It鈥檚 a bunny in bear鈥檚 clothing. By wrapping a rabbit up in a new textile inspired by polar bear hair, researchers have been able to keep all the animal鈥檚 body heat in, keeping it both cosy and invisible to infrared imaging.
The hollow interior structure of a polar bear鈥檚 fur helps the bears insulate themselves against harsh Arctic winters. Each hair is about 200 micrometre across, full of pores 15 to 20 micrometres wide. It is harder for heat to move through air than solid materials, so these pores make sure that the polar bear retains its body heat instead of radiating it away into the frigid air.
Porous structures like this often snap easily, but polar bear fur doesn鈥檛 suffer this problem because all of the elongated pores are aligned in layers in the same direction as the hair. This gives the hairs mechanical strength that they wouldn鈥檛 have if the pores were spread about randomly.
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at Zhejiang University in Hangzhou,聽China, and her colleagues processed silk to create a textile that mimics the pores of polar bear fur. They dissolved the silk in water and dripped a constant stream of the solution through a cold copper ring.
When the liquid silk 鈥渢hread鈥� passes through the ring, ice crystals grow within the thread in layers similar to the layers of pores in polar bear hairs. The frozen fibre is then freeze-dried to leave gaps where the ice was, resulting in well-aligned pores that fill up to 87 per cent of the silk fibres鈥� volume with air. The researchers can then weave the fibres together into a textile.
Cosy and invisible
They found that this new fabric changed temperature quite slowly when a thin layer was subjected to varying temperatures. A layer about 0.4 millimetres was subjected to temperatures between -22掳C and 80掳C. The fabric only reached temperatures between -10掳C and 64掳C, indicating good thermal insulation.
This slow rate of temperature change means that the textile is mostly reflecting heat rather than absorbing it. Like polar bear fur, it also reflects infrared light, making it practically invisible in those wavelengths. So, anyone wearing this fabric would be hard to see with night-vision goggles.
The researchers tested this property by wrapping a rabbit in a layer of their tactical silk. The rabbit became almost invisible to an infrared camera at temperatures ranging from -10掳C and 40掳C. It reflected the light up to twice as well as other commercial textiles.
As the outside of the fabric reflects the background heat and infrared light back into the environment, the inside reflects the rabbit鈥檚 body heat, keeping it warm. Plus, the fabric is soft and breathable, and it could someday be incorporated into clothing for humans.
Advanced Materials
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