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  • OUTLOOK

CageCapture: designing a molecule to filter out pollutants

A gram of diamond-shaped cage crystals in a vial.

A vial containing crystals of ‘cage’ molecules used to trap pollutants.Credit: Univ. Liverpool

CageCapture is a spin-off from the University of Liverpool, UK

Fresh air is big business. The worldwide market for air-purifying systems was valued at more than US$8 billion last year. And it is expected to grow by more than 10% annually over the next seven years, according to a report by the market research firm Grand View Research in San Francisco, California. The World Health Organization, the US Environmental Protection Agency (EPA), the American Lung Association and the American Cancer Society all recognize that indoor air pollution is a significant threat to human health, contributing to cancer and lung disease. CageCapture, a company spun out of the University of Liverpool, UK, is looking for a share of the air-purifier market with a technology designed to grab pollutants from the air.

CageCapture designs tiny, porous structures that can trap specific molecules. By carefully controlling the size and shape of these structures, and incorporating particular functional chemical groups, the company can create ‘cage’ molecules that capture whatever pollutant it wants. Although other materials, such as activated charcoal, are already used to trap gases, “we have more control of the profiles of the molecules, so we can do some very fine tuning”, says chemist Ming Liu, CageCapture co-founder and chief technical officer.

Liu helped to develop the technology during postdoctoral research at the University of Liverpool with chemist Andrew Cooper, who directs the university’s Materials Innovation Factory, and who came up with porous organic cages in 20091. The cages are made of carbon, hydrogen and nitrogen, bonded together to form four-sided pyramids (see ‘It’s a trap!’). Functional molecules, such as methyl groups, can be added to alter the structure of the cages. Some of these additions change the size and shape of the cavities to fit chemically inert gas molecules, whereas others add binding sites to make the cages ‘stickier’ for more-reactive molecules. Various molecules can enter the cages and stick to their inner surfaces, becoming trapped. Each cage’s particular chemistry determines which molecule it will capture.

The company’s first target is formaldehyde, a common indoor pollutant that can irritate the skin, eyes and nose, and that, in high concentrations, can cause cancer. Formaldehyde gas leaks out of common building materials, such as plywood, wood composites and insulation, as well as various glues, fabrics and paints. “It can be released everywhere in your house,” Liu says. “There are formaldehyde gases that slowly release into the air from the curtain, the paint on the wall, and the sofa fabric.”

In tests, the researchers found that their molecular cages could capture roughly 500 times as much formaldehyde as the same amount of activated charcoal2 — the main absorbent material used in air filters. Even though the cage material is more expensive to make than activated charcoal, Liu says, a lower volume is required so the cost should be about the same. And it might work better. During the summer, when it gets hot and humid, activated charcoal can release captured formaldehyde back into the air, becoming a source of pollution. The molecular cages hold onto the trapped gas indefinitely, unless exposed to temperatures above 300 °C or to very strong acid.

Increasing production

The cage material can be processed in a solution, then dried into powder. That makes it easy to incorporate into existing filtration systems, Liu says — it’s just a matter of replacing the charcoal with the cage material. The company still needs to work out how to scale up production of the material. In the lab it can make it at a rate of about 5 kilograms per month, but it is working to increase that by 20-fold. CageCapture also needs to evaluate its material’s performance in real-world tests in a complete filtration system, rather than on its own in lab tests. The team had hoped to have a prototype within a year, but disruptions as a result of the coronavirus pandemic will probably stretch that timeline to 18 months, Liu says.

CageCapture’s technology has already sparked some interest from potential collaborators. The company is working with activated-charcoal manufacturer Jacobi Carbons in Kalmar, Sweden. It has also had discussions with car manufacturers Nissan and Renault — a more compact air filter could prove useful in car design, Liu says.

The company is also developing cages that target other pollutants besides formaldehyde. One would capture radon, a radioactive gas that can seep into homes from the ground. The EPA says radon is the second leading cause of lung cancer after smoking, responsible for about 21,000 deaths annually in the United States. Another design is aimed at acetaldehyde, a carcinogen that can leak out of building materials and paint and can also be found in tobacco smoke. Other cages could be used for the microfiltration of water. “We have a powerful tool, which can develop customized materials for different pollutants,” Liu says.

Other porous materials exist that can be used for filtration, including zeolites, which are crystals made of silica and aluminium; and metal–organic frameworks — polymers that combine metal atoms and organic ligands that can be built into various shapes. But Liu says that molecular cages can be tuned to their applications more finely than these materials, and that they are more easily processed in solution.

Molecular cages could have other applications, which the company will consider later, Liu says. For instance, he and Cooper have shown that the cages can be used to separate isotopes of hydrogen to make nuclear fuel, and to make proton-exchange membranes for hydrogen fuel cells3.

CageCapture is still in its infancy, even by start-up standards. It was founded in December and has three full-time employees, including Liu and Cooper, who is chief science adviser, and one part-time employee. The company has two patents pending, one for formaldehyde capture in the United States, China, Europe and Japan, and another for radon capture in the United States. It is supported by £300,000 (US$375,000) in funding from Innovate UK’s Innovation and Commercialisation of University Research programme.

Emily MacKay, who heads artificial intelligence and data strategy at the clinical software company Congenica in Cambridge, UK, and who was one of the judges for the Nature Research Spin-off Prize, says she’s optimistic about CageCapture’s technology and its potential impact. “Air pollution is a major environmental risk to human health, so there’s a large problem to resolve,” she says.

She thinks the company needs to hire more people with business expertise. It will also need to address the issue of scaling up manufacturing, but overall the company seems to have a solid idea, she says. “They create a clear vision of much more effective air filtering.”

doi: https://doi.org/10.1038/d41586-020-01797-5

This article is part of Nature Outlook: The Spinoff Prize 2020, an editorially independent supplement produced with the financial support of third parties. About this content.

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