(3) polyurethane hydrogel prepolymers which can be crosslinked in aqueous solution by reaction with --NH, --SH, --OH, --COOH or H2 O. The following is an idealized structure of a representative polyurethane hydrogel prepolymer which is made by taking a water soluble polyether polyol having greater than two hydroxyl groups per molecule and a molecular weight greater than 3,000 and capping each hydr