon films had an initial compressive stress of about 340 MPa, and the residual stress was relaxed quickly after a few cycles of RTA at the higher temperatures. The stress variation with annealing time at temperatures of 900-1150 °C was analyzed. Using X-ray diffraction (XRD), micro-Raman spectroscopy and transmission electron microscopy (TEM), the changes in the microstructure of the thin films, induced by the RTA, during the stress relaxation were studied. Compared to regular furnace annealing, rapid thermal annealing can reduce stress in a short time and is an effective method for releasing the residual stress in polysilicon thin films.
• Zhang, X., Zhang, T., Zohar, Y., & Lee, S. (1997). Investigation on subsurface damage in silicon wafers. Materials Research Society Symposium - Proceedings, 442, 199-204.
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  Abstract: Micro-Raman spectroscopy and chemical etching were applied to determine the depth of subsurface damage in silicon wafers undergoing different machining processes: cutting, grinding, polishing and lapping. In comparison with the Raman spectrum of perfect single crystal silicon, both the shape and intensity at the shoulder (500 cm-1) and the subpeak (300 cm-1) spectral regions were changed in all the machined wafers. The intensities at shoulder and subpeak gradually decreased and finally resumed to normal, as the depth of the investigated layer increased. According to the chemical etch rate, the depth of the subsurface damage was thus evaluated for the different wafers. TEM observations further confirmed the obtained results.
• Zhang, X., Zohar, Y., & Zhang, T. (1997). Measurements of residual stresses in low-stress silicon nitride thin films using micro-rotating structures. Materials Research Society Symposium - Proceedings, 444, 111-116.
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  Abstract: A variety of rotating micro structures were designed, fabricated and characterized for residual-stress (or strain) measurements in low-stress silicon nitride thin films, deposited by LPCVD on silicon wafers. The sensitivities of the micro structures were calculated by finite element method (FEM) and verified experimentally. The results were further confirmed by utilizing the wafer-curvature method for stress measurements. The size of the structures enables local residual-stress (or strain) measurement. The stress level depends on both the film thickness and the gas ratio and also varies with the location on the wafer.
• Zohar, Y., & Ho, C. (1997). The PVC technique – a method to estimate the dissipation length scale in turbulent flows. Journal of Fluid Mechanics. doi:10.1017/s0022112097007180
• Zohar, Y., Zhang, X., & Zhang, T. (1997). FEM Simulation of Micro-rotating-structures and Their Applications in Measurement of Residual Stresses in Thin Films. MRS Proceedings, 505(1), 21-26. doi:10.1557/proc-505-21
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  FEM simulation of micro-rotating-structures was performed for local measurement of residual stresses in thin films. A sensitivity factor is introduced, studied and tabulated from the simulation results. The residual stress can be evaluated from the rotating deflection, the lengths of rotating and fixed beams, and the sensitivity factor. The micro-structure technique was applied to measure residual stresses in both silicon nitride and polysilicon thin films, before and after rapid thermal annealing (RTA), and further confirmed by wafer curvature method. Residual stresses in polysilicon films at different RTA stages were also characterized by micro-Raman spectroscopy (MRS). The experimental results indicate that micro-rotating-structures indeed have the ability to measure spatially and locally residual stresses in MEMS thin films with appropriate sensitivities.
• Zohar, Y., & Chih-Ming, H. (1996). Dissipation scale and control of fine-scale turbulence in a plane mixing layer. Journal of Fluid Mechanics, 320, 139-161.
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  Abstract: The entrainment of fluids from two streams into the shear region of an incompressible mixing layer is dominated by the evolution of large coherent structures. However, fine-scale mixing of the entrained fluids mainly occurs at the interfaces of the small-scale turbulence. In this investigation, experiments were conducted to understand the properties of the small scales and to explore a method for controlling the population of the fine-scale turbulence. Furthermore, a dissipation scale, ζ, is found from the zero-crossing of the time derivative of the velocity fluctuations. This scale characterizes the most probable size of fine-scale turbulence, which produces most of the viscous dissipation.
• Zohar, Y., & Ho, C. (1996). Dissipation scale and control of fine-scale turbulence in a plane mixing layer. Journal of Fluid Mechanics. doi:10.1017/s0022112096007483
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  The entrainment of fluids from two streams into the shear region of an incompressible mixing layer is dominated by the evolution of large coherent structures. However, fine-scale mixing of the entrained fluids mainly occurs at the interfaces of the small-scale turbulence. In this investigation, experiments were conducted to understand the properties of the small scales and to explore a method for controlling the population of the fine-scale turbulence. Furthermore, a dissipation scale, ζ, is found from the zero-crossing of the time derivative of the velocity fluctuations. This scale characterizes the most probable size of fine-scale turbulence, which produces most of the viscous dissipation.
• Zohar, Y., Zhang, X., & Zhang, T. (1996). Measurements of Residual Stresses in Low-Stress Silicon Nitride Thin Films Using Micro-Rotating Structures. MRS Proceedings, 444, 111-116. doi:10.1557/proc-444-111
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  AbstractA variety of rotating micro structures were designed, fabricated and characterized for residual-stress (or strain) measurements in low-stress silicon nitride thin films, deposited by LPCVD on silicon wafers. The sensitivities of the micro structures were calculated by finite element method (FEM) and verified experimentally. The results were further confirmed by utilizing the wafer-curvature method for stress measurements. The size of the structures enables local residual-stress (or strain) measurement. The stress level depends on both the film thickness and the gas ratio and also varies with the location on the wafer.
• Zohar, Y., Zhang, X., Zhang, T., & Lee, S. (1996). Investigation On Subsurface Damage In Silicon Wafers. MRS Proceedings, 442, 199-204. doi:10.1557/proc-442-199
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  Micro-Raman spectroscopy and chemical etching were applied to determine the depth of subsurface damage in silicon wafers undergoing different machining processes: cutting, grinding, polishing and lapping. In comparison with the Raman spectrum of perfect single crystal silicon, both the shape and intensity at the shoulder (500 cm −1 ) and the subpeak (300 cm −1 ) spectral regions were changed in all the machined wafers. The intensities at shoulder and subpeak gradually decreased and finally resumed to normal, as the depth of the investigated layer increased. According to the chemical etch rate, the depth of the subsurface damage was thus evaluated for the different wafers. TEM observations further confirmed the obtained results.
• Wang, Y., Zohar, Y., & Wang, M. (1995). Microelectromechanical systems. Tien Tzu Hsueh Pao/Acta Electronica Sinica, 23(10), 37-42.
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  Abstract: A survey of current international research on MEMS is presented. The principal technology involved in MEMS and its industrial application are introduced. The research of MEMS conducted by leading international institutes was analyzed and some suggestions are proposed for future development of MEMS technology in China.
• Liu, J., Tai, Y., Lee, J., Pong, K., Zohar, Y., & Ho, C. (1993). In situ monitoring and universal modelling of sacrificial PSG etching using hydrofluoric acid. IEEE Micro Electro Mechanical Systems, 71-76.
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  Abstract: A video system is designed to monitor the etching of sacrificial phosphosilicate-glass (PSG) microchannels in-situ accurately using hydrofluoric acid (HF). An universal model, which predicts accurately the etching length vs. time over a wide range of HF concentration (3-49 wt%), is identified. In addition to diffusion, this model is based on a first-and-second order chemical reaction mechanism. It is found that the PSG microchannel etching rate is HF is sensitive to channel thickness but not width. Finally, bubble formation and movement inside the etched microchannels are observed. Most of the generated bubbles are mobile and can enhance the etching rate.
• Ho, C., Zohar, Y., Foss, J. K., & Buell, J. C. (1991). Phase decorrelation of coherent structures in a free shear layer. Journal of Fluid Mechanics, 230, 319-337.
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  Abstract: The vortices near the origin of an initially laminar mixing layer have a single frequency with a well-defined phase; i.e. there is little phase jitter. Further downstream, however, the phase jitter increases suddenly. Even when the flow is forced, this same transition is observed. The forcing partially loses its influence because of the decorrelation of the phase between the forcing signal and the passing coherent structures. In the present investigation, this phenomenon is documented and the physical mechanism responsible for the phase decorrelation is identified.
• Zohar, Y., Buell, J. C., Foss, J. K., & Ho, C. (1991). Phase decorrelation of coherent structures in a free shear layer. Journal of Fluid Mechanics. doi:10.1017/s0022112091000800
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  The vortices near the origin of an initially laminar mixing layer have a single frequency with a well-defined phase; i.e. there is little phase jitter. Further downstream, however, the phase jitter increases suddenly. Even when the flow is forced, this same transition is observed. The forcing partially loses its influence because of the decorrelation of the phase between the forcing signal and the passing coherent structures. In the present investigation, this phenomenon is documented and the physical mechanism responsible for the phase decorrelation is identified.

Proceedings Publications

• Zohar, Y. (2018, December). Application of a microfluidic-based model of a human prostate gland for cancer research. In 12th International Conference on Nano/Molecular Medicine and Engineering, 109-112.
• Zohar, Y. (2018, December). Characterizing A549 cell line as an epithelial cell monolayer model for pharmacokinetic applications. In 12th International Conference on Nano/Molecular Medicine and Engineering, 27-30.
• Zohar, Y. (2018, June). Characterizing steady molecular transport between a microchannel pair separated by a porous membrane. In 2018 Synthetic Biology: Engineering, Evolution & Design (SEED), 51.
• Zohar, Y. (2018, November). Development of a microfluidic-based model of a human prostate gland. In 22nd International Conference on Miniaturized Systems for Chemistry and Life Sciences, 1625-1627.
• Zohar, Y. (2018, November). Protein transport through a separation membrane in a microfluidic device. In 22nd International Conference on Miniaturized Systems for Chemistry and Life Sciences, 1976-1978.
• Zohar, Y., Miranti, C. K., Frank, S. B., Tran, M., Ivich, F., & Jiang, L. (2018). Development of a microfluidic-based model of a human prostate gland. In 22nd International Conference on Miniaturized Systems for Chemistry and Life Sciences.
• Zohar, Y. (2017, June 2017). The capillary number effect on cell viability in microfluidic elasto-filtration devices for viable circulating tumor cell isolation. In 19th International Conference on Solid-State Sensors, Actuators and Microsystems, 488-491.
• Hu, L., Jeong, S. Y., Lee, Y., Lee, J., Song, K., Su, H., Yu, X., Zhang, Z., Zhao, C., & Zohar, Y. (2016).

  Capillary number effect on the depletion of leucocytes of blood in microfiltration chips for the isolation of circulating tumor cells

  . In 2016 IEEE 11th Annual International Conference on Nano/Micro Engineered and Molecular Systems (NEMS), 346-349.
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  To increase the purity of isolated circulating tumor cells (CTCs) in microfiltration chips, the depletion of leucocytes in human blood was systematically studied as a function of two dimensionless parameters, Capillary number Ca and the normalized leucocyte diameter. An optimized Ca* of 0.038 was identified to differentiate the capturing of cancer cells and leucocytes. Such Ca effect can be applied as a guideline for designing an optimized microfiltration system with both high capture efficiency and purity for isolating CTCs from human blood.
• Lee, Y., Liang, C., Liu, D., Ma, S., Song, K., Tang, C., Xu, R., Zhao, C., & Zohar, Y. (2015).

  The Capillary number effect on the capture efficiency of cancer cells on composite microfluidic filtration chips

  . In 2015 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 2015, 459-462.
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  We present a systematic study of the Capillary number (Ca) effect on the capture efficiency of cancer cells on a composite microfluidic filtration chip. By altering the Ca in microchip experiments, the balance between the viscous force and the cell cortical tension affecting the capture efficiency has been investigated experimentally and analyzed theoretically. A ‘Phase Diagram’ for the capture efficiency of microfiltration chips is presented, for the first time, as a function of the normalized cell diameter and Ca. A critical value of Ca, around 0.03∼0.04, has been identified for enhancing the capture efficiency of cancer cells. The phase diagram is found to be consistent with the results of cancer-cell capture in microfiltration systems reported previously by others. The diagram can be a useful tool for designing the next generation microfiltration devices for isolating circulating tumor cells.
• Ma, W., Zeng, R., Liu, D., Zohar, Y., & Lee, Y. K. (2014, April 13-16). SOI Technology-based Microfiltration System for Circulating Tumor Cells Isolatio and Enumeration. In 9th Annual IEEE International Conference on Nano/Micro Engineered and Molecular Systems, 333-336.
• Jiang, L., Cheung, L. S., Gudipaty, T., Stamm, M. T., & Zohar, Y. (2009).

  Cluster Dynamics in Flow of Suspended Particles in Microchannels

  . In 2009 IEEE 22nd International Conference on Micro Electro Mechanical Systems, 379-382.
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  Cluster dynamics in microchannels due to flow of dilute suspensions of polystyrene spherical particles has been studied experimentally. Cluster-cluster interaction as well as the functional dependence of cluster growth rate on several control parameters has been studied. Destructive, e.g. cluster collision, and constructive modes, e.g. cluster merging, of cluster-cluster interaction have been observed. Cluster growth rate with time is found to increase with either particle concentration or shear strain rate, and decrease with channel-height to particle-diameter ratio.
• Cheung, L. S., Zheng, X., Stopa, A., Schroeder, J. A., Heimark, R. L., Baygents, J. C., Guzman, R. Z., & Zohar, Y. (2008). ATTACHMENT & DETACHMENT OF PROSTATE CANCER CELLS IN A MICROFLUIDIC SYSTEM. In 12th International Conference on Miniaturized Systems for Chemistry and Life Sciences.
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  Cadherins make up a family of cell adhesion molecules, and one cadherin subtype only interacts with its counter receptor. Cancer cells typically down-regulate Ecadherin and up-regulate N-cadherin. Hence, a microchannel-based system is developed, with immobilized anti-N-cadherin antibodies, for highly specific capture of prostate cancer cells under both static (no-flow) and dynamic (flow) conditions. The maximum number of detached cells, after capture, is found to depend not only on the flow-induced shear stress but also on the rate of change of the applied shear stress.
• Gudipaty, T., Cheung, L. S., Jiang, L., & Zohar, Y. (2008). CLUSTER FORMATION AND EVOLUTION IN PARTICLE-LADEN MICROCHANNEL FLOW. In First Sensor, Signal and Information (SenSIP) Workshop.
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  Microchannels are susceptible to blockage by solid particles. Based on the present experiments, aggregation of clusters was observed for flow of liquid with 0.01 volume concentration of polystyrene particles, about 1.5µm in nominal diameter, through a microchannel 15µm high. The phenomenon of interest is the formation and growth of clusters in the flow of a dilute suspension of hard spheres. In general, the clusters initially grow almost linearly in time under various conditions. The spatial distribution and time evolution of clusters along the microchannel have been characterized. The effects of flow rate, volume fraction and channel-height to particle-diameter ratio are discussed as well.
• Jiang, L., Cheung, L. S., Gudipaty, T., & Zohar, Y. (2008).

  Cluster Formation and Growth in Flow of Dilute Particle Suspension in Microchannels

  . In Volume 4: Fatigue and Fracture; Fluids Engineering; Heat Transfer; Mechatronics; Micro and Nano Technology; Optical Engineering; Robotics; Systems Engineering; Industrial Applications, 357-362.
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  Microchannels are susceptible to blockage by solid particles. The lifetime of microfluidic devices depends on their ability to maintain flow without interruption, while certain applications require microdevices for transport of liquids containing particles. In this work, the phenomenon of interest is the formation and growth of clusters in the flow of a dilute suspension of hard spheres. Based on the present experiments, aggregation of clusters was observed for particle-laden flow in a microchannel with particle void fraction as low as 0.001 and particle diameter to channel height ratio as low as 0.1. The incipience and growth of a single cluster is discussed, and the spatial distribution and time evolution of clusters along the microchannel is presented. Although the cluster size seems to be independent of location, higher number of clusters is found at the inlet/outlet regions than in the microchannel center. Similar to individual cluster, the total cluster area in the microchannel grows almost linearly in time. The effects of flow rate, particle size and concentration are also reported.Copyright © 2008 by ASME
• Chan, Y. C., Lee, Y., Lee, Y., Wong, M., Zohar, Y., & Chan, Y. C. (2005). Effect of sub-micron pillar array on DNA kinetics in a free-solution capillary electrophoresis microsystem. In Ninth International Conference on Miniaturized Systems for Chemistry and Life Sciences.
• Zohar, Y., Lee, Y., & Lee, M. (2005). Fabrication and Characterization of an Integrated Thermal Microsystem. In 18th International Micro Electro Mechanical Systems Conference.
• Zohar, Y., Lee, Y., Lee, L. M., & Hau, W. L. (2003). Electrically Driven Vortical Motion for Mixing of Liquids in a Microchannel. In 7th International Conference on Miniaturized Chemical and BioChemical Analysis Systems.
• Zohar, Y., Lee, Y., & Yu, Z. T. (2002). Hele-Shaw Flow in a Microchannel with Cavities. In 2002 Am. Phys. Soc. meeting, 47, 130.
• Zohar, Y., Wong, M., Wang, M., & Meng, Z. (2000). A new polycrystalline silicon technology for integrated sensor applications. In Proceedings IEEE Thirteenth Annual International Conference on Micro Electro Mechanical Systems (Cat. No.00CH36308), 114-119.
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  A novel metal-induced lateral crystallization (MILC) technique has been applied to the formation of polycrystalline silicon (poly-Si) with greatly enhanced material characteristics. Compared to conventional poly-Si, MILC poly-Si gives rise to much improved performance not only in sensors but also in thin film transistors. A variety of sensing and electronic devices can be realized simultaneously on MILC poly-Si, thus making MILC an enabling technology for integrated silicon micro-machining.

Presentations

• Zohar, Y. (2014, February 17). Guiding Signals in Life Sciences: Cancer Metatasis. Seminar. University of Arizona, Tucson, AZ: Cancer Biology Program.
• Zohar, Y. (2014, January 29). Micro/Nanotechnology for Applications in Life Sciences. Seminar. University of Arizona, Tucson, AZ: College of Pharmacy.
• Zohar, Y. (2014, November 17). Micro/Nanotechnology for Cancer Metastasis Research. Seminar. University of Arizona, Tucson, AZ: Arizona Cancer Center: Cancer Biology Research Conference.

Poster Presentations

• Zohar, Y. (2017, November 2017). Characterizing diffusion in a microfluidic system for lung-on-a-chip applications. Junior Investigator Poster Forum. Tucson, AZ, USA: UA College of Medicine.
• Zohar, Y. (2017, November 2017). Development of a microfluidic model of a human prostate gland for Cancer Research. Junior Investigator Poster Forum. Tucson, AZ, USA: UA College of Medicine.
• Zohar, Y. (2017, October 2017). A microfluidic-based model of a human prostate gland. UACC Scientific Symposium & Retreat. Phoenix, AZ, USA: UA Cancer Center.