Differential diffusion driven far-from-equilibrium shape-shifting of hydrogels

Abstract

Far-from-equilibrium (FFE) conditions give rise to many unusual phenomena in nature. In contrast, synthetic shape-shifting materials typically rely on monotonic evolution between equilibrium states, limiting inherently the richness of the shape-shifting behaviors. Here we report an unanticipated shape-shifting behavior for a hydrogel that can be programmed to operate FFE-like behavior. During its temperature triggered shape-shifting event, the programmed stress induces uneven water diffusion, which pushes the hydrogel off the equilibrium based natural pathway. The resulting geometric change enhances the diffusion contrast in return, creating a self-amplifying sequence that drives the system into an FFE condition. Consequently, the hydrogel exhibits counterintuitive two opposite shape-shifting events under one single stimulation, at a speed accelerated by more than one order magnitude. Our discovery points to a future direction in creating FFE conditions to access otherwise unattainable shape-shifting behaviors, with potential implications for many engineering applications including soft robotics and medical devices.

Bioinspired Self-Resettable Hydrogel Actuators Powered by a Chemical Fuel

“…Inspired by the movements of soft living tissues, hydrogel-based soft actuators that can mimic biological functions have been an active area of research and discussion. − The mechanical actuation of hydrogels is usually achieved by volume change through absorbing and releasing water in and out of their networks in response to external stimuli. Along this line, diverse elegant hydrogel actuators have been created under the control of environmental parameters like pH, − temperature, − light, − electric field, − ions, − and magnetic field. − However, so far, the vast majority of the reported examples are switched between different thermodynamic equilibrium states by sequentially turning on/off the external stimuli, , showing limited autonomous capability. ,, In stark contrast, the actuation of soft living tissues is highly autonomous, which is usually realized by nonequilibrium chemical reaction networks (CRNs) powered by high-energy biomolecules, such as adenosine triphosphate (ATP). For example, muscles contract by consuming the energy released by the conversion of ATP into adenosine diphosphate (ADP) and spontaneously relax to the original state once ATP is used (Figure a). , Thus, access to autonomous hydrogel actuators powered by chemical fuels, analogous to living tissues, would be extremely advantageous for lifelike soft robotics yet remains a formidable task.…”

A double-network strategy for the tough tissue adhesion of hydrogels with long-term stability under physiological environment

“…PNIPAM is a widely studied polymer with tunable hydrophilicity, which possesses a lower critical solution temperature (LCST, B32 1C) close to the physiological body temperature. [27][28][29] A calcium alginate hydrogel is also widely used in biomedical fields, which has good biocompatibility and tissue-reactive carboxyl groups. [30][31][32] The PNIPAM/Ca-alginate hydrogel combines the swelling resistance of PNIPAM and the reactive groups of calcium alginate.…”

Self-Sensing Actuators Based on a Stiffness Variable Reversible Shape Memory Polymer Enabled by a Phase Change Material

“…Stimuli-responsive materials, mainly including hydrogels, dielectric elastomers, liquid crystal elastomers, and shape memory polymers (SMPs), − are promising soft materials with actuation ability for constructing soft robots. Among them, reversible SMPs (RSMPs) or two-way SMPs can provide reversible deformation without the action of external force in response to external stimuli (mostly heat) through the melting and crystallization of the actuation domain, − being attractive materials for constructing soft robots.…”