A novel environment-tolerant hydrogel via a combination effect of a polyurethane coating and hygroscopic salt for underwater monitoring

In recent years, hydrogels have been intensively studied to develop wearable devices with various functionalities. However, the further development of such devices is severely retarded by the inherent weaknesses of hydrogels, for example, signal instability or even failure caused by irreversible water evaporation, unbearable swelling, and low temperature. Herein, a simple and versatile strategy is proposed that combines the effect of a polyurethane coating and hygroscopic lithium chloride (LiCl) to enable polyacrylamide (PAAm) hydrogels with satisfactory stability of water content in wide ranges of relative humidity (30–80%), anti-swelling capacity in various liquid media (deionized water, seawater, acid (pH ≈ 2) and base (pH ≈ 12) solutions) and freezing tolerance. Furthermore, it is proved that our proposed strategy is also applicable to various hydrogels such as polyvinyl alcohol (PVA) and polyzwitterionic hydrogels, resulting in excellent environmental tolerance. Moreover, hydrogel-based electronic devices are developed to demonstrate their prospect in underwater communication and monitoring of human and aquatic animal movements.