is then washed with ethanol to remove any lipid content and excess water in preparation for the solvent defatting process the collagen is defatted by treatment with 1 1 ethanol ethyl ether mixture and recovered as a viscous solid usually by cheesecloth filtering it is then air dried and subsequently converted to a gel by solubilization in acidified water at a ph of about 3 0 the pellet formed by the ultrafugation step contains between 0 5 and 20 collagen proteins preferably about 1 to 8 collagen protein most preferably about 4 collagen protein with the balance water other biocompatible materials can also be included in the prosthesis compositions of the percent invention including polyhydroxyethylmethacrylate poly hema vitrosin a fibrous protein found in mammalian vitreous humor and other additives known in the art such as plasticizers to reduce brittleness and increase flexibility silicones to extend the collagen and other materials known in the art such materials may tend to prevent cell migration into the prosthesis such materials may be incorporated in any amount which does not adversely affect transparency of the resultant compositions or cause loss of biocompatibility e g the material may cease to be non immunogenic preferably the additional material is added in amounts of 0 01 to 50 percent more preferably from about 0 9 to about 20 by weight the prosthesis material of the present.