GM (41, 48, 59, 60) were synthesized from 11.1% (w/v) gelatin type A (Sigma-Aldrich) using a water-in-oil single emulsion technique and cross-linked for 4 hours with 1% (w/v) genipin (Wako USA, Richmond, VA) (60). Hydrated GM were light blue in color and spherical in shape with an average diameter of 52.9 ± 40.2 μm and a cross-linking density of 25.5 ± 7.0% (48). Growth factor–loaded microspheres were prepared by soaking cross-linked, ultraviolet (UV)–sterilized GM in a solution (80 μg/ml) of rhTGF-β1 (PeproTech, Rocky Hill, NJ) in PBS for 2 hours at 37°C. Unloaded microspheres without growth factor were hydrated similarly using only PBS.