Enhancing the biocompatibility of coronary artery stents

Abstract

Cardiovascular disease, in particular coronary artery disease is a leading cause of morbidity and mortality worldwide. Coronary artery disease occurs as a consequence of atherosclerosis. Impediment to coronary blood flow causes myocardial ischemia, manifesting clinically as angina. Plaque rupture can lead to rapid vessel occlusion and myocardial necrosis. Revascularization (restoration of normal blood flow) can be achieved percutaneously with balloon angioplasty and coronary stent placement. Coronary stents are the most commonly implanted medical prostheses. To date the majority of commercially available stents are constructed from metallic alloys. Implantation of stents in the vasculature has two main biocompatibility issues 1) metals are thrombogenic, 2) stent deployment injures the vessel wall. Stent thrombosis and in stent restenosis are clinical consequences of stent thrombogenicity and vessel injury, respectively. Potent anti‐platelet agents and use of anti‐proliferative drug eluting stents reduce thrombogenicity and restenosis at the cost of increased bleeding and retardation of stent strut endothelialization leading to very late stent thrombosis. The interaction of biological systems with biomaterials is highly dependent on surface properties. Modification of the physical and chemical properties of the surface offers a simple and effective meanings to modulate the biological response to stents without altering the mechanical benefits that metallic alloys offer. Recently, plasma polymer deposition of thin films has been adapted for metallic substrates and three dimensional structures. These films reduce thrombogenicity and can attach biomolecules. One such molecule, recombinant human tropoelastin (rhTE) when attached to the films has been shown to enhance endothelial activity. Increasingly endothelial progenitor cells (EPCs) have been implicated in the maintenance of vascular health. Of particular interest is the capacity of these cells to participate in the healing of injured endothelium. Animal models demonstrate the ability of these cells to home in to sites of iatrogenic vessel injury and contribute to re‐endothelialisation. The goal of this thesis is to design a reproducible and scalable biocompatible coating platform for coronary stents. We explored the capacity of rhTE to support EPC activity followed by a mechanistic study of the nature of this interaction. Following this, we purpose built a plasma polymer film deposition chamber, optimizing for consistent and predictable film production. Nitrogen content of the plasma polymer films were progressively increased. A detailed physical, chemical and biological characterization of these nitrogenized films was carried out. There were several key findings from this thesis. We found rhTE supported EPC attachment, spreading and proliferation via an integrin mediated process. Using truncated rhTE constructs we were able to narrow down the site of interaction on rhTE to a region between N‐terminal domains 10 and 18. By increasing the flow of nitrogen during plasma film deposition we successfully created films with increasing concentration of nitrogen. Physical and chemical analyses demonstrated an amorphous carbon structure with smooth topography, containing nanoscale p‐conjugated graphite‐ like clusters, independent of nitrogen content. In contrast, nitrogen doping increased surface wettability and the amount of polar functional groups. In thrombogenicity assays lower thrombus formation, platelet adhesion and activation correlated with higher nitrogen concentration. Surprisingly, highly nitrogenized films also enhanced endothelial cell and EPC attachment and proliferation independent of rhTE functionalization. Nitrogenization of the plasma polymers did not impact on the capacity to covalently attach proteins. Nitrogenized plasma films is a promising platform to improve the biocompatibility of existing coronary stents.