Fuel-Driven Rapid Swelling and Shrinking in Porous Hydrogels

Abstract Biological systems are known for their transient and autonomous responses, such as muscle actuation and cell division. Soft materials that mimic these responses include materials that actuate or morph when exposed to chemical fuels. However, their reconfiguration times extend to tens of hours, which is 10–100× slower than biological systems. Here, for the first time, we demonstrate autonomous responses (swelling and shrinking) in macroscopic porous hydrogels that occur in <1 h. The gels are cross-linked networks of acrylic acid and acrylamide copolymers containing ∼100 μm interconnected pores. When a carbodiimide chemical fuel is added to a water-swollen gel, anionic carboxylates are converted to nonpolar anhydrides, causing the gels to shrink by ∼50% in 5 min. The anhydrides then spontaneously hydrolyze and revert to their anionic form, causing the gels to autonomously reswell in <1 h. The reswelling time increases with decreasing porosity and increasing fuel concentration. Non-reactive acrylamide groups prevent the formation of extended insoluble polyanhydride domains and ensure that gels retain sufficient water affinity after fuel addition, that together with porosity enables rapid reswelling. We show that rapid gel shrinkage upon fuel exposure occurs via convective water transport out of pores, while reswelling occurs via rapid bulk hydrolysis of anhydrides. This work thereby uncovers general mechanisms for quickening the response of soft materials to chemical triggers.