Collagen bioinks redefined: Optimizing ionic strength and growth factor delivery for cartilage tissue engineering

Tissue engineering of hyaline cartilage for regenerative medicine and the treatment of osteoarthritis has advanced significantly over the past decade, driven by developments in 3D bioprinting and biomaterials science. Despite these advances, standardized biofabrication protocols approved for clinical applications remain elusive, underscoring the need for research into widely accessible, non-immunogenic, and biocompatible bioinks that support chondrogenesis. This study proposes a strategy to improve the gelation kinetics of collagen bioinks by fine-tuning their ionic strength and reports a highly efficient sequestration of TGF-β1 within them, alongside their compatibility with bioprinting live chondrocytes and adipose-derived stem cells for cartilage tissue engineering. By adjusting sodium chloride and phosphate-buffered saline (PBS) concentrations, we demonstrate that reduced ionic strengths accelerate gelation, facilitating high-fidelity bioprinting while supporting high cell viability and proliferation. Furthermore, at 1% collagen concentration, the hydrogel effectively immobilized TGF-β1, with less than 0.5% released over two weeks, indicating potent sequestration capability. Using adipose-derived mesenchymal stromal cells, histomorphological and transcriptomic analyses reveal that the presence of TGF-β1 significantly enhances chondrogenesis. These results underscore the neglected role of ionic strength in optimizing collagen ink properties for advanced bioprinting applications and highlight the potential of collagen hydrogels as effective carriers for sustained growth factor delivery, paving the way for successful cartilage tissue engineering strategies.