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Exposure to respirable coal dust and diesel exhaust in underground coal mines can cause detrimental airway diseases such as coal worker's pneumoconiosis (CWP), silicosis, and lung cancer. In this paper, we present the design, fabrication, and experimental evaluation of a low-cost wearable respirable dust monitor (WEARDM) which uses a dual-resonator gravimetric sensing approach for real-time measurement of respirable airborne particulate matter (PM) concentrations. The sensor selects for the ISO respirable dust fraction using a miniature virtual impactor and removes moisture from the collected dust to ensure accurate mass measurement. WEARDM uses a novel dual-resonator mass sensor which is composed of a quartz crystal microbalance (QCM) and a film bulk acoustic resonator (FBAR). The QCM measures the mass concentration of particles generated from coal mining operations (typically >2.5 µm A.D.), separated using inertial impaction. Thermophoretic precipitation is used to deposit the fine and ultrafine particles, such as emitted from diesel sources (typically <0.1 µm A.D.) on FBAR. This allows the WEARDM system to maintain large dynamic range and uniform collection efficiency across the entire respirable fraction. The WEARDM system is optimized for a low flow rate of 250 ml/min which results in low power usage and a small form factor, and is an order of magnitude smaller and less expensive than currently available devices.

Inertial impaction is frequently used for the collection and subsequent measurement of aerosol particles in resonator-based airborne particulate matter (PM) sensors. However, particle bounce is known to significantly reduce particle collection efficiency (CE) on surfaces exhibiting low roughness, such as those present in quartz crystal microbalance (QCM) PM sensors. This paper shows that the addition of micropillars to impaction surfaces can significantly enhance their particle collection. Similarities in particle capture mechanisms between fibrous filters and pillar-enhanced surfaces are explained, and we show the adaptability of fibrous filter theory to pillared surface collection efficiency. Experiments confirm that the micropillar cross section and spacing have a significant role in particle capture. Pillars with circular, rectangular, and cross-shaped horizontal cross sections with 15 μm height and 12 μm (dense), 20 μm (nominal), and 27 μm (sparse) center-to-center spacings were printed using two-photon micro stereolithography. The efficiency increased by 35%–52% in the dense case, while the effect of the pillar shape was negligible. At nominal spacing, CE depended heavily on the pillar shape. The cross-shaped and circular pillars improved the CE by 26%–29%, although the rectangular pillars were as efficient as the bare surface. No significant difference between the bare and pillar-enhanced surfaces was visible in sparse spacing. We further show that, upon addition of a nominal distribution of micropillars to the surface of a QCM sensor for real-time mass measurements, the sensor response improved significantly (approximately 2.5 times) compared to a QCM with a bare surface.