A research team at City University of Hong Kong (CityU) says it engineered a breath-to-charge electrostatic face mask.

The researchers say the mask can charge itself, continuously replenishing its electrostatic charge as the user wears and breathes through the mask. This increases the filtering performance in prolonged use of the mask for up to four hours.

Electrostatic adsorption is an important complement to mechanical filtration in the use of facemasks. This electrostatic charge of the filters generally decays over time, particularly in humid conditions.

Not only does the mask outlast the conventional four-hour wear of a surgical mask, but it also benefits the environment, CityU said.

“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,” said Dr. Yang Zhengbao, 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.”

Yang is an associate professor in the Department of Mechanical Engineering and the Department of Materials Science and Engineering at CityU. He and the team introduced a self-charging air filter (SAF) to leverage the triboelectric effect and achieve efficient and prolonged airborne particle removal.

How the mask works

The mask sandwiches the electrospun polyvinylidene fluoride (PVDF) nanofiber filter medium between two triboelectric nylon fabric layers. With this, the SAF continuously generates electrostatic charges excited by breathing. The researchers say it maintained a high efficiency of 95.8% after 60 hours of testing. That testing included 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,” said Yang. “This self-charging process enables the continuous replenishment of the electrostatic charges and prolonged electrostatic adsorption.”

The researchers also stressed cost-effectiveness. Raw material costs for SAF for making one mask comes to about 6¢ (HK$0.47). They said it’s cheaper than the cost to manufacture surgical, N95, KF94 and KN95 masks.

According to CityU, the strategy could pave a new path for high-efficiency, long-lifespan air filtering techniques.

“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 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.”