When using face masks, electrostatic adsorption is a crucial addition to mechanical filtration for high-efficiency air filtering. However, over time and especially in humid environments, the electrostatic charge of the filters degrades.
A breath-to-charge electrostatic face mask that can “self-charge” through the user’s breathing and continually replenish its electrostatic charge as the user wears and breathes through the mask has been successfully created by a research team at City University of Hong Kong (CityU).
This considerably improves the filtering efficiency when the mask is used for an extended period of time up to 60 hours as opposed to just four hours when using a standard surgical mask. This also benefits the environment.
COVID-19 and other airborne infections can be prevented easily and affordably with face masks. Most face masks have three functional layers: a core melt-blown polypropylene (PP) layer as the filter medium and two spun-bonding nonwoven fabrics (generally PP or polyethylene (PE)) as the supporting layers, including a hydrophilic layer, worn inwards, to absorb moisture from breathing and a hydrophobic layer, worn outwards, to repel fluid.
Tackling the problem of electrostatic adsorption efficacy decline
Compared to the widely utilized melt-blown method, electrospinning offers superior mechanical filtration. But mechanical filtration alone does not provide enough protection.
Surgical masks are suggested to be changed every four hours in a high-risk environment, but the vast number of discarded masks results in severe environmental challenges. We expect this self-charging strategy to significantly prolong the service life of face masks, enhance the protection effectiveness against the coronavirus, and reduce the environmental burden caused by discarded masks.
Professor Dr. Yang Zhengbao
The filter material can be given an electrostatic charge to help in ultrafine particle entrapment by applying an electric field. However, the effectiveness of electrostatic adsorption decreases over time, particularly in humid environments where respiration exhales moisture.
“Although many reports work on replenishing the charge for long-lasting electrostatic adsorption efficacy, an extra power source is generally needed, which is cumbersome and inconvenient,” explained Dr. Yang Zhengbao, Associate Professor in the Department of Mechanical Engineering and the Department of Materials Science and Engineering at CityU, who led the research. “We have developed an efficient, durable, low-cost air filter that can continuously replenish the electrostatic charge in a self-charging manner.”
The study team developed a self-charging air filter (SAF), which takes advantage of the triboelectric effect to effectively and continuously remove airborne particles. By sandwiching the electrospun polyvinylidene fluoride (PVDF) nanofiber filter medium between two triboelectric nylon fabric layers, the SAF continuously generates electrostatic charges excited by breathing.
As a result, the SAF provides durable particle removal performance, maintaining high efficiency of 95.8% after 60 hours of testing (including 30 hours of wearing).
“As the middle layer moves forth and back between the lateral layers with breathing, a charge transfer occurs between PVDF and nylon due to their large difference in electron affinity, resulting in the PVDF layer being negatively charged and the nylon layers positively charged,” explained Dr. Yang. “This self-charging process enables the continuous replenishment of the electrostatic charges and prolonged electrostatic adsorption.”
“Self-charging” driven by the user’s breath
The raw material cost of SAF for making one mask is as low as HK$0.47, making it the most cost-effective option among the commonly used masks, such as surgical, N95, KF94 and KN95 masks.
The findings were published in Nature Communications under the title “Self-charging electrostatic face masks leveraging triboelectrification for prolonged air filtration.”
A new route to the creation of high-efficiency, long-lasting air-filtering procedures has been opened up by this intriguing self-charging method that takes advantage of the triboelectric effect.
“Surgical masks are suggested to be changed every four hours in a high-risk environment, but the vast number of discarded masks results in severe environmental challenges,” said Dr. Yang. “We expect this self-charging strategy to significantly prolong the service life of face masks, enhance the protection effectiveness against the coronavirus, and reduce the environmental burden caused by discarded masks.”
For standardized, high-efficiency industrial production, the research also established a quantifiable link between surface electrostatic potential and filtration efficiency.
The first author of the paper is Dr. Peng Zehua, CityU postdoc in the Department of Mechanical Engineering. Dr. Yang is the corresponding author. Other collaborators from CityU include Professor Michael Leung Kwok-hi, Shun Hing Education and Charity Fund Professor of Energy and Environment, Professor Li Wenjung, Chair Professor and Mr. Shi Jihong, Ph.D. student in the Department of Mechanical Engineering, and researchers from Dr. Yang’s group, including Dr. Hong Ying, Dr. Li Xuemu and Dr. Zhang Weiwei.
CityU, the Hong Kong Research Grants Council, the Hong Kong Innovation and Technology Fund, the Shenzhen Fundamental Research Program, and the National Natural Science Foundation of China funded the research.
