Comprehensive Study of Capaciflector-Based Respiration Sensor Performance: Measurement Principle and Influence of Operating Frequency and Electrode Size

Respiratory rate is an important physiological parameter and its continuous monitoring plays a vital role in health management. This study investigates the application of a planar capacitive sensor equipped with active shielding, the capaciflector, in monitoring respiratory activity. Previous studies have assumed that changes in sensor capacitance are mainly caused by skin deformation during breathing, but this theory has not been experimentally confirmed. In this study, an integrated wireless capaciflector-based respiratory sensor was developed based on the Texas Instruments FDC2214 capacitance-to-digital converter (CDC). Experimental tests in 10 adult male volunteers showed that skin stretching led to an increase in sensor capacitance, confirming that skin stretching is the main driver of the respiratory signal. In addition, the study examined the effect of the operating frequency (0.23 to 7.41 MHz) on sensor sensitivity. The sensor sensitivity at an operating frequency of 230 kHz was found to be 1.2 times higher than that at other experimental frequencies, which has the potential to be used as an optimized operating frequency. The study also explored the effect of different electrode sizes in the capaciflector on its sensitivity. The results showed that the change of electrode size did not significantly affect the sensitivity of the sensor, but the design of electrode length of 16mm, spacing of 4mm and electrode length of 12mm, spacing of 2mm showed relatively high sensitivity in some subjects, which may become potential optimized size design. This study provides an important reference for the design of respiratory sensor based on capaciflector.