Polyimide-Coated S-Shaped Optical Fiber Interferometer for Non-Contact Skin Humidity Detection

Skin humidity detection is a vital biomarker for monitoring human health and safety conditions. This study presented the development of a non-contact skin humidity sensor that utilizes an S-shaped optical fiber coated with polyimide and employs the Sagnac interferometric technique. To position the polyimide coating closer to the fiber core, two channels were machined into the cladding, forming an S-shape while maintaining the fiber strength. The sensor operated based on the moisture absorption and swelling of the polyimide film, which induced a wavelength shift in the interference spectrum. The sensor demonstrated the sensitivity of −1.49 nm/%RH over the RH range of 30%−95%, where the RH is relative humidity. To compensate for temperature variations, a fiber Bragg grating was integrated into the exposed section of the S-shaped fiber, providing the temperature sensitivity of 10 pm/℃. The sensor exhibited excellent accuracy, linearity, reversibility, and low hysteresis. Moreover, non-contact skin humidity sensing was evaluated on both human and prosthetic hands, highlighting its potential applications in biomedical fields.