Construction of injectable silk fibroin/polydopamine hydrogel for treatment of spinal cord injury

Injectable hydrogels have potential applications for regenerating the injured spinal cord, but their performance needs to be further improved. In this study, an injectable silk fibroin/polydopamine (SF/PDA) composite hydrogel was fabricated via first Schiff base reaction between the amino groups of silk fibroin and the quinone structure of oxidized dopamine (DA), and then self-polymerization of free DA with SF-conjugated DA. The physicochemical properties of the hydrogel were characterized. The cytotoxicity and biocompatibility were evaluated using L929 fibroblasts and hippocampal neurons, respectively, and the expression levels of neurites-related proteins were analyzed by western blot. Finally, the effect of hydrogel on SCI repair was initially evaluated by in vivo animal experiment. With the increase of DA concentration, the hydrogels color became darker, the swelling rate, water uptake and the porosity decreased. Meanwhile, the Young's modulus continued to decrease when the DA concentration was increased from 0.5 mg/mL to 10 mg/mL, whereas rheological test showed the good injectability of the prepared hydrogels. NMR, FTIR and XPS confirmed that dopamine could combine with silk fibroin via Schiff base reaction and then polymerize to form the SF/PDA composite hydrogel. The hydrogels showed no cytotoxicity and the hippocampal neurons on hydrogel with 2 mg/mL DA displayed the largest axon length, the highest cell density and expression level of neuritis-related proteins. Further on, immunohistochemical staining showed initially that the prepared hydrogel possessed the capability of promoting SCI repair. In summary, the developed injectable SF/PDA hydrogel here may contribute to the potential application for promoting functional recovery of SCI, and provide important experimental and theoretical basis for developing SCI implants.