116. A Novel Injectable Composite Collagen Hydrogel That Induces Long Lasting New Tissue Formation

PURPOSE: Soft tissue fillers are commonly used in both reconstructive and aesthetic procedures. Among the most popular filler materials are autologous fat and hyaluronic acid, but the former provides inconsistent graft survival and causes donor site morbidity. Although hyaluronic acid (HA) is available off-the-shelf, it requires reapplication every 1-2 years since it largely gets absorbed without forming new tissue. Pre-clinical porcine studies using composite collagen hydrogels (CCH), which consist of high-density collagen microspheres within a low-density collagen matrix, demonstrated significantly enhanced cellular infiltration and vascularization allowing for simultaneous skin grafting and neodermis formation in one surgical step. Herein we evaluate a flowable form of the CCH to determine if it would similarly promote local tissue integration and facilitate the development of new tissue at the site of injection. METHODS: Sterile 1% bovine atelocollagen microspheres (80-300um in diameter) were mixed with 0.3% telocollagen bulk at different microsphere-to-bulk ratios, including 70:30, 50:50, 30:70, to fabricate a homogeneous viscous solution for injection. 500ul of the mixed collagen suspension was then injected subcutaneously bilaterally on the dorsa of wildtype mice. The mixed collagen suspension was cured into a hydrogel in situ after injection. Quantification of volume retention and histologic analysis of the injected substrates were performed. RESULTS: The CCH flowed easily, allowing for straightforward subcutaneous injection via an 18G needle without leakage. After 6 months in vivo, the shape and projection of the injected collagen hydrogel was well-maintained across all groups, especially in the 70:30 composite collagen group (70:30: 88.3%, 50:50: 81.0% of initial volume). Histologic analysis demonstrated a uniform distribution of collagen microspheres throughout the injected composite hydrogel, with notable cell infiltration and vascularization observed within the bulk collagen after only 1 month. The vessel formation within the CCH increased over 6 months, particularly in the 70:30 group. The average size of collagen microspheres significantly decreased from 3 to 6 months, presumably from degradation. Picrosirius red staining demonstrated an increase in host collagen deposition and maturation over time, accounting for the maintenance of tissue volume seen above. Notably, the amount of M1 and M2 macrophages significantly decreased over 6 months, indicating minimal inflammation. Further, there was no evidence of an adverse foreign body reaction/chronic inflammation at any point during the study. CONCLUSIONS: In contrast to the rapid degradation observed with monophase collagen solutions or slower volume loss associated with HA, this simple but unique composite collagen hydrogel promotes neovascularization and deposition of new host collagen, resulting in excellent maintenance of the volume and projection of the injected material. Longer term studies are ongoing to verify the permanence of the newly-integrated tissue.