Modeling spider silk supercontraction as a hydration-driven solid–solid phase transition

Abstract

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 transition energy modulated by humidity. Based on the methods of Statistical Mechanics, we deduce that supercontraction can be interpreted as a solid–solid phase transition. We elucidate the important role of thermal fluctuations. In particular, the decrease of the critical humidity needed for supercontraction as temperature grows results as an effect of entropic stabilization of the softer rubbery phase. Our model quantitatively predicts the observed experimental behavior, capturing the temperature dependence of humidity-induced supercontraction effects and related cooperative properties.