Influence of Microsphere‐to‐Hydrogel Matrix Composition on Long‐Term Volume Retention and Host Tissue Remodeling of Injectable Composite Collagen Hydrogels

The most commonly used materials for soft tissue augmentation, including autologous fat and hyaluronic acid, remain limited by inconsistent graft survival or repeated application to maintain volume. Composite collagen hydrogels (CCH) containing high-density crosslinked collagen microspheres embedded within a low-density collagen hydrogel, have demonstrated significantly enhanced vascularization, enabling simultaneous skin grafting and neodermis formation in one surgical step in preclinical models. Herein we evaluated an injectable form of the CCH (iCCH) to determine its long-term volume retention, host tissue integration, and remodeling following subcutaneous implantation. One percent collagen microspheres were mixed with 0.3% collagen hydrogel at microsphere-to-hydrogel ratios of 70:30 (MS70), 50:50 (MS50), and 30:70 (MS30). A microsphere-only formulation (MS-Only; 70:30 microsphere:PBS) and a 1% hyaluronic acid (HA) vehicle were included as controls. Each wild-type mouse received two 0.5 mL dorsal subcutaneous administrations of the assigned material. The iCCH formulations demonstrated cellular infiltration, neovascularization, and progressive inter-microsphere tissue remodeling, while MS70 showed near-complete construct volume retention at 12 months. Across the iCCH formulations, a mild and transient inflammatory response without evidence of an adverse foreign body reaction was observed. Overall, these findings support the potential of iCCH as a durable collagen scaffold for soft tissue augmentation.