Antiswelling Magnetic-Gradient Hydrogel via Synergistic Magnetically Induced and Ionic Cross-Linking

Magnetically responsive hydrogels offer considerable promise for underwater biomimetic actuators, owing to their inherent flexibility and remote controllability. However, their practical application is constrained by swelling and magnetic loss in aqueous environments, as well as the inability of conventional uniformly magnetized structures to achieve complex, segmented deformation. Herein, a synergistic strategy is proposed to construct an antiswelling magnetic-gradient hydrogel based on a 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS)/2-hydroxypropyl methacrylate (HPMA)/2-phenoxyethyl acrylate (PEA) copolymer system by integrating citric acid-modified Fe 3 O 4 nanoparticles (Fe 3 O 4 @CA) with magnetic field induction and Zr 4+ coordination fixation. Under an external magnetic field, Fe 3 O 4 @CA particles assume a spatial gradient distribution along the magnetic field direction. Subsequently, a structurally robust gradient network is constructed through multipoint coordination between Zr 4+ ions and the carboxylic/sulfonic acid groups, together with aromatic-ring-related secondary interfacial interactions within the network. The resulting hydrogel possesses robust mechanical properties, with a toughness of 1239.5 kJ m –3 and excellent fatigue resistance. Remarkably, the hydrogel demonstrates exceptional stability and a pronounced magnetic-gradient structure in aqueous environments. It has a low swelling ratio of 2.7% and an Fe 3 O 4 @CA particle leaching rate as low as 1.27% over 15 days. The pronounced magnetic-gradient structure (saturation magnetization, M s, max/ M s, min ≈ 16.5, 16.87/1.02 emu g –1 ) enables the gradient hydrogel to achieve both rapid actuation responsiveness (53.5° s –1 ) and controlled, segmented deformation. Consequently, bioinspired underwater actuators fabricated from this hydrogel demonstrate magnetic-field-controlled behaviors in water, including magnetically driven locomotion, conformal grasping, and controllable deformation-based actuation. Overall, this study proposes a synergistic strategy for preparing antiswelling magnetic-gradient hydrogels with actuation capability and shows their potential for underwater soft actuators.