Correlation of thrombosis growth rate to pathological wall shear rate during platelet accumulation

The rapid change of shear rate gradient is beneficial to platelet activation

Fluid shear plays a key role in hemostasis and thrombosis, and the purpose of this study was to investigate the effect of shear gradient change rate (SGCR) on platelet reactivity and von Willebrand factor (vWF) activity and its mechanism. In this study, we developed a set of microfluidic chips capable of generating different shear gradients and simulated the shear rate distribution in the flow field by COMSOL Multiphysics software. Molecular markers of platelet activation (P-selectin, activated GPIIb/IIIa, phosphatidylserine exposure, and monocyte-platelet aggregate formation) were analyzed by flow cytometry. Platelet aggregation induced by shear gradient was studied by a microfluidic experimental platform, and plasma vWF ristocetin cofactor (vWF: RCO) activity was investigated by flow cytometry. The expression of p-Akt was studied by Western blotting. The results showed that the faster the SGCR, the higher the expression of platelet p-Akt, and the stronger the platelet reactivity and vWF activity. This indicates that fluid shear stress can activate platelets and vWF in a shear gradient-dependent manner through the PI3K/AKT signal pathway, and the faster the SGCR, the more significant the activation effect.

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.

Impact of rise and fall phases of shear on platelet activation and aggregation using microfluidics

Blood flow disorders are often the result of the non-physiological narrowing of blood arteries caused by atherosclerosis and thrombus. The blood then proceeds through rising-peak-decreasing phases as it passes through the narrow area. Although abnormally high shear is known to activate platelets, the shear process that platelets undergo in small arteries is complex. Thus, understanding how each shear phase affects platelet activation can be used to improve antiplatelet therapy and decrease the risk of side effects like bleeding. Blood samples were sheared (68.8 ms,5200 s-1) in vitro by the microfluidic technique, and platelet activation levels (P-selectin and integrin αIIbβ3) and von Willebrand factor (vWF) binding to platelets were analyzed by flow cytometry. Post-stenosis platelet aggregation was dynamically detected using microfluidic technology. We studied TXA2, P2Y12-ADP, and integrin αIIbβ3-fibrinogen receptor pathways by adding antiplatelet drugs, such as acetylsalicylic acid (ASA, an active ingredient of aspirin that inhibits platelet metabolism), ticagrelor (hinders platelet activation), and tirofiban (blocks integrin αIIbβ3 receptor) in vitro, respectively, to determine platelet activation function mediated by transient non-physiological high shear rates. We demonstrated that platelets can be activated under transient pathological high shear rates. The shear rise and fall phases influenced shear-induced platelet activation by regulating the binding of vWF to platelets. The degree of platelet activation and aggregation increased with multiple shear rise and fall phases. ASA did not inhibit shear-mediated platelet activation, but ticagrelor and tirofiban effectively inhibited shear-mediated platelet activation. Our data demonstrated that the shear rise and fall phases play an important role in shear-mediated platelet activation and promote platelet activation and aggregation in a vWF-dependent manner. Blocking integrin αIIbβ3 receptor and hindering P2Y12-ADP were beneficial to reducing shear-mediated platelet activation.

Platelet biology and function: plaque erosion vs. rupture

The leading cause of heart disease in developed countries is coronary atherosclerosis, which is not simply a result of ageing but a chronic inflammatory process that can lead to acute clinical events upon atherosclerotic plaque rupture or erosion and arterial thrombus formation. The composition and location of atherosclerotic plaques determine the phenotype of the lesion and whether it is more likely to rupture or to erode. Although plaque rupture and erosion both initiate platelet activation on the exposed vascular surface, the contribution of platelets to thrombus formation differs between the two phenotypes. In this review, plaque phenotype is discussed in relation to thrombus composition, and an overview of important mediators (haemodynamics, matrix components, and soluble factors) in plaque-induced platelet activation is given. As thrombus formation on disrupted plaques does not necessarily result in complete vessel occlusion, plaque healing can occur. Therefore, the latest findings on plaque healing and the potential role of platelets in this process are summarized. Finally, the clinical need for more effective antithrombotic agents is highlighted.

Evaluating the impact of transient shear stress on platelet activation, adhesion, and aggregation with microfluidic chip technique

BACKGROUND

When nonphysiological stenosis occurs, the transient high shear stress formed in vessels increases the risk of thrombosis and is a potential factor for cardiovascular diseases. But the platelet adhesion and aggregation behavior at nonphysiological post-stenosis and its affecting factors are not fully understood yet.

METHODS

In this experiment, platelet aggregation on collagen and fibrinogen at different shear stresses and different hematocrits were observed by microfluidic technology. Platelet activation (P-selectin, glycoprotein IIb/IIIa) and monocyte-platelet aggregate (MPA) levels under different shear stresses were analyzed by flow cytometry.

RESULTS

On fibrinogen, platelets aggregate more at higher shear stress conditions. While on collagen, it becomes more difficult for platelets to form stable aggregation at higher shear stress conditions. If platelets adhere initially at low shear stress, stable platelet aggregation can be formed at subsequent high shear stress. Moreover, when the shear stress increases, platelet activity markers (P-selectin, glycoprotein IIb/IIIa and MPAs) increase significantly. Hematocrit affects the degree of platelet aggregation, and the influence of hematocrit is obvious at high shear stress.

CONCLUSION

Transient high shear stress (46 ms) can effectively activate platelets. Platelet aggregation behavior was different for coated fibrinogen and collagen protein. Stable platelet adhesion at post-stenosis is more dependent on fibrinogen and platelet aggregation is stable on both fibrinogen and collagen. Hematocrit can significantly affect the formation of platelet aggregation.

Decoding thrombosis through code: a review of computational models

From the molecular level up to a blood vessel, thrombosis and hemostasis involves many interconnected biochemical and biophysical processes over a wide range of length and time scales. Computational modeling has gained eminence in offering insights into these processes beyond what can be obtained from in vitro or in vivo experiments, or clinical measurements. The multiscale and multiphysics nature of thrombosis has inspired a wide range of modeling approaches that aim to address how a thrombus forms and dismantles. Here, we review recent advances in computational modeling with a focus on platelet-based thrombosis. We attempt to summarize the diverse range of modeling efforts straddling the wide-spectrum of physical phenomena, length scales, and time scales; highlighting key advancements and insights from existing studies. Potential information gleaned from models is discussed, ranging from identification of thrombus-prone regions in patient-specific vasculature to modeling thrombus deformation and embolization in response to fluid forces. Furthermore, we highlight several limitations of current models, future directions in the field, and opportunities for clinical translation, to illustrate the state-of-the-art. There are a plethora of opportunity areas for which models can be expanded, ranging from topics of thromboinflammation to platelet production and clearance. Through successes demonstrated in existing studies described here, as well as continued advancements in computational methodologies and computer processing speeds and memory, in silico investigations in thrombosis are poised to bring about significant knowledge growth in the years to come.

A microfluidic method to investigate platelet mechanotransduction under extensional strain

Background

Blood platelets have evolved a complex mechanotransduction machinery to rapidly respond to hemodynamic conditions. A variety of microfluidic flow-based approaches have been developed to explore platelet mechanotransduction; however, these experimental models primarily focus on the effects of increased wall shear stress on platelet adhesion events and do not consider the critical effects of extensional strain on platelet activation in free flow.

Objectives

We report the development and application of a hyperbolic microfluidic assay that allows for investigation of platelet mechanotransduction under quasi-homogenous extensional strain rates in the absence of surface adhesions.

Methods

Using a combined computational fluid dynamic and experimental microfluidic approach, we explore 5 extensional strain regimes (geometries) and their effect on platelet calcium signal transduction.

Results

We demonstrate that in the absence of canonical adhesion, receptor engagement platelets are highly sensitive to both initial increase and subsequent decrease in extensional strain rates within the range of 747 to 3319/s. Furthermore, we demonstrate that platelets rapidly respond to the rate of change in extensional strain and define a threshold of ≥7.33 × 10⁶/s/m, with an optimal range of 9.21 × 10⁷ to 1.32 × 10⁸/s/m. In addition, we demonstrate a key role of both the actin-based cytoskeleton and annular microtubules in the modulation of extensional strain-mediated platelet mechanotransduction.

Conclusion

This method opens a window onto a novel platelet signal transduction mechanism and may have potential diagnostic utility in the identification of patients who are prone to thromboembolic complications associated with high-grade arterial stenosis or are on mechanical circulatory support systems, for which the extensional strain rate is a predominant hemodynamic driver.

von Willebrand factor unfolding mediates platelet deposition in a model of high-shear thrombosis

Thrombosis under high-shear conditions is mediated by the mechanosensitive blood glycoprotein von Willebrand factor (vWF). vWF unfolds in response to strong flow gradients and facilitates rapid recruitment of platelets in flowing blood. While the thrombogenic effect of vWF is well recognized, its conformational response in complex flows has largely been omitted from numerical models of thrombosis. We recently presented a continuum model for the unfolding of vWF, where we represented vWF transport and its flow-induced conformational change using convection-diffusion-reaction equations. Here, we incorporate the vWF component into our multi-constituent model of thrombosis, where the local concentration of stretched vWF amplifies the deposition rate of free-flowing platelets and reduces the shear cleaning of deposited platelets. We validate the model using three benchmarks: in vitro model of atherothrombosis, a stagnation point flow, and the PFA-100, a clinical blood test commonly used for screening for von Willebrand disease (vWD). The simulations reproduced the key aspects of vWF-mediated thrombosis observed in these experiments, such as the thrombus location, thrombus growth dynamics, and the effect of blocking platelet-vWF interactions. The PFA-100 simulations closely matched the reported occlusion times for normal blood and several hemostatic deficiencies, namely, thrombocytopenia, vWD type 1, and vWD type 3. Overall, this multi-constituent model of thrombosis enables macro-scale 3D simulations of thrombus formation in complex geometries over a wide range of shear rates and accounts for qualitative and quantitative hemostatic deficiencies in patient blood.

Atherothrombosis-on-Chip: A Site-Specific Microfluidic Model for Thrombus Formation and Drug Discovery

Atherothrombosis, an atherosclerotic plaque disruption condition with superimposed thrombosis, is the underlying cause of cardiovascular episodes. Herein, a unique design is presented to develop a microfluidic site-specific atherothrombosis-on-chip model, providing a universal platform for studying the crosstalk between blood cells and plaque components. The device consists of two interconnected microchannels, namely main and supporting channels: the former mimics the vessel geometry with different stenosis, and the latter introduces plaque components to the circulation simultaneously. The unique design allows the site-specific introduction of plaque components in stenosed channels ranging from 0% to above 50%, resulting in thrombosis, which has not been achieved previously. The device successfully explains the correlation between vessel geometry and thrombus formation phenomenon as well as the influence of shear rate on platelet aggregation, confirming the reliability and the effectiveness of the design. The device exhibits significant sensitivity to aspirin. In therapeutic doses (50 × 10^-6 and 100 × 10^-6 m), aspirin delays and prevents platelet adhesion, thereby reducing the thrombus area in a dose-dependent manner. Finally, the device is effectively employed in testing the targeted binding of the RGD (arginyl-glycyl-aspartic acid) labeled polymeric nanoparticles on the thrombus, extending the use of the device to examine targeted drug carriers.

Polymer Coating Integrity, Thrombogenicity and Computational Fluid Dynamics Analysis of Provisional Stenting Technique in the Left Main Bifurcation Setting: Insights from an In-Vitro Model

Currently, the provisional stenting technique is the gold standard in revascularization of lesions located in the left main (LM) bifurcation. The benefit of the routine kissing balloon technique (KBI) in bifurcation lesions is still debated, particularly following the single stent treatment. We compared the latest-generation drug-eluting stent (DES) with no side branch (SB) dilatation “keep it open” technique (KIO) vs. KBI technique vs. bifurcation dedicated drug-eluting stent (BD-DES) implantation. In vitro testing was performed under a static condition in bifurcation silicone vessel models. All the devices were implanted in accordance with the manufacturers’ recommendations. As a result, computational fluid dynamics (CFD) analysis demonstrated a statistically higher area of high shear rate in the KIO group when compared to KBI. Likewise, the maximal shear rate was higher in number in the KIO group. Floating strut count based on the OCT imaging was significantly higher in KIO than in KBI and BD-DES. Furthermore, according to OTC analysis, the thrombus area was numerically higher in both KIO and KBI than in the BD-DES. Scanning electron microscopy (SEM) analysis shows the highest degree of strut coating damage in the KBI group. This model demonstrated significant differences in CFD analysis at SB ostia with and without KBI optimization in the LM setting. The adoption of KBI was related to a meaningful reduction of flow disturbances in conventional DES and achieved results similar to BD-DES.

Structure of shear-induced platelet aggregated clot formed in an in vitro arterial thrombosis model

The structure of occlusive arterial thrombi is described in this paper. Macroscopic thrombi are made from whole blood in a collagen-coated, large-scale stenosis with high shear flow conditions similar to an atherosclerotic artery. The millimeter-sized thrombi are harvested for histology and scanning electron microscopy. Histological images showed 3 distinctive structures of the thrombus. A) The upstream region showed string-like platelet aggregates growing out from the wall to protrude into the central lumen, while RBCs were trapped between the strings. The strings were >10x as long as they were wide and reached out to join the strings from the opposite wall. B) Near the apex, the platelet strings coalesced into a dense mass with microchannels that effectively occludes the lumen. C) In the expansion region, the thrombus ended abruptly with an annulus of free blood in the flow separation zone. Scanning electron microscopy showed dense clusters of spherical platelets upstream and downstream, with amorphous platelets in the occluded throat consistent with prior activation. The total clot is estimated to contain 1.23 billion platelets with pores on the order of 10-100 microns. The results reveal a complex structure of arterial thrombi that grow from their tips under high shear stress to bridge the 2.5 mm lumen quickly with VWF-platelet strings. The occlusion leaves many microchannels that allow some flow through the bulk of the thrombus. This architecture can create occlusion or hemostasis rapidly with minimal material, yet remain porous for potential delivery of lytic agents to the core of 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.

A two-fluid blood stasis model for false lumen thrombosis after type B dissection repair

The formation of thrombosis is a major concern in the false lumen (FL) for post-TEVAR (thoracic endovascular aortic repair) patients. Blood stasis is one of the key factors which lead to the formation of thrombosis in the arterial systems. This study proposed a computational model for blood stasis, using a two-fluid principle to track the locations of blood residual over time. The current study applied this novel model to evaluate blood stasis and thrombosis potential in four patient-specific post-TEVAR FLs of type B aortic dissection, with their follow-up in-vivo observations two years after TEVAR. The locations and topologies of residual blood in the FL predicted by the model agreed well with the in-vivo observations of thrombus. In addition, the results corresponded better with clinical observations in terms of interpatient comparison of degree of thrombosis, compared with conventional hemodynamic parameters. The blood stasis model serves as a valuable addition to conventional metrics to better predict thrombosis potential. Collectively, these metrics can provide an efficient non-invasive method for evaluating blood stasis and thrombosis potential in arterial system, and useful guidance for clinicians' operative planning and postoperative evaluation.

Method article: an in vitro blood flow model to advance the study of platelet adhesion utilizing a damaged endothelium

In vitro flow assays utilizing microfluidic devices are often used to study human platelets as an alternative to the costly animal models of hemostasis and thrombosis that may not accurately represent human platelet behavior in vivo. Here, we present a tunable in vitro model to study platelet behavior in human whole blood flow that includes both an inflamed, damaged endothelium and exposed extracellular matrix. We demonstrate that the model is adaptable across various anticoagulants, shear rates, and proteins for endothelial cell culture without the need for a complicated, custom-designed device. Furthermore, we verified the ability of this 'damaged endothelium' model as a screening method for potential anti-platelet or anti-thrombotic compounds using a P2Y₁₂ receptor antagonist (ticagrelor), a pan-selectin inhibitor (Bimosiamose), and a histamine receptor antagonist (Cimetidine). These compounds significantly decreased platelet adhesion to the damaged endothelium, highlighting that this model can successfully screen anti-platelet compounds that target platelets directly or the endothelium indirectly.

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.

Microstructure aware modeling of biochemical transport in arterial blood clots

Flow-mediated transport of biochemical species is central to thrombotic phenomena. Comprehensive three-dimensional modeling of flow-mediated transport around realistic macroscale thrombi poses challenges owing to their arbitrary heterogeneous microstructure. Here, we develop a microstructure aware model for species transport within and around a macroscale thrombus by devising a custom preconditioned fictitious domain formulation for thrombus-hemodynamics interactions, and coupling it with a fictitious domain advection-diffusion formulation for transport. Microstructural heterogeneities are accounted through a hybrid discrete particle-continuum approach for the thrombus interior. We present systematic numerical investigations on unsteady arterial flow within and around a three-dimensional macroscale thrombus; demonstrate the formation of coherent flow structures around the thrombus which organize advective transport; illustrate the role of the permeation processes at the thrombus boundary and subsequent intra-thrombus transport; and characterize species transport from bulk flow to the thrombus boundary and vice versa.

A Continuum Model for the Unfolding of von Willebrand Factor

von Willebrand Factor is a mechano-sensitive protein circulating in blood that mediates platelet adhesion to subendothelial collagen and platelet aggregation at high shear rates. Its hemostatic function and thrombogenic effect, as well as susceptibility to enzymatic cleavage, are regulated by a conformational change from a collapsed globular state to a stretched state. Therefore, it is essential to account for the conformation of the vWF multimers when modeling vWF-mediated thrombosis or vWF degradation. We introduce a continuum model of vWF unfolding that is developed within the framework of our multi-constituent model of platelet-mediated thrombosis. The model considers two interconvertible vWF species corresponding to the collapsed and stretched conformational states. vWF unfolding takes place via two regimes: tumbling in simple shear and strong unfolding in flows with dominant extensional component. These two regimes were demonstrated in a Couette flow between parallel plates and an extensional flow in a cross-slot geometry. The vWF unfolding model was then verified in several microfluidic systems designed for inducing high-shear vWF-mediated thrombosis and screening for von Willebrand Disease. The model predicted high concentration of stretched vWF in key regions where occlusive thrombosis was observed experimentally. Strong unfolding caused by the extensional flow was limited to the center axis or middle plane of the channels, whereas vWF unfolding near the channel walls relied upon the shear tumbling mechanism. The continuum model of vWF unfolding presented in this work can be employed in numerical simulations of vWF-mediated thrombosis or vWF degradation in complex geometries. However, extending the model to 3-D arbitrary flows and turbulent flows will pose considerable challenges.

Flow Dynamics and Mixing in Extracorporeal Support: A Study of the Return Cannula

Cannulation strategies in medical treatment such as in extracorporeal life support along with the associated cannula position, orientation and design, affects the mixing and the mechanical shear stress appearing in the flow field. This in turn influences platelet activation state and blood cell destruction. In this study, a co-flowing confined jet similar to a return cannula flow configuration found in extracorporeal membrane oxygenation was investigated experimentally. Cannula diameters, flow rate ratios between the jet and the co-flow and cannula position were studied using Particle Image Velocimetry and Planar Laser Induced Fluorescence. The jet was turbulent for all but two cases, in which a transitional regime was observed. The mixing, governed by flow entrainment, shear layer induced vortices and a backflow along the vessel wall, was found to require 9–12 cannula diameters to reach a fully homogeneous mixture. This can be compared to the 22–30 cannula diameters needed to obtain a fully developed flow. Although not significantly affecting mixing characteristics, cannula position altered the development of the flow structures, and hence the shear stress characteristics.

Haemodynamic flow conditions at the initiation of high-shear platelet aggregation: a combined in vitro and cellular in silico study

The influence of the flow environment on platelet aggregation is not fully understood in high-shear thrombosis. The objective of this study is to investigate the role of a high shear rate in initial platelet aggregation. The haemodynamic conditions in a microfluidic device are studied using cell-based blood flow simulations. The results are compared with in vitro platelet aggregation experiments performed with porcine whole blood (WB) and platelet-rich-plasma (PRP). We studied whether the cell-depleted layer in combination with high shear and high platelet flux can account for the distribution of platelet aggregates. High platelet fluxes at the wall were found in silico. In WB, the platelet flux was about twice as high as in PRP. Additionally, initial platelet aggregation and occlusion were observed in vitro in the stenotic region. In PRP, the position of the occlusive thrombus was located more downstream than in WB. Furthermore, the shear rates and stresses in cell-based and continuum simulations were studied. We found that a continuum simulation is a good approximation for PRP. For WB, it cannot predict the correct values near the wall.

Computational investigation of blood flow and flow-mediated transport in arterial thrombus neighborhood

A pathologically formed blood clot or thrombus is central to major cardiovascular diseases like heart attack and stroke. Detailed quantitative evaluation of flow and flow-mediated transport processes in the thrombus neighborhood within large artery hemodynamics is crucial for understanding disease progression and assessing treatment efficacy. This, however, remains a challenging task owing to the complexity of pulsatile viscous flow interactions with arbitrary shape and heterogeneous microstructure of realistic thrombi. Here, we address this challenge by conducting a systematic parametric simulation-based study on characterizing unsteady hemodynamics and flow-mediated transport in the neighborhood of an arterial thrombus. We use a hybrid particle-continuum-based finite element approach to handle arbitrary thrombus shape and microstructural variations. Results from a cohort of 50 different unsteady flow scenarios are presented, including unsteady vortical structures, pressure gradient across the thrombus boundary, finite time Lyapunov exponents, and dynamic coherent structures that organize advective transport. We clearly illustrate the combined influence of three key parameters-thrombus shape, microstructure, and extent of wall disease-in terms of: (a) determining hemodynamic features in the thrombus neighborhood and (b) governing the balance between advection, permeation, and diffusion to regulate transport processes in the thrombus neighborhood.

Establishment of an in vitro thrombogenicity test system with cyclic olefin copolymer substrate for endothelial layer formation

In vitro thrombogenicity test systems require co-cultivation of endothelial cells and platelets under blood flow-like conditions. Here, a commercially available perfusion system is explored using plasma-treated cyclic olefin copolymer (COC) as a substrate for the endothelial cell layer. COC was characterized prior to endothelialization and co-cultivation with platelets under static or flow conditions. COC exhibits a low roughness and a moderate hydrophilicity. Flow promoted endothelial cell growth and prevented platelet adherence. These findings show the suitability of COC as substrate and the importance of blood flow-like conditions for the assessment of the thrombogenic risk of drugs or cardiovascular implant materials.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1557/s43579-021-00072-6.

A two-phase flow approach for modeling blood stasis and estimating the thrombosis potential of a ventricular assist device

Thrombosis and its related events have become a major concern during the development and optimization of ventricular assist devices (VADs, also called blood pumps), and limit their clinical use and economic benefits. Attempts have been made to model the thrombosis formation, considering hemodynamic and biochemical processes. However, the complexities and computational expenses are prohibitive. Blood stasis is one of the key factors which may lead to the formation of thrombosis and excessive thromboembolic risks for patients. This study proposed a novel approach for modeling blood stasis, based on a two-phase flow principle. The locations of blood residual can be tracked over time, so that regions of blood stasis can be identified. The blood stasis in an axial blood pump is simulated under various working conditions, the results agree well with the experimental results. In contrast, conventional hemodynamic metrics such as velocity, time-averaged wall shear stress (TAWSS), and relative residence time (RRT), were contradictory in judging risk of blood stasis and thrombosis, and inconsistent with experimental results. We also found that the pump operating at the designed rotational speed is less prone to blood stasis. The model provides an efficient and fast alternative for evaluating blood stasis and thrombosis potential in blood pumps, and will be a valuable addition to the tools to support the design and improvement of VADs.

Comparison of overexpansion capabilities and thrombogenicity at the side branch ostia after implantation of four different drug eluting stents

Interventions in bifurcation lesions often requires aggressive overexpansion of stent diameter in the setting of long tapering vessel segment. Overhanging struts in front of the side branch (SB) ostium are thought to act as a focal point for thrombi formation and consequently possible stent thrombosis. This study aimed to evaluate the overexpansion capabilities and thrombogenicity at the SB ostia after implantation of four latest generation drug-eluting stents (DES) in an in-vitro bifurcation model. Four clinically available modern DES were utilized: one bifurcation dedicated DES (Bioss LIM C) and three conventional DES (Ultimaster, Xience Sierra, Biomime). All devices were implanted in bifurcation models with proximal optimization ensuring expansion before perfusing with porcine blood. Optical coherence tomography (OCT), immunofluorescence (IF) and scanning electron microscope analysis were done to determine thrombogenicity and polymer coating integrity at the over-expanded part of the stents. Computational fluid dynamics (CFD) was performed to study the flow disruption. OCT (p = 0.113) and IF analysis (p = 0.007) demonstrated lowest thrombus area at SB ostia in bifurcation dedicated DES with favorable biomechanical properties compared to conventional DES. The bifurcated DES also resulted in reduced area of high shear rate and maximum shear rate in the CFD analysis. This study demonstrated numerical differences in terms of mechanical properties and acute thrombogenicity at SB ostia between tested devices.

Biorheology of occlusive thrombi formation under high shear: in vitro growth and shrinkage

Occlusive thrombi formed under high flow shear rates develop very rapidly in arteries and may lead to myocardial infarction or stroke. Rapid platelet accumulation (RPA) and occlusion of platelet-rich thrombi and clot shrinkage have been studied after flow arrest. However, the influence of margination and shear rate on occlusive clot formation is not fully understood yet. In this study, the influence of flow on the growth and shrinkage of a clot is investigated. Whole blood (WB) and platelet-rich plasma (PRP) were perfused at high shear rates (> 3,000 s⁻¹) through two microfluidic systems with a stenotic section under constant pressure. The stenotic section of the two devices are different in stenotic length (1,000 vs 150 μm) and contraction angle of the stenosis (15° vs 80°). In all experiments, the flow chamber occluded in the stenotic section. Besides a significantly increased lag time and decreased RPA rate for PRP compared to WB (p < 0.01), the device with a shorter stenotic section and steeper contraction angle showed a shear-dependent occlusion and lag time for both PRP and WB. This shear-dependent behavior of the platelet aggregate formation might be caused by the stenotic geometry.

Shear rate gradients promote a bi-phasic thrombus formation on weak adhesive proteins, such as fibrinogen in a VWF-dependent manner

Blood flow profoundly varies throughout the vascular tree due to its pulsatile nature and to the complex vessel geometry. While thrombus formation has been extensively studied in vitro under steady flow, and in vivo under normal blood flow conditions, the impact of complex hemodynamics such as flow acceleration found in stenosed arteries has gained increased appreciation. We investigated the effect of flow acceleration, characterized by shear rate gradients, on the function of platelets adhering to fibrinogen, a plasma protein which plays a key role in hemostais and thrombosis. While we confirmed that under steady flow, fibrinogen only supports single platelet adhesion, we observed that under shear rate gradients, this surface becomes highly thrombogenic, supporting efficient platelet aggregation leading to occlusive thrombus formation. This shear rate gradient-driven thrombosis is biphasic with an initial step of slow platelet recruitment supported by direct plasma VWF adsorption to immobilized fibrinogen and followed by a second phase of explosive thrombosis initiated by VWF fiber formation on platelet monolayers. In vivo experiments confirmed that shear rate gradients accelerate thrombosis in a VWF-dependent manner. Together, this study characterizes a process of plasma VWF-dependent accelerated thrombosis on immobilized fibrinogen in the presence of shear rate gradients.

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.

In-vitro characterization of self-expandable textile transcatheter aortic valves

OBJECTIVE

This study aims at assessing the global dynamic behavior, closing energy and turbulence characteristics of self-expandable textile (inclined and straight yarn) transcatheter aortic valves (TAV) versus bioprosthetic TAVs.

METHODS

Two self-expandable textile TAVs one with inclined yarn textile and another with straight yarn textile leaflets were assessed in a pulse duplicator and compared with a self-expandable commercial bioprosthetic TAV under physiological pressure and flow. Particle Image Velocimetry and high-speed imaging were performed. Effective orifice areas (EOA), leakage fractions (LF), Pinwheeling indices (PI), closing energy (E), viscous shear stresses (VSS) and Reynolds shear stresses (RSS) were calculated.

RESULTS

(a) EOAs and LFs were 2.27 ± 0.03 cm2, 31.7 ± 0.6%; 2.25 ± 0.08 cm2, 26.6 ± 0.7%; and 1.63 ± 0.01 cm2, 29.1 ± 1.25% for inclined textile, bioprosthetic and straight textile TAV respectively (p < 0.0001). (b) Following same order, PIs were significantly different going from 1.16 ± 0.21%, 8.48 ± 0.8% and 8.865 ± 0.58% with the exception of CoreValve and straight yarn valve (p = 0.37); (c) E is lowest for straight textile TAV (0.0024 ± 0.0017 J), followed by bioprosthetic valve (0.00259 ± 0.0011 J) and then 45° Oriented Yarn Valve (0.00334 ± 0.03 J) (d) At peak systole, the highest RSS distribution was with the Straight textile TAV reaching up to 330Pa. The bioprosthetic TAV shows the smallest range with RSS reaching around 230Pa and the inclined textile TAV up to 280Pa. VSS limits were comparable among the 3 valves ranging between 5.2Pa and 5.7Pa.

CONCLUSION

Hemodynamic similarities were found between the textile self-expandable valves and the bioprosthetic valve. This study constitutes another step towards showing the potential that textile valves have to become an alternative for the biological ones.

Endothelial shear stress 5 years after implantation of a coronary bioresorbable scaffold

Aims

As a sine qua non for arterial wall physiology, local hemodynamic forces such as endothelial shear stress (ESS) may influence long-term vessel changes as bioabsorbable scaffolds dissolve. The aim of this study was to perform serial computational fluid dynamic (CFD) simulations to examine immediate and long-term haemodynamic and vascular changes following bioresorbable scaffold placement.

Methods and results

Coronary arterial models with long-term serial assessment (baseline and 5 years) were reconstructed through fusion of intravascular optical coherence tomography and angiography. Pulsatile non-Newtonian CFD simulations were performed to calculate the ESS and relative blood viscosity. Time-averaged, systolic, and diastolic results were compared between follow-ups. Seven patients (seven lesions) were included in this analysis. A marked heterogeneity in ESS and localised regions of high blood viscosity were observed post-implantation. Percent vessel area exposed to low averaged ESS (<1 Pa) significantly decreased over 5 years (15.92% vs. 4.99%, P < 0.0001) whereas moderate (1-7 Pa) and high ESS (>7 Pa) did not significantly change (moderate ESS: 76.93% vs. 80.7%, P = 0.546; high ESS: 7.15% vs. 14.31%, P = 0.281), leading to higher ESS at follow-up. A positive correlation was observed between baseline ESS and change in lumen area at 5 years (P < 0.0001). Maximum blood viscosity significantly decreased over 5 years (4.30 ± 1.54 vs. 3.21± 0.57, P = 0.028).

Conclusion

Immediately after scaffold implantation, coronary arteries demonstrate an alternans of extremely low and high ESS values and localized areas of high blood viscosity. These initial local haemodynamic disturbances may trigger fibrin deposition and thrombosis. Also, low ESS can promote neointimal hyperplasia, but may also contribute to appropriate scaffold healing with normalisation of ESS and reduction in peak blood viscosity by 5 years.

Occlusive thrombosis in arteries

Thrombus formation in major arteries is life threatening. In this review article, we discuss how an arterial thrombus can form under pathologically high shear stresses, with bonding rates estimated to be the fastest Kon values in biochemistry. During occlusive thrombosis in arteries, the growth rate of the thrombus explodes to capture a billion platelets in about 10 min. Close to 100% of all platelets passing the thrombus are captured by long von Willebrand factor (vWF) strands that quickly form tethered nets. The nets grow in patches where shear stress is high, and the local concentration of vWF is elevated due to α-granule release by previously captured platelets. This rapidly formed thrombus has few red blood cells and so has a white appearance and is much stronger and more porous than clots formed through coagulation. Understanding and modeling the biophysics of this event can predict totally new approaches to prevent and treat heart attacks and strokes.

Thrombus growth modelling and stenosis prediction in the cerebral microvasculature

Cerebral microvascular occlusions cause restriction of blood supply to the brain, thus potentially severely impacting cognitive abilities. Thus, accurate prediction of thrombus growth in realistic geometries is important. Thrombi growth in an existing 13-generation cerebral microvasculature network is simulated here to study the haemodynamic effects of single and multiple blockages on the occlusion of the network. Compared to a single vessel, in a network, the occlusion probability is found to be different. It is the downstream/smaller arterioles (i.e. the 3rd, 4th, 5th, 6th generation arterioles in this study) that tend to reach occlusion first in a network and thus are the critical vessels. Simulations of simultaneous growth of two independent thrombi in the network (referred to here as the two-block case) show a close coupling between the locations of the various blocks in the network, each influencing the other's growth. The presence of the lead block (LB) slows the growth of the trailing block (TB). In some cases, it stops the TB's growth thereby preventing it from occluding the vessel. Findings in this work thus indicate that, to prevent ischaemia, blocks in the smaller arterioles need to be identified and treated first, and that this is more critical if the number of simultaneous blocks is higher.

Hemocompatibility of Super-Repellent surfaces: Current and Future

Virtually all blood-contacting medical implants and devices initiate immunological events in the form of thrombosis and inflammation. Typically, patients receiving such implants are also given large doses of anticoagulants, which pose a high risk and a high cost to the patient. Thus, the design and development of surfaces with improved hemocompatibility and reduced dependence on anticoagulation treatments is paramount for the success of blood-contacting medical implants and devices. In the past decade, the hemocompatibility of super-repellent surfaces (i.e., surfaces that are extremely repellent to liquids) has been extensively investigated because such surfaces greatly reduce the blood-material contact area, which in turn reduces the area available for protein adsorption and blood cell or platelet adhesion, thereby offering the potential for improved hemocompatibility. In this review, we critically examine the progress made in characterizing the hemocompatibility of super-repellent surfaces, identify the unresolved challenges and highlight the opportunities for future research on developing medical implants and devices with super-repellent surfaces.

Coronary Shear Stress after Implantation of Bioresorbable Scaffolds – a Modern Interdisciplinary Concept at the Border between Interventional Cardiology and Cardiac Imaging

Bioresorbable scaffolds/stents offer new and exciting perspectives in the treatment of patients with acute coronary syndromes, especially after the recent development of invasive imaging techniques, such as optical coherence tomography, which allow complete assessment of vascu-lar segments. A particular advantage of bioresorbable scaffolds is that once the biosorption of the scaffold is complete, the vascular segment regains its normal physiological functions, thus eliminating the risk of late complications. New studies show the importance of shear stress in the progression of vascular atherosclerosis or in accelerating endothelial turnover. Based on the current knowledge in this field, a future standardized determination of shear stress may help in the long-term follow-up of patients that have suffered or are at risk of developing an acute coronary syndrome.

Coagulopathy implications using a multiscale model of traumatic bleeding matching macro- and microcirculation

Quantifying the relationship between vascular injury and the dynamic bleeding rate requires a multiscale model that accounts for changing and coupled hemodynamics between the global and microvascular levels. A lumped, global hemodynamic model of the human cardiovascular system with baroreflex control was coupled to a local 24-level bifurcating vascular network that spanned diameters from the muscular artery scale (0.1-1.3 mm) to capillaries (5-10 μm) via conservation of momentum and conservation of mass boundary conditions. For defined injuries of severing all vessels at each nth-level, the changing pressures and flowrates were calculated using prescribed shear-dependent hemostatic clot growth rates (normal or coagulopathic). Key results were as follows: 1) the upstream vascular network rapidly depressurizes to reduce blood loss; 2) wall shear rates at the hemorrhaging wound exit are sufficiently high (~10,000 s^-1) to drive von Willebrand factor unfolding; 3) full coagulopathy results in >2-liter blood loss in 2 h for severing all vessels of 0.13- to 0.005-mm diameter within the bifurcating network, whereas full hemostasis limits blood loss to <100 ml within 2 min; and 4) hemodilution from transcapillary refill increases blood loss and could be implicated in trauma-induced coagulopathy. A sensitivity analysis on length-to-diameter ratio and branching exponent demonstrated that bleeding was strongly dependent on these tissue-dependent network parameters. This is the first bleeding model that prescribes the geometry of the injury to calculate the rate of pressure-driven blood loss and local wall shear rate in the presence or absence of coagulopathic blood. NEW & NOTEWORTHY We developed a multiscale model that couples a lumped, global hemodynamic model of a patient to resolved, single-vessel wounds ranging from the small artery to capillary scale. The model is able to quantify wall shear rates, seal rates, and blood loss rates in the presence and absence of baroreflex control and hemodilution.

In vivo measurement of blood clot mechanics from computational fluid dynamics based on intravital microscopy images

Ischemia which leads to heart attacks and strokes is one of the major causes of death in the world. Whether an occlusion occurs or not depends on the ability of a growing thrombus to resist flow forces exerted on its structure. This manuscript provides the first known in vivo measurement of how much stress a clot can withstand, before yielding to the surrounding blood flow. Namely, Lattice-Boltzmann Method flow simulations are performed based on 3D clot geometries, which are estimated from intravital microscopy images of laser-induced injuries in cremaster microvasculature of live mice. In addition to reporting the blood clot yield stresses, we also show that the thrombus "core" does not experience significant deformation, while its "shell" does. This indicates that the shell is more prone to embolization. Therefore, drugs should be designed to target the shell selectively, while leaving the core intact to minimize excessive bleeding. Finally, we laid down a foundation for a nondimensionalization procedure which unraveled a relationship between clot mechanics and biology. Hence, the proposed framework could ultimately lead to a unified theory of thrombogenesis, capable of explaining all clotting events. Thus, the findings presented herein will be beneficial to the understanding and treatment of heart attacks, strokes and hemophilia.

Sinus Hemodynamics Variation with Tilted Transcatheter Aortic Valve Deployments

Leaflet thrombosis is a complication associated with transcatheter aortic valve (TAV) replacement (TAVR) correlated with sinus flow stasis. Sinus hemodynamics are important because they dictate shear stress and washout necessary to avoid stasis on TAV leaflets. Sinus flow is controlled by TAV axial deployment position but little is known regarding TAV axis misalignment effect. This study aims to elucidate TAV angular misalignment with respect to aortic root axis effect on sinus flow stasis potentially leading to leaflet thrombosis. Sinus hemodynamics were assessed in vitro using particle-image velocimetry in three different angular misalignments with respect to aorta axis: untilted, tilted away from the sinus and tilted towards sinus. A 26 mm Edwards SAPIEN3 was implanted in a 3D printed model of an anatomically realistic aortic root. TAV hemodynamics, sinus vortex tracking, leaflet shear stress probability density functions, and sinus blood time to washout were calculated. While pressure gradients differed insignificantly, blood velocity and vorticity decreased significantly in both tilted cases sinuses. Shear stress probability near the leaflet decreases with tilt indicating stasis. TAV tilted away from the sinus is the most unfavorable scenario with poor washout. TAV axial misalignment adds to factors list that could influence leaflet thrombosis risk through modifying sinus hemodynamics and washout.

Sinus Hemodynamics in Representative Stenotic Native Bicuspid and Tricuspid Aortic Valves: An In-Vitro Study

(1) The study’s objective is to assess sinus hemodynamics differences between stenotic native bicuspid aortic valve (BAV) and native tricuspid aortic valve (TrAV) sinuses in order to assess sinus flow shear and vorticity dynamics in these common pathological states of the aortic valve. (2) Representative patient-specific aortic roots with BAV and TrAV were selected, segmented, and 3D printed. The flow dynamics within the sinus were assessed in-vitro using particle image velocimetry in a left heart simulator at physiological pressure and flow conditions. Hemodynamic data calculations, vortex tracking, shear stress probability density functions and sinus washout calculations based on Lagrangian particle tracking were performed. (3) (a) At peak systole, velocity and vorticity in BAV reach 0.67 ± 0.02 m/s and 374 ± 5 s−1 versus 0.49 ± 0.03 m/s and 293 ± 3 s−1 in TrAV; (b) Aortic sinus vortex is slower to form but conserved in BAV sinus; (c) BAV shear stresses exceed those of TrAV (1.05 Pa versus 0.8 Pa); (d) Complete TrAV washout was achieved after 1.5 cycles while it was not for BAV. 4) In conclusion, sinus hemodynamics dependence on the different native aortic valve types and sinus morphologies was clearly highlighted in this study.

Past and future of blood damage modelling in a view of translational research

Anatomic pathologies such as stenosed or regurgitating heart valves and artificial organs such as heart assist devices or heart valve prostheses are associated with non-physiological flow. This regime is associated with regions of spatially high-velocity gradients, high-velocity and/or pressure fluctuations as well as neighbouring regions with stagnant flow associated with high residence time. These hemodynamic conditions cause destruction and/or activation of blood components and their accumulation in regions with high residence time. The development of next-generation artificial organs, which allow long-term patient care by reducing adverse events and improve quality of life, requires the development of blood damage models serving as a cost function for device optimization. We summarized the studies underlining the key findings with subsequent elaboration of the requirements for blood damage models as well as a decision tree based on the classification of existing blood damage models. The four major classes are Lagrangian or Eulerian approaches with stress- or strain-based blood damage. Key challenges were identified and future steps towards the translation of blood damage models into the device development pipeline were formulated. The integration of blood damage caused by turbulence into models as well as in vitro and in vivo validation of models remain the major challenges for future developments. Both require the development of novel experimental setups to provide reliable and well-documented experimental data.

Implantation Depth and Rotational Orientation Effect on Valve-in-Valve Hemodynamics and Sinus Flow

BACKGROUND

This study evaluated the effect of transcatheter aortic valve implantation depth and rotation on pressure gradient (PG), leakage fractions (LF), leaflet shear stress, and sinus washout in an effort to understand factors that may dictate optimal positioning for valve-in-valve (ViV) procedures. Sinus flow stasis is often associated with prosthetic leaflet thrombosis. Although recent ViV in vitro studies highlighted potential benefits of transcatheter aortic valve supraannular implantation to minimize PGs, the relationship between transcatheter aortic valve depth and other determinates of valve function remains unknown. Among these, LFs, shear stress, and poor sinus washout have been associated with poorer valve outcomes.

METHODS

ViV hemodynamic performance was evaluated in vitro vs axial positions -9.8, -6.2, 0, and +6 mm and angular orientations 0, 30, 60, and 90 degrees in a degenerated surgical aortic valve. PGs, LFs, and sinus shear stress and washout were compared. Leaflet high-speed imaging and particle-image velocimetry were performed to elucidate hemodynamic mechanisms.

RESULTS

(1) The PG varies as a function of axial position, with supraannular deployments yielding a maximum benefit of 7.85 mm Hg less than PGs for subannular deployments irrespective of commissural alignment (p < 0.01); (2) in contrast, LF decreased in relationship to subannular deployment; and (3) at peak systole, sinus flow shear stress increased with deployment depth as did sinus washout with and without coronary flow.

CONCLUSIONS

First, supraannular axial deployment is associated with lower PGs irrespective of commissural alignment. Second, subannular deployment is associated with more favorable sinus hemodynamics and less LF. Further in vivo studies are needed to substantiate these observations and facilitate optimal prosthesis positioning during ViV procedures.

Impact of patient-specific morphologies on sinus flow stasis in transcatheter aortic valve replacement: An in vitro study

OBJECTIVE

The goal of this study is to evaluate how sinus flow patterns after transcatheter aortic valve replacement in realistic representative patient roots vary. Sinus flow can affect transcatheter aortic valve operation and likely leaflet thrombosis occurrence due to stasis and poor washout. How the interaction between transcatheter aortic valve and representative patient aortic roots affects sinus hemodynamics is important to establish for future individualization of transcatheter aortic valve replacement therapy.

METHODS

Two representative patient aortic roots were selected, segmented and 3-dimensional printed followed by deployment of Medtronic CoreValve (Medtronic Inc, Irvine, Calif) and Edwards SAPIEN (Edwards Lifesciences, Irvine Calif) transcatheter aortic valves. Sinus hemodynamics were assessed in vitro using high spatio-temporal resolution particle-image-velocimetry. Detailed sinus vortex tracking, shear stress probability density functions, and sinus washout were evaluated and assessed as a function of valve type and representative patient morphology as independent case studies.

RESULTS

Peak velocity in the sinus with SAPIEN valve was approximately 3 times higher than with CoreValve for both models (0.30 ± 0.02 m/s and 0.34 ± 0.041 m/s vs 0.13 ± 0.01 m/s and 0.10 ± 0.02 m/s) (P < .01). Between representative patient models, vorticity magnitudes were significantly different (75 ± 1.1 s^-1, 77 ± 3.2 s^-1, 109 ± 2.3 s^-1, and 250 ± 4.1 s^-1) (P < .01) regardless of valve type. Sinus blood washout characteristic as a function of cardiac cycles was strongly both patient related and valve specific. Fluid dynamics favored shear stresses and washout characteristics due to a smaller sinus and sinotubular junction, further amplified by the SAPIEN valve.

CONCLUSIONS

Sinus flow dynamics are highly sensitive to aortic root characteristics and transcatheter aortic valve aortic root interaction. Differences in sinus-flow washout and stasis regions between representative patient models may be reflected in different risks of leaflet thrombosis or valve degeneration.

Computational Assessment of the Relation Between Embolism Source and Embolus Distribution to the Circle of Willis for Improved Understanding of Stroke Etiology

Stroke caused by an embolism accounts for about a third of all stroke cases. Understanding the source and cause of the embolism is critical for diagnosis and long-term treatment of such stroke cases. The complex nature of the transport of an embolus within large arteries is a primary hindrance to a clear understanding of embolic stroke etiology. Recent advances in medical image-based computational hemodynamics modeling have rendered increasing utility to such techniques as a probe into the complex flow and transport phenomena in large arteries. In this work, we present a novel, patient-specific, computational framework for understanding embolic stroke etiology, by combining image-based hemodynamics with discrete particle dynamics and a sampling-based analysis. The framework allows us to explore the important question of how embolism source manifests itself in embolus distribution across the various major cerebral arteries. Our investigations illustrate prominent numerical evidence regarding (i) the size/inertia-dependent trends in embolus distribution to the brain; (ii) the relative distribution of cardiogenic versus aortogenic emboli among the anterior, middle, and posterior cerebral arteries; (iii) the left versus right brain preference in cardio-emboli and aortic-emboli transport; and (iv) the source-destination relationship for embolisms affecting the brain.

Local Hemodynamic Forces After Stenting: Implications on Restenosis and Thrombosis

Local hemodynamic forces are well-known to modulate atherosclerotic evolution, which remains one of the largest cause of death worldwide. Percutaneous coronary interventions with stent implantation restores blood flow to the downstream myocardium and is only limited by stent failure caused by restenosis, stent thrombosis, or neoatherosclerosis. Cumulative evidence has shown that local hemodynamic forces affect restenosis and the platelet activation process, modulating the pathophysiological mechanisms that lead to stent failure. This article first covers the pathophysiological mechanisms through which wall shear stress regulates arterial disease formation/neointima proliferation and the role of shear rate on stent thrombosis. Subsequently, the article reviews the current evidence on (1) the implications of stent design on the local hemodynamic forces, and (2) how stent/scaffold expansion can influence local flow, thereby affecting the risk of adverse events.

Predicting false lumen thrombosis in patient-specific models of aortic dissection

Aortic dissection causes splitting of the aortic wall layers, allowing blood to enter a ‘false lumen’ (FL). For type B dissection, a significant predictor of patient outcomes is patency or thrombosis of the FL. Yet, no methods are currently available to assess the chances of FL thrombosis. In this study, we present a new computational model that is capable of predicting thrombus formation, growth and its effects on blood flow under physiological conditions. Predictions of thrombus formation and growth are based on fluid shear rate, residence time and platelet distribution, which are evaluated through convection–diffusion–reaction transport equations. The model is applied to a patient-specific type B dissection for which multiple follow-up scans are available. The predicted thrombus formation and growth patterns are in good qualitative agreement with clinical data, demonstrating the potential applicability of the model in predicting FL thrombosis for individual patients. Our results show that the extent and location of thrombosis are strongly influenced by aortic dissection geometry that may change over time. The high computational efficiency of our model makes it feasible for clinical applications. By predicting which aortic dissection patient is more likely to develop FL thrombosis, the model has great potential to be used as part of a clinical decision-making tool to assess the need for early endovascular intervention for individual dissection patients.

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.