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Paul Day and Alan Asbeck worked on adhesives for the feet of the gecko-like Stickybot.
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A Stanford
mechanical engineer is using the biology of a gecko's sticky foot to create a
robot that climbs. In the same way the small reptile can scale a wall of slick
glass, the Stickybot can climb smooth surfaces with feet modeled on the
intricate design of gecko toes.
Mark Cutkosky,
the lead designer of the Stickybot, a professor of mechanical engineering and
co-director of the Center for Design Research, has been collaborating with
scientists around the nation for the last five years to build climbing robots.
After
designing a robot that could conquer rough vertical surfaces such as brick
walls and concrete, Cutkosky moved on to smooth surfaces such as glass and
metal. He turned to the gecko for ideas.
"Unless
you use suction cups, which are kind of slow and inefficient, the other
solution out there is to use dry adhesion, which is the technique the gecko
uses," Cutkosky said.
Wonders
of the gecko toe
The toe of a gecko's foot contains hundreds of flap-like ridges called
lamellae. On each ridge are millions of hairs called setae, which are 10 times
thinner than a human's. Under a microscope, you can see that each hair divides
into smaller strands called spatulae, making it look like a bundle of split
ends. These split ends are so tiny (a few hundred nanometers) that they
interact with the molecules of the climbing surface.
The
interaction between the molecules of gecko toe hair and the wall is a molecular
attraction called van der Waals force. A gecko can hang and support its whole
weight on one toe by placing it on the glass and then pulling it back. It only
sticks when you pull in one direction—their toes are a kind of one-way
adhesive, Cutkosky said.
"It's
very different from Scotch tape or duct tape, where, if you press it on, you
then have to peel it off. You can lightly brush a directional adhesive against
the surface and then pull in a certain direction, and it sticks itself. But if
you pull in a different direction, it comes right off without any effort,"
he said.
Robots
with gecko feet
One-way adhesive is important for climbing because it requires little effort to
attach and detach a robot's foot.
"Other
adhesives are sort of like walking around with chewing gum on your feet: You
have to press it into the surface and then you have to work to pull it off. But
with directional adhesion, it's almost like you can sort of hook and unhook
yourself from the surface," Cutkosky said.
After the
breakthrough insight that direction matters, Cutkosky and his team began asking
how to build artificial materials for robots that create the same effect. They
came up with a rubber-like material with tiny polymer hairs made from a
micro-scale mold.
The designers
attach a layer of adhesive cut to the shape of Stickybot's four feet, which are
about the size of a child's hand. As it steadily moves up the wall, the robot
peels and sticks its feet to the surface with ease, resembling a mechanical
lizard.
The newest
versions of the adhesive, developed in 2009, have a two-layer system, similar
to the gecko's lamellae and setae. The "hairs" are even smaller than the
ones on the first version—about 20 micrometers wide, which is five times
thinner than a human hair. These versions support higher loads and allow
Stickybot to climb surfaces such as wood paneling, painted metal and glass.
The material
is strong and reusable, and leaves behind no residue or damage. Robots that
scale vertical walls could be useful for accessing dangerous or hard to reach
places.
Next
steps
The team's new project involves scaling up the material for humans. A
technology called Z-Man, which would allow humans to climb with gecko adhesive,
is in the works.
Cutkosky and
his team are also working on a Stickybot successor: one that turns in the
middle of a climb. Because the adhesive only sticks in one direction, turning
requires rotating the foot.
"The new
Stickybot that we're working on right now has rotating ankles, which is also
what geckos have," he said.
"Next
time you see a gecko upside down or walking down a wall head first, look
carefully at the back feet, they'll be turned around backward. They have to be;
otherwise they'll fall."
Cutkosky has
collaborated with scientists from Lewis & Clark
College, the University
of California-Berkeley, the University of Pennsylvania,
Carnegie Mellon University
and a robot-building company called Boston Dynamics. His project is funded by
the National Science Foundation and the Defense Advanced Research Projects
Agency. The research is described in a paper published online Aug. 2 in Applied Physics Letters, "Effect
of fibril shape on adhesive properties."
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