Optical Fiber Sensor with Dynamically Responsive Cladding for Real-Time Breath Pattern Monitoring.

Rapid and real-time monitoring of humidity changes is critical, as they impact human health, material storage, and stability, industrial fabrication, microbial transmission, and agriculture. In recent years, optical fiber-based sensors have emerged as promising candidates for monitoring exhaled and inhaled breath humidity to assess respiratory rate, with the goal of supporting clinical diagnosis and patient care. Conventional multimode plastic optical fibers utilize a high refractive index core and lower refractive index cladding for effective total internal reflection (TIR) to achieve waveguiding. In contrast, here, we fabricated an optical fiber-based sensor with a higher refractive index (1.52) biopolymer-based cladding onto a lower refractive index (1.49) poly-(methyl methacrylate) (PMMA) core. We demonstrate that the cladding dynamically responds to a rapid humidity change by altering the refractive index and improving the TIR, allowing real-time humidity monitoring. Our results show that the sensitivity was characterized by a second-order polynomial fit, with a sensitivity of 0.18 dB/%RH (in the range of 40-70% RH) and an overall attenuation reduction of 9.4 dB at 70% RH. A proof-of-concept device, by ready integration of a fiber into a miniaturized platform, allowed for rapid and real-time exhaled breath humidity detection and differentiation of nasal and oral breathing patterns with a time resolution of 1.3 s. Notably, the device does not exhibit saturation under high humidity conditions, suggesting the robustness and enormous potential of optical fiber-based humidity sensors for long-term breath humidity monitoring, early detection of abnormal breath patterns, and deployment in next-generation healthcare monitoring.