Humidity-induced supercontraction and twist in spider silk

Spider silk is an extraordinary protein material that exhibits counterintuitive mechanical behaviors such as a reduction in stiffness of several orders of magnitude, supercontraction (i.e. a shortening of up to ~60% in length), and twist upon exposure to high humidity. These non-trivial responses originate from a unique polymeric structure made of crystalline domains that are embedded in a highly aligned amorphous matrix. Broadly, high humidity leads to water uptake by the silk, which in turn motivates the dissociation of intermolecular hydrogen cross-linking bonds. In this talk, a microscopically motivated model for the constitutive response of spider silk is presented. I will show that the dissociation of intermolecular bonds results in a transition from a glassy to a rubbery phase and an increase in entropy (and a decrease in free energy). These factors explain the experimentally observed behaviors. The proposed model is found to be in excellent agreement with several experimental findings. The merit of this work is two-fold: (1) it accounts for the microstructural evolution of spider silk in response to water uptake and (2) it provides a method to characterize the microstructural evolution of hydrogen-bond dominated networks. The insights from this work pave the way to the design of novel biomimetic fibers with non-trivial properties.

1) N. Cohen and C.D. Eisenbach, “Humidity-Driven Supercontraction and Twist in Spider Silk”, Physical Review Letters, 128:098101, 2022.

2) N. Cohen, M. Levin, and C.D. Eisenbach, “On the origin of supercontraction in spider silk”, Biomacromolecules, 22:993-1000, 2021.