Switchable, Recyclable, and Time-Programmable Hydrogels via Counterion-Engineered Differential Metal-Ion Coordination

Multifunctional hydrogels applied in flexible electronics and smart sensing face prominent challenges including functionality integration, environmental adaptability, and sustainability. This paper presents an anti-ion-regulated differential metal-ion coordination strategy to fabricate a multifunctional hydrogel (SP-M n+ X y− ) with switchable modules, recyclability, and time-dependent encryption. The SA-PAM double-network hydrogel, modulated by Fe 3+ and Ca 2+ (with Cl − or Ac − as counterions), exhibits distinct coordination behaviors: Fe 3+ demonstrates strong coordination due to high charge density, while Ca 2+ ’s binding is counterion-dependent–rapid network densification with CaCl 2 (fully dissociated Cl − ) versus controlled release with CaAc 2 (weakly dissociated Ac − ). The hydrogel achieves outstanding mechanical strength (0.52 MPa), wide sensing range (300%), fast response (114 ms), and freeze resistance (−20 °C). Reversible chelation of ethylene diamine tetraacetic acid enables efficient switching among conductive, adhesive, and sensing modules (performance decay < 17.95% after cycles). Leveraging Fe 3+ /Ca 2+ coordination kinetics, time-dependent color changes enable dynamic encryption. With high conductivity (15.1 S·m −1 ) and self-adhesion (9.5 kPa), the hydrogel serves as a wearable sensor for motion tracking, speech recognition, and electrocardiograph monitoring. Key innovations are achieved in this work, including: anti-ion-mediated precise property control, green fabrication of recyclable, multi-modal hydrogels, and kinetic-driven encryption, which advances hydrogel design for flexible electronics, sensing, and information security. Graphical Counterion-Mediated Differential Metal Coordination enables recyclable, multifunctional hydrogels (SP-M n+ X y− ) with switchable conductive/adhesive/sensing modules and time-dependent encryption, which yields advancing wearable electronics with the capability of motion/speech/ECG monitoring, self-power supply, and secure time-dependent information coding.