Self-powered and degradable humidity sensors based on silk nanofibers and its wearable and human–machine interaction applications

Humidity sensors have received increasing attention due to their broad potential application in wearable devices, health monitoring, intelligent control, and other fields. However, traditional humidity sensors are prone to produce various toxic substances to pollute the environment after discarding. Also, they are prone to damage during use, which shortens their service life. Therefore, the development of eco-friendly, degradable, and repairable humidity sensors is of great significance for reducing e-waste and protecting the environment. Here, we started with the selection of materials by using silk nanofiber (SNF) film as the substrate material, copper/aluminum as electrodes, and calcium chloride as the electrolyte, combined with the working mechanism of the primary battery, to construct a self-powered humidity sensor. The humidity sensor exhibits excellent humidity sensing performance and good linearity within the range of 30–90 % relative humidity (RH). A single humidity sensor can output 346 mV at 90 % RH. After being damaged, the humidity sensor can be repaired with the help of water. When the ruptured SNF film encounters water molecules, the water molecules can prompt the hydrogen bonds between the SNFs to rearrange and connect, connecting the broken area. Young's modulus retention rate of the repaired SNF film is over 85 %. The repaired humidity sensor can also output 328 mV. Additionally, an alkaline solution can completely degrade the humidity sensor in less than 80 min. The proposed humidity sensors have been applied to wearable self-powered wristbands, wireless monitoring systems, and non-contact human–machine interaction systems. Hence, we provide comprehensive research from the material level (SNFs) to the functional device level (the self-powered humidity sensor), and then to the system level (wearable and human–machine interface system). The research will open a new way for the design of the self-powered humidity sensor and have great potential applications in wearable electronics and human–machine interface applications.