Scientists have created a new lightweight metal foam that can filter out tiny particulates, including droplets and aerosols containing the coronavirus, and is just as breathable as filters in current N95 masks.

The metal foams "exhibit outstanding filtration, near 100% efficiency" for particulate sizes that are key to combating COVID-19, according to a new study published Wednesday in Nano Letters. The copper-based metal foam is more sustainable than current filters in masks, as it can easily be cleaned — and decontaminated — before being reused or recycled. These foam filters can be used in a wide range of products, from respirators to air filters in personal homes.

Ultra-small particulates that are just microns in size (that is, one-millionth of a meter or one-hundredth of a human hair) play a huge role in spreading respiratory infections. They can be suspended in the air for hours or days, and travel over long distances. 

While droplets a few microns in size are relatively easy to filter, the smaller aerosols are harder to capture — especially those less than 0.3 micrometers. These "lung-penetrating particles pose the most health risks to humans," but their size makes air filtration difficult, lead author and Georgetown University professor Kai Liu told The Academic Times. Filtering ultra-small particulates is also vital to addressing air pollution, which the World Health Organization identified as the largest environmental health risk before COVID-19. 

Liu, who has researched low-density metallic materials since 2010, came up with the idea of using these metal foams for air filtration. The copper foams start with ultra-thin copper nanowires and are fabricated using an electrodeposition process, where the metal is deposited through electric currents. 

The foams get their lightweight strength from a 3D scaffold of interconnected copper nanowires and are 2 to 30% as dense as copper itself. "It took us a lot of effort to find the optimum structures that are both stable and effective for filtration," Liu said, but the researchers were pleasantly surprised by the results. 

The copper foams have excellent mechanical properties. The material's large surface area makes it a great fit to filter a broad range of particle sizes, especially airborne particulates. Previous studies have found that thinner fibers filter more efficiently, which makes this copper foam able to capture small particles, comparable to current N95 masks. The metal foams are resistant to oils and corrosive chemicals and can survive high temperatures and large pressures without being deformed — unlike many filters on the market now.

"Materials currently used for air filtration have various limitations," the Georgetown University researchers said. For example, fiberglass fibers "are fragile, challenging to clean and use as durable personal protective equipment (PPE), and deteriorate under high temperatures." Even filters in N95 masks can "degrade upon exposure to organic solvents, making them difficult to decontaminate and reuse," Liu explained. These new nanowire-based metal foams are the opposite: The material is easy to clean and possible to reuse before eventually being recycled. 

A sustainable alternative to single-use masks is needed as the COVID-19 pandemic continues. Recyclable masks could significantly lower "the sheer volume of the waste face masks and other PPE materials generate on the global scale," Liu said. Other researchers have predicted "significant environmental challenges" from plastic and hazardous waste, and many current filters take a long time to decompose.

Liu sees great potential in metal foams as durable, reusable and recyclable filters. The foam filters are promising not only because of their breathability but also because they can be produced at scale at a relatively low cost. Liu said an early estimate for the materials cost of one metal foam filter is $2 per mask, which is close to price parity. Reusing the masks "will substantially extend the[ir] lifetime, and the eventual cost per use will be quite competitive with that of conventional filters and masks," he said.

In the future, Liu is excited to research the "fascinating potential" of lightweight metal foams in filters and other applications. "As physicists," Liu said, "We want to gain a quantitative understanding of the mechanisms." The team's next step is to explore other types of filtration that can improve the efficiency of the metal foam filters.

Liu acknowledged his late colleague Tom Cahill, who used his physics background to solve environmental issues, as his inspiration for this project. 

The study, "Efficient and Robust Metallic Nanowire Foams for Deep Submicrometer Particulate Filtration," published March 24 in Nano Letters, was authored by Kai Liu and James Malloy, Georgetown University and University of California, Davis;  and Alberto Quintana and Christopher J. Jensen, Georgetown University.