If a room is cold, you have a choice. Pull on a jumper or a jacket, or turn up the heating. If it is hot, the obverse choice is not so easy to make. There is a limit to how much disrobing is permissible, and even the wearing of light garments such as T-shirts, in order to stay cool, is frowned on in some business circles. The default is therefore to switch on the air conditioning.
That may change if a discovery published this week in Science comes to commercial fruition. Yi Cui and his colleagues at Stanford University have discovered a fabric that keeps skin 2°C cooler than a cotton T-shirt. In terms of comfort, this is a significant drop—and one which would be good not only for the wearer but also for energy bills. If widely adopted it would mean buildings could be kept warmer than at present, saving huge amounts of electricity in the summer months.
Cui's goal was to cool the wearers of clothing by tinkering with the way heat radiates from their bodies. More than half of body heat is in the infrared (IR) part of the spectrum. This means its wavelength is longer than that of visible light. Materials like polyethylene are transparent to both, making them useless for weaving into clothing for anyone other than exhibitionists—though such clothes would dissipate IR effectively.
Conversely, wool, cotton, silk and so on are transparent to neither part of the spectrum, so retain IR even as they preserve the wearer's modesty. In what seems to have been a classic "Eureka!" moment, however, Dr Cui realised the answer was staring him in the face.
Borrowing from batteries
His day job is as a battery researcher. One material commonly used in modern batteries is called nanoPE. It is a species of polyethylene sheet perforated by pores 50-1,000 nanometres (billionths of a metre) across. These pores are there to regulate the passage of ions within a battery, but they are also of exactly the right dimensions to make nanoPE opaque to visible light. They do not, however, affect the infrared part of the spectrum, meaning nanoPE blocks less than 10% of IR incident upon it. Cotton, by contrast, blocks more than 95% of IR.
Straight out of the factory, nanoPE resembles a plastic sheet and is, unsurprisingly, not that comfortable to wear. To overcome this, the team made three improvements. First, they punctured it at regular intervals using a tiny needle, to let air move in and out. Second, they added a substance called polydopamine, which made the plastic more hydrophilic.
This meant that instead of repelling sweat and causing it to accumulate on the skin, the modified nanoPE absorbed perspiration and wicked it to the fabric's outer surface, whence it evaporated. Third, to improve the material's mechanical properties, the final product consisted of two nanoPE sheets sandwiched above and below a widely spaced cotton mesh. Better and better
The team then tested how the added bells and whistles affected nanoPE's performance. In a room kept at 23.5°C they found that the temperature of bare skin was 33.5°C. Skin covered by cotton warmed to 37°C, while skin covered with nanoPE alone clocked in at 34.3°C. The variant with perforations, mesh and doping did not do quite as well as this, but it still reduced skin temperature by 2°C compared with cotton, to 35°C.
Fashionistas are not, it is true, likely to be rushing out to buy garments made of battery membrane, however cleverly it has been treated. But what Dr Cui's work does do is create a new way of thinking about cooling the body. Manipulating its emission spectrum is a clever idea. It clearly works. So the search should be on for materials that do it better and more comfortably than nanoPE can manage.
Not that such considerations will apply to all customers. Besides being worn indoors, cool fabric of the sort Cui has made will have many outdoor applications. There is, after all, no air-conditioning in the desert. And those who must work there—soldiers, for example—are less likely than the office-bound to worry about sartorial niceties.
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