Will "Gecko Tape" Let Humans Climb Walls?
By John Roach
for National Geographic News
|June 2, 2003|
Humans may someday be able to scuttle up walls, scamper across ceilings, and scurry out windows with the agility of a startled gecko in the tropical nightthanks to a new adhesive tape that mimics the lizard's sticky feet.
"Geckoman is less than science fiction these days," said Andre Geim, a physicist at the University of Manchester, England, who is part of a team that overcame considerable engineering challenges to produce the first synthetic "gecko tape."
The breakthrough comes just a few years after scientists resolved the centuries-old mystery of how geckos climb up and down trees and cling from the underside of branches.
The geckos' secret is millions of microscopic hairs on the pads of their feet. Each hair, or seta, provides a miniscule adhesive force called van der Waals, which operate over very small distances but bond to just about anything.
Since geckos have millions of these hairs on each foot, their combined adhesive force is hundreds of times greater than what is required for the gecko to hang from a ceiling by one foot.
Inspired by this biological design, Geim and his colleagues etched a plastic mold with a scanning electron microscope that they used to make little hair-like pillars for a 0.4 inch by 0.4 inch (1 centimeter by 1 centimeter) piece of tape.
Once they make the tape more durable and scale up the technology, Geim and colleagues calculate that a human could hang from a ceiling by one tape-covered palm.
"I think it is really exciting to see every step toward the design of an adhesive that can be as effective as a gecko," said Robert Full, a biologist at the University of California at Berkeley. "It shows the importance of biological inspiration."
Full is part of the team that discovered in 2000 the adhesive force of the hairs on the pads of gecko feet. He and his colleagues are also developing synthetic setae based on the biological design of the gecko.
To make tape stick like a gecko, Geim and his colleagues etched several molds to make various sized plastic pillars that resembled the hairs on gecko feet. "We tried to find out what would be optimal geometry," said Geim.
Through trial and error, the researchers found that no matter what the diameter of the pillar, the adhesive force created does not change.
"Take a pin and stick it to a solid surface and it is a point of contact," explained Geim. "Now take a stick, a much larger stick, and touch it to the surface and eventually you find out that the thin pin and big stick would stick only at one point to a surface."
Once the researchers determined the optimal geometry, they densely packed the pillars to get as many individual points of contact as possible, as demonstrated by nature on the pads of the gecko foot.
They attached the pillars onto a rigid wafer and then tried to stick the wafer to a surface. Contrary to the gecko theory, however, they found that very few of the hairs actually made contact with the surface, and thus the wafer did not stick very well.
Through more trial and error, the researchers determined the reason for their failure. Owing to the inherent unevenness of even the seemingly smoothest of surfaces, very few of the hairs actually come into contact with the surface.
The researchers overcame this obstacle by making the hairs flexible and attaching them to a pliable, ribbon-like backing. The flexibility of the hairs and backing made the tape "stick to the surface quite nicely, using most hairs instead of just a fraction of them," said Geim.
Full's colleagues Ron Fearing and Metin Sitti at the University of California at Berkeley have also designed a flexible array of gecko-like hairs. Their hairs are longer and of thicker diameter than the hairs on the tape created by Geim and colleagues.
"What these guys did was design narrower, but shorter hair. Therefore they were able to pack more together to increase adhesive ability," said Full. "However, the geometry of the real gecko hairs that allows adhesion to rough surfaces will be a real challenge."
Real gecko hairs are longer and more flexible than the ones reported on by Geim and colleagues in the June 1 online edition of Nature Materials.
Geim said that he and his colleagues were inspired to try and manufacture the gecko tape after reading the paper by Full and colleagues in the June 8, 2000 Nature on how the gecko employs millions of hairs on its feet to scurry up walls.
As experts in the physics of microscopic structures such as tiny transistors in a computer processor, Geim said he and his colleagues "immediately realized it would be easy for us to contribute to this area from the point of view of physics."
Despite being quite different from an operational principle, gecko hairs and modern microelectronic devices can be produced by the same techniques, he added.
Full agrees that this collaboration between the various scientific disciplines is allowing the development of gecko tape to proceed much faster than he originally thought. His team, he notes, includes biologists, chemists, and engineers.
"It is the beginning of a new age among all the different disciplines," said Full. As a result, he predicts that within three to five years "gecko tape" will be available at the neighborhood hardware store. Just a few years ago, he thought the feat would take 20 years.
And what are the applications of a roll of gecko tape?
"It is nearly unlimited," said Full. "In addition to a general adhesive, it can be used to move computer chips in a vacuum, pick up small fibers. It can be like Velcro without needing the other side."
Geim foresees all of these applications as well and then some. "Don't forget about a gecko glove for climbers and window cleaners," he said. "Maybe that is too far of a speculation, but the physics tells us it is possible."
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