Soft/hard double network supramolecular hydrogel with multifunctional properties for flexible strain sensor

Abstract

Flexible sensors are widely used in wearable devices, embedded equipment, bionic skin, and soft robots due to their flexibility, extensibility, and bendability. Supramolecular gels play a crucial role in the field of flexible sensors due to their structural designability, tunable mechanical properties, and multi-stimulus responsiveness. However, they face challenges such as poor stability, low mechanical strength, and complex preparation processes, which currently limit their potential applications. In this study, 5,10,15,20-tetra-(4-carboxyphenyl) porphyrin (TCPP) and Li⁺ ions construct a rigid, cross-linked skeleton via metal coordination bonds, forming a hard network. The introduction of PVA benefits the gel by enhancing its flexibility through energy dissipation. The interaction between the rigid and flexible networks enhances the mechanical strength of the PVA/TCPP/LiCl (PTL) supramolecular gel. The gel's elongation at break reaches 1000 %, and its maximum tensile strength attains 3.93 MPa. Notably, the hydrogel nearly fully recovers at 200 % strain after five consecutive loading-unloading cycles. The hydrogel demonstrates excellent self-healing properties, with crack traces healing almost full recovery within 60 min. Additionally, it exhibits strong adhesion to a variety of hard and soft substrates. The hydrogel also maintains its soft properties at −40 °C, possesses thermal stability, UV shielding, and electrical conductivity. These properties can be applied to flexible sensors that accurately monitor human movement, including vocal cord vibration and limb bending, and can be applied in extreme conditions such as cold environments.