Toughening of anti-freezing ionic hydrogels with Zr4+-dicarboxylic acid coordination complex for low temperature sensing applications

Abstract

Conductive hydrogels are widely used in electronic skin, wearable sensing devices, human–machine interfaces, and soft robots because of their high elasticity, biocompatibility, and conformability in interfacial contact. However, hydrogels lose electrical conductivity and flexibility due to water crystallization at low temperatures, which severely limits their applications in the frigid regions. In this study, we propose to design a metal–ligand ionic hydrogel (PAASP-Zr-LiCl) through the formation of a stable coordination bond between the dicarboxylic acid group monomer poly(N-acryloyl aspartic acid) (PAASP) and zirconium ion (Zr⁴⁺). Zr⁴⁺–COO^- metal-coordination complex as physical cross-linking points of the network can effectively improve the mechanical properties of hydrogels. By introducing lithium chloride (LiCl), the hydrogel obtained excellent anti-freezing properties (crystallization temperature < -80 °C) and high ionic conductivity (8.45 S/m). The LiCl molecules enhance the interaction between the polymer network and water molecules. The ionic hydrogel-based strain sensors exhibited a high gauge factor of 3.21. Combining hydrogel sensors with soft grippers can realize continuous and stable monitoring of grasping objects at low temperature of −30 °C. By integration of excellent flexibility (elongation at break 837.4 %), good ionic conductivity, high sensitivity, and excellent anti-freezing properties, the anti-freezing ionic hydrogel has a wide range of applications in the frigid regions of the plateau.