One of the main reasons we should all be proponents of preserving biodiversity is that evolution has done a vast amount of design work for us, and we still have so much to learn from it.
The latest example is thin, highly insulating fibers that are stretchable and flexible. If you want the best design for something like that, why not look at a successful system from nature, one that has been refined by millions of years of evolution?
The polar bear keeps warm because each fiber of its fur has a very porous interior, so that means a lot of air. We know that air is a great insulator because there just aren’t very many molecules to bang into surfaces or each other to transfer heat.
It’s easy to use air as an insulator, such as in a double-pane window, which has very rigid walls, or in a natural material like down, which has a lot of bulk. But the polar bear has the key to doing this in a thin, flexible fiber, something that up to now has eluded us humans. Here’s an extreme closeup of the inside of a polar-bear fur fiber:
But now, Drs. Hao Bai and Weiwei Gao at Zhejiang University in China have devised an inexpensive way to make polar-bearlike fibers that perform as well as a layer of down five times as thick. They showed that these fibers can be stretched to ten times their original length without breaking, they can be dyed like any other textile fiber, and they can be repeatedly washed. Sweaters made out of these fibers held warmth much better than wool or cotton. It’s all laid out for us in the December 22 issue of Science.
A great way to use air’s insulating ability in a solid form is to make an aerogel. Containing up to 99.8% air, aerogels are the lightest solids known. If Travis Kelce were made of aerogel, he would weigh about as much as a hamster. NASA has been using aerogels as ultralight insulation on Mars rovers, including Perseverance, for years.
As you can (barely) see, aerogels are blocky like styrofoam. It’s hard to imagine weaving fibers out of anything like that. The really smart innovation we’re talking about today is dialing down the brittleness of an aerogel and encasing it inside a flexible but tough coating, just like the polar bear does.
There’s no doubt aerogels have supreme insulating ability. They boast photo ops like this:
But let’s start with the “reasonable cost” angle. We can get a pretty good starting material for our aerogel called chitin (KYE-tin) from the shells of crab, lobster, and shrimp. If we use some lye to remove its acetyl groups (shown in blue below), then we get chitosan, which has amino groups (in red) instead:
Chitin isn’t soluble in water at all (otherwise a lobster would dissolve!), but chitosan is, especially when we add a little bit of acid like vinegar to the water. We want to start with long chains of chitosan, where n in the figure above is 1,000 or more, so that when we dissolve it in water the chains get all tangled together and we get a kind of goop, or gel.
Chitosan is great for us to use, not only because it’s cheap, but because it will form a gel in water by itself. Its amino (—NH2) and hydroxyl (—OH) groups can form loose (hydrogen) bonds with each other like this:
Although each individual bond isn’t very strong, our long intertwined chains mean that there are lots and lots of bonds. So the goop actually holds together pretty well, so well that we can spin it into a fiber. We freeze the fiber as it’s spun, and believe it or not, this frozen rope holds together well enough that we can roll it onto a spool. Then, when we freeze-dry it, all the ice will be replaced by air (as in, say, freeze-dried strawberries), and we will be left with an aerogel fiber. I know this sounds kind of nuts, but it actually works.
We could have forced stronger (covalent) bonds to form between the chains by adding a linker molecule of some kind, a practice called crosslinking, which is done to a lot of polymers to make them stronger. The classic example is vulcanizing rubber, which makes it go from soft and stretchy to tough and rigid. That’s good for tires, or for hard aerogels, but not for stretchy fabrics. So we don’t do that here.
Our aerogel will be stretchy, but it’ll also still be pretty fragile because it’s so porous. And the other big problem with it, of course, is that it will dissolve in slightly acidic water. I don’t think we want our sweater to melt when we go out in the rain!
So here’s where we do what the polar bear does and encase our porous fiber within a stronger shell. This part is actually pretty simple, because remember our uncoated aerogel fiber is already on a spool, so we just roll it over to another spool with a tank of molten polyurethane in between:
We can roll this at different speeds and get different coating thicknesses, so we try these out and arrive at something that works for us. We end up approximately mimicking the polar-bear core-to-shell ratio, but we have a thicker, yarnlike fiber:
The amazing thing about these fibers is that they can stretch to ten times their original length over and over again and recover. Here we have a machine that tests materials for their ability to stretch and recover, working on an encapsulated chitosan aerogel fiber:
An aerogel just did that? Let me just interject here that that is INSANE.
OK, fine, but how does it perform as an insulator? We can compare it to traditional materials with a thermal camera that measures surface temperature to see how we do. The warmer the surface temperature, the more heat is escaping from the wearer:
Wow! Even better than a down jacket five times as thick! And way better than wool or cotton.
One challenge left here is how to do this just a little faster. Freezing a fiber as it spins isn’t hard or expensive to do, but it limits you to speeds considerably slower than for traditional fiber spinning. So if you’re Bai and Gao, you’ve got your next challenge set up for you. Given their track record so far, I’ll say I’m pretty confident they’ll work it out.
Meanwhile, I gotta pick out my sweater color.
Whaddya think? The gold is on point, but the fuchsia looks pretty compelling…..