Quadruple-Functional MXene Enables Ultrafast, Green Fabrication of High-Performance Polyacrylamide Hydrogels for Flexible Strain Sensors

Polyacrylamide (PAM) hydrogels are promising candidates for flexible wearable sensors but suffer from limitations including toxic cross-linkers(N,N′-methylenebis(acrylamide), BIS), prolonged gelation times, high-temperature requirements, and inadequate mechanical properties. Herein, a quadruple-functional MXene was employed to fabricate high-performance PAM hydrogels within 5 min at ambient temperature. MXenes served simultaneously as an initiating accelerator (replacing toxic BIS), physical cross-linker, conductive agent, and reinforcing nanofiller. Reductive MXene nanosheets catalyzed ultrafast polymerization of acrylamide monomers via redox reactions with ammonium persulfate(APS), facilitating a green, low-energy fabrication process and preventing MXene nanosheets reaggregation within the hydrogel network. By adjusting the MXene content, the optimized hydrogel (6 mg/mL MXene) demonstrated exceptional mechanical performance, with 1558% elongation at break and 119 kPa tensile strength, exceeding traditional BIS/PAM hydrogels (1081% elongation, 74 kPa strength). This enhancement is attributed to the uniform honeycomb-like porous structure and dual-network architecture comprising covalent PAM backbone and physical hydrogen-bonded MXene cross-links. Additionally, the MXene incorporation confers electrical conductivity (0.3 S/m) and photothermal performance, with temperature increase up to 21.8 °C under near-infrared (NIR) irradiation and good photothermal stability. As a strain sensor, the hydrogel exhibits two-stage sensing response with gauge factor (GF) of 1.1 (0–235%) and 2.4 (235–400%), rapid response time (35 ms), recovery time (98 ms), and excellent cyclic stability. In L929 cytotoxicity assays, MXene/PAM hydrogels demonstrate improved biocompatibility compared to BIS/PAM hydrogels. The sensor accurately monitors both large human movements (finger, wrist, bending, handwriting) and subtle physiological activities (pulse and throat vibrations). This study introduces a rapid, eco-friendly method for fabricating multifunctional PAM hydrogels with enhanced mechanical, conductive, photothermal, and biocompatible properties for flexible electronic skin and biomedical devices.