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

Abstract 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.