Optimizing Flexible FBG Sensors: Effect of Embedding Materials on Respiratory and BCG Signal Detection

Continuous monitoring of physiological parameters is essential in applications ranging from preventive healthcare and clinical diagnosis to remote monitoring. Wearable technologies offer a non-invasive alternative to capture these vital signs in real-world settings; however, challenges remain in achieving both accuracy and comfort. Fiber Bragg grating (FBG) sensors represent a promising alternative due to their miniature size, immunity to electromagnetic interference, and high strain sensitivity. This study investigates the performance of FBG sensors embedded in two distinct encapsulation materials—Dragon Skin 10 (DS10) and Thermoplastic Polyurethane 85A (TPU 85A)—for monitoring respiratory and cardiac activity. The sensors were metrologically characterized and tested in six healthy volunteers in standing position across three breathing patterns: slow (SB), quiet (QB), and fast (FB), with intermediate apnea (AP) phases. Respiratory rate (RR) and heart rate (HR) signals acquired from the FBG sensors were compared against reference devices: BioHarness for RR and Movesense MD for HR. Results demonstrated that both sensors could reliably detect physiological signals with a minimally invasive setup. However, TPU 85A outperformed DS10, especially for HR monitoring, reaching lower mean absolute error (MAE), and narrower limits of agreement in Bland-Altman analysis. While both materials maintained RR errors below 0.3 apm and under 3% MAPE, DS10 showed greater variability in HR, occasionally exceeding 10 bpm and 20% MAPE. TPU 85A, in contrast, maintained HR errors generally below 5 bpm and 10% MAPE. These findings support the feasibility of FBG-based systems for reliable physiological monitoring.