When bitter, cold winds whip around outside, it’s best to dress in layers to stay warm. But when you have to go out into the sweltering heat, social norms put a hard limit on the opposite strategy: You still have to wear one layer of clothing.
A team of researchers at Stanford University may now have a strategy for keeping us cool with that one layer. They report a nanoporous fabric that could help our bodies cool themselves by allowing body heat to escape (Science 2016, DOI:10.1126/science.aaf5471). The researchers think that clothing made from such fabric could reduce energy use for air conditioning.
Our bodies naturally release heat, in part, through emission of mid-infrared radiation in the 7- to 14-µm wavelength range. To help our bodies cool, Yi Cui and his Stanford colleagues wanted to design fabric that would allow that IR radiation to pass through. Basically, they were looking for a material that was transparent to mid-IR radiation.
Unfortunately, many of the fabrics we wear, such as cotton, have chemical groups that absorb radiation in that range. One material that is mid-IR transparent is polyethylene. But it’s also transparent to visible light—not such a desirable property for clothing.
The Stanford team reasoned that if the polyethylene had pores of the right size—between 50 and 1,000 nm in diameter—it would scatter visible light, making it opaque, but continue to allow mid-IR to pass through. Fortunately for them, such a material already exists. Battery makers use nanoporous polyethylene as a separator in lithium-ion batteries.
Cui and coworkers tested the material on a device that simulates how skin radiates heat. Compared with a piece of cotton, the nanoporous polyethylene kept the “skin” 2.7 °C cooler.
“That may not sound like much, but in terms of energy savings it actually could be huge,” says Svetlana V. Boriskina of Massachusetts Institute of Technology, who wrote a perspective on the work in Science. She points out that setting a building’s or home’s thermostat just a few degrees higher can cut energy use by up to 45%.
The Stanford researchers processed the nanoporous polyethylene in a few ways to give it more appealing, fabriclike properties. For example, they coated it with polydopamine to allow it to wick away moisture.
Boriskina says the team has demonstrated a multifunctional fabric, but that there are still a couple issues to work out, including testing the material for comfort and durability, as well as finding ways to color the material with dyes that don’t block mid-IR radiation.