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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