Electrochemical phase transition of ionic hydrogels

Surface-patterned phase transition on network-homogeneous hydrogels remains a challenge. Herein, a general electrochemistry protocol is proposed which changes the local pH by electrode reactions, and enables a spatially patterned phase transition on the surface of hydrogels. We introduce bis-quaternary ammonium (BQA) salts with cationic groups of (C₂H₅)₃N⁺ into a hydrogel network containing carboxylic acid moieties. Upon applying electricity at 3 V, the cathode reaction generates OH⁻ ions that neutralize carboxylic acid (COOH) moieties into carboxylate (COO⁻) groups. The electrostatic interaction between COO⁻ and (C₂H₅)₃N⁺ decreases the local hydrophilicity of the network, and results in the phase transition of the hydrogel into a white state. By applying acid solution in the phase-transition areas, the carboxylate groups are protonated into carboxylic acid groups, which breaks the electrostatic interaction, and brings the phase-transition areas back to the transparent state. Such a reversible phase transition allows for writing, storing, and erasing of complicated information on the hydrogel surface, showing great potential for application in informatics, cryptology, and security areas.