Weighted collagen microsponges having a highly crosslinked collagen matrix are described suitable for use in culturing organisms in motive reactor systems. The microsponges have an open to the surface pore structure, and pore sizes and volumes suitable for immobilizing a variety of bioactive materials. The microsponges also have an average particle size in the range of about 100 to about 1000 microns and a specific gravity above about 1.05.