Tough and elastic hydrogels based on robust hydrophobicity-assisted metal ion coordination for flexible wearable devices

Abstract

Flexible wearable sensors that combine excellent flexibility, high elasticity, sensing capabilities, and outstanding biocompatibility are gaining increasing attention. In this study, we successfully develop a robust and elastic hydrogel-based flexible wearable sensor by modulating molecular structures combined with metal ion coordination. We leverage three N-acryloyl amino acid monomers, including N-acryloyl glycine (AG), N-acryloyl alanine (AA), and N-acryloyl valine (AV) with different hydrophobic groups adjacent to the carboxyl group, to copolymerize with acrylamide (AM) in the presence of Zr⁴⁺ for hydrogel preparation in one step (P(AM₃–AG/AA/AV_0.06)–Zr_0.03⁴⁺ hydrogels). Our investigation reveals that the P(AM₃–AV_0.06)–Zr_0.03⁴⁺ hydrogel with the most hydrophobic side group demonstrates superior mechanical properties (1.1 MPa tensile stress, 3566 kJ m⁻³ toughness and 1.3 kJ m⁻² fracture energy) and resilience to multiple tensile (30% strain, 500 cycles) and compression cycling (50% strain, 500 cycles). Moreover, the P(AM₃–AV_0.06)–Zr_0.03⁴⁺ hydrogel exhibits good biocompatibility and high conductivity (1.1 S m⁻¹) and responsivity (GF = 16.21), and is proved to be suitable as a flexible wearable sensor for comprehensive human activity monitoring.