Abstract: Described herein are injectable in vivo crosslinking materials for use as soft tissue filler comprising (a) a reactive multi-arm polymer that comprises a plurality of hydrophilic polymeric arms, at least a portion of the hydrophilic polymeric arms comprising one or more reactive end groups and (b) a multifunctional compound that comprises functional groups that are reactive with the reactive end groups of the reactive multi-arm polymer. Also described herein are systems and methods that are based on such materials.