High sensitivity fiber optic flexible sensor based on tapered optical fiber with coated CaAlg hydrogel

Flexible fiber optic sensors serve as prototypes for advanced photonic skin fiber sensors (PSFS), which exhibit high sensitivity and rapid response times, paving the way for the development of advanced wearable medical devices. PSFS demonstrate transformative potential in next-gen wearables for biomarker surveillance. In this work, we propose a PSFS based on a single-mode tapered fiber (SMTF) structure coated with Calcium alginate (CaAlg) hydrogel for the first time to improve the performance of sensors. The structure consists of a tapered fiber optic as the core sensing node, with biocompatible CaAlg hydrogel as the sensing material. The CaAlg hydrogel modifies the surface of the waist and cone of the tapered fiber optic, enabling both static and dynamic sensing measurements for relative humidity (RH) with high sensitivity and excellent biocompatibility. By leveraging the photoelastic effect of the SMTF and its unique structure, the sensitivity of the material to RH is significantly enhanced, reaching 0.994 dBm/%RH. Furthermore, the sensor exhibits a response time of 0.71s and a recovery time of 1.65s. In addition, this work explores the influence of film thickness on sensor sensitivity and time response. The flexible fiber optic sensors proposed in our work provide a novel approach to constructing highly sensitive PSFS, specifically optimized for healthcare monitoring applications.