Heating and cooling is the largest energy expenditure for most homes in the United States, making up some 40% of home energy use. So, researchers have begun searching for other ways to control the mercury—like clothing. The warming part is easy: Design insulating materials that better trap the body’s own heat. Cooling has been tougher, because textiles normally trap heat next to the skin. One recent strategy is to create materials that are transparent to infrared radiation (IR) produced by the body—which accounts for between 40% and 60% of lost body heat—thereby allowing it to pass through to the outside air.
Now, Yi Cui, a materials scientist at Stanford University in Palo Alto, California, and colleagues have combined the insulating and IR approaches in a layered fabric that heats or cools depending on which side faces the wearer. The bulk of the 45-micron-thick material is made from polyethylene (PE), an inexpensive plastic, which in this case is shot through with nanometer-sized holes. This nano-PE blocks visible light, so you can’t see through it as you might polyethylene-based plastic wrap. But it allows the body’s IR to pass right through. Sandwiched inside is a two-part material: nano-PE that is coated on one side with IR-absorbing (and emitting) black carbon, and on the other side an ultrathin layer of copper that only weakly absorbs and emits IR.
Crucially, this double layer doesn’t sit right in the middle of the fabric, but closer to one side than the other. When the double layer is nearer the skin, the carbon layer is on the outside. As it absorbs body heat, it reradiates it out into the surrounding air, cooling the wearer; the copper layer on the inside blocks it from radiating back toward the skin. But if the garment is turned inside out, the double layer is farther from the body. That prevents the carbon layer from absorbing as much heat from the skin. And whatever heat is absorbed and reradiated is reflected back toward the skin by the copper layer that now sits toward the outside, effectively trapping body heat.
The technology works so well that it can create a 6.5°C temperature difference depending on which way it is worn, Cui and his colleagues report today in Science Advances. That could be enough to set the thermostat a few degrees cooler in the winter and warmer in the summer, yet still have everyone in the building be comfortable. What is more, computer modeling suggests that further improvements should be able to more than double this temperature swing.
“It’s an exciting paper,” says Evelyn Wang, a mechanical engineer at the Massachusetts Institute of Technology in Cambridge. “I think it’s an elegant concept and pretty simple.” Unlike some other heat-regulating technologies, this material relies on a passive heating and cooling effect, which means it doesn’t need outside power to do the job, which should make it easier to produce and maintain.
Cui and his colleagues have already begun efforts to commercialize the technology and plan to form a company to bring it to market in the next few years. The carbon and copper layers are so thin that, like the polyethylene, they should be cheap, Cui says. But the future fabric will have to leap other practical hurdles, such as proving to be durable and stable after washing.