Anion-π Interaction-Regulated Ion Generation and High Piezoionic Response in Ultrasoft Low-Salinity Hydrogels

Anion-π interactions play important roles in underwater cohesion and selective ion recognition and transport in biological systems. Inspired by these functions, we report the first hydrogel system employing anion-π interactions as a design motif for piezoionic energy conversion. Unlike conventional piezoionic hydrogels that typically employ relatively stiff matrices (10^-7-10^-4 Pa^-1) and concentrated salts (≥1 M), our system achieves efficient mechanoelectrical conversion in an ultrasoft hydrogel under low-salinity conditions. Anion-π interactions serve as dynamic cohesive crosslinking motifs, imparting injectability, self-healing, and ultrasoft tissue compliance (10^-3-10^-2 Pa^-1), while the cooperative aromatic-phosphate environment is proposed to regulate the separation of water-derived ionic species. Hydrated protonic species may associate with aromatic motifs through cation-π interactions strengthened by adjacent phosphate groups, increasing the lifetime of charge-separated protonic and hydroxide-containing ions. Under deformation-induced pressure gradients, poroelastic solvent redistribution is proposed to preferentially transport mobile hydroxide-containing species, producing ionic transport asymmetry and a high piezoionic coefficient of 7.8 mV kPa^-1, with 70.6 mV voltage and 32.1 µA current at ≤0.15 M salt. This combination enables minimally invasive delivery, conformal tissue contact, biomechanical monitoring, and stimulation-level voltage generation. This work highlights anion-π interactions as a versatile strategy for developing next-generation soft ion-conducting materials with combined mechanical and electromechanical properties.