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The team, from Georgetown University in Washington DC, said the recyclable and easily decontaminated material could also be used in air-filtration systems.
When a person with an illness such as Covid-19 coughs or sneezes, they release small droplets and aerosolised particles into the air. Particles smaller than 0.3μm can stay airborne for hours, so materials that trap these tiny particles are ideal for use in face masks and air filters.
Some existing filter materials have drawbacks, however. Glass fibre, carbon nanotubes and polypropylene fibres are not durable enough for repeated decontamination procedures, the researchers said. Other materials rely on electrostatics so they cannot be washed, leading to large amounts of waste.
Researchers recently developed metallic foams with microscopic pores that are stronger and more resistant to deformation, solvents, and high temperatures and pressures. Georgetown researcher Kai Liu and colleagues wanted to develop and 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 copper foam held its form when pressurised and at high air speeds, suggesting it is durable for reusable face masks or air filters, and could be cleaned with washing or compressed air.
The team found the metal foams had ‘excellent’ filtration efficiency for particles within the 0.1-1.6μm size range, relevant for filtering out viral particles that cause Covid-19. The most effective version was 2.5mm-thick, with copper making up 15% of the volume. This foam had a large surface area and trapped 97% of 0.1-0.4μm aerosolised salt particles, which are commonly used in face-mask tests.
According to the team's calculations, the breathability of the foams was ‘generally comparable’ to commercially available polypropylene N95 masks.
“Because the new material is copper based, the filters should be resistant to cleaning agents, allowing for many disinfection options, and its antimicrobial properties will help kill trapped bacteria and viruses,” the researchers said.
The team estimated that the materials would cost $2 per mask. Disinfection and reuse of the masks would make them economically competitive with other products.
The research was published in the American Chemical Society’s Nano Letters.
The authors acknowledged funding from the Georgetown Environmental Initiative Impact Programme Award, the McDevitt bequest to Georgetown University, and Tom and Ginny Cahill's Fund for Environmental Physics at the University of California Davis.
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