Ultra-Stretchable,\nAdhesive, Conductive, and Antifreezing\nMultinetwork Borate Ester-Based Hydrogel for Wearable Strain Sensor\nand VOC Absorption

Abstract

Hydrogels based on borate ester bonds exhibit remarkable\ntensile\nstrength and self-healing ability, which make them a promising material\nfor various biological research and strain sensor applications. However,\nin order to meet the practical application of hydrogel strain sensors,\nthey must also show high conductivity, frost resistance, and proper\nadhesion, which is still a continuous challenge. Herein, a triple\nnetwork hydrogel was prepared using poly(vinyl alcohol) (PVA) as the\nfirst network, ethylene imine polymer (PEI) as the second network,\nand poly(acrylamide-<i>co</i>-acrylic acid) copolymer (denoted\nas P(AM-Co-AA)) as the third network. 3-Carboxy-4-fluorophenylboronic\nacid (CFBS) was used as the cross-linking agent, glycerol (GL) was\nadded to improve low-temperature resistance, and sodium chloride (NaCl)\nwas incorporated to enhance electrical conductivity. The resulting\nPVA-CFBS@PEI@P(AM-Co-AA) triple network hydrogel exhibited impressive\nmechanical properties, including ultra tensile strength (4100%, 266.8\nkPa), high toughness (6.5 MJ/m³), and low-temperature resistance\n(−60 °C). Additionally, it demonstrated high conductivity\n(σ = 1.83 mS/cm). The incorporation of CFBS endowed the hydrogel\nwith excellent self-healing ability, while GL improved low-temperature\nresistance and strain sensing sensitivity (gauge factor (GF) = 2.8\n(0–300%), GF = 5.6 (300–600%), GF = 8.7 (600–1000%)).\nThe prepared hydrogel sensor can repetitively detect and differentiate\nbetween a wide range of human activities such as joint movements,\nfrowning, and smiling. Additionally, the hydrogel demonstrated favorable\nmechanical properties at −20 °C (good adhesion, tensile\nstrength: 1169.8%, 1.2 MPa; conductivity: 0.71 mS/cm, and strain sensing\ncoefficient: GF = 1.3), making it suitable for applications in low-temperature\nenvironments. Furthermore, it also functions as an exceptional adsorbent,\ncapable of selectively absorbing volatile organic compounds at high\ncapacity (e.g., methanol: 1.80 g/g; acetone: 1.50 g/g).