Recently, there has been a great deal of interest in creating smart fabrics to facilitate more integrated wearable tech. One of the hurdles developers have come across is how to power a wearable in a way that effectively manages heat. This is an extraordinarily important facet to wearable development because overheating wearables can be uniquely dangerous as they're, by definition, worn by the consumer on their body.
Even when wearable tech doesn't overheat, the tiny amounts of heat created by power systems can easily get trapped close to the skin by fabrics like cotton. Some researchers are handling this issue by creating tech powered by body heat, itself.
Other wearable developers, however, may be interested in this new material from Stanford.
The Challenge of Body Heat
According to Shanhui Fan, a professor of electrical engineering at Stanford University, 40-60% of human body heat is released through infrared radiations when at rest. That is why when we bundle up under a blanket, we get warmer. The blanket traps the infrared radiations around our body. The same radiations make us visible in the dark through night-vision goggles.
Unfortunately, the infrared radiations of the body, which are in the range of 7- to 14-µm wavelength, are absorbed by the traditional materials such as cotton. Therefore, these materials are not desirable to keep the body cool.
The study shows that the material the Stanford team developed can keep the person nearly 4 degrees Fahrenheit cooler compared to cotton clothing.
Fan notes that there are a few studies about designing the thermal radiation properties of textiles and that this invention can be a good start to designing our clothes more efficiently.
The Evolution of the Cooling Material
The basic idea of the cooling material is to design a material which allows infrared radiation. The first way to achieve this goal is a clear material. However, a clear material will not be a modest option for clothing! Therefore, the new textile needs to be transparent to infrared radiations while being opaque to the visible light.
With this set of parameters before them, researchers had to resort to a combination of nanotechnology, photonics, and chemistry in order to engineer a new material. They realized that a special form of a well-known clear material, polyethylene—the clingy plastic used as kitchen wrap—could lead to an acceptable solution.
The ordinary polyethylene food wrap is already transparent to the infrared radiations; however, it is see-through and impervious to water vapor and air. Yi Cui, a materials scientist at Stanford University, noticed that a special form of the plastic wrap, nanoporous polyethylene, lets infrared radiation pass through whereas it is opaque to visible light. Nanoporous polyethylene, or "nanoPE" for short, is widely used in lithium-ion batteries to separate the positive and negative sides and prevent electrical shorting.
The nanosized pores in a piece of polyethylene. Image courtesy of Chemical & Engineering News.
Prabhakar Bandaru, a professor of nanoengineering at the University of California-San Diego, calls the research team’s choice of material innovative and definitely fascinating. Although nanoPE is a widely-used and readily-available material, it is a non-fabric material and it took the research team a long time to think about the possibilities of nanoPE as a textile. After all, the people working in the textile industry almost never overlap with the people working on batteries.
Fortunately, Cui’s research is on batteries and, two years ago, he decided to save energy by reducing AC and designing thermally-efficient materials. After considering many options, it occurred to him that nanoPE might be the right choice.
The specific nanostructure of the new material uses pores of the right size—between 50 and 1,000 nm in diameter—to scatter visible light but let the infrared radiations of the body through.
Since polyethylene traps moisture, the research team had to modify it. To this end, they applied a chemical called polydopamine and made it permeable to water, like cotton is. They also punched holes in the material to make it more breathable.
The modified nanoPE can now cool the body via the natural mechanism of sweat evaporating into the surrounding air. Moreover, in order to make the product stronger and thicker, researchers created a three-ply version and stuck one sheet of cotton mesh between layers of nanoPE.
The nanoPE textile. Image courtesy of Yi Cui Group/Stanford University.
To experimentally verify the cooling potential of the material, researchers compared the three-ply version of the material with a cotton fabric of comparable thickness. They used a small swatch of each material to cover a device which simulates how skin radiates heat. The comparison showed that experimenting with the cotton fabric, the temperature will be 3.6 degrees F warmer than the case in which the new material was used.
By cooling the person passively, the new clothing can increase the setpoint of the air conditioning system. Svetlana V. Boriskina of Massachusetts Institute of Technology points out that increasing the home’s thermostat during warm months by just a few degrees may not sound like much but, in terms of energy savings, it could be huge. She expects that the new material can cut the energy use by up to 45%.
Cui insists that the new material feels very much like normal fabric and hopes that it will be commercialized within two years. The team is further processing the material to give it more appealing and fabric-like properties. For example, they will produce it in different colors and textures.
A Connection to Saharan Silver Ants
It is interesting to note that the Saharan silver ant employs a similar mechanism to survive in the sweltering heat of the desert. These ants are covered on the top and sides of their bodies with very tiny hairs.
The unique shape of these hairs keeps the ant cooler in two ways. Firstly, the tiny hairs are highly reflective for the visible light. Scattering the sunlight, the hair coat prevents the heat of the sunlight from being absorbed. Secondly, the hairs are highly emissive for the infrared wavelengths. Acting as an antireflection layer, the hairs allow the insect to emit its body heat away.
This latter method of passive cooling is similar to what the new material employs, allowing body-generated heat to escape away from the skin.
The Saharan silver ant. Image courtesy of Bjørn Christian Tørrissen [CC BY-SA 3.0]
The Stanford team had previously published a paper about employing metallic nanowires to produce clothes which can trap the infrared radiations and keep the wearer warmer than the clothes woven from traditional materials. However, according to Cui, cooling is much harder than warming. That's a lesson electronics developers know well.
The new fabric and its cooling properties are discussed in the journal Science.