The present invention provides a polymer membrane enhanced with cured epoxy resin for use as the outer membrane of biosensors. The membrane includes approximately 30-80% epoxy resin adhesives, 10-60% polymer such as poly(vinyl chloride), polycarbonate and polyurethane and 0-30% plasticizers and 5-15% surface modifier reagent such as polyethylene oxide-containing block copolymers. Utilizing the polymer membrane of the present invention, a three-layered sensing element has been developed. This sensing element will be particularly useful for miniaturized biosensors used for in vitro blood measurements or for continuous in vivo monitoring such as implantable biosensors. This element includes an enzyme layer, an interference-eliminating layer and the novel polymer member of the present invention as the outer polymer layer. This novel sensing element shows excellent response characteristics in solutions and has an extremely long lifetime. This technology is particularly useful for improving the lifetime of implantable biosensors.