Throughout the COVID-19 pandemic, now entering its seventh month, a simple piece of personal protective equipment has been in short supply: N95 masks.
N95 and other medical-grade masks rely on two filtration methods: mechanical filtering by mask fibers, and electrostatic filtering, in which stationary electric charges attract and ensnare tiny 0.3-micron particles such fluid droplets containing viruses. The masks are specified for single-use only because even after a day, the electrostatic charges in the mask leak out into the air and the mask becomes less effective at filtering out particles. That gradual loss of efficiency is even worse in countries like India where high humidity speeds the loss of static charge to the air.
The problem is exacerbated when healthcare workers turn to procedures to decontaminate and reuse masks, such as baking or boiling, UV light towers, even large fumigation machines, all of which can extinguish a mask’s electrostatic charge.
“We wondered, why can’t we recharge it?” says Dov Levine, a physicist at Technion-IIT in Haifa, Israel. “Well, it turns out you can.”
In a paper published this week in the journal Physics of Fluids, Levine, along with Shankar Ghosh and colleagues at Tata Institute of Fundamental Research in Mumbai, India, show that decontaminated N95 masks can be rejuvenated to have off-the-shelf filtration levels with a jolt of electricity that restores electrostatic charges. The team also made a prototype, battery-powered, smart mask with a removable filter that can be continuously charged to maintain high levels of filtration even after hours of use.
First, the team measured the filtration efficiency of several brands of N95 masks, which ranged from 95 to 98 percent efficiency. Then, they sanitized a set of masks either with ethanol, boiling water, steam, or simply threw them into a washing machine. As expected, after each decontamination procedure, the masks became less efficient at filtering out particles.
A new KN95 mask is roughly 95% efficient at filtering out fine particles (left). After washing, that efficiency drops to about 75% (middle), but can be restored to 95% (right) by recharging the mask for 60 minutes. Image: Tata Institute of Fundamental Research
To recharge a mask, the group sandwiched it between two metal plate electrodes, then zapped the masks with an electric field strength of 800 volts per millimeter. The field needed to be high enough to cause the polypropylene mask fabric to become conductive so charges could be deposited on it, says Levine. Other researchers have suggested using a hair dryer to recharge a mask, but Levine calls that procedure “wishful thinking,” as it is unlikely to sufficiently restore the charge.
When the current was switched off, those extra charges remained on the fabric. It took about an hour to charge a mask fully. When the team tested the filtering ability of the recharged masks, they found that charging restored the filtration efficacy of a decontaminated KN95 mask to 95 percent, the same level as if the mask were brand-new.
Illustration: Tata Institute of Fundamental Research
Peter Tsai, the inventor of the N95 mask, questions whether the process for testing the degree of filtration was sufficient. “Without using a reliable commercial filtration tester, the tested data from a rough apparatus are likely questionable,” says Tsai, who was not involved in the study. While the team would love to have used a commercial filtration tester, they stand by the results from their lab-made filtration device, says Levine. “Both rely on light scattering to detect particles, and the theory and practice is well known to physicists: these are standard methods.”
The researchers also built a prototype mask fitted with a 3D-printed cartridge containing layers of N95 mask material and two porous metal screens attached to a small battery. When the battery was turned on, the mask maintained a continuous electric field on the filter. “If you do that, it turns out that you don’t have any degradation in the filtration efficiency, as far as we were able to observe,” says Levine.
As the amount of current required to maintain a continuously charged filter is small, Levine imagines it should be possible to make a smart mask that works at peak efficiency all day and only periodically needs to be decontaminated and recharged by plugging into a power source, or even given a quick charge with a smartphone.
The team is currently applying for a patent, and exploring other applications, such as applying the recharging method to HVACs and other air filtering systems. “The idea that you can recharge filtration materials will be useful in all sorts of settings—wherever you have a lot of air handling,” says Levine.
Megan is an award-winning freelance journalist based in Boston, Massachusetts, specializing in the life sciences and biotechnology. She was previously a health columnist for the Boston Globe and has contributed to Newsweek, Scientific American, and Nature, among others. She is the co-author of a college biology textbook, “Biology Now,” published by W.W. Norton. Megan received an M.S. from the Graduate Program in Science Writing at the Massachusetts Institute of Technology, a B.A. at Boston College, and worked as an educator at the Museum of Science, Boston.