Dynamic metal–ligand cross‐link promoted mechanically robust and pH responsive hydrogels for shape memory, programmable actuation and resistive sensing application

Employing Fe3+‐dicarboxylate dynamic metal–ligand cross‐links in combination with low density chemical cross‐links, herein we have developed a mechanically robust and stretchable poly(acrylamide‐co‐maleic acid) hydrogel. The mechanical properties of these hydrogels can be controlled by altering the concentrations of maleic acid and Fe3+ ions. The hydrogels showed high tensile strength (~1–2 MPa), toughness (~3.3 MJ m−3) and stretchability (up to ~5 times of their original length) along with high compressive strength (~13.5 MPa). Because of the dynamic nature of the metal–ligand cross‐links, the hydrogels exhibited good self‐recovery, anti‐fatigue properties and fast self‐healing capacity. Since the Fe3+‐dicarboxylate cross‐links can be dissociated at low pH, the mechanical properties could further be tuned in response to the pH of the medium. This property is exploited to demonstrate shape memory behavior of these hydrogel materials. A temperature responsive bilayer actuator is reported, with poly(N‐isopropylacrylamide) as the thermoresponsive layer and the Fe3+ cross‐linked poly(acrylamide‐co‐maleic acid) hydrogel as the memorizing layer, demonstrating programmable and reversible shape transition. The application of these ionically conducting hydrogels as highly sensitive and flexible resistive strain sensor is also demonstrated. Schematic illustration of synthesis and application of dynamic metal‐ligand cross‐linked, mechanically strong and multi stimuli‐responsive flexible hydrogel materials.