The N95 respirator mask is, for many medical care professionals, the first line of defense against COVID-19 and dwindling supplies are one of the biggest threats to health systems during this pandemic.
University of Michigan engineers are working quickly to address the shortage by developing efficient, effective and scalable ways to disinfect masks, which are typically discarded after one use. As part of the effort, they’re testing whether the masks still work—and fit well—after repeated rounds of treatment. A viable means of getting multiple uses from masks would help protect doctors and nurses until more masks can be produced.
“There’s research on how coronaviruses behave in fluids, but we don’t know if they behave the same way on masks. It is important for experiments to be conducted to address these knowledge gaps,” said Nancy Love, the Borchardt and Glysson Collegiate Professor of Civil and Environmental Engineering who is involved in the testing. “And few if any studies are doing all of the above—biological testing, mask particle reduction performance, and human fit testing.”
This effort is a result of a partnership between the College of Engineering and Michigan Medicine to identify problems and assign U-M engineers to address them. Deactivating viruses on N95 masks is among the first, most-pressing issues doctors on this COVID-19 Rapid Reaction Steering Committee identified.
Dozens of researchers from both schools are approaching this task on two fronts. One team is assessing the methods that best inactivate virus particles on N95 masks. A second team is discerning how many times masks can be treated by those methods and still retain their protective capabilities.
“This is a really comprehensive team that’s shifting and working in real time,” said Krista Wigginton, an associate professor in civil and environmental engineering. “So, if the integrity of these masks fail due to one kind of treatment, we know not to put our time and effort into studying the viruses in that treatment and we instead move on to something different. It’s all leading to a really efficient and thorough approach that I think is unique.”
U-M is one of the few major U.S. research institutions with both engineering and medical schools on its campus.
Testing various N95 mask treatments
Michigan Medicine officials asked engineers to test the effectiveness of heat, ultraviolet light and hydrogen peroxide—all things hospitals have on hand.
Initial testing provides evidence that a combination of ultraviolet light and humid heat may work, as may hydrogen peroxide.
“Heat is certainly a known way to inactivate many viruses, but the information out there is somewhat mixed,” Love said. “If you just heat it, an oven will reduce humidity, and what we’ve found is that dry heat may not provide adequate virus deactivation.”
The team is using a suite of surrogate viruses to determine if the treatments will be effective with the SARS-CoV-2 virus that causes the illness COVID-19. To conduct experiments, the researchers spray or place droplets of the test viruses on masks and let them dry.
Once the drying is completed, they transport the masks to the hospital where they are exposed to ultraviolet light, heat, hydrogen peroxide, or a combination of these. Each of these treatments could be scaled up to treat the thousands of masks every day. Following treatments at the hospital, the masks are returned to the engineering lab to assess how well the treatments worked.
To do that, the researchers suspend the masks in a solution containing proteins and shake the solution to knock off viruses on the mask material. The solution is then examined to determine how many infective viruses remain.
How much can a mask take?
In a separate U-M research space, Herek Clack and Mirko Gamba are stress testing the masks after treatment.
“We need to verify that the mask can still perform its function,” said Gamba, an associate professor of aerospace engineering. “Its function is to avoid the penetration of particles of given sizes.”
To do that, researchers fire an air stream of fine particles at the mask and measure the percentage that pass through. And that requires a machine designed specifically for the job.
“With the mask mounted in what is essentially a small wind tunnel, we measure flow speed, pressure, temperature and relative humidity upstream and downstream,” said Clack, an associate professor of civil and environmental engineering.
“Most importantly, we measure particle size distribution. Sensors measure the numbers of particles counted upstream and downstream of the mask. These measures allow us to calculate the percentage of particles that penetrate the mask.”
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Masks are rated for safety under specified conditions by the American Society for Testing and Materials. The organization’s standard for N95 masks and similar size particles to what U-M is testing with is a penetration rate of no more than 5%.
“And if you look at a mask that’s been treated once, five times and then ten times and you don’t see an increase in penetration, that gives us confidence that the mask really isn’t being degraded by these decontamination processes,” Clack said.
As a final step, the decontaminated masks are tested for their ability to form a seal on a person’s face using a fit test. This testing is conducted at the hospital.