Derek: This is the largest amount of spider silk that's ever been put together for a test like this. So I'm gonna put it to the ultimate test, and try to be the first person ever to swing from spider silk. I don't know if it's going to hold my weight, so let's see. In three, two, one.

Henry: In "Spider-Man Two," Peter Parker is on a runaway train full of passengers. And to stop it from derailing, he stands in front of the train, fires webs at the buildings around him, and holds on. The webs stretch, some of them snap, but they do just enough to slow down the train and save everyone on board. It sounds like something straight out of a comic book. But real spider silk, the fiber that makes up a spider's web, well, it truly might be this incredible. It's reportedly stronger than steel. It's pretty strong. That's crazy! And 10 times tougher than Kevlar. To find out if that's really true, I went to the Blackledge Spider Lab at the University of Akron, one of the world's leading research centers studying spider silk. Before I went to the lab, I'd always assume that a spiderweb was made from just one kind of silk. But that's actually not the case. One type of silk anchors the web to a surface. Another forms this stretchy spiral, and a different sticky silk catches prey. Once something's trapped, the spider switches to yet another silk to wrap it up.

Todd Blackledge: The threads are all thinner than your hair, so they're going to look pretty similar to each other unless you use a microscope.

Henry: All in all, there are seven different types of silk, but the strongest is the dragline silk, which holds the web in its place.

Todd Blackledge: We've got the two main structural threads in this web. So the spokes on the wagon wheel and the outer framework here, that's all the dragline silk. It's called major ampullate silk if you want the fancy word.

Henry: The major ampullate is that one that commonly compared to steel?

Todd Blackledge: Yes!

Henry: Okay.

Todd Blackledge: Absolutely. And that's partly because it's function in a web like this. A bug hits it, to stop that moving bug, most of the work is done by the dragline silk.

Henry: To do this, the dragline silk has to be incredibly strong. So to find out how it compares to steel, we wanted to run a test, but it turns out that getting the sample isn't so easy. All right, we've taken a garden variety spider and knocked it unconscious using CO2. We brought it over here and we put it down with tape. I then started pulling out the dragline silk by hand. It's so light that you can barely feel it. We've attached it to this big rotating mechanism which they custom made to milk spiders. No spiders were harmed in the making of this video. Now that we've got our silk, we can put one strand into a machine that slowly pulls on our sample. And as it pulls, we'll monitor two key properties. When you pull on the silk, that force is spread over a tiny cross-sectional area. If you divide the force by that area, you get the stress. Now, as you pull, the silk is also stretching. To see just how much, we divide the change in length by the original length to get a measure called the strain. At first, as the stress increases, the strain increases roughly in proportion. And if we stop pulling here, the silk would spring back to its original length. But keep pulling and eventually it passes a point where some of that stretching becomes permanent. And it's pulling.

Videographer: All right. You film that, I'm going to be filming this.

Henry: And it broke.

Videographer: Wow!

Henry: Eventually, if you keep pulling on the silk, it will break. And at that point, the force applied, divided by the cross-sectional area is called its ultimate tensile strength.

Todd Blackledge: We're at about 600 megapascals.

Henry: To put that in perspective, if you had a rope of spider silk that had a cross-sectional area of one square centimeter, roughly the size of your little finger, it could support about 60,000 newtons of force. That's the weight of a fully grown African elephant. And that's not even the strongest spider silk out there.

Todd Blackledge: Breaking stress wasn't incredible for this piece of silk.

Henry: Deep in the jungles of Madagascar lives the Darwin's bark spider. Its diet consists of flying insects that zip over wide rivers. So to catch its prey, the Darwin's bark spider has to spin giant webs that stretch up to 25 meters over open water. And to survive that kind of distance and tension, its silk has to be incredibly strong. It has an ultimate tensile strength of around 1,600 megapascals. That's more than twice the strength of our sample. But experimental ultra high strength steels can reach an ultimate tensile strength of nearly 3,000 megapascals. That's about three times that of typical dragline silk, but steel is also about six times denser. So for two ropes with the same mass and length, the silk rope has six times the cross-sectional area. Six times the area at one third the strength, well, that gives a rope that can take twice the force before it snaps. This measure of strength relative to mass is called specific strength. And it means that for the same mass and length, spiderwebs can withstand roughly twice the force of ultra high strength steel before breaking.

Todd Blackledge: So that's where ounce for ounce, spider silk is quite strong. You got to think about what is the application you're talking about using this material in. Is it somewhere where you don't care about weight, and you need the strength? Then steel's going to be perfectly fine. But if weight matters, spider silk is really the champion there for tensile strength.

Henry: But strength alone isn't enough. To see why, look at rock climbers. If a rock climber was just hanging, all we would care about was if the rope could support their weight. That's strength. But climbers don't just hang, they fall. And now the rope has to bring them to a stop, which means absorbing that kinetic energy. Stop the fall in a short distance, and the energy has to be absorbed quickly. The forces are enormous. Stretch it out and that same energy is absorbed gradually, so the forces are lower. So to absorb more energy without snapping, it has to stretch further. And that's actually not so different from spider silk.

Todd Blackledge: So a bug hits this web, you've got to stop the flight of that bug pretty quickly. And you could do it by just building a really stiff structure. Think like a brick wall, you throw a tennis ball against it. You're gonna stop that ball instantly. But that doesn't work very well if you want to catch a meal.

Henry: It'll bounce off.

Todd Blackledge: Because it'll just fall out. And so it helps to slowly decelerate the insect so that the web has time to stick to it, and keep the meal for the spider to subdue.

Henry: So the spider silk has to be quite stretchy.

Todd Blackledge: This piece of silk stretched to 70% of its length before it broke, which is really exceptional for dragline silk.

Henry: Now, every tiny bit of stretching absorbs a little bit of energy. If you add all these tiny amounts together, the result is the total energy the material absorbs before breaking. On a stress/strain graph, it's proportional to the area under the curve. This is the material's toughness. Kevlar, the material used in bulletproof vests, whose whole job is to take the energy of a bullet and spread it out before it reaches the body, well, it has a toughness of up to 50 megajoules per cubic meter. Experimental ultra-high strength steel has a toughness of around 170 megajoules per cubic meter. But spider silk?

Todd Blackledge: This piece of silk had a toughness of about 205 megajoules per cubic meter.

Henry: Oh, okay. So that might be like four times Kevlar. But the silk from the Darwin's bark spider can have a peak toughness of 520 megajoules per cubic meter. That's roughly three times the strongest steel out there, and 10 times as tough as Kevlar. See, Kevlar has a very high tensile strength, but it's also quite stiff. Compared with stretchy spider silk, that means that the stress/strain graph rises sharply and fails before it can stretch very far. So it's not able to absorb as much energy. And that's why spider silk wins on toughness.

Todd Blackledge: But it's the ultimate in green chemistry. They're not using the high temperature, or the caustic solvents that most of the polymer industry uses to make high performance threads. They're doing it at room temperature, inside a living body, with the same basic building blocks that you use to build your hair and your skin.

Henry: Now, the secret to spider silk's unbelievable natural strength lies in how those building blocks are arranged. If you look inside a silk fiber, you'll find spider silk proteins called spidroins. In some regions, the spidroins are arranged in similar size and shape, and they're closely aligned along its length. They're kind of like stacked egg trays. These are nanocrystals. In other areas, the spidroins aren't so uniform. Here they're less ordered, and they can move and change shape more freely. These are called the amorphous regions. Together, they work like a network of tiny rigid blocks connected by elastic cords, which I have in front of me. When I pull on the network, the cords stretch, and they absorb energy. The blocks don't stretch much, but each one connects to several cords, which spreads the pull across the network. So the amorphous regions let the silk stretch while the nanocrystals help it withstand a much larger pull. Together, they allow the silk to absorb a tremendous amount of energy before breaking, which makes it so tough. The result is a material that's stronger than steel by weight and also tougher than Kevlar. It sounds perfect for making climbing ropes, parachute cords, airbags, even bulletproof vests. But we actually aren't using spider silk for any of that, even though we've been trying to for centuries.

Henry: In 1709, a Frenchman named Francois Xavier Bon painstakingly collected hundreds of spider egg sacs, boiled them, combed them like wool, spun them into thread, and then used them to make stockings that attracted the attention at the royal court. But this was harder than it sounds. As he later said, "The only difficulty now lies in procuring a sufficient quantity of spider bags to make any considerable work of it." Three centuries later, Simon Peers and Nicholas Godley led a years long project in Madagascar, in which teams of Malagasy workers gathered more than a million golden orb weaver spiders in the jungle by hand. They extracted their silk and painstakingly wove and embroidered it into a single golden cape. Why hadn't they tried farming the spiders instead? Well, attempts to do that all run into the same problem. Spiders are cannibals. The big problem with spiders is that they eat each other?

Todd Blackledge: That's one of the big problems. And then you can't just keep them in a tiny little container. You're going to have to have a lot of real estate if you're going to have a farm with 10,000 spiders on it.

Henry: Even if you could get your hands on that many spiders, as we've seen, milking them is difficult, and the spiders don't always cooperate.

Todd Blackledge: Really cooperative spider will give you 300 yards of silk. 100's a lot more typical, but they kind of have their own individual personalities. Some of them will just break the thread right away, and not even cooperate with you.

Henry: That's why natural spider silk is so scarce. One supplier charges $700 for 100 milligrams, which works out to roughly $7 million per kilogram. That's roughly 50 times the price of gold. Okay, so we can't farm spiders for their silk, but we know what the proteins are, so surely we can just make them? Well, scientists have tried. By the late 1990s, researchers at DuPont took the genes that code for spider silk proteins and put them into E. coli and yeast. These living cells are like protein factories. One engineered cell quickly becomes millions. It worked, but scientists wanted an even more efficient method. So in 2001, a team in Germany tried implanting spider silk genes into plants like tobacco and potatoes. But if you really want to produce a lot of protein in a concentrated form, look at milk. To feed their young, mothers produce milk with high concentrations of proteins. So around the same time, a Canadian company called Nexia tried something much stranger. They put spider silk genes into goats, creating genetically modified spider goats. No, no, really, they made spider goats. These goats produce milk chock-full of spider silk proteins. In one sense, it worked. All the experiments were able to produce spider silk proteins, but they ran into the same problem. From E. coli or yeast, these proteins were purified as a fine white powder. From plants, the purified proteins formed a viscous, gelatin-like liquid. And from goats, it was dissolved in the milk. None of these even vaguely resemble spider silk.

Todd Blackledge: And that's one of the secrets that any company that really wants to mimic spider silk at an industrial scale needs to kind of figure out is how to process silk proteins the way spiders do.

Henry: But that's easier said than done. In a spider, it all starts in the silk gland in a region called tail. Cells here make proteins called spidroins, and they secrete them into the fluid inside the gland. Every spidroin has three main parts. One end is called the N-terminal, and the other the C-terminal. And between them is a long repeating sequence of amino acids. This is called the repetitive region. Once inside the tail, the C-terminal ends pair up, which links neighboring spidroins together. But the N-terminal ends stay mostly separate. So the spidroins are still loose enough to bend, coil, and move around. Next, they move into the storage sac, where they're packed together at high concentrations. Normally, proteins this concentrated would stick to each other, and clump into a solid, but this can be modified with a careful balance of salts and by changing the pH. The pH is a measure of the concentration of hydronium ions in a solution. And because hydrogen ions are little positive charges, you can think of the pH like a dial that controls the electrical charge on different parts of the protein. In the storage sac, the pH is tuned so that the N-terminal ends carry a net negative charge. Every protein carries the same charge. And since like charges repel, when you crowd them together, they shove apart instead of sticking together. Now, the spidroins are still crowded in this aqueous solution, and at such high concentrations, they begin to gradually rearrange. The N-terminals are attracted to water. These are hydrophilic. While the repetitive region is repelled by it, this is hydrophobic. So the spidroins rearrange, and you end up having these hydrophilic parts facing outward towards the water. And hydrophobic parts tuck themselves inside away from it. They then form these tiny structures called micelles. And the micelles cluster together into larger globules. These globules next enter the spinning duct.

Todd Blackledge: For the dragline silk, the duct is kind of S-shaped, but it gets skinnier and skinnier.

Henry: As the globules go through the narrow duct, the liquid near the walls rubs against the surface, and it moves more slowly, while the liquid in the middle moves faster. So the layers slide past each other, which is known as shear. These shear forces stretch and deform the globule.

Todd Blackledge: You also remove some water, so water goes out. You get a drop in pH, so it becomes a little bit more acidic as you go down this duct.

Henry: So the proteins are then being changed in two ways at once. Mechanically, they're being pulled and aligned. And chemically, that drop in pH means that there are now more positive hydronium ions in the solution. These bind to the negative parts of the proteins, neutralizing them. Now all the proteins don't repel each other as strongly, and so the N-terminal ends begin pairing, linking the proteins into long chains. These chains are pulled through a tiny narrowing called the drawdown taper, which leads to the spigot. Here, the spider pulls the liquid through a small opening. This forces the liquid to speed up. And as it speeds up, it stretches. It's this final stretch that pulls neighboring spidroins tightly alongside one another. Now, depending on the sequence of amino acids, some sections fold into zigzag strands. These strands then interlock and rows of hydrogen bonds stitch them into rigid, sheet-like structures, which stack together into the nanocrystals that give the spider silk its strength. In other sections, the spidroins don't fold as neatly. That's how you get the amorphous regions which give the silk its stretch. Exactly how the proteins assemble inside the spinning duct is still being studied. No one fully understands it yet, and that makes recreating the process in a lab quite difficult. So scientists wondered, "What if instead of starting from scratch, you used an animal that already has a spinner?" This is exactly what researchers at Kraig BioCraft Laboratories, a biotech company in Michigan, have been working on. What's more, they're doing it with an insect that humans have been farming for nearly 5,000 years.

Henry: According to legend, a Chinese empress was relaxing in her garden, when something fell into her teacup. She looked inside, and saw a cocoon. The hot water loosened it. And when she lifted it out, it unraveled into a single shimmering thread. So long that it stretched across her entire garden. This was silk. Whether or not that story's true, people eventually learned to farm the insect that made the thread. Silkworms. They're the caterpillars of the domesticated silk moth. Silk quickly became one of the most valuable materials in the land. It dressed emperors and spurred trade connecting East Asia and Europe. So much so that it gave the trade route its name, the Silk Road. For nearly 2,000 years, China guarded the secret. But of course, people were always trying to steal it. By the mid-sixth century, the Byzantine Empire was in trouble. Plague had devastated the population. And on top of that, the purchase of silk was draining its gold. The problem was that Byzantines couldn't make silk. Most of it had to come through Persia, the very empire it was fighting. So Emperor Justinian made a secret deal with two monks. They would travel east, find the source of silk, and then smuggle it back. They returned with a secret hidden inside their hollow canes. Silkworm eggs. This may be the first recorded case of industrial espionage in history. With that, Byzantium could finally make its own silk. Nowadays, we farm them by the millions.

Jon: One moth lays 500 eggs.

Henry: Okay.

Jon: So when you talk about growing productions, right? Like my wife and I had a baby. These guys, they have 500 babies. And they do it every 30 days.

Henry: So the researchers at Kraig wondered, "What if we could make the silkworm spin spider silk instead?"

Jon: When we create a transgenic silkworm, we're taking the genetics of a silkworm, and the genetics of a spider, and we're putting those together. We're taking the best aspects of both. And that's what we're doing to create our fibers.

Henry: They start by gluing eggs onto a slide, which they line up under a microscope.

Jon: So this is the microinjection station. We're talking about eggs that are the size of the head of a pen. We're talking about needles that are half a micron in diameter.

Technician: This part is the, I want to inject. So as you see, this is the tungsten needle and this is the glass capillary. And first we poke a hole with the tungsten needle. I will actually move the eggs to the capillary and poke.

Henry: The fluid carrying the spider DNA is then injected into the part of the egg that will eventually become the silkworm. But at this point, the genes are still separate. They have to become part of the silkworm's own DNA. And luckily, nature already has a way of doing that. If you take an insect like a moth and look inside its genome, you'll see some stretches of DNA that move around. They jump from one place in the genome to another. These are called jumping genes, or transposons. One specific one is called piggyBac, and it was first identified in a cabbage looper moth. PiggyBac is a piece of DNA with two short sequences at its ends, and it works together with an enzyme that recognizes these ends, cuts the whole piece out, and inserts it somewhere else in the genome, kind of like a natural cut and paste system. What scientists realized is that if the enzyme only looks at the two ends, then you can keep those ends the same, but change what sits in the middle, and use that to implant the DNA of your choosing. So in the lab, the DNA in the middle is replaced with the spider silk gene. This, together with separate instructions for making the enzyme, is what's then injected into the silkworm egg. And inside the egg, the enzyme recognizes the familiar end sequences and cuts and inserts the spider silk gene into the silkworm's DNA. If everything works, it becomes a permanent part of the silkworm's genome. Now with silkworm where the edit worked looks almost exactly like one where it didn't.

Jon: The first approach we used was creating transgenics with the glowing cocoon. A green fluorescent protein is what's making these guys glow that green color under this particular wavelength with that particular light filter in front of it.

Henry: It feels magical. And there's a living thing inside of it.

Jon: Mm hmm, yep.

Henry: And it's creating the glowing effect.

Jon: Yeah.

Henry: But even if the DNA insertion is successful, it might not have ended up in the right place.

Jon: If I create the world's best recipe for chocolate chip cookies, and I stick it in the pasta section-

Henry: Yeah.

Jon: You're probably not going to make a lot of chocolate chip cookies, right? 'Cause you're not going to see it.

Henry: Yeah.

Jon: Same thing for this, right? We want the recipe in the right section of the right cookbook.

Henry: That's why getting the spider silk gene into the silkworm isn't enough.

Jon: We really want the spider silk get incorporated into their silk gland, not any other tissue.

Henry: But piggyBac only looks for a four-letter DNA sequence. TTAA.

Jon: There's so many TTAA in their genome. You couldn't really control where they were.

Henry: So it's like there are a lot of different places where they could park.

Jon: Yeah, yeah, that's true!

Henry: And that means that it won't spin pure spider silk. If you had to estimate, of the transgenic silk, like what percentage is spider DNA?

Jon: Like 6%, 10%.

Henry: That's insane to me! So even with this limited gene transfer, the fibers still achieve much of the mechanical performance of pure spider silk.

Jon: Our average samples are usually about 60% of spider silk, right? Which, show me any other place in the world where you can get materials that perform at 60% the strength of spider silk that you can make cost effectively, right?

Henry: To push the 60% average even higher, the insertion needs to be more targeted.

Jon: The next step is to create pure, what are called knock-in/knockout transgenics where we will completely remove all aspects of the silkworm's native DNA for the silk, and replace that completely with the spider silk protein.

Henry: And to do that, most of the leading labs in the field are exploring CRISPR-Cas9. This tool has two parts. The first is a guide, a short piece of genetic code designed to match one exact stretch of DNA. Unlike piggyBac, which recognizes a sequence just four DNA letters long, this guide matches about 20 letters, making it far more specific. The second is a cutting protein. When the guide finds its matching sequence, the protein cuts the DNA. Here, that spot is inside the silkworm's own gene for making silk. The cell immediately tries to repair the break, and scientists take advantage of this by also injecting donor DNA, carrying the spider silk gene. As the cell repairs the cut, it copies in that donor DNA, stitching the spider silk gene into this one precise location. But this process is still an active development. And Kraig is not the only group chasing the dream of spider silk. In Germany, AMSilk is making proteins for fibers, coatings, powders, and hydrogels. And in Japan, Spiber is brewing protein fibers through fermentation. Brands like Goldwin and The North Face have already used a version of these fibers in their clothing. And the military has been interested too. In 2016, the US Army funded Kraig to produce ballistic shoe packs, panels layered with transgenic silk that are designed as potential body armor. And it's even being explored in medicine. For centuries, surgeons have stitched wounds together with silk. But now a company called Newrotex is using spider silk to help repair damaged nerves. So if it can be produced cheaply, spider silk could find its way into clothing, armor, implants, even nerve repair. But to do that, we're going to need a lot of it.

Jon: And this is our specialized spider silk. There you go.

Henry: This is the transgenic spider silk.

Jon: This is spider silk. This is transgenic spider silk.

Henry: What? This is like a bale. This is a hay bale of spider silk.

Jon: So last year we produced about a half a ton of spider silk cocoon.

Henry: Half a ton.

Jon: And this is silk from that.

Henry: But what about our original question? Can you use transgenic spider silk to swing like Spider-Man? So Derek, I've got you here in a climbing gym for a very good reason. And the reason is underneath this dirty dish towel.

Derek: Unveil it, unveil it. Yeah, go. All right, okay. This looks very uninspiring.

Henry: So what's interesting is this is one continuous filament.

Derek: This is just one strand.

Henry: But each one of these strands is actually 10 individual silk filaments that are kind of connected.

Derek: All right. So you want me to hang from this?

Henry: I do want you to hang from it, yes. I actually want you to do more than just hang from it. I want you to swing.

Derek: This will be the first time ever anyone has ever swung from actual spider silk. Granted this spider silk came from other organisms, but... I don't know if it's going to hold my weight, so let's see. In three, two, one.

Henry: You really came close!

Derek: Don't tell anyone, but Spider-Man.

Henry: Okay, so we've seen it can hold your weight pretty reasonably. We could swing, but I do want to put it to the ultimate test and get you up there. How much do you trust spider silk?

Derek: Tensile strength doesn't lie.

Henry: All right, let's get you up there.

Derek: We used the title, "Risking my life to settle a physics debate" too early. It feels like it's going to dig into my fingers.

Speaker: This is crazy.

Derek: I'm going to go. I'm going to go!

Henry: Whoo! Let's go!

Derek: You got to see my finger, man!

Henry: Ooh, damn!

Derek: Yeah. This is the problem with spider silk. Just, it's thin, it's strong, and it just rips you open.

Henry: Now, if it isn't clear, we're not exactly web-swinging experts. But we'd love to get the silk into of the hands of someone who is.

Derek: Tom Holland, this is a challenge. Yeah, I'm calling out Tom Holland. I'm calling out all them guys. Andrew Garfield, Tobey McGuire. That's right. Come at me, bro.

Henry: See, spider silk is so strong and so thin that it actually cut Derek's skin. And that's actually a great way to explain the design flaw with modern razors. Thanks to Henson Shaving for sponsoring this video. See, facial hair resists more than you think. And many razor blades are engineered so thin that they bend under that resistance. This is called blade flex. Now, most razors actually build this flex in. They use springs, pivots, and flexible mounts that allow the blade to move. It's supposed to make the shave feel gentler, but it means that the cutting angle is constantly changing. When unsupported, the blade bends mid-stroke and instead of slicing cleanly, it tugs the hairs, micro-cutting the skin. That's what causes irritation. The usual fixes are lubrication strips or extra blades, but they don't solve the real problem. The blade is still allowed to flex. We assume that razor bumps and irritation are inevitable, but that's shaped by bad design. But Henson is designed by aerospace machinists who control cutting forces with high precision. And their razors use a single precisely-machined blade. Instead of letting it move, Henson's design supports the cutting edge with tight machining tolerances, holding the blade securely so the angle is consistent. Remove the flex, remove the irritation. Use code "Veritasium" for 100 free blades with any razor purchase. Just add both the razor and blades to your cart. Thanks to Henson Shaving, and as always, thanks for watching.