Background: For effective bone regeneration, it is necessary to implant a biocompatible scaffold that is capable of inducing cell growth and continuous osteogenic stimulation at the defected site. Here, we suggest an injectable hydrogel system using enzymatic cross-linkable gelatin (Gel) and functionalized gold nanoparticles (GNPs).
Methods: In this work, tyramine (Ty) was synthesized on the gelatin backbone (Gel-Ty) to enable a phenol crosslinking reaction with horseradish peroxidase (HRP). N-acetyl cysteine (NAC) was attached to the GNPs surface (G-NAC) for promoting osteo-differentiation. 
Results: The Gel-Ty hydrogels containing G-NAC (Gel-Ty/G-NAC) had suitable mechanical strength and biocompatibility to embed and support the growth of human adipose derived stem cells (hASCs) during a proliferation test for three days. In addition, G-NAC promoted osteo-differentiation both when it was included in Gel-Ty and when it was used directly in hASCs. The osteogenic effects were demonstrated by the alkaline phosphatase (ALP) activity test.
Conclusion: These findings indicate that the phenol crosslinking reaction is suitable for injectable hydrogels for tissue regeneration and G-NAC stimulate bone regeneration. Based on our results, we suggest that Gel-Ty/G-NAC hydrogels can serve both as a biodegradable graft material for bone defect treatment and as a good template for tissue engineering applications such as drug delivery, cell delivery, and various tissue regeneration uses.