Ultra-responsive soft matter from strain-stiffening hydrogels

TLDR

Hydrogel stiffness is critical for applications, and natural hydrogels stiffen under strain—a property cells exploit to modulate function; yet strain‑stiffening remains largely unexplored in synthetic gels because key design parameters are unknown. By varying concentration, temperature, and polymer length, the authors examine how these tunable parameters influence strain‑stiffening behavior. The study identifies a marginal critical point where minute stresses trigger a transition from a low‑viscous liquid to an elastic gel, and, combining experiments with network theory, derives universal design principles for strain‑stiffening materials.

Abstract

Abstract The stiffness of hydrogels is crucial for their application. Nature’s hydrogels become stiffer as they are strained. This stiffness is not constant but increases when the gel is strained. This stiffening is used, for instance, by cells that actively strain their environment to modulate their function. When optimized, such strain-stiffening materials become extremely sensitive and very responsive to stress. Strain stiffening, however, is unexplored in synthetic gels since the structural design parameters are unknown. Here we uncover how readily tuneable parameters such as concentration, temperature and polymer length impact the stiffening behaviour. Our work also reveals the marginal point, a well-described but never observed, critical point in the gelation process. Around this point, we observe a transition from a low-viscous liquid to an elastic gel upon applying minute stresses. Our experimental work in combination with network theory yields universal design principles for future strain-stiffening materials.