This invention relates to a hydrogel based on a copolymer comprising (a) at least one monomer comprising covalently bound iodine or bromine, and; (b) at least one hydrophilic monomer, wherein the hydrogel, when fully hydrated, has an elastic modulus between 0.01-100 MPa at 37.degree. C. The invention also relates to a prosthetic nucleus comprising the hydrogel according to the invention and a method for making the prosthesis.