Fiber Cladding Waveguide Phase-Shifted FBG Encapsulated in Flexible Material for Estimating Human Blood Pressure

A femtosecond (fs) laser-written cladding waveguide phase-shift FBG(CWPS-FBG) structure was demonstrated for high-precision blood pressure (BP) measurement. This structure primarily consists of phase-shift FBGs written within a single-mode fiber (SMF) cladding waveguide, encapsulated in flexible polydimethylsiloxane (PDMS). It efficiently couples optical energy from the fiber core into the cladding waveguide structure. When subjected to slight bending, the center wavelength of the phase-shift FBG undergoes significant shift, resulting in sensitivity two orders of magnitude higher compared to FBGs inscribed within the fiber core, enabling detection of surface deformations caused by pulsations in the skin layer. By parallel monitoring at two different pulse points on the human body, the time difference of pulse signals is obtained for determining pulse transit time (PTT) and further estimating systolic blood pressure (SBP). Experimental results indicate that the proposed sensor exhibits high consistency with commercial electronic blood pressure monitors, achieving comparable accuracy with a mean difference of only −1.357 ± 1.74 mmHg. The proposed BP monitoring system achieves high measurement accuracy, while its compact size, wearability, non-invasiveness, and painlessness offer promising advancements in the prevention and diagnosis of hypertension and cardiovascular diseases.