Self-healing hydrogel with multiple dynamic interactions for multifunctional epidermal sensor

With the fast development of flexible sensors, various materials are used to construct sensors. However, it is still a challenge to prepare a hydrogel sensor with the integrated functions of good mechanical and sensing performance, and excellent adhesion and self-healing properties. Herein, an all-in-one hydrogel sensor TG2P3 was constructed through the rational design of intermolecular interactions containing tannic acid (TA), graphene oxide (GO), glycerol, poly(allylamine hydrochloride) (PAH), poly(acrylic acid) (PAA), and Poly(N-isopropylacrylamide) (PNIPAM). The electrostatic interactions between the polymer chains of PAH and PAA, multiple hydrogen bonds, π-π interactions between GO and TA, and cation-π interactions between PAH and the aromatic benzene rings, all significantly contributed to the tremendous mechanical strain (1420%) of the hydrogel. Meanwhile, TG2P3 hydrogel showed an impressive self-healing efficiency of 87%, owing to the feature of multiple dynamic interactions. The addition of glycerol endowed TG2P3 hydrogel with outstanding anti-freezing and anti-drying properties, allowing the retainment of a desirable self-healing efficiency of 25.7% even at −20 ℃, as well as a high water-retention rate at room temperature (97.9%) and at 60 ℃ (95.8%). Notably, a remarkable stretchability of 572% could be observed even after 7 days of storage. Furthermore, the TG2P3 hydrogel sensor exhibited an expressive gauge factor (GF = 1.3) within a wide strain range (300%), owing to the synergistic effect of high mechanical and electrochemical compliance. By taking advantage of PNIPAM moiety's thermosensitivity, TG2P3 hydrogel could sensitively detect temperature variations. Lastly, TG2P3 hydrogel successfully demonstrated the applications in human motion monitoring sensors, flexible touch keyboards, and fever indicators, showing tremendous potential in the next generation of wearable epidermal electronics.