High strength, anti-freezing, and conductive poly(vinyl alcohol)/urea ionic hydrogels as soft sensor

Hydrogel-based sensors with excellent flexibility and stretchability have received extensive attention. However, fabricating anti-freezing, high-strength, and conductive hydrogel-based sensors remains a significant challenge. A stretchable sensor with high strength and anti-freezing herein is constructed using poly(vinyl alcohol)/urea (PVA/urea) ionic hydrogels. The PVA/urea ionic hydrogels were prepared by a simple strategy of soaking the freeze-thawed PVA hydrogels in sodium citrate (Na(3)Cit) water/urea aqueous solutions. The presence of Na(3)Cit in the hydrogel produces hydrophobic aggregation, endowing the PVA/urea ionic hydrogel with high mechanical properties (similar to 1.51 MPa) and makes the hydrogel have high strain gauge factors (GF = 1.94). The synergistic effect of Na(3)Cit and urea endows the PVA/urea ionic hydrogel with anti-freezing properties (similar to-20 degrees C). The obtained sensors have excellent stability (100 cycles at epsilon = 85%) and could detect various human activities even at -20 degrees C. More importantly, joint motion can be monitored wirelessly via Bluetooth at -20 degrees C. This work provides a feasible method to construct anti-freezing, high-strength, and conductive hydrogel. It paves the way for versatile applications in human-motion detection, intelligence device, and biomimetic skin.