Wearing a face mask is an effective and easy way to prevent the transmission of the virus, including the influenza virus and the current epidemic SARS-Cov-2. However, the service time of a face mask is limited, and thus, it commonly needs to change daily; prolonged use would lead to the failure to provide enough protection. Experts even suggest replacing the face mask every 4 hours in a high-risk environment1. It is frustrating for the wearer to change a mask frequently. Meanwhile, the overwhelming majority of commercial face masks are made of nondegradable thermoplastic plastics; the vast number of discarded masks brings about severe environmental challenges. To this end, we conceived a high-efficiency and long-lasting face mask leveraging self-charging technology.
A layer of filter medium (commonly made of melt-blown polypropylene nonwoven) is the core component determining the filtration performance of a face mask. The filter medium consists of numerous randomly crosslinked fibers in the micrometer scale. It contains numerous micro pores for particle capture and retention via four basic mechanical mechanisms, including inertial impaction, interception, sieving, and diffusion2. Seemingly, increasing filter medium thickness and decreasing pore size are straightforward ways to improve filtration efficiency; however, such ways are accompanied by raising respiratory resistance, which is a vital index of mask performance evaluation. Electrostatic adsorption is an important complement to mechanical filtration to break through the tradeoff between filtration efficiency and respiratory resistance. Removing particular matters via electrostatic adsorption contributes a large proportion (up to 80%3) to the overall filtration efficiency. Unfortunately, the electrostatic charge decays significantly with time, especially in a humid condition, leading to the reduced service time of a face mask. Hence, in this work, we targeted improving the electrostatic charge level and extending the charge reservation for a prolonged service time of the face mask.
Fig. 1. A self-charging air-filtering mask with prolonged electrostatic adsorption. a Schematic diagram of the proposed self-charging air-filtering mask. b Enhanced particle capture capacity with the PVDF/nylon pair (left) employed in the SAF compared with the PP/PP (or PP/PE) pair (right) in a surgical mask. The zoomed-in illustrations indicate the efficient electrostatic adsorption for fine particles with the SAF design.
First, we experimentally established the quantitative relation between filtration efficiency and surface electrostatic potential based on an electrospun PVDF filter medium. Our results showed that filtration efficiency increased with the ascending surface electrostatic potential (the potential was adjusted from 0 to −3.3 kV). The grade efficiencies for 0.3 μm, 1 μm, and 2.5 μm with a surface potential of −3.3 kV were increased by 7.39%, 6.86%, and 6.95%, respectively, compared with those of the filter solely relies on mechanical capture mechanism, which well reflects the effectiveness of electrostatic charge on filtration efficiency. It is worth noting that, even though several works had qualitatively demonstrated the positive influence of electrostatic charge on particle removal, this is the first time to uncover the quantitative relation between filtration efficiency and surface charge level. Furthermore, we studied the charge decay under various environmental humidity. A significant charge decline was observed under a humid environment, with 92.5% decay under 50% relative humidity after the initial charge injection via corona electret treatment. Therefore, finding a way to improve the charge reservation of a face mask is necessary and meaningful.
Fig. 2. Quantitative relation between surface potential and filtration efficiency. a Grade efficiencies for particles ranging from 0.3 μm to 10 μm under various surface potential. b Electrostatic potential attenuation under 20% and 50% relative humidity.
Previously reported methods to replenish the surface charge include equipping the filter medium with an external high-voltage power supply, an internally integrated power source, and in-situ charge generation with electromechanical technologies. Although these works made remarkable achievements in long-lasting electrostatic adsorption efficacy, academic questions and engineering challenges are still existing. First, the cumbersome external power source makes it inconvenient to use. Second, the liquid electrolyte of the battery may further cause safety issues. In addition, the poor efficiency performance or the substandard evaluation conditions make the face mask suspicious to provide enough protective efficacy. To overcome the abovementioned problems, we developed a self-charging air filter (SAF) that leverages the triboelectric effect and achieves efficient and prolonged airborne particle removal under the international standard NIOSH 42 CFR 84.
Targeting the maximum charge generation, the first challenge is how to improve the charge transfer between triboelectric materials. We used the electrospun PVDF filter medium as the negative tribo-material and selected a nylon fabric as the counterpart because of its large electron affinity difference with PVDF according to the literature4, which means a strong tendency of it to donate electrons when physically contacting with the PVDF layer. The second challenge is how to optimize the structure for sufficient and efficient contact and separation between tribo-layers. To avoid the net force in the asymmetrical structure, which is detrimental to the effective separation, we utilized a sandwich structure with nylon layers symmetrically located at both sides of the PVDF layer. As a result, the optimized SAF demonstrated excellent charge-reservation capacity, with a high potential of −0.75 kV reserved after 10-hour wearing, while the potential of the PP/PP pair (typical materials used in commercial surgical masks) attenuated to −0.27 kV after wearing for the same time span. Consequently, the SAF maintained high capture efficiency of 95.8% for 0.3-μm particles after 30 hours of wearing, which still meets the N95 standard. In addition, the SAF behaved well in terms of respiratory resistance, quality factor, and cost-effectiveness compared with commercial face masks.
Fig. 3. Triboelectric effect-enabled efficient and durable PM filtration. a Triboelectric series comparing the charge transfer capacity of studied materials4. Dashed bars represent qualitative ranking. b Triboelectric charge generation mechanism of the SAF. c Electrostatic potential attenuation with and without self-charging function, implying a prolonged electrostatic absorption capacity with self-charging. d Durability evaluation of the filtration efficiency and pressure drop. e Radar chart comparing the performance of the SAF with commercial face masks.
In summary, we have developed an efficient, durable, and low-cost air filter that can continuously replenish electrostatic charges in a self-charging manner. The SAF relieved the detrimental influence of charge decay of an electret mask to long-lasting protection against harmful airborne particles and achieved high filtration efficiency for 0.3-μm particles of 95.8% after 60 hours of testing (including 30 hours of wearing). This work provides a promising strategy for efficient and durable particular matter capture that benefits from its prolonged electrostatic adsorption efficacy.
- Barbosa, M. H. & Graziano, K. U. Influence of wearing time on efficacy of disposable surgical masks as microbial barrier. J. Microbiol. 37, 216–217 (2006).
- Guidance for gas turbine inlet air filtration systems (2010).
- Xu, J. et al. Air-filtering masks for respiratory protection from PM2.5 and pandemic pathogens. One Earth 3, 574–589 (2020).
- Zou, H. et al. Quantifying the triboelectric series. Commun. 10, 1427 (2019).