Ultralow-Concentration-Initiator-Enabled Acrylamide-Phosphoric Acid Ionic Hydrogel with High Toughness and Low Hysteresis for Multiple-Function Flexible Sensors

Phosphoric acid-based ion-conductive hydrogels face a long-standing challenge of balancing toughness, low hysteresis, and high conductivity, which are key requirements for advanced flexible sensing materials. In this work, a tough and low-hysteresis acrylamide-phosphoric acid (AP) hydrogel is proposed, using an ultralow-initiator-concentration (0.005 wt % relative to the monomer mass) polymerization method. Swelling, SAXS, and SEM experiments confirm the uniform chains, and FT-IR experiments demonstrate a strong hydrogen-bonding network of our ultralow-initiator-concentration-polymerized AP hydrogel. Uniaxial tensile tests show that the toughness and hysteresis of the AP hydrogel are inversely associated with the initiator concentration. Consequently, the ultralow-initiator-concentration-enabled AP hydrogel presents a tensile strength of 2.1 MPa, a toughness of 8.9 MJ m–3, and an ultralow hysteresis of 1.35% under 100% strain. Benefiting from the excellent electromechanical properties, the AP hydrogel-based flexible sensor exhibits high reliability as a flexible resistance strain sensor and an electric double-layer capacitive pressure sensor. This work not only resolves the inherent constraint of traditional phosphoric acid-based hydrogels but also provides a perspective for the structural regulation of functional hydrogels, enabling broader applications in flexible sensing and human–machine interactions.