Down feathers are among the best insulators that we have. In cold and dry conditions, nothing comes close: a quick look at the typical attire and sleeping bag for someone heading up an 8000 metre peak or to the Poles will attest to that. Despite years of humans developing synthetic materials, we still can't match the stuff that geese and ducks churn out without breaking a sweat. So what is it that makes down so effective, and why can't we recreate its properties? To find out, you've got to look close...
Down is made primarily of keratin, a versatile biopolymer that is found all over nature: it's in hooves, hair, wool, horn, reptile scales, feathers, and more. So, despite huge differences in the appearances of these materials, they are all made of very similar stuff. Keratins tend to be used to protect animals against their environment, so they may be resistant to weather, impact, or just wear and tear.
"The more you learn about it the more you realise that evolution has done a really very good job with it"
You could attack some of the world's finest down feathers with mankind's most powerful chemistry-based analytical techniques and you'd be hard-pushed to see much difference between down feathers and the mundane flight feathers of pigeons or seagulls. However, ten minutes in a sleeping bag filled with seagull feathers, or just a quick look by eye will tell you that down feathers are extremely different to most other feathers. It turns out that the chemistry of these materials is actually not that unusual. In other words, it's not what down feathers are made of, but how that stuff is put together.
"A typical down barb is about five times thinner than your average coat of paint"
How can we see what's inside a down feather?
So if down is just keratin (avian keratin, to be specific), then how this keratin is arranged must be unique, and this is when you need to start getting inside a down feather to have a look. The problem is that a typical down barb (a fibre that leaves the central feather core) is only about 20 micrometres across. That is five times thinner than your average coat of paint. Just handling the stuff is very difficult, and pinning it down for a look with a microscope can be infuriating. When it comes to slicing a down barb like a salami so you have thin sections to look at, you need a very good biology lab and technician, a lot of a practice, and even more patience. To do this at home, you need to suck all the water from the down and then dye and stain it (with uranium, no less) to make its features stand out. Then you need to embed it in a resin (a bit like that mosquito in Jurassic Park). Once it's in the resin it's then time to make some salami slices, but these need to be nanometres thick (smaller than small!) so you need an atomically-sharp glass or diamond knife. Very cool, very pointy. You then use an electron microscope to look at the very thin salami slices, and the way the electrons interact with these slices tells you about the makeup of the down.
So what is inside down?
The first remarkable thing about down is how similar its barbs are to those of wool fibres or human hair, two other keratinous fibres. These materials are all hierarchical composites, just like the stuff in carbon fibre bikes or F1 cars. Down barbs are made of hard cylinders that give strength and rigidity which are surrounded by a softer matrix that absorbs impacts and gives the material toughness. The resulting composite is extremely tough but also very lightweight.
The actual cross section of each barb is also perfectly-designed to resist compression (crucial in an insulating material) or to recover from compression: down feathers barbs are irregular shapes that combine buckle-resistant properties with strength in all directions. Down's cross section is also where goose and duck down can vary: some goose down is hollow in cross section, further adding to its compression resistance. No duck down barbs have yet been found to be hollow. The difference between goose and duck down (or eider, for that matter), is a whole other story though.
"Its barbs are perfectly-arranged to trap the maximum amount of air and are stiff enough to maintain the structure of the feather, and the barbules (even finer fibres that grow like twigs off the barbs) are a nearly-perfect diameter to block heat transfer by radiation"
How are the composites put together?
In the same way that the best composites in the world would never make a good F1 car if assembled by me, only because of the way the keratin composite is arranged on a macroscopic scale (a size we can just-about see) is down really amazing at insulating. Its barbs are perfectly-arranged to trap the maximum amount of air and are stiff enough to maintain the structure of the feather, and the barbules (even finer fibres that grow like twigs off the barbs) are a nearly-perfect diameter to block heat transfer by radiation. Just like the barbs, the barbules aren't just dumb cylinders: as they leave the barb they are quite flat and this gives them a predefined way in which they bend, stopping them tangling and helping them recover from being compressed. Further from the barb they are more cylindrical to give maximum stiffness. Basically, nature thought long and hard about this. The icing on the cake is the little hooks and nodules (prongs and nodes to be precise) that stick out of the barbules. These tiny structures further increase compression resistance. One hindrance of these is that they lock together once down has been compressed, and this is why, following compression, down should be shaken to restore its full loft. Geese and ducks shake you should too.
The surface of down
Down's surface is just as clever as the rest of it. Just like wool or hair, its barbs have an extremely thin cuticle that surrounds its core. This is the rough-looking thing that you see on the L'Oreal adverts. In down the cuticle is much smoother, as it doesnt need to help anchor the down feather in the birds skin, but it aids in chemical resistance and, of more relevance to us, water resistance. The cuticle is hydrophobic (water-hating) and this is why down, like hair, floats on water. However, given enough encouragement and soakings or sufficient contamination in oils or soaps, down will suddenly uptake a lot of water. This is because once the cuticle has been penetrated, the cortex inside it acts a bit like a sponge. This is one reason why down is fine in damp conditions and even heavy rain for a time before it might eventually give in quite suddenly. Long-term though, wet down is no problem, in the same way that wet hair is no problem. Just be really thorough when drying it and break up any lumps that form. The lumps form because the relatively stiff barbs and barbules go limp when wet.
So why can't we recreate down's properties?
All the above is just a small insight into why down is so amazing at insulating. The more you learn about it the more you realise that evolution has done a really very good job with it. We've been trying for a while to make down-like synthetic materials and significant improvements are being made all the time. Some of the biggest improvements in synthetic insulation manufacturing have coincided with big research efforts into down (Primaloft is one such example; they even hold a patent called synthetic down). However, at present, making fibres as small and complicated as down is really hard. Fibre extrusion methods have moved on significantly in recent years and new techniques continue to develop to make very fine fibres, but the manufacture of multi-cored composite fibres of the dimensions of down remains troublesome. The hardest thing of all, though, is making fibres that are arranged into little symmetrical trees like down feathers are. That's currently impossible. However, imagine if we had a material with the resilience, lifespan, warmth-to-weight ratio, compressibility, and softness of down in a product that was a tenth of the cost and absorbed no water. That would be very cool. Maybe a ground-up approach will work, perhaps using 3-D printing or growing crystals (neoprene under a microscope can look very similar to down). Someone will manage it one day: the gauntlet's been thrown down.
About Matthew Fuller
Matt is an experienced hiker, keen mountaineer, and terrible rock climber. Most weekends he can be found hiking, climbing, running or cycling in the hills. After 9 years of university, including the world's first PhD in down and its use in the outdoors, he decided to get a job. He is now lucky enough to work for Mountain Equipment as a Product Engineer where he works on R&D, marginal gains, number-crunching, and, obviously, down products.
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