Totally rad: Radiative materials redefine cool clothes for a hotter world

The T-shirt looks just like others you might find in a clothing store. It is soft to the touch and pleasant to wear, with just the right stretch. But it feels noticeably cool on the skin.

That’s because it uses a radical new way of lowering your temperature. Cotton fabrics circulate air to keep you comfortable, and athletic apparel wicks sweat away from skin. But this T-shirt, from the start-up i2Cool, does more: it transfers heat from your body to walls and furniture, even sending it straight into the cold depths of outer space.

The shirt was designed by materials scientists to use the principle of radiative cooling: the natural process by which heat, or infrared (IR) radiation, flows from a hotter object to a cooler one. Martin Chu, CEO and cofounder of i2Cool, a spin-off from City University of Hong Kong, says the shirt lowers the wearer’s skin temperature 2–3 °C by removing body heat and by reflecting most of the sunlight falling on it.

Many others are also making engineered textiles to cool people indoors and outdoors, and in urban settings. These radiative cooling fabrics play clever tricks with the electromagnetic radiation emitted by the body, the objects around us, and the sun. Some of these fabrics reduce skin temperatures by more than 15 °C in controlled experiments.

If the past two summers are an indication of the planet’s scorching trajectory, we will need all the help we can get to stay cool. Record-breaking temperatures in 2023 and 2024 led to over 47,000 deaths in Europe and were responsible for many heat-related tragedies in Asia and the Middle East.

In a world with more extreme heat waves, radiative cooling fabrics could keep people comfortable indoors without air conditioners blasting. “For every 1 °C you increase an air conditioner’s set point, you save 10% of indoor cooling energy use. It’s a lot,” says Yi Cui, a materials scientist at Stanford University and cofounder of the apparel start-up LifeLabs Design, another firm making radiative cooling products.

The fabrics could be lifesavers for older people, anyone vulnerable to heat because of health and living conditions, as well as for agriculture and construction workers, who cannot escape outdoor heat. “Just 3 °C can be the difference between heatstroke or not,” says Trisha L. Andrew, a chemistry professor at the University of Massachusetts Amherst.

For now, the materials are being tested primarily in laboratories. LifeLabs, i2Cool, and others are working on commercial products, but high costs could relegate them to a niche market for now. That would need to change for radiative cooling fabrics to have a widespread impact on health and energy use. “Things have to scale,” Andrew says. “Each person around the world is not going to be able to afford a $100 garment.”

The human factor

Radiative cooling has recently become a hot field. It all started at Stanford 10 years ago, when electrical engineer Shanhui Fan, graduate student Aaswath Raman, and colleagues developed a coating that reflected 97% of sunlight and emitted mid-IR radiation at wavelengths of 8–13 µm. Because Earth’s atmosphere is transparent to those wavelengths, they go straight into space through that window (Nature 2014, DOI: 10.1038/nature13883).

The coating was made of alternating layers of silicon dioxide and hafnium oxide on top of a thin layer of silver. Panels coated with the multilayer material cooled surfaces beneath them by up to 5 °C.

The work sparked a flurry of research on radiative cooling materials for buildings, but Fan and Cui wanted to translate the concept to clothing.

They started with a fundamental limitation of the clothing people have made for centuries: it relies on airflow for thermal comfort. Clothing design has ignored the body’s heat radiation, says Raman, who is now a materials science and engineering professor at the University of California, Los Angeles. Our normal skin temperature is about 33 °C. Indoors, or in mild weather outdoors, things around us are generally cooler than our skin, so we radiate heat to our surroundings. “Historically, what has been neglected is the fact that this radiation contributes to 50% of the body’s heat loss and therefore 50% of your perception of being comfortable,” Raman says.

Traditional clothing does not let the body’s IR radiation escape. In fact, most materials are not transparent to IR, according to Raman. To make its radiative cooling textiles, the Stanford team picked one of the few materials that does let IR radiation through: polyethylene (PE), the polymer that plastic wrap and garbage bags are made from.

Wearing a transparent plastic film is not appealing, of course. Clothing needs to be breathable and soft and to look nice, besides being strong and washable. People are picky about what they wear. “It is infinitely more complicated than building materials,” Raman says.

That didn’t daunt the Stanford researchers. Borrowing processing techniques from the battery industry, which uses porous PE sheets as electrode separators, the group made a nanoporous PE film that kept the temperature of simulated skin 2.7 °C lower than it was when covered with cotton (Science 2016, DOI: 10.1126/science.aaf5471). The film’s pores scatter light, so it looks white.

Having made a lightweight, flexible film that dissipated heat, the researchers now faced a bigger challenge of creating woven PE fabrics. People have been making polyester fabrics for decades, but PE yarn is stiff and can liquefy in weaving machines because of its low melting point. The researchers developed a process to manufacture nanoporous PE yarns that look and feel like cotton and can be run through industrial machines to weave fabrics.

Cui launched LifeLabs Design in 2021. The start-up’s clothing is a specialty product, though, and the company is trying to figure out the best market strategy. “I’m the first person to test these products,” Cui says with a laugh. “The T-shirt feels like a regular one, but you are cool right away.”

The outdoor factor

Cooling people outdoors is complicated. On a sunny summer day, our bodies soak up sunlight and convert it to heat. In addition, the ground and air are hotter than our skin, which makes it tough to stay cool, Raman says. “We’re being bombarded with heat from everywhere.”

As a result, radiative cooling fabrics must play a complex optical game. They need to reflect sunlight, which includes ultraviolet, visible, and near-IR wavelengths spanning a range of roughly 0.3–4 μm. And instead of simply allowing heat to pass through them—which would let outside heat get to the skin—the fabrics have to absorb body heat and efficiently emit it in the range of 8–13 μm wavelengths that go straight through the atmosphere. By doing this, the fabrics effectively pull heat from the body and dump it into space.

This is precisely what Raman achieved in 2014 with his cooling panels, but practicing such spectral engineering in textiles is challenging. It requires clever solutions that involve a mix of carefully selected polymers, nanomaterials, and porous microstructures.

Cui and colleagues made the first radiative textile for outdoor cooling by adding zinc oxide nanoparticles to their nanoporous PE fibers. ZnO and a few other metal oxides emit strongly in the wavelength range of the atmospheric window and are good at reflecting visible light. The researchers’ fabric kept simulated skin 5–13 °C cooler than cotton.

At the City University of Hong Kong, Edwin Chi-Yan Tso and colleagues also made fabrics from PE yarns doped with ZnO nanoparticles. Tso cofounded i2Cool 2 years ago with then-graduate student Chu to commercialize cooling paints, films, and fabrics. The company has combined its PE-based cooling yarns with nylon, spandex, cotton, and polyester to make textiles that also have “quick-drying and moisture-wicking properties, significantly improving comfort and heat dissipation,” Chu says in an email.

Others have chosen polymers such as polydimethylsiloxane and polyvinylidene fluoride (PVDF), which emit IR radiation in the atmospheric window range. They then commonly add oxides of metals such as titanium, aluminum, silicon, barium, and zinc.

The sky’s the limit when it comes to designing radiative cooling fabrics from these materials. One approach is to make layered fabrics with multiple polymers, such as nylon, PE, and PVDF for a cooling effect of up to 6.5 °C in direct sunlight. Another is to coat polymer fibers with thin metal layers or to embed titania or alumina particles into the polymer.

Fabric designs can be highly imaginative and complex. For instance, a team at Harbin Institute of Technology made a skin-inspired radiative cooling fabric composed of cotton and polyester fibers soaked in a mix of PVDF containing microparticles of titanium dioxide, barium sulfate, and silicon dioxide. The waterproof, washable fabric stayed as much as 12.6 °C cooler than white fabrics made of cotton, polyester, and linen.

With a focus on wearability, researchers at Donghua University combined moisture-wicking structures into radiative cooling fabrics to achieve both moisture and temperature regulation and keep simulated skin 16 °C cooler than traditional textiles. And researchers at Nanjing University attached alumina nanoparticles to silk to boost its radiative cooling properties while retaining its softness and comfort.

The urban factor

Besides being picky about clothes, humans are difficult to design radiative cooling fabrics for because we walk upright and often dwell in dense cities. Our vertical orientation means that most of the body does not face the sky and therefore cannot radiate its heat through the atmosperic window into space. And in urban settings, we soak up IR heat from pavements, roads, and buildings.

In 2020, Raman and others proposed a solution for radiative cooling in urban heat islands. Their idea was to use materials that reflect sunlight as well as the longer-wavelength IR coming from the ground and buildings, while selectively absorbing body heat and emitting in the sky window range of 8–13 µm.

“This takes advantage of the fact that the atmosphere is transparent to IR in a certain range of wavelengths, but in others it’s opaque so there’s no benefit in radiating at those wavelengths,” Raman says. The researchers pointed out that a commercially available polymer called polymethylpentene (PMP) can perform this selective emission.

A team led by Po-Chun Hsu, a materials scientist at the University of Chicago, followed through on this idea to make a cooling fabric for urban settings (Science 2024, DOI: 10.1126/science.adl0653). The fabric comprises three layers: The bottom layer, made of cotton, channels body heat to the middle layer, which contains silver nanowires and reflects solar radiation. The top layer is a superthin mat spun from PMP nanofibers. The researchers tested the fabrics outdoors in daytime Arizona heat in simulated urban settings. When vertically oriented, the fabric kept skin 2.3 °C cooler than outdoor sports fabrics and 8.9 °C cooler than silk.

The design needs a very thin layer of silver nanowires, which could be replaced with aluminum, says Hsu, who as a graduate student at Stanford worked with Cui and Fan on the 2016 research on nanoporous PE films. Nonetheless, he admits that translating this laboratory prototype to clothing that people find acceptable might be a tall order. “You can verify the cooling impact in the laboratory, but if it doesn’t feel good, you can’t really make a tangible impact.”

Finding one radiative cooling fabric that works in various human settings could also be difficult. “Our skin temperature is generally static, but our environment is not,” Raman says. He feels a fabric based on the PMP-like “selective emissivity” might be a good compromise. “They kind of balance across multiple scenarios. They’re never going to be optimal in every scenario, but they will be pretty good. And better than what we have today.”

The cost factor

Another challenge is lowering cost. Andrew at UMass Amherst is wary of fluorinated polymers, which often lead to waste that persists in the environment, and complicated methods of making radiative cooling fabrics. She and her students want to make such fabrics for the masses using environmentally friendly materials.

They produced a coating made from cheap and safe materials that can be applied to any commodity textile (ACS Appl. Mater. Interfaces 2024, DOI: 10.1021/acsami.4c15984). The key ingredients are calcium carbonate and barium sulfate, which are used as white pigments in cooling paints. But when the team just painted those compounds on fabrics, they did a poor job of reflecting sunlight and transmitting body heat, Andrew says.

The group figured out that a mix of both types of particles ranging in size from 5–50 nm did the trick, reflecting the various wavelengths in sunlight and allowing body heat to escape. To coat fabrics, the researchers use chemical vapor deposition to apply a thin layer of poly(2-hydroxyethyl acrylate); they then dip it into pigment solutions. The final 10 µm thick coating gives textiles a matte finish appearance and can’t be felt, Andrew says.

In open-air tests on a sunny summer day, a coated polyester fabric kept simulated skin underneath it 8 °C cooler than ambient air. And in a parking lot, with pounding heat from the sun and black asphalt, the skin under the fabric stayed 3.4 °C cooler than the air. Andrew estimates that it should cost about $1 to apply the coating to a linear yard of fabric. “Most finishes applied on textiles produced today, such as waterproof or stain-repellent finishes, cost roughly 50 cents per linear yard. So our estimated cost is comparable.”

Radiative cooling fabrics and finishes could be used for things other than garments. The materials could also be incorporated into temporary shelters to provide respite from the brutal heat for the millions who live near the equator in Asia, Africa, and the Middle East.

This high-tech approach to cooling without using energy shows promise. But like many technologies, it will most likely first appear in specialty products such as athletic apparel before further materials discoveries and development reduce costs, Raman says.

“My cynical view is that it will end up being a high-end, niche product that Westerners wear when traveling in hot places or doing extreme athletics,” he says. “Perhaps over time it will filter down to broader markets. But the encouraging thing is that people are thinking about this. We have to plan ahead for 10–20 years from now, when we get more extreme heat waves.”