Ion-Dipole-Mediated, Mechanosensitive Iontronic Skin with Ultrafast Self-Healing Both in Air and Underwater

The properties of the human skin such as self-healing and mechano-sensation even in aquatic environments has inspired the design of various underwater self-healing based iontronic sensors that respond to strain, stress, and pressure. Stretchable iontronic sensors that emulate these functionalities as well as high sensitivity can be utilized in wide range of applications including water-resistant human-machine interface, and wearable sensors in high humidity environment (water, sweet, etc.). However, it is usually challenging to design iontronic sensors with stretchability, high sensitivity, and self-healable ability both in-air and underwater simultaneously. Conventionally, normal self-healing is attained via dynamic bonds, but these bonds will be saturated or coordinated with water molecules in high humidity conditions causing breakdown of the reversible process and subsequent loss of self-healing properties. Moreover, the sensitivity to water molecules can defunctionalize the electrical properties and mechanical property of the sensor limiting its sensing ability in aquatic environment. Hence, it is necessary to design and develop materials with underwater self-healing ability and outstanding waterproof properties for highly sensitive iontronic to ensure stability in humid conditions as well as prolong the lifespan of the device.<br/>Here, we describe a highly deformable iontronic sensor from a novel designed polymer matrix with excellent waterproof property, high sensitivity and can autonomously self-heal in both dry and humid conditions. Our design strategy is the combination of fluorinated groups, glyceryl benzenediborate (GB) and hydroxyl-terminated polybutadiene (HTPB) in the design of a novel polyurethane (PU) structure together with compatible ionic species to create an iontronic sensor. First, the self-healing ability of the sensor originates from the dynamically reversible boronate bonds in the GB, which can undergo both bond exchange reaction in air and reversible hydrolysis underwater. The superb waterproof property was obtained due to the fluorinated groups within the hydrophobic soft segment of the PU that have stronger hydrophobic properties, which can ensure the stability of all performance in water. We also can maximize the underwater self-healing ability by adjusting the hydrophobicity to control the water intake. Moreover, the highly electronegative fluorine atoms interact with the cation of the ionic liquid via ion-dipole interaction establishing ion confinement effect. The ions are therefore trapped by the carbon-fluorine bonds generating ultra-low initial capacitance value. Under external stimuli such as stress or strain, the ion-dipole interactions are broken creating a high increase in capacitance change generating high sensitivity. As a result, the novel iontronic sensor can achieve excellent sensing performance and self-healing properties both in dry and wet conditions for the development of next-generation wearable sensors.