The world of materials science is increasingly turning to biology for inspiration and assistance in creating better materials. One of the Great Quests of materials science is the world’s strongest material: can we make something that is flexible, strong, and tough? Can we make a fiber of it, so we can weave it into a fabric? If we can get all these things to happen, the applications would be amazingly varied. We could cover our cars with it and get light cars that are safe in accidents and therefore way high gas mileage. We could make thinner, more comfortable and effective flack jackets and military armor, and therefore cover more of the body with it. We could use it to replace our torn tendons or to make stiches or scaffolding to grow new organs on, including skin. The applications in machinery and manufacturing are also diverse.

Spider silk has long been the strongest biological material we know of, but earlier this year we found a stronger one: limpet teeth. But limpet teeth, basically a really hard sea shell material, is not as flexible and useful as spider silk could be. It might be great for a building material, but we wouldn’t want to wear the stuff. And anyway, a new paper touts changes to spider silk that make it even more amazing, and even tougher than limpet teeth. Or anything else we’ve made ourselves. Ever.

A group of scientists from Europe, mainly Italy and the UK, were trying to think how they could improve on spider silk and thought it would be good to try to make it conduct electricity to expand its uses. Also, carbon nanotubes are just the sexy thing to work with right now, so they thought they’d start with those and some graphene. Rather than trying to coat the silk with these materials after it was made, they sprayed water laced with carbon nanotubes into the air where the spiders were doing their thing, and the spider inhaled the carbon and then, since their bodies didn’t like that much the stuff, expelled it in the silk. So the silk itself was then conductive as soon as the spiders made it. But it was also way tougher and stronger.

Toughness and strength are actually technical terms when we’re talking about materials. Toughness is how much energy a material can absorb before it breaks. Flexibility helps this – you don’t want something to be brittle and break right away – that wouldn’t be a tough material. It’s usually reported as energy/mass, or how much energy it can absorb per gram (or pound) of material. Strength is the ability of the material to bear a load – or hold things up – without deforming or stretching or otherwise getting bent out of shape. For more materials in the world, you either get one or the other, not both, because flexibility helps toughness and hurts strength. But at the very high end, there are some materials that do both to an amazing degree.

This figure is from the paper about this nanotube-infused spider silk. The green bars show toughness and the blue line shows strength. These are the strongest materials we know of. Limpet teeth are way on the right side (strong but not tough), and the metal spider silk is on the far left. Then there’s some pretty cool stuff in between, including some knotted materials. We know that adding slipknots to fibers makes them stronger, so presumably if they added knots to this spider silk it would get stronger, too.

Just to compare, Kevlar, which is the most common super-strong material and is now used to make bullet-proof vests (and light-weight canoes, of course), is way off the chart to the right on this scale. Depending on how it’s woven, it has a strength of 2-4 GPa but a toughness of only 20-30 J/g. So this spider silk is both stronger and tougher than Kevlar, pound for pound.

Oh. But it’s really hard to get a pound of spider silk, and it’s easy to get a pound of Kevlar. It’s difficult to farm spiders because they have territorial issues and tend to eat other spiders that wander into their personal space. Also, when prodded to spin silk when they’re not naturally inclined to do so, spiders produce weaker silk. So they’re not very cooperative when it comes to high-volume manufacturing.

But surely we can outsmart them and get our spider silk some other way, without the spiders? Yes! All sorts of intriguing solutions have been found. A few years ago, a group from Utah managed to do some genetic hoo doo and make goats produce spider silk in their milk, with apparently no side effects for the goat. That’s great, but the process of extracting the silk from the milk is finicky. There’s at least one company doing this commercially, though.

So then scientists tried inserting the spider silk protein gene in other things that we’re good at farming, like alfalfa and silk worms (who don’t eat each other, apparently). But these life forms didn’t produce very much spider silk, so it wasn’t viable commercially.

Just last year, it looks like we finally landed on a good solution: E. coli. Bacteria that live in our guts (and lots of other places!) can be made to produce spider silk. It’s a little rough: they have to be modified to make them capable of producing a huge molecule like the spider silk protein, but it looks like it’s been done. That’s super exciting and under development. But yes, there’s a drawback. The silk is strong but not as strong as the real thing from spiders.  Yet.

Then the news came this month that someone has finally been able to produce spider silk without involving biology at all. The protein is incredibly complicated, so no one’s been able to manufacture it before, but a group of scientists at MIT managed to make it with a teeny tiny micro 3D printer. They did what most of us would do when given spider silk: they started to make webs. They studied some properties of webs and compared different densities of lines and thicknesses of fiber. They were finally able to do this because – and you know I have to bring this up – they got a got computer simulation of the molecule and were able to design the protein and test it before they had to build it. Computer modeling to the rescue! Again, it’s not the same as the real thing, but it’s recognizable.

So now I need the metal-spider-silk people and the synthetic-spider-silk people (or e. coli-silk people) to get together and come up with an amazing material so that I can get to work on my Iron Woman suit.

Spider sources:

Paper on carbon nanotube spider silk

Paper in synthetic spider silk

Article on silk & bacteria