Design and Development of an Injectable Hydrogel for Cartilage Repair

Articular cartilage injuries present a significant clinical challenge due to the tissue's limited intrinsic capacity for self-repair. Without effective treatment, such defects progress to osteoarthritis, severely impacting the patient’s mobility and quality of life. Existing clinical strategies, including microfracture and autologous chondrocyte implantation (ACI), are often associated with complications such as fibrocartilage formation and donor site morbidity. Injectable hydrogels offer a minimally invasive strategy with defect-conforming properties with the ability to deliver bioactive cues, yet conventional bulk ‘filler’ hydrogels are hindered by nanoporosity that limits nutrient transport and cell migration.

To overcome these limitations, a granular injectable hydrogel system was developed, composed of microparticles assembled from fragmented bulk hydrogels, thereby introducing interstitial microporosity to enhance cellular interactions and tissue integration. The hydrogel formulation was composed of gelatin methacryloyl-, hyaluronic acid methacryloyl-, and methacryloyl-modified platelet lysate. Bulk formulations were first optimized through live/dead viability assays, DNA and glycosaminoglycan quantification, and immunofluorescent staining for hyaline cartilage-specific markers. Granular hydrogels were fabricated by extruding and fragmenting the optimized bulk gels into microparticles, followed by centrifugation and photoannealing (λ=365nm). These constructs were evaluated using the same in vitro assays and further tested in an ex vivo osteochondral defect model.

Fabrication yielded large and small microparticles of 230 ± 82 µm and 46 ± 18 µm, respectively, with interstitial porosity ranging from 3–11% depending on centrifugation speed. While the optimized bulk hydrogel supported chondrocyte viability (+90%) and hyaline-like matrix production, granular hydrogels substantially improved repair outcomes. By day 31, GAG/DNA ratios reached 0.79 and 0.72 for small particle and large particle groups, compared to 0.677 for bulk gels. We report enhanced microporosity facilitated cell infiltration, more extracellular matrix deposition, and better integration with host cartilage. Immunostaining confirmed that the regenerated tissue was rich in type II collagen and aggrecan, resembling native hyaline cartilage.