Biophysical mechanisms regulating Von Willebrand disease, arterial thrombosis, and deep vein thrombosis in microfluidic models of vascular injury

Abstract

Thrombus formation is regulated by biophysical mechanisms in ways that are not fully understood. Platelets are transported to injuries at rates that depend not only on the bulk flow, but also collisions with red blood cells (RBC). Their ability to tether to the subendothelium depends on shear stresses at the injury and can be impaired by deficiencies in Von Willebrand factor (VWF). The subsequent rate of fibrin formation is a function of the mass transfer of coagulation factors and of surface reaction rates. In this thesis, I detail studies of these biophysical mechanisms using microfluidic models of arterial thrombosis and a novel venous thrombosis model. In a flow chamber, I perfused whole blood from patients presenting with clinical bleeding over collagen. I found that at elevated shear rates, platelet accumulation was sensitive to VWF deficiencies in patients with low VWF levels and type I Von Willebrand Disease (VWD). From the assay, I was able to discriminate type I VWD patients from healthy controls, suggesting that microfluidic technologies can be adapted into a clinical setting. Using a low Reynolds number microfluidic mixer I developed, I showed that a clinically relevant increase in hematocrit increased platelet accumulation but not fibrin formation on a fibrillar collagen surface at an arterial shear rate. In concert with in vivo and in silico data, this result suggests that an elevated hematocrit increases the contact time platelets have with a growing thrombus, leading to more bond formations and an accelerated thrombus growth. This result provides a rationale for antiplatelet therapy for patients exhibited elevated hematocrit. Venous thrombosis is less characterized than arterial thrombosis. To my knowledge, I created the first microfluidic system that includes secondary flows and coagulation as a way to model the propagation of a venous thrombus out of a valve pocket. While traditionally thought of as a coagulation dependent system, my model shows the critical importance of platelets and platelet-RBC collisions in this propagation. This study justifies antiplatelet therapy for deep vein thrombosis, and provides a novel framework for future mechanistic studies of platelet activation and function in venous thrombosis.