Spider-Man's sticky power in human reach

Spider Man, it seems, got it wrong. By the end of the decade, humans might well be able to climb like insects up the face of any building. Yet the inspiration behind such wall-scaling technology will not be spindly-legged arachnids, but rather hairy-toed geckos.

In this month's issue of Nature Materials, a team of scientists reports that it has prodcued a dry, glueless adhesive that would allow humans to scurry across the living-room ceiling - supported solely by the feeble molecular attractions that occur every time two objects touch.

There is more work to be done before "gecko tape" is ready for human use. The first piece is smaller than a postage stamp and can only hold a Spider-Man action figure to the underside of a piece of glass.

But the promises are astounding: from gecko gloves to easy-peel bandages to one-sided velcro. Now, scientists believe they have passed perhaps the most significant stumbling block.

"There are technological challenges, but I see them only as challenges," says André Geim, a physicist at the University of Manchester in England and author of the new study. "There is nothing that is fundamental ... that could stop us from achieving this."

Studying molecular forces

How gecko tape works is a glimpse into what was - until recently - a mystery of modern biology. Scientists previously assumed that geckos used some sort of sticky residue or suction to scuttle across sheer glass. Three years ago, though, a team of biologists discovered that geckos used something far more unusual: molecular attraction.

Weak molecular forces, called van der Waal forces, attract any two touching objects to one another. These forces are so small they usually go unnoticed. When a human puts his hand against a wall, for example, it doesn't stick because only a small fraction of the hand touches the wall - perhaps several thousand points of contact.

The feet of geckos have billions of microscopic hairs per square inch, with anywhere from several to thousands of split-ends touching any surface. The resulting molecular forces are strong enough to defy gravity.

The plastic hairs on Dr. Geim's gecko tape are not nearly as concentrated as the hairs on a gecko's feet. But he says that bigger strips of his gecko tape have the potential to harness van der Waal forces to hold up larger things - like humans. "We have shown that this mechanism is scalable," he says, whereas the mechanism for insects is not.

The effort is just one example of research in the field of biomimicry, where scientists examine conch shells for ways to improve tank armor and analyze spider webs to create stronger construction materials.

Improvements still needed

However, it is uncertain when gecko tape will be strong enough to help rock climbers scale mountains or window-washers cling to skyscrapers.

For one, Geim's gecko tape is too small. A Spider Man wannabe that sought to hang from the ceiling would need at least a hand-size swath of tape. Geim's small piece of tape is only as sticky as Scotch Tape and costs $500 (See Editor's note below).

Moreover, the tape is about as durable as a croissant. Geim speculates that a student could take two or three paces up a wall before the tape began falling apart.

Scientists' next goal is to find a material that is durable - strong enough to keep the hairs from becoming tangled, yet flexible enough to ensure the hairs stick to any surface.

"There's a long way to go," says Robert Full, a biologist at the University of California at Berkeley who helped to unravel the gecko climbing mystery in 2000. "[But] the biological inspiration is going to work."

He envisions robots that can go anywhere on their gecko-like feet; bandages that won't hurt to remove; and a more efficient way to handle computer chips in vacuum environments, where sticky residues can't be used.

"It's revolutionary at all scales," Dr. Full says. "What takes longer is that we want to move bigger things, but that's not far off - maybe five years."

(Editor's note: The original version of this story misstated the cost of the scientists' small piece of gecko tape.)

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