process for forming a multilayer blood vessel prosthesis . each layer is formed from bioreplaceable materials which include those produced by contacting collagen with an aminopolysaccharide and subsequently covalently crosslinking the resulting polymer , polymers of hydroxyacetic acid and the like . cross flow filtration molding and wet extrusion molding are two processes which are particularly useful for forming the inner layer of the blood vessel prosthesis . the outer layer of the blood vessel prosthesis is preferably formed by freeze drying a dispersion of the bioreplaceable material onto the inner layer . the disclosed blood vessel prosthesis is a multilayer structure with each layer having a porosity and other physicochemical and mechanical characteristics selected to maximize the effectiveness of the blood vessel . the prosthesis functions initially as a thromboresistant conduit with mechanical properties which match those of the adjacent natural blood vessel . eventually , the prosthesis functions as a regeneration template which is replaced by new connective tissue that forms during the healing process following attachment of the prosthesis .