Microfluidic system for simultaneous optical measurement of platelet aggregation at multiple shear rates in whole blood

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.

A single-particle mechanofluorescent sensor

Monitoring mechanical stresses in microchannels is challenging. Herein, we report the development of a mechanofluorescence sensor system featuring a fluorogenic single polydiacetylene (PDA) particle, fabricated using a co-flow microfluidic method. We construct a stenotic vessel-mimicking capillary channel, in which the hydrodynamically captured PDA particle is subjected to controlled fluid flows. Fluorescence responses of the PDA particle are directly monitored in real time using fluorescent microscopy. The PDA particle displays significant nonlinear fluorescence emissions influenced by fluid viscosity and the presence of nanoparticles and biomolecules in the fluid. This nonlinear response is likely attributed to the torsion energy along the PDA's main chain backbone. Computational fluid dynamic simulations indicate that the complete blue-to-red transition necessitates ~307 μJ, aligning with prior research. We believe this study offers a unique advantage for simulating specific problematic regions of the human body in an in vitro environment, potentially paving the way for future exploration of difficult-to-access areas within the body.

Microfluidic-based cardiovascular systems for advanced study of atherosclerosis

Atherosclerosis (AS) is a significant global health concern due to its high morbidity and mortality rates. Extensive efforts have been made to replicate the cardiovascular system and explore the pathogenesis, diagnosis, and treatment of AS. Microfluidics has emerged as a valuable technology for modeling the cardiovascular system and studying AS. Here a brief review of the advances of microfluidic-based cardiovascular systems for AS research is presented. The critical pathogenetic mechanisms of AS investigated by microfluidic-based cardiovascular systems are categorized and reviewed, with a detailed summary of accurate diagnostic methods for detecting biomarkers using microfluidics represented. Furthermore, the review covers the evaluation and screening of AS drugs assisted by microfluidic systems, along with the fabrication of novel drug delivery carriers. Finally, the challenges and future prospects for advancing microfluidic-based cardiovascular systems in AS research are discussed and proposed, particularly regarding new opportunities in multi-disciplinary fundamental research and therapeutic applications for a broader range of disease treatments.

Shear Histories Alter Local Shear Effects on Thrombus Nucleation and Growth

Our goal was to determine the impact of physiological and pathological shear histories on platelet nucleation and thrombus growth at various local shear rates. We designed and characterized a microfluidic device capable of subjecting platelets to shear histories reaching as high as 6700 s- 1 in a single passage. Time-lapse videos of platelets and thrombi are captured using fluorescence microscopy. Thrombi are tracked, and the degree of thrombosis is evaluated through surface coverage, platelet nucleation maps, and ensemble-averaged aggregate areas and intensities. Surface coverage rates were the lowest when platelets deposited at high shear rates following a pathological shear history and were highest at low shear rates following a pathological shear history. Early aggregate area growth rates were significantly larger for thrombi developing at high shear following physiological shear history than at high shear following a pathological shear history. Aggregate vertical growth was restricted when depositing at low shear following a pathological shear history. In contrast, thrombi grew faster vertically following physiological shear histories. These results show that physiological shear histories pose thrombotic risks via volumetric growth, and pathological shear histories drastically promote nucleation. These findings may inform region-based geometries for biomedical devices and refine thrombosis simulations.

Engineering Organ-on-a-Chip Systems for Vascular Diseases

Vascular diseases, such as atherosclerosis and thrombosis, are major causes of morbidity and mortality worldwide. Traditional in vitro models for studying vascular diseases have limitations, as they do not fully recapitulate the complexity of the in vivo microenvironment. Organ-on-a-chip systems have emerged as a promising approach for modeling vascular diseases by incorporating multiple cell types, mechanical and biochemical cues, and fluid flow in a microscale platform. This review provides an overview of recent advancements in engineering organ-on-a-chip systems for modeling vascular diseases, including the use of microfluidic channels, ECM (extracellular matrix) scaffolds, and patient-specific cells. We also discuss the limitations and future perspectives of organ-on-a-chip for modeling vascular diseases.

Flow chamber staining modality for real-time inspection of dynamic phenotypes in multiple histological stains

Traditional histological stains, such as hematoxylin-eosin (HE), special stains, and immunofluorescence (IF), have defined myriads of cellular phenotypes and tissue structures in a separate stained section. However, the precise connection of information conveyed by the various stains in the same section, which may be important for diagnosis, is absent. Here, we present a new staining modality-Flow chamber stain, which complies with the current staining workflow but possesses newly additional features non-seen in conventional stains, allowing for (1) quickly switching staining modes between destain and restain for multiplex staining in one single section from routinely histological preparation, (2) real-time inspecting and digitally capturing each specific stained phenotype, and (3) efficiently synthesizing graphs containing the tissue multiple-stained components at site-specific regions. Comparisons of its stains with those by the conventional staining fashions using the microscopic images of mouse tissues (lung, heart, liver, kidney, esophagus, and brain), involving stains of HE, Periodic acid-Schiff, Sirius red, and IF for Human IgG, and mouse CD45, hemoglobin, and CD31, showed no major discordance. Repetitive experiments testing on targeted areas of stained sections confirmed the method is reliable with accuracy and high reproducibility. Using the technique, the targets of IF were easily localized and seen structurally in HE- or special-stained sections, and the unknown or suspected components or structures in HE-stained sections were further determined in histological special stains or IF. By the technique, staining processing was videoed and made a backup for off-site pathologists, which facilitates tele-consultation or -education in current digital pathology. Mistakes, which might occur during the staining process, can be immediately found and amended accordingly. With the technique, a single section can provide much more information than the traditional stained counterpart. The staining mode bears great potential to become a common supplementary tool for traditional histopathology.

Quantitative image analysis of thrombus formation in microfluidic in-vitro models

In this study, we present a method to quantitatively analyze the thrombus formation process through image analysis in an in vitro thrombus model with a circular cross section. The thrombus model used was designed based on the mechanism between the physical principle of wall shear rate (WSR) and thrombus formation. Image analysis was used to help visualize the thrombus formation process and calculate the thrombus area. Through this method, the thrombus formation and growth from the channel wall was demonstrated without the use of fluorescence. In addition, by dividing the image into sub-sections, the accuracy of the thrombus growth pattern was improved. The departing blood clots which are called embolus, were observed being separated from the thrombus.

A three-dimensional multiscale model for the prediction of thrombus growth under flow with single-platelet resolution

Modeling thrombus growth in pathological flows allows evaluation of risk under patient-specific pharmacological, hematological, and hemodynamical conditions. We have developed a 3D multiscale framework for the prediction of thrombus growth under flow on a spatially resolved surface presenting collagen and tissue factor (TF). The multiscale framework is composed of four coupled modules: a Neural Network (NN) that accounts for platelet signaling, a Lattice Kinetic Monte Carlo (LKMC) simulation for tracking platelet positions, a Finite Volume Method (FVM) simulator for solving convection-diffusion-reaction equations describing agonist release and transport, and a Lattice Boltzmann (LB) flow solver for computing the blood flow field over the growing thrombus. A reduced model of the coagulation cascade was embedded into the framework to account for TF-driven thrombin production. The 3D model was first tested against in vitro microfluidics experiments of whole blood perfusion with various antiplatelet agents targeting COX-1, P₂Y₁, or the IP receptor. The model was able to accurately capture the evolution and morphology of the growing thrombus. Certain problems of 2D models for thrombus growth (artifactual dendritic growth) were naturally avoided with realistic trajectories of platelets in 3D flow. The generalizability of the 3D multiscale solver enabled simulations of important clinical situations, such as cylindrical blood vessels and acute flow narrowing (stenosis). Enhanced platelet-platelet bonding at pathologically high shear rates (e.g., von Willebrand factor unfolding) was required for accurately describing thrombus growth in stenotic flows. Overall, the approach allows consideration of patient-specific platelet signaling and vascular geometry for the prediction of thrombotic episodes.

The excessive formation of blood clots under flow within the circulatory system (thrombosis) is known to initiate heart attacks and strokes. Therefore, obtaining insights into the formation and progression of these clots will be useful in evaluating pharmacological options. To this end, we have developed a 3D computational model that tracks the growth of a blood clot under flow from initial platelet deposition to full vessel occlusion in the presence of soluble platelet agonists. We first validated the model against experimental predictions of blood clots formed in vitro. Due to the construction of the model in 3D, we were able to carry out simulations of clot formation under important clinical situations, namely cylindrical blood vessels and acute flow narrowings (stenoses). To our knowledge, our model is the first of its kind that can account for patient-specific platelet phenotypes to perform robust 3D simulations of thrombus growth in geometries of clinical relevance.

Emerging Microfluidic Approaches for Platelet Mechanobiology and Interplay With Circulatory Systems

Understanding how platelets can sense and respond to hemodynamic forces in disturbed blood flow and complexed vasculature is crucial to the development of more effective and safer antithrombotic therapeutics. By incorporating diverse structural and functional designs, microfluidic technologies have emerged to mimic microvascular anatomies and hemodynamic microenvironments, which open the floodgates for fascinating platelet mechanobiology investigations. The latest endothelialized microfluidics can even recapitulate the crosstalk between platelets and the circulatory system, including the vessel walls and plasma proteins such as von Willebrand factor. Hereby, we highlight these exciting microfluidic applications to platelet mechanobiology and platelet–circulatory system interplay as implicated in thrombosis. Last but not least, we discuss the need for microfluidic standardization and summarize the commercially available microfluidic platforms for researchers to obtain reproducible and consistent results in the field.

Microfluidic models of the human circulatory system: versatile platforms for exploring mechanobiology and disease modeling

The human circulatory system is a marvelous fluidic system, which is very sensitive to biophysical and biochemical cues. The current animal and cell culture models do not recapitulate the functional properties of the human circulatory system, limiting our ability to fully understand the complex biological processes underlying the dysfunction of this multifaceted system. In this review, we discuss the unique ability of microfluidic systems to recapitulate the biophysical, biochemical, and functional properties of the human circulatory system. We also describe the remarkable capacity of microfluidic technologies for exploring the complex mechanobiology of the cardiovascular system, mechanistic studying of cardiovascular diseases, and screening cardiovascular drugs with the additional benefit of reducing the need for animal models. We also discuss opportunities for further advancement in this exciting field.

An "occlusive thrombosis-on-a-chip" microfluidic device for investigating the effect of anti-thrombotic drugs

Cardiovascular disease remains one of the world's leading causes of death. Myocardial infarction (heart attack) is triggered by occlusion of coronary arteries by platelet-rich thrombi (clots). The development of new anti-platelet drugs to prevent myocardial infarction continues to be an active area of research and is dependent on accurately modelling the process of clot formation. Occlusive thrombi can be generated in vivo in a range of species, but these models are limited by variability and lack of relevance to human disease. Although in vitro models using human blood can overcome species-specific differences and improve translatability, many models do not generate occlusive thrombi. In those models that do achieve occlusion, time to occlusion is difficult to measure in an unbiased and objective manner. In this study we developed a simple and robust approach to determine occlusion time of a novel in vitro microfluidic assay. This highlighted the potential for occlusion to occur in thrombosis microfluidic devices through off-site coagulation, obscuring the effect of anti-platelet drugs. We therefore designed a novel occlusive thrombosis-on-a-chip microfluidic device that reliably generates occlusive thrombi at arterial shear rates by quenching downstream coagulation. We further validated our device and methods by using the approved anti-platelet drug, eptifibatide, recording a significant difference in the "time to occlude" in treated devices compared to control conditions. These results demonstrate that this device can be used to monitor the effect of antithrombotic drugs on time to occlude, and, for the first time, delivers this essential data in an unbiased and objective manner.

Important Regulatory Roles of Erythrocytes on Platelet Adhesion to the von Willebrand Factor on the Wall Under Blood Flow Conditions

The roles of erythrocytes on platelet adhesion to von Willebrand factor (VWF) on the vessel wall through their membrane glycoprotein (GP)Ibα under blood flow condition is still to be elucidated. Blood specimens containing fluorescently labeled platelet and native, biochemically fixed, or artificial erythrocytes, at various hematocrits were perfused on a surface of VWF immobilized on the wall at a shear rate of 1,500 s-1. Rates of platelet adhesions were measured in each condition. Computer simulation of platelet adhesion to the VWF on the wall at the same shear rate was conducted by solving governing equations with a finite-difference method on K-computer. The rates of platelet adhesion were calculated at various hematocrits conditions in the computational domain of 100 µm (x-axis) x 400 µm (y-axis) x 100 µm (z-axis). Biological experiments demonstrated the positive correlation between the rates of platelet adhesion and hematocrit values in native, fixed, and artificial erythrocytes. (r=0.992, 0.934, and 0.825 respectively, p<0.05 for all). The computer simulation results supported the hematocrit dependent increase in platelet adhesion rates on VWF (94.3/sec at 10%, 185.2/sec at 20%, and 327.9/sec at 30%, respectively). These results suggest the important contributing role of erythrocytes on platelet adhesion to the VWF. The augmented z-axis fluctuation of flowing platelet caused by the physical presence of erythrocytes is speculated as the cause for hematocrit dependent increase in platelet adhesion.

Modeling Thrombus Shell: Linking Adhesion Receptor Properties and Macroscopic Dynamics

Damage to arterial vessel walls leads to the formation of platelet aggregate, which acts as a physical obstacle for bleeding. An arterial thrombus is heterogeneous; it has a dense inner part (core) and an unstable outer part (shell). The thrombus shell is very dynamic, being composed of loosely connected discoid platelets. The mechanisms underlying the observed mobility of the shell and its (patho)physiological implications are unclear. To investigate arterial thrombus mechanics, we developed a novel, to our knowledge, two-dimensional particle-based computational model of microvessel thrombosis. The model considers two types of interplatelet interactions: primary reversible (glycoprotein Ib (GPIb)-mediated) and stronger integrin-mediated interaction, which intensifies with platelet activation. At high shear rates, the former interaction leads to adhesion, and the latter is primarily responsible for stable platelet aggregation. Using a stochastic model of GPIb-mediated interaction, we initially reproduced experimental curves that characterize individual platelet interactions with a von Willebrand factor-coated surface. The addition of the second stabilizing interaction results in thrombus formation. The comparison of thrombus dynamics with experimental data allowed us to estimate the magnitude of critical interplatelet forces in the thrombus shell and the characteristic time of platelet activation. The model predicts moderate dependence of maximal thrombus height on the injury size in the absence of thrombin activity. We demonstrate that the developed stochastic model reproduces the observed highly dynamic behavior of the thrombus shell. The presence of primary stochastic interaction between platelets leads to the properties of thrombus consistent with in vivo findings; it does not grow upstream of the injury site and covers the whole injury from the first seconds of the formation. А simplified model, in which GPIb-mediated interaction is deterministic, does not reproduce these features. Thus, the stochasticity of platelet interactions is critical for thrombus plasticity, suggesting that interaction via a small number of bonds drives the dynamics of arterial thrombus shell.

Mechanobiology of shear-induced platelet aggregation leading to occlusive arterial thrombosis: A multiscale in silico analysis

Occlusive thrombosis in arteries causes heart attacks and strokes. The rapid growth of thrombus at elevated shear rates (~10,000 1/s) relies on shear-induced platelet aggregation (SIPA) thought to come about from the entanglement of von Willebrand factor (VWF) molecules. The mechanism for SIPA is not yet understood in terms of cell- and molecule-level dynamics in fast flowing bloodstreams. Towards this end, we develop a multiscale computational model to recreate SIPA in silico, where the suspension dynamics and interactions of individual platelets and VWF multimers are resolved directly. The platelet-VWF interaction via GP1b-A1 bonds is prescribed with intrinsic binding rates theoretically derived and informed by single-molecule measurements. The model is validated against existing microfluidic SIPA experiments, showing good agreement with the in vitro observations in terms of the morphology, traveling distance and capture time of the platelet aggregates. Particularly, the capture of aggregates can occur in a few milliseconds, comparable to the platelet transit time through pathologic arterial stenotic sections and much shorter than the time for shear-induced platelet activation. The multiscale SIPA simulator provides a cross-scale tool for exploring the biophysical mechanisms of SIPA in silico that are difficult to access with single-molecule measurements or micro-/macro-fluidic assays only.

A Three-Dimensional Microfluidic Device for Monitoring Cancer and Chemotherapy-Associated Platelet Activation

Platelet activation and the risk of thrombosis are increased in cancer patients, especially after chemotherapy. Our previous studies indicated that chemotherapy-induced platelet activation is largely due to endothelial cell damage. Thus, simple in vitro tests, such as aggregometry, are not desirable tests to predict platelet responsiveness to different chemotherapeutic agents because other contributory factors, such as tumor cells, endothelial cells, and the flow rate of platelets, also contribute to the formation of cancer-associated thrombosis. Therefore, developing a platelet detection system, which includes all possible risk parameters, is necessary. In the present study, we described a microengineered microfluidic system that contained a drug concentration generator, cancer cell culture chip, and three-dimensional (3D) circular microvascular model covered with a confluent endothelial layer and perfused with human platelets at a stable flow rate. Doxorubicin was injected through two injection sites. Endothelial cell injury was evaluated by counting, cell cytoskeleton observation, and the level of IACM1 and ET-1 in endothelial cells or a culture medium. Prestained platelets were perfused into the artificial blood vessel, and platelet-endothelial cell adhesion was measured. We found that (i) MCF7 cell-released factors had a cytotoxicity effect on both endothelial cells and platelets. (ii) We confirmed that doxorubicin-induced platelet activation was endothelial cell-dependent. (iii) A lower dosage of doxorubicin (0-2.0 μM) induced platelet activation, while a higher dosage of doxorubicin (2.0-4.0 μM) led to platelet death. Our findings indicated that platelet-endothelial cell adhesion could be used as a diagnostic marker of platelet activation, providing a simple and rapid detective way to predict platelet responsiveness before or during chemotherapy.

Label-free multimodal quantitative imaging flow assay for intrathrombus formation in vitro

Microfluidics in vitro assays recapitulate a blood vessel microenvironment using surface-immobilized agonists under biofluidic flows. However, these assays do not quantify intrathrombus mass and activities of adhesive platelets at the agonist margin and use fluorescence labeling, therefore limiting clinical translation potential. Here, we describe a label-free multimodal quantitative imaging flow assay that combines rotating optical coherent scattering microscopy and quantitative phase microscopy. The combined imaging platform enables real-time evaluation of membrane fluctuations of adhesive-only platelets and total intrathrombus mass under physiological flow rates in vitro. We call this multimodal quantitative imaging flow assay coherent optical scattering and phase interferometry (COSI). COSI records intrathrombus mass to picogram accuracy and shape changes to a platelet membrane with high spatial-temporal resolution (0.4 μm/s) under physiological and pathophysiological fluid shear stress (1800 and 7500 s^-1). With COSI, we generate an axial slice of 4 μm from the coverslip surface, approximately equivalent to the thickness of a single platelet, which permits nanoscale quantification of membrane fluctuation (activity) of adhesive platelets during initial adhesion, spreading, and recruitment into a developing thrombus (mass). Under fluid shear, pretreatment with a broad range metalloproteinase inhibitor (250 μM GM6001) blocked shedding of platelet adhesion receptors that shown elevated adhesive platelet activity at average of 42.1 μm/s and minimal change in intrathrombus mass.

Microfluidic assay of antiplatelet agents for inhibition of shear-induced platelet adhesion and activation

We have developed a microfluidic system to perfuse whole blood through a flow channel with an upstream stenotic region and a downstream protein capture region. This flow-based system was used to assay how effectively antiplatelet agents suppress shear-induced platelet adhesion and activation downstream of the stenotic region. Microcontact printing was used to covalently attach one of three platelet binding proteins [fibrinogen, collagen, or von Willebrand factor (vWf)] to the surface of the downstream capture region. Whole blood with an antiplatelet agent was transiently exposed to an upstream high wall shear rate (either 4860 s-1 or 11 560 s-1), and subsequently flowed over the downstream capture region where the platelet adhesion was measured. Several antiplatelet agents (acetylsalicylic acid, tirofiban, eptifibatide, anti-vWf, and anti-GPIbα) were evaluated for their efficacy in attenuating downstream adhesion. Following antibody blocking of vWf or GPIbα, downstream platelet activation was also assessed in perfused blood by flow cytometry using two activation markers (active GPIIb/IIIa and P-selectin). Acetylsalicylic acid demonstrated its inability to diminish shear-induced platelet adhesion to all three binding proteins. GPIIb/IIIa inhibitors (tirofiban and eptifibatide) significantly reduced platelet adhesion to fibrinogen. Antibody blocking of vWf or GPIbα effectively diminished platelet adhesion to all three capture proteins as well as platelet activation in perfused blood, indicating an essential role of vWf-GPIbα interaction in mediating shear-induced platelet aggregation.

Image-guided simulation in comparison with laser speckle contrast imaging for full-field observation of blood flow in a microvasculature model

The in vitro reconstruction of the microvascular network model provides a reproducible platform for hemodynamic study with great biological relevance. In the present study, microvascular models with different parametric features were designed under the guidance of Murray's law and derived from representative natural vascular network topography in vivo. Computational fluid dynamics (CFD) was used to numerically simulate blood velocity distributions inside of the designed microvasculature models. Full-field blood flow in the vascular network was visualized in vivo using a laser speckle contrast imaging (LSCI) system, from which the measured relative velocity was compared with CFD computed flow distribution. The results have shown that, in comparison with the simplified flow patterns obtained from idealized geometries, the irregular vascular topography is expected to lead to nonuniform and poor regional blood velocity distribution. The velocity distribution acquired by in vivo LSCI experiment is in good agreement with that of numerical simulation, indicating the technical feasibility of using biomimetic microchannels as a reasonable approximation of the microcirculatory flow conditions. This study provides a new paradigm that can be well suited to the study of microvascular blood flow properties and can further expand to mimic other in-vivo scenarios for accurately recapitulating the physical and hemodynamic environment of the microcirculation.

Predictive modelling of thrombus formation in diabetic retinal microaneurysms

Microaneurysms (MAs) are one of the earliest clinically visible signs of diabetic retinopathy (DR). Vision can be reduced at any stage of DR by MAs, which may enlarge, rupture and leak fluid into the neural retina. Recent advances in ophthalmic imaging techniques enable reconstruction of the geometries of MAs and quantification of the corresponding haemodynamic metrics, such as shear rate and wall shear stress, but there is lack of computational models that can predict thrombus formation in individual MAs. In this study, we couple a particle model to a continuum model to simulate the platelet aggregation in MAs with different shapes. Our simulation results show that under a physiologically relevant blood flow rate, thrombosis is more pronounced in saccular-shaped MAs than fusiform-shaped MAs, in agreement with recent clinical findings. Our model predictions of the size and shape of the thrombi in MAs are consistent with experimental observations, suggesting that our model is capable of predicting the formation of thrombus for newly detected MAs. This is the first quantitative study of thrombosis in MAs through simulating platelet aggregation, and our results suggest that computational models can be used to predict initiation and development of intraluminal thrombus in MAs as well as provide insights into their role in the pathophysiology of DR.

A three-dimensional phase-field model for multiscale modeling of thrombus biomechanics in blood vessels

Mechanical interactions between flowing and coagulated blood (thrombus) are crucial in dictating the deformation and remodeling of a thrombus after its formation in hemostasis. We propose a fully-Eulerian, three-dimensional, phase-field model of thrombus that is calibrated with existing in vitro experimental data. This phase-field model considers spatial variations in permeability and material properties within a single unified mathematical framework derived from an energy perspective, thereby allowing us to study effects of thrombus microstructure and properties on its deformation and possible release of emboli under different hemodynamic conditions. Moreover, we combine this proposed thrombus model with a particle-based model which simulates the initiation of the thrombus. The volume fraction of a thrombus obtained from the particle simulation is mapped to an input variable in the proposed phase-field thrombus model. The present work is thus the first computational study to integrate the initiation of a thrombus through platelet aggregation with its subsequent viscoelastic responses to various shear flows. This framework can be informed by clinical data and potentially be used to predict the risk of diverse thromboembolic events under physiological and pathological conditions.

Microfluidics in Haemostasis: A Review

Haemostatic disorders are both complex and costly in relation to both their treatment and subsequent management. As leading causes of mortality worldwide, there is an ever-increasing drive to improve the diagnosis and prevention of haemostatic disorders. The field of microfluidic and Lab on a Chip (LOC) technologies is rapidly advancing and the important role of miniaturised diagnostics is becoming more evident in the healthcare system, with particular importance in near patient testing (NPT) and point of care (POC) settings. Microfluidic technologies present innovative solutions to diagnostic and clinical challenges which have the knock-on effect of improving health care and quality of life. In this review, both advanced microfluidic devices (R&D) and commercially available devices for the diagnosis and monitoring of haemostasis-related disorders and antithrombotic therapies, respectively, are discussed. Innovative design specifications, fabrication techniques, and modes of detection in addition to the materials used in developing micro-channels are reviewed in the context of application to the field of haemostasis.

Vascularized Lower Respiratory-Physiology-On-A-Chip

Recently, respiratory systems are increasingly threatened by high levels of environmental pollution. Organ-on-a-chip technology has the advantage of enabling more accurate preclinical experiments by reproducing in vivo organ physiology. To investigate disease mechanisms and treatment options, respiratory-physiology-on-a-chip systems have been studied for the last decade. Here, we delineate the strategic approaches to develop respiratory-physiology-on-a-chip that can recapitulate respiratory system in vitro. The state-of-the-art biofabrication methods and biomaterials are considered as key contributions to constructing the chips. We also explore the vascularization strategies to investigate complicated pathophysiological phenomena including inflammation and immune responses, which are the critical aggravating factors causing the complications in the respiratory diseases. In addition, challenges and future research directions are delineated to improve the mimicry of respiratory systems in terms of both structural and biological behaviors.

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.

METALLOPROTEASE ADAMTS-13

Introduction. The signifi cance of ADAMTS-13 extends beyond its key role in the pathogenesis of thrombotic thrombocytopenic purpura (TTP); there is evidence of a relationship between a decrease in the ADAMTS-13 activity and thrombotic events in acute myocardial infarction and ischemic stroke.

Aim. To generalise available information on the structure and function of the metalloprotease ADAMTS-13.

General findings. The biological function of ADAMTS-13 consists in the cleavage of ultra-large von Willebrand factor (vWF) multimers. The fact that its defi ciency causes the development of TTP provides a basis for understanding the function of vWF–cleaving protease. ADAMTS-13 has a domain structure. The functional roles of most ADAMTS-13 domains, as well as the key role of the ADAMTS-13-vWF interaction in the regulation of haemostasis, are defi ned. The conformational activation of ADAMTS-13 by vWF constitutes an important aspect of its function. After getting into the bloodstream, ultra-large vWF multimers quickly adopt a closed conformation, which becomes very resistant to ADAMTS-13 proteolysis in the absence of shear stress. Ultra-large plasma vWF multimers regain their sensitivity to ADAMTS-13 after being exposed to high fl uid shear stress, which unfolds the central vWF-A2 domain. The unfolding of a vWF molecule under shear stress conditions reveals previously hidden exosites in domain A2, which gradually increase the binding affi nity between ADAMTS-13 and vWF. The mechanism underlying the production of autoantibodies against ADAMTS-13 is unknown and requires further study. The masking of cryptic epitopes in the closed conformation of ADAMTS-13 prevents the formation of autoantibodies. Early antigen recognition of ADAMTS-13 occurs through surface-exposed epitopes in the C-terminal domains. More detailed information on the mechanisms underlying the interaction between ADAMTS-13 and the vWF can improve the understanding of mechanisms involved in the regulation of the coagulation system.

Conflict of interest: the authors declare no confl ict of interest.

Financial disclosure: the study had no sponsorship.

Engineering a Bi-Conical Microchip as Vascular Stenosis Model

Vascular stenosis is always associated with hemodynamic changes, especially shear stress alterations. Herein, bi-conical shaped microvessels were developed through flexibly and precisely controlled templated methods for hydrogel blood-vessel-like microchip. The blood-vessel-like microvessels demonstrated tunable dimensions, perfusable ability, and good cytocompatibility. The microchips showed blood-vessel-like lumens through fine embeddedness of human umbilical vein endothelial cells (HUVECs) on the interior surface of hydrogel microchannels, which closely reproduced the morphology and functions of human blood vessels. In the gradual narrowing region of bi-conical shape, fluid flow generated wall shear stress, which caused cell morphology variations. Wall shear rates at the gradual narrowing region were simulated by FLUENT software. The results showed that our microchannels qualified for performance as a vascular stenosis-like model in evaluating blood hydrodynamics. In general, our blood-vessel-on-a-chip could offer potential applications in the prevention, diagnosis, and therapy of arterial thrombosis.

Shear-induced platelet aggregation: 3D-grayscale microfluidics for repeatable and localized occlusive thrombosis

Atherothrombosis leads to complications of myocardial infarction and stroke as a result of shear-induced platelet aggregation (SIPA). Clinicians and researchers may benefit from diagnostic and benchtop microfluidic assays that assess the thrombotic activity of an individual. Currently, there are several different proposed point-of-care diagnostics and microfluidic thrombosis assays with different design parameters and end points. The microfluidic geometry, surface coatings, and anticoagulation may strongly influence the precision of these assays. Variability in selected end points also persists, leading to ambiguous results. This study aims to assess the effects of three physiologically relevant extrinsic design factors on the variability of a single end point to provide a quantified rationale for design parameter and end-point standardization. Using a design of experiments approach, we show that the methods of channel fabrication and collagen surface coating significantly impact the variability of occlusion time from porcine whole blood, while anticoagulant selection between heparin and citrate did not significantly impact the variability. No factor was determined to significantly impact the mean occlusion time within the assay. Occlusive thrombus was found to consistently form in the first third (333 μm) of the high shear zone and not in the shear gradient regions. The selection of these factors in the design of point-of-care diagnostics and experimental SIPA assays may lead to increased precision and specificity in high shear thrombosis studies.

Hemostasis-On-a-Chip: Impedance Spectroscopy Meets Microfluidics for Hemostasis Evaluation

In the case of vascular injury, a complex process (of clotting) starts, involving mainly platelets and coagulation factors. This process in healthy humans is known as hemostasis, but when it is deregulated (thrombosis), it can be the cause of important cardiovascular diseases. Nowadays, the aging of the population and unhealthy lifestyles increase the impact of thrombosis, and therefore there is a need for tools to provide a better understanding of the hemostasis mechanisms, as well as more cost-effective diagnosis and control devices. This study proposes a novel microflow chamber, with interchangeable biomimetic surfaces to evaluate global hemostasis, using reduced amounts of blood sample and reagents, and also a minimized time required to do the test. To validate the performance of this novel device, a study on the new oral anticoagulant Apixaban (APIX) has been performed and compared to previous conventional techniques. The test shows an excellent agreement, while the amount of the required sample has been reduced (only 100 µL is used), and the amount of reagent as well. An imprinted electrode embedded in the chamber in order to measure the impedance during the coagulation process. This approach distinguishes the impedance behavior of plasma poor in platelets (PPP) and plasma rich in platelets (PRP) for the first time.

The Margination of Particles in Areas of Constricted Blood Flow

Stroke is a leading cause of death globally and is caused by stenoses, abnormal narrowings of blood vessels. Recently, there has been an increased interest in shear-activated particle clusters for the treatment of stenosis, but there is a lack of literature investigating the impact of different stenosis geometries on particle margination. Margination refers to the movement of particles toward the blood vessel wall and is desirable for drug delivery. The current study investigated ten different geometries and their effects on margination. Microfluidic devices with a constricted area were fabricated to mimic a stenosed blood vessel with different extent of occlusion, constricted length, and eccentricity (gradualness of the constriction and expansion). Spherical fluorescent particles with a diameter of 2.11 μm were suspended in blood and tracked as they moved into, through, and out of the constricted area. A margination parameter, M, was used to quantify margination based on the particle distribution after velocity normalization. Experimental results suggested that a constriction leads to an enhanced margination, whereas an expansion is responsible for a decrease in margination. Further, margination was found to increase with increasing percent occlusion and constriction length, likely a result of higher shear rate and longer residence time, respectively. Margination decreases as the stenosis geometry becomes more gradual (eccentricity increases) with the exception of a sudden constriction/expansion geometry. The findings demonstrate the importance of geometric effects on margination and call for detailed numerical modeling and geometric characterization of the stenosed areas to fully understand the underlying physics.

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.

Downstream platelet adhesion and activation under highly elevated upstream shear forces

Elevated shear force caused by an anastomotic stenosis is a common complication at the blood vessel-vascular implant interface. Although elevated shear forces were found to cause platelet aggregation around a stenotic region, transient platelet exposure to elevated shear forces and subsequent downstream events occurring under lower shear force were not extensively studied. We hypothesize that effects of elevated shear forces on pre-activation of platelets for downstream adhesion and activation are relevant in understanding the increased thrombotic risk associated with blood-contacting devices. We designed a microfluidic flow system to mimic the hemodynamic environment of vasculature with an upstream anastomotic stenosis with five wall shear strain rates ranging from 1620 s^-1 to 11560 s^-1. Under shear flow conditions, transient exposure of whole blood to elevated shear forces resulted in higher downstream platelet adhesion onto three different immobilized platelet agonists: fibrinogen, collagen, or von Willebrand factor. Platelet expression of four activation markers (P-selectin, GPIIb/IIIa, lysosomal glycoprotein, and phosphatidylserine) significantly increased after transient exposure to higher upstream wall shear strain rates of 2975-11560 s^-1. A significant lysis was observed when platelets were primed by upstream wall shear strain rate of 11560 s^-1. These experimental results could be helpful to understand how altered hemodynamics around an anastomotic stenosis promotes thrombus formation downstream. STATEMENT OF SIGNIFICANCE: Studying the downstream response of platelets following transient exposure to an upstream agonist is important because of significant clinical implications to the implantation of vascular devices. Due to intimal fibrous hyperplasia, vascular biomaterials such as synthetic small-diameter vascular grafts sometimes become stenotic (narrow), leading to transient platelet exposure to elevated shear forces. In this study, a microfluidic flow system was developed to mimic a stenosed vascular graft and to investigate how highly elevated, transient upstream shear forces, typically found in severe stenosis, results in the pre-activation of platelets for downstream adhesion and activation. The findings of the present study have implications for optimizing the design of blood-contacting biomaterials in order to minimize thrombotic risk associated with transiently elevated shear forces. The findings also provide additional insights into the mechanisms of thrombus formation at the post-stenotic regions of vascular implants.

Effect of estetrol, a selective nuclear estrogen receptor modulator, in mouse models of arterial and venous thrombosis

Estetrol (E4) is a natural estrogen synthesized exclusively during pregnancy by the human fetal liver, and the physiological role of this hormone is unknown. Interestingly, E4 was recently evaluated in preclinical and phase II-III clinical studies in combination with a progestin, with the advantage to not increase the circulating level of coagulation factors, at variance to oral estradiol or ethinylestradiol. Here, we evaluated the effect of E4 on hemostasis and thrombosis in mouse. Following chronic E4 treatment, mice exhibited a prolonged tail-bleeding time and were protected from arterial and also venous thrombosis in vivo. In addition, E4 treatment decreased ex vivo thrombus growth on collagen under arterial flow conditions. We recently showed that E4 activates uterine epithelial proliferation through nuclear estrogen receptor (ER) α. To analyze the impact of nuclear ERα actions on hemostasis and thrombosis, we generated hematopoietic chimera with bone marrow cells deficient for nuclear ERα. E4-induced protection against thromboembolism was significantly reduced in the absence of hematopoietic nuclear ERα activation, while the increased tail-bleeding time was not impacted by this deletion. In addition to its "liver friendly" profile described in women, our data shows that E4 has anti-thrombotic properties in various mouse models. Altogether, the natural fetal estrogen E4 could represent an attractive alternative to classic estrogens in oral contraception and treatment of menopause.

Effects of upstream shear forces on priming of platelets for downstream adhesion and activation

Platelets in flowing blood are sometimes exposed to elevated shear forces caused by anastomotic stenosis at the blood vessel-vascular implant interface. The objective of this study was to determine how effective upstream shear forces are in priming platelets for downstream adhesion and activation. Flow chambers with upstream stenotic regions (shear rates of 400-1000 s^-1) were manufactured by relief molding of polydimethylsiloxane. Downstream from the stenotic regions, microcontact printing was used to covalently immobilize three different proteins (fibrinogen, collagen, or von Willebrand factor) to serve as platelet capture agents. Anticoagulated whole blood was perfused through the flow chambers and platelet adhesion to the downstream capture region was quantified. It was found that transient exposure of platelets to increased shear forces resulted in higher platelet adhesion on all three proteins. The duration of the platelet exposure to elevated shear forces was varied by changing the length of the stenotic regions. The results indicated that, in addition to the magnitude of shear forces, the duration of exposure to these forces was also an important factor in priming platelets. The effect of upstream shear forces on platelet activation was assessed by quantifying P-selectin, integrin α_IIbβ₃, lysosomal glycoprotein, and phosphatidylserine exposure using flow cytometry. The results suggested that increased shear forces were capable of increasing the priming of platelets for downstream activation. This study implicates the anastomotic region(s) of vascular implants as a locus of platelet pre-activation that may lead to thrombus formation downstream.

STATEMENT OF SIGNIFICANCE

A synthetic small-diameter vascular graft can often become stenotic due to intimal fibrous hyperplasia, either generally along the inside of the graft or at the anastomotic regions, leading to an increased shear force on flowing platelets. Our lab is studying how the upstream platelet preactivation (aka "priming") in flowing blood affects their downstream adhesion and activation. This manuscript describes a study in which priming of platelets is achieved by upstream stenotic narrowing in a microfluidic flow chamber. Such experimental design was intended to mimic a vascular implant with stenotic upstream anastomosis and downstream exposed platelet protein agonists. Understanding how the pre-activated platelets respond to imperfect vascular implant surfaces downstream is an important factor in designing better vascular implants.

Fibrinogen Motif Discriminates Platelet and Cell Capture in Peptide-Modified Gold Micropore Arrays

Human blood platelets and SK-N-AS neuroblastoma cancer-cell capture at spontaneously adsorbed monolayers of fibrinogen-binding motifs, GRGDS (generic integrin adhesion), HHLGGAKQAGDV (exclusive to platelet integrin α_IIbβ₃), or octanethiol (adhesion inhibitor) at planar gold and ordered 1.6 μm diameter spherical cap gold cavity arrays were compared. In all cases, arginine/glycine/aspartic acid (RGD) promoted capture, whereas alkanethiol monolayers inhibited adhesion. Conversely only platelets adhered to alanine/glycine/aspartic acid (AGD)-modified surfaces, indicating that the AGD motif is recognized preferentially by the platelet-specific integrin, α_IIbβ₃. Microstructuring of the surface effectively eliminated nonspecific platelet/cell adsorption and dramatically enhanced capture compared to RGD/AGD-modified planar surfaces. In all cases, adhesion was reversible. Platelets and cells underwent morphological change on capture, the extent of which depended on the topography of the underlying substrate. This work demonstrates that both the nature of the modified interface and its underlying topography influence the capture of cancer cells and platelets. These insights may be useful in developing cell-based cancer diagnostics as well as in identifying strategies for the disruption of platelet cloaks around circulating tumor cells.

Impedance biosensor for real-time monitoring and prediction of thrombotic individual profile in flowing blood

A new biosensor for the real-time analysis of thrombus formation is reported. The fast and accurate monitoring of the individual thrombotic risk represents a challenge in cardiovascular diagnostics and in treatment of hemostatic diseases. Thrombus volume, as representative index of the related thrombotic status, is usually estimated with confocal microscope at the end of each in vitro experiment, without providing a useful behavioral information of the biological sample such as platelets adhesion and aggregation in flowing blood. Our device has been developed to work either independently or integrated with the microscopy system; thus, images of the fluorescently labeled platelets are acquired in real-time during the whole blood perfusion, while the global electrical impedance of the blood sample is simultaneously monitored between a pair of specifically designed gold microelectrodes. Fusing optical and electrical data with a novel technique, the dynamic of thrombus formation events in flowing blood can be reconstructed in real-time, allowing an accurate extrapolation of the three-dimensional shape and the spatial distribution of platelet thrombi forming and growing within artificial capillaries. This biosensor is accurate and it has been used to discriminate different hemostatic conditions and to identify weakening and detaching platelet aggregates. The results obtained appear compatible with those quantified with the traditional optical method. With advantages in terms of small size, user-friendliness and promptness of response, it is a promising device for the fast and automatic individual health monitoring at the Point of Care (POC).

Flow chamber and microfluidic approaches for measuring thrombus formation in genetic bleeding disorders

Platelet adhesion and aggregation, coagulation, fibrin formation, and fibrinolysis are regulated by the forces and flows imposed by blood at the site of a vascular injury. Flow chambers designed to observe these events are an indispensable part of doing hemostasis and thrombosis research, especially with human blood. Microfluidic methods have provided the flexibility to design flow chambers with complex geometries and features that more closely mimic the anatomy and physiology of blood vessels. Additionally, microfluidic systems with integrated optics and/or pressure sensors and on-board signal processing could transform what have been primarily research tools into clinical assays. Here, we describe a historical review of how flow-based approaches have informed biophysical mechanisms in genetic bleeding disorders, challenges and potential solutions for developing models of bleeding in vitro, and outstanding issues that need to be addressed prior to their use in clinical settings.

Hemodynamic studies of platelet thrombosis using microfluidics

Platelets contribute to thrombus formation in a variety of ways. Platelet adhesion, activation, and thrombus growth depend greatly on the type of hemodynamic environment surrounding an inciting event. Microfluidic systems may be used to explore these relationships. In this review, we describe some important considerations required in the design of a microfluidic system and identify some limitations that may require use of a macroscale system.

Prediction of Thrombus Growth: Effect of Stenosis and Reynolds Number

Shear stresses play a major role in platelet-substrate interactions and thrombus formation and growth in blood flow, where under both pathological and physiological conditions platelet adhesion and accumulation occur. In this study, a shear-dependent continuum model for platelet activation, adhesion and aggregation is presented. The model was first verified under three different shear conditions and at two heparin levels. Three-dimensional simulations were then carried out to evaluate the performance of the model for severely damaged (stripped) aortas with mild and severe stenosis degrees in laminar flow regime. For these cases, linear shear-dependent functions were developed for platelet-surface and platelet-platelet adhesion rates. It was confirmed that the platelet adhesion rate is not only a function of Reynolds number (or wall shear rate) but also the stenosis severity of the vessel. General correlations for adhesion rates of platelets as functions of stenosis and Reynolds number were obtained based on these cases. Finally using the new platelet adhesion rates, the model was applied to different experimental systems and shown to agree well with measured platelet deposition.

Utility of microfluidic devices to study the platelet-endothelium interface

The integration of biomaterials and understanding of vascular biology has led to the development of perfusable endothelialized flow models, which have been used as valuable tools to study the platelet-endothelium interface under shear. In these models, the parameters of geometry, compliance, biorheology, and cellular complexity are varied to recapitulate the physical biology of platelet recruitment and activation under physiologically relevant conditions of blood flow. In this review, we summarize the mechanistic insights learned from perfusable microvessel models and discuss the potential utility as well as challenges of endothelialized microfluidic devices to study platelet function in the bloodstream in vitro.

Microfluidic measurement for blood flow and platelet adhesion around a stenotic channel: Effects of tile size on the detection of platelet adhesion in a correlation map

Platelet aggregation affects the surrounding blood flow and usually occurs where a blood vessel is narrowed as a result of atherosclerosis. The relationship between blood flow and platelet aggregation is not yet fully understood. This study proposes a microfluidic method to measure the velocity and platelet aggregation simultaneously by combining the micro-particle image velocimetry technique and a correlation mapping method. The blood flow and platelet adhesion procedure in a stenotic micro-channel with 90% severity were observed for a relatively long period of 4 min. In order to investigate the effect of tile size on the detection of platelet adhesion, 2D correlation coefficients were evaluated with binary images obtained by manual labeling and the correlation mapping method with different sizes of the square tile ranging from 3 to 50 pixels. The maximum 2D correlation coefficient occurred with the optimum tile size of 5 × 5 pixels. Since the blood flow and platelet aggregation are mutually influenced by each other, blood flow and platelet adhesion were continuously varied. When there was no platelet adhesion (t = 0 min), typical blood flow is observed. The blood flow passes through the whole channel smoothly, and jet-like flow occurs in the post-stenosis region. However, the flow pattern changes when platelet adhesion starts at the stenosis apex and after the stenosis. These adhesions induce narrow high velocity regions to become wider over a range of area from upstream to downstream of the stenosis. Separated jet-like flows with two high velocity regions are also created. The changes in flow patterns may alter the patterns of platelet adhesion. As the area of the plate adhesion increases, the platelets plug the micro-channel and there is only a small amount of blood flow, finally. The microfluidic method could provide new insights for better understanding of the interactions between platelet aggregation and blood flow in various physiological conditions.

A General Shear-Dependent Model for Thrombus Formation

Modeling the transport, activation, and adhesion of platelets is crucial in predicting thrombus formation and growth following a thrombotic event in normal or pathological conditions. We propose a shear-dependent platelet adhesive model based on the Morse potential that is calibrated by existing invivo and invitro experimental data and can be used over a wide range of flow shear rates (100<γ˙<28,000s-1). We introduce an Eulerian-Lagrangian model where hemodynamics is solved on a fixed Eulerian grid, while platelets are tracked using a Lagrangian framework. A force coupling method is introduced for bidirectional coupling of platelet motion with blood flow. Further, we couple the calibrated platelet aggregation model with a tissue-factor/contact pathway coagulation cascade, representing the relevant biology of thrombin generation and the subsequent fibrin deposition. The range of shear rates covered by the proposed model encompass venous and arterial thrombosis, ranging from low-shear-rate conditions in abdominal aortic aneurysms and thoracic aortic dissections to thrombosis in stenotic arteries following plaque rupture, where local shear rates are extremely high.

Hemostasis (thrombus formation) is the normal physiological response that prevents significant blood loss after vascular injury. The resulting clots can form under different flow conditions in the veins as well as the arteries. The excessive and undesirable formation of clots (i.e., thrombosis) in our circulatory system may lead to significant morbidity and mortality. Some of these pathologies are deep vein thrombosis and pulmonary embolism and atherothrombosis (thrombosis triggered by plaque rupture) in coronary arteries, to name a few. The process of clot formation and growth at a site on a blood vessel wall involves a number of simultaneous processes including: multiple chemical reactions in the coagulation cascade, species transport and platelet adhesion all of which are strongly influenced by the hydrodynamic forces. Numerical models for blood clotting normally focus on one of the processes under a specific flow condition. Here, we propose a general numerical model that encompass a wide range of hemodynamic conditions in the veins and arteries, with individual platelets and their adhesive dynamics included explicitly in the models. Further, we include the biochemistry of coagulation cascade, which is essential to modeling thrombus formation, and couple that to our platelet aggregation model. The simulation results—tested against three different experiments—demonstrate that the proposed model is effective in capturing the invivo and invitro experimental observations.

Microfabricated Physiological Models for In Vitro Drug Screening Applications

Microfluidics and microfabrication have recently been established as promising tools for developing a new generation of in vitro cell culture microdevices. The reduced amounts of reagents employed within cell culture microdevices make them particularly appealing to drug screening processes. In addition, latest advancements in recreating physiologically relevant cell culture conditions within microfabricated devices encourage the idea of using such advanced biological models in improving the screening of drug candidates prior to in vivo testing. In this review, we discuss microfluidics-based models employed for chemical/drug screening and the strategies to mimic various physiological conditions: fine control of 3D extra-cellular matrix environment, physical and chemical cues provided to cells and organization of co-cultures. We also envision future directions for achieving multi-organ microfluidic devices.

Peptide-Mediated Platelet Capture at Gold Micropore Arrays

Ordered spherical cap gold cavity arrays with 5.4, 1.6, and 0.98 μm diameter apertures were explored as capture surfaces for human blood platelets to investigate the impact of surface geometry and chemical modification on platelet capture efficiency and their potential as platforms for surface enhanced Raman spectroscopy of single platelets. The substrates were chemically modified with single-constituent self-assembled monolayers (SAM) or mixed SAMs comprised of thiol-functionalized arginine-glycine-aspartic acid (RGD, a platelet integrin target) with or without 1-octanethiol (adhesion inhibitor). As expected, platelet adhesion was promoted and inhibited at RGD and alkanethiol modified surfaces, respectively. Platelet adhesion was reversible, and binding efficiency at the peptide modified substrates correlated inversely with pore diameter. Captured platelets underwent morphological change on capture, the extent of which depended on the topology of the underlying substrate. Regioselective capture of the platelets enabled study for the first time of the surface enhanced Raman spectroscopy of single blood platelets, yielding high quality Raman spectroscopy of individual platelets at 1.6 μm diameter pore arrays. Given the medical importance of blood platelets across a range of diseases from cancer to psychiatric illness, such approaches to platelet capture may provide a useful route to Raman spectroscopy for platelet related diagnostics.

A portable microfluidic system for rapid measurement of the erythrocyte sedimentation rate

The erythrocyte sedimentation rate (ESR) is a frequently used 30 min or 60 min clinical test for screening of several inflammatory conditions, infections, trauma, and malignant diseases, as well as non-inflammatory conditions including prostate cancer and stroke. Erythrocyte aggregation (EA) is a physiological process where erythrocytes form face-to-face linear structures, called rouleaux, at stasis or low shear rates. In this work, we proposed a method for ESR measurement from EA. We developed a microfluidic opto-electro-mechanical system, using which we experimentally showed a significant correlation (R² = 0.86) between ESR and EA. The microfluidic system was shown to measure ESR from EA using fingerprick blood in 2 min. 40 μl of whole blood is filled in a disposable polycarbonate cartridge which is illuminated with a near infrared emitting diode. Erythrocytes were disaggregated under the effect of a mechanical shear force using a solenoid pinch valve. Following complete disaggregation, transmitted light through the cartridge was measured using a photodetector for 1.5 min. The intensity level is at its lowest at complete disaggregation and highest at complete aggregation. We calculated ESR from the transmitted signal profile. We also developed another microfluidic cartridge specifically for monitoring the EA process in real-time during ESR measurement. The presented system is suitable for ultrafast, low-cost, and low-sample volume measurement of ESR at the point-of-care.