Small calibre polyhedral oligomeric silsesquioxane nanocomposite cardiovascular grafts: Influence of porosity on the structure, haemocompatibility and mechanical properties

There is a significant worldwide demand for a small calibre vascular graft for use as a bypass or replacement conduit. An important feature in determining the success of a graft is the wall structure, which includes porosity, pore size and pore interconnectivity, as these play a crucial role in determining the long-term patency of a bypass graft. In this study we fabricate a small diameter (<5 mm) vascular graft from polyhedral oligomeric silsesquioxane–poly(carbonate urea)urethane (POSS–PCU) via an extrusion, phase inversion method using an automated, custom built machine. Through the dispersion of a porogen, sodium bicarbonate (NaHCO3), in controlled concentrations (0–55%) we were able to produce grafts with well-defined pore morphologies. The impact of NaHCO3 concentration on the structure of the graft wall and its influence on the mechanical and haemocompatibility properties are evaluated here. Scanning electron microscopy and mercury porosimetry were used to characterise graft structure. Atomic force microscopy elucidated any changes in surface morphology. The addition of NaHCO3 improved the pore interconnectivity and increasing the concentration of NaHCO3 led to grafts with rougher surfaces and larger pore sizes. The ultimate tensile strength and suture retention decreased with increasing concentrations of NaHCO3, while graft compliance increased. To evaluate haemocompatibility platelets and peripheral blood mononuclear cells (PBMC) were incubated on a range of different graft samples. Platelet adhesion, PBMC surface receptor expression (CD14, CD86, CD69 and HLA-DR) and cytokine release (PF4, IL-1β, IL-6, IL-10, TNFα) were all measured. Increasing numbers of platelets adhered to grafts produced with no NaHCO3, which exhibited a smooth surface morphology, and PBMC adherent on these grafts expressed higher levels of CD14 and CD86. Whilst the different graft samples induced varying levels of cytokine secretion in vitro, no distinct pattern suggesting a non-trivial relationship was observed.