Modeling Spider Silk Supercontraction as a Hydration-Driven Solid–Solid Phase Transition

Spider silks have attracted significant interest due to their exceptional mechanical properties, which include a unique combination of high strength, ultimate strain, and toughness. A notable characteristic of spider silk, still debated from both mechanical and functional viewpoints, is supercontraction –a pronounced contraction of up to half its original length when an unconstrained silk thread is exposed to a wet environment. We propose a predictive model for the hygro-thermo-mechanical behavior of spider silks, conceptualizing this phenomenon as a solid–solid phase transition, similar to the glass transition in rubber, but driven by humidity. As wetting increases, the system undergoes a transition, at the network scale, from a hard, dry state –where the material behavior is governed by stiff chains elongated along the fiber axis– to a soft, wet state, regulated by a rubber-like response. We model these states using a two-well free energy function dependent on molecular stretch, with...