Stimuli-Responsive Iron-Cross-Linked Hydrogels That Undergo Redox-Driven Switching between Hard and Soft States

A unique class of stimuli-responsive hydrogels, termed electroplastic elastomers (EPEs), whose mechanical properties can be reversibly tuned between hard and soft states with the application of an electric potential, is described. Electrochemically reversible cross-links formed within a permanent, covalently cross-linked polymeric hydrogel network are switched between strongly binding Fe 3+ and weak to nonbinding Fe 2+, as determined by potentiometric titration. With the incorporation of graphene oxide (GO) into the EPE, a significant enhancement in modulus and toughness was observed, allowing for the preparation of thinner EPE samples, 80–100 μm in thickness, which could be reversibly cycled between soft (Young’s modulus: ∼0.38 MPa) and hard (∼2.3 MPa) states over 30 min. Further characterization of EPE samples by magnetic susceptibility measurements suggests the formation of multinuclear iron clusters within the gel.