micromachines Article Microfluidic Airborne Metal Particle Sensor Using Oil Microcirculation for Real-Time and Continuous Monitoring of Metal Particle Emission Jong-Seo Yoon, Jiwon Park, Hye-Rin Ahn, Seong-Jae Yoo and Yong-Jun Kim *   Citation: Yoon, J.-S.; Park, J.; Ahn, H.-R.; Yoo, S.-J.; Kim, Y.-J. Microfluidic Airborne Metal Particle Sensor Using Oil Microcirculation for Real-Time and Continuous Monitoring of Metal Particle Emission. Micromachines 2021, 12, 825. https://doi.org/10.3390/mi12070825 Academic Editor: Dimitris Tsoukalas Received: 5 June 2021 Accepted: 11 July 2021 Published: 14 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea; jsyoon129@yonsei.ac.kr (J.-S.Y.); prion1548@yonsei.ac.kr (J.P.); hyerinahn@yonsei.ac.kr (H.-R.A.); sj92@yonsei.ac.kr (S.-J.Y.) * Correspondence: yjk@yonsei.ac.kr; Tel.: +82-2-2123-7212 Abstract: Airborne metal particles (MPs; particle size > 10 μm) in workplaces result in a loss in production yield if not detected in time. The demand for compact and cost-efficient MP sensors to monitor airborne MP generation is increasing. However, contemporary instruments and laboratory- grade sensors exhibit certain limitations in real-time and on-site monitoring of airborne MPs. This paper presents a microfluidic MP detection chip to address these limitations. By combining the proposed system with microcirculation-based particle-to-liquid collection and a capacitive sensing method, the continuous detection of airborne MPs can be achieved. A few microfabrication processes were realized, resulting in a compact system, which can be easily replaced after contamination with a low-priced microfluidic chip. In our experiments, the frequency-dependent capacitive changes were characterized using MP (aluminum) samples (sizes ranging from 10 μm to 40 μm). Performance evaluation of the proposed system under test-bed conditions indicated that it is capable of real-time and continuous monitoring of airborne MPs (minimum size 10 μm) under an optimal frequency, with superior sensitivity and responsivity. Therefore, the proposed system can be used as an on-site MP sensor for unexpected airborne MP generation in precise manufacturing facilities where metal sources are used. Keywords: microfluidics; microcirculation; particle-to-liquid collection; airborne metal particle; continuous and real-time monitoring; capacitive detection 1. Introduction The development of automotive technologies has been rapidly expanding owing to an increased focus on reducing environmental impact [1,2]. Various electronics have been embedded into autonomous systems for different operations, such as supplying power and controlling steering systems [3]. Following this ongoing trend regarding driving devices, there is a demand for the miniaturization and integration of electronics to achieve spatial savings, low power consumption, and high electrical performance [4,5]. For packaged semiconductor chips, rechargeable batteries, and other components that aim at the same goal, greater safety and quality are essential [6]. However, tremendous effort should be invested because these electronic components have a wide range of production failures [7]. Among the common issues, airborne metal particles (MPs; particles larger than 10 μm) at manufacturing sites are one of the root causes of the degradation of the lifespan and reliability of the products, because they induce internal short circuits or stress points [8,9]. There have been advances in manufacturing procedures such as encapsulation packaging, electrode welding, and cutting; however, micrometer-sized MPs break down from the metal sources, scatter in the air, and settle on multiple spots that may be electrically connected [10,11]. In addition, when the deterioration of manufacturing machinery begins, the numbers and sizes of MPs become larger [12,13]. Thus, airborne MPs must be monitored to improve the current production yield. Micromachines 2021, 12, 825. https://doi.org/10.3390/mi12070825 https://www.mdpi.com/journal/micromachines