Supercontraction-Induced Twist in Spider Silk Is a Dual Poynting Effect

TL;DR

This paper explains how humidity-induced supercontraction causes spider silk to twist through a dual Poynting effect, combining experimental observations with a physical model based on nonlinear anisotropic elasticity.

Contribution

It introduces a minimal physical model that explains supercontraction-induced torsion as a dual Poynting effect driven by humidity, supported by quantitative experimental validation.

Findings

Model reproduces monotonic and cyclic torsional behavior

Torsion arises from nonlinear anisotropic elasticity

Humidity-driven matrix remodeling causes twist

Abstract

Spider dragline silk supercontracts as humidity increases, displaying large axial shortening together with a reproducible macroscopic twist. The physical origin of this torsion remains debated and is often attributed to helically arranged load-bearing elements, despite the lack of direct evidence for helicity in the native fiber. Here we show that torsion can arise generically from nonlinear anisotropic elasticity: humidity-driven shortening of the amorphous matrix, mechanically constrained by stiff, axially aligned $\beta$-sheet--rich load-bearing segments and their experimentally induced prestretch, drives the system into a dual Poynting regime in which axial shortening couples to spontaneous twist. Coupling a diffusion-based water-uptake law to irreversible matrix remodeling and fiber plasticity, the model quantitatively reproduces monotonic and cyclic torsional measurements using parameter values consistent with available experimental material parameters. These results identify supercontraction-induced torsion in spider silk as a manifestation of a dual Poynting effect and provide a minimal, physically grounded framework for humidity-driven torsional actuation in matrix--fiber architectures.