Pressure- and humidity-induced structural transition of silk fibroin

Abstract

Spiders and silkworms convert silk fibroin solution into fibers by adjusting physiological conditions such as water content and internal pressure. Water molecules are known to plasticize molecular chains of silk fibroin, affording β-sheet structures. However, the effect of pressure on the crystallization of silk fibroin and the differences between pressure- and water-induced crystallization in spider and silkworm silk remain unexplained. Herein, the effect of pressure and humidity on the crystallization of spider and silkworm silk amorphous films is investigated. Despite not inducing irreversible crystallization of spider silk, pressure promotes β-sheet formation of silkworm silk. In contrast, water induces crystallization of both spider and silkworm silk. Pressure-induced crystallization results in larger crystallinity but smaller crystallite size in comparison to water-induced crystallization. The formation of β-sheet structures enhances breaking strength and modulus of the silk films, which paves the way toward silk-based structural materials.