In accordance with one aspect of the invention, injectable compositions are provided, which contain temperature-sensitive hydrogel particles. The hydrogel particles may be provided in dry form, or they may be provided in hydrated form in an aqueous fluid. The temperature-sensitive hydrogel particles may have an upper critical solution temperature (UCST) below normal body temperature, they may have a lower critical solution temperature (LCST) above normal body temperature, or they may have a LCST that changes from below normal body temperature to above normal body temperature after injection into a subject.