An injectable collagen-genipin-carbon dot hydrogel combined with photodynamic therapy to enhance chondrogenesis

Collagen has been widely used for cartilage repair, but its low stiffness and rapid degradation disfavor chondrogenesis. Here we conjugated biocompatible carbon dot nanoparticles (CD NPs) onto collagen through a natural product crosslinker (genipin) to prepare an injectable hydrogel (termed collagen-genipin-CD nanoparticles, CGN). The CGN hydrogel showed increased stiffness due to the cross-linking effect of genipin and the presence of CD NPs, and could produce a moderate amount of reactive oxygen species (ROS) by photodynamic therapy (PDT). Both the stiffness enhancement and ROS generation resulted in improved chondrogenic differentiation of bone marrow-derived stem cells (BMSCs) and the subsequent enhanced cartilage regeneration for cartilage defect repair. Specifically, the CGN hydrogel presented a 21-fold higher compression modulus and a 39.3% lower degradation rate than the pure collagen hydrogel. A combination of both PDT and CGN hydrogel increased the BMSCs proliferation by 50.3%, upregulated their expression of cartilage-specific genes by multiple folds, and enhanced GAG secretion by 205.1% on day 21. This combination also accelerated the cartilage regeneration within as short as 8 weeks. The stiffness enhancement and ROS generation synergistically contributed to chondrogenic differentiation by regulating the TGF-β/SMAD and mTOR signaling pathway, respectively. The combination of CD-modified hydrogel injection and PDT treatment represents a new strategy for minimally invasive repair of cartilage defects.