Ultra-stretchable, fast self-healing, adhesive, and strain-sensitive wearable sensors based on ionic conductive hydrogels

Conductive hydrogels have been widely studied in many fields, such as wearable electronic materials, soft robotics, and human medical monitoring, due to their good water retention, elasticity, softness, flexibility, and sensory properties. However, it is still a great challenge to make all-in-one hydrogels with good mechanical properties, rapid self-healing ability, self-adhesion, and sensitive sensing abilities like natural skin. Thus, a multifunctional ionic conductive hydrogel containing hydroxypropyl methylcellulose/acrylic acid/acrylamide/tea polyphenol and Al3+ (HPMC/AA/AM/TP/Al3+) was prepared by a simple method. Owing to multiple dynamic noncovalent interactions, such as hydrogen bonds, electrostatic interactions, and metal coordination, in the ionic conductive hydrogel network, the hydrogel exhibited excellent mechanical properties (breaking stress: 0.032 MPa, toughness: 0.689 MJ m-3), skin-like elastic modulus (0.065 kPa), and stretchability (2225%). The electric conductivity could rapidly self-heal in a dramatically short period of time (5.1 s). Stress and strain self-healing efficiencies reached 93.8% and 99.4% in 60 min, respectively. Its adhesion strengths on wood and pigskin were as high as 13.12 kPa and 9.98 kPa, respectively, due to phenolic hydroxyl groups in TP. Moreover, the hydrogel exhibits good conductivity (3.03 S m-1). Hence, the hydrogel was assembled as a human motion sensor and high sensitivity (GF: 5.13) was obtained over the strain range of 200-1000% with a fast response/recovery time (response time: 0.4 s; recovery time: 0.75 s). Furthermore, the hydrogel sensor can monitor various human movements, including joint movement and vocal cord vibration. The as-prepared hydrogel achieved multiple functions in a single hydrogel system (all-in-one) and has great potential for application in flexible wearable sensors. A wearable hydrogel-based sensor has been developed by constructing various dynamic interactions to balance mechanical strength and conductivity as well as improve the self-healing and self-adhesive properties.