Reinforcement and chemical cross-linking in collagen-based scaffolds in cartilage tissue engineering: a comparative study

Mechanical strength and biocompatibility are issues of most concern for scaffolds in cartilage tissue engineering. Collagen modification is always used to strengthen scaffolds. There are mainly two ways for collagen modification: inclusion of reinforcing phase to form composites and chemical cross-linking. To explore an alternative approach, the collagen hydrogel modified by a reinforcement phase was compared with cross-linking. Collagen-alginate hydrogel (CAH) and collagen hydrogel cross-linked by genipin (CGH), which were different in modification methods, were chosen candidates. A comprehensive study was carried out on mechanical, structural and biological properties including swelling ratio measurement, in vitro degradation, AFM, mechanical test, thermogravimetric analysis, and in vitro cartilage tissue engineering. The results showed that mechanical strength of collagen was more enhanced for CGH than CAH, as evidenced by analysis of swelling ratio, in vitro degradation, AFM, mechanical test and thermostability. MTT and histological results showed that CGH was superior to CAH with less cytotoxicity and more chondrocytes distributed as well as more aggrecan secreted. With the increase in culture time, the cytotoxicity of cross-linker may be alleviated. CGH may provide a more favorable biomimetic environment for cartilage growth. All these indicated that selecting a cross-linker with a minimal cytotoxicity could be more promising for collagen modification, with improvements observed in both physical and biological properties. For reinforcement, it was required that the incorporated component should be equipped with better or equivalent properties compared with collagen. This study provided important implications to engineering collagen-based hydrogels for cartilage graft applications.