Scientists have successfully used nanotechnology to produce a form of silk that stays cooler than the surrounding environment when exposed to direct sunlight.
“When you place a piece of cotton or native silk under the sun, it absorbs light and becomes hotter than the surrounding environment. Our material, instead, gets 3.5℃ cooler than ambient air,” says Shanhui Fan, a professor at Stanford University and a senior author on the paper published this week in Nature Nanotechnology. “No other wearable material can currently do that.”
This sounds very much like that scene from Back to the Future when Marty McFly gets soaked, but his self-drying jacket kicks in and saves the day. But how did the researchers achieve such an amazing result?
They dipped the silk in a chemical bath that contained aluminium oxide nanoparticles, tiny balls of material with diameters a million times smaller than a human hair. Aluminium oxide is a mineral found in many common pottery items, and the scientists chose it because it reflects UV light, which is the component of solar radiation that causes our skin to burn but also causes a heating effect in surfaces that absorb it. Adding aluminium oxide selectively enhances the reflective properties of silk against UV but, being otherwise transparent, does not affect the colour of the silk, or its performance as a smooth and cool textile.
To make the nanoparticles stick to the silk fibres, the researchers used a chemical bonding agent. “To see how well it worked, we performed many washing machine cycles, and confirmed that the nanoparticles did not get washed away from the silk,” says Fan. This is an important test to prove that the cooling effect of the textile does not wear off with use and that nanomaterials are not released into the environment with unpredictable impacts.
The researchers tested the cooling properties of the nanoprocessed silk in two ways. “In the first test, we simply put a piece of the silk out in the sun and compared its temperature to that of the surrounding air. In that case, we found that the silk was 3.5℃ cooler than the air,” explains Fan.
Then, they compared the performance of the nanoprocessed silk to normal silk by placing both textiles onto a piece of artificial skin under direct sunlight. Artificial skin is just a patch of heated material that simulates the way human skin interacts with light and heat. When they measured the temperature of the silks, they found an 8℃ difference between them. This is because not only the nanoprocessed silk reflects sunlight better than normal silk, but it also allows through more of the radiative heat coming from the skin, and so it keeps itself cooler.
“We have already made a t-shirt-sized patch of this new special material,” says Fan, who sees this new nanotechnology as a way to help people cope with intense heat when walking outside in the sun, and as a more efficient way to keep our bodies cool compared to air conditioning an entire room.
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