New research, published Tuesday in the journal Physics of Fluids, suggests the use of nanofibers can improve the filtration efficiency of face masks, but they must be replaced fairly frequently.

With COVID-19 case and hospitalization rates spiking across the United States, health officials are once again encouraging people to wear face masks indoors, regardless of vaccination status.

Since the pandemic began, health experts and material scientists have been trying determine the best ways to filter out viral particles. While N95 masks remain the standard for healthcare workers, single-use masks aren't always practical for the general public.

While several studies have shown multiple cloth layers -- or combinations of cloth, silk and chiffon -- provide significant filtration, mask makers have also deployed nanofibers in order to beef up protection against aerosolized virus particles.

While nanofibers have proven effective at boosting filtration, they're resiliency hasn't been thoroughly tested -- until now.

"When COVID-19 first hit, face masks were in extremely short supply everywhere, and people came up with all kinds of ways to 'rejuvenize' used face masks," study co-author Boyang Yu said in a press release.

"It was like a chef's contest, with boiling, steaming, grilling, and even smoking involved. Our intuition told us this can't be right. We have to look into it and see what exactly happened with the nanofibers," said Yu, engineer at the Southern University of Science and Technology in China.

To better understand the filtration abilities and durability of nanofiber masks, researchers used high-speed microscopic video to film different polymer layers under water aerosol exposure.

"Filming nanofibers is like taking portraits of babies," said Yu. "They don't like to stay in place for the camera. This is because nanofibers are very soft and flimsy, especially with the aerosol flow blowing through. But with enough care, patience and luck, we eventually got nice shots for our analysis."

The video footage and images showed that fibers irreversibly coalesce during the "droplet capture stage," thus reducing their filtration capabilities. Scientists found fiber-coalescence could be reduced by adding hydrophobic and orthogonally woven fibers.

"We confirmed three things," said co-author Weiwei Deng, also an engineer at the Southern University of Science and Technology. "One, nanofibers are superb at capturing droplets in aerosol. Two, the nanofibers are bonded together after the aerosol is captured. And three, this bonding is tight and irreversible, even after the captured droplets evaporate."

"Wetted fibers tend to bond to each other in the same way that wet hairs tend to bundle together," Deng said. "It is because of the capillary force, which becomes dominant as the size scale shrinks, and it is extremely strong for nanofibers."

Researchers suggest nanofiber masks be replaced more frequently during the winter. When it's cold, exhaled breaths contain more water droplets.

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