Nanoporous polyethylene reflects visible sunlight, but allows heat to escape © Carla Schaffer/AAAS
Rumour has it an Indian summer is just around the corner, but if you’re looking for novel new ways to beat the heat, then researchers at Stanford University might have a cool solution for you – they have developed a fabric that will keep you a full three degrees cooler than regular cotton.
According to the team’s simulations, the new cooling fabric, which is a developed version of nanoporous ethylene (nanoPE), raises your skin temperature only 0.8°C above what it would be without any clothing at all, compared with a 2.9°C rise for existing polyethylenes, and 3.5°C for cotton.
Normally, your body emits 40 to 60 per cent of its heat as radiation in the infra-red (IR) and visible light spectra. The material clothes are made of stops heat from escaping by blocking this radiation, which is great when you’re snuggled under a blanket, but is what makes you uncomfortable in hotter climates.
The new fabric has a network of interconnected pores that let 96 per cent of IR through, huge compared to cotton’s measly 1.5 per cent, but it still reflects and scatters visible light, making it, thankfully, appear opaque, as Shanhui Fan, a professor of electrical engineering points out: "Wearing anything traps some heat and makes the skin warmer. If dissipating thermal radiation were our only concern, then it would be best to wear nothing." The secret is in the diameter of the pores, which at 50 to 1,000 nanometres is about the same size as a visible light wave but much smaller than IR.
Strangely, the team started with a type of polyethylene usually used in batteries. The material was already opaque to visible light, but allowed IR to pass through, so heat could escape. After some chemical treatment, the material could also let water vapour through, so other cooling mechanisms like sweating work just as well.
Since the starting material is already widely used, the fabric is already low cost and easy to manufacture. Furthermore, as Yi Cui, an associate professor of materials science, engineering and photon science at Stanford explains: "If you can cool the person rather than the building where they work or live, that will save energy.”
The study is available in full in the journal Science.