Nanomaterials 2022, 12, 3152. https://doi.org/10.3390/nano12183152 www.mdpi.com/journal/nanomaterials Article Washable Fabric Triboelectric Nanogenerators for Potential Application in Face Masks Sang-Hwa Jeon 1,† , Yongju Lee 2,† , Swarup Biswas 2 , Hyojeong Choi 2 , Selim Han 2,3 , Minseo Kim 4 , Dong-Wook Lee 3 , Sohee Lee 5, *, Hyeok Kim 2, * and Jin-Hyuk Bae 1,6, * 1 School of Electronic and Electrical Engineering, Kyungpook National University, Daegu 41566, Korea 2 School of Electrical and Computer Engineering, Center for Smart Sensor System of Seoul (CS4), University of Seoul, Seoul 02504, Korea 3 AI Robot R&D Department, Korea Institute of Industrial Technology (KITECH), Ansan 15588, Korea 4 School of Chemical Engineering, Pusan National University, Busan 56241, Korea 5 Department of Clothing and Textiles, Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Korea 6 School of Electronics Engineering, Kyungpook National University, Daegu 41566, Korea * Correspondence: sohee.lee@gnu.ac.kr (S.L.); hyeok.kim@uos.ac.kr (H.K.); jhbae@ee.knu.ac.kr (J.-H.B.) † These authors contributed equally to this work. Abstract: In order to counteract the COVID-19 pandemic by wearing face masks, we examine wash- able fabric-based triboelectric nanogenerators (FTENGs). We applied the flash-spun nonwoven fab- ric (FS fabric) into the FTENGs, comparing the melt-blown nonwoven fabric (MB fabric) based FTENGs, which is conventionally studied in the field of energy harvesting. For reusability, all our proposed FTENGs are systematically investigated by controlling the washing conditions. After washing, the degradation ratio of the obtained output voltage is found to be only 12.5% for FS FTENGs, compared to the ratio of about 50% for the typical MB FTENGs. A rather small degradation ratio for FS fabric cases has resulted from less changed fabric structure after washing due to more dense fabric nature. Additionally, in order to improve the electrical characteristics of FS FTENGs. Note that the output voltage of FTENGs exhibits as much as 600 V. Keywords: washability; fabric-triboelectric nanogenerators; degradation ratio; elastomer; flash-spun nonwoven 1. Introduction Triboelectric nanogenerators (TENGs) have attracted increasing attention over the past 10 years as self-powered sensors or eco-friendly energy harvesters [1–8]. The funda- mental concept behind TENGs is the conversion of irregular motion energy, which is gen- erally wasted, into useful electrical energy based on contact electrification and electro- static induction phenomena [9]. TENGs can be fabricated using a variety of materials [10]; however, fabric-based TENGs (FTENGs) are particularly promising as wearable elec- tronic devices because they are flexible, stretchable, and produced from materials that are similar to those used in existing clothing [11–16]. Meanwhile, the recent COVID-19 out- break has necessitated the use of masks. Apart from COVID-19, growing concerns over air pollution and respiratory health also contribute to the demand for masks. This has motivated researchers to actively improve the functionality and reusability of masks. Ad- ditionally, they have also received attention as form factors in the wearable electronics field [17–19]. Given that several people currently wear quarantine masks made of fabric, FTENGs demonstrate significant potential as energy sources or sensors for wearable elec- tronic devices. In fact, following the COVID-19 pandemic, triboelectric nanogenerator face mask research is currently underway [20–22]. Citation: Jeon, S.-H.; Lee, Y.; Biswas, S.; Choi, H.; Han, S.; Kim, M.; Lee, D.-W.; Lee, S.; Kim, H.; Bae, J.-H. Washable Fabric Triboelectric Nanogenerators for Potential Application into Face Mask. Nanomaterials 2022, 12, 3152. https:// doi.org/10.3390/nano12183152 Academic Editor: Jie Wang Received: 8 August 2022 Accepted: 7 September 2022 Published: 11 September 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distrib- uted under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecom- mons.org/licenses/by/4.0/).