Self-assembled silk fibroin hydrogels with precisely engineered viscoelasticity enhance BMSC chondrogenesis and cartilage repair.

The repair of articular cartilage damage is a major challenge in the biomedical field. Silk fibroin hydrogels have garnered considerable attention in cartilage repair due to the excellent biocompatibility. However, a critical challenge remains in decoupling their elastic modulus and stress relaxation rate-two core viscoelastic properties that jointly regulate cell fate-thus limiting their tailored application in cartilage regeneration. In this work, we achieved precise control over the elastic modulus and stress relaxation rate of silk fibroin hydrogels for the first time by modulating the self-assembly of silk fibroin. We elucidated the mechanisms by which self-assembly process impacts the viscoelastic properties of the hydrogels and successfully established a silk fibroin-based hydrogel system with independently tunable stress relaxation rates and elastic moduli. The effects of silk fibroin viscoelasticity on BMSC differentiation and cartilage regeneration were evaluated in vitro and in vivo. The synergistic combination of a low elastic modulus (E ∼ 6.5 kPa) and fast stress relaxation (τ1/2-12 s) significantly upregulated the expression of cartilage-related genes, including SOX9 and COL2A1, and enhanced the secretion of glycosaminoglycans (GAGs). The newly formed tissue exhibited a smooth surface and tight integration with the surrounding cartilage tissue. Histological analysis revealed a high degree of structural similarity to native cartilage. This study provides a robust theoretical foundation for the development of novel cartilage repair materials and holds the potential to advance the field of tissue engineering and regenerative medicine.