- 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 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 in 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 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.
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