Ice-regenerated flame retardant and robust film of silk fibroin and POSS nano-cages.

In this study, we present a simple method to prepare and control the structure of regenerated hybrid silkworm silk films through icing. A regenerated hybrid silk (RHS) film consisting of a micro-fibrillar structure was obtained by partially dissolving amino-functionalized polyhedral oligomeric silsesquioxanes (POSS) and silk fibers in a CaCl-formic acid solution. After immersion in water and icing, the obtained films of RHS showed polymorphic and strain-stiffening behaviors with mechanical properties that were better than those observed in dry or wet-regenerated silk. It was also found that POSS endowed the burning regenerated silk film with anti-dripping properties. The higher β-sheet content observed in the ice-regenerated hybrid micro-fibrils indicates a useful route to fabricate regenerated silk with physical and functional properties, strain-stiffening, similar to those observed to date in natural spider silk counterpart and synthetic rubbers, and anti-dripping of the flaming melt. Related carbon nanotube composites are considered for comparison.