New fabric passively cools everything it covers, including you

Rising temperatures around the world run the risk of creating a dangerous cycle where more people benefit from air conditioning, which causes increased demand for energy and leads to increased carbon emissions, pushing up even more the temperatures. Renewable energy is an option to break this cycle, but people have also been studying materials that allow what is called passive cooling. Without using energy, these materials absorb heat from everything they cover and diffuse it into 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 ambient air. But now a team based in China has taken the same principles and applied them to the fabric, creating a vest that keeps its users around 3 ° C cooler than they would otherwise be.

Built to relax

Every time something is exposed to the sun, it will absorb some of these photons, which will be converted into heat. This heat can then be returned in infrared wavelengths. The problem is, that doesn’t cool things down very much. Many gases in the atmosphere immediately absorb infrared light, trapping energy in the form of heat in the immediate vicinity of the object. If the object is a person, there is the additional problem of the heat generated by their metabolism, which is also radiated in the infrared at the same time.

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

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

No material does it all on its own. But with our growing ability to structure multiple materials on a small scale, it is possible to find material combinations that do the trick. The result is a coating that cools things down without requiring energy beyond what is needed for its manufacture and installation.

Now make clothes

Clothing obviously adds some complications to this task. They should be soft and washable to start. 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 clothes reflective, the researchers used titanium dioxide powder, which is highly reflective and often used to bleach things like paint. Obviously, a powder on its own wouldn’t make good clothes. But the researchers took titanium dioxide nanoparticles and embedded them into polymer fibers, choosing the particle size based on computer modeling to maximize reflection.

The polymer used, polylactic acid, emits in the mid-infrared, which is exactly what it takes to send photons into space through the atmospheric window. The researchers also proudly advertise that the polymer is biodegradable, although I’d like to see long-term data on how it works after a few years of rubbing shoulders with the bacterial population in human skin.

This material is woven so that there are pores large enough for air exchange. It is then covered with a thin layer of another polymer, polytetrafluoroethylene. This serves two purposes. The polymer effectively reflects UV light, handling certain wavelengths that titanium dioxide does not. It is also hydrophobic, which means it will repel water. Combine that with a carefully chosen pore size, and it allows for breathability while keeping things waterproof.

This last feature deals with the sweat problem. As sweat evaporates from our skin, it goes into a vapor phase, allowing it to pass through the pores of the material. It works even if the fabric rejects liquid water due to its hydrophobic nature.

Do all the demos

The researchers did their best to subject their wonderful tissue to a whole host of tests and demonstrations. They showed that the breathable / waterproof suit worked by using fabric to seal the bottom of a water container and then pumping air through it. (Oddly, the image of this in the paper shows that they are putting fish in the water for … I’m not sure why.) The fabric also reflected well over 90 percent of the incoming sunlight.

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

And, most importantly, the researchers have shown that the fabric manages heat as expected. They placed a variety of fabrics on a copper plate and glued them in direct sunlight. To make this test reflect normal tissue use, they also injected the amount of heat normally dissipated by the human body (which is somewhat disturbing, they called it a “skin simulator”). The plaque was found to be 5 ° C cooler than cotton and almost 7 ° C cooler than elastane.

As a final test, the researchers made a vest half covered with this fabric, glued it to one of their students, and sat the student in the sun. By recording the person’s temperature with an infrared camera, they found that the half covered with their structured material was usually about 3 ° C cooler than the one that was not.

This material has obvious limitations, one of which is that dyeing it would immediately eliminate much of its function. But as someone who suffers a lot from the summer heat, I would be more than happy to accept a “you can have any color as long as it’s white” situation for my shirt if that keeps me a few degrees in the dark. fresh. So hopefully there aren’t too many obstacles to marketing on this one.

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