Flexible keratin hydrogels obtained by a reductive method†

Keratin derived materials are still underexploited due to the little understanding of their chemical versatility. Whereas many protein based materials achieve flexibility by crosslinking or interpenetrating with synthetic polymers, we assessed the effect of reductive treatments on aqueous media. Hydrazine sulphate (HZN) and ascorbic acid reduction were compared. The reduced material is bendable and stretchable, whereas the original keratin hydrogel is brittle. This would imply a technological leap in protein materials. Both reductive treatments would achieve reduced keratins by the reduction of oxidised cysteines which leads to a change in the polypeptide chain interaction by a decrease in electrostatic repulsion and swelling. Moreover, in contrast with the ascorbic acid treatment, when higher levels of HZN are employed, the effect of residual sulphates lead to the interchain closeness of the more mobile domains acting as physical crosslinkers, leading to compressed structures with narrower pores. This suggests that the flexible properties of the hydrogel could be related not only to the reduction of the hydrogel but also to the interaction of the sulphate ions with the keratin structure. As a result, the reduction of sulfinic and sulfenic groups to thiol, along with the incorporation of sulphate ions into the structure, impart the material with an elongation at break ranging between 10–25%, nano-scale pores approximately 2 nm in size, swelling capacity of around 50%, all while preserving the biocompatibility observed in the original material tested across two cell lines comprising fibroblasts and keratinocytes.