Tough and responsive oppositely charged nanocomposite hydrogels for use as bilayer actuators assembled through interfacial electrostatic attraction

Responsive nanocomposite hydrogels are widely recognized as strong and tough soft materials for smart devices, but there are still challenges in their synthesis and in the fabrication of devices. We report here the versatile synthesis of nanocomposite polyelectrolyte hydrogels with high strength and toughness using either cationic or anionic monomers and demonstrate the simple and versatile fabrication of bilayer actuators by assembling the oppositely charged hydrogels through electrostatic attraction. Exfoliated sodium montmorillonite nanosheets were used as cross-linkers through the adsorption of monomers and initiators before in situ free radical polymerization. Nanocomposite hydrogels with negative charges were obtained by the copolymerization of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid, whereas positively charged nanocomposite gels were obtained using acrylamide and dimethylaminoethyl methacrylate methylchloride. The nanocomposite polyelectrolyte hydrogels were responsive to pH and the ionic strength of buffer solutions. They also showed outstanding fatigue resistance against cyclic compression loading and high tensile strength and toughness. The gels were able to recover at room temperature after tensile testing. The oppositely charged hydrogels were assembled with a robust interface based on the electrostatic attraction between the opposite charges. These hydrogels were actuated under an electric field. The oppositely charged hydrogels were assembled into bilayers that were reversibly actuated as a result of the contrasting responsiveness of each gel to the ionic strength.