Research by Physics Professor Shows Copper Foam Could Be Highly Efficient, Durable Filter for Reusable Masks and Air Cleaners

A copper-based foam filter that could someday be used in facemasks or air cleaners sits on the bristles of a plant, illustrating its light-weight nature.
A copper-based foam filter that could someday be used in facemasks or air cleaners sits on the bristles of a plant, illustrating its light-weight nature.

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During the COVID-19 pandemic, people have grown accustomed to wearing face masks, but many coverings are fragile and not easily disinfected. In a recent  Nano Letters article, Department of Physics professor Kai Liu and his group of researchers have transformed copper nanowires into metal foams that could be used in face masks and air filtration systems, which are more durable and have the potential to effectively filter out microbes. 

“When a person with a respiratory infection, such as COVID-19, coughs or sneezes, they release small droplets and aerosolized particles into the air,” says graduate student James Malloy (C’22), first author of the study, “Minuscule particles can stay airborne for hours, so materials that can trap these tiny particles are ideal for use in face masks and air filters. The foams filter efficiently and can be easily decontaminated, reused repeatedly, and recycled.”

Making a Better Filter

Though current filtration systems such as those in N95 masks are helpful in reducing the spread of COVID-19, the materials they are made with contain some drawbacks.  

Fiberglass, carbon nanotubes and polypropylene fibers are not durable enough to undergo repeated decontamination procedures and some further rely on electrostatics that precludes washing, which leads to large amounts of waste from single-time use. 

Liu’s team recently developed metallic foams with microscopic pores that are stronger and more resistant to deformation, solvents and high temperatures and pressures. As such, they wanted to test copper foams to see if they could effectively remove submicron-sized aerosols while also being durable enough to be decontaminated and reused.

The researchers fabricated metal foams by harvesting electrodeposited copper nanowires and casting them into a free-standing 3D network, which was solidified with heat to form strong bonds. A second copper layer was added to further strengthen the material. In tests, the light-weight copper foam held its form when pressurized and at high air speeds, suggesting it’s durable for reusable face masks or air filters and could be cleaned with washing or compressed air. 

“We find excellent filtration efficiency in these metal foams for particulates in the micron size range and below, which is quite relevant for the fight against COVID-19 as well as air pollution in general”, says Liu. 

Their most effective material was 2.5 millimeters thick and trapped 97% of deep submicron aerosolized salt particles, which are commonly used in face mask tests. 

According to the team’s calculations, the breathability of their foams was generally comparable to that of commercially available polypropylene N95 face masks. Because the new material is copper-based, the filters should be resistant to cleaning agents, which would allow for many disinfection options and its antimicrobial properties will help kill trapped bacteria and viruses. In addition, they are recyclable, unlike polymer-based filters that go into waste streams. 

The researchers estimate that the materials would cost around $2 per mask currently, and disinfection and reuse would extend their lifetime, making them economically competitive with current products.

Other researchers involved include postdoctoral fellow Alberto Quintana and graduate student Christopher Jensen (C’24). This research was made possible through funding from the Georgetown Environmental Initiative Impact Program Award, the McDevitt bequest to Georgetown University and Tom and Ginny Cahill’s Fund for Environmental Physics at University of California Davis

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