PROJECT SUMMARY/ABSTRACT Mechanical circulatory support (MCS) is a critical tool to treat heart or lung failure, in the form of extracorporeal circulation through membrane oxygenation or through a ventricular assist device. Thrombosis and bleeding remain major complications with MCS. As a result, patients receive systemic anticoagulation to prevent thrombosis. However, this can increase the risk for bleeding, which is the most common complication in MCS. To counter this issue, there has been a large effort to eliminate or minimize the need for anticoagulation. Surprisingly, even if anticoagulation is eliminated, studies demonstrate that bleeding remains highly prevalent, while thrombosis remains relatively unaffected. Therefore, there is a need to focus on alternative pathways to bleeding. Almost all patients on MCS experience the bleeding disorder acquired von Willebrand syndrome. Furthermore, patients, especially pediatric patients, experience platelet dysfunction and can exhibit low platelet counts. We attribute these events to the flow environment in MCS. While many groups have focused on the effect of shear stress on blood, our group discovered an unprecedented role for turbulence in driving loss of high and even intermediate molecular weight von Willebrand factor (VWF) multimers, reducing the ability for VWF to bind to platelets and to collagen. Furthermore, there is strong evidence that flow in MCS is causing signals for platelet activation, but also clearance and cell death, with an unknown effect of turbulence. The combination of signals in response to flow can lead to both thrombosis and hemorrhage, depending on the balance of events. Our goal is identify what specific conditions lead to VWF or platelet functional loss in response to flow by pursuing three aims. 1) We will quantify changes in thrombus growth in response to turbulence relative to laminar shear conditions for various anticoagulants. 2) We will quantify the increased cleavage occurring in turbulence relative to laminar flow for similar shear stress conditions and how VWF function varies after flow exposure with and without flow-induced extension. 3) We will assess platelet state after exposure to different flow regimes and how this changes with the presence of VWF or with potential new therapeutic targets. Altogether, this work will distinguish the impact of turbulence relative to shear stress on blood, which could lead to improved design criteria for blood-contacting medical devices and potential therapeutics if we identify specific pathways leading to dysfunctional hemostasis.