Face masks have become a common method of protection during the COVID-19 pandemic. A new copper “foam” could make those masks work even better.

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Face masks have become a vital tool in slowing the spread of the virus that causes COVID-19. They help filter or block spit or mucus droplets that carry infectious particles. Even homemade fabric masks can do a good job. But many are not very durable. Now, researchers have come up with a new sort of filter for use in masks. Made of copper, it’s sturdy and lightweight. The sponge-like material also is easy to clean and can be recycled. In tests, it performed as well in filtering as a standard N95 mask. It might even trap and kill bacteria, its developers say.

Masks to guard against viruses can be made of many different materials. Some fabric ones even use extra layers — often cotton, silk or some synthetic — to boost their filtering prowess. Others use paper similar to coffee filters. With so many people now being asked to wear masks during the pandemic, researchers began scrambling to identify new and better filters. Kai Liu was among them.

This materials scientist thought his team at Georgetown University in Washington, D.C., had a head start. They already had been testing materials to filter small particles out of polluted air.

Recalls Liu, “We saw that small droplets carrying viruses were the same size as some atmospheric pollutants.” Right away, he says, “we thought we should check our materials to see if they might make good filters for face masks.”

Liu’s team soon began cranking out new batches of a material they call copper foam.

They started with templates to make copper nanowires. The diameter of each wire was typically about 200 nanometers, says Liu — or less than one ten-millionth of an inch. After dumping those wires into ultrapure water, they flash-froze the mix in liquid nitrogen. Afterward, they put the copper-filled ice in a vacuum chamber. It drove off the ice to freeze dry the now loosely packed mass of tiny copper wires. Finally, they heated the mass of wires to 300° Celsius (572° Fahrenheit). This fostered chemical reactions that helped bind them into a mesh.

Unfortunately, that mesh was super flimsy, says Liu. Tests showed it would collapse if someone breathed on it. Obviously, that would not work well in masks. So, the researchers kept tweaking the process.

They bathed the weak mesh in a liquid that included copper ions. Then they sent an electric current through this chemical bath. That deposited more copper onto the nanowires, thickening them. Liu says it also helped weld the wires at points where they touched. In tests, some samples of this material could now support about 10,000 times their own weight without collapsing. That was true even when the material was 85 percent air.

More importantly, this 85-percent-air foam filtered out tiny particles. A sample 2.5 millimeters (0.1 inch) thick captured 97 percent of particles between 0.1 to 0.4 micrometers in diameter. Such super-small particles not only are the hardest to trap but also the size of the smallest aerosol droplets that can carry virus particles. These particles don’t just get trapped by the material’s tiny pores, Liu explains. The particles are particularly attracted to the enormous surface area that the nanowires provide. They get stuck there on it as they try to move through the wire maze between the outer and inner edges of the filter. Liu and his colleagues described their innovative new foam April 14 in Nano Letters.

The Georgetown team developed “an interesting and innovative way to produce their material,” says Semali Perera. She’s a chemical engineer at the University of Bath in England. She wonders, however, if it would be hard to scale up the process to make really big batches and large pieces of the thin foam for use in masks.

Perera’s team is taking a different approach to germ filters. Theirs were initially targeted to collect and kill bacteria. Now they’re being designed to trap viruses, too.

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One promising material her team is exploring is a plastic-like foam made out of a polymer called polyimide. To give it a germ-killing punch, the researchers added copper and nickel. Nickel helps slow the growth of bacteria, and copper helps kill them. Those metals make up about 80 percent of the material, says Perera. The plastic-like polymer helps bind the metal atoms together.

Instead of using a multi-step process to make its material, this group mixes its ingredients in one container all at once. A chemical reaction that generates large amounts of carbon dioxide makes the material frothy, Perera notes. As it foams, it expands into a mold. Within three seconds it hardens into its final shape. To make big batches, the researchers merely mix more of the ingredients and then cook them up in a bigger pot.

Perera and her team are working with companies to design new products. One potential use for their material might be filters for home air-conditioners.

UPDATE: In June, Dr. Liu’s team’s innovation was judged one of the top 10 entries in Phase I of a government-sponsored competition to design low-cost, easy-to-use masks. If they also are judged one of five winners in Phase II of the challenge, they’ll earn a share of $400,000. More info on the competition can be found at https://drive.hhs.gov/mask_challenge.html.

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

aerosol: (adj. aerosolized) A tiny solid or liquid particle suspended in air or as a gas. Aerosols can be natural, such as fog or gas from volcanic eruptions, or artificial, such as smoke from burning fossil fuels.

atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bacteria: (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth.

carbon dioxide: (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten.

chemical engineer: A researcher who uses chemistry to solve problems related to the production of food, fuel, medicines and many other products.

chemical reaction: A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

colleague: Someone who works with another; a co-worker or team member.

copper: A metallic chemical element in the same family as silver and gold. Because it is a good conductor of electricity, it is widely used in electronic devices.

COVID-19: A name given to the disease that caused a massive global outbreak. It first emerged in December 2019 and is caused by a new coronavirus known as SARS-CoV-2. Symptoms can include pneumonia, trouble breathing, feeling too tired to walk more than a few steps, fever, headaches, low blood-oxygen levels, blood clots and brain “fog.”

develop: To emerge or to make come into being, either naturally or through human intervention, such as by manufacturing.

diameter: The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.

electric current: A flow of electric charge — electricity — usually from the movement of negatively charged particles, called electrons.

engineer: A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need. (v.) To perform these tasks, or the name for a person who performs such tasks.

fabric: Any flexible material that is woven, knitted or can be fused into a sheet by heat.

factor: Something that plays a role in a particular condition or event; a contributor.

filter: (n.) Something that allows some materials to pass through but not others, based on their size or some other feature. (v.) The process of screening some things out on the basis of traits such as size, density, electric charge. (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.

germ: Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

infectious: An adjective that describes a type of germ that can be transmitted to people, animals or other living things.

ion: An atom or molecule with an electric charge due to the loss or gain of one or more electrons.

liquid nitrogen: A colorless, odorless, nonflammable refrigerant used in research to keep things amazingly cold or frozen. It can chill things down and hold them at temperatures as low as -196° Celsius (-320.8° Fahrenheit).

mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

materials scientist: A researcher who studies how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application.

metal: Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure). 

micrometer: (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.

N95 mask: A type of close-fitting high-filtration facial mask. It takes its name from a requirement that such masks filter out 95 percent of small particles. The edges of these masks should form a tight seal around the nose and mouth. They’re designed to be used once and then discarded. These are not appropriate for children or men with facial hair because they will not allow a proper seal. Hospital workers wear them when they’re around people who may be very infectious. These masks filter smaller particles from the air than do surgical masks. And because they fit the face better, they are better at limiting inhalation of small particles.

nanowire: A wire or rod on the order of a billionth of a meter in cross-section or in circumference. It is usually made from some type of semiconducting material. However some bacteria make string-like anchoring structures on the same size scale. Like the semiconductor wire, the bacterial ones also can transport electrons.

nickel: Number 28 on the periodic table of elements, this hard, silvery element resists oxidation and corrosion. That makes it a good coating for many other elements or for use in multi-metal alloys.

nitrogen: A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn. It comes in two stable forms. Both have 14 protons in the nucleus. But one has 14 neutrons in that nucleus; the other has 15. For that difference, they are known, respectively, as nitrogen-14 and nitrogen-15 (or 14N and 15N).

pandemic: An epidemic that affects a large proportion of the population across a country or the world.

particle: A minute amount of something.

polymer: A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

range: The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.

recycle: To find new uses for something — or parts of something — that might otherwise be discarded, or treated as waste.

surface area: The area of some material’s surface. In general, smaller materials and ones with rougher or more convoluted surfaces have a greater exterior surface area — per unit mass — than larger items or ones with smoother exteriors. That becomes important when chemical, biological or physical processes occur on a surface.

synthetic: An adjective that describes something that did not arise naturally, but was instead created by people. Many synthetic materials have been developed to stand in for natural materials, such as synthetic rubber, synthetic diamond or a synthetic hormone. Some may even have a chemical makeup and structure identical to the original.

tool: An object that a person or other animal makes or obtains and then uses to carry out some purpose such as reaching food, defending itself or grooming.

vacuum: Space with little or no matter in it. Laboratories or manufacturing plants may use vacuum equipment to pump out air, creating an area known as a vacuum chamber.

virus: Tiny infectious particles consisting of RNA or DNA surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.

Journal:​ ​​J. Malloy et al. Efficient and robust metallic nanowire foams for deep submicrometer particulate filtration. Nano Letters. Vol. 21, April 14, 2021, p. 2968. doi:​ ​10.1021/acs.nanolet.1c00050.

Journal:​ G. Ramya et al. Bactericidal–bacteriostatic foam filters for air treatment. ACS Applied Polymer Materials. Vol. 2, April 10, 2020, p. 1569. doi: 10.1021/acsapm.9b01235.

Sid Perkins is an award-winning science writer who lives in Crossville, Tenn., with his wife, two dogs and three cats. He enjoys cooking and woodworking, and he really, really wants to get better at golf.

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