While we humans have known for a long time how to use fabrics to stay warm, designing practical, cooler clothing for summer can still be a challenge because of the way our own body heat is retained by traditional textiles. Now researchers at the American Association for the Advancement of Science have developed a cloth that reflects sunlight but also allows heat radiating from a person's body to escape.
Overheating has as much to do with output as input. In order to cool down, our bodies naturally give off heat in the form of infrared radiation, an invisible and harmless wavelength of energy. Blankets and clothes keep us warm by trapping these emissions close to the body. "Forty to 60% of our body heat is dissipated as infrared radiation when we are sitting in an office," says Shanhui Fan, a specialist in photonics — the study of visible and invisible light. "But until now there has been little or no research on designing the thermal radiation characteristics of textiles."
At normal skin temperature of 93.2F (34C), the wavelength range of the body's radiation partially overlaps with that of visible light. Essentially, this means fabrics that block the sun's rays — like those cool summer whites — can often also trap the most body heat. The new material works in a number of ways to prevent overheating. First, it cools by letting perspiration evaporate through the material. Although this is something ordinary fabrics do already, the new material provides a second cooling mechanism by also allowing the majority of body heat to pass through.
Yi Cui, along with Po-Chun Hsu and other colleagues, decided to look at nanoporous polyethylene (nanoPE), a material which has tiny interconnected pores that are 50 to 1000 nanometers in diameter. These minuscule holes are comparable in size to the wavelength of visible light, and so the researchers realized the material could scatter or reflect back most of the light that fell on it while the pores would still permit the body's natural radiation to pass through.
NanoPE is a material used in battery making with a specific structure that blocks visible light but which is transparent to infrared radiation. The researchers modified this base material by chemically treating it to enable water vapor molecules to evaporate through the pores in the plastic. This allows the plastic to breathe like a natural fiber, says Hsu. They then created a practical textile by layering the synthetic material with a cotton mesh to give it strength and thickness. This gave the researchers a single-sheet material that met their criteria for a cooling fabric.
When the team compared the artificial material to natural summer fibers, they discovered that, while cotton only permits about 1.5% of infrared waves through, a whopping 96% of the body's infrared radiation can pass through nanoPE. It was also found that, while regular polyethylene allows similar amounts of IR to escape, the nanoPE reflects significantly more visible light — 99% compared to only 20%.
The researchers used a special device to mimic the heat output of human skin and found that nanoPE only allows an increase in temperature beneath the clothing of around 1.4F, compared with 6.3F for cotton and 5.2F for traditional polyethylene. "Wearing anything traps some heat and makes the skin warmer," says Fan. "If dissipating thermal radiation were our only concern, then it would be best to wear nothing."
The researchers — whose work is published in Science — believe this new material, as well as keeping us cool outside, could reduce "energy costs associated with keeping body temperatures cool" — such as air conditioning. "If you can cool the person rather than the building where they work or live, that will save energy," said Cui. In addition, where normal polyethylene does not effectively "wick" away moisture, the researchers were able to enhance draining in the nanoPE by adopting a microneedle injection technique and by coating the material with a water-repellent agent.
The researchers are continuing their work, hoping to add more colors, textures and cloth-like characteristics to their material. Adapting a material already mass produced for the battery industry could make it easier to create products. "If you want to make a textile, you have to be able to make huge volumes inexpensively," Cui says. Fan believes that this research opens up new avenues of inquiry to cool or heat things, passively, without the use of outside energy, by tuning materials to dissipate or trap infrared radiation. The team suggests this new family of fabrics could have further applications in tents, vehicles and buildings.