Vertically-stacked MEMS PM2.5 sensor for wearable applications

[Display omitted] •Miniaturized air-microfluidic real-time gravimetric PM sensor.•Stacked multi-level channels supporting reduction in size/footprint.•Novel dual fractionating system to select PM2.5 mass fraction and reduce fouling.•Experimentally-verified PM2.5 selectivity using opto-gravimetry.•Highest sensitivity to date of direct mass MEMS PM2.5 sensors. Exposure to fine airborne particulate matter (PM), i.e., PM with an aerodynamic diameter (AD) less than 2.5 μm, is associated with many adverse health effects, including impaired pulmonary function, asthma, cardiovascular diseases, and Alzheimer. Consequently, there is a strong need for the development of small, low-cost wearable PM2.5 sensors that can be used by regular citizens to monitor their PM exposure. This work presents the design, fabrication, and experimental evaluation of a stacked-channel wearable direct-read micro-electro-mechanical system (MEMS) PM2.5 sensor that directly measures the mass concentration of the ambient PM by deposition on a mass-sensing resonator. The sensor employs a vertically stacked air-microfluidic channel geometry and an out-of-plane vertical virtual impactor (VVI), fabricated using a novel delayed deep reactive ion etch (DDRIE) process. As a result, this new device not only allows for a greater level of miniaturization than the previously reported air-microfluidic PM sensors, but also significantly reduces internal fouling due to PM accumulation in its channels. The small footprint of the sensor (27 mm × 14 mm × 2 mm) allows for the integration of the sensor into a wearable or cellular platform. An opto-gravimetric method was used to evaluate the collection efficiency of the VVI, which confirmed its 50% cutpoint at 2.5 μm. The experimental evaluation of the functionality of the sensor confirmed the sensitivity of the new design at 7 Hz/min per μg/m3, which is the highest sensitivity of direct-read mass-based PM2.5 sensor presented to this date. Such sensitivity allows for the limit of detection (LOD) of one μg/m3 within 7 min of integration time at a noise level of 50 Hz.