Flexible Li+/agar/pHEAA double-network conductive hydrogels with self-adhesive and self-repairing properties as strain sensors for human motion monitoring

Hydrogel-based flexible strain sensors have drawn significant interests due to their potential applications in the fields of wearable devices, electronic skins, soft robotics. Herein, novel Li+ ions doped agar/(N-poly(hydroxyethyl)acrylamide) (pHEAA) conductive double-network hydrogels (Li+/agar/pHEAA DN hydrogels) were synthesized and assembled as strain sensors for human motion monitoring. The incorporation of Li+ not only endowed high conductivity of the hydrogels, but also increased the interaction between the agar and pHEAA networks. At proper content of Li+ (0.8 wt%), transparent Li+/agar/pHEAA DN hydrogels with excellent mechanical performances (fracture stress of 1.18 MPa and fracture strain of 860%), good self-recovery and self-repairing properties were obtained. Ascribe to the plenty of amide and hydroxyl functional groups in pHEAA networks, the as-prepared hydrogels displayed strong self-adhesiveness on various nonporous solid surfaces (e.g., aluminum, glass, titanium, and stainless steel) with high interfacial toughness of 920–1130 J/m2 due to the multiple hydrogen bonding between the hydrogels and substrates. More impressively, the DN hydrogels can be self-adhered on human skins. Furthermore, the hydrogels also showed stretching and temperature responsiveness with high sensitivity and reliability in the form of resistance variation. Taking advantage of excellent toughness, surface adhesiveness, self-repairing and stimulus responsiveness, the as-obtained hydrogels can be applied as strain sensors for monitoring human motion in large amplitude joint movement and feeble physiological signals. We envision that the proposed Li+/agar/pHEAA DN hydrogels-based strain sensors hold great potential in health care management, disease diagnosis and artificial electronic skins.