Injectable Silk Fibroin Hydrogels with Ionic Strength and pH Response for Tissue Engineering and Regenerative Medicine Applications

The traditional methods to prepare silk fibroin (SF) hydrogels take advantage of the conformation transition from amorphous to β-sheet in aqueous SF solution. During this procedure, the gelation time normally varies from tens of minutes to months depending on the methods used. The relative long gelation time of those methods limits their practicality as in situ injectable systems for incorporation of cells or drugs. Furthermore, most of these approaches are not suitable for cell/drug incorporation in the silk hydrogel. The current study provided an approach to develop SF hydrogel within a few minutes in physiological conditions via peroxidase mediated cross-linking. The influences of the silk concentration,and the content of peroxidase and hydrogen peroxide on the physicochemical properties of the hydrogels were studied. The results showed that the gelation time of the silk hydrogel decreased as increasing silk concentration and peroxidase content, and can be tuned between 4 to 50 minutes. The storage moduli of the hydrogels improved via increasing the hydrogen peroxide content and silk concentration, ranging from 0.25 to 5.20 kPa. The fast formed hydrogels showed extreme elasticity and transparent appearance. There were no differences of the silk hydrogel in the visible light absorbance, before and after the gelation. The dominant conformation of the formed silk hydrogels was amorphous, confirmed by Fourier Transform Infrared Spectroscopy. Interestingly, the prepared hydrogels were of ionic strength and pH responsive properties. Their sizes or wet weights increased in solutions of low ionic strength or basic pH, and vice versa. Cells can be incorporated into the hydrogels and were viable up to 11 days. The cytotoxicity result demonstrated that these hdyrogels were non-cytoxic. After subcutaneous implantation in mice for 2 and 4 weeks, the SF hydrogels induced no inflammation reactions in vivo. This study provides a facile approach to prepare injectable SF hydrogels with dual stimuli responsive properties. The distinctive properties of these hydrogels exhibit innumerous potential applications, such as for cell/drug delivery and optical device.