Low Hysteresis Hydrogel Induced by Spatial Confinement

Abstract Stress concentration and hysteresis often occur in polymer hydrogels under large deformation, affecting their mechanical properties, and durability. Here, a new spatial confinement gelation strategy is proposed for fabrication of hysteresis‐free polymer hydrogels, and stress concentration is avoided. Experimental results and theoretical calculations confirm that the hydratability difference between salts and polymer chains leads to spatial confinement of the polymer chains by control over the ratio of bound to free water. Spatial confinement acts in place of physical cross‐linking of polymer chains, which slide to dissipate energy and effectively avoid stress concentration and hysteresis, resulting in ultra‐tough hydrogels with fatigue resistance. This is also a universal strategy for preparing polymer hydrogels with different monomers and salts. Taking the prepared PAM‐CaCl 2 hydrogel, it displays a hysteresis of only 0.13% during load‐unload cycles, even under 1000% strain. This is the lowest value among hydrogels reported to date. Along with excellent fatigue resistance, water retention, antifreeze, and water‐assisted healing ability, this hydrogel shows good performance in fatigue resistant capacitive strain sensors. Moreover, a fishtail‐shaped PAM‐CaCl 2 hydrogel operates as a bionic muscle to drive a bionic fish at a forward speed of 16.3 cm s −1 , when a voltage of 0.1 V is applied.