In this thesis a flexible leaflet polymeric prosthetic aortic heart valve was designed, manufactured, and tested. The prosthesis was designed with the aim of overcoming the need for anticoagulant therapy, which is required for current mechanical prostheses; while also having lifelong durability, which current bioprosthetic heart valves are not able to achieve. Inspired by the anisotropic architecture of collagen in the natural valve, a shortlist of polystyrene based block copolymers (BCPs), which can be processed to yield mechanically anisotropic materials, was proposed. The shortlist was evaluated based upon processability, biostability, ex vivo haemocompatibility, and a novel material performance index comprising the flexural modulus and the cyclic fatigue stress predicted by fracture mechanics methods. Polystyrene-block-polyethylene-polypropylene-block-polystyrene with 22 mol% polystyrene (SEPS22) was selected for further testing and use in the design. Haemocompatibility and calcification of the BCPs was assessed against reference materials. In measures of coagulation and thrombogenicity the BCPs were better than polyester, but worse than expanded polytetrafluoroethylene and pericardium graft materials. In measures of inflammation, the BCPs and polytetrafluoroethylene were better than polyester and pericardium. A durable heparin coating gave SEPS22 superior haemocompatibility compared to all the reference materials. The BCPs calcified less than pericardium, but calcification still accelerated failure. The technique of injection moulding discs of the BCP from a point was used to create a novel biaxial structure of cylindrical polystyrene domains. A combination of modelling and bench-scale injection moulding was used to select a point from which the prosthetic heart valve injection tool cavity should be filled. By simultaneously injecting at a point at the centre of the free edge of each leaflet, a bioinspired orientation was produced. Based upon hydrodynamic testing, a spherical form leaflet design was selected. The hydrodynamic performance of the complied with the ISO 5840 standard for cardiac valve prostheses, but the fatigue performance was inadequate due to the leaflets being thinner than specified due to manufacturer error. Fatigue prediction and finite element analysis were used to conjecture that correctly manufactured polymeric valves could theoretically reach the ISO limit, indicating that there is potential for polymeric prostheses to overcome the issues of durability and need for anticoagulation.