Wearable breath monitoring based on a flexible fiber-optic humidity sensor

Breath, as an important health monitoring indicator, provides valuable diagnostic information for cardiovascular disease and pulmonary function. Humidity can act as a bridge between breath and sensing signals. Current monitoring methods depend on humidity-sensitive material characteristics. In this work, an all fiber-optic flexible humidity sensor for wearable breath monitoring is reported. An eccentric fiber Bragg grating (EFBG) is inscribed in a single mode fiber to excite a stable core mode and sensitive cladding modes. The core mode is shown to maintain stable spectral features under a high-humidity atmosphere and can be used to calibrate the wavelength and power of the system. Importantly, the interface evanescent field of the cladding mode is highly sensitive to the ambient refractive index (RI) and even humidity-induced RI variation. Without combining any sensitized material, EFBG can directly perceive humidity fluctuations during breath with fast response (92 ms) and recovery times (100 ms). Different breathing patterns can be recognized, and breathing frequency can be extracted by sensor responses. The EFBG humidity sensor demonstrates great reproducibility, fast response, high flexibility, excellent robustness, and self-compensation capability, showing promising potential for wearable breath monitoring.