@article{yao_walker_gamcsik_2023, title={Assessing MTT and sulforhodamine B cell proliferation assays under multiple oxygen environments}, ISSN={["1573-0778"]}, DOI={10.1007/s10616-023-00584-0}, journal={CYTOTECHNOLOGY}, author={Yao, Ming and Walker, Glenn and Gamcsik, Michael P.}, year={2023}, month={Jul} }
 @article{yao_walker_gamcsik_2021, title={A multiwell plate-based system for toxicity screening under multiple static or cycling oxygen environments}, volume={11}, ISSN={["2045-2322"]}, DOI={10.1038/s41598-021-83579-1}, abstractNote={Abstract}, number={1}, journal={SCIENTIFIC REPORTS}, author={Yao, Ming and Walker, Glenn and Gamcsik, Michael P.}, year={2021}, month={Feb} }
 @article{rich_mohd_ligler_walker_2019, title={Characterization of glass frit capillary pumps for microfluidic devices}, volume={23}, ISSN={["1613-4990"]}, DOI={10.1007/s10404-019-2238-6}, number={5}, journal={MICROFLUIDICS AND NANOFLUIDICS}, author={Rich, Matthew and Mohd, Omar and Ligler, Frances S. and Walker, Glenn M.}, year={2019}, month={May} }
 @article{yao_rabbani_sattler_nguyen_zaharoff_walker_gamcsik_2019, title={Flow-Encoded Oxygen Control to Track the Time-Dependence of Molecular Changes Induced by Static or Cycling Hypoxia}, volume={91}, ISSN={["1520-6882"]}, DOI={10.1021/acs.analchem.9b03709}, abstractNote={Detecting the effects of low oxygen on cell function is often dependent on monitoring the expression of a number of hypoxia markers. The time dependence of the appearance and stability of these markers varies between cell lines. Assessing cellular marker dynamics is also critical to determining how quickly cells respond to transient changes in oxygen levels that occurs with cycling hypoxia. We fabricated a manifold designed to use flow-encoding to produce sequential changes in gas mixtures delivered to a permeable-bottom 96-well plate. We show how this manifold and plate design can be used to expose cells to either static or cycling hypoxic conditions for eight different time periods thereby facilitating the study of the time-response of cells to altered oxygen environments. Using this device, we monitored the time-dependence of molecular changes in human PANC-1 pancreatic carcinoma and Caco-2 colon adenocarcinoma cells exposed to increasing periods of static or cycling hypoxia. Using immunohistochemistry, both cell lines show detectable levels of the marker protein hypoxia-inducible factor-1α (HIF-1α) after 3 h of exposure to static hypoxia. Cycling hypoxia increased the expression level of HIF-1α compared to static hypoxia. Both static and cycling hypoxia also increased glucose uptake and aldehyde dehydrogenase activity. This new device offers a facile screening approach to determine the kinetics of cellular alterations under varying oxygen conditions.}, number={23}, journal={ANALYTICAL CHEMISTRY}, author={Yao, Ming and Rabbani, Zahid N. and Sattler, Tyler and Nguyen, Khue G. and Zaharoff, David A. and Walker, Glenn and Gamcsik, Michael P.}, year={2019}, month={Dec}, pages={15032–15039} }
 @article{sotoudegan_mohd_ligler_walker_2019, title={Paper-based passive pumps to generate controllable whole blood flow through microfluidic devices}, volume={19}, ISSN={["1473-0189"]}, DOI={10.1039/c9lc00822e}, abstractNote={Grooved paper pumps provide controllable flow of complex biofluids within microfluidic devices.}, number={22}, journal={LAB ON A CHIP}, author={Sotoudegan, Mohamad S. and Mohd, Omar and Ligler, Frances S. and Walker, Glenn M.}, year={2019}, month={Nov}, pages={3787–3795} }
 @article{yao_sattler_rabbani_pulliam_walker_gamcsik_2018, title={Mixing and delivery of multiple controlled oxygen environments to a single multiwell culture plate}, volume={315}, ISSN={["1522-1563"]}, DOI={10.1152/ajpcell.00276.2018}, abstractNote={ Precise oxygen control is critical to evaluating cell growth, molecular content, and stress response in cultured cells. We have designed, fabricated, and characterized a 96-well plate-based device that is capable of delivering eight static or dynamically changing oxygen environments to different rows on a single plate. The device incorporates a gas-mixing tree that combines two input gases to generate the eight gas mixtures that supply each row of the plate with a different gas atmosphere via a removable manifold. Using air and nitrogen as feed gases, a single 96-well plate can culture cells in applied gas atmospheres with Po2 levels ranging from 1 to 135 mmHg. Human cancer cell lines MCF-7, PANC-1, and Caco-2 were grown on a single plate under this range of oxygen levels. Only cells grown in wells exposed to Po2 ≤37 mmHg express the endogenous hypoxia markers hypoxia-inducible factor-1α and carbonic anhydrase IX. This design is amenable to multiwell plate-based molecular assays or drug dose-response studies in static or cycling hypoxia conditions. }, number={5}, journal={AMERICAN JOURNAL OF PHYSIOLOGY-CELL PHYSIOLOGY}, author={Yao, Ming and Sattler, Tyler and Rabbani, Zahid N. and Pulliam, Thomas and Walker, Glenn and Gamcsik, Michael P.}, year={2018}, month={Nov}, pages={C766–C775} }
 @article{cummins_chinthapatla_lenin_ligler_walker_2017, title={Modular pumps as programmable hydraulic batteries for microfluidic devices}, volume={5}, ISSN={["2345-7740"]}, DOI={10.1142/s2339547817200011}, abstractNote={ Simple fluid pumps have been developed to improve microfluidic device portability, but they cannot be easily programmed, produce repeatable pumping performance, or generate complex flow profiles — key requirements for increasing the functionality of portable microdevices. We present a detachable, paper-based, “hydraulic battery” that can be connected to the outlet of a microfluidic channel to pump fluid at varying flow rates over time, including step changes, ramping flows, and oscillating flows. }, number={1}, journal={TECHNOLOGY}, author={Cummins, Brian M. and Chinthapatla, Rukesh and Lenin, Balaji and Ligler, Frances S. and Walker, Glenn M.}, year={2017}, month={Mar}, pages={21–30} }
 @article{cummins_chinthapatla_ligler_walker_2017, title={Time-Dependent Model for Fluid Flow in Porous Materials with Multiple Pore Sizes}, volume={89}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ACS.ANALCHEM.6B04717}, DOI={10.1021/acs.analchem.6b04717}, abstractNote={An understanding of fluid transport through porous materials is critical for the development of lateral flow assays and analytical devices based on paper microfluidics. Models of fluid transport within porous materials often assume a single capillary pressure and permeability value for the material, implying that the material comprises a single pore size and that the porous material is fully saturated behind the visible wetted front. As a result, current models can lead to inaccuracies when modeling transport over long distances and/or times. A new transport model is presented that incorporates a range of pore sizes to more accurately predict the capillary transport of fluid in porous materials. The model effectively predicts the time-dependent saturation of rectangular strips of Whatman filter no. 1 paper using the manufacturer's data, published pore-size distribution measurements, and the fluid's properties.}, number={8}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Cummins, Brian M. and Chinthapatla, Rukesh and Ligler, Frances S. and Walker, Glenn M.}, year={2017}, month={Mar}, pages={4377–4381} }
 @article{ye_yu_wang_nguyen_walker_buse_gu_2016, title={Drug Delivery: Microneedles Integrated with Pancreatic Cells and Synthetic Glucose-Signal Amplifiers for Smart Insulin Delivery (Adv. Mater. 16/2016)}, volume={28}, ISSN={0935-9648}, url={http://dx.doi.org/10.1002/ADMA.201670112}, DOI={10.1002/ADMA.201670112}, abstractNote={A bio-responsive microneedle-based patch, integrated with a rhodamine-stained glucose-signal amplifier and calcein-AM-stained pancreatic β-cell capsules, is developed by Z. Gu and co-workers. This "smart cell patch", described on page 3115, effectively regulates the blood glucose level of type-1 diabetic mice, achieving a reduction for over 10 h. Image credit: Yanqi Ye.}, number={16}, journal={Advanced Materials}, publisher={Wiley}, author={Ye, Yanqi and Yu, Jicheng and Wang, Chao and Nguyen, Nhu-Y and Walker, Glenn M. and Buse, John B. and Gu, Zhen}, year={2016}, month={Apr}, pages={3223–3223} }
 @article{murray_walker_bereman_2016, title={Improving the analytical performance and versatility of paper spray mass spectrometry via paper microfluidics}, volume={141}, ISSN={["1364-5528"]}, DOI={10.1039/c6an00649c}, abstractNote={Paper-based microfluidic techniques were explored to increase paper spray mass spectrometry's performance and versatility.}, number={13}, journal={ANALYST}, author={Murray, Ian and Walker, Glenn and Bereman, Michael S.}, year={2016}, pages={4065–4073} }
 @article{ye_yu_wang_nguyen_walker_buse_gu_2016, title={Microneedles Integrated with Pancreatic Cells and Synthetic Glucose-Signal Amplifiers for Smart Insulin Delivery}, volume={28}, ISSN={0935-9648}, url={http://dx.doi.org/10.1002/ADMA.201506025}, DOI={10.1002/ADMA.201506025}, abstractNote={An innovative microneedle (MN)-based cell therapy is developed for glucose-responsive regulation of the insulin secretion from exogenous pancreatic β-cells without implantation. One MN patch can quickly reduce the blood-sugar levels (BGLs) of chemically induced type-1 diabetic mice and stabilize BGLs at a reduced level for over 10 h.}, number={16}, journal={Advanced Materials}, publisher={Wiley}, author={Ye, Yanqi and Yu, Jicheng and Wang, Chao and Nguyen, Nhu-Y and Walker, Glenn M. and Buse, John B. and Gu, Zhen}, year={2016}, month={Mar}, pages={3115–3121} }
 @article{cummins_ligler_walker_2016, title={Point-of-care diagnostics for niche applications}, volume={34}, ISSN={0734-9750}, url={http://dx.doi.org/10.1016/J.BIOTECHADV.2016.01.005}, DOI={10.1016/j.biotechadv.2016.01.005}, abstractNote={Point-of-care or point-of-use diagnostics are analytical devices that provide clinically relevant information without the need for a core clinical laboratory. In this review we define point-of-care diagnostics as portable versions of assays performed in a traditional clinical chemistry laboratory. This review discusses five areas relevant to human and animal health where increased attention could produce significant impact: veterinary medicine, space travel, sports medicine, emergency medicine, and operating room efficiency. For each of these areas, clinical need, available commercial products, and ongoing research into new devices are highlighted.}, number={3}, journal={Biotechnology Advances}, publisher={Elsevier BV}, author={Cummins, Brian M. and Ligler, Frances S. and Walker, Glenn M.}, year={2016}, month={May}, pages={161–176} }
 @misc{permana_grant_walker_yoder_2016, title={Review of Automated Microinjection Systems for Single Cells in the Embryogenesis Stage}, volume={21}, ISSN={["1941-014X"]}, DOI={10.1109/tmech.2016.2574871}, abstractNote={Modern genetics research has resulted in significant advances in cell in vitro microinjection systems. Such systems provide biological and medical practitioners with high volume cell throughput and statistically relevant data. This paper provides the reader with a comprehensive review of the major research technologies used in automated cell microinjection and of their individual subsystems. Microinjection subsystems reviewed include machine vision, nonvision sensors and user interface (input), cell modeling, piercing mechanisms and injection control loop (control), cell holder and manipulator, and microinjection (output). The interdisciplinary technologies reviewed for microinjection sensing, automation, and control include microfluidic actuation, optical field actuation (optical trapping and optical guidance), electrical field actuation (electrorotation, electrowetting, and dielectrophoresis), and ultrasonic vibration. The survey concludes that research into automated microinjection systems will focus on reducing the scale of microinjection systems and developing appropriate controllers.}, number={5}, journal={IEEE-ASME TRANSACTIONS ON MECHATRONICS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Permana, Sofie and Grant, Edward and Walker, Glenn M. and Yoder, Jeffrey A.}, year={2016}, month={Oct}, pages={2391–2404} }
 @article{pacardo_neupane_wang_gu_walker_ligler_2015, title={A temperature microsensor for measuring laser-induced heating in gold nanorods}, volume={407}, ISSN={["1618-2650"]}, DOI={10.1007/s00216-014-8222-9}, abstractNote={Measuring temperature is an extensively explored field of analysis, but measuring a temperature change in a nanoparticle is a new challenge. Here, a microsensor is configured to measure temperature changes in gold nanorods in solution upon laser irradiation. The device consists of a silicon wafer coated with silicon nitride in which a microfabricated resistance temperature detector was embedded and attached to a digital multimeter. A polydimethylsiloxane mold served as a microcontainer for the sample attached on top of the silicon membrane. This enables laser irradiation of the gold nanorods and subsequent measurement of temperature changes. The results showed a temperature increase of 8 to 10 °C and good correlation with theoretical calculations and bulk sample direct temperature measurements. These results demonstrate the suitability of this simple temperature microsensor for determining laser-induced heating profiles of metallic nanomaterials; such measurements will be essential for optimizing therapeutic and catalytic applications.}, number={3}, journal={ANALYTICAL AND BIOANALYTICAL CHEMISTRY}, author={Pacardo, Dennis B. and Neupane, Bhanu and Wang, Gufeng and Gu, Zhen and Walker, Glenn M. and Ligler, Frances S.}, year={2015}, month={Jan}, pages={719–725} }
 @article{jiang_jeffries_acosta_tikunov_macdonald_walker_gamcsik_2015, title={Biocompatibility of Tygon® tubing in microfluidic cell culture}, volume={17}, ISSN={1387-2176 1572-8781}, url={http://dx.doi.org/10.1007/S10544-015-9938-9}, DOI={10.1007/S10544-015-9938-9}, abstractNote={Growth of the MDA-MB-231 breast cancer cell line in microfluidic channels was inhibited when culture media was delivered to the channels via microbore Tygon® tubing. Culture media incubated within this tubing also inhibited growth of these cells in conventional 96-well plates. These detrimental effects were not due to depletion of critical nutrients due to adsorption of media components onto the tubing surface. A pH change was also ruled out as a cause. Nuclear magnetic resonance spectroscopy of the cell growth media before and after incubation in the tubing confirmed no detectable loss of media components but did detect the presence of additional unidentified signals in the aliphatic region of the spectrum. These results indicate leaching of a chemical species from microbore Tygon® tubing that can affect cell growth in microfluidic devices.}, number={1}, journal={Biomedical Microdevices}, publisher={Springer Science and Business Media LLC}, author={Jiang, Xiao and Jeffries, Rex E. and Acosta, Miguel A. and Tikunov, Andrey P. and Macdonald, Jeffrey M. and Walker, Glenn M. and Gamcsik, Michael P.}, year={2015}, month={Feb} }
 @article{vutha_davaji_lee_walker_2014, title={A microfluidic device for thermal particle detection}, volume={17}, ISSN={1613-4982 1613-4990}, url={http://dx.doi.org/10.1007/S10404-014-1369-Z}, DOI={10.1007/S10404-014-1369-Z}, abstractNote={We demonstrate the use of heat to count microscopic particles. A thermal particle detector (TPD) was fabricated by combining a 500-nm-thick silicon nitride membrane containing a thin-film resistive temperature detector with a silicone elastomer microchannel. Particles with diameters of 90 and 200 μm created relative temperature changes of 0.11 and −0.44 K, respectively, as they flowed by the sensor. A first-order lumped thermal model was developed to predict the temperature changes. Multiple particles were counted in series to demonstrate the utility of the TPD as a particle counter.}, number={5}, journal={Microfluidics and Nanofluidics}, publisher={Springer Science and Business Media LLC}, author={Vutha, Ashwin Kumar and Davaji, Benyamin and Lee, Chung Hoon and Walker, Glenn M.}, year={2014}, month={Mar}, pages={871–878} }
 @article{acosta_jiang_huang_cutler_grant_walker_gamcsik_2014, title={A microfluidic device to study cancer metastasis under chronic and intermittent hypoxia}, volume={8}, ISSN={1932-1058}, url={http://dx.doi.org/10.1063/1.4898788}, DOI={10.1063/1.4898788}, abstractNote={Metastatic cancer cells must traverse a microenvironment ranging from extremely hypoxic, within the tumor, to highly oxygenated, within the host's vasculature. Tumor hypoxia can be further characterized by regions of both chronic and intermittent hypoxia. We present the design and characterization of a microfluidic device that can simultaneously mimic the oxygenation conditions observed within the tumor and model the cell migration and intravasation processes. This device can generate spatial oxygen gradients of chronic hypoxia and produce dynamically changing hypoxic microenvironments in long-term culture of cancer cells.}, number={5}, journal={Biomicrofluidics}, publisher={AIP Publishing}, author={Acosta, Miguel A. and Jiang, Xiao and Huang, Pin-Kang and Cutler, Kyle B. and Grant, Christine S. and Walker, Glenn M. and Gamcsik, Michael P.}, year={2014}, month={Sep}, pages={054117} }
 @article{mcpherson_walker_2012, title={A photo-defined membrane for precisely patterned cellular and microparticle arrays}, volume={2}, ISSN={2158-3226}, url={http://dx.doi.org/10.1063/1.3690966}, DOI={10.1063/1.3690966}, abstractNote={The ability to pattern particles in well-defined arrays enhances microfluidic devices. A low-fluorescence optically transparent photo-curable resist (1002F) was characterized for use as a mechanical sieve in a microfluidic chip. Films of thickness 10 μm and 25 μm were created containing pores 6–10 μm in diameter with pitches ranging from 5–300 μm. The uniform photo-defined pores had diameters with standard deviations of 3%. Integrated with microfluidic devices, the films were used to trap polystyrene microspheres, and in a different experiment, MCF7 human epithelial adenocarcinoma cells (ATCC HTB-22). A mechanical sieve was used to trap two types of fluorescent particles and, separately MCF7 cells with NIH/3T3 murine fibroblast cells (ATCC CRL-1658) as a proof-of-concept for striated cellular co-culture.}, number={1}, journal={AIP Advances}, publisher={AIP Publishing}, author={McPherson, A. L. and Walker, G. M.}, year={2012}, month={Mar}, pages={012153} }
 @article{dong_walker_2012, title={Adjustable stiffness tubes via thermal modulation of a low melting point polymer}, volume={21}, ISSN={0964-1726 1361-665X}, url={http://dx.doi.org/10.1088/0964-1726/21/4/042001}, DOI={10.1088/0964-1726/21/4/042001}, abstractNote={Stent procedures require a range of catheters, each with a different stiffness, to deliver a stent to the site of a lesion. A single catheter with adjustable stiffness is a promising way to simplify stenting procedures. Here we report on an adjustable stiffness tube composed of high melting point polymers for the inner and outer layer and a low melting point polymer in the middle that could be incorporated into an adjustable stiffness catheter. Tube stiffness is controlled by thermal modulation of the low melting point polymer via an embedded heater. Seven tubes were tested and their effective Young’s modulus was calculated. The largest change in modulus within a single tube was from 0.53 to 0.09 GPa, or an 83% decrease. Over all tube dimensions tested, we recorded stiffnesses ranging from 1.34 to 0.09 GPa after applying current for 300 s. In contrast, 2 French and 8 French commercial catheters had effective Young’s moduli of 0.583 GPa and 0.513 GPa, respectively, demonstrating that the adjustable stiffness tube is a viable candidate for use as an adjustable stiffness catheter.}, number={4}, journal={Smart Materials and Structures}, publisher={IOP Publishing}, author={Dong, Hua and Walker, Glenn M}, year={2012}, month={Mar}, pages={042001} }
 @article{soohoo_herr_ramsey_walker_2012, title={Microfluidic Cytometer for the Characterization of Cell Lysis}, volume={84}, ISSN={["0003-2700"]}, DOI={10.1021/ac202461h}, abstractNote={Blood cytometry and intercellular analysis typically requires lysis as a preparatory step, which can alter the results of downstream analyses. We fabricated a microfluidic cytometer to characterize erythrocyte lysis kinetics. Forward light scatter from erythrocytes was used for enumeration at specific locations on a microfluidic chip. Diffusive transport coupled with laminar flow was used to control the concentration and exposure time of the lysis reagent Zap-OGLOBIN II to erythrocytes. Standard clinical practice is to expose erythrocytes to lysis reagent for 10 min. Under optimum conditions, we achieved complete erythrocyte lysis of a blood sample in 0.7 s. A maximum lysis reaction rate of 1.55 s(-1) was extrapolated from the data. Lysis began after 0.2 s and could be initiated with a lysis reagent concentration of 1.0% (68.5 mM). An equation that related lysis reagent concentration, [A], to erythrocyte lysis, [B], was determined to be [B] = -0.77[A](0.29)t.}, number={5}, journal={ANALYTICAL CHEMISTRY}, author={SooHoo, Jeffrey R. and Herr, Joshua K. and Ramsey, J. Michael and Walker, Glenn M.}, year={2012}, month={Mar}, pages={2195–2201} }
 @article{walker_2011, title={PENSIONING OFF PIPETTES: Pensioning off pipettes}, volume={3}, ISSN={["1755-4349"]}, DOI={10.1038/nchem.1060}, number={6}, journal={NATURE CHEMISTRY}, author={Walker, Glenn M.}, year={2011}, month={Jun}, pages={428–429} }
 @article{mcpherson_walker_2010, title={A microfluidic passive pumping Coulter counter}, volume={9}, ISSN={["1613-4990"]}, DOI={10.1007/s10404-010-0609-0}, abstractNote={A microfluidic device using on-chip passive pumping was characterized for use as a particle counter. Flow occurred due to a Young-Laplace pressure gradient between two 1.2 mm diameter inlets and a 4 mm diameter reservoir when 0.5μ L fluid droplets were applied to the inlets using a micropipette. Polystyrene particles (10μm diameter) were enumerated using the resistive pulse technique. Particle counts using passive pumping were within 13% of counts from a device using syringe pumping. All pumping methods produced particle counts that were within 16% of those obtained with a hemocytometer. The effect of intermediate wash steps on particle counts within the passive pumping device was determined. Zero, one, or two wash droplets were loaded after the first of two sample droplets. No statistical difference was detected in the mean particle counts among the loading patterns (p > 0.05). Hydrodynamic focusing using passive pumping was also demonstrated.}, number={4-5}, journal={MICROFLUIDICS AND NANOFLUIDICS}, author={McPherson, Amy L. and Walker, Glenn M.}, year={2010}, month={Oct}, pages={897–904} }
 @article{dong_walker_2010, title={Adjustable-Stiffness Films via Integrated Thermal Modulation}, volume={295}, ISSN={["1439-2054"]}, DOI={10.1002/mame.201000097}, abstractNote={Abstract}, number={8}, journal={MACROMOLECULAR MATERIALS AND ENGINEERING}, author={Dong, Hua and Walker, Glenn M.}, year={2010}, month={Aug}, pages={735–741} }
 @article{adrian t. o'neill_monteiro-riviere_walker_2009, title={Microfabricated curtains for controlled cell seeding in high throughput microfluidic systems}, volume={9}, ISSN={["1473-0189"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000266616000016&KeyUID=WOS:000266616000016}, DOI={10.1039/b819622b}, abstractNote={A microfabricated cell curtain is presented that facilitates cellular assays. The cell curtain is defined as a poly(dimethylsiloxane) (PDMS) wall that extends from the ceiling of a cell culture microchamber to within microns of the chamber floor. Curtain use is demonstrated by observing monolayer human epidermal keratinocyte (HEK) colonies for 48 h longer than possible with non-curtained microfluidic chambers. The curtains were further characterized by integrating them into a 96 chamber high throughput microfluidic cell culture device. As proof of concept, this device was used to assay a range of ethanol dilutions spanning 0-22% in cell culture medium. Cells exposed to 12% ethanol or less for 30 min would recover to 85% viability at 24 h, while cells exposed to higher concentrations had viabilities below 10%. The data also showed that cells exposed to 6% ethanol or less grew in population size, 8% ethanol exposure stunted growth, and higher concentrations led to population loss. Curtain use permitted high initial cell seeding densities and increased the amount of time cells can be cultured compared to multi-well plates.}, number={12}, journal={LAB ON A CHIP}, author={Adrian T. O'Neill and Monteiro-Riviere, Nancy A. and Walker, Glenn M.}, year={2009}, pages={1756–1762} }
 @article{soohoo_walker_2009, title={Microfluidic aqueous two phase system for leukocyte concentration from whole blood}, volume={11}, ISSN={["1387-2176"]}, DOI={10.1007/s10544-008-9238-8}, abstractNote={Leukocytes from a whole blood sample were concentrated using a microfluidic aqueous two phase system (μATPS). Whole blood was simultaneously exposed to polyethylene glycol (PEG) and dextran (Dex) phase streams and cells were partitioned based on their differential affinity for the streams. The laminar flow characteristic of microfluidic devices was used to create zero, one, and two stable interfaces between the polymer streams. Three different patterns of three polymer streams each were evaluated for their effectiveness in concentrating leukocytes: immiscible PEG-PEG-Dex, immiscible Dex-PEG-Dex, and miscible PEG-PBS-Dex. The most effective configuration was the Dex-PEG-Dex stream pattern which on average increased the ratio of leukocytes to erythrocytes by a factor of 9.13 over unconcentrated blood.}, number={2}, journal={BIOMEDICAL MICRODEVICES}, author={SooHoo, Jeffrey R. and Walker, Glenn M.}, year={2009}, month={Apr}, pages={323–329} }
 @article{adrian t. o'neill_monteiro-riviere_walker_2008, title={Characterization of microfluidic human epidermal keratinocyte culture}, volume={56}, ISSN={["0920-9069"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000257370900007&KeyUID=WOS:000257370900007}, DOI={10.1007/s10616-008-9149-9}, abstractNote={Human epidermal keratinocytes (HEK) are skin cells of primary importance in maintaining the body's defensive barrier and are used in vitro to assess the irritation potential and toxicity of chemical compounds. Microfluidic systems hold promise for high throughput irritant and toxicity assays, but HEK growth kinetics have yet to be characterized within microscale culture chambers. This research demonstrates HEK patterning on microscale patches of Type I collagen within microfluidic channels and maintenance of these cells under constant medium perfusion for 72 h. HEK were shown to maintain 93.0%-99.6% viability at 72 h under medium perfusion ranging from 0.025-0.4 mul min(-1). HEK maintained this viability while approximately 100% confluent-a level not possible in 96 well plates. Microscale HEK cultures offer the ability to precisely examine the morphology, behavior and viability of individual cells which may open the door to new discoveries in toxicological screening methods and wound healing techniques.}, number={3}, journal={CYTOTECHNOLOGY}, author={Adrian T. O'Neill and Monteiro-Riviere, Nancy A. and Walker, Glenn M.}, year={2008}, month={Mar}, pages={197–207} }
 @article{walker_monteiro-riviere_rouse_adrian t. o'neill_2007, title={A linear dilution microfluidic device for cytotoxicity assays}, volume={7}, ISSN={["1473-0189"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000244616300011&KeyUID=WOS:000244616300011}, DOI={10.1039/b608990a}, abstractNote={A two-layer polymer microfluidic device is presented which creates nine linear dilutions from two input fluid streams mixed in varying volumetric proportions. The linearity of the nine dilutions is conserved when the flow rate is held constant at 1.0 microl min(-1) (R(2) = 0.9995) and when it is varied from 0.5-16 microl min(-1) (R(2) = 0.9998). An analytical expression is presented for designing microfluidic devices with arbitrary numbers of linear dilutions. To demonstrate the efficacy of this device, primary human epidermal keratinocytes (HEK) were stained with nine dilutions of calcein, resulting in a linear spread of fluorescent intensities (R(2) = 0.94). The operating principles of the device can be scaled up to incorporate any number of linear dilutions. This scalability, coupled with an intrinsic ability to create linear dilutions under a variety of operating conditions, makes the device applicable to high throughput screening applications such as combinatorial chemistry or cytotoxicity assays.}, number={2}, journal={LAB ON A CHIP}, author={Walker, Glenn M. and Monteiro-Riviere, Nancy and Rouse, Jillian and Adrian T. O'Neill}, year={2007}, pages={226–232} }
 @inbook{o'neill_monteiro-riviere_walker_2006, title={A serial dilution microfluidic device for cytotoxicity assays}, booktitle={28th annual International Conference of the IEEE Engineering in Medicine and Biology Society}, publisher={Piscataway, NJ: IEEE}, author={O'Neill, A. T. and Monteiro-Riviere, N. A. and Walker, G. M.}, year={2006}, pages={2836–2839} }
 @article{sai_walker_wikswo_richmond_2006, title={The IL Sequence in the LLKIL Motif in CXCR2 Is Required for Full Ligand-induced Activation of Erk, Akt, and Chemotaxis in HL60 Cells}, volume={281}, ISSN={0021-9258 1083-351X}, url={http://dx.doi.org/10.1074/jbc.M605883200}, DOI={10.1074/jbc.M605883200}, abstractNote={The chemotaxis of differentiated HL60 cells stably expressing CXCR2 was examined in a microfluidic gradient device where the steepness of the CXCL8 chemokine gradient was varied from 2 pg/ml/μm (0-1 ng/ml over a width of 500 μm) to 50 pg/ml/μm (0-25 ng/ml over 500 μm). The differentiated HL60 cells stably expressing CXCR2 exhibited little chemotaxis in response to a 0-1 ng/ml gradient, but displayed an increasing chemotactic response as the gradient steepness increased from 0 to 5, 0 to 10, and 0 to 25 ng/ml, demonstrating that steepness of gradient is a major determinant of the relative ability of cells to persistently migrate up a chemotactic gradient. When HL60 cells expressed CXCR2 mutated in the C terminus LLKIL motif (IL to AA), ligand-induced internalization of receptors was reduced 50%, whereas cell migration along the gradient of CXCL8 was completely lost. Although both mutant and wild-type receptors could mediate Akt and Erk activation in response to CXCL8, the level of activation of these two kinases was much lower in the cell line expressing the mutant receptors. These data imply that the IL amino acid residues in the LLKIL motif are very important for activation of the signal transduction cascade, which is necessary for cells to sense the chemokine gradient and respond with chemotaxis. Moreover, because mutation of the IL residues in the LLKIL motif resulted in only 50% reduction in receptor internalization, and a 50% reduction in Akt and Erk phosphorylation, but a complete loss of chemotactic response, the data imply that IL amino acid residues in the LLKIL motif are key either for amplification or oscillation of crucial signaling events or for establishment of a threshold for signals required for chemotaxis.}, number={47}, journal={Journal of Biological Chemistry}, publisher={American Society for Biochemistry & Molecular Biology (ASBMB)}, author={Sai, Jiqing and Walker, Glenn and Wikswo, John and Richmond, Ann}, year={2006}, month={Sep}, pages={35931–35941} }
 @article{walker_sai_richmond_stremler_chung_wikswo_2005, title={Effects of flow and diffusion on chemotaxis studies in a microfabricated gradient generator}, volume={5}, ISSN={1473-0197 1473-0189}, url={http://dx.doi.org/10.1039/b417245k}, DOI={10.1039/b417245k}, abstractNote={An understanding of chemotaxis at the level of cell-molecule interactions is important because of its relevance in cancer, immunology, and microbiology, just to name a few. This study quantifies the effects of flow on cell migration during chemotaxis in a microfluidic device. The chemotaxis gradient within the device was modeled and compared to experimental results. Chemotaxis experiments were performed using the chemokine CXCL8 under different flow rates with human HL60 promyelocytic leukemia cells expressing a transfected CXCR2 chemokine receptor. Cell trajectories were separated into x and y axis components. When the microchannel flow rates were increased, cell trajectories along the x axis were found to be significantly affected (p < 0.05). Total migration distances were not affected. These results should be considered when using similar microfluidic devices for chemotaxis studies so that flow bias can be minimized. It may be possible to use this effect to estimate the total tractile force exerted by a cell during chemotaxis, which would be particularly valuable for cells whose tractile forces are below the level of detection with standard techniques of traction-force microscopy.}, number={6}, journal={Lab on a Chip}, publisher={Royal Society of Chemistry (RSC)}, author={Walker, Glenn M. and Sai, Jiqing and Richmond, Ann and Stremler, Mark and Chung, Chang Y. and Wikswo, John P.}, year={2005}, pages={611} }
 @article{walker_ozers_beebe_2004, title={Cell infection within a microfluidic device using virus gradients}, volume={98}, ISSN={0925-4005}, url={http://dx.doi.org/10.1016/j.snb.2003.10.014}, DOI={10.1016/j.snb.2003.10.014}, abstractNote={A microfluidic device has been developed which allows cells to be infected at many different concentrations of virus within a microscale environment. Diffusion and laminar flow were used to create a concentration gradient of virus particles over cells attached to the bottom of a microchannel. The expression of green fluorescent protein (GFP) was monitored optically in situ over several days. In characterizing non-gradient infection at the microscale, average multiplicity of infections (MOIs) ranging from 8 to 25 were used and reduced GFP expression levels, compared to accepted macroscale values, were observed. Cells infected with virus gradients also displayed reduced expression levels but their location within the microchannel followed the same trend as the virus gradient.}, number={2-3}, journal={Sensors and Actuators B: Chemical}, publisher={Elsevier BV}, author={Walker, G and Ozers, M.S. and Beebe, D.J.}, year={2004}, month={Mar}, pages={347–355} }
 @article{walker_zeringue_beebe_2004, title={Microenvironment design considerations for cellular scale studies}, volume={4}, ISSN={1473-0197 1473-0189}, url={http://dx.doi.org/10.1039/b311214d}, DOI={10.1039/b311214d}, abstractNote={In vivo cellular microenvironments are not well-mimicked in present in vitro cell culture systems. Microtechnology, and microfluidics in particular, provides the tools to create in vivo-like cellular microenvironments in vitro. Features of in vitro cellular microenvironments are discussed and compared to macroscale cell culture environments; the concept of an effective culture volume (ECV) is introduced to facilitate the comparison. Current research using microtechnology to investigate in vitro cellular microenvironments is presented and areas where more research is needed in characterizing the in vitro microenvironment are outlined.}, number={2}, journal={Lab on a Chip}, publisher={Royal Society of Chemistry (RSC)}, author={Walker, Glenn M. and Zeringue, Henry C. and Beebe, David J.}, year={2004}, pages={91} }
 @article{rocheleau_walker_head_mcguinness_piston_2004, title={Microfluidic glucose stimulation reveals limited coordination of intracellular Ca2+ activity oscillations in pancreatic islets}, volume={101}, ISSN={0027-8424 1091-6490}, url={http://dx.doi.org/10.1073/pnas.0405149101}, DOI={10.1073/pnas.0405149101}, abstractNote={
            The pancreatic islet is a functional microorgan involved in maintaining normoglycemia through regulated secretion of insulin and other hormones. Extracellular glucose stimulates insulin secretion from islet β cells through an increase in redox state, which can be measured by NAD(P)H autofluorescence. Glucose concentrations over ≈7 mM generate synchronous oscillations in β cell intracellular Ca
            2+
            concentration ([Ca
            2+
            ]
            i
            ), which lead to pulsatile insulin secretion. Prevailing models assume that the pancreatic islet acts as a functional syncytium, and the whole islet [Ca
            2+
            ]
            i
            response has been modeled in terms of islet bursting and pacemaker models. To test these models, we developed a microfluidic device capable of partially stimulating an islet, while allowing observation of the NAD(P)H and [Ca
            2+
            ]
            i
            responses. We show that β cell [Ca
            2+
            ]
            i
            oscillations occur only within regions stimulated with more than ≈6.6 mM glucose. Furthermore, we show that tolbutamide, an antagonist of the ATP-sensitive K
            +
            channel, allows these oscillations to travel farther into the nonstimulated regions of the islet. Our approach shows that the extent of Ca
            2+
            propagation across the islet depends on a delicate interaction between the degree of coupling and the extent of ATP-sensitive K
            +
            -channel activation and illustrates an experimental paradigm that will have utility for many other biological systems.
          }, number={35}, journal={Proceedings of the National Academy of Sciences}, publisher={Proceedings of the National Academy of Sciences}, author={Rocheleau, J. V. and Walker, G. M. and Head, W. S. and McGuinness, O. P. and Piston, D. W.}, year={2004}, month={Aug}, pages={12899–12903} }