Silk Resin with Hydrated Dual Chemical-Physical Cross-Links Achieves High Strength and Toughness.

Native silk fibers are known to demonstrate excellent mechanical properties such as high strength and ductility. However, regenerated silk material has not yet been used as a tough structural material in our everyday life. To recreate the mechanical properties with regenerated silk material, the network structure and hydration state of silk materials are studied and optimized in this study. This is the first to demonstrate the effect of chemical and physical cross-links in hydrated and dehydrated silk materials, namely, silk hydrogels and resins. Mild hydration conditions (relative humidity 20-60%) realizes tough and strong silk materials with chemical and physical cross-links. In the case of relatively high concentrations of silk molecules, contributions to the high strength and toughness of silk-based materials are considered to come not only from β-sheet cross-links and chemical dityrosine links but also from entanglements and assembly via the hydrophobic interactions of silk molecules. In addition, dehydration treatment does not disturb the biodegradability of the silk resins in natural environments. Based on the overall results, the silk resins with controlled network structures and hydration state have successfully achieved the highest toughness possible for a bulk silk material while maintaining favorable biodegradability.