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11/21/2002

Artificial Sticky Hair

Old-timers may remember a movie in which Fred Astaire dances
up and down walls and perhaps even on the ceiling? With his
remarkable agility and grace, it seemed quite plausible that he
could accomplish such a feat but we know that cinematic trickery
was involved. With today''s computers, much more spectacular
special effects are now commonplace. However, as I discussed
in a column two years ago, the gecko requires no special effects
to walk on walls or ceilings. In that column I discussed the work
of Kellar Autumn, Robert Full and their colleagues at the
University of California, Berkeley on what makes it possible for
these geckos to nonchalantly defy gravity.

They found that the gecko can walk on walls because of split
ends. The gecko''s feet harbor hundreds of thousands of fine
hairs known as setae. In turn, each seta ends in roughly a
thousand split ends known as spatulae. Each spatula has a tiny
pad on its end. These pads are really tiny, only a couple hundred
nanometers wide. (You''ll remember that a nanometer is one
billionth of a meter.) Before reading about the Berkeley work, I
thought that animals that could walk on glass used tiny suction
cups on the bottoms of their feet. I was unaware that others
thought water molecules and capillary action were involved.

Autumn, Full and their co-workers discounted both of these
possibilities. They proposed that the millions of tiny hairs on the
gecko''s feet are attracted to the surface of a wall or ceiling by so-
called van der Waals forces. The van der Waals force is a very
weak attractive force between molecules that comes into play at
very small distances. In graphite, for example, the carbon atoms
form hexagons that line up in layers or sheets. The sheets are
held together weakly by the van der Waals force. When you
write with your pencil you put enough pressure on the tip to
overcome this weak force of attraction. Consequently, sheets of
graphite slide off and onto your paper.

For the geckos, each of its hairs is assumed to be attracted
weakly to the molecules comprising the wall. With so many
hairs, the cumulative effect of all these weak forces adds up to
enough attraction to stick the gecko to the wall. In the earlier
column, I noted that the Berkeley group hoped to mimic the
gecko''s trick and come up with new adhesives. At the time, two
years ago, it seemed like a formidable task, if only because of the
challenge in duplicating so many tiny fibers in a reproducible
way. Questions also remained as to whether the attraction was
really due to van der Waals forces.

My December 2002 issue of Discover magazine contains a small
item by Maia Winestock about more recent work by Autumn and
his team. Autumn has moved to Lewis and Clark College but
still collaborates with Full and Ronald Fearing at Berkeley and
with Thomas Kenny at Stanford. Visits to their Web sites
fleshed out the details of their research. As you might suspect
from the size of the tiny gecko hairs, their work now falls into
the sphere of the red-hot field of nanotechnology. In a paper
presented in August at the 2nd IEEE Conference on
Nanotechnology, Metin Sitti and Fearing described a couple of
neat ways to try to mimic the gecko''s foot hair structure.

One approach is to use the tiny tip of an atomic force microscope
(AFM) to poke very tiny indentations in a flat wax surface. The
AFM is one of the scanning probe microscopes we''ve discussed
earlier. It essentially drags a probe with a tiny point across a
surface and you can even resolve individual atoms on the most
sensitive machines. Using the point to poke indentations in wax
is new to me. Silicone rubber is then molded over the wax
surface. When you peel off the rubber sheet, its surface has
many small cone-shaped protrusions where the rubber flowed
into the indentations. These protrusions are a good first
approximation to the gecko spatulae.

In a second approach, Sitti and Fearing used ceramic membranes
containing numerous small pores as a base. These days it''s
possible to buy ceramics that contain remarkably uniform pores
in size and distribution. Using the ceramic avoids having to
poke indentations one at a time with the AFM. When the
silicone rubber is molded over the ceramic, you again have a
rubber sheet with protrusions where the rubber flowed into the
pores.

Now that we have these rubber sheets what do we do with them?
We want to measure the force of attraction between these
artificial "hairs" and a surface. To do this we use an AFM again,
this time with a silicon tip. We bring the tip in contact with a
"hair" of rubber and, having the right equipment, we measure
how much force we need to pull off the tip from the rubber hair.
This pull-off force is compared with the pull-off force for a real
gecko hair. We also have a colleague with the smarts to come up
with mathematical calculations predicting what van der Waals
force we expect for this kind of rubber and the geometry of the
"hair".

I won''t burden you, or myself, with the details of the calculations
except to say that van der Waals forces were found to be
responsible for about half to two thirds of the pull-off forces that
were measured. If some of you say that rubber is a sticky
substance, let''s try other polymers such as polyester. With the
other polymers we find the same sort of agreement with theory
and even find a polymer whose pull-off force is in the same
range as that for a real gecko hair.

In a joint paper appearing in the August 27 Proceedings of the
National Academy of Science, Autumn and his co-workers show
that it’s the size and shape of the tips of the artificial foot hairs
that determine how forcefully the hairs stick to the surface. So,
what does this portend for the future? The gecko crowd foresees
an adhesive made by just breaking up the adhesive''s surface into
smaller and smaller units so as to maximize the van der Waals
attraction. A sticky tape of this type would be a dry adhesive
that would leave no residue on removal, a self-cleaning adhesive.
What are some potential applications? Astronauts working in
space could use dry adhesives to hang up tools while working on
space walks. Applications might be found in surgery or perhaps
in handling silicon chips in a production line.

One potential application is in the design of robots that not only
walk but also climb, like a gecko. The gecko group is
collaborating with the iRobot Corporation in trying to design
such a robot. Gecko robots could be used in exploring Mars or
dangerous environments such as burning buildings. Military and
covert operations offer a wide range of James Bond type
scenarios for such robots. A visit to the Berkeley Web site
reveals Ronald Fearing''s obvious interest in robotics. There is a
course listed in which the students build mobile robots to race
against competitors from other universities. Autumn has been
interested in animal locomotion for some time. In 1990, he
discovered a gecko species in China that has the best fuel
economy of any animal studied to date. This gecko apparently
gets three times as much movement for a given amount of energy
than any animal its size. With their backgrounds, I wouldn''t be
surprised to see these guys come up with climbing robots in the
not-too-distant future.

If the gecko is attached so strongly to the wall or ceiling, how
does he move at all? Autumn et al. found that all they have to do
is change the angle of a hair to the surface and off it pops. It also
seems that the gecko doesn''t use all his little hairs at once. That
would be overkill. While the gecko''s hundreds of millions of
spatulae might not be able to hold up even the newly thinned Al
Roker, they could probably come close to lifting Katie Couric!

Allen F. Bortrum



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-11/21/2002-      
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Dr. Bortrum

11/21/2002

Artificial Sticky Hair

Old-timers may remember a movie in which Fred Astaire dances
up and down walls and perhaps even on the ceiling? With his
remarkable agility and grace, it seemed quite plausible that he
could accomplish such a feat but we know that cinematic trickery
was involved. With today''s computers, much more spectacular
special effects are now commonplace. However, as I discussed
in a column two years ago, the gecko requires no special effects
to walk on walls or ceilings. In that column I discussed the work
of Kellar Autumn, Robert Full and their colleagues at the
University of California, Berkeley on what makes it possible for
these geckos to nonchalantly defy gravity.

They found that the gecko can walk on walls because of split
ends. The gecko''s feet harbor hundreds of thousands of fine
hairs known as setae. In turn, each seta ends in roughly a
thousand split ends known as spatulae. Each spatula has a tiny
pad on its end. These pads are really tiny, only a couple hundred
nanometers wide. (You''ll remember that a nanometer is one
billionth of a meter.) Before reading about the Berkeley work, I
thought that animals that could walk on glass used tiny suction
cups on the bottoms of their feet. I was unaware that others
thought water molecules and capillary action were involved.

Autumn, Full and their co-workers discounted both of these
possibilities. They proposed that the millions of tiny hairs on the
gecko''s feet are attracted to the surface of a wall or ceiling by so-
called van der Waals forces. The van der Waals force is a very
weak attractive force between molecules that comes into play at
very small distances. In graphite, for example, the carbon atoms
form hexagons that line up in layers or sheets. The sheets are
held together weakly by the van der Waals force. When you
write with your pencil you put enough pressure on the tip to
overcome this weak force of attraction. Consequently, sheets of
graphite slide off and onto your paper.

For the geckos, each of its hairs is assumed to be attracted
weakly to the molecules comprising the wall. With so many
hairs, the cumulative effect of all these weak forces adds up to
enough attraction to stick the gecko to the wall. In the earlier
column, I noted that the Berkeley group hoped to mimic the
gecko''s trick and come up with new adhesives. At the time, two
years ago, it seemed like a formidable task, if only because of the
challenge in duplicating so many tiny fibers in a reproducible
way. Questions also remained as to whether the attraction was
really due to van der Waals forces.

My December 2002 issue of Discover magazine contains a small
item by Maia Winestock about more recent work by Autumn and
his team. Autumn has moved to Lewis and Clark College but
still collaborates with Full and Ronald Fearing at Berkeley and
with Thomas Kenny at Stanford. Visits to their Web sites
fleshed out the details of their research. As you might suspect
from the size of the tiny gecko hairs, their work now falls into
the sphere of the red-hot field of nanotechnology. In a paper
presented in August at the 2nd IEEE Conference on
Nanotechnology, Metin Sitti and Fearing described a couple of
neat ways to try to mimic the gecko''s foot hair structure.

One approach is to use the tiny tip of an atomic force microscope
(AFM) to poke very tiny indentations in a flat wax surface. The
AFM is one of the scanning probe microscopes we''ve discussed
earlier. It essentially drags a probe with a tiny point across a
surface and you can even resolve individual atoms on the most
sensitive machines. Using the point to poke indentations in wax
is new to me. Silicone rubber is then molded over the wax
surface. When you peel off the rubber sheet, its surface has
many small cone-shaped protrusions where the rubber flowed
into the indentations. These protrusions are a good first
approximation to the gecko spatulae.

In a second approach, Sitti and Fearing used ceramic membranes
containing numerous small pores as a base. These days it''s
possible to buy ceramics that contain remarkably uniform pores
in size and distribution. Using the ceramic avoids having to
poke indentations one at a time with the AFM. When the
silicone rubber is molded over the ceramic, you again have a
rubber sheet with protrusions where the rubber flowed into the
pores.

Now that we have these rubber sheets what do we do with them?
We want to measure the force of attraction between these
artificial "hairs" and a surface. To do this we use an AFM again,
this time with a silicon tip. We bring the tip in contact with a
"hair" of rubber and, having the right equipment, we measure
how much force we need to pull off the tip from the rubber hair.
This pull-off force is compared with the pull-off force for a real
gecko hair. We also have a colleague with the smarts to come up
with mathematical calculations predicting what van der Waals
force we expect for this kind of rubber and the geometry of the
"hair".

I won''t burden you, or myself, with the details of the calculations
except to say that van der Waals forces were found to be
responsible for about half to two thirds of the pull-off forces that
were measured. If some of you say that rubber is a sticky
substance, let''s try other polymers such as polyester. With the
other polymers we find the same sort of agreement with theory
and even find a polymer whose pull-off force is in the same
range as that for a real gecko hair.

In a joint paper appearing in the August 27 Proceedings of the
National Academy of Science, Autumn and his co-workers show
that it’s the size and shape of the tips of the artificial foot hairs
that determine how forcefully the hairs stick to the surface. So,
what does this portend for the future? The gecko crowd foresees
an adhesive made by just breaking up the adhesive''s surface into
smaller and smaller units so as to maximize the van der Waals
attraction. A sticky tape of this type would be a dry adhesive
that would leave no residue on removal, a self-cleaning adhesive.
What are some potential applications? Astronauts working in
space could use dry adhesives to hang up tools while working on
space walks. Applications might be found in surgery or perhaps
in handling silicon chips in a production line.

One potential application is in the design of robots that not only
walk but also climb, like a gecko. The gecko group is
collaborating with the iRobot Corporation in trying to design
such a robot. Gecko robots could be used in exploring Mars or
dangerous environments such as burning buildings. Military and
covert operations offer a wide range of James Bond type
scenarios for such robots. A visit to the Berkeley Web site
reveals Ronald Fearing''s obvious interest in robotics. There is a
course listed in which the students build mobile robots to race
against competitors from other universities. Autumn has been
interested in animal locomotion for some time. In 1990, he
discovered a gecko species in China that has the best fuel
economy of any animal studied to date. This gecko apparently
gets three times as much movement for a given amount of energy
than any animal its size. With their backgrounds, I wouldn''t be
surprised to see these guys come up with climbing robots in the
not-too-distant future.

If the gecko is attached so strongly to the wall or ceiling, how
does he move at all? Autumn et al. found that all they have to do
is change the angle of a hair to the surface and off it pops. It also
seems that the gecko doesn''t use all his little hairs at once. That
would be overkill. While the gecko''s hundreds of millions of
spatulae might not be able to hold up even the newly thinned Al
Roker, they could probably come close to lifting Katie Couric!

Allen F. Bortrum