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 crosslinked 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. ■ INTRODUCTIONWe report herein the synthesis and extensive characterization of a unique class of stimuli-responsive hydrogels, which we term electroplastic elastomers (EPEs). These iron-cross-linked hydrogels can be transitioned through a continuum from soft to hard by the application of an electrochemical stimulus that reversibly switches the iron between +2 and +3 oxidation states. A portion of this work has been previously communicated. 1 As has been established in the past few decades, stimuliresponsive hydrogels are attractive candidates for a broad range of applications such as tissue engineering, 2 drug and protein delivery, 3,4 sensors, 5 actuators, 6 and energy storage. 7 A variety of stimuli, including light, 8 temperature, 9 pH, ionic strength, electric field, 10 magnetic field, 11 enzyme catalysis, 12 and redox reactions, 13 have been utilized to elicit changes in one or more properties of these materials, with typical responses being degradation, reversible swelling/deswelling, actuation, sol−gel transition, 14 change in wavelength of diffracted light, changes in mechanical properties, 15 and self-healing. 16 Among these many stimuli, we have chosen to focus our attention on electricity because the addition or removal of reagents or heat, which is particularly suitable for some applications, is not always desirable or achievable.In our responsive EPE hydrogels we harness the unique ability of metal ions to act as reversible cross-links in polymeric systems. There are numerous examples of the exploitation of this characteristic in previously reported stimuli-responsive metal-containing polymers and supramolecular materials. 7,17−20 Of particular relevance to our work are those systems that rely on redox-driven changes in coordination around the metal center. Iron, due to the accessibility and stability of the +2 and +3 oxidation states, is one of the most widely exploited metals.Tong and co-workers have, for example, reported on the reversible sol−gel transitioning of poly(acrylic acid) (p(AA)) using the Fe 2+ /Fe 3+ redox couple as controlled by light, air oxidation, 21 or an applied p...