The Stickiest *Non-Sticky* Substance

This is one of the strangest materials I have ever seen. It is not sticky at all. You can't even stick regular tape to it. But if I drape it over this tomato, it holds it up, unless you turn it upside down, in which case it just falls off. Now does it only stick to fruit? No, it'll stick to a water bottle or a bag of chips, basically any approximately smooth surface. And that's because this material is made to mimic gecko skin. Artificial gecko skin comes out of Professor Mark Cutkosky's Stanford lab and it has now been used on robotic grippers, on tiny robots that can pull way more than their weight, on a robot that floats around the International Space Station and even enabling a person to scale a glass wall, Spider-man style. It all started with a competition to make a robot that climbs a vertical wall without suction.

Here's sticky type. Everybody's familiar with it. You press it on and it sticks. Sticks pretty hard actually. And then you peel it off. Here's gecko material. And the first thing you notice is that it's actually... It's not sticky at all, but if I lift it from the middle I can lift a football. You might say, "Well you could do that if you had this other sticky tape", right? Well, yes, but if I had the other sticky tape it wouldn't do that. And that's important because if you're a small robot trying to climb a wall, you can't afford to have something like chewing gum on your feet, 'cause then every step you take is effort. What you want is something that only grabs when you need it to, and that's kind of the main principle of our gecko-inspired adhesive. Here this is an early prototype of the sticky bot gecko-inspired robot. This got us into a long effort to understand what is it that makes gecko adhesion work, having something which grips but is not sticky.

[Derek] Geckos are incredible climbers. They scale walls and even walk on ceilings, but for a long time we didn't know how they did it. They don't use hairs or spikes like spiders or other insects. You might think they'd have tiny little suction cups at the end of their feet, but their adhesion is actually way stronger than suction cups.

[Mark] A gecko can easily hang its entire weight from just part of one toe.

[Derek] The clue to how this works can be seen if you zoom in on a gecko's toe.

[Mark] These are called lamella. They're regions of stalks, which are called seta. And the seta branch into these incredibly fine structures called spatula, which are less than one micrometer across.

[Derek] And that's important because the physical principle they use to stick is actually incredibly weak. It's not like an ionic bond where atoms are attracted to each other because one is positive and the other is negative. It's not even a hydrogen bond like in water where part of the molecule is slightly positive and the other part is slightly negative. No, geckos rely on the attraction between neutral atoms. So how does that work? Say a gecko is climbing up a glass window. The gecko atoms are neutral and the glass atoms are neutral, but at any particular instant the electrons around an atom are not perfectly evenly distributed about the nucleus. They may be a little bit more on one side than the other. And this makes the atom momentarily a little positively charged on one side and a little negatively charged on the other side. Now if there's a neighboring atom really close to it, say an atom for the glass is within a few nanometers, this charge can induce a complimentary charge imbalance on the glass atom. So now the electrons from the glass atom are attracted to the nucleus of the gecko atom and vice versa. There is a very weak force of attraction between gecko and glass. And this is known as a Van der Waal's Force. Van der Waal's Forces are, at least in part, why gauge blocks, smooth flat pieces of steel, can stick together.

It's there all the time. If you have a car with, you know, you can put these vinyl sheets on the back window so that your kids don't get like excessive sun when they're in the car seat. That's also Van der Waal's Force.

[Derek] But generally we don't notice Van der Waal's Forces. We don't feel them with our fingers because at least on the molecular scale, our skin is incredibly bumpy. Touching a piece of glass is like putting a mountain range on top of a flat plane. There aren't many points of close contact. Geckos overcome this using all those tiny branches.

So this means that when the gecko puts its foot down, it has a very large intimate area of contact. It's almost as if you were pouring glue on the surface and letting it flow. And that's what makes the adhesion work.

[Derek] Replicating the gecko's intricate branching structures is currently impossible.

We cannot make what the gecko has. We can't.

[Derek] That structure, that really fine branching structure.

That sophisticated branching structure. But we can make a more crude approximation.

[Derek] Under a microscope, you can see that the artificial gecko adhesive is covered in rows of sharp wedges. The tips are around one or two micrometers wide. That's a hundred times narrower than a human hair. To create such fine structures requires a labor-intensive process. A block of wax is used as the base for a mold and then a razor blade is repeatedly pressed into the block creating wedge-shaped indents. A silicone polymer is poured into the mold and a backing material is attached.

This is silicone?

[Lab Worker] So it's called Sylgard 170. And it is a type of silicone, yes.

[Derek] And after about 24 hours, the adhesive is cured and ready to go. And that's what it looks like. But the mold can only be used a few times before the quality of the adhesive declines and you have to start all over again.

This sharp wedge structure, which has this interesting property that when you first bring it up to a surface, the only part in contact is these tips. So therefore it's not sticky because there's no Van der Waal's Force worth mentioning. As you load it in shear, these things all bend over and we get a much larger, in fact almost continuous contact area. And so like the gecko, we have some Van der Waal's Forces to work with and that's what give us adhesion.

[Derek] So to make the adhesive stick you have to pull it parallel to the surface, that's known as a shear force, and you have to pull it in the direction that will bend the wedges so that they make contact with the surface. If you pull it in any other direction, it won't stick. This makes it easy to pull the adhesive straight off the surface. I mean, there is basically no contact force.

[Lab Worker] And so the way we clean this is just with tape, because tape doesn't stick to the Sylgard, but the dust particles do.

[Derek] They used this property to create an incredibly small and lightweight robot called MicroTug. It weighs just 17 grams, but is able to pull a 20 kilogram weight. That's the equivalent of a human towing a blue whale. Just six of these tiny robots can tow a car. The gecko adhesive is on the underside of the MicroTugs. So when they're pulling the car, the material is in shear and it sticks tight to the ground. But as the car moves forward, the shear force decreases and so it's easy for the robot to pick itself up and move itself forward. The amount of pulling force attainable depends on the area of adhesive that's in contact with the surface. So they've developed a method for measuring this by shining light through acrylic.

[Tony] So what it does is shines the LED across the acrylic and whenever there's a contact with the surface, it frustrates the ray and it shows you exactly where the contact area is.

[Derek] One square inch of contact area can support the weight of about four-and-a-half kilograms or 10 pounds. Since the adhesive is directional, to grip an object two pieces of adhesive are attached in opposite directions. So when you pull up, both sides are in shear and so they both stick. Two opposing pads of adhesive were actually tested on a robot on the international space station.

It's called Astrobee. Think of it as like a drone, a UAV, only actually uses a fan to push itself around inside the International Space Station. And the idea is that it could be there to take video or fetch something for astronauts. Our proposal was, "How about it should be able to float along gently and stick itself to a wall, for example. Or pick up a big box.

[Astronaut] Our goal is to get that blue LED to turn green.

Because there's no gravity. You don't actually need a lot of force but you do want to be able to grab very gently and release easily. And so we got the gecko adhesives up at International Space Station and it works just as well up there as it does down here.

[Derek] This went to space, this was in the space station?

This was in Space station for about a year-and-a-half.

Wow. This principle can be extended to three pieces of adhesive so that when you pull them all in they can stick to a flat surface. When it's at rest, these things are sticking up just a little bit and then when you pull there, then they become flatter right?

Yeah, the entire pad doesn't touch because it's both surfaces is not perfectly flat. And then once you activate it.

[Derek] Oh yeah, I could totally see that.

[Tony] So I'm going to unplug the power so there's absolutely no power to it at all and it's still attach the surface.

[Derek] Yeah, I was thinking that you'd have to keep the motor on but it just sort of like locks in position or what?

[Tony] Yeah, as long as there's tiny bit of tension it just stays there forever.

[Derek] There are lots of potential applications for this material, but the most obvious one is for robotic grippers. Since the adhesive sticks well with just a small shear force, it's great for picking up delicate items like produce.

These are meant to be smart gecko palms, basically. So we're trying to grasp with minimal squeezing.

We're not even really engaging the ratchet mechanism there.

[Lab Worker] Yeah, not at all.

So it's like less than a Newton. It can also grab bulky items or palm a basketball. Now can the gecko adhesive pull a car? They have hooked this one up to this rope, which is connected to a winch here, but the winch is not actually secured. It is tethered back to four pieces of gecko adhesive which are just sitting on this pipe. So the question is, can these four pieces of gecko adhesive anchor the winch as it pulls that car back? (winch grinding) Oh yeah, it's moving. (winch grinding) Wow. Could I pull these off?

[All] Yep, yeah.

How much does... like there was nothing holding it on. That's wild. But of course the application everyone wants is to be able to climb a building. And this was actually a PhD project for grad student Elliot Hawks. It was a challenge because he had to get enough gecko adhesive in contact with the glass at any one time to support his whole weight. Let me know if you want me to try to climb a building with gecko adhesive and maybe even go a little bit faster. (electronic beeping) Hey, this video was sponsored by Brilliant, the online learning tool that helps you understand game-changing technologies. Not only in the physical sciences, but also in computer science. You know, these days everyone's talking about neural networks. How AI can win any game or write sonnets with ChatGPT. Tech like this is already changing the world and you should really know how it works. Brilliant has courses that delve into the inner machinery of neural networks to discover how a computer system can actually learn. You'll be able to apply what you've learned and build a neural network yourself to recognize numbers, shapes, and objects. And if you need to refresh your math or computer science skills first, Brilliant has a whole library of courses, ranging from beginner to intermediate to advanced. Personally, I use Brilliant as a mental gym. The other day I was going through a lesson on group theory, and the thing Brilliant does better than any other platform is it forces you to think for yourself. It asks you questions every step of the way and the material is carefully curated to increase in complexity as you go. I find it really satisfying to work things out for myself. That's the only way to truly understand something. And you can try Brilliant out for free by going to brilliant.org/veritasium. And if you want to sign up for an annual premium subscription, the first 200 people to join get 20% off through that link. So I will put it down in the description. I want to thank Brilliant for supporting Veritasium and I want to thank you for watching.