Ion-conductive hydrogel sensor prepared with alginate crosslinker for wide-range motion and temperature monitoring.

Flexible wearable electronic devices have gained widespread attention due to their comfort and real-time information transmission capabilities. However, they often exhibit poor stretchability, low sensitivity, and limited functionality, which restricts their practical applications. Herein, a conductive hydrogel (PNAL, a polymer synthesized from polyvinyl alcohol, nipaam, acrylic acid, and lithium chloride) that responds to both strain and temperature simultaneously was developed using a natural polysaccharide composite (SA-AGE) synthesized from sodium alginate (SA) and allyl glycidyl ether (AGE) as a crosslinking agent. These hydrogels were fabricated into multifunctional sensors capable of detecting strain and temperature with high sensitivity (gauge factor up to 1.12 and temperature coefficients of resistance up to -1.20 %/°C) and reliable stability (1000 cycles). Importantly, by adjusting the molar ratio of NIPAAm to AA, the volume phase transition temperature (VPTT) of the hydrogel can be easily controlled. When the temperature increases, the hydrogel changes from translucent to white and opaque within 20 s; when the temperature decreases, it returns to its original state. Based on the color-changing property of the hydrogel in response to temperature variations, it can serve as a visible color-changing sticker, providing early warning for patients with fever, thus significantly improving traditional temperature measurement methods.