6-amino caproic acid regulated mechanical performance of ionically conductive ultra-stretchable hydrogels for flexible strain and epidermis sensors

In wearable sensors, flexible and stretchable hydrogels attract much attention from researchers however, weak mechanical properties, low toughness, poor self-recovery, and large response time limit the practical applicability of hydrogels. Here, ultra-stretchable, tough, high antifatigue, and fast responsive hydrogels are prepared using 6-amino caproic acid (ACA) as a dynamic bridge between the cationic surfactant and hydrophobically associated acrylamide (Amm) hydrogels. The hydrophobic monomer lauryl methacrylate (LMA) stabilized in the micelles of surfactant which work as hydrophobic associations and keep the integrity of the hydrogel network. While the addition of ACA significantly regulates the mechanical properties of hydrogels, where ACA work as a bridge between the micelles and polymer chains through ionic interactions with cationic surfactant and hydrogen bonding with Amm. These non-covalent interactions endow excellent mechanical properties, high toughness, and good self-recovery in the hydrogel network. Moreover, the excellent mechanical performance and good conductivity of the hydrogels provide durability and antifatigue properties into hydrogels which make the hydrogels suitable for strain sensors and human motion monitoring. Our fabricated hydrogels have the capability of performing more than 350 cyclic loading and unloading cycles without any damage during strain sensing and also senses different large and small strains with a highest gauge factor (GF) of 12.8 (at 400% strain). Besides, the hydrogels have also the ability to monitor different large and subtle human motions that include wrist, neck, finger, swallowing, speaking, and writing with a very fast response and recovery time of 100 ms and 80 ms respectively. Our prepared hydrogels would discover a wide spread application in the field of soft robotics, electronic skin, human health care monitoring, strain, and pressure sensors.