Lignin-Ethylene Glycol Improved Hydrogel with Antifreezing and Antiswelling Properties As a Flexible Sensor for Underwater Motion Monitoring

Conductive hydrogels have great potential as flexible sensors for motion monitoring. However, their practical applications are still limited due to the poor functional stability of high-water-content hydrogels at low temperatures and in wet environments. In this paper, a hydrogel composite system was synthesized with various hydrogen-bonding interactions. The main strategy was to introduce ethylene glycol (EG) and alkaline lignin (AL) into a polymer network. Except for EG, a moderate amount of AL (2 wt %) significantly enhanced the freeze resistance of the hydrogel. Moreover, the antiswelling ability of hydrogel improved significantly (the optimal swelling rate was 10%) with the introduction of AL. After a long-term immersion of the hydrogel in water, no change in mechanical properties (elongation at break > 300%, tensile strength > 0.13 MPa), frost resistance (−25.88 °C), and conductivity (0.18 S/m) were observed. Notably, the hydrogel sensor could accurately detect human and aquatic organisms’ movements in water. The sensor was also assembled into a wireless sensing device to transmit information via Bluetooth technology and display an electromyography signal equivalent to that of a commercial Ag/AgCl gel electrode. This green, low-cost, and sustainable antifreezing and antiswelling hydrogel sensor exhibits great potential in low-temperature and underwater sensing applications.