Injectable Hydroxyapatite-Reinforced Methacrylated Recombinant Type III Collagen Microgels for Soft-Tissue Filling

Injectable fillers that combine immediate volume restoration with a sustained biological response remain of considerable interest in minimally invasive aesthetic medicine. In this study, a hydroxyapatite-loaded methacrylated recombinant type III collagen microgel (HAp@rhCol III-MA) was prepared by in situ coprecipitation and photocrosslinking. Recombinant type III collagen (rhCol III) was functionalized with methacryloyl groups to obtain rhCol III-MA, after which a calcium phosphate phase was mineralized in the presence of the modified collagen and the collagen phase was crosslinked under ultraviolet irradiation. More than 80% of the microgel particles prepared at 900 rpm were 20-80 μm in diameter. Spectroscopic, elemental, thermal, and diffraction analyses supported the incorporation of a poorly crystalline, HAp-compatible calcium phosphate phase, while rheological measurements showed higher storage and loss moduli than those of rhCol III-MA gel. HAp@rhCol III-MA did not reduce NIH-3T3 cell viability at the tested concentrations and enhanced HUVEC scratch closure and tube-network formation in vitro. Following subcutaneous implantation in rats, the microgel retained more volume than rhCol III-MA during the early and intermediate observation periods, with residual volumes of 56.58 ± 3.03 mm³ for HAp@rhCol III-MA and 52.80 ± 1.79 mm³ for rhCol III-MA at day 59; the commercial type I collagen comparator retained 123.23 ± 2.80 mm³. The composite caused no evident tissue injury and was associated with progressive collagen deposition and a low, declining CD68-positive response. These findings support further investigation of HAp@rhCol III-MA as an injectable dermal-filling material, while long-term persistence and clinical injection performance remain to be established.