Flow-induced Platelet Activation Research Articles

A predictive multiscale model for simulating flow-induced platelet activation: Correlating in silico results with in vitro results

Flow-induced platelet activation prompts complex filopodial formation. Continuum methods fail to capture such molecular-scale mechanisms. A multiscale numerical model was developed to simulate this activation process, where a Dissipative Particle Dynamics (DPD) model of viscous blood flow is interfaced with a Coarse Grained Molecular Dynamics (CGMD) platelet model. Embedded in DPD blood flow, the macroscopic dynamic stresses are interactively transferred to the CGMD model, inducing intra-platelet associated events. The platelets activate by a biomechanical transductive linkage chain and dynamically change their shape in response. The models are fully coupled via a hybrid-potential interface and multiple time-stepping (MTS) schemes for handling the disparity between the spatiotemporal scales. Cumulative hemodynamic stresses that may lead to platelet activation are mapped on the surface membrane and simultaneously transmitted to the cytoplasm and cytoskeleton. Upon activation, the flowing platelets lose their quiescent discoid shape and evolve by forming filopodia.The model predictions were validated by a set of in vitro experiments, Platelets were exposed to various combinations of shear stresses and durations in our programmable hemodynamic shearing device (HSD). Their shape change was measured at multiple time points using scanning electron microscopy (SEM). The CGMD model parameters were fine-tuned by interrogating a parameter space established in these experiments. Segmentation of the SEM imaging streams was conducted by a deep machine learning system. This model can be further employed to simulate shear mediated platelet activation thrombosis initiation and to study the effects of modulating platelet properties to enhance their shear resistance via mechanotransduction pathways.

Flow-induced platelet activation in components of the extracorporeal membrane oxygenation circuit

Extracorporeal membrane oxygenation (ECMO) is used for rescue in severe respiratory and/or circulatory failure. The patient’s blood is pumped over artificial surfaces in the ECMO circuit. A platelet activation model was applied to study the potential thrombogenicity of ECMO circuit components: the centrifugal blood pump, cannulae, and tubing connectors. Based on the accumulated effect of the scalar form of the stress acting on the platelet over time, the activation model enables assessment of platelet activation and pinpoints regions of elevated activation risk in a component. Numerical simulations of the flow in different components of the ECMO circuit was carried out where the activation level is a function of the impact of local stress and its history along the path that the platelets follow. The results showed that the pump carried the largest risk for platelet activation followed by the reinfusion cannula and lastly the connectors. Pump thrombogenicity was mainly due to long residence time and high shear-rate while the connector showed a high level of non-stationary shear-rate that in turn may contribute to the formation of aggregates through direct platelet activation or through high shear-rate modulation of the vWF multimers.

Multiscale Particle-Based Modeling of Flowing Platelets in Blood Plasma Using Dissipative Particle Dynamics and Coarse Grained Molecular Dynamics.

We developed a multiscale particle-based model of platelets, to study the transport dynamics of shear stresses between the surrounding fluid and the platelet membrane. This model facilitates a more accurate prediction of the activation potential of platelets by viscous shear stresses - one of the major mechanisms leading to thrombus formation in cardiovascular diseases and in prosthetic cardiovascular devices. The interface of the model couples coarse-grained molecular dynamics (CGMD) with dissipative particle dynamics (DPD). The CGMD handles individual platelets while the DPD models the macroscopic transport of blood plasma in vessels. A hybrid force field is formulated for establishing a functional interface between the platelet membrane and the surrounding fluid, in which the microstructural changes of platelets may respond to the extracellular viscous shear stresses transferred to them. The interaction between the two systems preserves dynamic properties of the flowing platelets, such as the flipping motion. Using this multiscale particle-based approach, we have further studied the effects of the platelet elastic modulus by comparing the action of the flow-induced shear stresses on rigid and deformable platelet models. The results indicate that neglecting the platelet deformability may overestimate the stress on the platelet membrane, which in turn may lead to erroneous predictions of the platelet activation under viscous shear flow conditions. This particle-based fluid-structure interaction multiscale model offers for the first time a computationally feasible approach for simulating deformable platelets interacting with viscous blood flow, aimed at predicting flow induced platelet activation by using a highly resolved mapping of the stress distribution on the platelet membrane under dynamic flow conditions.

Multiscale Modeling of Flow Induced Thrombogenicity With Dissipative Particle Dynamics and Molecular Dynamics.

The coagulation cascade of blood may be initiated by flow induced platelet activation, which prompts clot formation in prosthetic cardiovascular devices and arterial disease processes. While platelet activation may be induced by biochemical agonists, shear stresses arising from pathological flow patterns enhance the propensity of platelets to activate and initiate the intrinsic pathway of coagulation, leading to thrombosis. Upon activation platelets undergo complex biochemical and morphological changes: organelles are centralized, membrane glycoproteins undergo conformational changes, and adhesive pseudopods are extended. Activated platelets polymerize fibrinogen into a fibrin network that enmeshes red blood cells. Activated platelets also cross-talk and aggregate to form thrombi. Current numerical simulations to model this complex process mostly treat blood as a continuum and solve the Navier-Stokes equations governing blood flow, coupled with diffusion-convection-reaction equations. It requires various complex constitutive relations or simplifying assumptions, and is limited to μm level scales. However, molecular mechanisms governing platelet shape change upon activation and their effect on rheological properties can be in the nm level scales. To address this challenge, a multiscale approach which departs from continuum approaches, may offer an effective means to bridge the gap between macroscopic flow and cellular scales. Molecular dynamics (MD) and dissipative particle dynamics (DPD) methods have been employed in recent years to simulate complex processes at the molecular scales, and various viscous fluids at low-to-high Reynolds numbers at mesoscopic scales. Such particle methods possess important properties at the mesoscopic scale: complex fluids with heterogeneous particles can be modeled, allowing the simulation of processes which are otherwise very difficult to solve by continuum approaches. It is becoming a powerful tool for simulating complex blood flow, red blood cells interactions, and platelet-mediated thrombosis involving platelet activation, aggregation, and adhesion.Copyright © 2013 by ASME

A novel mathematical model of activation and sensitization of platelets subjected to dynamic stress histories

Blood recirculating devices, such as ventricular assist devices and prosthetic heart valves, are burdened by thromboembolic complications requiring complex and lifelong anticoagulant therapy with its inherent hemorrhagic risks. Pathologic flow patterns occurring in such devices chronically activate platelets, and the optimization of their thrombogenic performance requires the development of flow-induced platelet activation models. However, existing models are based on empirical correlations using the well-established power law paradigm of constant levels of shear stress during certain exposure times as factors for mechanical platelet activation. These models are limited by their range of application and do not account for other relevant phenomena, such as loading rate dependence and platelet sensitization to high stress conditions, which characterize the dynamic flow conditions in devices. These limitations were addressed by developing a new class of phenomenological stress-induced platelet activation models that specifies the rate of platelet activation as a function of the entire stress history and results in a differential equation that can be directly integrated to calculate the cumulative levels of activation. The proposed model reverts to the power law under constant shear stress conditions and is able to describe experimental results in response to a diverse range of highly dynamic stress conditions found in blood recirculating devices. The model was tested in vitro under emulated device flow conditions and correlates well with experimental results. This new model provides a reliable and robust mathematical tool that can be incorporated into computational fluid dynamic studies in order to optimize design, with the goal of improving the thrombogenic performance of blood recirculating devices.

Computational evaluation of the thrombogenic potential of a hollow-fiber oxygenator with integrated heat exchanger during extracorporeal circulation

The onset of thromboembolic phenomena in blood oxygenators, even in the presence of adequate anticoagulant strategies, is a relevant concern during extracorporeal circulation (ECC). For this reason, the evaluation of the thrombogenic potential associated with extracorporeal membrane oxygenators should play a critical role into the preclinical design process of these devices. This study extends the use of computational fluid dynamics simulations to guide the hemodynamic design optimization of oxygenators and evaluate their thrombogenic potential during ECC. The computational analysis accounted for both macro- (i.e., vortex formation) and micro-scale (i.e., flow-induced platelet activation) phenomena affecting the performances of a hollow-fiber membrane oxygenator with integrated heat exchanger. A multiscale Lagrangian approach was adopted to infer the trajectory and loading history experienced by platelet-like particles in the entire device and in a repetitive subunit of the fiber bundles. The loading history was incorporated into a damage accumulation model in order to estimate the platelet activation state (PAS) associated with repeated passes of the blood within the device. Our results highlighted the presence of blood stagnation areas in the inlet section that significantly increased the platelet activation levels in particles remaining trapped in this region. The order of magnitude of PAS in the device was the same as the one calculated for the components of the ECC tubing system, chosen as a term of comparison for their extensive diffusion. Interpolating the mean PAS values with respect to the number of passes, we obtained a straightforward prediction of the thrombogenic potential as a function of the duration of ECC.

Quantifying the effects of shear stress and shear exposure duration regulation on flow induced platelet activation and aggregation

A heightened flow shear stress magnitude or increased exposure duration at any shear stress magnitude enhances platelet activation. It has been suggested that increased circulating activated platelets are an indication of enhanced cardiovascular risk and that activated platelets can perpetuate cardiovascular diseases. However, the combined effects that shear stress and shear exposure duration have on platelet activation and aggregation are not clear. Our objective was to identify a new parameter ("shear stress-exposure time") to predict platelet functional changes better than the absolute magnitude of shear stress. Platelets were exposed to shear stress waveforms, with different combinations of shear stress magnitude and exposure duration, in a circulation flow loop. Timed samples were removed from the flow loop to quantify thrombogenicity (modified prothrombinase assay), aggregation potential (optical platelet aggregometry), and cell surface marker expression (flow cytometry). Flow induced platelet activation and aggregation was enhanced with increased shear stress-exposure time. Platelets that were exposed to waveforms with the same shear stress-exposure time had the same thrombogenicity and aggregation potential. This was true even for waveforms that exposed platelets to shear stress magnitudes varying from 4 to 40 dynes/cm(2). These results indicate that shear stress-exposure time is a good predictor of platelet activation and aggregation levels and it may be able to predict the likelihood of cardiovascular disease onset.

Dose effect of shear stress on platelet complement activation

Altered blood shear stress is one of the risk factors that can induce platelet activation, thrombosis, atherosclerosis and many other cardiovascular diseases. Complement activation, an important contributor in inflammation, has also been reported to be related to blood flow induced platelet activation. In this study, the dose effect of shear stress (magnitude and shear exposure time) on platelet surface complement activation was investigated, using a dynamic cone and plate shearing device. Washed platelets were exposed to uniformly distributed shear stresses at 2.4 or 9 dyne/cm2 for 30 min. Timed samples at 0, 10, 20 and 30 min were taken and assayed for complement activation (C4d, C3b and C5b‐9 deposition) using a solid phase ELISA approach. In parallel, platelet activation was examined using flow cytometry for CD62P expression, and a modified prothrombinase assay for thrombin generation. Results indicate that activated platelets support complement activation to completion, with the production of membrane attack complex (C5b‐9). The generation of complement activation products, C4d, C3b and C5b‐9, increased significantly as shear stress magnitude and shear exposure time increased, correlating to platelet activation levels. Therefore, platelet activation and its associated complement activation may interact and promote cardiovascular disease development under pathological shear conditions.

Dose Effect of Shear Stress on Platelet Complement Activation in a Cone and Plate Shearing Device

Altered blood shear stress is one of the primary risk factors that can induce platelet activation, thrombosis, atherosclerosis, and many other cardiovascular diseases. Complement activation, an important contributor in inflammation, has also been reported to be related to blood flow-induced platelet activation. In this study, the dose effect of shear stress (magnitude and shear exposure time) on platelet surface complement activation was investigated using a dynamic cone and plate shearing device. Washed platelets were exposed to uniformly distributed shear stresses at 2.4 or 9 dyne/cm2 for 30 min. Timed samples at 0, 10, 20, and 30 min were taken and assayed for complement activation (C4d and C5b-9 deposition) using a solid-phase ELISA approach. In parallel, platelet activation was examined using flow cytometry for CD62P expression, and a modified prothrombinase assay for thrombin generation. Results indicated that activated platelets support complement activation to completion with the production of membrane attack complex C5b-9. The generation of complement activation products, C4d and C5b-9, increased significantly as shear stress magnitude and shear exposure time increased, correlating with the enhanced platelet activation levels. Therefore, platelet activation and its associated complement activation may interact and promote cardiovascular disease development under pathological shear conditions.

Glycated albumin modulates platelet susceptibility to flow induced activation and aggregation

A diabetic vasculature promotes cardiovascular diseases via endothelial cell activation induced by advanced glycation end products. It has recently become clear that activated platelets are a hallmark of cardiovascular disease and diabetes progression, by initiating and/or perpetuating the endothelial cell response. However, the role that platelets play in diabetic cardiovascular diseases remains elusive. Our objective was to evaluate the effects of glycated serum albumin on flow induced platelet activation and platelet aggregation. Albumin was glycated for up to 8 weeks. Timed samples of glycated or non-glycated albumin were removed to determine the effects of the extent of glycation on platelet functional changes. Thrombin receptor agonist peptide 6 (TRAP6, residues 42-47 of the thrombin receptor) and collagen I induced platelet aggregation was measured as a time course of glycated albumin incubation. The thrombogenicity of platelets incubated with glycated albumin was also measured under static and dynamic flow conditions using the modified prothrombinase assay. CD41 and CD62P expression was examined using flow cytometry to validate aggregation and activation studies. Platelets subjected to glycated albumin were more susceptible to TRAP6- and collagen-induced aggregation and flow induced activation. The extent of albumin glycation modulates these changes. As the albumin glycation time increased, this enhancement in platelet function was more pronounced. These results indicate that under diabetic conditions activated platelets may act to promote cardiovascular disease progression.

Platelet Activation Due to Hemodynamic Shear Stresses: Damage Accumulation Model and Comparison to In Vitro Measurements

The need to optimize the thrombogenic performance of blood recirculating cardiovascular devices, e.g., prosthetic heart valves (PHV) and ventricular assist devices (VAD), is accentuated by the fact that most of them require lifelong anticoagulation therapy that does not eliminate the risk of thromboembolic complications. The formation of thromboemboli in the flow field of these devices is potentiated by contact with foreign surfaces and regional flow phenomena that stimulate blood clotting, especially platelets. With the lack of appropriate methodology, device manufacturers do not specifically optimize for thrombogenic performance. Such optimization can be facilitated by formulating a robust numerical methodology with predictive capabilities of flow-induced platelet activation. In this study, a phenomenological model for platelet cumulative damage, identified by means of genetic algorithms (GAs), was correlated with in vitro experiments conducted in a Hemodynamic Shearing Device (HSD). Platelets were uniformly exposed to flow shear representing the lower end of the stress levels encountered in devices, and platelet activity state (PAS) was measured in response to six dynamic shear stress waveforms representing repeated passages through a device, and correlated to the predictions of the damage accumulation model. Experimental results demonstrated an increase in PAS with a decrease in "relaxation" time between pulses. The model predictions were in very good agreement with the experimental results.

DPIV Prediction of Flow Induced Platelet Activation—Comparison to Numerical Predictions

Flow induced platelet activation (PA) can lead to platelet aggregation, deposition onto the blood vessel wall, and thrombus formation. PA was thoroughly studied under unidirectional flow conditions. However, in regions of complex flow, where the platelet is exposed to varying levels of shear stress for varying durations, the relationship between flow and PA is not well understood. Numerical models were developed for studying flow induced PA resulting from stress histories along Lagrangian trajectories in the flow field. However, experimental validation techniques such as Digital Particle Image Velocimetry (DPIV) were not extended to include such models. In this study, a general experimental tool for PA analysis by means of continuous DPIV was utilized and compared to numerical simulation in a model of coronary stenosis. A scaled up (5:1) 84% eccentric and axisymetric coronary stenosis model was used for analysis of shear stress and exposure time along particle trajectories. Flow induced PA was measured using the PA State (PAS) assay. An algorithm for computing the PA level in pertinent trajectories was developed as a tool for extracting information from DPIV measurements for predicting the flow induced thrombogenic potential. CFD, DPIV and PAS assay results agreed well in predicting the level of PA. In addition, the same trend predicted by the DPIV was measured in vitro using the Platelet Activity State (PAS) assay, namely, that the symmetric stenosis activated the platelets more as compared to the eccentric stenosis.

PREDICTION OF OXYGENATION, HEMOLYSIS AND THROMBOSIS IN BLOOD MEMBRANE OXYGENATORS USING COMPUTATIONAL FLUID DYNAMICS

The fundamental engineering problems to develop a functional and biocompatible blood oxygenating device involve optimizing the flow path to efficiently transfer gas between blood and gas phases, to reduce pressure drop, and to minimize flow-induced blood trauma and thrombosis. All of them are closely related to the local fluid dynamics. It is difficult to experimentally investigate the effects of fiber buddle and device structure on localized blood flow, gas transfer and biocompatibility due to their complex structure of contemporary hollow fiber membrane oxygenators. To obviate these difficulties, we developed computational fluid dynamics (CFD) based mathematical models to predict gas transfer, shear-induced hemolysis and thrombosis in hollow fiber membrane blood oxygenators. The gas transfer model includes transport between blood and gas phases, oxygen binding to hemoglobin and reaction with blood. The hemolysis model is based on an Eulerian approach to consider both the shear stress and exposure time as the primary factors. These models were tested by comparing computationally predicted results with experimentally measured data. They are in excellent agreement. Our on-going activities will be including flow-induced platelet activation and deposition modeling. The advantage of CFD modeling is the ability to optimize the expended design to avoid the time-consuming and costly process of trial and error. By incorporating gas transport, blood damage, and thrombosis models, it will be a very attractive design paradigm to optimize the design of blood contacting devices.