Researchers in the School of Engineering and Applied Sciences have developed a method of mass-producing effective and reusable cloth masks. The masks owe their effectiveness as air filters to chemical compounds called metal-organic frameworks, or MOFs.
Asst. Chemical Engineering Prof. Gaurav Giri, the principal investigator of the lab developing the technology, explained that MOFs are empty crystals formed from metal ions attached to organic compounds. These MOFs have very high porosity which can be put to use. For example, MOFs can store drugs inside for drug delivery, or they can store gasses inside for carbon capture. The high porosity of MOFs can also be used to filter out airborne particles.
Graduate Engineering student Sangeun Jung has been working with MOFs in Giri’s lab since beginning graduate school.
“Think of MOFs as sponges with very tiny holes,” Jung said. “It’s going to be working as a molecular sieve.”
This high porosity means that MOFs have a very high surface area, approximately 1000 square meters per gram of the material.
“Within a gram of material, your surface area is equal to about that of a football field,” Giri said.
Giri is originally from Nepal. After returning there in 2016 to get married, he noticed that Nepal’s air pollution had worsened. He had the idea that if MOFs could be put into cloth — which is cheap and used by everyone — this cloth could be used to make cheap and effective air filters.
The process of making MOF fabrics is very similar to the process of dyeing fabrics. Fourth-year Engineering student Emily Beyer has been working in Giri’s lab since her first year at the University. Her current duties in the lab include producing the MOF fabric samples by means of a technique called dip-coating.
Beyer first dips a sample of any common household fabric — such as cotton or polyester — into a solution of organic molecules dissolved in water. She dries the sample with paper towels, then dips it into a solution of metal ions — including zirconium — dissolved in water. A reaction takes place in the pores of the fabric to integrate the MOFs into the material. Beyer repeats this cycle six times and lets it dry for 24 hours. A completed sample still looks and feels like ordinary fabric. The only apparent changes are a slight whitish or yellowish tinge and powdery look due to the properties of the MOFs.
Initially, Giri’s lab focused on making MOF fabrics to combat air pollution. But when the COVID-19 pandemic locked down the University and the world, the work shifted gears. The need was high for air filters and data about their efficacy.
“Masks were running out,” Giri said. “We had no idea what works, what doesn't. People were doing everything, in terms of making a filter on your face that could prevent particles from going in.”
Giri’s lab was already designing air filters and had the equipment to test them, so the lab obtained emergency approval to stay open. Over the next six months, Giri’s lab tested the efficacy of 50 to 100 different filters.
Beyer continued to help with the research remotely the summer of 2020 but was able to return to the lab when she came back for her third year that fall. Testing MOF fabric samples for efficacy is also part of Beyer’s duties in the lab.
Giri explained that filters block particles in two different ways. For particles smaller than 0.3 microns, filters are “electrostatically sticky.” Particles stick to the filter instead of passing through it. For particles larger than 0.3 microns, filters act as a simple barrier that they can’t get through. But particles the size of 0.3 microns are too big to stick well to the filter and too small for the filter to block them well. A mask’s efficacy is measured by how well it blocks particles with this tricky size.
There are no federal regulations regarding mask efficacy. The Centers for Disease Control and Prevention emphasize that “all masks and respirators provide some level of protection” but encourage people to “wear the most protective mask you can that you will wear consistently.”
N95 masks have 95 percent efficacy, while cloth masks have only 5 to 10 percent efficacy. The MOF fabrics have been shown by Nelson Labs — a leading provider of microbiological and analytical lab testing — to be 25 to 30 percent effective against 0.3 micron particles.
MOF masks are about twice as effective — 50-60 percent — against COVID-19 viral particles because they are smaller — about 0.1 microns — and thus more likely to stick to the filter.
Although MOF masks are not as effective as N95 masks, it is important to take another factor into account — breathability.
“A brick wall can stop 100 percent of particles from going through, right?” Giri said. “It's also unbreathable.”
Giri explained that N95 masks are very effective but have low breathability. Cloth masks are not very effective but much more breathable. MOF masks are three times as effective as cloth masks while still retaining about the same breathability.
“People are much more likely to wear cloth masks for a six or eight or 10 hour workday, compared to wearing an N95 mask for that period of time,” Giri said. “How much protection are you willing to sacrifice for comfort? Because if the choice is either you're comfortable or you don't wear it, [then] it doesn't matter if it's really good because then nobody's gonna wear it.”
MOF masks are also reusable. According to Giri, they can be washed about 25 times without losing efficacy. However, the lab continues to test how washing affects MOFs, Jung said.
Over the last year and a half, Giri’s group has been increasing the size of the MOF fabrics.
Generally, MOF fabrics are produced in lab scale — about the size of the circle you can make with your thumb and forefinger. Giri’s group has now successfully scaled the production size up to 50 square meters of material. The next step is 1000 square meters — the size in which a standard fabric company would make fabric. This will be the first time MOF fabrics are produced on this scale.
When this is achieved, MOF masks can be mass manufactured using factories’ pre-existing machinery. Giri predicted that the first batch of about 5,000 to 10,000 MOF masks should be available to the public within the next two months, if all goes well.
“We've looked at the physics, we’ve looked at the fluid dynamics, we think it's going to work, but that's not the same as saying it's going to work,” Giri said.
Due to the relatively small number of masks that will be produced at first, the first batch of masks will only be sold around Charlottesville. Giri could not give a price estimate at this point, but the existing market for masks will be taken into account.
Giri is already wearing some of the MOF masks that the lab made as a proof of concept, Beyer said. She was asked if she plans to get a MOF mask once they are available to the public.
“After spending four years on it?” Beyer said. “Absolutely.”
Masks to protect against COVID-19 are “the most salient and significant point right now” for MOF fabrics, Giri said. But this work has an even broader reach. Potential applications for MOF fabrics include technologies like clothing that can monitor a pulse or curtains that can capture carbon.
“All of these varied applications that people have already used MOFs for, we are providing a pathway to scale it up,” Giri said. “Getting it from just science in a lab to useful for humanity is a bridge that we're trying to cross.”