Transparent and conductive amino acid-tackified hydrogels as wearable strain sensors

Abstract Currently, flexible and conductive hydrogels have become promising materials for wearable strain sensors. However, adhering to skin seamlessly without any adhesive tape or bandage still remains a challenge. Moreover, the transparency of hydrogel was highly desired for satisfying the diverse requirement of electronic devices. Here, an adhesive polyacrylamide hydrogel driven by amino acids was designed by using lysine. The lysine-mediated hydrogel exhibited excellent adhesive behavior for diverse material substrates (e.g., metal, stainless steel, plastic, glass, rubber, silica rubber) and even for skin. The maximum peeling strength of hydrogel on aluminium could achieve 202.2 N/m, which exceeded that of PAAm hydrogel with 36.4 N/m. Intriguingly, by introducing lithium chloride into the system, the adhesive hydrogel exhibited high conductivity (0.0736 S/cm), transparency (92%) and freezing-resistant property (−31.8 °C). Moreover, the hydrogel-based strain sensor could be sensitive to both large activities (e.g., joint bending) and tiny motions (e.g., swallowing, speaking and breathing). This work provided an inspiration for the design of novel adhesive and conductive hydrogels as flexible strain sensors.