Silk has been modified through the addition of nanoparticles to reflect 95% of sunlight, which means the material stays extra cool on a hot day.
A fabric made of engineered silk keeps skin about 12.5°C cooler than cotton clothing and provides relief from hot weather. Approximately 15% of global electricity goes towards keeping us cool. To reduce this energy demand, scientists have been searching for passive ways of cooling us that don’t require electricity.
Jia Zhu at Nanjing University in China and Shanhui Fan at Stanford University and their colleagues were inspired by silk, which feels cool against the skin because it reflects most of the sunlight that strikes it – mainly the infrared and visible wavelengths – and also readily radiates heat.
They were able to engineer silk to block even more sunlight – about 95% – by embedding the fibres with aluminium oxide nanoparticles that reflect the ultraviolet wavelengths of sunlight.
When the researchers bathed this engineered silk in sunlight, they found that it stayed 3.5°C cooler than the surrounding air because of its ability to reflect most sunlight and radiate heat. It is the first fabric to be developed that stays colder than the surrounding air when in sunlight.
The researchers also found that when they draped the engineered silk over a surface designed to simulate skin, it kept the skin 8°C cooler under direct sunlight than natural silk did – and it kept the skin 12.5°C cooler than cotton did. The simulated skin was made of silicone rubber that was wrapped around a heater to mimic body warmth.
In the final part of their experiments, they made a collared long-sleeved shirt from the engineered silk and asked a volunteer to wear it while standing out in the sun on a 37°C day. Infrared images revealed that the shirt stayed cool. Similar infrared images captured of the volunteer wearing shirts made of natural silk or cotton showed that these fabrics warmed up. “Wearing the engineered silk on a hot day under sunlight, one feels much cooler than wearing normal textiles such as cotton,” says Zhu.
The engineered silk is comfortable to wear, with good breathability, and can be washed and dried repeatedly without falling apart, says Zhu. It is cost-effective to make and could be mass produced, he says.
The fabric is mainly designed to keep people cool when they are outdoors and exposed to sun, rather than in indoor settings like homes and office buildings, says Fan.