Silk fibroin/carboxymethyl chitosan hydrogel with tunable biomechanical properties has application potential as cartilage scaffold

Tissue engineering is a promising strategy for cartilage repair and regeneration. However, an ideal scaffolding material that not only mimics the biomechanical properties of the native cartilage, but also supports the chondrogenic phenotype of the seeding cells is in need. In this study, we developed a silk fibroin (SF) and carboxymethyl chitosan (CMCS) composite hydrogel with enzymatic cross-links (horseradish peroxidase and hydrogen peroxide) and β-sheet cross-links (ethanol treatment). Results of Fourier transform infrared (FTIR), thermal gravimetric analysis (TGA), and X-ray diffraction (XRD) verified that SF/CMCS composite hydrogels had a tunable β-sheet structure. Therefore, by increasing the time of ethanol treatment from 0 h to 8 h, a series of parameters including pore size (from 50 to 300 μm), equilibrium swelling (from 78.1 ± 2.6% to 91.9 ± 0.9%), degradation (from 100% to 9% reduction in mass over 56 days), rheological properties (storage modulus from 177 Pa to 88,904 Pa), and mechanical properties (compressive modulus from 13 to 829 kPa) of the hydrogels were adjusted. In particular, the material parameters of the hydrogels with 2 h ethanol treatment appeared most suitable for engineered cartilage. Furthermore, the in vitro cellular experiments showed that the hydrogels supported the adhesion, proliferation, glycosaminoglycan synthesis, and chondrogenic phenotype of rabbit articular chondrocytes. Finally, subcutaneous implantation of the hydrogels in mice showed no infections or local inflammatory responses, indicating a good biocompatibility in vivo. In conclusion, the chemical-physical cross-linking SF/CMCS composite hydrogels, with tunable material properties and degradation rate, good biocompatibility, are promising scaffolds for cartilage tissue engineering.