Nanotopography and Plasma Treatment: Redesigning the Surface for Vascular Graft Endothelialisation

Abstract

Introduction: Current vascular graft materials in clinical use, such as PTFE and Dacron®, do not endothelialise and have low unacceptable patency rates. The importance of an endothelial cell layer on the luminal surface of a vascular graft is well-known. The influence of topographical features and surface chemistry on cellular adhesion and proliferation is recognised and under investigation. A nanocomposite polymer has been developed which has shown promise as a vascular graft material due to its compliant, biocompatible and anti-thrombogenic properties. However, despite these benefits a lack of endothelialisation is still a cause for major concern. Our aim in this work is to investigate the potential of plasma treatment and topographical structures on the luminal graft surface to enhance the self-endothelialisation potential of a nanocomposite vascular graft material. Methods: POSS-PCU is a polycarbonate urea urethane (PCU) with a nanoparticle, polyhedral oligomeric silsesquioxane (POSS) incorporated within it and fabricated according to published protocols. Microgrooves (MG) of pitch 25 μm were fabricated using photolithography and nanopits, Near-Square (NSQ), were fabricated using electron beam lithography. These were then embossed onto the POSS-PCU polymer and replication fidelity was confirmed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The samples then underwent oxygen plasma treatment at different powers at a fixed time (40 W, 60 W, 80 W at 60 seconds). Successful plasma treatment was confirmed by water contact angle (WCA) measurements. Human Umbilical Vein Endothelial Cells (HUVECs) were seeded onto the treated polymer samples and cell proliferation was measured using Live/Dead Cell® staining. Immunostaining of vinculin and actin was conducted to observe cell morphology and adhesion. Results: The embossing of the micro- and nanostructures were replicated with high fidelity, as seen by SEM and AFM. The microgrooves have a pitch size of 25 μm. NSQ was also verified to be 120nm pits with centre-centre spacing of 300 nm with ±50 nm offset in pit placement. Oxygen plasma treatment of the different samples, show that increase in power increased significantly the hydrophilicity of the samples (p < 0.05). These had a direct impact on giving the optimal surface on which HUVECs preferentially proliferate and adhere, with an average WCA of 68°, giving the highest HUVEC growth. HUVEC proliferation was seen to increase on NSQ surfaces over MG and planar samples, retaining both morphology and function. Conclusion: These exciting observations indicate an important role for nanotopography and plasma treatment in the development of vascular grafts.