This thesis describes a fatigue lifetime prediction model for anisotropic styrenic block copolymers. A shortlist of cylinder forming block copolymers has been tested for a flexible leaflet aortic prosthetic heart valve, designed to mimic the native structure of the aortic leaflet. Polymeric valves have the potential to overcome the limit of current available prosthesis, however no such valve is available clinically at present. Durability and calcification are among the main issues with polymeric valves designed and tested previously. The solution to these problems resides in the correct choice of material. In this study, a fatigue model was validated for anisotropic styrenic block copolymers to assist in the selection of the most durable material. The prediction is based on a unified approach of crack growth and nucleation tests. The model correctly predicted the lifetime of the material with same microstructure orientation, thickness and geometry. Material comparison based on the fatigue results highlighted the most durable material, a poly(styrene-b-butadiene-b-styrene) with 20% weight of styrene. The material was manufactured into a valve, which comfortably exceeded ISO standards for in vitro durability and hydrodynamic perfomance. Calcification and its effect on durability were tested. The calcification test was conducted in a simulated body fluid solution. Calcium levels on the styrenic block copolymers were significantly lower than on bovine pericardium, which is one of the materials used in clinical prosthesis. Durability of the calcified valve was not affected, indicating that the selected materials have great potential for biomedical applications. Finally, the effect of heparin coating was measured for both durability and calcification. The coating increased the level of calcification but did not affect durability in either unitensile specimens or valve prototypes.