Flexible body-integrated breathing monitoring system based on near-field coupling printed sensor

Body-integrated sensors have gained increasing attention for their potential to monitor physiological signals unobtrusively in real-time, offering a promising approach for early detection of health changes. This paper introduces an innovative non-contact sensor system designed for unobtrusive monitoring of respiration patterns. The sensor system, comprised of a loop antenna and a split-ring resonator (SRR) tag, accurately detects and distinguishes signals generated by human breathing. Placed in proximity to the diaphragm, the sensors concurrently capture breathing-induced motion and muscle stretch with high precision. The body-worn RF tag is fabricated using inkjet and extrusion printing technologies, which deposit conductive ink onto flexible substrates. The sensor components utilize polyimide for the loop antenna and PET for the SRR tag. The sensor system operates based on near-field electromagnetic coupling between the loop antenna and the SRR tag, detecting changes in resonance frequency induced by breathing motion. Through analysis of near-field coupling, different breathing patterns are analyzed, enabling the identification of subtle and rapid changes in chest movement. To validate the proposed sensor system, a comparative study was conducted using a BIOPAC breathing belt sensor as a reference. Experimental results demonstrate a strong correlation between the measurements obtained from the proposed sensor and those from the BIOPAC system, confirming its accuracy in tracking respiratory motion. The sensor system demonstrates the ability to detect subtle changes in chest motion with a sensitivity of 1.7 MHz/mm. The wireless nature of the proposed system, with a tag area of (50 mm × 50 mm), ensures body-conformable operation and enables unobtrusive and accurate monitoring of respiration patterns in real-time, especially for early respiratory change detection, a key indicator of declining health.