Ultra-flexible, transparent, adhesive, healable, freezing-tolerant and long-term stable cryogels for wearable sensors

Multifunctional hydrogels are highly desirable for soft devices. However, their complex functional constituents pose safe and environmental concerns in the application. Herein, a single salt, LiCl, simultaneously serves as multiple roles in the construction of hierarchical structures for freeze-thawed polyvinyl alcohol (PVA) hydrogels, triggering unprecedented multifunctionality and long-term stability. First, the antifreezing LiCl suppresses the growth of ice crystals and thus creates an unusually weak approach of polymer chains upon freezing due to the weakened expelling of ice, achieving the amorphization of polymer network and releasing some free hydroxyl groups. Second, contributed by bound water with LiCl, the selected evaporation of free water not only densifies polymer networks but also endows the hydrogel with long-term stability. Third, the incorporation of LiCl also adds its intrinsic features to hydrogels. Thus, the ultimate PVA hydrogels exhibit integrated properties of superior flexibility (Young’s modulus of 18.8 kPa), extensibility (704%), transparency (84%), adhesion, self-healing, freezing-tolerance (−43°C) and long-term stability (95% width and 87% thickness of the initial size after five months). With these particular hierarchical structures and performances, the capacity of using single cryogels in the wearable sensors is demonstrated. We anticipate that the new generation of PVA hydrogels will provide more opportunities in various soft devices.