Development of hydrogel-based strategies for tissue vascularisation

Abstract

The tissue engineering field and regenerative medicine has emerged as a promising strategy to overcome the lack of availability of organs and tissue grafts for transplantation. One of the main challenges in the field is to generate tissue analogues of relevant size with adequate nutrient and oxygen supply that promote graft integration and long-term functionality. The main strategies to promote tissue vascularisation are to either deliver bioactive cues such as growth factors (GFs), which attract the host vessels towards the area of interest, or to directly generate tissue analogues including a vessel network that can anastomose with the host blood vessels after implantation. This thesis aims to develop novel platforms to improve tissue vascularisation by delivering GFs and generating in vitro vascularised tissues. Firstly, a literature review summarises the main strategies to deliver GFs and to generate in vitro vascularised tissues, together with their present limitations for clinical translation. Next, a GF delivery system based on tyraminated poly(vinyl alcohol) (PVA-Tyr) is developed, which has the ability to covalently incorporate a range of native GFs and then release them in a controlled profile and during a tailorable time frame. The flexibility of this GF delivery system allowed its adaptation to the revascularisation of the femoral head in an in vivo model of avascular necrosis, which was done by adjusting the incorporated GF, the GF release time, the cell-material interactions of the system and its delivery method. This research is followed by the development of a gelatin-norbornene (Gel-NOR) platform that allows biofabrication of large vascularised tissues. The fabricated tissues combined a controllable pore structure, which mimicked larger vessels in native tissues, with encapsulated endothelial cells that were able to assemble into micro-capillary structures, mimicking the capillary beds in native tissues. These fabricated tissues were used as a model to study how design parameters can affect the behaviour of endothelial cells, demonstrating that pore diameter and fibre diameter could have an impact on the ability of endothelial cells to form micro-capillaries. Finally, the developed platforms were combined using bioassembly, which allowed understanding how different cell arrangements can affect endothelial cell behaviour within fabricated scaffolds and to develop tissue analogues that combine GF delivery with pre-vascularisation techniques. Overall, this thesis combines materials chemistry and design with advanced fabrication techniques to develop new strategies that address current limitations in the field of tissue vascularisation.