pH-tunable electro-responsive sodium alginate/chitosan-based Janus polyelectrolyte hydrogel with reversible bidirectional bending

Abstract

Polyelectrolyte hydrogels are ideal for soft actuators, but conventional chitosan/sodium alginate (CS/SA) ionic hydrogels face a critical trade-off among flexibility, ionic content, and actuation speed, hindering their practical applications. Herein, dimethylaminoethyl methacrylate (DMAEMA) and sodium acrylate (AAS) were used to modify CS-based polycationic and SA-based polyanionic networks, respectively. A Janus polyelectrolyte hydrogel (PCDM/PSSM) with seamless interface was fabricated via one-pot synthesis, electrohydrodynamic (EHD) printing, and in-situ photopolymerization. Microstructural characterization verified successful monomer grafting, amorphous/low-crystallinity networks, and robust interlocked interfaces. The hydrogel exhibited superior mechanical properties (PSSM achieving a tensile stress of 78.6 kPa and a strain of 775.6%, and PCDM achieving a tensile stress of 52.2 kPa and a strain of 808.9%, respectively), high ionic conductivity (13.81 × 10⁻³ S/cm for PSSM), and efficient electro-actuation (155° bending at 10 V). Its core advantage is pH-programmable reversible bidirectional bending: −145° toward the polyanionic layer at pH = 2 and + 151° toward the polycationic layer at pH ≥ 7, originating from pH-regulated asymmetric swelling of protonated/deprotonated functional groups and enhanced by electric-field-induced ion migration. Demonstrated applications included an underwater soft gripper for fragile object manipulation and a biomimetic flower for pH-responsive petal closure. This work resolves the longstanding trade-off of CS/SA hydrogels and provides a scalable strategy for high-performance stimuli-responsive soft actuators, promising broad use in soft robotics, bioinspired systems, and biomedical engineering.