In one embodiment, the invention relates to a composition suitable for preparing a hydrogel useful as replacement material for all or part of a disc nucleus. The composition comprises: a curable, cross-linkable, non-immunogenic, non-toxic protein comprising a minimum of about 10% and a maximum of about 25% of the hydrogel by weight; a metal contrast agent comprising a minimum of about 2% and a maximum of about 70% of the hydrogel by weight; and a crosslinker comprising a minimum of about 0.1% and a maximum of about 10% of the hydrogel by weight; wherein the composition has a discernable radiopacity level.