Adhesive, transparent, stretchable, and strain-sensitive hydrogel as flexible strain sensor

In recent years, flexible hydrogels have shown potential applications as strain sensors in artificial intelligence, such as medical monitoring, human motion detection, and intelligent robotics. However, these flexible sensors require additional adhesive tapes due to a lack of adhesion so that they cannot be seamlessly combined with the contacted substrates, making it difficult for long-term practical applications. In addition, transmittance is an important indicator of the visualization for wearable flexible sensors and the addition of conductive fillers reduces transparency. It is a challenge for flexible sensors to maintain both light transmittance and conductivity. In this study, an adhesive, transparent, stretchable, and strain-sensitive conductive hydrogel was prepared from 2-acrylamido-2-methyl propane sulfonic acid (AMPS), N,N′-methylenebisacrylamide (MBA), and ammonium persulfate (APS) via a one-step in-situ polymerization and freeze-thaw method. Because the covalent bond and hydrogen bond cooperate, the hydrogel exhibited excellent stretchability. Meanwhile, the hydrogel could adhere to the surfaces of various objects (e.g., wood, plastic, rubbers, metal, glasses, and human skin) without residue and achieved repeated adhesion. Moreover, the hydrogel exhibited superior transparency as well as strain-sensitive conductivity that could be used to monitor body movements (e.g., bending of fingers, wrists, and elbows), and could be applied in wearable devices without impeding optical sign.