Shear-mediated platelet activation in the free flow: Perspectives on the emerging spectrum of cell mechanobiological mechanisms mediating cardiovascular implant thrombosis

Shear Stress Promotes Remodeling of Platelet Glycosylation via Upregulation of Platelet Glycosidase Activity: One More Thing

BACKGROUND

 Mechanical circulatory support (MCS) is a mainstay of therapy for advanced and end-stage heart failure. Accompanied by systemic anticoagulation, contemporary MCS has become less thrombogenic, with bleeding complications emerging as a major cause of readmission and 1-year mortality. Shear-mediated platelet dysfunction and thrombocytopenia of undefined etiology are primary drivers of MCS-related bleeding. Recently, it has been demonstrated that deprivation of platelet surface glycosylation is associated with the decline of hemostatic function, microvesiculation, and premature apoptosis. We test the hypothesis that shear stress induces remodeling of platelet surface glycosylation via upregulation of glycosidase activity, thus facilitating platelet count decline and intense microvesiculation.

METHODS

 Human gel-filtered platelets were exposed to continuous shear stress in vitro. Platelets and platelet-derived microparticles (PDMPs) were quantified via flow cytometry using size standard fluorescent nanobeads. Platelet surface glycosylation and NEU1 expression were evaluated using lectin- or immune-staining and multicolor flow cytometry; lectin blotting was utilized to verify glycosylation of individual glycoproteins. Platelet neuraminidase, galactosidase, hexosaminidase, and mannosidase activities were quantified using 4-methylumbelliferone-based fluorogenic substrates.

RESULTS

 We demonstrate that shear stress promotes selective remodeling of platelet glycosylation via downregulation of 2,6-sialylation, terminal galactose, and mannose, while 2,3-sialylation remains largely unchanged. Shear-mediated deglycosylation is partially attenuated by neuraminidase inhibitors, strongly suggesting the involvement of platelet neuraminidase in observed phenomena. Shear stress increases platelet NEU1 surface expression and potentiates generation of numerous NEU1+ PDMPs. Platelets exhibit high basal hexosaminidase and mannosidase activities; basal activities of platelet neuraminidase and galactosidase are rather low and are significantly upregulated by shear stress. Shear stress of increased magnitude and duration promotes an incremental decline of platelet count and immense microvesiculation, both being further exacerbated by neuraminidase and partially attenuated by neuraminidase inhibition.

CONCLUSION

 Our data indicate that shear stress accumulation, consistent with supraphysiologic conditions of device-supported circulation, promotes remodeling of platelet glycosylation via selective upregulation of platelet glycosidase activity. Shear-mediated platelet deglycosylation is associated with platelet count drop and increased microvesiculation, thus offering a direct link between deglycosylation and thrombocytopenia observed in device-supported patients. Based on our findings, we propose a panel of molecular markers to be used for reliable detection of shear-mediated platelet deglycosylation in MCS.

The impact of the Impeller's hub design on the performance and blood damage in a microaxial mechanical circulatory support device - A numerical study

This study uses CFD methods to investigate the effects of the impeller's geometry on the hemodynamic characteristics, pump performance, and blood damage parameters, in a percutaneous microaxial Mechanical Circulatory Support (MCS) device. The numerical simulations employ the steady state Reynolds-Averaged Navier-Stokes approximation using the SST k-ω turbulent model. Three different impeller models are examined with different hub conversion angles (α = 0○, 3○ and 5○). The analysis includes 23 cases for different pressure heads (Δp = 60-80 mmHg) and angular velocities (ω = 30-52 kRPM). The obtained flow rate is compared between the cases to assess the effect of the impeller's design and working conditions on the pump performance. The comparative risk of shear-induced platelet activation is estimated using the statistical median of the stress-accumulation values calculated along streamlines. The risk of hemolysis is estimated using the average exposure time to shear stress above a threshold (τ > 425 Pa). The results reveal that the shape of the impeller's hub has a great impact on the flow patterns, performance, and risk of blood damage, as well as the angular velocity. The highest flow rate (Q = 3.7 L/min) and efficiency (η = 11.3 %) were achieved using a straight hub (α = 0○). Similarly, for the same condition of flow and pressure, the straight hub impeller has the lowest blood damage risk parameters. This study shed light on the effect of pump design on the performance and risk of blood damage, indicating the roles of the hub shape and angular velocity as dominant parameters.

Investigation of the role of von Willebrand factor in shear-induced platelet activation and functional alteration under high non-physiological shear stress

BACKGROUND

von Willebrand factor (vWF) plays a crucial role in physiological hemostasis through platelet and subendothelial collagen adhesion. However, its role in shear-induced platelet activation and functional alteration under non-physiological conditions common to blood-contacting medical devices (BCMDs) is not well investigated.

METHODS

Fresh healthy human blood was treated with an anti-vWF antibody to block vWF-GPIbα interaction. Untreated blood was used as a control. They were exposed to three levels of non-physiological shear stress (NPSS) (75, 125, and 175 Pa) through a shearing device with an exposure time of 0.5 s to mimic typical shear conditions in BCMDs. Flow cytometric assays were used to measure the expression levels of PAC-1 and P-Selectin and platelet aggregates for platelet activation and the expression levels of GPIbα, GPIIb/IIIa, and GPVI for receptor shedding. Collagen/ristocetin-induced platelet aggregation capacity was characterized by aggregometry.

RESULTS

The levels of platelet activation and aggregates increased with increasing NPSS in the untreated blood. More receptors were lost with increasing NPSS, resulting in a decreased capacity of collagen/ristocetin-induced platelet aggregation. In contrast, the increase in platelet activation and aggregates after exposure to NPSS, even at the highest level of NPSS, was significantly lower in treated blood. Nevertheless, there was no notable difference in receptor shedding, especially for GPIIb/IIIa and GPVI, between the two blood groups at the same level of NPSS. The block of vWF exacerbated the decreased capacity of collagen/ristocetin-induced platelet aggregation.

CONCLUSIONS

High NPSS activates platelets mainly by enhancing the vWF-GPIbα interaction. Platelet activation and receptor shedding induced by high NPSS likely occur through different pathways.

Platelet Biorheology and Mechanobiology in Thrombosis and Hemostasis: Perspectives from Multiscale Computation

Thrombosis is the pathological clot formation under abnormal hemodynamic conditions, which can result in vascular obstruction, causing ischemic strokes and myocardial infarction. Thrombus growth under moderate to low shear (<1000 s-1) relies on platelet activation and coagulation. Thrombosis at elevated high shear rates (>10,000 s-1) is predominantly driven by unactivated platelet binding and aggregating mediated by von Willebrand factor (VWF), while platelet activation and coagulation are secondary in supporting and reinforcing the thrombus. Given the molecular and cellular level information it can access, multiscale computational modeling informed by biology can provide new pathophysiological mechanisms that are otherwise not accessible experimentally, holding promise for novel first-principle-based therapeutics. In this review, we summarize the key aspects of platelet biorheology and mechanobiology, focusing on the molecular and cellular scale events and how they build up to thrombosis through platelet adhesion and aggregation in the presence or absence of platelet activation. In particular, we highlight recent advancements in multiscale modeling of platelet biorheology and mechanobiology and how they can lead to the better prediction and quantification of thrombus formation, exemplifying the exciting paradigm of digital medicine.

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.

Incorporating Unresolved Stresses in Blood Damage Modeling: Energy Dissipation More Accurate Than Reynolds Stress Formulation

OBJECTIVE

Reynolds Averaged Navier Stokes (RANS) models are often used as the basis for modeling blood damage in turbulent flows. To predict blood damage by turbulence stresses that are not resolved in RANS, a stress formulation that represents the corresponding scales is required. Here, we compare two commonly employed stress formulations: a scalar stress representation that uses Reynolds stresses as a surrogate for unresolved fluid stresses, and an effective stress formulation based on energy dissipation.

METHODS

We conducted unsteady RANS simulations of the CentriMag blood pump with three different closure models and a Large Eddy Simulation (LES) for reference. We implemented both stress representations in all models and compared the resulting total stress distributions in Eulerian and Lagrangian frameworks.

RESULTS

The Reynolds-stress-based approach overestimated the contribution of unresolved stresses in RANS, with differences between closure models of up to several orders of magnitude. With the dissipation-based approach, the total stresses predicted with RANS deviated by about 50% from the LES reference, which was more accurate than only considering resolved stresses.

CONCLUSION

The Reynolds-stress-based formulation proved unreliable for estimating scalar stresses in our RANS simulations, while the dissipation-based approach provided an accuracy improvement over simply neglecting unresolved stresses.

SIGNIFICANCE

Our results suggest that dissipation-based inclusion of unresolved stresses should be the preferred choice for blood damage modeling in RANS.

Vibro-Acoustic Platelet Activation: An Additive Mechanism of Prothrombosis with Applicability to Snoring and Obstructive Sleep Apnea

Introduction: Obstructive sleep apnea (OSA) and loud snoring are conditions with increased cardiovascular risk and notably an association with stroke. Central in stroke are thrombosis and thromboembolism, all related to and initiaing with platelet activation. Platelet activation in OSA has been felt to be driven by biochemical and inflammatory means, including intermittent catecholamine exposure and transient hypoxia. We hypothesized that snore-associated acoustic vibration (SAAV) is an activator of platelets that synergizes with catecholamines and hypoxia to further amplify platelet activation. Methods: Gel-filtered human platelets were exposed to snoring utilizing a designed vibro-acoustic exposure device, varying the time and intensity of exposure and frequency content. Platelet activation was assessed via thrombin generation using the Platelet Activity State assay and scanning electron microscopy. Comparative activation induced by epinephrine and hypoxia were assessed individually as well as additively with SAAV, as well as the inhibitory effect of aspirin. Results: We demonstrate that snore-associated acoustic vibration is an independent activator of platelets, which is dependent upon the dose of exposure, i.e., intensity x time. In snoring, acoustic vibrations associated with low-frequency sound content (200 Hz) are more activating than those associated with high frequencies (900 Hz) (53.05% vs. 22.08%, p = 0.001). Furthermore, SAAV is additive to both catecholamines and hypoxia-mediated activation, inducing synergistic activation. Finally, aspirin, a known inhibitor of platelet activation, has no significant effect in limiting SAAV platelet activation. Conclusion: Snore-associated acoustic vibration is a mechanical means of platelet activation, which may drive prothrombosis and thrombotic risk clinically observed in loud snoring and OSA.

Sodium bicarbonate as a local adjunctive agent for limiting platelet activation, aggregation, and adhesion within cardiovascular therapeutic devices

Cardiovascular therapeutic devices (CTDs) remain limited by thrombotic adverse events. Current antithrombotic agents limit thrombosis partially, often adding to bleeding. The Impella® blood pump utilizes heparin in 5% dextrose (D5W) as an internal purge to limit thrombosis. While effective, exogenous heparin often complicates overall anticoagulation management, increasing bleeding tendency. Recent clinical studies suggest sodium bicarbonate (bicarb) may be an effective alternative to heparin for local anti-thrombosis. We examined the effect of sodium bicarbonate on human platelet morphology and function to better understand its translational utility. Human platelets were incubated (60:40) with D5W + 25 mEq/L, 50 mEq/L, or 100 mEq/L sodium bicarbonate versus D5W or D5W + Heparin 50 U/mL as controls. pH of platelet-bicarbonate solutions mixtures was measured. Platelet morphology was examined via transmission electron microscopy; activation assessed via P-selectin expression, phosphatidylserine exposure and thrombin generation; and aggregation with TRAP-6, calcium ionophore, ADP and collagen quantified; adhesion to glass measured via fluorescence microscopy. Sodium bicarbonate did not alter platelet morphology but did significantly inhibit activation, aggregation, and adhesion. Phosphatidylserine exposure and thrombin generation were both reduced in a concentration-dependent manner-between 26.6 ± 8.2% (p = 0.01) and 70.7 ± 5.6% (p < 0.0001); and 14.0 ± 6.2% (p = 0.15) and 41.7 ± 6.8% (p = 0.03), respectively, compared to D5W control. Platelet aggregation via all agonists was also reduced, particularly at higher concentrations of bicarb. Platelet adhesion to glass was similarly reduced, between 0.04 ± 0.03% (p = 0.61) and 0.11 ± 0.04% (p = 0.05). Sodium bicarbonate has direct, local, dose-dependent effects limiting platelet activation and adhesion. Our results highlight the potential utility of sodium bicarbonate as a locally acting agent to limit device thrombosis.

A robust mixed-charge zwitterionic polyurethane coating integrated with antibacterial and anticoagulant functions for interventional blood-contacting devices

Antifouling coatings based on zwitterionic polymers have been widely applied for surface modification of interventional blood-contacting devices to combat thrombosis and infection. However, the weak adhesion stability of the zwitterionic coating to the device surface is still the key challenge. In this work, biocompatible mixed-charge zwitterionic polyurethane (MPU) polymers, that bear equal amounts of cationic quaternary amine groups and anionic carboxyl groups, were developed and further uniformly dip-coated onto a thermoplastic polyurethane (TPU) substrate with a commercial aliphatic isocyanate cross-linker (AIC). During the curing process, AIC not only crosslinks MPU chains into a polymer network but also reacts with hydroxyl groups of TPU to interlink the polymer network to the substrate, resulting in a cross-linking reinforced MPU coating (CMPU) with excellent mechanical robustness and adhesion strength. Taking advantage of the mixed-charge feature, the final zwitterionic CMPU coating exhibits both excellent antifouling and antibacterial activities against protein adsorption and bacterial growth, respectively, which is beneficial for effectively inhibiting the occurrence of in vivo infection. Moreover, anticoagulation studies show that CMPU-coated TPU catheters can also prevent the formation of blood clots in ex vivo rabbit blood circuits without anticoagulants. Hence, the designed CMPU coating has immense potential to address thrombosis and infection for interventional blood-contacting devices.

Novel Antiplatelet Activity of Ginsenoside Re Through the Inhibition of High Shear Stress-Induced Platelet Aggregation

ABSTRACT

Bleeding is one of the most serious side effects of antiplatelet drugs. Efforts have been made to find new antiplatelet agents without bleeding complications. Shear-induced platelet aggregation (SIPA) occurs only under pathological conditions and is a promising target for overcoming bleeding problems. This work demonstrates that the ginsenoside Re selectively inhibits platelet aggregation induced by high shear stress. Human platelets were exposed to high shear stress using microfluidic chip technology, and aggregation, activation, and phosphatidylserine (PS) exposure were measured. The Von Willebrand Ristocetin Cofactor (vWF:RCo) assay and western blot were used to evaluate the effect of the vWF-GPⅠb/PI3K/Akt signal pathway. The coagulation and bleeding risk were evaluated by measuring the coagulation parameters PT, APTT, TT, and thromboelastography. The 3-dimensional morphology of platelet aggregates was observed by a microscopic 3-dimensional imaging. Re was a potent inhibitor of SIPA, with an IC 50 of 0.071 mg/mL. It effectively blocked shear stress-induced platelet activation without any significant toxicity. It was highly selective against SIPA, effectively inhibiting vWF-GPIb and the downstream PI3K/Akt signaling pathway. Most importantly, Re did not affect normal blood coagulation and did not increase the risk of bleeding. In conclusion, Re inhibits platelet activation through the inhibition of the vWF-GPIb/PI3K/Akt pathway. Thus, it might be considered as a new antiplatelet drug in the prevention of thrombosis without increasing the risk of bleeding.

Shear-Mediated Platelet Microparticles Demonstrate Phenotypic Heterogeneity as to Morphology, Receptor Distribution, and Hemostatic Function

Implantable Cardiovascular Therapeutic Devices (CTD), while lifesaving, impart supraphysiologic shear stress to platelets, resulting in thrombotic and bleeding coagulopathy. We previously demonstrated that shear-mediated platelet dysfunction is associated with downregulation of platelet GPIb-IX-V and αIIbβ3 receptors via generation of Platelet-Derived MicroParticles (PDMPs). Here, we test the hypothesis that sheared PDMPs manifest phenotypical heterogeneity of morphology and receptor surface expression and modulate platelet hemostatic function. Human gel-filtered platelets were exposed to continuous shear stress. Alterations of platelet morphology were visualized using transmission electron microscopy. Surface expression of platelet receptors and PDMP generation were quantified by flow cytometry. Thrombin generation was quantified spectrophotometrically, and platelet aggregation was measured by optical aggregometry. Shear stress promotes notable alterations in platelet morphology and ejection of distinctive types of PDMPs. Shear-mediated microvesiculation is associated with the remodeling of platelet receptors, with PDMPs expressing significantly higher levels of adhesion receptors (αIIbβ3, GPIX, PECAM-1, P-selectin, and PSGL-1) and agonist receptors (P2Y12 and PAR1). Sheared PDMPs promote thrombin generation and inhibit platelet aggregation induced by collagen and ADP. Sheared PDMPs demonstrate phenotypic heterogeneity as to morphology and defined patterns of surface receptors and impose a bidirectional effect on platelet hemostatic function. PDMP heterogeneity suggests that a range of mechanisms are operative in the microvesiculation process, contributing to CTD coagulopathy and posing opportunities for therapeutic manipulation.

Shear-Mediated Platelet Microparticles Demonstrate Phenotypic Heterogeneity as to Morphology, Receptor Distribution, and Hemostatic Function

Objective

Implantable cardiovascular therapeutic devices (CTD) including stents, percutaneous heart valves and ventricular assist devices, while lifesaving, impart supraphysiologic shear stress to platelets resulting in thrombotic and bleeding device-related coagulopathy. We previously demonstrated that shear-mediated platelet dysfunction is associated with downregulation of platelet GPIb-IX-V and αIIbβ3 receptors via generation of platelet-derived microparticles (PDMPs). Here, we test the hypothesis that shear-generated PDMPs manifest phenotypical heterogeneity of their morphology and surface expression of platelet receptors, and modulate platelet hemostatic function.

Approach and Results

Human gel-filtered platelets were exposed to continuous shear stress and sonication. Alterations of platelet morphology were visualized using transmission electron microscopy. Surface expression of platelet receptors and PDMP generation were quantified by flow cytometry. Thrombin generation was quantified spectrophotometrically, and platelet aggregation in plasma was measured by optical aggregometry. We demonstrate that platelet exposure to shear stress promotes notable alterations in platelet morphology and ejection of several distinctive types of PDMPs. Shear-mediated microvesiculation is associated with the differential remodeling of platelet receptors with PDMPs expressing significantly higher levels of both adhesion (α _IIb β ₃ , GPIX, PECAM-1, P-selectin, and PSGL-1) and agonist-evoked receptors (P ₂ Y ₁₂ & PAR1). Shear-mediated PDMPs have a bidirectional effect on platelet hemostatic function, promoting thrombin generation and inhibiting platelet aggregation induced by collagen and ADP.

Conclusions

Shear-generated PDMPs demonstrate phenotypic heterogeneity as to morphologic features and defined patterns of surface receptor alteration, and impose a bidirectional effect on platelet hemostatic function. PDMP heterogeneity suggests that a range of mechanisms are operative in the microvesiculation process, contributing to CTD coagulopathy and posing opportunities for therapeutic manipulation.

Patient specific approach to analysis of shear-induced platelet activation in haemodialysis arteriovenous fistula

Shear-induced platelet activation (SIPAct) is an important mechanism of thrombosis initiation under high blood flow. This mechanism relies on the interaction of platelets with the von Willebrand factor (VWF) capable of unfolding under high shear stress. High shear stress occurs in the arteriovenous fistula (AVF) commonly used for haemodialysis. A novel patient-specific approach for the modelling of SIPAct in the AVF was proposed. This enabled us to estimate the SIPAct level via computational fluid dynamics. The suggested approach was applied for the SIPAct analysis in AVF geometries reconstructed from medical images. The approach facilitates the determination of the SIPAct level dependence on both biomechanical (AVF flow rate) and biochemical factors (VWF multimer size). It was found that the dependence of the SIPAct level on the AVF flow rate can be approximated by a power law. The critical flow rate was a decreasing function of the VWF multimer size. Moreover, the critical AVF flow rate highly depended on patient-specific factors, e.g., the vessel geometry. This indicates that the approach may be adopted to elucidate patient-specific thrombosis risk factors in haemodialysis patients.

DNA Origami-Platelet Adducts: Nanoconstruct Binding without Platelet Activation

Objective. Platelets are small, mechanosensitive blood cells responsible for maintaining vascular integrity and activatable on demand to limit bleeding and facilitate thrombosis. While circulating in the blood, platelets are exposed to a range of mechanical and chemical stimuli, with the platelet membrane being the primary interface and transducer of outside-in signaling. Sensing and modulating these interface signals would be useful to study mechanochemical interactions; yet, to date, no methods have been defined to attach adducts for sensor fabrication to platelets without triggering platelet activation. We hypothesized that DNA origami, and methods for its attachment, could be optimized to enable nonactivating instrumentation of the platelet membrane. Approach and Results. We designed and fabricated multivalent DNA origami nanotile constructs to investigate nanotile hybridization to membrane-embedded single-stranded DNA-tetraethylene glycol cholesteryl linkers. Two hybridization protocols were developed and validated (Methods I and II) for rendering high-density binding of DNA origami nanotiles to human platelets. Using quantitative flow cytometry, we showed that DNA origami binding efficacy was significantly improved when the number of binding overhangs was increased from two to six. However, no additional binding benefit was observed when increasing the number of nanotile overhangs further to 12. Using flow cytometry and transmission electron microscopy, we verified that hybridization with DNA origami constructs did not cause alterations in the platelet morphology, activation, aggregation, or generation of platelet-derived microparticles. Conclusions. Herein, we demonstrate that platelets can be successfully instrumented with DNA origami constructs with no or minimal effect on the platelet morphology and function. Our protocol allows for efficient high-density binding of DNA origami to platelets using low quantities of the DNA material to label a large number of platelets in a timely manner. Nonactivating platelet-nanotile adducts afford a path for advancing the development of DNA origami nanoconstructs for cell-adherent mechanosensing and therapeutic agent delivery.

Models of Shear-Induced Platelet Activation and Numerical Implementation With Computational Fluid Dynamics Approaches

Shear-induced platelet activation is one of the critical outcomes when blood is exposed to elevated shear stress. Excessively activated platelets in the circulation can lead to thrombus formation and platelet consumption, resulting in serious adverse events such as thromboembolism and bleeding. While experimental observations reveal that it is related to the shear stress level and exposure time, the underlying mechanism of shear-induced platelet activation is not fully understood. Various models have been proposed to relate shear stress levels to platelet activation, yet most are modified from the empirically calibrated power-law model. Newly developed multiscale platelet models are tested as a promising approach to capture a single platelet's dynamic shape during activation, but it would be computationally expensive to employ it for a large-scale analysis. This paper summarizes the current numerical models used to study the shear-induced platelet activation and their computational applications in the risk assessment of a particular flow pattern and clot formation prediction.

Discrete responses of erythrocytes, platelets, and von Willebrand factor to shear

Despite decades of technological advancements in blood-contacting medical devices, complications related to shear flow-induced blood trauma are still frequently observed in clinic. Blood trauma includes haemolysis, platelet activation, and degradation of High Molecular Weight von Willebrand Factor (HMW vWF) multimers, all of which are dependent on the exposure time and magnitude of shear stress. Specifically, accumulating evidence supports that when blood is exposed to shear stresses above a certain threshold, blood trauma ensues; however, it remains unclear how various constituents of blood are affected by discrete shears experimentally. The aim of this study was to expose blood to discrete shear stresses and evaluate blood trauma indices that reflect red cell, platelet, and vWF structure. Citrated human whole blood (n = 6) was collected and its haematocrit was adjusted to 30 ± 2% by adding either phosphate buffered saline (PBS) or polyvinylpyrrolidone (PVP). Viscosity of whole blood was adjusted to 3.0, 12.5, 22.5 and 37.5 mPa·s to yield stresses of 3, 6, 9, 12, 50, 90 and 150 Pa in a custom-developed shearing system. Blood samples were exposed to shear for 0, 300, 600 and 900 s. Haemolysis was measured using spectrophotometry, platelet activation using flow cytometry, and HMW vWF multimer degradation was quantified with gel electrophoresis and immunoblotting. For tolerance to 300, 600 and 900 s of exposure time, the critical threshold of haemolysis was reached after blood was exposed to 90 Pa for 600 s (P < 0.05), platelet activation and HMW vWF multimer degradation were 50 Pa for 600 s and 12 Pa for 300 s respectively (P < 0.05). Our experimental results provide simultaneous comparison of blood trauma indices and thus also the relation between shear duration and magnitude required to induce damage to red cells, platelets, and vWF. Our results also demonstrate that near-physiological shear stress (<12 Pa) is needed in order to completely avoid any form of blood trauma. Therefore, there is an urgent need to design low shear-flow medical devices in order to avoid blood trauma in this blood-contacting medical device field.

Shear-Mediated Platelet Activation is Accompanied by Unique Alterations in Platelet Release of Lipids

INTRODUCTION

Platelet activation by mechanical means such as shear stress exposure, is a vital driver of thrombotic risk in implantable blood-contacting devices used in the treatment of heart failure. Lipids are essential in platelets activation and have been studied following biochemical activation. However, little is known regarding lipid alterations occurring with mechanical shear-mediated platelet activation.

METHODS

Here, we determined if shear-activation of platelets induced lipidome changes that differ from those associated with biochemically-mediated platelet activation. We performed high-resolution lipidomic analysis on purified platelets from four healthy human donors. For each donor, we compared the lipidome of platelets that were non-activated or activated by shear, ADP, or thrombin treatment.

RESULTS

We found that shear activation altered cell-associated lipids and led to the release of lipids into the extracellular environment. Shear-activated platelets released 21 phospholipids and sphingomyelins at levels statistically higher than platelets activated by biochemical stimulation.

CONCLUSIONS

We conclude that shear-mediated activation of platelets alters the basal platelet lipidome. Further, these alterations differ and are unique in comparison to the lipidome of biochemically activated platelets. Many of the released phospholipids contained an arachidonic acid tail or were phosphatidylserine lipids, which have known procoagulant properties. Our findings suggest that lipids released by shear-activated platelets may contribute to altered thrombosis in patients with implanted cardiovascular therapeutic devices.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12195-021-00692-x.

Mathematical and Computational Modeling of Device-Induced Thrombosis

Given the extensive and routine use of cardiovascular devices, a major limiting factor to their success is the thrombotic rate that occurs. This both poses direct risk to the patient and requires counterbalancing with anticoagulation and other treatment strategies, contributing additional risks. Developing a better understanding of the mechanisms of device-induced thrombosis to aid in device design and medical management of patients is critical to advance the ubiquitous use and durability. Thus, mathematical and computational modelling of device-induced thrombosis has received significant attention recently, but challenges remain. Additional areas that need to be explored include microscopic/macroscopic approaches, reconciling physical and numerical timescales, immune/inflammatory responses, experimental validation, and incorporating pathologies and blood conditions. Addressing these areas will provide engineers and clinicians the tools to provide safe and effective cardiovascular devices.

The effect of turbulence modelling on the assessment of platelet activation

Pathological platelet activation by abnormal shear stresses is regarded as a main clinical complication in recipients of cardiovascular mechanical devices. In order to improve their performance computational fluid dynamics (CFD) are used to evaluate flow fields and related shear stresses. CFD models are coupled with mathematical models that describe the relation between fluid dynamics variables, and in particular shear stresses, and the platelet activation state (PAS). These models typically use a Lagrangian approach to compute the shear stresses along possible platelet trajectories. However, in the case of turbulent flow, the choice of the proper turbulence closure is still debated for both concerning its effect on shear stress calculation and Lagrangian statistics. In this study different numerical simulations of the flow through a mechanical heart valve were performed and then compared in terms of Eulerian and Lagrangian quantities: a direct numerical simulation (DNS), a large eddy simulation (LES), two Reynolds-averaged Navier-Stokes (RANS) simulations (SST k-ω and RSM) and a "laminar" (no turbulence modelling) simulation. Results exhibit a large variability in the PAS assessment depending on the turbulence model adopted. "Laminar" and RSM estimates of platelet activation are about 60% below DNS, while LES is 16% less. Surprisingly, PAS estimated from the SST k- ω velocity field is only 8% less than from DNS data. This appears more artificial than physical as can be inferred after comparing frequency distributions of PAS and of the different Lagrangian variables of the mechano-biological model of platelet activation. Our study indicates how much turbulence closures may affect platelet activation estimates, in comparison to an accurate DNS, when assessing blood damage in blood contacting devices.

Predicting the In Vivo Performance of Cardiovascular Biomaterials: Current Approaches In Vitro Evaluation of Blood-Biomaterial Interactions

The therapeutic efficacy of a cardiovascular device after implantation is highly dependent on the host-initiated complement and coagulation cascade. Both can eventually trigger thrombosis and inflammation. Therefore, understanding these initial responses of the body is of great importance for newly developed biomaterials. Subtle modulation of the associated biological processes could optimize clinical outcomes. However, our failure to produce truly blood compatible materials may reflect our inability to properly understand the mechanisms of thrombosis and inflammation associated with biomaterials. In vitro models mimicking these processes provide valuable insights into the mechanisms of biomaterial-induced complement activation and coagulation. Here, we review (i) the influence of biomaterials on complement and coagulation cascades, (ii) the significance of complement-coagulation interactions for the clinical success of cardiovascular implants, (iii) the modulation of complement activation by surface modifications, and (iv) in vitro testing strategies.

A Uniform and Robust Bioinspired Zwitterion Coating for Use in Blood-Contacting Catheters with Improved Anti-Inflammatory and Antithrombotic Properties

Inflammation and thrombosis are two major complications of blood-contacting catheters that are used as extracorporeal circuits for hemodialysis and life-support systems. In clinical applications, complications can lead to increased mortality and morbidity rates. In this work, a biomimetic erythrocyte membrane zwitterion coating based on poly(2-methacryloyloxyethyl phosphorylcholine-co-dopamine methacrylate) (pMPCDA) copolymers is uniformly and robustly modified onto a polyvinyl chloride (PVC) catheter via mussel-inspired surface chemistry. The zwitterionic pMPCDA coating exhibits excellent antifouling activity and resists bacterial adhesion, fibrinogen adsorption, and platelet adhesion/activation. The material also demonstrates great hemocompatibility, cytocompatibility, and anticoagulation properties in vitro. Additionally, this biocompatible pMPCDA coating reduces in vivo foreign-body reactions by mitigating inflammatory response and collagen capsule formation, due to its outstanding ability to resist nonspecific protein adsorption. More importantly, when compared with a bare PVC catheter, the pMPCDA coating exhibits outstanding antithrombotic properties when tested in an ex vivo rabbit perfusion model. Thus, it is envisioned that this biomimetic erythrocyte membrane surface strategy will provide a promising way to mitigate inflammation and thrombosis caused by the use of blood-contacting catheters.

The Role of Biodegradable Poly-(L-lactide)-Based Polymers in Blood Cell Activation and Platelet-Monocyte Interaction

The main purpose of new stent technologies is to overcome unfavorable material-related incompatibilities by producing bio- and hemo-compatible polymers with anti-inflammatory and anti-thrombogenic properties. In this context, wettability is an important surface property, which has a major impact on the biological response of blood cells. However, the influence of local hemodynamic changes also influences blood cell activation. Therefore, we investigated biodegradable polymers with different wettability to identify possible aspects for a better prediction of blood compatibility. We applied shear rates of 100 s⁻¹ and 1500 s⁻¹ and assessed platelet and monocyte activation as well as the formation of CD62P+ monocyte-bound platelets via flow cytometry. Aggregation of circulating platelets induced by collagen was assessed by light transmission aggregometry. Via live cell imaging, leukocytes were tracked on biomaterial surfaces to assess their average velocity. Monocyte adhesion on biomaterials was determined by fluorescence microscopy. In response to low shear rates of 100 s⁻¹, activation of circulating platelets and monocytes as well as the formation of CD62P+ monocyte-bound platelets corresponded to the wettability of the underlying material with the most favorable conditions on more hydrophilic surfaces. Under high shear rates, however, blood compatibility cannot only be predicted by the concept of wettability. We assume that the mechanisms of blood cell-polymer interactions do not allow for a rule-of-thumb prediction of the blood compatibility of a material, which makes extensive in vitro testing mandatory.

A multiscale model for multiple platelet aggregation in shear flow

We developed a multiscale model for simulating aggregation of multiple, free-flowing platelets in low-intermediate shear viscous flow, in which aggregation is mediated by the interaction of αIIbβ3 receptors on the platelet membrane and fibrinogen (Fg). This multiscale model uses coarse grained molecular dynamics (CGMD) for platelets at the microscales and dissipative particle dynamics (DPD) for the shear flow at the macroscales, employing our hybrid aggregation force field for modeling molecular level receptor ligand bonds. We define an aggregation tensor and use it to quantify the molecular level contact characteristics between platelets in an aggregate. We perform numerical studies under different flow conditions for platelet doublets and triplets and evaluate the contact area, detaching force and minimum distance between different pairs of platelets in an aggregate. We also present the dynamics of applied stress and velocity magnitude distributions on the platelet membrane during aggregation and quantify the increase in stress in the contact region under different flow conditions. Integrating the knowledge from our previously validated models, together with new aggregation scenarios, our model can dynamically quantify aggregation characteristics and map stress and velocity distribution on the platelet membrane which are difficult to measure in vitro, thus providing an insight into mechanotransduction bond formation response of platelets to flow-induced shear stresses. This modeling framework, together with the tensor method for quantifying inter-platelet contact, can be extended to simulate and analyze larger aggregates and their adhesive properties.

Shear-mediated platelet activation in the free flow II: Evolving mechanobiological mechanisms reveal an identifiable signature of activation and a bi-directional platelet dyscrasia with thrombotic and bleeding features

Shear-mediated platelet activation (SMPA) in the "free flow" is the net result of a range of cell mechanobiological mechanisms. Previously, we outlined three main groups of mechanisms including: 1) mechano-destruction - i.e. additive platelet (membrane) damage; 2) mechano-activation - i.e. activation of shear-sensitive ion channels and pores; and 3) mechano-transduction - i.e. "outside-in" signaling via a range of transducers. Here, we report on recent advances since our original report which describes additional features of SMPA. A clear "signature" of SMPA has been defined, allowing differentiation from biochemically-mediated activation. Notably, SMPA is characterized by mitochondrial dysfunction, platelet membrane eversion, externalization of anionic phospholipids, and increased thrombin generation on the platelet surface. However, SMPA does not lead to integrin αIIbβ3 activation or P-selectin exposure due to platelet degranulation, as is commonly observed in biochemical activation. Rather, downregulation of GPIb, αIIbβ3, and P-selectin surface expression is evident. Furthermore, SMPA is accompanied by a decrease in overall platelet size coupled with a concomitant, progressive increase in microparticle generation. Shear-ejected microparticles are highly enriched in GPIb and αIIbβ3. These observations indicate the enhanced diffusion, migration, or otherwise dispersion of platelet adhesion receptors to membrane zones, which are ultimately shed as receptor-rich PDMPs. The pathophysiological consequence of this progressive shear accumulation phenomenon is an associated dyscrasia of remaining platelets - being both reduced in size and less activatable via biochemical means - a tendency to favor bleeding, while concomitantly shed microparticles are highly prothrombotic and increase the tendency for thrombosis in both local and systemic milieu. These mechanisms and observations offer direct clinical utility in allowing measurement and guidance of the net balance of platelet driven events in patients with implanted cardiovascular therapeutic devices.

Characterization of the competing role of surface-contact and shear stress on platelet activation in the setting of blood contacting devices

Supraphysiological shear stress and surface-contact are recognized as driving mechanisms of platelet activation (PA) in blood contacting devices (BCDs). However, the competing role of these mechanisms in triggering thrombogenic events is poorly understood. Here, we characterized the dynamics of PA in response to the combined effect of shear stress and material exposure. Human platelets were stimulated with different levels of shear stress (500, 750, 1000 dynes/cm²) over a range of exposure times (10, 20, and 30 min) within capillary tubes made of various polymeric materials. Polyethylene (PE), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), and polyether ether ketone (PEEK), used for BCDs fabrication, were investigated as compared to glass and thromboresistant Sigma™-coated glass. PA was quantified using the Platelet Activity State assay. Our results indicate that mechanical stimulation and polymer surface-contact both significantly contribute to PA. Notably, the contribution of the mechanical stimulus ranges between +36% and +43%, while that associated with polymer surface-contact ranges from +48% to +59%, depending on the exposure time. In more detail, our results indicate that: (i) PA increases with increasing shear stress magnitude; (ii) PA has a non-linear, time-dependent relationship to exposure time; (iii) PA is largely influenced by biomaterials, with PE and PEEK having respectively the lowest and highest prothrombotic potential; (iv) the effects of polymer surface-contact and shear stress are not correlated and can be studied separately. Our results suggest the importance of incorporating the evaluation of platelet activation driven by the combined effect of shear stress and polymer surface-contact for the comprehensive assessment, and eventually minimization, of BCDs thrombogenic potential.

Platelet Dysfunction During Mechanical Circulatory Support: Elevated Shear Stress Promotes Downregulation of αIIbβ3 and GPIb via Microparticle Shedding Decreasing Platelet Aggregability

[Figure: see text].

Shear-Induced Platelet Activation is Sensitive to Age and Calcium Availability: A Comparison of Adult and Cord Blood

INTRODUCTION

Antiplatelet therapy for neonates and infants is often extrapolated from the adult experience, based on limited observation of agonist-induced neonatal platelet hypoactivity and poor understanding of flow shear-mediated platelet activation. Therefore, thrombotic events due to device-associated disturbed flow are inadequately mitigated in critically ill neonates with indwelling umbilical catheters and infants receiving cardiovascular implants.

METHODS

Whole blood (WB), platelet-rich plasma (PRP), and gel-filtered platelets (GFP) were prepared from umbilical cord and adult blood, and exposed to biochemical agonists or pathological shear stress of 70 dyne/cm2. We evaluated α-granule release, phosphatidylserine (PS) scrambling, and procoagulant response using P-selectin expression, Annexin V binding, and thrombin generation (PAS), respectively. Activation modulation due to depletion of intracellular and extracellular calcium, requisite second messengers, was also examined.

RESULTS

Similar P-selectin expression was observed for sheared adult and cord platelets, with concordant inhibition due to intracellular and extracellular calcium depletion. Sheared cord platelet Annexin V binding and PAS activity was similar to adult values in GFP, but lower in PRP and WB. Annexin V on sheared cord platelets was calcium-independent, with PAS slightly reduced by intracellular calcium depletion.

CONCLUSIONS

Increased PS activity on purified sheared cord platelets suggest that their intrinsic function under pathological flow conditions is suppressed by cell-cell or plasmatic components. Although secretory functions of adult and cord platelets retain comparable calcium-dependence, PS exposure in sheared cord platelets is uniquely calcium-independent and distinct from adults. Identification of calcium-regulated developmental disparities in shear-mediated platelet function may provide novel targets for age-specific antiplatelet therapy.

Comprehensive review of hemolysis in ventricular assist devices

Ventricular assist devices (VADs) have played an important role in altering the natural history of end-stage heart failure. Low-grade hemolysis has been traditionally described in patients with VADs, indicating effective device functionality. However, clinically significant hemolysis could be crucial in terms of prognosis, calling for prompt therapeutic actions. The absence of solid and widely approved diagnostic criteria for clinically significant hemolysis, render the utilization of hemolysis laboratory markers challenging. Hemolysis incidence varies (5%-18%) depending on definition and among different VAD generations, being slightly higher in continuous-flow devices than in pulsatile devices. Increased shear stress of red blood cells and underlying device thrombosis appear to be the main pathogenetic pathways. No certain algorithm is available for the management of hemolysis in patients with VADs, while close clinical and laboratory monitoring remains the cornerstone of management. Imaging examinations such as echocardiography ramp test or computed tomography scan could play a role in revealing the underlying cause. Treatment should be strictly personalized, including either pharmacological (antithrombotic treatment) or surgical interventions.

Imaging Platelet Processes and Function-Current and Emerging Approaches for Imaging in vitro and in vivo

Platelets are small anucleate cells that are essential for many biological processes including hemostasis, thrombosis, inflammation, innate immunity, tumor metastasis, and wound healing. Platelets circulate in the blood and in order to perform all of their biological roles, platelets must be able to arrest their movement at an appropriate site and time. Our knowledge of how platelets achieve this has expanded as our ability to visualize and quantify discreet platelet events has improved. Platelets are exquisitely sensitive to changes in blood flow parameters and so the visualization of rapid intricate platelet processes under conditions found in flowing blood provides a substantial challenge to the platelet imaging field. The platelet's size (~2 μm), rapid activation (milliseconds), and unsuitability for genetic manipulation, means that appropriate imaging tools are limited. However, with the application of modern imaging systems to study platelet function, our understanding of molecular events mediating platelet adhesion from a single-cell perspective, to platelet recruitment and activation, leading to thrombus (clot) formation has expanded dramatically. This review will discuss current platelet imaging techniques in vitro and in vivo, describing how the advancements in imaging have helped answer/expand on platelet biology with a particular focus on hemostasis. We will focus on platelet aggregation and thrombus formation, and how platelet imaging has enhanced our understanding of key events, highlighting the knowledge gained through the application of imaging modalities to experimental models in vitro and in vivo. Furthermore, we will review the limitations of current imaging techniques, and questions in thrombosis research that remain to be addressed. Finally, we will speculate how the same imaging advancements might be applied to the imaging of other vascular cell biological functions and visualization of dynamic cell-cell interactions.

Circulatory loop design and components introduce artifacts impacting in vitro evaluation of ventricular assist device thrombogenicity: A call for caution

Mechanical circulatory support (MCS) devices continue to be hampered by thrombotic adverse events (AEs), a consequence of device-imparted supraphysiologic shear stresses, leading to shear-mediated platelet activation (SMPA). In advancing MCS devices from design to clinical use, in vitro circulatory loops containing the device under development and testing are utilized as a means of assessing device thrombogenicity. Physical characteristics of these test circulatory loops may also contribute to inadvertent platelet activation through imparted shear stress, adding inadvertent error in evaluating MCS device thrombogenicity. While investigators normally control for the effect of a loop, inadvertent addition of what are considered innocuous connectors may impact test results. Here, we tested the effect of common, additive components of in vitro circulatory test loops, that is, connectors and loop geometry, as to their additive contribution to shear stress via both in silico and in vitro models. A series of test circulatory loops containing a ventricular assist device (VAD) with differing constituent components, were established in silico including: loops with 0~5 Luer connectors, a loop with a T-connector creating 90° angulation, and a loop with 90° angulation. Computational fluid dynamics (CFD) simulations were performed using a k - ω shear stress transport turbulence model to platelet activation index (PAI) based on a power law model. VAD-operated loops replicating in silico designs were assembled in vitro and gel-filtered human platelets were recirculated within (1 hour) and SMPA was determined. CFD simulations demonstrated high shear being introduced at non-smooth regions such as edge-connector boundaries, tubing, and at Luer holes. Noticeable peaks' shifts of scalar shear stress (sss) distributions toward high shear-region existed with increasing loop complexity. Platelet activation also increased with increasing shear exposure time, being statistically higher when platelets were exposed to connector-employed loop designs. The extent of platelet activation in vitro could be successfully predicted by CFD simulations. Loops employing additional components (non-physiological flow pattern connectors) resulted in higher PAI. Loops with more components (5-connector loop and 90° T-connector) showed 63% and 128% higher platelet activation levels, respectively, versus those with fewer (0-connector (P = .023) and a 90° heat-bend loop (P = .0041). Our results underscore the importance of careful consideration of all component elements, and suggest the need for standardization in designing in vitro circulatory loops for MCS device evaluation to avoid inadvertent additive SMPA during device evaluation, confounding overall results. Specifically, we caution on the use and inadvertent introduction of additional connectors, ports, and other shear-generating elements which introduce artifact, clouding primary device evaluation via introduction of additive SMPA.

A Flow Induced Autoimmune Response and Accelerated Senescence of Red Blood Cells in Cardiovascular Devices

Red blood cells (RBCs) passing through heart pumps, prosthetic heart valves and other cardiovascular devices undergo early senescence attributed to non-physiologic forces. We hypothesized that mechanical trauma accelerates aging by deformation of membrane proteins to cause binding of naturally occurring IgG. RBCs isolated from blood of healthy volunteers were exposed to high shear stress in a viscometer or microfluidics channel to mimic mechanical trauma and then incubated with autologous plasma. Increased binding of IgG was observed indicating forces caused conformational changes in a membrane protein exposing an epitope(s), probably the senescent cell antigen of band 3. The binding of immunoglobulin suggests it plays a role in the premature sequestration and phagocytosis of RBCs in the spleen. Measurement of IgG holds promise as a marker foreshadowing complications in cardiovascular patients and as a means to improve the design of medical devices in which RBCs are susceptible to sublethal trauma.

Antithrombotic Effects of Paeoniflorin from Paeonia suffruticosa by Selective Inhibition on Shear Stress-Induced Platelet Aggregation

Antiplatelet agents are important in the pharmacotherapeutic regime for many cardiovascular diseases, including thrombotic disorders. However, bleeding, the most serious adverse effect associated with current antiplatelet therapy, has led to many efforts to discover novel anti-platelet drugs without bleeding issues. Of note, shear stress-induced platelet aggregation (SIPA) is a promising target to overcome bleeding since SIPA happens only in pathological conditions. Accordingly, this study was carried out to discover antiplatelet agents selectively targeting SIPA. By screening various herbal extracts, Paeonia suffruticosa and its major bioactive constituent, paeoniflorin, were identified to have significant inhibitory effects against shear-induced aggregation in human platelets. The effects of paeoniflorin on intraplatelet calcium levels, platelet degranulation, and integrin activation in high shear stress conditions were evaluated by a range of in vitro experiments using human platelets. The inhibitory effect of paeoniflorin was determined to be highly selective against SIPA, through modulating von Willebrand Factor (vWF)-platelet glycoprotein Ib (GP Ib) interaction. The effects of paeoniflorin on platelet functions under high shear stress were confirmed in the ex vivo SIPA models in rats, showing the good accordance with the anti-SIPA effects on human platelets. Treatment with paeoniflorin significantly prevented arterial thrombosis in vivo from the dose of 10 mg/kg without prolonging bleeding time or blood clotting time in rats. Collectively, our results demonstrated that paeoniflorin can be a novel anti-platelet agent selectively targeting SIPA with an improved safety profile.

Active Micropump-Mixer for Rapid Antiplatelet Drug Screening in Whole Blood

There is a need for scalable automated lab-on-chip systems incorporating precise hemodynamic control that can be applied to high-content screening of new more efficacious antiplatelet therapies. This paper reports on the development and characterization of a novel active micropump-mixer microfluidic to address this need. Using a novel reciprocating elastomeric micropump design, we take advantage of the flexible structural and actuation properties of this framework to manage the hemodynamics for on-chip platelet thrombosis assay on type 1 fibrillar collagen, using whole blood. By characterizing and harnessing the complex three-dimensional hemodynamics of the micropump operation in conjunction with a microvalve controlled reagent injection system we demonstrate that this prototype can act as a real-time assay of antiplatelet drug pharmacokinetics. In a proof-of-concept preclinical application, we utilize this system to investigate the way in which rapid dosing of human whole blood with isoform selective inhibitors of phosphatidylinositol 3-kinase dose dependently modulate platelet thrombus dynamics. This modular system exhibits utility as an automated multiplexable assay system with applications to high-content chemical library screening of new antiplatelet therapies.

Platelet Function During Extracorporeal Membrane Oxygenation in Adult Patients

Objective: Hemorrhagic and thromboembolic complications are common during support with extracorporeal membrane oxygenation (ECMO). As platelets play a pivotal role in hemostasis, we aimed to clarify how ECMO support affects platelet function.

Methods: We included 33 adult patients undergoing ECMO support at a tertiary ECMO referral center at Aarhus University Hospital, Denmark. Blood samples were collected on the first morning following ECMO initiation, and subsequently every morning until the 7th (±1) day. Platelet aggregation was evaluated by whole blood impedance aggregometry (Multiplate^® Analyzer) using adenosine diphosphate (ADPtest), arachidonic acid (ASPItest), and thrombin-receptor-agonist-peptide-6 (TRAPtest) as agonists. A new model was applied, taking platelet count into consideration in interpretation of impedance aggregometry analyses. On the 1st and 3rd day, platelet activation was assessed by flow cytometry (Navios) using collagen-related peptide, ADP, TRAP, and arachidonic acid as agonists.

Results: Blood samples from all 33 patients were analyzed on day 1 of ECMO support; 24 patients were still receiving ECMO and analyzed on day 3; 12 patients were analyzed on day 7 (±1). After ECMO initiation, platelet counts decreased significantly (p < 0.002) and remained low during ECMO support. ECMO patients demonstrated significantly reduced platelet aggregation on day 1 compared with healthy controls (all p < 0.001). However, when taking platelet count into consideration, platelet aggregation relative to platelet count did not differ from healthy controls. Flow cytometry analyses demonstrated impaired platelet activation in ECMO patients on day 1 compared with healthy controls (all p < 0.03). No substantial difference was found in platelet activation from day 1 to day 3 on ECMO support.

Conclusions: Employing impedance aggregometry and flow cytometry, we found both impaired platelet aggregation and decreased platelet activation on day 1 of ECMO support compared with healthy controls. However, platelet aggregation was not impaired, when interpreted relative to the low platelet counts. Furthermore, levels of bound fibrinogen, on the surface of activated platelets in ECMO patients, were higher than in healthy controls. Together, these findings suggestively oppose that platelets are universally impaired during ECMO support. No marked difference in activation from day 1 to day 3 was seen during ECMO support.

Validated model of platelet slip at stenosis and device surfaces

Platelets are central to thrombosis. However, it is unknown whether platelets slip at vascular or device surfaces. The presence of platelet slip at a surface would interrupt physical contact between the platelet and that surface, and therefore diminish adhesion and thrombosis. Unfortunately, no existing technology can directly measure platelet slip in a biological environment. The objective of this study was to explore whether microspheres-modeling platelets-slip at different vascular and device surfaces in an acrylic scaled-up model coronary artery. The microspheres (3.12 µm diameter) were suspended in a transparent glycerol/water experimental fluid, which flowed continuously at Reynolds numbers typical of coronary flow (200-400) through the model artery. We placed a series of axisymmetric acrylic stenoses (cross-sectional area reduction [CSAr], 20-90%) into the model artery, both without and with a central cylinder present (modeling a percutaneous interventional guide wire, and with a scaled-up Doppler catheter mounted upstream). We used laser Doppler velocimetry (LDV) to measure microsphere velocities within, proximal and distal to each stenosis, and compared to computer simulations of fluid flow with no-slip. For validation, we replaced the acrylic with paraffin stenoses (more biologically relevant from a surface roughness perspective) and then analyzed the signal recorded by the scaled-up Doppler catheter. Using the LDV, we identified progressive microsphere slip proportional to CSAr inside entrances for stenoses ≥60% and ≥40% without and with cylinder present, respectively. Additionally, microsphere slip occurred universally along the cylinder surface. Computer simulations indicated increased fluid shear rates (velocity gradients) at these particular locations, and logistic regression analysis comparing microsphere slip with fluid shear rate resulted in a c-index of 0.989 at a cut-point fluid shear rate of (10.61 [cm^-1]×mean velocity [cm×sec^-1]). Moreover, the presence of the cylinder caused disordering of microsphere shear rates distal to higher grade stenoses, indicating a disturbance in their flow. Finally, despite lower precision, the signal recorded by the scaled-up Doppler catheter nonetheless indicated slip at the entry into and at most locations distal to the 90% stenosis. Our validated model establishes proof of concept for platelet slip, and platelet slip explains several important basic and clinical observations. If technological advances allow confirmation in a true biologic environment, then our results will likely influence the development of shear-dependent antiplatelet drugs. Also, adding shear rate information, our results provide a direct experimental fluid dynamic foundation for antiplatelet-focused antithrombotic therapy during coronary interventions directed towards higher grade atherosclerotic stenoses.

Novel Stenotic Microchannels to Study Thrombus Formation in Shear Gradients: Influence of Shear Forces and Human Platelet-Related Factors

Thrombus formation in hemostasis or thrombotic disease is initiated by the rapid adhesion, activation, and aggregation of circulating platelets in flowing blood. At arterial or pathological shear rates, for example due to vascular stenosis or circulatory support devices, platelets may be exposed to highly pulsatile blood flow, while even under constant flow platelets are exposed to pulsation due to thrombus growth or changes in vessel geometry. The aim of this study is to investigate platelet thrombus formation dynamics within flow conditions consisting of either constant or variable shear. Human platelets in anticoagulated whole blood were exposed ex vivo to collagen type I-coated microchannels subjected to constant shear in straight channels or variable shear gradients using different stenosis geometries (50%, 70%, and 90% by area). Base wall shears between 1800 and 6600 s⁻¹, and peak wall shears of 3700 to 29,000 s⁻¹ within stenoses were investigated, representing arterial-pathological shear conditions. Computational flow-field simulations and stenosis platelet thrombi total volume, average volume, and surface coverage were analysed. Interestingly, shear gradients dramatically changed platelet thrombi formation compared to constant base shear alone. Such shear gradients extended the range of shear at which thrombi were formed, that is, platelets became hyperthrombotic within shear gradients. Furthermore, individual healthy donors displayed quantifiable differences in extent/formation of thrombi within shear gradients, with implications for future development and testing of antiplatelet agents. In conclusion, here, we demonstrate a specific contribution of blood flow shear gradients to thrombus formation, and provide a novel platform for platelet functional testing under shear conditions.

The role of PI3K/Akt signaling pathway in non-physiological shear stress-induced platelet activation

The PI3K/Akt signaling pathway has been implicated in playing an important role in platelet activation during hemostasis and thrombosis involving platelet-matrix interaction and platelet aggregation. Its role in non-physiological shear stress (NPSS)-induced platelet activation relevant to high-shear blood contacting medical devices (BCMDs) is unclear. In the context of blood cells flowing in BCMDs, platelets are subjected to NPSS (>100 Pa) with very short exposure time (<1 s). In this study, we investigated whether NPSS with short exposure time induces platelet activation through the PI3K/Akt signaling pathway. Healthy donor blood treated with or without PI3K inhibitor was subjected to NPSS (150 Pa) with short exposure time (0.5 s). Platelet activation indicated by the surface P-selectin expression and activated glycoprotein (GP) IIb/IIIa was quantified using flow cytometry. The phosphorylation of Akt, activation of the PI3K signaling, was characterized by western blotting. Changes in adhesion behavior of NPSS-sheared platelets on fibrinogen, collagen, and von Willebrand factor (vWF) were quantified with fluorescent microscopy by perfusing the NPSS-sheared and PI3K inhibitor-treated blood through fibrinogen, collagen, and vWF-coated microcapillary tubes. The results showed that the PI3K/Akt signaling was involved with both NPSS-induced platelet activation and platelet-matrix interaction. NPSS-sheared platelets exhibited exacerbated platelet adhesion on fibrinogen, but had diminished platelet adhesion on collagen and vWF. The inhibition of PI3K signaling reduced P-selectin expression and GPIIb/IIIa activation with suppressed Akt phosphorylation and abolished NPSS-enhanced platelet adhesion on fibrinogen in NPSS-sheared blood. The inhibition of PI3K signaling can attenuate the adhesion of unsheared platelets (baseline) on collagen and vWF, while had no impact on adhesion of NPSS-sheared platelets on collagen and vWF. This study confirmed the important role of PI3K/Akt signaling pathway in NPSS-induced platelet activation. The finding of this study suggests that blocking PI3K/Akt signaling pathway could be a potential method to treat thrombosis in patients implanted with BCMDs.

Device-induced platelet dysfunction in mechanically assisted circulation increases the risks of thrombosis and bleeding

Thrombotic and bleeding complications are the major obstacles for expanding mechanical circulatory support (MCS) beyond the current use. While providing the needed hemodynamic support, those devices can induce damage to blood, particularly to platelets. In this study, we investigated device-induced alteration of three major platelet surface receptors, von Willebrand factor (VWF) and associated hemostatic functions relevant to thrombosis and bleeding. Fresh human whole blood was circulated in an extracorporeal circuit with a clinical rotary blood pump (CentriMag, Abbott, Chicago, IL, USA) under the clinically relevant operating condition for 4 hours. Blood samples were examined every hour for glycoprotein (GP) IIb/IIIa activation and receptor loss of GPVI and GPIbα on the platelet surface with flow cytometry. Soluble P-selectin in hourly collected blood samples was measured by enzyme linked immunosorbent assay to characterize platelet activation. Adhesion of device-injured platelets to fibrinogen, collagen, and VWF was quantified with fluorescent microscopy. Device-induced damage to VWF was characterized with western blotting. The CentriMag blood pump induced progressive platelet activation with blood circulating time. Particularly, GPIIb/IIIa activation increased from 1.1% (Base) to 11% (4 hours) and soluble P-selectin concentration increased from 14.1 ng/mL (Base) to 26.5 ng/mL (4 hours). Those device-activated platelets exhibited increased adhesion capacity to fibrinogen. Concurrently, the CentriMag blood pump caused progressive platelet receptor loss (GPVI and GPIbα) with blood circulating time. Specifically, MFI of the GPVI and GPIbα receptors decreased by 17.2% and 16.1% for the 4-hours sample compared to the baseline samples, respectively. The device-injured platelets exhibited reduced adhesion capacities to collagen and VWF. The high molecular weight multimers (HMWM) of VWF in the blood disappeared within the first hour of the circulation. Thereafter the multimeric patterns of VWF were stable. The change in the VWF multimeric pattern was different from the progressive structural and functional changes of platelets with the circulation time. This study suggested that the CentriMag blood pump could induce two opposite effects on platelets and associated hemostatic functions under a clinically relevant operating condition. The device-altered hemostatic function may contribute to thrombosis and bleeding simultaneously as occurring in patients supported by a rotary blood pump. Device-induced damage of platelets may be an important cause for bleeding in patients supported with rotary blood pump MCS systems relative to device-induced loss of HMWM-VWF.

Device Thrombogenicity Emulation: An In Silico Predictor of In Vitro and In Vivo Ventricular Assist Device Thrombogenicity

Ventricular assist devices (VAD), a mainstay of therapy for advanced and end-stage heart failure, remain plagued by device thrombogenicity. Combining advanced in silico and in vitro methods, Device Thrombogenicity Emulation (DTE) is a device design approach for enhancing VAD thromboresistance. Here we tested DTE efficacy in experimental VAD designs. DTE incorporates iterative design modifications with advanced CFD to compute the propensity of large populations of platelets to activate by flow-induced stresses (statistically representing the VAD ‘Thrombogenic Footprint’). The DTE approach was applied to a VAD (MIN_DTE) design with a favorable thromboresistance profile and compared against a design (MAX_DTE) that generated an intentionally poor thromboresistance profile. DTE predictions were confirmed by testing physical prototypes in vitro by measuring VAD thrombogenicity using the modified prothrombinase assay. Chronic in vivo studies in VAD implanted calves, revealed MIN_DTE calf surviving well with low platelet activation, whereas the MAX_DTE animal sustained thromboembolic strokes. DTE predictions were confirmed, correlating with in vitro and in vivo thrombogenicity, supporting utility in guiding device development, potentially reducing the need for animal studies.

Prothrombotic activity of cytokine-activated endothelial cells and shear-activated platelets in the setting of ventricular assist device support

BACKGROUND

We systematically analyzed the synergistic effect of: (i) cytokine-mediated inflammatory activation of endothelial cells (ECs) with and (ii) shear-mediated platelet activation (SMPA) as a potential contributory mechanism to intraventricular thrombus formation in the setting of left ventricular assist device (LVAD) support.

METHODS

Intact and shear-activated human platelets were exposed to non-activated and cytokine-activated ECs. To modulate the level of LVAD-related shear activation, platelets were exposed to shear stress patterns of varying magnitude (30, 50, and 70 dynes/cm², 10 minutes) via a hemodynamic shearing device. ECs were activated via exposure to inflammatory tumor necrosis factor-α (TNF-α 10 and 100 ng/ml, 24 hours), consistent with inflammatory activation recorded in patients on LVAD circulatory support.

RESULTS

Adhesivity of shear-activated platelets to ECs was significantly higher than that of intact/unactivated platelets, regardless of the initial activation level (70 dynes/cm² shear-activated platelets vs intact platelets: +80%, p < 0.001). Importantly, inflammatory activation of ECs amplified platelet prothrombinase activity progressively with increasing shear stress magnitude and TNF-α concentration: thrombin generation of 70 dynes/cm² shear-activated platelets was 2.6-fold higher after exposure and adhesion to 100 ng/ml TNF-α‒activated ECs (p < 0.0001).

CONCLUSIONS

We demonstrated synergistic effect of SMPA and cytokine-mediated EC inflammatory activation to enhance EC‒platelet adhesion and platelet prothrombotic function. These mechanisms may contribute to intraventricular thrombosis in the setting of mechanical circulatory support.

Use of Computational Fluid Dynamics to Analyze Blood Flow, Hemolysis and Sublethal Damage to Red Blood Cells in a Bileaflet Artificial Heart Valve

Artificial heart valves may expose blood to flow conditions that lead to unnaturally high stress and damage to blood cells as well as issues with thrombosis. The purpose of this research was to predict the trauma caused to red blood cells (RBCs), including hemolysis, from the stresses applied to them and their exposure time as determined by analysis of simulation results for blood flow through both a functioning and malfunctioning bileaflet artificial heart valve. The calculations provided the spatial distribution of the Kolmogorov length scales that were used to estimate the spatial and size distributions of the smallest turbulent flow eddies in the flow field. The number and surface area of these eddies in the blood were utilized to predict the amount of hemolysis experienced by RBCs. Results indicated that hemolysis levels are low while suggesting stresses at the leading edge of the leaflet may contribute to subhemolytic damage characterized by shortened circulatory lifetimes and reduced RBC deformability.

Blood damage in Left Ventricular Assist Devices: Pump thrombosis or system thrombosis?

Introduction:

Despite significant technical advancements in the design and manufacture of Left Ventricular Assist Devices, post-implant thrombotic and thromboembolic complications continue to affect long-term outcomes. Previous efforts, aimed at optimizing pump design as a means of reducing supraphysiologic shear stresses generated within the pump and associated prothrombotic shear-mediated platelet injury, have only partially altered the device hemocompatibility.

Methods:

We examined hemodynamic mechanisms that synergize with hypershear within the pump to contribute to the thrombogenic potential of the overall Left Ventricular Assist Device system.

Results:

Numerical simulations of blood flow in differing regions of the Left Ventricular Assist Device system, that is the diseased native left ventricle, the pump inflow cannula, the impeller, the outflow graft and the anastomosed downstream aorta, reveal that prothrombotic hemodynamic conditions might occur at these specific sites. Furthermore, we show that beyond hypershear, additional hemodynamic abnormalities exist within the pump, which may elicit platelet activation, such as recirculation zones and stagnant platelet trajectories. We also provide evidences that particular Left Ventricular Assist Device implantation configurations and specific post-implant patient management strategies, such as those allowing aortic valve opening, are more hemodynamically favorable and reduce the thrombotic risk.

Conclusion:

We extend the perspective of pump thrombosis secondary to the supraphysiologic shear stress environment of the pump to one of Left Ventricular Assist Device system thrombosis, raising the importance of comprehensive characterization of the different prothrombotic risk factors of the total system as the target to achieve enhanced hemocompatibility and improved clinical outcomes.

Couette shearing device for the investigation of shear-induced damage of the primary hemostasis by left ventricular assist devices

INTRODUCTION:

Continuous-flow left ventricular assist devices have evolved from short-time therapy into permanent or so-called destination therapy. One complication in long-term usage is bleeding, which is presumably attributed to shear-induced interference of left ventricular assist devices with the coagulation system.

METHODS:

The influence of dynamic shear stresses on primary hemostasis by single or multiple passes through left ventricular assist devices was investigated. A novel Couette-type shearing device, especially fitted to simulate left ventricular assist devices with highly dynamic and repetitive stresses, was developed. To evaluate the clotting ability of the blood and thus the bleeding tendency, the closure time of the platelet function analyzer (PFA-100^®, Dade Behring, Marburg, Germany) was used. The relationship of the PFA-100 closure time was fitted to measurement points with shear stress and exposure time as parameters.

RESULTS:

76 samples of human blood collected from four different healthy donors in sodium-citrate anticoagulant solution were tested, including 20 control samples. A damage model according to the power law approach could be developed. A linear correlation of shear stress and exposure time to the PFA-100 closure time could be determined. In addition, a model was developed to calculate the increase in the PFA closure time on the basis of shear stress over time curves.

DISCUSSION:

With the shearing device, half-sine-wave-shaped shear stress patterns relevant to rotary blood pumps can be achieved with very good repeatability. The proposed damage model could be used to compare and optimize left ventricular assist devices under development. The tests showed a significant decrease in coagulability after only a few repetitions.

Microfluidic flow-based platforms for induction and analysis of dynamic shear-mediated platelet activation-Initial validation versus the standardized hemodynamic shearing device

A microfluidic flow-based platform (μFP), able to stimulate platelets via exposure of shear stress patterns pertinent to cardiovascular devices and prostheses, was compared to the Hemodynamic Shearing Device (HSD)-a state-of-the-art bench-top system for exposure of platelets to defined levels and patterns of shear. Platelets were exposed to time-varying shear stress patterns in the two systems; in detail, platelets were recirculated in the μFP or stimulated in the HSD to replicate comparable exposure time. Shear-mediated platelet activation was evaluated via (i) the platelet activity state assay, allowing the measurement of platelet-mediated thrombin generation and associated prothrombotic tendencies, (ii) scanning electron microscopy to evaluate morphological changes of sheared platelets, and (iii) flow cytometry for the determination of platelet phosphatidylserine exposure as a marker of shear activation. The results revealed good matching and comparability between the two systems, with similar trends of platelet activation, formation of microaggregates, and analogous trends of activation marker exposure for both the HSD and microfluidic-stimulated samples. These findings support future translation of the microfluidic platform as a Point-of-Care facsimile system for the diagnosis of thrombotic risk in patients implanted with cardiovascular devices.