Rising temperatures around the world run the risk of creating a dangerous cycle where more people get air conditioning, which causes energy demand to surge and leads to higher carbon emissions, which makes temperatures rise even more. Renewable power is one option for breaking that cycle, but people have also been studying materials that enable what's called passive cooling. Without using energy, these materials take heat from whatever they're covering and radiate it out to space.

Most of these efforts have focused on building materials, with the goal of creating roofs that can keep buildings a few degrees cooler than the surrounding air. But now a team based in China has taken the same principles and applied them to fabric, creating a vest that keeps its users about 3º C cooler than they would be otherwise.

Built to chill

Whenever something's out in the sunlight, it's going to absorb some of those photons, which will get converted into heat. That heat can then be radiated back out in infrared wavelengths. The problem is that this doesn't actually cool things down much. Lots of the gasses in the atmosphere immediately absorb the infrared light, trapping the energy as heat in the immediate vicinity of the object. If the object is a person, there's the added issue of heat generated by their metabolism, which is also getting radiated away in the infrared at the same time.

The secret to passive cooling is the existence of what's called the atmospheric window. This is an area of the infrared spectrum that none of the gasses found in our atmosphere can absorb. Photons in this area of the spectrum are likely to make their way to space, effectively allowing the heat to escape permanently.

A passive cooling material is designed so that it reflects most of the incoming light, keeping stray photons from heating the object it covers. At the same time, the material will absorb some heat by contact with whatever it covers—either directly or via the intervening air. But the material is designed so that this heat is radiated away in the mid-infrared, allowing the photons to escape through the atmospheric window.

No materials do all of this on their own. But with our growing ability to structure multiple materials on small scales, it's possible to find combinations of materials that do the trick. The result is a covering that cools things without requiring any energy beyond what's needed for its manufacture and installation.

Now do clothes

Clothes obviously add a few complications to this task. They have to be flexible and washable to start with. And, if the goal is to keep someone cool, they have to deal with the body's built-in cooling system: sweat.

To make the clothing reflective, the researchers used a titanium dioxide powder, which is highly reflective and often used to turn things like paint white. Obviously, a powder on its own wouldn't make good clothing. But the researchers took titanium dioxide nanoparticles and embedded them in polymer fibers, choosing the size of the particles based on computer modeling to maximize reflection.

The polymer used, polylactic acid, emits in the mid-infrared, which is exactly what's needed to send photons out to space via the atmospheric window. The researchers also proudly announce that the polymer is biodegradable, though I'd like to see some long-term data on how well that works out after a few years of spending time brushing up against the bacterial population of human skin.

This material is woven so that there are pores large enough for air exchange. It's then coated with a thin layer of another polymer, polytetrafluoroethylene. That serves two purposes. The polymer reflects UV light efficiently, handling some wavelengths that titanium dioxide doesn't. It's also hydrophobic, meaning it will repel water. Combine that with a carefully chosen pore size, and it allows breathability while keeping things waterproof.

This last feature handles the sweat issue. As sweat evaporates from our skin, it goes into the vapor phase, allowing it to pass through the pores of the material. That works even as the fabric rejects liquid water due to its hydrophobic nature.

Do all the demos

The researchers did their best to put their wonder-fabric through a whole bunch of tests and demonstrations. They showed that the breathable/waterproof combination worked by using the fabric to seal the bottom of a container of water and then pumping air through it. (Oddly, the image of this in the paper shows that they put fish in the water for... I'm not entirely sure what.) The fabric also reflected well over 90 percent of incoming sunlight.

They also made a large roll of the fabric and showed that you could do the things you'd normally expect to do to clothes, including embroidering it with patterns and sending it through the washing machine.

And, critically, the researchers showed that the fabric managed heat as expected. They placed a variety of fabrics over a copper plate and stuck them in direct sunlight. To get this test to reflect normal fabric use, they also injected the amount of heat normally dissipated by the human body (somewhat disturbingly, they called this a "skin simulator"). The plate ended up 5º C cooler than cotton and nearly 7º C cooler than spandex.

As their final test, the researchers made a vest that was half-covered with this fabric, stuck it on one of their students, and sat the student out in the sun. Registering the person's temperature with an infrared camera, they found that the half that was covered in their structured material was typically about 3º C cooler than the one that wasn't.

This material has some obvious limitations, most notably that dyeing it would immediately eliminate much of its function. But as someone who suffers badly in summertime heat, I'd be more than happy to accept a "you can have any color you want as long as it's white" situation for my shirt if it would keep me a few degrees cooler. So here's hoping that there aren't too many barriers to commercialization on this one.

Science, 2021. DOI: 10.1126/science.abi5484  (About DOIs).