Temperature Sensor with a Water-Dissolvable Ionic Gel for Ionic Skin.

In the era of a trillion sensors, a tremendous number of sensors will be consumed to collect information for big data analysis. Once they are installed in a harsh environment or implanted in a human/animal body, we cannot easily retrieve the sensors; the sensors for these applications are left unattended but expected to decay after use. In this paper, a disposable temperature sensor that disappears with contact with water is reported. The gel electrolyte based on an ionic liquid and a water-soluble polymer, so-called ionic gel, exhibits a Young's modulus of 96 kPa, which is compatible with human muscle, skin, and organs, and can be a wearable device or in soft robotics. A study on electrical characteristics of the sensor with various temperatures reveals that the ionic conductivity and capacitance increased by 12 times and 4.8 times, respectively, when the temperature varies from 30 to 80 °C. The temperature sensor exhibits a short response time of 1.4 s, allowing real-time monitoring of temperature change. Furthermore, sensors in an array format can obtain the spatial distribution of temperature. The developed sensor was found to fully dissolve in water in 16 h. The water-dissolvability enables practical applications including healthcare, artificial intelligence, and environmental sensing.