Programming Homogeneous Hydrogels Using Directional Ion Transport toward Rapid 3D Reconfiguration

Environmentally adaptive hydrogels undergo reconfiguration under external stimuli, suitable for intelligent sensing, bioinspired actuation, and soft robotics. However, achieving programmable three-dimensional (3D) morphing in homogeneous hydrogels under constant stimuli remains quite challenging despite the tremendous research efforts. Herein, inspired by the directional ion-transport actuation of starfish, supramolecular poly(amic acid) salt (PAAS) hydrogels with predictable 3D structure formation were developed through directional metal ion transport imparted by seawater. These hydrogels were prepared through aqueous polymerization of 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) and p-phenylenediamine (PDA) in the presence of organic bases with imidazole moieties (1,2-dimethylimidazole (DMZ), imidazole (IM), and 1-(2-hydroxyethyl)imidazole (HIM)), followed by thermal treatment at 50 °C. The resultant hydrogels, featuring high-density carboxylates, enable programmable 3D shape-morphing under seawater stimulation through spatially asymmetric (Ca²⁺/Mg²⁺)-carboxylate cross-linking and swelling/contraction. The dynamic supramolecular networks provide remarkable reconfigurability, with repeated reconstruction of complex 3D architectures. Specifically, the hydrogels show exceptional stability with low equilibrium swelling ratios (<50%), increased tensile strength (up to 2.1 MPa), and all 180° deformations completed within 70 s. Overall, programming 3D morphologies of homogeneous hydrogels using a single stimulus has potential for advancing shape-morphing engineering.