New textile goes from warm to cool with a flip
Reversible clothing isn’t just for style anymore. A newly developed fabric can heat or cool the body depending on which side is facing out. It could lead to clothing that the wearer would simply turn inside out to regulate their temperature. By helping users adapt to large temperature fluctuations, the new climate-control textile could reduce the use of energy needed to heat and cool buildings, improve productivity, and personal health, says Yi Cui, a materials science and engineering professor at Stanford University, one of the developers of the fabric.
Textiles that cool or warm the body are already on the market. Warming fabrics like thermal blankets use thin metallic layers to reflect body heat back toward the body. Athletic wear that cools the user do so by wicking away moisture or by changing the material’s microstructure and open its pores. But there are no fabrics that can both heat and cool.
Two years ago, a team led by Cui developed a polyethylene fabric that could cool the wearer by passing the body’s infrared heat radiation through its nanometer-sized pores.
The team’s new dual-function fabric, reported in Science Advances, builds on earlier developed material. At its heart is a two-layer structure: one side emits a large amount of heat while the other side is a low heat emitter. The high-emitting side is a 9-µm-thick layer of carbon black, which is efficient at absorbing the body’s mid-infrared radiation. The coating is also rough and highly porous, which makes it good at releasing heat and permeable to air and vapor. The low-emissivity side is a 150 nm copper layer that is shiny and smooth so it reflects heat.
The researchers use a blade to coat carbon black slurry on a sheet of their nanoporous polyethylene, and sputter copper on another sheet. Then they press the carbon and copper sides together to form the bilayer emitter sandwiched between the polyethylene fabric. The polyethylene is 24nm-thick on the carbon side and 12nm-thick on the copper side. This thickness controls how far the bilayer emitter is from skin.
In cooling mode, the carbon side faces out and the emitter is closer to the skin. The thinner polyethylene layer transfers body heat to the emitter, which ejects it into the environment. In heating mode, the copper side faces out. The thicker polyethylene layer increases the distance between the skin and the emitter, reducing the heat discharged into the environment.
Laboratory tests on artificial skin showed that the textile could make it 6.5°C warmer or cooler than a normal textile would. “Radiative heat transfer between the human body and ambient air is very linear,” Cui says, “so the heat transfer coefficients do not change with ambient temperature.” In other words, the heating and cooling effect would work in freezing temperatures to sweltering heat.
Radiative cooling fabrics are hard to realize, says Svetlana Boriskina, a research scientist in the Mechanical Engineering Department at the Massachusetts Institute of Technology. Fabrics that can both heat and cool could be useful in situations when a person can be exposed to different types of environmental conditions, such as is the case for military personnel, park rangers, fishermen, campers, and backpackers. The fabrics could also find use in medical or emergency response. “If these fabrics can be made comfortable and visually appealing, they could potentially find a wide range of consumers,” Boriskina says.
Read the article in Science Advances.