A versatile, highly stretchable, and anti-freezing alginate/polyacrylamide/polyaniline multi-network hydrogel for flexible strain sensors and supercapacitors

Conductive hydrogels have great potential as electrolyte materials for flexible strain sensors and supercapacitors. However, it remains a challenge to develop multifunctional hydrogels with excellent frost resistance, toughness, ionic conductivity, and electrochemical properties using simple methods. Herein, a "chemical-physical-ionic" cross-linked sodium alginate/polyacrylamide/polyaniline (SA/PAM/Ca²⁺/PANI) multi-network hydrogel was developed by in situ polymerization of aniline monomer within a Ca²⁺-crosslinked SA/PAM hydrogel network. The SA/PAM/Ca²⁺/PANI hydrogel shows excellent mechanical properties, (tensile strength of 0.577 MPa at a strain of 1991 %), high toughness (5.52 KJ·m^-3), and high ionic conductivity (16.51 S·m^-1 at 25 °C and 11.08 S·m^-1 at -20 °C). The SA/PAM/Ca²⁺/PANI hydrogel-based strain sensor exhibited high sensitivity (gauge factor of 3.82 at 60-500 % strain), an extensive detection range (0-2000 %), and excellent frost resistance. The strain sensor can accurately monitor various human motions, as well as electrocardiograph (ECG) signals during both rest and exercise. The supercapacitor assembled with the SA/PAM/Ca²⁺/PANI hydrogel electrolyte exhibited a high surface capacitance (177.19 mF·cm^-2 at 2 mA·cm^-2), maximum energy density (21.93 Wh·kg^-1), and high power density (3089 W·kg^-1). Moreover, it maintained satisfactory electrochemical stability with 77.8 % capacitance retention after 4000 cycles. Therefore, the versatile SA/PAM/Ca²⁺/PANI hydrogel shows promising potential for applications in flexible wearable electronic devices.