methods for treating xenogenic tissue for implantation into a human body including in - situ polymerization of a hydrogel polymer in tissue , and tissue treated according to those methods , where the polymerization takes place in tissue that has not been fixed with glutaraldehyde . the polymerization may only fill the tissue , bind the polymer to the tissue , or cross - link the tissue through the polymer , depending on the embodiment . one method includes free radical polymerization of a first vinylic compound , and can include cross - linking through use of a second compound having at least two vinyl groups . another method utilizes nucleophilic addition polymerization of two compounds , one of which can include peg and can further include hydrolytically degradable regions . in one embodiment , applicants believe the in - situ polymerization inhibits calcification , and that the polymerization of tissue un - fixed by glutaraldehyde allows for improved penetration of the polymer . the methods find one use in the treatment of porcine heart valve tissue , intended to extend the useful life of the valves by inhibiting calcification . the incorporation of degradable hydrogel regions may initially fill the tissue and reduce any initial inflammatory response , but allow for later infiltration by cells to remodel the tissue .