Gelation and the antioxidant and antibacterial properties of silk fibroin/tannic acid/Zn2+ mixtures

Solutions of a mixture of water-soluble silk fibroin (SF) and tannic acid (TA) were prepared to investigate the phase behavior (solution or coacervate) according to the degree of hydrogen bonding between them. Further, ZnCl2 was added to the SF/TA solutions to form hydrogels via uniform crosslinking at a certain range of the Zn2+/TA ratio. The formation of the SF/TA/Zn2+ hydrogel is attributed to molecular crosslinking owing to a combination of the hydrogen-bonding of TA with SF, and the coordination of TA with Zn2+. The resultant hydrogel exhibited excellent antioxidative and antimicrobial properties owing to the residual polyphenolic groups of TA, which were not bonded to SF or Zn2+ ions. Furthermore, the SF/TA/Zn2+ hydrogels had spreadability and could be easily processed as a multifunctional cosmetic material.