Hot, hot, hot? New fabric could help you stay cool
This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.
When it’s cold outside, it’s pretty easy to put on more clothes to stay warm. But when it’s hot, there are only so many layers of clothing you can peel off. In most places, after all, it’s not polite to run around naked! Now, researchers have developed a new type of fabric that holds in very little body heat. That could keep the wearer cooler than is possible with other fabrics.
Clothes help warm the body. And they do it several ways. The first is by preventing a process known as convection. Here, fabric traps a layer of air next to the skin. Once warmed, this air cuts the flow of heat out of the body. If the clothes weren’t there, any air the body warmed would drift away, via convection. Cooler air would move in to take its place. That new air would now pick up more body heat and carry it away, too.
A second way that clothes keep us warm is by blocking the release of body heat through a process known as radiation. Like light, heat is a form of electromagnetic radiation. In winter, blocking the loss of body heat — and thus staying warm — is a good thing. But in summer, when overheating can be a problem, it would be nice to let heat leave the body, says Yi Cui. He’s a materials scientist at Stanford University in Palo Alto, Calif.
So Cui’s team came up with a fabric that’s transparent to body heat. It lets that energy just radiate away.
The researchers described their new fabric September 2 in Science.
Making a fabric largely invisible to body heat
Because its energy is lower than that of visible light, heat is known as infrared radiation. (The prefix infra, from Latin, means “below.”) This radiation also has longer wavelengths than visible light. (Wavelength is the distance from one peak of a wave to the next.) And those differences in wavelength were key to designing a fabric that would block visible light but not stand in the way of body heat, explains Cui.
When radiation (such as heat or light) reaches an object, it can bounce off or travel around it. Which happens will depend largely on two things: the size of the object and the wavelength of the radiation. If a wave is the same size as an object or smaller, it usually reflects off of the object. That’s why the sky is blue. Wavelengths of blue light are about the same size as gas molecules in the air. So, they hit the molecules and scatter in all directions. Other colors, including red light, have longer wavelengths. They pass right around the gas molecules.
When designing their new fabric, Cui and his team started with a common plastic. It’s known as polyethylene (PAHL-ee-ETH-ul-een). People turn millions of tons of it into bottles and packaging each year. A thin flexible film of it can also be found in batteries, notes Cui. (The type used in batteries is very similar to what’s used in grocery bags.) This plastic is cheap, costing only about $2 per square meter (about 10.5 square feet).
But the new material’s great advantage is its structure. Unlike the version of the plastic used in bottles and many bags, the new version includes tiny fused bubbles, or nanopores. Those bubbles — each around a billionth of a meter in diameter — are interconnected and let fluids (like air) flow from one side to another. Overall, when seen up close, the material looks like a very thin, very tiny version of a kitchen sponge.
Cui and his team modified the material to make its bubbles a particular size. If most are between 50 and 1,000 nanometers (or up to 39 millionths of an inch) across, they’ll scatter and block visible light. (The wavelengths of visible light vary between about 400 nanometers — red, and 700 nanometers — blue.) But infrared wavelengths are larger than the material’s nanopores. So heat will radiate right around the tiny bubbles.
For infrared wavelengths above 2,000 nanometers, more than 90 percent of this heat will pass through.
The team came up with a way to test how well heat (that infrared radiation) will pass through the new fabric. When wearing a cotton shirt, a man’s skin temperature would climb about 3.5° Celsius (6.3° Fahrenheit) higher than if he wore no shirt at all. But in a shirt made from the team’s new fabric, his skin would feel only 0.8 °C (about 1.4 °F) warmer than if he went shirtless. (Of course, they never made such a shirt. They didn't make enough of the new fabric.)
That’s a big temperature difference, says Cui. And it would likely make a big difference in household energy bills too. Someone who feels cool doesn’t need to set the thermostat for the air conditioning too low. And that would save money. For every 1 °C (1.8 °F) warmer the air-conditioning’s thermostat is set, residents will use about 10 percent less household energy, studies have shown.
And that’s a big deal. “Air conditioning is one of the biggest energy costs in the nation,” says Ping Liu. He’s a chemist at the University of California, San Diego. Reducing those bills would save people lots of money. It also would reduce the amount of electricity that power plants would need to provide. Many of those power plants generate electricity by burning coal or other fossil fuels. That burning emits carbon dioxide gas, a planet-warming greenhouse gas.
Overall, Cui notes, heating and cooling buildings accounts for about one-eighth of all energy used in the United States.
His team’s new fabric, for now, has only been tested in the lab. It hasn’t been made in large quantities. But it isn’t the only type of fabric that could let lots of body heat escape, notes Svetlana Boriskina. She’s a physicist at the Massachusetts Institute of Technology in Cambridge.
Last year, her team came up a different approach. This group, too, would use polyethylene. But it would start with bubble-free fibers of the plastic that were 1 micrometer across (about one-seventeenth the width of the finest human hair). Then, they would wind these fibers together to make strands of yarn about 30 micrometers across. Their fabric would be woven from this yarn.
Boriskina and her teammates described their fabric last year in ACS Photonics. They haven’t made any of this cool fabric yet. But their computer analyses suggest such a fabric would be even more transparent to heat than the one Cui’s group has designed. A mere 3 percent of the body heat would be retained by the material, her team estimates. So wearing it might keep people even somewhat cooler than the Stanford fabric.
Polyethylene is normally clear, says Boriskina. But it’s possible to add pigments or dyes to the material to give it color. Because those colors are inside the material, it would make them less likely to rub off and fade, she notes. But researchers would need to be very careful, she adds, to select pigments that didn’t block infrared radiation.
(for more about Power Words, click here)
carbon dioxide (or CO2 ) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
convection The rising and falling of material in a fluid or gas due to uneven temperatures. This process occurs in the outer layers of some stars.
electromagnetic radiation Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.
fabric Any flexible material that is woven, knitted or can be fused into a sheet by heat.
fiber Something whose shape resembles a thread or filament of some kind. (in nutrition) Components of many fibrous plant-based foods. These so-called non-digestible fiber tends to come from cellulose, lignin, and pectin — all plant constituents that resist breakdown by the body’s digestive enzymes.
fossil fuel Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed in the Earth over millions of years from the decayed remains of bacteria, plants or animals.
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
micrometer (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nano A prefix indicating a billionth. In the metric system of measurements, it’s often used as an abbreviation to refer to objects that are a billionth of a meter long or in diameter.
online A term that refers to things that can be found or done on the Internet.
photonics Technology and research on the properties and transmission of light particles, called photons.
physicist A scientist who studies the nature and properties of matter and energy.
pigment A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
plastic Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.
polyethylene A plastic made from chemicals that have been refined (produced from) crude oil and/or natural gas. The most common plastic in the world, it is flexible and tough. It also can resist radiation.
power plant An industrial facility for generating electricity.
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
thermostat A temperature sensor that allows a system to know when a change — either heating or cooling — is called for.
transparent Allowing light to pass through so that objects behind can be distinctly seen.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Journal: P.-C.Hsu et al. Radiative body heat cooling by nanoporous polyethylene textile. Science. Vol. 353, September 2, 2016, p. 1019. doi: 10.1126/science.aaf5471.
Journal: J.K. Tong et al. Infrared-transparent visible-opaque fabrics for wearable personal thermal management. ACS Photonics. Vol. 2, June 17, 2015, p. 769. doi: 10.1021/acsphotonics.5b00140.