Biodegradable, Efficient, and Breathable Multi-Use Face Mask Filter

Abstract

The demand for face masks is increasing exponentially due to the coronavirus pandemic and issues associated with airborne particulate matter (PM). However, both conventional electrostatic- and nanosieve-based mask filters are single-use and are not degradable or recyclable, which creates serious waste problems. In addition, the former loses function under humid conditions, while the latter operates with a significant air-pressure drop and suffers from relatively fast pore blockage. Herein, a biodegradable, moisture-resistant, highly breathable, and high-performance fibrous mask filter is developed. Briefly, two biodegradable microfiber and nanofiber mats are integrated into a Janus membrane filter and then coated by cationically charged chitosan nanowhiskers. This filter is as efficient as the commercial N95 filter and removes 98.3% of 2.5 µm PM. The nanofiber physically sieves fine PM and the microfiber provides a low pressure differential of 59 Pa, which is comfortable for human breathing. In contrast to the dramatic performance decline of the commercial N95 filter when exposed to moisture, this filter exhibits negligible performance loss and is therefore multi-usable because the permanent dipoles of the chitosan adsorb ultrafine PM (e.g., nitrogen and sulfur oxides). Importantly, this filter completely decomposes within 4 weeks in composting soil.

Keywords: biodegradability; chitosan; face masks; particulate matter; polybutylene succinate.

Figure 1

Filtration mechanisms for particulate matter (PM), characteristics in use, and environmental impact of conventional and developed mask filters. a) Two different representative PM capturing mechanisms of conventional nonwoven filters, and their consequential shortcomings: temporary charge of a microfiber‐based electrostatic filter, the high pressure drop of a nanofiber‐based physical filter, and environmental pollution (as the masks are disposable). b) Outstanding characteristics of the developed chitosan‐coated PBS nanofiber/microfiber integrated Janus membrane filter: permanently preserved ionic charges, low pressure drop facilitates comfortable breathing by the user, and biodegradability (Movie S1, Supporting Information).

Figure 2

Fabrication process, morphologies, and PM removal performance of PBS microfibers and nanofibers produced from solutions of different concentration. a) Schematically illustrating the electrospinning setup and the production of a PBS fiber mat. b) Fabricated PBS nanofiber mat. c) SEM image of the fabricated microfiber mat and its average fiber diameter and pore size. d) SEM image of the fabricated nanofiber mat and its average fiber diameter and pore size. e) Basis weight (g m⁻²) ≈ thickness of the PBS microfiber (gray) and nanofiber (red) mats prepared for various spinning durations. f) PM removal efficiencies of the microfiber (gray) and nanofiber (red) mats with similar basis weights (2.0 and 2.5 g m⁻², respectively) for various particle sizes. The removal efficiency was determined at an air velocity of 1.0 m s⁻¹.

Figure 3

CsW‐coated filter and its filtration performance. a) PBS fiber mat dip‐coated using the CsW dispersion, and an SEM image of CsWs ≈200 nm in length. b) EDS maps used to investigate pure PBS fibers and a uniformly well‐coated CsW‐PBS fiber. c) Effect of CsW on the removal efficiency for PM_1.0, PM_2.5, and PM₁₀ using the microfiber (left graph) and nanofiber (right graph) mats, before (dark gray: M2.0 and dark red: N2.5) and after (light gray: ChM2.0 and light red: ChN2.5) CsW‐coating. d) Pressure differential before and after coating the microfiber (gray: ChM2.0) and nanofiber (red: ChN2.5) mats with CsW. The removal efficiency and pressure drop were determined at an air velocity of 1.0 m s⁻¹.

Figure 4

Integrated filters with superior characteristics, such as low pressure drops, high‐efficiencies, and permanent charges. Comparing a) pressure drops, b) PM removal efficiencies, and c) quality factors, of single‐layered and integrated filters. d) Successfully blocking the PM from a cigarette source as demonstrated by Tyndall light scattering (Movie S2, Supporting Information). e) SEM images showing the physical and electrostatic PM capturing abilities of the integrated filter. f) Efficiency declines of commercial N95 and the integrated filter after exposure to moisture.

Figure 5

Biodegradability of the developed filter. a) Time‐dependent enzymatic degradation images of the CsW‐coated PBS filter and the corresponding weight loss as a function of time (enzyme: lipase from Thermomyces lanuginosus; Movie S3, Supporting Information). b) Images showing the degradation of the CsW‐coated PBS filter in the composting soil over time.