The hotter the world gets, the more people will turn up the air-conditioning. But cooling air requires energy, which generates greenhouse gas emissions. In the U.S., air conditioners contribute more than 100 million metric tons of carbon dioxide to the atmosphere every year.
Yi Cui, a professor of materials science and engineering at Stanford University, wants to cool people with clothing instead. Even lightweight clothes made of cotton fabric absorb infrared radiation emitted by our bodies, trapping heat. Cui and his team found that a material called nanoporous polyethylene, or nanoPE, which is used in lithium-ion batteries, lets that radiation escape. And unlike high-tech exercise clothes, which depend on perspiration to keep their wearers cool, nanoPE works with no sweat required.
NanoPE, which is roughly the same price as cotton fabric, comes in thin sheets perforated with interconnected pores ranging from 50 to 1,000 nanometers wide. Pores this size let infrared radiation escape while scattering visible light, making the material opaque. (Regular polyethylene is transparent, which is an obvious drawback for a clothing material.) A sheet of nanoPE looks like a flimsy piece of plastic—not a natural clothing material. Cui's team turned it into a passable textile by coating it with a chemical that wicks away water, sandwiching a layer of cotton mesh between two sheets of nanoPE and poking tiny holes in the fabric with a microneedle so that air could flow through it more easily. After these adjustments, Cui found that nanoPE cooled simulated human skin by two degrees Celsius more than cotton. The group reported its findings this past September in Science.
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“If you wear nanoPE, as long as the external temperature is lower than your body temp, you'll feel cooler,” Cui explains. On hot days you might still want to run the AC, but you will be able to turn the thermostat up—and research shows that raising thermostat set points by just a few degrees can cut energy use by almost half.
The team still needs to test the durability, comfort and cooling effect of nanoPE on real human skin, and the researchers have yet to determine how dyes will change its performance. If the material passes these tests, Cui envisions its being used in uniforms and scrubs for workers in factories and hospitals.