A Turbine-Based MEMS Sensor for Spirometry with Wearable Devices

This paper presents a turbine-based microelectromechanical systems (MEMS) sensor for spirometry application. The compact sensor dimensions allow integration into smart wearable devices for mobile spirometry without a dedicated hand-held component. The sensor is composed of a turbine with embedded magnets, which is surrounded by two stators with micro coils in a PMMA package. Tangential gas flow actuates the turbine on ball bearings and results in voltage generation due to electromagnetic induction. The sensor was initially characterized with pressurized nitrogen in the range of breathing flow rates. Sensitivity values up to 0.35 V/lpm were demonstrated. The device was then tested with actual breathing by a healthy subject. Sensor output voltages sampled under varying breath strengths ranged from 0.68 to 2.94 V, corresponding to breathing flow rates of 3.5 and 22.4 lpm, respectively. The device was also tested for power generation for a potential self-powered operation. A maximum output power of $67~\mu $ W was achieved on a resistive load. Energy storage tests using a 1-mF capacitor showed that normal breathing can raise the capacitor voltage to 0.2 V, while a deep breath resulted in 0.6 V. The difference in capacitor voltage poses a potential alternative method for lung volume measurements. The results presented here demonstrate the potential of this sensor to perform spirometry tests in wearable electronic devices, such as smart watches and smart bands.