Highly Stretchable, Self-Adhesive, Direction-Aware Wireless Hydrogel-MMT Strain Sensors via a Gradient Structure of Intersecting Networks

Hydrogels are considered as an ideal material for bioelectronic devices to fulfill the requirements of various applications. However, integrating self-adhesion properties, high stretchability, good sensing performance, and direction-aware functionality into a single hydrogel strain sensor remains a significant challenge and has been seldom reported. In this study, through the characteristics of montmorillonite (MMT) adsorption, we designed a gradient intersecting strategy based on bifunctional MMT nanoparticles. These nanoparticles act as both cross-linking agents and conductive components to strongly intersect the hydrogel and the conductive networks, preventing them from sliding against each other. Thus, the hydrogel sensor had a wide range of adjustable mechanical properties and good adhesion on various substrates. Moreover, the hydrogel sensor can detect both large- and small-scale human activities and achieves direction awareness. Importantly, the hydrogel sensor can be used to construct a wireless sensor system to monitor human movement in real time. This work provides a strategy for the design of the next-generation flexible strain sensors and a method for the development of multifunctional MMT materials, expanding the application of both MMT materials and hydrogel sensors.