Abstract: The present invention provides a composition comprising a polymer and a natural or synthetic peptide or protein (NSPP). The composition forms a hydrogel with water. The composition is useful as a filler for cosmetic and therapeutic applications. Embodiments of the invention provide methods of treating certain conditions using the composition or hydrogel, and surgical kits for the simultaneous or sequential administration of the respective components of the composition, enabling the formation of the hydrogel in situ.