Personal heating products have always scared me. The idea of getting into a bed with an electric blanket appeals about as much as a romantic bath with my toaster. So when researchers propose a shirt that you plug in to the mains, my response is: yeah, nah.

But the research is quite interesting, made even better by the inclusion of a gratuitous polar bear picture. 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 off your left foot. Equally famously, polar bears seem to be able to cope with some rather 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 weavable thread.

Polar bear fur faces some tough challenges. Being insulating is not enough: it also needs to maintain that insulation when the bear's coat is soaked through after swimming from one ice floe to another. For fair-weather animals, this swimming would be deadly, because their insulation mostly relies on the air trapped between hairs. Since air is a poor heat conductor, trapped air provides remarkably good insulation. Unfortunately, a prolonged swim will remove all the air, replacing it with water. Water is a much better heat conductor, so body heat is sucked away from a sodden animal at a deadly rate in cold conditions.

Polar bear fur has evolved to maintain 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 coating to provide mechanical strength and a core with a set of pores that is aligned along the fiber, creating a kind of honeycomb structure that provides insulating air-pockets that are protected from water. Between the fur and a useful layer of fat, polar bear insulation is so good that, apparently, it doesn't show up on infrared cameras. That's pretty cool.

Silky polar bear cloth

To mimic polar bear hair and to turn it into a weavable 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 that was cooled. The idea is that the cold ring freezes the silk as it passes through. Depending on the temperature of the ring and the speed at which the fiber is drawn through the ring, however, the silk can start to freeze before it reaches the ring. So researchers tried to hit a sweet spot in balancing temperature and draw speed. If the ring is too cold, everything freezes in place and the pores will be oriented in random directions. If the ring is not cold enough, then the silk protein has time to close up and remove the pores entirely.

After a bit of searching, the researchers found a range of temperatures that produce pores that are aligned to the direction of the fiber. Unlike the polar bear hair that the researchers were imitating, this fiber does not have a thick outer coating. Even though aligning the pores increased the strength of the fiber by about a factor of two, it is still about 1,000 times weaker than commercial clothing fibers.

Nevertheless, the researchers spun the story out by weaving their new threads into a cloth. They put the fabric onto a heatable stage and measured the temperature difference between the top and bottom side of the fabric. A single layer of the fabric was able to introduce about a four-degree 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 kind of final demonstration, a rabbit was dressed in a single layer of the fabric. That single layer was almost sufficient to hide the rabbit from an infrared camera. The edges of the fabric showed some leakage, however, revealing a line image of the rabbit. But the fabric-covered body was the same as the background. This could be due to the nature of the test, or it could be that bending the fabric changes its thermal properties. (We shall never know since that was not tested and the edge leakage wasn't even mentioned.)

The researchers also tested the fabric's water resistance, but they did not test if the material's insulating properties were preserved when it was wet. Since that seems a pretty obvious test, the lack of reported results suggests that it probably isn't that good in the wet.

Time to cook

Because the insulating properties are so good, it makes sense to also be able to heat or cool the fabric. 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 thread. The researchers hooked a bit of cloth up to a 5V power supply, and it rapidly heated but did not catch fire—always a bonus for a wearable. Unfortunately, the current 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 quote research that suggests that nearly half the energy we use to heat our homes is to pander to our body's desire to be comfortably warm. I must admit that I can't think of another reason to heat a house, so I am still wondering what the other half is used for: maybe they separated heating and cooling? In any case, the researchers are suggesting that, if there was a light and comfortable fabric that could either keep us warm or be heated to warm us, there might be a substantial energy savings.

That may be correct, but I think human psychology plays a big role here. We already have a large portion of the population that would rather turn the heater up and walk around in a T-shirt than wear a long-sleeved shirt or sweater. I'm pretty sure that a new fabric will not change that.

And, even though I like the research, I really wish the researchers had reported their results in a more straightforward manner. For instance, they define the thermal conductivity of their insulating fabric, but they never actually report a value, so we can't compare it to existing materials. Similarly, their fabric is exposed to all sorts of different conditions, but it's only compared to existing fabrics twice. So, even though I think this might be a material with a future (assuming that it can be strengthened), there is simply not enough information to speculate.

Advanced Materials, DOI: 10.1002/adma.201706807 (About DOIs).