Personal heating products have always scared me. The idea of getting into a bed with an electric blanket appeals to me about as much as a romantic bath with my toaster. So when researchers suggest a shirt you plug in, my answer is yes, no.
But the research is quite interesting, made even better by the inclusion of a free polar bear photo. Polar bears, according to Chris’ compendium of tru facs (patent pending), are most famous for being the animal least likely to express regret after accidentally chewing your left foot. Equally famously, polar bears seem to be able to handle fairly cold weather. It is this latter fact that has caught the attention of researchers who have replicated the insulating properties of polar bear fur in a weaveable thread.
Polar bear fur faces some tough challenges. Insulation isn’t enough: it needs to maintain that insulation even if the bear’s fur is soaked from swimming from one ice floe to another. For fair-weather animals, this swim would be deadly, as their insulation largely relies on the air trapped between their hairs. Because air is a poor conductor of heat, trapped air provides remarkably good insulation. Unfortunately, a prolonged swim will remove all air and replace it with water. Water is a much better conductor of heat, so body heat is sucked away from a soaked animal at deadly speed in cold conditions.
Polar bear fur has evolved to retain some of its insulating properties, even when soaked, by hollowing out the interior of the fiber. So a cross-section of polar bear hair would show it has a thick outer layer to provide mechanical strength and a core with a series of pores aligned along the fiber, creating a sort of honeycomb structure that creates insulating air pockets that are protected from water. Between the fur and a useful layer of fat, the polar bear’s insulation is so good that it apparently cannot be seen on infrared cameras. That’s pretty cool.
Silky Polar Bear Cloth
To mimic polar bear hair and turn it into a weaveable thread, the researchers turned to another natural fiber: silk. Instead of using the silk fiber directly, the silk was purified and dissolved in a liquid. The solution was then spun and extruded through a ring which was cooled. The idea is that the cold ring freezes the silk as it passes through it. However, depending on the temperature of the ring and the rate at which the fiber is drawn through the ring, the silk may freeze before reaching the ring. So researchers tried to find a sweet spot in balancing temperature and pulling speed. If the ring is too cold, everything will freeze in place and the pores will be oriented in random directions. If the ring is not cold enough, the silk protein has time to close and clear the pores completely.
After a bit of searching, the researchers found a range of temperatures that produce pores aligned with the direction of the fiber. Unlike the polar bear hair that the researchers imitated, this fiber does not have a thick outer layer. Although aligning the pores increased the fiber’s strength by about a factor of two, it is still about 1,000 times weaker than commercial clothing fibers.
Still, the researchers spun out the story by weaving their new threads into a cloth. They placed the fabric on a heatable stage and measured the temperature difference between the top and bottom of the fabric. A single layer of the fabric was able to introduce about four degrees of temperature difference when the stage was cooled to -20 degrees Celsius, while five layers increased the temperature difference to about 14 degrees Celsius.
As a sort of final demonstration, a rabbit was clothed in a single layer of cloth. That single layer was almost enough to hide the rabbit from an infrared camera. However, the edges of the fabric showed some leakage, revealing a line image of the rabbit. But the cloth-covered body was the same as the background. This may be due to the nature of the test, or it may be that bending the fabric changes its thermal properties. (We’ll never know because that hasn’t been tested and the edge leakage wasn’t even mentioned.)
The researchers also tested the fabric’s water resistance, but they didn’t test whether the material’s insulating properties were maintained when wet. Since that seems like a pretty obvious test, the lack of reported results suggests it probably isn’t as good on wet surfaces.
Time to cook
Because the insulating properties are so good, it makes sense to be able to heat or cool the fabric as well. To do this, the researchers added carbon nanotubes, which are conductors, to the mixture. The tubes ended up embedded in the fiber without destroying the aligned pore structure of the wire. The researchers plugged a piece of cloth into a 5V power supply and it quickly heated up but didn’t catch fire – always a bonus for a wearable. Unfortunately the power was not reported so I have no idea how much power was used to heat the fabric.
The researchers have an interesting take on heatable clothing. They cite research suggesting that nearly half of the energy we use to heat our homes is to meet our bodies’ desire to be comfortably warm. I have to admit that I can’t think of any other reason to heat a house, so I’m still wondering what the other half is used for: maybe they separated heating and cooling? Regardless, the researchers suggest that if there were a light and comfortable fabric that could keep us warm or be heated to keep us warm, there could be significant energy savings.
That may be right, but I think human psychology plays a big role here. We already have a large part of the population that would rather turn up the heat and walk around in a T-shirt than wear a shirt or sweater with long sleeves. I’m pretty sure a new fabric won’t change that.
And while I like the research, I really wish the researchers had reported their results in a more straightforward way. For example, they determine the thermal conductivity of their insulating material, but they never report a value, so we can’t compare it to existing materials. Likewise, their fabric is exposed to all sorts of different conditions, but it’s only twice compared to existing fabrics. So even though I think this material could be future-proof (assuming it can be amplified), there’s just not enough information to speculate.
Advanced materialsDOI: 10.1002/adma.201706807 (About DOIs).