Multifunctional Ionic Skin with Sensing, UV-Filtering, Water-Retaining, and Anti-Freezing Capabilities

Conductive polymer hydrogels are receiving considerable attention in applications such as soft robots and human-machine interfaces. Herein, a transparent and highly ionically conductive hydrogel that integrates sensing, UV-filtering, water-retaining, and anti-freezing performances is achieved by the organic combination of tannic acid-coated hydroxyapatite nanowires (TA@HAP NWs), polyvinyl alcohol (PVA) chains, ethylene glycol (EG), and metal ions. The highly ionic conductivity of the hydrogel enables tensile strain, pressure, and temperature sensing capabilities. In particular, in terms of the hydrogel strain sensors based on ionic conduction, it has high sensitivity (GF = 2.84) within a wide strain range (350%), high linearity (R-2 = 0.99003), fast response (approximate to 50 ms) and excellent cycle stability. In addition, the incorporated TA@HAP NWs act as a nano-reinforced filler to improve the mechanical properties and confer a UV-shielding ability upon the hydrogel due to its size effect and the characteristics of absorbing ultraviolet light waves, which can reflect and absorb short ultraviolet rays and transmit visible light. Meanwhile, owing to the water-locking effect between EG and water molecules, the hydrogel exhibits freezing resistance at low temperatures and moisture retention at high temperatures. This biocompatible and multifunctional conductive hydrogel provides new ideas for the design of novel ionic skin devices.