A polysiloxane prepolymer that is useful for forming biomedical devices has the formula: M(*Dii*PS).sub.x*Dii*M (I) wherein: each M is independently a polymerizable ethylenically unsaturated radical; each Dii is independently a diradical residue of a diisocyanate; each PS is independently a diradical residue of a polysiloxane-diol or a polysiloxane-diamine; each * is independently --NH--CO--NH--, --NH--COO-- or --OCO--NH--; and x is at least 2.