Transparent, highly stretchable, adhesive, and sensitive ionic conductive hydrogel strain sensor for human motion monitoring

Due to the flexibility, ductility and tensile properties, hydrogel is considered as promising candidates for the new generation of wearable flexible devices. However, it is still a challenge to endow multiple functions such as excellent conductivity, high stretchability, self-healing, self-adhesion, and biocompatibility in one hydrogel network. Herein, a two-step method was used to prepare multifunctional transparent tannic acid/sodium alginate/polyacrylic acid/carboxymethyl cellulose/aluminum trichloride (SA/TA/PAA/CMC/Al(III)) hydrogels with high stretchability, toughness, adhesion, and conductivity. This hydrogel can adhere to various substrates both organic and inorganic materials. The highest adhesion strength of the hydrogels to the iron sheet reaches 22.33 kPa with the fracture stress being 0.16 MPa and the toughness 1.01 MJ/m3. Moreover, the hydrogel exhibited repeatable and durable adhesion to human skin and could be peeled off completely without any residual, irritation or allergic reactions. Additionally, the hydrogels also have good repetitive adhesion and strain sensitivity. It was demonstrated that both large-scale and small-scale movements of the human body could be monitored by directly attached to various parts of the human body by the self-adhesion. This work provides a new prospect for the design of the biocompatible hydrogels with transparent, stretchable, self-adhesive, and strain-sensitive properties for potential applications in wearable electronic sensors.