Thermoresponsive, magnetic, adhesive and conductive nanocomposite hydrogels for wireless and non-contact flexible sensors

Hydrogel sensors are increasingly investigated and applied in emerging flexible electronics, but the exploration of hydrogel sensors with multiple functionalities remains a challenge. In this work, thermoresponsive, magnetic, adhesive and conductive nanocomposite hydrogels are developed and further applied in fabricating multi-functional flexible sensors. In such nanocomposite hydrogels, poly(N-isopropyl acrylamide-co-acrylamide) (P(NIPAAm-co-Am)) and Fe3O4 nanoparticles are designed as the thermoresponsive and magnetic component respectively, poly(vinyl alcohol) (PVA) is introduced in a semi-interpenetrating way to enhance the adhesiveness and mechanical property, and potassium chloride (KCl) works as the conductive component. Interestingly, the adhesion property is switchable by temperature based on the thermoresponsiveness of the nanocomposite hydrogels. Resistive strain sensors assembled from the nanocomposite hydrogels achieve a gauge factor (GF) of 4.21, a response time of 140 ms, stability of 2000 sensing cycles and excellent reversibility. Such nanocomposite hydrogel sensors are used as wearable sensors for monitoring diverse human motions and further integrated with Bluetooth signal transmission and reception system to fabricate wireless wearable sensors. Because of the deformation of nanocomposite hydrogels in switching magnetic field, the nanocomposite hydrogel sensors also have the ability to respond in a non-contact way to switching magnetic fields.