The Tokay gecko is the envy of every serious rock climber and Spiderman wannabe. This tropical lizard runs up walls and upside down across ceilings as readily as across floors. It can hang from one toe pad--that's akin to holding oneself in midair by a fingertip. The gecko's secret, says Kellar Autumn, a biomechanist at Lewis and Clark College in Portland, Oregon, is that each toe pad has rows of tiny hairs with multiple split ends.
The hairy feet by themselves aren't news. The soles of gecko toe pads are famous for being covered with rows of hairs, called setae. Each seta's curved shaft ends in many hundreds of stubby tendrils--too small to see with a regular microscope--with rounded ends. To figure out how these hairs might help geckos hang upside down, Autumn--then at the University of California, Berkeley--and his colleagues used a microelectrical mechanical sensor to measure the lateral and perpendicular forces exerted by a single hair that had been removed from a gecko's foot.
At first the hair didn't stick well to the sensor surface. But that changed after the researchers gently pressed the hair into the surface and then began to drag it across and nearly parallel to the sensor--movements that resemble how intact setae work as the gecko puts its foot down.
Previously, researchers had measured the overall adhesive forces of a gecko foot and calculated the contributions of individual hairs. But amazingly, the new study shows that "each [hair] was 10 times more adhesive than we would have predicted," Autumn says. One seta is strong enough to hold up an ant, and a million could support a small child. Autumn and other researchers have ruled out suction, glue, or even electrostatic forces; instead, they think that as the tendrils get close enough to the surface, they generate weak intermolecular forces, akin to van der Waals forces, that add up to a secure foothold.
The work has delighted other scientists. "It's great to look at how evolution has solved mechanical problems," marvels Bruce Jayne, a functional morphologist at the University of Cincinnati. The researchers hope that further studies will help them design simplified setae that can be manufactured. Eventually, Full and Autumn envision an all-purpose, reusable, gecko tape--one that leaves no residue behind.
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