Innovative bioactive scaffold technologies for vascular tissue engineering: influences of morphology and composition

Abstract

Vascular disease is currently the leading cause of mortality worldwide, with coronary heart disease, peripheral arterial disease and strokes accounting for upwards of 30% of all deaths in Europe. Bypass grafting is one of the major approaches utilised in the treatment of vascular disease. This technique involves diverting blood flow around an arterial blockage by grafting in an alternative path. This surgery is widely used in the treatment of coronary heart disease and peripheral arterial disease. This is either done with a synthetic material, such as PTFE, or by using an autologous vessel such as the saphenous vein or the internal thoracic artery. The grafting of autologous vessels is considered the gold standard for small diameter bypass grafts due to the higher patency rates compared to their synthetic counterparts. However, there are only a finite amount of vessels that can be harvested and in many cases the vessels are not of a high enough quality for use in surgery. Therefore, there is an urgent need for the development of novel biomaterials that can improve patency rates in bypass grafts. As a push for solutions to this problem, the field has placed a focus on improving the biofunctionality of materials in vitro as a means of testing the translatability of the material. This thesis presents four different methods of improving the bioactivity of scaffolds for vascular tissue engineering through alteration of the morphology and composition of the scaffold: 1) Altering the fibre diameter of electrospun polycaprolactone (PCL) scaffolds to enhance the morphology for seeded cells. 2) Native aortic and heart bovine extracellular matrices (ECM) were incorporated into PCL scaffolds. 3) Cell secretome was bound to the PCL scaffolds to enhance the bioactivity of the scaffold. 4) Altering the cell’s response through environmental factors such as hydrostatic pressure and hypoxia. Scaffolds were either seeded with human umbilical vein endothelial cells (HUVECs) or human umbilical vein smooth muscle cells (HUVSMCs) and their response to their microenvironments were analysed. Gene expression analysis and immunohistochemical analysis showed that altering the fibre diameter of the scaffold had evident effects on cellular response. For example, increasing fibre diameter had the effect of increasing cellular infiltration for both cell types. HUVECs upregulated key phenotypic genes when seeded on the largest fibre diameter. On the contrary, the HUVSMCs upregulated key genes when cultured on the smallest fibre diameter. Likewise, the incorporation of ECMs into the scaffold altered their mechanical properties and changed the biological response of the seeded cells. Incorporating ECM had the effect of decreasing stiffness and increasing scaffold elasticity. Furthermore, the incorporation of aortic ECM into the fibres led to higher cell viability for both cell types. Additionally, the binding of proteins released by cultured cells to the PCL scaffold was shown to alter gene expression and cell survival. Each method undertaken altered cellular responses such as gene expression and cell viability, indicating that these methods provide a viable and translatable platform for vascular tissue engineering. These scaffold have great potential for vascular tissue engineering and offer translatability to other tissue types.