Air-Filtering Masks for Respiratory Protection from PM2.5 and Pandemic Pathogens

Abstract

Air-filtering masks, also known as respirators, protect wearers from inhaling fine particulate matter (PM_2.5) in polluted air, as well as airborne pathogens during a pandemic, such as the ongoing COVID-19 pandemic. Fibrous medium, used as the filtration layer, is the most essential component of an air-filtering mask. This article presents an overview of the development of fibrous media for air filtration. We first synthesize the literature on several key factors that affect the filtration performance of fibrous media. We then concentrate on two major techniques for fabricating fibrous media, namely, meltblown and electrospinning. In addition, we underscore the importance of electret filters by reviewing various methods for imparting electrostatic charge on fibrous media. Finally, this article concludes with a perspective on the emerging research opportunities amid the COVID-19 crisis.

Keywords: PM2.5; air filtration; air pollution; electrospinning; mask; meltblown; pandemic; respirator.

Figure 1

Masks Protecting People from PM_2.5 and Pandemic Pathogens (A) Annual average concentration of PM_2.5 around the globe in 2016. Image courtesy of Ritchie et al., CC BY 4.0, via Our World in Data. Data source, World Bank. (B) Transmission of the SARS-CoV-2 virus through viral aerosols. (C) Peeling apart an N95 mask revealing multiple layers. (D) Cross-sectional SEM image of an N95 mask showing that the filtration layer is sandwiched between the supportive layers. (B), (C), and (D) are adapted from Liao et al. with permission.

Figure 2

Filtration Mechanisms (A) Schematic illustration of the single-fiber model. Inertial impaction, interception, and diffusion are collectively known as mechanical filtration mechanisms. (B) Electrostatic filtration mechanisms for electret filters. (C) Contribution of each mechanism toward the total filtration efficiency. Note: Only mechanical filtration mechanisms are included here. Refer to Figure 3E for the contribution of electrostatic filtration mechanisms. Adapted from Mukhopadhyay et al. with permission.

Figure 3

Intrinsic Factors Affecting Filtration Performance (A) Schematic comparison of a microfiber medium (1-μm fiber diameter) and a nanofiber medium (100-nm fiber diameter) at the same packing density (2.5%). (B) Quality factor of fibrous media with different average fiber diameters. Adapted from Leung et al. with permission. (C) Schematic illustration of the slip flow effect. (D) Quality factor, packing density, and thickness of nanofiber media with different fabrication times (i.e., with different total mass of nanofibers per unit area). Adapted from Leung et al. with permission. (E) Filtration efficiency of three commercial electret filters, initially and after discharge in isopropyl alcohol. Adapted from Kilic et al. with permission. (F) Filtration efficiency of fibrous media made of different materials and the calculated dipole moment of the repeating unit of each polymer. All the samples have the same transmittance (~70%). Error bars, mean ± SD (n = 3). Adapted from Liu et al. with permission.

Figure 4

Extrinsic Factors Affecting Filtration Performance (A) Percentage of particle penetration through an N95 mask under different inhalation flow rates. Error bars, mean ± SD (n = 3). Adapted from Eninger et al. with permission. (B) Decay of the electrostatic charge, reflected by the surface potential, of an electret filter in ambient conditions. Adapted from Motyl et al. with permission. (C) Reduction of the quality factor and the surface potential versus moisture regain of electret filters made of different polymers. Error bars, mean ± SD (n = 3). Adapted from Lee et al., CC BY 4.0. (D) Change of the filtration efficiency of an N95 mask after cycles of sterilization treatment with different sterilization methods. Error bars, mean ± SD (n = 3). Adapted from Liao et al. with permission.

Figure 5

Fabrication of Fibrous Media (A) Schematic illustration of meltblown. (B) Photograph of a working slot die in a commercial meltblown system. Image courtesy of 4FFF, CC BY-SA 4.0, via Wikimedia Commons. (C) SEM image of typical meltblown microfibers. Adapted from Liao et al. with permission. (D) Schematic illustration of electrospinning. (E) Photograph of the whipping electrified jet in an electrospinning process. Adapted from Han et al. with permission. (F) SEM image of typical electrospun nanofibers. Adapted from Xu et al. with permission.

Figure 6

Conventional Methods for Making Electret Filters (A) Schematic illustration of a typical electrode configuration for corona discharge: a biased wire above a grounded roller. A fibrous medium is charged as it travels through the gap. (B) Schematic illustration of an electrode configuration for corona discharge, which is integrated with a meltblown process: two biased wire electrodes at the exit of a grounded slot die. Adapted from Moosmayer et al. (C) Triboelectric series of textile yarns. Adapted from Smith et al. with permission. (D) Schematic illustration of hydro-charging, which utilizes the triboelectric effect between water and polymers. Adapted from Angadjivand et al.

Figure 7

Recent Development in Making Electret Filters (A) Filter medium charged by an R-TENG. The working mechanism of the R-TENG relies on the triboelectric effect. Adapted from Gu et al. with permission. (B) Working principle of a self-charged mask. The filter medium, which was made of electrospun polyvinylidene fluoride nanofibers (PVDF-ESNF), was charged by the contact electrification with a copper foil under the periodic force exerted by the air flow of breathing. Adapted from Liu et al. with permission. (C) Decay of the residual charge, reflected by the surface potential, in electrospun polycarbonate nanofibers at 25°C and 50% relative humidity. Adapted from Cho et al. with permission. (D) Decay of the residual charge, reflected by the normalized surface potential, in electrospun polyetherimide (PEI) nanofibers doped with different electret nanoparticles. Adapted from Li et al. with permission.

Figure 8

Research Opportunities Amid the COVID-19 Pandemic (A) Schematic illustration of an antiviral mask, which is composed of two external spunbond polypropylene layers (layers A and D) containing 2.2 wt % copper oxide particles, one internal meltblown polypropylene layer (layer B) containing 2 wt % copper oxide particles, and one mechanically supportive layer made of plain polyester. Adapted from Borkow et al., CC BY 4.0. (B) Thermal imaging of a bare face (left panel), a face covered with a radiative-cooling mask (middle panel), and a face covered with a commercial mask (right panel) demonstrating that the radiative-cooling mask is transparent to mid-infrared light. Adapted from Yang et al. with permission. (C) Schematic illustration of a mask for respiration monitoring with the embedded moisture sensor and electronics (left panel). Photograph of a tablet computer running an Android app that can display and analyze the incoming data stream from the data acquisition electronics (right panel). Adapted from Güder et al. with permission.