<p>A polysiloxane prepolymer that is useful for forming biomedical devices has the formula: M(*Dii*PS)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.</p>
