Creating Stiff, Tough, and Functional Hydrogel Composites with Low-Melting-Point Alloys

Abstract

Reinforcing hydrogels with a rigid scaffold is a promising method to greatly expand the mechanical and physical properties of hydrogels. One of the challenges of creating hydrogel composites is the significant stress that occurs due to swelling mismatch between the water-swollen hydrogel matrix and the rigid skeleton in aqueous media. This stress can cause physical deformation (wrinkling, buckling, or fracture), preventing the fabrication of robust composites. Here, a simple yet versatile method is introduced to create "macroscale" hydrogel composites, by utilizing a rigid reinforcing phase that can relieve stress-induced deformation. A low-melting-point alloy that can transform from a load-bearing solid state to a free-deformable liquid state at relatively low temperature is used as a reinforcing skeleton, which enables the release of any swelling mismatch, regardless of the matrix swelling degree in liquid media. This design can generally provide hydrogels with hybridized functions, including excellent mechanical properties, shape memory, and thermal healing, which are often difficult or impossible to achieve with single-component hydrogel systems. Furthermore, this technique enables controlled electrochemical reactions and channel-structure templating in hydrogel matrices. This work may play an important role in the future design of soft robots, wearable electronics, and biocompatible functional materials.

"Macroscale" double-network hydrogel composites, consisting of a mesh-like rigid skeleton within a soft hydrogel matrix, are reported. A low-melting-point alloy frame is used as the functional skeleton to introduce properties activated via thermal stimulus (e.g., releasing swelling mismatch, shape memory, and thermal healing). The resulting composites exhibit excellent mechanical properties based on the macroscopic "double-network" effect.

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