It's a common enough experience: the weather is warm but you expect it to cool down later, so you don't know how much clothing you need. A new fabric could solve this problem. One side keeps you warm, while the other is cool. If clothing made of this material is shaped to be reversible, a quick trip to a changing room is all it would take to stay comfortable.
According to a paper in Science Advances, 40-60 percent of heat loss from the human body comes in the form of infrared radiation with a wavelength between 7 and 14 micrometers. We already know that ordinary fabrics coated with metallic nanowires reflect most of this radiation, making for very warm items of clothing.
On the other hand, nanoporous polyethylene (nanoPE) blocks visible light but is transparent at the wavelengths important for body heat.
As the paper notes, however; “It remains a grand challenge how to design a single textile to perform both heating and cooling functions.” Nevertheless, that is what the authors claim to have done. They embedded a layer of copper nanowires coated on one side with carbon inside a nanoPE textile. The carbon side of this bilayer radiates heat easily, while the copper holds it in. The nanoPE was also made thinner on the copper side.
When the material is placed so the copper is towards the skin, heat flows easily through the nanoPE to the bilayer emitter, where it's radiated to the surrounding environment. When the material is reversed, more heat is held in by the thicker nanoPE, and what does reach the emitter is less likely to escape.
When a layer of artificial skin at 31ºC (89ºF) was placed inside a 22ºC (72ºF) chamber, a traditional textile warmed the skin to 36.9ºC (98.4ºF). In cooling mode, the material kept the skin's temperature at 33.8ºC (92.8ºF), but when flipped for warming this temperature became 40.3ºC (104.5ºF).
As a result, someone equipped only with clothing made from the material tested here could be comfortable, without needing to turn on heating or air conditioning, in ambient temperatures from 13.9ºC (57ºF) to 24.3ºC (75.7ºF). That's a much wider range than we're used to getting from a single item of clothing – all it would take is to flip the clothing inside out when the temperature changed. Indoors, this could majorly reduce energy use for heating and cooling.
The authors note the nanoPE is hard to replace, but a range of materials could substitute for the bilayer, potentially tuning clothing for specific conditions.