Flexible and Environmentally Friendly Calcium Polyphosphate Hydrogels: Toward Inorganic Functional Materials for Wearable Devices and Soft Actuators

Inorganic hydrogels have attracted significant interest in materials science. However, it is a big challenge to fabricate flexible and multifunctional inorganic hydrogels due to the inherent rigidity of traditional inorganic materials. Herein, a flexible inorganic hydrogel is proposed, which is prepared by crosslinking long-chain polyphosphate (LPP) with Ca²⁺ ions. This pure-inorganic CaLPP hydrogel exhibits excellent self-healing ability, arbitrarily shapable ability, conductivity, degradability and biocompatibility. Furthermore, the CaLPP hydrogel can be used as strain sensors to monitor dynamic deformations (e.g., stretching and bending) with high sensitivity and reliability. The CaLPP hydrogel can also be used as ionic skins to detect human motions, such as bending of the joints and facial expressions. After functionalization, the CaLPP hydrogel can be used as a magnetic actuator. This fundamental work provides an environmentally friendly soft material with a purely inorganic composition, which can be used as a complement to organic-based soft materials for wearable devices and actuators.