Micelle-Micelle Cross-Linked Highly Stretchable Conductive Hydrogels for Potential Applications of Strain and Electronic Skin Sensors

Wearable sensors made of flexible and stretchable hydrogelshavegarnered significant attention. However, their use has been limitedby poor mechanical performance, such as poor toughness, poor self-recovery,and a large response-recovery time. To overcome these limitations,we have developed a novel cross-linking agent-based hydrogel withhigh stretchability, high toughness, antifatigue properties, and goodconductivity. These hydrogels were developed by introducing l-glutamic acid (LGA) into hydrophobically cross-linked polyacrylamide(PAmm) chains. In this system, LGA dynamically cross-linked the micelle-micelleand micelle-polymer chains and greatly regulates the mechanicalproperties of the hydrogels. The noncovalent synergistic interactionsthat came with the insertion of LGA enable the hydrogels to achievehigh stretchability and high stress values, with fast self-recoveryand antifatigue behaviors without the help of foreign stimuli. Additionally,LGA-based hydrogels can function as durable and highly sensitive strainsensors for detecting various mechanical deformations with a fastresponse-recovery time and high gauge factor value. As a result,the hydrogels have the capability to be designed as wearable strainsensors that are capable of detecting large human joint motions, suchas neck twisting, neck bending, and wrist, finger, and elbow movements.Similarly, these hydrogels are capable of monitoring different subtlehuman motions such as speaking and differentiating between differentwords, swallowing, and drinking through larynx vibrations. Besidesthese large and subtle human motions, hydrogels have the ability todifferentiate and reproduce different written words with reliability.These LGA-regulated hydrogels have potential applications in electricskins, medical monitoring, soft robotics, and flexible touch panels.