Maybe you’re scared of spiders.

I mean, I am, so I’m not judging! But whether their many legs and eyes charm you or creep you out, the fact is that spider webs are a materials science miracle.

The silk that spider webs are made of is stronger than steel by weight and stretchier at the same time, unlike anything we can mass produce. If we could make lots of the stuff, we could construct all kinds of things – from stronger bulletproof clothing, to more effective surgical sutures, to maybe even space elevators! Sadly, we’ve yet to replicate on a large scale the kind of nanoengineering that goes on in spider butts worldwide, day after day.

But that might finally have changed, because a research team has reported a new way of making spider silk that might scale up to make lots of it. And in the process, they’ve busted a longstanding materials science mystery wide open. And how’d they do it?

Well, with genetically engineered worms. [♪ INTRO] Before we get to those bioengineered spider-silk worms, it’s worth understanding what gives spider silk its enormous hype in the world of engineering and manufacturing. Spider silk is really strong for its weight. It’s stronger than high tensile strength steel when stretched.

It’s also tougher than Kevlar, the stuff that bulletproof vests are made out of. But it’s also also stretchier and more lightweight than those materials for the same amount of durability. And because of that it would d be great for situations where we need strong, lightweight materials, like surgical sutures and even artificial tendons used to repair muscles.

Spider silk is biocompatible, meaning you can use it in and around the human body without much risk of toxicity or immune response. The other great thing about spider silk is that it doesn’t change shape very much when you try to compress or twist it. In fact, that property is why some scientists have suggested that it could help us to construct space elevators, since they can provide a lot of durability for the relatively tiny amount of weight needed.

Finally, in comparison to other strong fibers like nylon, which is made of petroleum, spider silk has the potential to be relatively sustainable. And that’s not even getting into its funky thermal, electrical and optical properties that get scientists all worked up. So, if you’re thinking “it’s hard to believe a single material could do all that,” you’re in good company because scientists don’t really get it either.

Like, I said that spider silk is both “strong” and “tough,” which in everyday terms mean basically the same thing. But in materials science, strong and tough are sort of opposites! Technically speaking a strong material is one that can handle a whole lot of force without changing its shape, while a tough material is one that takes a lot of force to break.

To really see the difference, imagine hanging a weight off a pencil compared to a rubber band. The pencil would be strong because it can handle the force of the weight without changing shape. But on the other hand, pencils are brittle, and if the weight were large enough, the pencil would probably snap.

Now picture the rubber band. The band might stretch a lot, but it doesn’t break. And it takes a lot of force, maybe with a heavier weight, for the band to snap.

That’s what makes it tough. In short, strength is about being difficult to change shape, while toughness is about being difficult to break, which might actually be because it can change shape to handle a force, like a rubber band. Pencils and rubber bands have very different properties, and it turns out for most materials there’s the same kind of tradeoff.

Either it can be a brittle but “strong” material, that doesn’t easily change shape, or it’s a bendy but “tough” material that can give a long way before breaking. That’s also the tradeoff for most fibers like nylon, which is tough and stretchy, and Kevlar, which is strong but brittle. But spider silk is somehow both, which has been a real head scratcher for scientists, and the clue to solving it might hide in its chemistry.

On a fundamental level, spider silk consists of long chains of protein molecules. Those proteins are alternately organized in neat arrangements and jumbled up, which gives the silk its mechanical properties. But it’s the way those molecules are bonded to each other that might hold the clue to the “strong-and-tough” paradox.

It has to do with a certain kind of chemical bond between molecules called non-covalent bonds: a sort of loose alliance where two molecules are connected, but don’t formally swap electrons. It’s kind of like the molecules are magnetically attracted to each other when they come close. In fact, they’re literally electromagnetically attracted.

These non-covalent bonds are associated with materials being both stronger and tougher, but only up to a point! And that observation was the key to the breakthrough in question. Scientists at Southwest and Donghua Universities in China suspected something called average single-molecule intermolecular non-covalent bond energy density might be key to spider silk.

We’ll call it ASM-INCBED for short. ASM-INCBED essentially measures the average effect of non-covalent bonds that create a sort of internal friction between themselves inside a fiber as it’s stretched out. The chains of proteins inside the fiber have lots of these non-covalent bonds attracting each other, kind of like tiny magnets stuck on ropes, that attract each other as they slide past one another.

That creates an internal friction which makes it harder to stretch. On the other hand, when some force holds the fiber in constant tension, the “average single-molecule” bit in ASM-INCBED isn’t relevant anymore. That’s because it’s just the overall bond strength between protein chains that are fixed in place, rather than averaging how each molecule affects all the molecules it passes as it slides.

The researchers call this just INCBED. So, INCBED is basically what keeps a fiber strong as it’s stretched out but static, like a steel cable in a suspension bridge. ASM-INCBED, which is generated by the same non-covalent bonds but now as they slide past each other, is what makes the fiber tough like in a bungee cord being stretched out.

The “ASM” bit describes what happens when the molecules are sliding past each other, while “INCBED” is just how attracted to one another the protein molecules are while fixed in place under tension. And critically, the researchers reckon there’s a kind of sweet spot for both ASM-INCBED and INCBED, the proportion of these “internal friction” bonds, that make a material both tough and strong. But only up to a limit.

If there’s too much of both properties, the material will be strong, because it resists changing shape, but not tough, since it can’t stretch to accommodate forces, so it becomes brittle like Kevlar. But on the flipside, if the INCBED and ASM-INCBED in a chain of proteins, like spider silk, is just under that critical level, the friction of the chemical bonds along the fiber will make it both strong and tough, with the characteristic stretchiness of spider silk. Now, it seems like the easiest way to get lots of spider silk might be to just, you know, farm a whole bunch of spiders for it.

However, spiders have the unfortunate habit of eating each other when you stick a whole bunch of them in a dense environment like the kind needed to produce lots of silk. So instead of farming spiders, which is a job you’d have to pay me a lot of money to take, over the years, scientists have toyed around with other ways of producing spider silk, or something really similar to it, over the years. For instance, a 2022 study published in the journal Matter tried reconstituting ordinary silk, which we know how to make lots of, with a bit of zinc, which actually makes it as strong as spider silk – albeit in small quantities.

But the approach developed by Southwest and Donghua goes to a source that seems so obvious in retrospect, you might find yourself facepalming: silkworms! Silkworms are the larvae of silk moths, which have been kept by humans for millennia for making the silk we use in all kinds of soft, luxurious items from scarves to bed sheets. The worms make the silk to form little cocoons around themselves before they turn into moths, except that for us to harvest it, we kind of have to halt the process in the middle and steal their cocoons.

It’s a raw deal for the worms but, unlike spiders, we know how to keep a lot of these critters around and make a ton of usable silk from them. Now, silk is also pretty strong, but it’s a far cry from spider silk. So the researchers figured if their INCBED theory was true, and more of these “frictitious” chemical bonds really can increase the strength and toughness of the fibers up to a point. …then by hacking the silkworms to produce silk with a higher INCBED and ASM-INCBED they’d basically have spider silk, with all its wonderful properties.

Which is exactly what they pulled off by genetically engineering silkworms. The technique they used is called CRISPR-Cas9, and it relies on a special kind of protein that can carry around little snippets of DNA, and can be made to chop and change pieces around in various kinds of cells. Basically, they’re kind of like little helper drones for changing bits of DNA, and in the last decade scientists have gotten pretty good at using CRISPR-CAS9 to edit the genetic makeup of different organisms.

In their 2023 study, also in the journal Matter, the Southwest and Donghua team reported genetically modifying silkworm silk glands to produce more spidroins: the molecules in spider silk. That should theoretically increase the INCBED and ASM-INCBED in ordinary silk. And sure enough, after hacking some silkworm DNA, they tested the silk that the modified worms produced.

Just as they suspected, it was nearly as strong and as tough as spider silk! What’s more, by stretching and straining the resulting silk under a microscope, they measured how the forces were transmitted along the length of the silk and found that it follows the expected mechanical properties predicted by the INCBED theory. It’s like, two scientific breakthroughs for the price of one!

Not only have we created silk worms that can produce full length spider silk for the first time ever, but it seems like we also know better than before what it is that gives spider silk its toughness and strength. So we might be able to make a whole lot more of this stuff, because we can rely on what we already know about harvesting silk from silkworms and play around with the chemistry of the silk by tinkering with silkworm DNA to make the silk even tougher, or stronger, or maybe both! Now, we should qualify that it’s still really early days, and for now at least, the research hasn’t yet been replicated or repeated.

So whether this approach really will allow us to harvest full on spider silk in the same quantities we currently produce normal silk is still an open, if promising, question. But if the results really do bear out, it might not be long until you can find ultra-tough, spider-silk gowns hanging from the racks at a retail outlet near you. And goths everywhere rejoice.

Now, aside from silk, worms and spiders have more in common than you might think. Sometimes their eating habits are shockingly similar, which we explain over on our sister channel: Bizarre Beasts! Bizarre Beasts is another awesome YouTube channel made by the same group that makes these SciShow videos.

We all work at Complexly together and hold our videos to the same standards. So if you like this stuff, you’ll also like that stuff. And it really does mean a lot to us that you like what we do at Complexly, so thanks! [♪ OUTRO]