Characterizing blood protein surface interactions for the development of thromboresistant fluoropolymer coatings

Abstract

Patients with long term blood-contacting medical devices will continue to require risky systemic anticoagulant administration until the effects of device thrombogenicity can be adequately addressed. Adsorption of the protein, fibrinogen, to biomaterials is broadly acknowledged as the primary mediator of platelet adhesion and aggregation, yet fibrinogen in its native soluble form is inactive and circulates harmoniously with platelets in the bloodstream. Therefore, the adsorption process induces a structural change to the protein that exposes platelet-binding epitopes. Complete elimination of protein adsorption on biomaterial surfaces for extended durations has proven a significant challenge, and even ultralow levels of surface fibrinogen can initiate full platelet activation and thromboembolism formation. However, a body of evidence suggests that platelet membranes themselves are inert against continued exposure to blood after initial degranulation. Thus, perhaps a more realistic approach to developing long-term blood contacting materials is to engineer surfaces that adsorb fibrinogen in a layer that rapidly promotes uniform platelet adhesion and spreading to form a smooth passivating layer against further activation by circulating blood components. Separately, fluoropolymers hold a long history of delivering favorable outcomes for blood-contacting applications. Our group possesses expertise in glow-discharge plasma polymerized fluorocarbons, which have historically demonstrated exceptionally high patency rates, high blood flow, and low embolization rates in in vitro studies and ex vivo primate shunt models compared to other materials of their class. However, the mechanism behind their favorable adsorption of blood protein constituents or conformations to resist long-term activation of blood cellular components deserves more investigation to properly characterize their mode of action. In this dissertation, we present several studies of blood protein interactions on a custom plasma-polymerized fluorocarbon (ppC3F6) developed in our research group in comparison with standard commercial fluoropolymer counterparts to further explore the platelet membrane passivation mechanism. We begin with an overview of the historical challenges in addressing hemocompatibility and the viability of engineered fluoropolymers to fulfill long-term blood contacting needs (Chapter 1). We then delve into preparation and characterization of a small selection of fluoropolymers (Chapter 2), followed by assessments on differential total protein adsorption using quartz crystal microbalance with dissipation (Chapter 3). Given that the origins of thrombus formation depend on adsorption-induced structural changes of fibrinogen in addition to total adsorption, we pursued a series of studies investigating these changes across our fluoropolymer materials (Chapter 4). We then discuss our effort to better understand structurally sensitive regions in the fibrinogen platelet-binding domain that give rise to bleeding disorders including thrombosis through molecular dynamics simulations (Chapter 5). Finally, we assess platelet adhesion and activation in relation to our fluoropolymers preadsorbed with fibrinogen to identify relationships between fluoropolymer surface chemistry, adsorbed blood protein surface composition and structural bioactivity, and cellular response (Chapter 6), concluding with proposed mechanisms of improved blood compatibility and remarks on future directions for the application of plasma-polymerized fluoropolymers in blood-contacting devices (Chapter 7).