Miniaturized Respiratory Monitoring System With Subsecond Response Enabled by Microresonator Sensing Technology

Existing respiratory sensing techniques face challenges including low sensitivity, slow response, and bulky size, which limit their capability of recognizing abnormal breathing patterns. In this work, a miniaturized and portable respiratory monitoring system featuring high sensitivity and subsecond response speed is proposed based on a capacitive micromachined ultrasonic transducer (CMUT). The CMUT microresonator with a 520-nm-thick resonant nitride membrane is designed and implemented by the nitride-to-oxide wafer bonding process. Without any coatings for functionalization, the ultrathin membrane allows the device to identify the loaded mass of exhaled water molecules at the fg level. Based on the modified Butterworth-Van Dyke (BVD) equivalent circuit model, a compact signal processing unit is developed to stably track the CMUT’s resonance frequency in real time with a high signal-to-noise ratio (SNR) of 47.96 dB. The experimental results demonstrate that the device can distinguish various respiratory rates and patterns, including abnormal rates of up to 120 breaths per minute (BPM). Investigation into the sensing mechanism confirms that the frequency response of the device is triggered by local humidity changes rather than variations in pressure or temperature.