We present a novel type of injectable biomaterial with an elastic softening transition. The material enables in-vivo shaping, followed by induction of 3D stable vascularized tissue adopting the desired shape. We establish the necessary geometrical and physical parameters by extensive numerical simulation. Irregular particle shape dramatically enhances yield strain for in-vivo stability against deformation, while friction and porosity provide the elastic softening transition as an emergent meta-material property. Accordingly, we synthesize our injectable meta-biomaterial as a suspension of irregularly fragmented, highly porous sponge-like microgels. The meta-biomaterial exhibits both high yield strain, and the desired novel elastic softening transition for in-situ shaping and unprecedented dynamic matching of adipose tissue mechanics. In vivo, predetermined shapes can be sculpted manually after subcutaneous injection in mice. The 3D shape is maintained during excellent host tissue integration into the particle pore space. The meta-biomaterial sustains vascularized connective tissue to the end of one-year follow-up.