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 Fe3+ and weak to nonbinding Fe2+, 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 mu m in thickness, which could be reversibly cycled between soft (Youngs modulus: similar to 0.38 MPa) and hard (similar to 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.