Matrix protein microarrays for spatially and compositionally controlled microspot thrombosis under laminar flow

Point of care whole blood microfluidics for detecting and managing thrombotic and bleeding risks

Point-of-care diagnostics of platelet and coagulation function present demanding challenges. Current clinical diagnostics often use centrifuged plasmas or platelets and frozen plasma standards, recombinant protein standards, or even venoms. Almost all commercialized tests of blood do not recreate the in vivo hemodynamics where platelets accumulate to high densities and thrombin is generated from a procoagulant surface. Despite numerous drugs that target platelets, insufficient coagulation, or excess coagulation, POC blood testing is essentially limited to viscoelastic methods that provide a clotting time, clot strength, and clot lysis, while used mostly in trauma centers with specialized capabilities. Microfluidics now allows small volumes of whole blood (<1 mL) to be tested under venous or arterial shear rates with multi-color readouts to follow platelet function, thrombin generation, fibrin production, and clot stability. Injection molded chips containing pre-patterned fibrillar collagen and lipidated tissue factor can be stored dry for 6 months at 4C, thus allowing rapid blood testing on single-use disposable chips. Using only a small imaging microscope and micropump, these microfluidic devices can detect platelet inhibitors, direct oral anticoagulants (DOACs) and their reversal agents. POC microfluidics are ideal for neonatal surgical applications that involve small blood samples, rapid DOAC testing in stroke or bleeding or emergency surgery situations with patients presenting high risk cofactors for either bleeding or thrombosis.

Point-of-Care Diagnostic Assays and Novel Preclinical Technologies for Hemostasis and Thrombosis

Hemostasis is a complex wound-healing process involving numerous mechanical and biochemical mechanisms and influenced by many factors including platelets, coagulation factors, and endothelial components. Slight alterations in these mechanisms can lead to either prothrombotic or bleeding consequences, and such hemostatic imbalances can lead to significant clinical consequences with resultant morbidity and mortality. An ideal hemostasis assay would not only address all the unique processes involved in clot formation and resolution but also take place under flow conditions to account for endothelial involvement. Global assays do exist; however, these assays are not flow based. Flow-based assays have been limited secondary to their large blood volume requirements and low throughput, limiting potential clinical applications. Microfluidic-based assays address the aforementioned limitations of both global and flow-based assays by utilizing standardized devices that require low blood volumes, offer reproducible analysis, and have functionality under a range of shear stresses and flow conditions. While still largely confined to the preclinical space, here we aim to discuss these novel technologies and potential clinical implications, particularly in comparison to the current, commercially available point-of-care assays.

Adsorption and Absorption of Collagen Peptides to Polydimethlysiloxane and Its Influence on Platelet Adhesion Flow Assays

Collagen peptides are an alternative to animal derived collagens for platelet function studies under flow. The purpose of this study was to examine the use of collagen peptides in polydimethylsiloxane (PDMS) devices. Three collagen peptides with amino acid sequences and structures that capture von Willebrand factor and bind it with the platelet receptors integrin α₂β₁ and glycoprotein VI were patterned on glass, silicon, and PDMS. Each of these surfaces was also functionalized with tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS). Surfaces were characterized by their ability to support platelet adhesion, topology by atomic force microscopy, contact angle, and peptides absorption. PDMS readily absorbs collagen peptides, depleting them from solution, thus reducing their adsorption to glass and silicon substrates when used for micropatterning. Treatment of PDMS with FOTS, but not bovine serum albumin or poloxamer 407, inhibits collagen peptide absorption and supports adsorption and platelet adhesion at venous and arterial shear rates. Similarly, FOTS treatment of glass or silicon supports collagen peptide adsorption even in the presence of untreated PDMS. In conclusion, PDMS acts as an absorptive sink for collagen peptides, rendering a non-adhesive surface for platelet adhesion and competing for peptides when used for micropatterning. The absorption of collagen peptides can be overcome by functionalization of PDMS with a fluorinated alkyl silane, thus allowing its use as a material for micropatterning or as a surface for platelet adhesion flow assays.

Platelet Mechanotransduction

The vasculature is a dynamic environment in which blood platelets constantly survey the endothelium for sites of vessel damage. The formation of a mechanically coherent hemostatic plug to prevent blood loss relies on a coordinated series of ligand-receptor interactions governing the recruitment, activation, and aggregation of platelets. The physical biology of each step is distinct in that the recruitment of platelets depends on the mechanosensing of the platelet receptor glycoprotein Ib for the adhesive protein von Willebrand factor, whereas platelet activation and aggregation are responsive to the mechanical forces sensed at adhesive junctions between platelets and at the platelet-matrix interface. Herein we take a biophysical perspective to discuss the current understanding of platelet mechanotransduction as well as the measurement techniques used to quantify the physical biology of platelets in the context of thrombus formation under flow.

Recent advances in microfluidic platelet function assays: Moving microfluidics into clinical applications

The analysis of platelet aggregation and thrombosis kinetics has significantly advanced with progress in microfluidic technology. However, the results of platelet aggregation tests do not fully reflect the observed clinical outcomes. To address the present unmet clinical needs, the basic but essential biology of platelets should be reconsidered in relation to the characteristics of microfluidic systems employed for platelet tests. To this end, the present article provides an overview of commercially available point of care devices and focuses on recent microfluidic studies, describing their measurement principles. We critically discuss the characteristics of the microfluidics systems used to evaluate the complex processes underlying platelet aggregation, and that are specifically designed to mimic the pathophysiological environment of blood vessels, including hemodynamic factors as well as blood vessel injury. To this end, we summarize unsolved issues related to the application of platelet function tests based on microfluidics. Overall, we confirm that platelet function tests based on microfluidics provide a versatile platform that encompasses a variety of basic research, as well as clinical diagnostic applications.

Systems Analysis of Thrombus Formation

The systems analysis of thrombosis seeks to quantitatively predict blood function in a given vascular wall and hemodynamic context. Relevant to both venous and arterial thrombosis, a Blood Systems Biology approach should provide metrics for rate and molecular mechanisms of clot growth, thrombotic risk, pharmacological response, and utility of new therapeutic targets. As a rapidly created multicellular aggregate with a polymerized fibrin matrix, blood clots result from hundreds of unique reactions within and around platelets propagating in space and time under hemodynamic conditions. Coronary artery thrombosis is dominated by atherosclerotic plaque rupture, complex pulsatile flows through stenotic regions producing high wall shear stresses, and plaque-derived tissue factor driving thrombin production. In contrast, venous thrombosis is dominated by stasis or depressed flows, endothelial inflammation, white blood cell-derived tissue factor, and ample red blood cell incorporation. By imaging vessels, patient-specific assessment using computational fluid dynamics provides an estimate of local hemodynamics and fractional flow reserve. High-dimensional ex vivo phenotyping of platelet and coagulation can now power multiscale computer simulations at the subcellular to cellular to whole vessel scale of heart attacks or strokes. In addition, an integrated systems biology approach can rank safety and efficacy metrics of various pharmacological interventions or clinical trial designs.

In microfluidico: Recreating in vivo hemodynamics using miniaturized devices

Microfluidic devices create precisely controlled reactive blood flows and typically involve: (i) validated anticoagulation/pharmacology protocols, (ii) defined reactive surfaces, (iii) defined flow-transport regimes, and (iv) optical imaging. An 8-channel device can be run at constant flow rate or constant pressure drop for blood perfusion over a patterned collagen, collagen/kaolin, or collagen/tissue factor (TF) to measure platelet, thrombin, and fibrin dynamics during clot growth. A membrane-flow device delivers a constant flux of platelet agonists or coagulation enzymes into flowing blood. A trifurcated device sheaths a central blood flow on both sides with buffer, an ideal approach for on-chip recalcification of citrated blood or drug delivery. A side-view device allows clotting on a porous collagen/TF plug at constant pressure differential across the developing clot. The core-shell architecture of clots made in mouse models can be replicated in this device using human blood. For pathological flows, a stenosis device achieves shear rates of >100,000 s(-1) to drive plasma von Willebrand factor (VWF) to form thick long fibers on collagen. Similarly, a micropost-impingement device creates extreme elongational and shear flows for VWF fiber formation without collagen. Overall, microfluidics are ideal for studies of clotting, bleeding, fibrin polymerization/fibrinolysis, cell/clot mechanics, adhesion, mechanobiology, and reaction-transport dynamics.

Microfluidic technology as an emerging clinical tool to evaluate thrombosis and hemostasis

Assessment of platelet function and coagulation under flow conditions can augment traditional static assays used to evaluate patients with suspected hemostatic or thrombotic disorders. Among the available flow-based assays, microfluidic devices require the smallest blood volume and provide multiple output options. These assays are based on the presence of wall shear stress that mimics in vivo interactions between blood components and vessel walls. Microfluidic devices can generate essential information regarding homeostatic regulation of platelet activation and subsequent engagement of the coagulation cascade leading to fibrin deposition and clot formation. Emerging data suggest that microfluidic assays may also reveal consistent patterns of hemostatic or thrombotic pathology, and could aid in assessing and monitoring patient-specific effects of coagulation-modifying therapies.

Assays of different aspects of haemostasis - what do they measure?

Haemostasis is a complex process affected by many factors including both cellular and plasma components. It is a multistep process starting with platelet adhesion to damaged endothelium and ending in clot fibrinolysis. There are several methods available to study different aspects of haemostasis including adhesion, aggregation, coagulation and fibrinolysis. This review describes the different methods, what aspects of haemostasis they measure and their limitations. Methods discussed include methods to study adhesion (e.g. PFA-100, cone and platelet(let) analyzer and perfusion chambers) and aggregation (e.g. Multiplate, VerifyNow and Plateletworks). Furthermore the principles behind viscoelastic haemostatic assays are presented as well as methods that can analyse aspects of haemostasis in plasma or platelet-rich-plasma samples (thrombin generation, overall haemostasis potential and Thrombodynamics Analyzer).

Identification of platelet function defects by multi-parameter assessment of thrombus formation

Assays measuring platelet aggregation (thrombus formation) at arterial shear rate mostly use collagen as only platelet-adhesive surface. Here we report a multi-surface and multi-parameter flow assay to characterize thrombus formation in whole blood from healthy subjects and patients with platelet function deficiencies. A systematic comparison is made of 52 adhesive surfaces with components activating the main platelet-adhesive receptors, and of eight output parameters reflecting distinct stages of thrombus formation. Three types of thrombus formation can be identified with a predicted hierarchy of the following receptors: glycoprotein (GP)VI, C-type lectin-like receptor-2 (CLEC-2)>GPIb>α6β1, αIIbβ3>α2β1>CD36, α5β1, αvβ3. Application with patient blood reveals distinct abnormalities in thrombus formation in patients with severe combined immune deficiency, Glanzmann's thrombasthenia, Hermansky-Pudlak syndrome, May-Hegglin anomaly or grey platelet syndrome. We suggest this test may be useful for the diagnosis of patients with suspected bleeding disorders or a pro-thrombotic tendency.

The use of microfluidics in hemostasis: clinical diagnostics and biomimetic models of vascular injury

PURPOSE OF REVIEW

This article reviews the application of microfluidic technologies in hemostasis. The emphasis is on promising developments in devices for clinical applications and novel approaches to modeling complex hemodynamics.

RECENT FINDINGS

Microfluidics combined with micropatterning of prothrombotic substrates provides devices for measuring platelet function and coagulation with low blood volumes (∼100 μl) over a wide range of shear stresses. This technology has been applied to the diagnosis of bleeding and thrombotic disorders, as well as to dosing and monitoring of anticoagulation and antiplatelet agents. Microfluidic devices that mimic vascular geometries such as bifurcations, stenosis, and complex interconnected networks yield complex flow fields that have revealed new mechanisms of platelet adhesion and aggregation. Applying techniques from tissue engineering by endothelializing these networks is beginning to close the gap between in-vitro and in-vivo models of vascular injury.

SUMMARY

Microfluidic technology enables researchers to create in-vitro models of vascular disease with unprecedented control of the biochemical and biophysical conditions. Two promising directions are flow-dependent clinical assays and biomimetic vascular networks. These approaches are particularly well suited for modeling the microvasculature. However, caution should be used when extrapolating results from microfluidic channels to the pathophysiology of thrombosis in large arteries and veins.

Sources of variability in platelet accumulation on type 1 fibrillar collagen in microfluidic flow assays

Microfluidic flow assays (MFA) that measure shear dependent platelet function have potential clinical applications in the diagnosis and treatment of bleeding and thrombotic disorders. As a step towards clinical application, the objective of this study was to measure how phenotypic and genetic factors, as well as experimental conditions, affect the variability of platelet accumulation on type 1 collagen within a MFA. Whole blood was perfused over type 1 fibrillar collagen at wall shear rates of 150, 300, 750 and 1500 s⁻¹ through four independent channels with a height of 50 µm and a width of 500 µm. The accumulation of platelets was characterized by the lag time to 1% platelet surface coverage (Lag_T), the rate of platelet accumulation (V_PLT), and platelet surface coverage (SC). A cohort of normal donors was tested and the results were correlated to plasma von Willebrand factor (VWF) levels, platelet count, hematocrit, sex, and collagen receptors genotypes. VWF levels were the strongest determinant of platelet accumulation. VWF levels were positively correlated to V_PLT and SC at all wall shear rates. A longer Lag_T for platelet accumulation at arterial shear rates compared to venous shear rates was attributed to the time required for plasma proteins to adsorb to collagen. There was no association between platelet accumulation and hematocrit or platelet count. Individuals with the AG genotype of the GP6 gene had lower platelet accumulation than individuals with the AA genotype at 150 s⁻¹ and 300 s⁻¹. Recalcified blood collected into sodium citrate and corn trypsin inhibitor (CTI) resulted in diminished platelet accumulation compared to CTI alone, suggesting that citrate irreversibly diminishes platelet function. This study the largest association study of MFA in healthy donors (n = 104) and will likely set up the basis for the determination of the normal range of platelet responses in this type of assay.

The role of fibrinogen spacing and patch size on platelet adhesion under flow

Platelet adhesion to the vessel wall during vascular injury is mediated by platelet glycoproteins binding to their respective ligands on the vascular wall. In this study we investigated the roles that ligand patch spacing and size play in regulating platelet interactions with fibrinogen under hemodynamic flow conditions. To regulate the size and distance between patches of fibrinogen we developed a photolithography-based technique to fabricate patterns of proteins surrounded by a protein-repellant layer of poly(ethylene glycol). We demonstrate that when mepacrine labeled whole blood is perfused at a shear rate of 100 s ⁻¹ over substrates patterned with micron-sized wide lines of fibrinogen, platelets selectively adhere to the areas of patterned fibrinogen. Using fluorescent and scanning electron microscopy we demonstrate that the degree of platelet coverage (3-35%) and the ability of platelet aggregates to grow laterally are dependent upon the distance (6-30 μm) between parallel lines of fibrinogen. We also report on the effects of fibrinogen patch size on platelet adhesion by varying the size of the protein patch (2-20 μm) available for adhesion, demonstrating that the downstream length of the ligand patch is a critical parameter in platelet adhesion under flow. We expect that these results and protein patterning surfaces will be useful in understanding the spatial and temporal dynamics of platelet adhesion under physiologic flow, and in the development of novel platelet adhesion assays.

High content evaluation of shear dependent platelet function in a microfluidic flow assay

The high blood volume requirements and low throughput of conventional flow assays for measuring platelet function are unsuitable for drug screening and clinical applications. In this study, we describe a microfluidic flow assay that uses 50 μL of whole blood to measure platelet function on ~300 micropatterned spots of collagen over a range of physiologic shear rates (50-920 s(-1)). Patterning of collagen thin films (CTF) was achieved using a novel hydrated microcontact stamping method. CTF spots of 20, 50, and 100 μm were defined on glass substrates and consisted of a dense mat of nanoscale collagen fibers (3.74 ± 0.75 nm). We found that a spot size of greater than 20 μm was necessary to support platelet adhesion under flow, suggesting a threshold injury size is necessary for stable platelet adhesion. Integrating 50 μm CTF microspots into a multishear microfluidic device yielded a high content assay from which we extracted platelet accumulation metrics (lag time, growth rate, total accumulation) on the spots using Hoffman modulation contrast microscopy. This method has potential broad application in identifying platelet function defects and screening, monitoring, and dosing antiplatelet agents.

Measurement of whole blood thrombus formation using parallel-plate flow chambers - a practical guide

Custom-made and commercial parallel-plate flow chambers are widely used for studies of platelet activation and thrombus formation in whole blood at defined shear rates. When used in a reproducible way, such flow chamber devices give valuable information on the thrombogenic potential of human, mouse, or rat blood. This article aims to provide a practical guide for the use of parallel-plate flow chambers in combination with routine microscopic imaging techniques. The following methodological aspects are addressed: preparation of surface coatings, calculation of blood flow and shear rate, control of pre-analytical variables, protocols for routine performing of flow chamber tests with non-coagulating or coagulating blood, and procedures for real-time and end-point analysis of thrombus formation. Frequently encountered experimental problems and artifacts are discussed, as well as possibilities for using flow chamber devices as a diagnostic tool to test antithrombotic medication.

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.

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.

The Prostaglandin E 2 Receptor EP4 Is Expressed by Human Platelets and Potently Inhibits Platelet Aggregation and Thrombus Formation

Objective— Low concentrations of prostaglandin (PG) E 2 enhance platelet aggregation, whereas high concentrations inhibit it. The effects of PGE 2 are mediated through 4 G protein-coupled receptors, termed E-type prostaglindin (EP) receptor EP1, EP2, EP3, and EP4. The platelet-stimulating effect of PGE 2 has been suggested to involve EP3 receptors. Here we analyzed the receptor usage relating to the inhibitory effect of PGE 2 .

Methods and Results— Using flow cytometry, we found that human platelets expressed EP4 receptor protein. A selective EP4 agonist (ONO AE1-329) potently inhibited the platelet aggregation as induced by ADP or collagen. This effect could be completely reversed by an EP4 antagonist, but not by PGI 2 , PGD 2 , and thromboxane receptor antagonists or cyclooxygenase inhibition. Moreover, an EP4 antagonist enhanced the PGE 2 -induced stimulation of platelet aggregation, indicating a physiological antiaggregatory activity of EP4 receptors. The inhibitory effect of the EP4 agonist was accompanied by attenuated Ca 2+ flux, inhibition of glycoprotein IIb/IIIa, and downregulation of P-selectin. Most importantly, adhesion of platelets to fibrinogen under flow and in vitro thrombus formation were effectively prevented by the EP4 agonist. In this respect, the EP4 agonist synergized with acetylsalicylic acid.

Conclusion— These results are suggestive of EP4 receptor activation as a novel antithrombotic strategy.

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.

Microfluidic focal thrombosis model for measuring murine platelet deposition and stability: PAR4 signaling enhances shear-resistance of platelet aggregates

BACKGROUND

Flow chambers allow the ex vivo study of platelet response to defined surfaces at controlled wall shear stresses. However, most assays require 1-10 mL of blood and are poorly suited for murine whole blood experiments.

OBJECTIVE

To measure murine platelet deposition and stability in response to focal zones of prothrombotic stimuli using 100 microL of whole blood and controlled flow exposure.

METHODS

Microfluidic methods were used for patterning acid-soluble collagen in 100 microm x 100 microm patches and creating flow channels with a volume of 150 nL. Within 1 min of collection into PPACK and fluorescent anti-mouse CD41 mAb, whole blood from normal mice or from mice deficient in the integrin alpha(2) subunit was perfused for 5 min over the patterned collagen. Platelet accumulation was measured at venous and arterial wall shear rates. After 5 min, thrombus stability was measured with a 'shear step-up' to 8000 s(-1).

RESULTS

Wild-type murine platelets adhered and aggregated on collagen in a biphasic shear-dependent manner with increased deposition from 100 to 400 s(-1), but decreased deposition at 1000 s(-1). Adhesion to patterned collagen was severely diminished for platelets lacking a functional alpha(2)beta(1) integrin. Those integrin alpha(2)-deficient platelets that did adhere were removed from the surface when challenged to shear step-up. PAR4 agonist (AYPGKF) treatment of the thrombus at 5 min enhanced aggregate stability during the shear step-up.

CONCLUSIONS

PAR4 signaling enhances aggregate stability by mechanisms independent of other thrombin-dependent pathways such as fibrin formation.

Multifunctional surfaces with discrete functionalized regions for biological applications

In this paper we describe a method for creating multifunctional glass surfaces presenting discrete patches of different proteins on an inert PEG-functionalized background. Microcontact printing is used to stamp the substrate with octadecyltrichlorosilane to define the active regions. The substrate is then back-filled with PEG-silane {[[2-methoxypoly(ethyleneoxy)]propyl]trimethoxysilane} to define passive regions. A microfluidics device is subsequently affixed to the substrate to deliver proteins to the active regions, with as many channels as there are proteins to be patterned. Examples of trifunctional surfaces are given which present three terminating functional groups, i.e., protein 1, protein 2, and PEG. These surfaces should be broadly useful in biological studies, as patch size is well established to influence cell viability, growth, and differentiation. Three examples of cellular interactions with the surfaces are demonstrated, including the capture of cells from a single cell suspension, the selective sorting of cells from a mixed suspension, and the adhesion of cells to ligand micropatches at critical shear stresses. Within these examples, we demonstrate that the patterned immobilized proteins are active, as they retain their ability to interact with either antibodies in solution or receptors presented by cells. When appropriate (e.g., for E-selectin), proteins are patterned in their physiological orientations using a sandwich immobilization technique, which is readily accommodated within our method. The protein surface densities are highly reproducible in the patches, as supported by fluorescence intensity measurements. Potential applications include biosensors based on the interaction of cells or of marker proteins with protein patches, fundamental studies of cell adhesion as a function of patch size and shear stress, and studies of cell differentiation as a function of surface cues.

A membrane-based microfluidic device for controlling the flux of platelet agonists into flowing blood

The flux of platelet agonists into flowing blood is a critical event in thrombosis and hemostasis. However, few in vitro methods exist for examining and controlling the role of platelet agonists on clot formation and stability under hemodynamic conditions. In this paper, we describe a membrane-based method for introducing a solute into flowing blood at a defined flux. The device consisted of a track-etched polycarbonate membrane reversibly sealed between two microfluidic channels; one channel contained blood flowing at a physiologically relevant shear rate, and the other channel contained the agonist(s). An analytical model described the solute flux as a function of the membrane permeability and transmembrane pressure. The model was validated using luciferase as a model solute for transmembrane pressures of 50-400 Pa. As a proof-of-concept, the weak platelet agonist ADP was introduced into whole blood flowing at 250 s(-1) at three fluxes (1.5, 2.4, and 4.4 x 10(-18) mol microm(-2) s(-1)). Platelet aggregation was monitored by fluorescence microscopy during the experiment and the morphology of aggregates was determined by post hoc confocal and electron microscopy. At the lowest flux (1.5 x 10(-18) mol microm(-2) s(-1)), we observed little to no aggregation. At the higher fluxes, we observed monolayer (2.4 x 10(-18) mol microm(-2) s(-1)) and multilayer (4.4 x 10(-18) mol microm(-2) s(-1)) aggregates of platelets and found that the platelet density within an aggregate increased with increasing ADP flux. We expect this device to be a useful tool in unraveling the role of platelet agonists on clot formation and stability.

Determination of surface tissue factor thresholds that trigger coagulation at venous and arterial shear rates: amplification of 100 fM circulating tissue factor requires flow

Protein microarrays presenting spots of collagen and lipidated tissue factor (TF) allowed a determination of the critical surface concentration of TF required to trigger coagulation under flow. Whole blood supplemented with corn trypsin inhibitor (to inhibit factor XIIa) was perfused over microarrays for 5 minutes. Immunofluorescence staining of platelet glycoprotein GPIbalpha and fibrin(ogen) revealed a critical TF concentration (EC50) of 3.6, 8.4, and 10.2 molecules-TF/microm2 at wall shear rates of 100, 500, and 1000 s(-1), respectively. For collagen arrays where only the center lane of spots (in the direction of flow) contained TF, a downstream distance of 14 mm was required for the thrombus to widen enough to reach across a 300-micrometer gap to the adjacent TF-free lanes of collagen spots, in agreement with numerical simulation. To investigate the effect of low levels of circulating TF, whole blood (+/-100 fM added TF) was tested under static and flow conditions. After 5 minutes, the addition of 100 fM TF to whole blood had negligible effect under static conditions, but caused a 2.5-fold increase in fibrin formation under flow. This report defines the threshold concentrations of surface TF required to trigger coagulation under flow.