A robust and sensitive flexible strain sensor based on polyurethane cross-linked composite hydrogels for the detection of human motion

Abstract

Although conventional hydrogel sensors have excellent flexibility and conductivity, they suffer from low mechanical strength and are not able to withstand large stresses. Herein, a mechanically enhanced hydrogel doped with an MXene was prepared to further expand the application of hydrogels as flexible sensors in large-stress scenarios. The MXene-doped hydrogel (PPMD) was obtained via the copolymerization of a double-bond-capped polyurethane with acrylamide (AM), and acrylic acid (AA) monomers under UV light. In this sophisticated structure, polyurethane was introduced into the hydrogel system as a cross-linking point, which greatly improves the mechanical properties of the hydrogel. Due to the presence of a considerable amount of hydrogen bonds of polyurethane and polyacrylamide, an MXene modified by carboxylate can be well dispersed in hydrogels. Therefore, the hydrogel was endowed with excellent electrical conductivity. Based on the synergistic effect of the MXene and polyurethane, the hydrogel exhibited excellent mechanical properties (tensile strength = 1.1 MPa, strain at break = 780%). In addition, the hydrogel showed no significant signal interference during testing over 1000 tensile cycles, indicating its excellent fatigue resistance and resilience within the stipulated test period. Benefiting from these merits, the hydrogel can be assembled with a Bluetooth module as a wireless system to monitor the movement of the human body with good responsiveness (as a stress sensor, S₁ = 9.21 kPa⁻¹; as a strain sensor, S₁ = 0.25).