Interactions between multiple cell types in parallel microfluidic channels: monitoring platelet adhesion to an endothelium in the presence of an anti-adhesion drug

Recent advances in microfluidic technology of arterial thrombosis investigations

Microfluidic technology has emerged as a powerful tool in studying arterial thrombosis, allowing researchers to construct artificial blood vessels and replicate the hemodynamics of blood flow. This technology has led to significant advancements in understanding thrombosis and platelet adhesion and aggregation. Microfluidic models have various types and functions, and by studying the fabrication methods and working principles of microfluidic chips, applicable methods can be selected according to specific needs. The rapid development of microfluidic integrated system and modular microfluidic system makes arterial thrombosis research more diversified and automated, but its standardization still needs to be solved urgently. One key advantage of microfluidic technology is the ability to precisely control fluid flow in microchannels and to analyze platelet behavior under different shear forces and flow rates. This allows researchers to study the physiological and pathological processes of blood flow, shedding light on the underlying mechanisms of arterial thrombosis. In conclusion, microfluidic technology has revolutionized the study of arterial thrombosis by enabling the construction of artificial blood vessels and accurately reproducing hemodynamics. In the future, microfluidics will place greater emphasis on versatility and automation, holding great promise for advancing antithrombotic therapeutic and prophylactic measures.

Microfluidic Chip with Integrated Electrophoretic Immunoassay for Investigating Cell-Cell Interactions

Microfluidics have been used to create "body-on-chip" systems to mimic in vivo cellular interactions with a high level of control. Most such systems rely on optical observation of cells as a readout. In this work we integrated a cell-cell interaction chip with online microchip electrophoresis immunoassay to monitor the effects of the interaction on protein secretion dynamics. The system was used to investigate the effects of adipocytes on insulin secretion. Chips were loaded with 190 000 3T3-L1 adipocytes and a single islet of Langerhans in separate chambers. The chambers were perfused at 300-600 nL/min so that adipocyte secretions flowed over the islets for 3 h. Adipocytes produced 80 μM of nonesterified fatty acids (NEFAs), a factor known to impact insulin secretion, at the islets. After perfusion, islets were challenged with a step change in glucose from 3 to 11 mM while monitoring insulin secretion at 8 s intervals by online immunoassay. Adipocyte treatment augmented insulin secretion by 6-fold compared to controls. The effect was far greater than comparable concentrations of NEFA applied to the islets demonstrating that adipocytes release multiple factors that can strongly potentiate insulin secretion. The experiments reveal that integration of chemical analysis with cell-cell interaction can provide valuable insights into cellular functions.

Combinatorial in Vitro and in Silico Approach To Describe Shear-Force Dependent Uptake of Nanoparticles in Microfluidic Vascular Models

In the present work, we combine experimental and computational methods to define the critical shear stress as an alternative parameter for nanotoxicological and nanomedical evaluations using an in vitro microfluidic vascular model. We demonstrate that our complementary in vitro and in silico approach is well suited to assess the fluid flow velocity above which clathrin-mediated (active) nanoparticle uptake per cell decreases drastically although higher numbers of nanoparticles per cell are introduced. Results of our study revealed a critical shear stress of 1.8 dyn/cm², where maximum active cellular nanoparticle uptake took place, followed by a 70% decrease in uptake of 249 nm nanoparticles at 10 dyn/cm², respectively. The observed nonlinear relationship between flow velocity and nanoparticle uptake strongly suggests that fluid mechanical forces also need to be considered in order to predict potential in vivo distribution, bioaccumulation, and clearance of nanomaterials and novel nanodrugs.

Nanodiagnostics, nanopharmacology and nanotoxicology of platelet–vessel wall interactions

In physiological conditions, the interactions between blood platelets and endothelial cells play a major role in vascular reactivity and hemostasis. By contrast, increased platelet activation contributes to the pathogenesis of vascular pathology such as atherosclerosis, thrombosis, diabetes mellitus, hypertension and carcinogenesis. Nanomedicine, including nanodiagnostics and nanotherapeutics is poised to be used in the management of vascular diseases. However, the inherent risk and potential toxicity resultant from the use of nanosized (<100 nm) materials need to be carefully considered. This review, basing on a systematic search of literature provides state-of-the-art and focuses on new discoveries, as well as the potential benefits and threats in the field of nanodiagnostics, nanopharmacology and nanotoxicology of platelet–vessel wall interactions.

C-peptide and zinc delivery to erythrocytes requires the presence of albumin: implications in diabetes explored with a 3D-printed fluidic device

People with type 1 diabetes (T1D) must administer insulin exogenously due to the destruction of their pancreatic β-cells. Endogenous insulin is stored in β-cell granules along with C-peptide, a 31 amino acid peptide that is secreted from these granules in amounts equal to insulin. Exogenous co-administration of C-peptide with insulin has proven to reduce diabetes-associated complications in animals and humans. The exact mechanism of C-peptide's beneficial effects after secretion from the β-cell granules is not completely understood, thus hindering its development as an exogenously administered hormone. Monitoring tissue-to-tissue communication using a 3D-printed microfluidic device revealed that zinc and C-peptide are being delivered to erythrocytes by albumin. Upon delivery, erythrocyte-derived ATP increased by >50%, as did endothelium-derived NO, which was measured downstream in the 3D-printed device. Our results suggest that hormone replacement therapy in diabetes may be improved by exogenous administration of a C-peptide ensemble that includes zinc and albumin.

Adhesion patterns in the microvasculature are dependent on bifurcation angle

Particle adhesion in vivo is highly dependent on the microvascular environment comprising of unique anatomical, geometrical, physiological fluid flow conditions and cell-particle and cell-cell interactions. Hence, proper design of vascular-targeted drug carriers that efficiently deliver therapeutics to the targeted cells or tissue at effective concentrations must account for these complex conditions observed in vivo. In this study, we build upon our previous results with the goal of characterizing the effects of bifurcations and their corresponding angle on adhesion of functionalized particles and neutrophils to activated endothelium. Our hypothesis is that adhesion is significantly affected by the type of biochemical interactions between particles and vessel wall as well as the presence of bifurcations and their corresponding angle. Here, we investigate adhesion of functionalized particles (2 μm and 7 μm microparticles) to protein coated channels as well as adhesion of human neutrophils to human endothelial cells under various physiological flow conditions in microfluidic bifurcating channels comprising of different contained angles (30°, 60°, 90°, or 120°). Our findings indicate that both functionalized particle and neutrophil adhesion propensity increase with a larger bifurcation angle. Moreover, the difference in the adhesion patterns of neutrophils and rigid, similar sized (7 μm) particles is more apparent in the junction regions with a larger contained angle. By selecting the right particle size range, enhanced targeted binding of vascular drug carriers can be achieved along with a higher efficacy at optimal drug dosage. Hence, vascular drug particle design needs to be tailored to account for higher binding propensity at larger bifurcation angles.

Successes and future outlook for microfluidics-based cardiovascular drug discovery

INTRODUCTION

The greatest advantage of using microfluidics as a platform for the assessment of cardiovascular drug action is its ability to finely regulate fluid flow conditions, including flow rate, shear stress and pulsatile flow. At the same time, microfluidics provide means for modifying the vessel geometry (bifurcations, stenoses, complex networks), the type of surface of the vessel walls, and for patterning cells in 3D tissue-like architecture, including generation of lumen walls lined with cells and heart-on-a-chip structures for mimicking ventricular cardiomyocyte physiology. In addition, owing to the small volume of required specimens, microfluidics is ideally suited to clinical situations whereby monitoring of drug dosing or efficacy needs to be coupled with minimal phlebotomy-related drug loss.

AREAS COVERED

In this review, the authors highlight potential applications for the currently existing and emerging technologies and offer several suggestions on how to close the development cycle of microfluidic devices for cardiovascular drug discovery.

EXPERT OPINION

The ultimate goal in microfluidics research for drug discovery is to develop 'human-on-a-chip' systems, whereby several organ cultures, including the vasculature and the heart, can mimic complex interactions between the organs and body systems. This would provide in vivo-like pharmacokinetics and pharmacodynamics for drug ADMET assessment. At present, however, the great variety of available designs does not go hand in hand with their use by the pharmaceutical community.

Integrated hybrid polystyrene-polydimethylsiloxane device for monitoring cellular release with microchip electrophoresis and electrochemical detection

In this work, a polystyrene (PS)-polydimethylsiloxane (PDMS) hybrid device was developed to enable the integration of cell culture with analysis by microchip electrophoresis and electrochemical detection. It is shown that this approach combines the fundamental advantages of PDMS devices (the ability to integrate pumps and valves) and PS devices (the ability to permanently embed fluidic tubing and electrodes). The embedded fused-silica capillary enables high temporal resolution measurements from off-chip cell culture dishes and the embedded electrodes provide close to real-time analysis of small molecule neurotransmitters. A novel surface treatment for improved (reversible) adhesion between PS and PDMS is described using a chlorotrimethylsilane stamping method. It is demonstrated that a Pd decoupler is efficient at handling the high current (and cathodic hydrogen production) resulting from use of high ionic strength buffers needed for cellular analysis; thus allowing an electrophoretic separation and in-channel detection. The separation of norepinephrine (NE) and dopamine (DA) in highly conductive biological buffers was optimized using a mixed surfactant system. This PS-PDMS hybrid device integrates multiple processes including continuous sampling from a cell culture dish, on-chip pump and valving technologies, microchip electrophoresis, and electrochemical detection to monitor neurotransmitter release from PC 12 cells.

Endothelium-derived nitric oxide production is increased by ATP released from red blood cells incubated with hydroxyurea

Red blood cells (RBCs) release adenosine triphosphate (ATP) in response to a variety of stimuli, including flow-induced deformation. Hydroxyurea (HU), a proven therapy for individuals with sickle cell disease (SCD), is known to improve blood flow. However, the exact mechanism leading to the improved blood flow is incomplete. Here, we report that the incubation of human RBCs with HU enhances ATP release from these cells and that this ATP is capable of stimulating nitric oxide (NO) production in an endothelium. RBCs incubated with HU were pumped through micron-size flow channels in a microfluidic device. The release of ATP from the RBCs was measured using the luciferin-luciferase assay in detection wells on the device that were separated from the flow channels by a porous polycarbonate membrane. NO released from a layer of bovine artery endothelial cells (bPAECs) cultured on the polycarbonate membrane was also measured using the extracellular NO probe DAF-FM. ATP release from human RBCs incubated with 100 μM HU was observed to be 2.06±0.37-fold larger than control samples without HU (p<0.05, N ≥ 3). When HU-incubated RBCs were flowed under a layer of bPAECs, NO released from the bPAEC layer was measured to be 1.34±0.10-fold higher than controls. An antagonist of the P2Y receptor established that this extra 30% increase in NO release is ATP mediated. Furthermore, when RBCs were incubated with L-NAME, a significant decrease in endothelium-derived NO production was observed. Control experiments suggest that RBC-generated NO indirectly affects endothelial NO production via its effects on RBC-derived ATP release.

Microvascular platforms for the study of platelet-vessel wall interactions

Platelets interact with the endothelium to regulate vascular integrity and barrier function, mediate inflammation and immune response, and prevent and arrest hemorrhage. In this review, we describe existing tools to study the flow-dependent interactions of platelets with the vessel wall. We also discuss our work on building engineered microvessels to study the roles of platelets on endothelial barrier function, endothelial sprouting, and thrombus formation on both quiescent and stimulated endothelium. In particular, we will show the advantage of using a cell-remodelable system in the studies of platelet-vessel wall interactions.

Effects of microfluidic channel geometry on leukocyte rolling assays

Microfluidic cell adhesion assays have emerged as a means to increase throughput as well as reduce the amount of costly reagents. However as dimensions of the flow chamber are reduced and approach the diameter of a cell (D(c)), theoretical models have predicted that mechanical stress, force, and torque on a cell will be amplified. We fabricated a series of microfluidic devices that have a constant width:height ratio (10:1) but with varying heights. The smallest microfluidic device (200 μm ×20 μm) requires perfusion rates as low as 40 nL/min to generate wall shear stresses of 0.5 dynes/cm(2). When neutrophils were perfused through P-selectin coated chambers at equivalent wall shear stress, rolling velocities decreased by approximately 70 % as the ratio of cell diameter to chamber height (D(c)/H) increased from 0.08 (H = 100 μm) to 0.40 (H = 20 μm). Three-dimensional numerical simulations of neutrophil rolling in channels of different heights showed a similar trend. Complementary studies with PSGL-1 coated microspheres and paraformaldehyde-fixed neutrophils suggested that changes in rolling velocity were related to cell deformability. Using interference reflection microscopy, we observed increases in neutrophil contact area with increasing chamber height (9-33 %) and increasing wall shear stress (28-56 %). Our results suggest that rolling velocity is dependent not only on wall shear stress but also on the shear stress gradient experienced by the rolling cell. These results point to the D(c)/H ratio as an important design parameter of leukocyte microfluidic assays, and should be applicable to rolling assays that involve other cell types such as platelets or cancer cells.

Microchip-based electrochemical detection for monitoring cellular systems

The use of microchip devices to study cellular systems is a rapidly growing research area. There are numerous advantages of using on-chip integrated electrodes to monitor various cellular processes. The purpose of this review is to give examples of advancements in microchip-based cellular analysis, specifically where electrochemistry is used for the detection scheme. These examples include on-chip detection of single-cell quantal exocytosis, electrochemical analysis of intracellular contents, the ability to integrate cell culture/immobilization with electrochemistry, and the use of integrated electrodes to ensure cell confluency in longer-term cell culture experiments. A perspective on future trends in this area is also given.

C-peptide-stimulated nitric oxide production in a cultured pulmonary artery endothelium is erythrocyte mediated and requires Zn(2+)

BACKGROUND

C-peptide has been shown to stimulate the production of nitric oxide (NO) in aortic endothelial cells via activation of endothelial nitric oxide synthase (eNOS) through an increased calcium influx. Here, results obtained using cultured bovine pulmonary artery endothelial cells (bPAECs) suggest that C-peptide does not induce eNOS activation directly in cultured pulmonary artery endothelium. However, C-peptide has been shown to stimulate the release of ATP from erythrocytes, a well-documented stimulus of eNOS activity in the pulmonary endothelium. Therefore, studies were performed to examine if C-peptide can indirectly stimulate NO production in a cultured pulmonary endothelium that is erythrocyte mediated.

METHODS

NO production and free intracellular calcium changes were monitored in immobilized bPAECs using specific intracellular fluorescent probes after stimulation with adenosine triphosphate (ATP), calcium ionophore A23187, or C-peptide. A microfluidic device enabled immobilized bPAECs to interact with flowing erythrocytes in the presence and absence of C-peptide to determine the role of the erythrocyte in C-peptide-stimulated NO production in cultured bPAECs.

RESULTS

ATP and the calcium ionophore stimulate significant increases in both intracellular NO production and influx of free calcium in cultured bPAECs. In contrast, C-peptide, ranging from physiological to above physiological concentrations, was unable to stimulate NO production or calcium influx in the bPAECs. However, when erythrocytes were pre-incubated with a mixture containing physiological concentrations of C-peptide with Zn(2+) and haemodynamically pumped beneath bPAECs cultured on a microfluidic device, an 88.6 ± 7.5% increase in endothelial NO production was observed.

CONCLUSIONS

C-peptide does not affect NO production in bPAECs directly but can impact NO production through an erythrocyte-mediated mechanism. Furthermore, in the absence of Zn(2+), C-peptide does not stimulate this NO production directly or indirectly. These results suggest that C-peptide, in the presence of Zn(2+), may be a determinant in purinergic receptor signalling via its ability to stimulate the release of ATP from erythrocytes.

Integration of multiple components in polystyrene-based microfluidic devices part I: fabrication and characterization

In Part I of a two-part series, we describe a simple and inexpensive approach to fabricate polystyrene devices that is based upon melting polystyrene (from either a Petri dish or powder form) against PDMS molds or around electrode materials. The ability to incorporate microchannels in polystyrene and integrate the resulting device with standard laboratory equipment such as an optical plate reader for analyte readout and pipets for fluid propulsion is first described. A simple approach for sample and reagent delivery to the device channels using a standard, multi-channel micropipette and a PDMS-based injection block is detailed. Integration of the microfluidic device with these off-chip functions (sample delivery and readout) enables high-throughput screens and analyses. An approach to fabricate polystyrene-based devices with embedded electrodes is also demonstrated, thereby enabling the integration of microchip electrophoresis with electrochemical detection through the use of a palladium electrode (for a decoupler) and carbon-fiber bundle (for detection). The device was sealed against a PDMS-based microchannel and used for the electrophoretic separation and amperometric detection of dopamine, epinephrine, catechol, and 3,4-dihydroxyphenylacetic acid. Finally, these devices were compared against PDMS-based microchips in terms of their optical transparency and absorption of an anti-platelet drug, clopidogrel. Part I of this series lays the foundation for Part II, where these devices were utilized for various on-chip cellular analysis.

Monitoring in vitro thrombus formation with novel microfluidic devices

Cardiovascular disease is a major cause of mortality globally and is subject to ongoing research to improve clinical treatment. It is established that activation of platelets and coagulation are central to thrombosis, yet at different extents in the arterial and venous system. In vitro perfusion chamber technology has contributed significant knowledge on the function of platelets in the thrombotic process under shear conditions. Recent efforts to downscale this technique with a variety of microfluidic devices has opened new possibilities to study this process under precisely controlled flow conditions. Such microfluidic devices possess the capability to execute platelet function tests more quickly than current assays, while using small blood samples. Gradually becoming available to the clinic now, they may provide a new means to manage the treatment of cardiovascular diseases, although accurate validation studies still are missing. This review highlights the progress that has been made in monitoring aspects of thrombus formation using microfluidic devices.

Elastomeric microposts integrated into microfluidics for flow-mediated endothelial mechanotransduction analysis

Mechanotransduction is known as the cellular mechanism converting insoluble biophysical signals in the local cellular microenvironment (e.g. matrix rigidity, external mechanical forces, and fluid shear) into intracellular signalling to regulate cellular behaviours. While microfluidic technologies support a precise and independent control of soluble factors in the cellular microenvironment (e.g. growth factors, nutrients, and dissolved gases), the regulation of insoluble biophysical signals in microfluidics, especially matrix rigidity and adhesive pattern, has not yet been achieved. Here we reported an integrated soft lithography-compatible microfluidic methodology that could enable independent controls and modulations of fluid shear, substrate rigidity, and adhesive pattern in a microfluidic environment, by integrating micromolded elastomeric micropost arrays and microcontact printing with microfluidics. The geometry of the elastomeric micropost array could be regulated to mediate substrate rigidity and adhesive pattern, and further the elastomeric microposts could be utilized as force sensors to map live-cell subcellular contractile forces. To illustrate the general application of our methodology, we investigated the flow-mediated endothelial mechanotransduction process and examined specifically the involvement of subcellular contractile forces in the morphological realignment process of endothelial cells under a sustained directional fluid shear. Our results showed that the cytoskeletal contractile forces of endothelial cells were spatiotemporally regulated and coordinated to facilitate their morphology elongation process along the direction of flow. Together, our study provided an integrated microfluidic strategy to modulate the in vitro cellular microenvironment with both defined soluble and insoluble signals, and we demonstrated its application to investigate quantitatively the involvement of cytoskeletal contractile forces in the flow-mediated mechanotransduction process of endothelial cells.

Engineers are from PDMS-land, Biologists are from Polystyrenia

As the integration of microfluidics into cell biology research proceeds at an ever-increasing pace, a critical question for those working at the interface of both disciplines is which device material to use for a given application. While PDMS and soft lithography methods offer the engineer rapid prototyping capabilities, PDMS as a material has characteristics that have known adverse effects on cell-based experiments. In contrast, while polystyrene (PS), the most commonly used thermoplastic for laboratory cultureware, has provided decades of grounded and validated research conclusions in cell behavior and function, PS as a material has posed significant challenges in microfabrication. These competing issues have forced microfluidics engineers and biologists to make compromises in how they approach specific research questions, and furthermore, have attenuated the impact of microfluidics on biological research. In this review, we provide a comparison of the attributes of PDMS and PS, and discuss reasons for their popularity in their respective fields. We provide a critical evaluation of the strengths and limitations of PDMS and PS in relation to the advancement and future impact on microfluidic cell-based studies and applications. We believe that engineers have a responsibility to overcome any challenges associated with microfabrication, whether with PS or other materials, and that engineers should provide options and solutions that assist biologists in their experimental design. Our goal is not to advocate for any specific material, but provide guidelines for researchers who desire to choose the most suitable material for their application, and suggest important research directions for engineers working at the interface between microfabrication technology and biological application.

A digital microfluidic platform for primary cell culture and analysis

Digital microfluidics (DMF) is a technology that facilitates electrostatic manipulation of discrete nano- and micro-litre droplets across an array of electrodes, which provides the advantages of single sample addressability, automation, and parallelization. There has been considerable interest in recent years in using DMF for cell culture and analysis, but previous studies have used immortalized cell lines. We report here the first digital microfluidic method for primary cell culture and analysis. A new mode of "upside-down" cell culture was implemented by patterning the top plate of a device using a fluorocarbon liftoff technique. This method was useful for culturing three different primary cell types for up to one week, as well as implementing a fixation, permeabilization, and staining procedure for F-actin and nuclei. A multistep assay for monocyte adhesion to endothelial cells (ECs) was performed to evaluate functionality in DMF-cultured primary cells and to demonstrate co-culture using a DMF platform. Monocytes were observed to adhere in significantly greater numbers to ECs exposed to tumor necrosis factor (TNF)-α than those that were not, confirming that ECs cultured in this format maintain in vivo-like properties. The ability to manipulate, maintain, and assay primary cells demonstrates a useful application for DMF in studies involving precious samples of cells from small animals or human patients.

On-chip evaluation of shear stress effect on cytotoxicity of mesoporous silica nanoparticles

In this work, nanotoxicity in the bloodstream was modeled, and the cytotoxicity of sub-50 nm mesoporous silica nanoparticles to human endothelial cells was investigated under microfluidic flow conditions. Compared to traditional in vitro cytotoxicity assays performed under static conditions, unmodified mesoporous silica nanoparticles show higher and shear stress-dependent toxicity to endothelial cells under flow conditions. Interestingly, even under flow conditions, highly organo-modified mesoporous silica nanoparticles show no significant toxicity to endothelial cells. This paper clearly demonstrates that shear stress is an important factor to be considered in in vitro nanotoxicology assessments and provides a simple device for pursuing this consideration.

Effective hydrodynamic shaping of sample streams in a microfluidic parallel-plate flow-assay device: matching whole blood dynamic viscosity

We report the development of an aqueous buffer system tailored to the fluidic and hemodynamic requirements of our recently reported microfluidic platelet dynamic assay device, which uses hydrodynamic focusing to "shape" a blood sample into a thin flowing layer adjacent to its protein-functionalized surface. By matching the dynamic viscosity of whole blood (3.13 ± 0.08 mPa·s, from healthy donors), the selected buffer minimizes interfacial fluid mixing and better controls shear rate within the device, permitting platelet/protein-surface interaction assays with as little as 50 μL of whole blood. Buffers containing the viscosity-enhancing components bovine serum albumin (BSA), gelofusine/glycine, or histopaque (Ficoll gradient solution) were found not to activate platelets when incubated with blood at concentrations up to 50%, as assessed by flow cytometry quantitation of P-selectin expression and αIIbβ (3) activation. In contrast, glycerol-based buffer activated platelets (two-fold increase in P-selectin levels) at concentrations as low as 10% by volume. BSA- and gelofusine/glycine-based buffers were problematic in preparation and use, and therefore, were not used beyond initial characterization. The histopaque solution selected as the best choice for flow studies stabilizes sample contact with the device's thrombogenic surface, does not activate platelets, and does not interfere with the action of agonists added to deliberately activate platelets.

Well plate microfluidic system for investigation of dynamic platelet behavior under variable shear loads

The study of platelet behavior in real-time under controlled shear stress offers insights into the underlying mechanisms of many vascular diseases and enables evaluation of platelet-focused therapeutics. The two most common methods used to study platelet behavior at the vessel wall under uniform shear flow are parallel plate flow chambers and cone-plate viscometers. Typically, these methods are difficult to use, lack experimental flexibility, provide low data content, are low in throughput, and require large reagent volumes. Here, we report a well plate microfluidic (WPM)-based system that offers high throughput, low reagent consumption, and high experimental flexibility in an easy to use well plate format. The system consists of well plates with an integrated array of microfluidic channels, a pneumatic interface, an automated microscope, and software. This WPM system was used to investigate dynamic platelet behavior under shear stress. Multiple channel designs are presented and tested for shear loads with whole blood to determine their applicability to study thrombus formation. Normal physiological shear (0.1-20 dyn/cm(2) ) and pathological shear (20-200 dyn/cm(2) ) devices were used to study platelet behavior in vitro under various shear, matrix coating, and monolayer conditions. The high physiological relevance, low blood consumption, and increased throughput create a valuable technique available to vascular biology researchers. The approach also has extensibility to other research areas including inflammation, cancer biology, and developmental/stem cell research.

Microfluidic transendothelial electrical resistance measurement device that enables blood flow and postgrowth experiments

Transendothelial electronic resistance (TEER) measurements are performed across a cell layer immobilized on a microfluidic device that also enables the cell layer to interact with a flowing stream of red blood cells (RBCs). A bipolar pulsed square wave potential is applied across a monolayer of bovine pulmonary artery endothelial cells, and the resulting current response is measured and integrated. The overall impedance of the cell layer provides an indicator of cell layer integrity. After cell seeding on the device, a decrease in TEER signal from 22.3 ± 1.6 μC to 3.5 ± 0.4 μC (corresponding to a resistance of 40.9 ± 2.9 Ω·cm(2) to 259.1 ± 27.4 Ω·cm(2)) was observed after 8 h of cell growth. Intracellular nitric oxide (NO) production by the immobilized endothelial cells that had reached confluence was 34% higher than those cells that had not reached confluence, as indicated by the integrated TEER system. Importantly, this NO production by the confluent endothelium was stimulated by ATP released from RBCs flowing under the endothelial cells. In this construct, the described microfluidic device enables both a TEER-based evaluation of cell layer integrity and molecularly communicated interactions of these cells with a flowing stream of blood components.

Rapid prototyping of arrayed microfluidic systems in polystyrene for cell-based assays

Microfluidic cell-based systems have enabled the study of cellular phenomena with improved spatiotemporal control of the microenvironment and at increased throughput. While poly(dimethylsiloxane) (PDMS) has emerged as the most popular material in microfluidics research, it has specific limitations that prevent microfluidic platforms from achieving their full potential. We present here a complete process, ranging from mold design to embossing and bonding, that describes the fabrication of polystyrene (PS) microfluidic devices with similar cost and time expenditures as PDMS-based devices. Emphasis was placed on creating methods that can compete with PDMS fabrication methods in terms of robustness, complexity, and time requirements. To achieve this goal, several improvements were made to remove critical bottlenecks in existing PS embossing methods. First, traditional lithographic techniques were adapted to fabricate bulk epoxy molds capable of resisting high temperatures and pressures. Second, a method was developed to emboss through-holes in a PS layer, enabling creation of large arrays of independent microfluidic systems on a single device without need to manually create access ports. Third, thermal bonding of PS layers was optimized in order to achieve quality bonding over large arrays of microsystems. The choice of materials and methods was validated for biological function in two different cell-based applications to demonstrate the versatility of our streamlined fabrication process.

Evaluating the effects of estradiol on endothelial nitric oxide stimulated by erythrocyte-derived ATP using a microfluidic approach

Recently, estrogens have been reported to have protective effects against experimental autoimmune encephalomyelitis (EAE), a murine model of multiple sclerosis (MS). Although the molecular mechanism for such a protective effect is currently incomplete, we hypothesized that estradiol may reduce the release of ATP from erythrocytes (ERYs), thereby lowering the production of nitric oxide (NO) by endothelial cells. Here, we report on the use of a microfluidic device to investigate the direct effects of the estrogen estradiol on endothelial cell nitric oxide production. In addition, the incorporation of a thin polycarbonate membrane into the device enabled the passage of ERYs through the device to determine indirect effects of estradiol on NO production that may be meditated by ERYs.When these ERYs were incubated with increasing concentrations of estradiol, the NO production from the endothelial cells was attenuated to a value that was only 59 +/- 7% of ERYs in the absence of estradiol. This decrease in NO production coincides with reductions in ERY-derived ATP release in the presence of estradiol. Estradiol is typically reported to have NO-stimulating effects; however, such reports have employed in vitro experimental designs that include only a single cell type. To demonstrate the potential importance of this attenuation of ATP from ERYs, results from a small-scale study show that the ATP release obtained from healthy controls was 138 +/- 21 nM (n=18) while the release from the ERYs obtained from people with MS was 375 +/- 51 nM (n=11). The studies reported here involving multiple cells types (endothelial cells and ERYs) may lead to a reappraisal of the in vivo activities of estradiol.

Adhesion assays of endothelial cells on nanopatterned surfaces within a microfluidic channel

We present a simple analytical method to measure adhesion of human umbilical vein endothelial cells (HUVECs) and calf pulmonary artery endothelial cells (CPAEs) using nanopatterned, biodegradable poly(lactic-co-glycolic acid) (PLGA) surfaces for potential applications to artificial tissue-engineered blood vessel. Various nanostructured PLGA surfaces (350 nm wide ridges/350 nm grooves, 350 nm ridges/700 nm grooves, 350 nm ridges/1750 nm grooves, 700 nm ridges/350 nm grooves, 1050 nm ridges/350 nm grooves, 1750 nm ridges/350 nm grooves) and flat (unpatterned) surfaces were fabricated on the bottom of polydimethylsiloxane (PDMS) microfluidic channel of 2 mm width and 60 microm height by using thermal imprinting and irreversible channel bonding. To measure adhesion strength of HUVECs and CPAEs, the cells were exposed to a range of shear stress (12, 40, and 80 dyn/cm(2)) within the channels for 20 min after a preculture for 3 days and the remaining cells were counted under each condition. The highest adhesion strength was found on the surface of 700 nm wide ridges, 350 nm wide grooves for both cell types. The enhanced adhesion on nanopatterned surfaces can be attributed to two aspects: (i) contact guidance along the line direction and (ii) clustered focal adhesions. In particular, the contact guidance induced cell alignment along the line directions, which in turn lowers wall shear stress applied to the cell surface, as supported by a simple hydrodynamic model based on cell morphology.

Microfluidic cell culture systems for drug research

In pharmaceutical research, an adequate cell-based assay scheme to efficiently screen and to validate potential drug candidates in the initial stage of drug discovery is crucial. In order to better predict the clinical response to drug compounds, a cell culture model that is faithful to in vivo behavior is required. With the recent advances in microfluidic technology, the utilization of a microfluidic-based cell culture has several advantages, making it a promising alternative to the conventional cell culture methods. This review starts with a comprehensive discussion on the general process for drug discovery and development, the role of cell culture in drug research, and the characteristics of the cell culture formats commonly used in current microfluidic-based, cell-culture practices. Due to the significant differences in several physical phenomena between microscale and macroscale devices, microfluidic technology provides unique functionality, which is not previously possible by using traditional techniques. In a subsequent section, the niches for using microfluidic-based cell culture systems for drug research are discussed. Moreover, some critical issues such as cell immobilization, medium pumping or gradient generation in microfluidic-based, cell-culture systems are also reviewed. Finally, some practical applications of microfluidic-based, cell-culture systems in drug research particularly those pertaining to drug toxicity testing and those with a high-throughput capability are highlighted.

P2Y12 or P2Y1 inhibitors reduce platelet deposition in a microfluidic model of thrombosis while apyrase lacks efficacy under flow conditions

Determination of the patient-specific response to antiplatelet agents facilitates proper dosing for both acute and chronic prophylaxis. "Closed" systems (with or without flow) may fail to predict pharmacological potency in situations where platelets rapidly accumulate under flow conditions at a site of thrombosis ("Open" systems). Using an 8-channel microfluidic flow assay of human whole blood with corn trypsin inhibitor (+/- PPACK) perfused over focal zones of collagen, dose-response curves were measured for pharmacological agents at a wall shear rate of 210 s(-1). The P2Y(1) inhibitor MRS 2179 (IC(50) = 0.233 +/- 0.132 microM) and P2Y(12) inhibitor 2-MeSAMP (IC(50) = 2.558 +/- 0.799 microM) were potent blockers of secondary platelet accumulation under flow, while the P2X(1) inhibitor (NF 449) and apyrase failed to reduce platelet accumulation. MRS 2179 and 2-MeSAMP had undetectable effects on initial platelet adhesion to collagen. Numerical simulation of convective-diffusive transport and apyrase-mediated catalytic degradation of ADP indicated that ultra-high concentrations of apyrase ( approximately 2000 U mL(-1)) would be required to have the same effect under flow as much lower concentrations (1 U mL(-1)) currently used in closed systems (aggregometry or cone-and-plate viscometer). This is the first evaluation of IC(50) values for P2Y(12) and P2Y(1) antagonists under controlled flow conditions. Evaluation of antiplatelet agents in open flow systems demonstrates that inhibition of either ADP by apyrase or antagonism of P2X(1) signaling had no inhibitory effect on platelet accumulation. This technique provides a platform for rapidly investigating effects of antithrombotic therapies simultaneously in a model injury system.

A Molecular Level Understanding of Zinc Activation of C-peptide and its Effects on Cellular Communication in the Bloodstream

Inspired by previous reports, our group has recently demonstrated that C-peptide exerts beneficial effects upon interactions with red blood cells (RBCs). These effects can be measured in RBCs obtained from animal models of both type 1 diabetes and type 2 diabetes, though to different extents. To date, the key metrics that have been measured involving C-peptide and RBCs include an increase in glucose uptake by these cells and a subsequent increase in adenosine triphosphate (ATP) release. Importantly, to date, our group has only been able to elicit these beneficial effects when the C-peptide is prepared in the presence of Zn2+. The C-peptide-induced release of ATP is of interest when considering that ATP is a purinergic signaling molecule known to stimulate the production of nitric oxide (NO) in the endothelium and in platelets. This NO production has been shown to participate in smooth muscle relaxation and subsequent vessel dilation. Furthermore, NO is a well-established platelet inhibitor. The objective of this review is to provide information pertaining to C-peptide activity on RBCs. Special attention is paid to the necessity of Zn2+ activation, and the origin of that activation in vivo. Finally, a mechanism is proposed that explains how C-peptide is exerting its effects on other cells in the bloodstream, particularly on endothelial cells and platelets, via its ability to stimulate the release of ATP from RBCs.

Microfluidic technology in vascular research

Vascular cell biology is an area of research with great biomedical relevance. Vascular dysfunction is involved in major diseases such as atherosclerosis, diabetes, and cancer. However, when studying vascular cell biology in the laboratory, it is difficult to mimic the dynamic, three-dimensional microenvironment that is found in vivo. Microfluidic technology offers unique possibilities to overcome this difficulty. In this review, an overview of the recent applications of microfluidic technology in the field of vascular biological research will be given. Examples of how microfluidics can be used to generate shear stresses, growth factor gradients, cocultures, and migration assays will be provided. The use of microfluidic devices in studying three-dimensional models of vascular tissue will be discussed. It is concluded that microfluidic technology offers great possibilities to systematically study vascular cell biology with setups that more closely mimic the in vivo situation than those that are generated with conventional methods.

The dual nature of extracellular ATP as a concentration-dependent platelet P2X1 agonist and antagonist

Patient groups subject to higher occurrence of stroke (e.g., people with diabetes, cystic fibrosis, pulmonary hypertension) have reduced release of ATP from their erythrocytes (ERYs) when subjected to flow-induced deformation or pharmacological stimuli. These same groups also have platelets that are more adhesive in comparison to controls. Here we show platelet aggregation, and inhibition of that aggregation, is affected by free Ca(2+) entering the platelet through the ATP-gated P2X1 receptor. The addition of ATP (10 microM) increased the platelet NO by 26.7 +/- 7.7%. This value was decreased significantly to below basal levels in the presence of NF 449 (p < 0.001), an inhibitor of the P2X1 receptor on the platelet. Aggregation profiles measured in the presence of ATP revealed that when the P2X1 receptor was blocked, or when the measurements were performed in Ca(2+) free buffer, platelet aggregation was nearly eliminated. Our findings employing standard aggregation measurements suggest that ATP behaves as a platelet inhibitor below 1.6 x 10(-19) moles ATP per platelet; however, above this value, ATP behaves as a platelet activator. These findings suggesting a dual nature of ATP with regard to platelet behavior were confirmed by passing platelets over endothelial cells that were coated in the channels of a microfluidic device. Importantly, it was determined that ERY-derived ATP release was a major determinant of platelet adhesion to the endothelium. These findings may have implications in anti-platelet drug design as most current therapies focus on the inhibition of P2Y-type receptors. Moreover, through the use of microfluidic technologies, we have provided in vitro evidence for a possible relationship between ERY properties and platelet behavior in vivo.

Zinc-activated C-peptide resistance to the type 2 diabetic erythrocyte is associated with hyperglycemia-induced phosphatidylserine externalization and reversed by metformin

Insulin resistance can broadly be defined as the diminished ability of cells to respond to the action of insulin in transporting glucose from the bloodstream into cells and tissues. Here, we report that erythrocytes (ERYs) obtained from type 2 diabetic rats display an apparent resistance to Zn(2+)-activated C-peptide. Thus, the aims of this study were to demonstrate that Zn(2+)-activated C-peptide exerts potentially beneficial effects on healthy ERYs and that these same effects on type 2 diabetic ERYs are enhanced in the presence of metformin. Incubation of ERYs (obtained from type 2 diabetic BBZDR/Wor-rats) with Zn(2+)-activated C-peptide followed by chemiluminescence measurements of ATP resulted in a 31.2 +/- 4.0% increase in ATP release from these ERYs compared to a 78.4 +/- 4.9% increase from control ERYs. Glucose accumulation in diabetic ERYs, measured by scintillation counting of (14)C-labeled glucose, increased by 35.8 +/- 1.3% in the presence of the Zn(2+)-activated C-peptide, a value significantly lower than results obtained from control ERYs (64.3 +/- 5.1%). When Zn(2+)-activated C-peptide was exogenously added to diabetic ERYs, immunoassays revealed a 32.5 +/- 8.2% increase in C-peptide absorbance compared to a 64.4 +/- 10.3% increase in control ERYs. Phosphatidylserine (PS) externalization and metformin sensitization of Zn(2+)-activated C-peptide were examined spectrofluorometrically by measuring the binding of FITC-labeled annexin to PS. The incubation of diabetic ERYs with metformin prior to the addition of Zn(2+)-activated C-peptide resulted in values that were statistically equivalent to those of controls. Summarily, data obtained here demonstrate an apparent resistance to Zn(2+)-activated C-peptide by the ERY that is corrected by metformin.

An easy to assemble microfluidic perfusion device with a magnetic clamp

We have built and characterized a magnetic clamp for reversible sealing of PDMS microfluidic chips against cover glasses with cell cultures and a microfluidic chip for experiments on shear stress response of endothelial cells. The magnetic clamp exerts a reproducible uniform pressure on the microfluidic chip, achieving fast and reliable sealing for liquid pressures up to 40 kPa inside the chip with <10% deformations of microchannels and minimal variations of the substrate shear stress in perfusion flow. The microfluidic chip has 8 test regions with the substrate shear stress varying by a factor of 2 between each region, thus covering a 128-fold range from low venous to arterial. The perfusion is driven by differential pressure, which makes it possible to create pulsatile flows mimicking pulsing in the vasculature. The setup is tested by 15-40 hours perfusions over endothelial monolayers with shear stress in the range of 0.07-9 dyn/cm(2). Excellent cell viability at all shear stresses and alignment of cells along the flow at high shear stresses are repeatedly observed. A scratch wound healing assay under a shear flow is demonstrated and cell migration velocities are measured. Transfection of cells with a fluorescent protein is performed, and migrating fluorescent cells are imaged at a high resolution under shear flow in real time. The magnetic clamp can be closed with minimal mechanical perturbation to cells on the substrate and used with a variety of microfluidic chips for experiments with adherent and non-adherent cells.

Microfluidics as a functional tool for cell mechanics

Living cells are a fascinating demonstration of nature's most intricate and well-coordinated micromechanical objects. They crawl, spread, contract, and relax-thus performing a multitude of complex mechanical functions. Alternatively, they also respond to physical and chemical cues that lead to remodeling of the cytoskeleton. To understand this intricate coupling between mechanical properties, mechanical function and force-induced biochemical signaling requires tools that are capable of both controlling and manipulating the cell microenvironment and measuring the resulting mechanical response. In this review, the power of microfluidics as a functional tool for research in cell mechanics is highlighted. In particular, current literature is discussed to show that microfluidics powered by soft lithographic techniques offers the following capabilities that are of significance for understanding the mechanical behavior of cells: (i) Microfluidics enables the creation of in vitro models of physiological environments in which cell mechanics can be probed. (ii) Microfluidics is an excellent means to deliver physical cues that affect cell mechanics, such as cell shape, fluid flow, substrate topography, and stiffness. (iii) Microfluidics can also expose cells to chemical cues, such as growth factors and drugs, which alter their mechanical behavior. Moreover, these chemical cues can be delivered either at the whole cell or subcellular level. (iv) Microfluidic devices offer the possibility of measuring the intrinsic mechanical properties of cells in a high throughput fashion. (v) Finally, microfluidic methods provide exquisite control over drop size, generation, and manipulation. As a result, droplets are being increasingly used to control the physicochemical environment of cells and as biomimetic analogs of living cells. These powerful attributes of microfluidics should further stimulate novel means of investigating the link between physicochemical cues and the biomechanical response of cells. Insights from such studies will have implications in areas such as drug delivery, medicine, tissue engineering, and biomedical diagnostics.