Ultra-sensitive humidity QCM sensor based on sodium alginate/ polyacrylonitrile composite film for contactless Morse code communication

The double-layer structure of sodium alginate/polyacrylonitrile (SA/PAN) humidity sensor was fabricated. The humidity sensitivity of the sensors was evaluated while characterizing the films using scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FT-IR), water contact angle tester, and X-ray photoelectron spectroscopy (XPS). The results showed that the 2-SA/PAN sensor has low hysteresis (1.6% RH), excellent repeatability, short response/recovery time (14.5 s/2.5 s), satisfactory stability and selectivity. The maximum frequency shift was up to - 6960.55 Hz (0-97% RH). As a hydrophilic polymer in the upper layer of the bilayer film, SA has hydrophilic functional groups, and the water-insoluble electrospun fiber PAN in the lower layer is the diffusion channel of water molecules, which not only plays a supporting role, but also reduces the load of SA loaded water molecules, and shortens the response/recovery time. Furthermore, the 2-SA/PAN sensor can quickly and accurately monitor skin humidity in different parts of the human body, capture changes in wrist humidity before and after exercise, and monitor fingertip movements. Furthermore, the fingertip movement is used to generate Morse code, and an app is built to decode and display the original code.