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Sticky Situation

INSPIRED BY A GECKO, UCSB SCIENTISTS GET A GRIP ON ADHESIVE RESEARCH

Creatures' feet are "beautifully engineered structures"

By ANNA DAVISON, NEWS-PRESS STAFF WRITER

May 18, 2006

An orange-spotted gecko dubbed Dude, who spends his days firmly stuck to the glass wall of a terrarium, is providing inspiration for UCSB scientists trying to create new adhesives.

"We observe that gecko a lot," said Kimberly Turner, an associate professor in the Mechanical Engineering Department. His feet, she added, are "really, really beautifully engineered structures."

Dude, a tokay gecko that lives with Michael Northen, provides inspiration for the UCSB researcher's work.
Mr. Northen and other scientists at the university are trying to create adhesives modeled on the gecko's sticky toes.

They're so sticky that geckos can hang upside-down from a single toe, or catch themselves on a leaf as they fall. Scientists have long been fascinated by gecko feet and in recent years these remarkable structures have grabbed attention in scientific journals otherwise devoted to the likes of nanoelectronics. Not only do they stick hard, but they unstick -- repeatedly.

While some researchers have been figuring out how geckos achieve their superior stickiness, others are hoping to create a material with those very same properties.

"You're not just making something that looks like the gecko, but something that has the functionality," Ms. Turner said. "We'd love to be able to get a structure that can run around and stick and unstick."

Such an adhesive could enable diminutive robots to scuttle up walls and hang off ceilings in a burning building, or to explore other inhospitable environments -- maybe Mars. It could stick a coffee mug to the dashboard of a car, or could assist "Navy SEALS scaling the side of a ship," said Michael Northen, a postdoctoral researcher in Ms. Turner's lab -- and owner of Dude. "That's pretty out there," he added. "Whether that's possible is debatable."

Ms. Turner and her team are among the swelling ranks of scientists who are looking to nature to create "bio-inspired materials." Some are studying how spiders spin silk in an effort to produce new fibers with useful properties. Also at UCSB, Daniel Morse, a professor in the Department of Molecular, Cellular and Developmental Biology, is looking at how sea sponges produce their elaborate silica skeletons and harnessing those processes to produce new materials.

Dude, a tokay gecko, is "a huge inspiration," Mr. Northen said. Although he's fetchingly adorned with orange spots and big yellow eyes, Dude is hardly an affectionate pet. Tokay geckos are renowned for their nastiness, and they express their displeasure with interfering humans by barking loudly, and biting -- hard. The bite marks on Mr. Northen's leather gloves are proof of that.

Nonetheless, you can't beat those feet.

Scientists' fascination with gecko appendages goes back a long way -- to Aristotle, who marveled at their climbing ability in a manuscript written in the 4th century B.C. Since then, Mr. Northen said, there's been "a huge debate" on how geckos achieve their stickiness -- "suction cups, sticky secretions, mechanical interlocking, some kind of short-range force . . . ?"

In fact, geckos aren't accessorized with sticky goop, or octopus-style suction pads. The secret to their stickiness is in the intricate structure of their feet.

Each toe bears a series of overlapping layers, which are comprised of a multitude of tiny hairs. Each hair is split into hundreds of even smaller structures, each with a broad tip, like a spatula. These "spatulae" are so small and so plentiful that researchers believe that atoms within them interact with those in the surface the gecko is sticking to, increasing the adhesion. Although there's still plenty of debate over the forces that may be at work in a gecko's foot, Mr. Northen said, "I don't really care. I have the gecko. The gecko works."

Intricate structures on silicon wafers

Kimberly Turner, an associate professor in UCSB's Department of Mechanical Engineering, and postdoctoral researcher Michael Northen are creating adhesives inspired by geckos' sticky feet. They begin with a silicon wafer--like this one--and etch it to create a structure similar to the gecko's foot.

There's more to the function of a gecko foot than simply sticking, though. Unsticking is just as important. After all, geckos "don't stick there like a Post-It note," Ms. Turner said, "they run up and down walls and ceilings."

Geckos can unstick their feet in a flash and they do it not by pulling, but by peeling them off a surface -- the angle of the hairs to the surface is important.

While others in the gecko community have produced collections of miniscule rods modeled on those on a gecko's toes, Mr. Northen said the UCSB gecko team's approach is different because they're creating something inspired by the whole structure of the gecko's foot -- the increasingly small divisions.

They begin with a silicon wafer, four inches across, then etch its surface to produce an intricate pattern of subdivided parts, a process that takes about an hour, Mr. Northen said. Then a coating is added and tiny rods are sculpted from that.

After a lot of trial and error, and boxes of wafers, they've created an adhesive that's very sticky, although it doesn't reach the gecko's gold standard.

"Oh, geckos, they're good," Mr. Northen remarked.

Scientists have calculated that the four feet of a tokay gecko could support the weight of a small child. But that's not to say that humans will be hanging from ceilings anytime soon.

"Why can't we just be Spiderman and climb up the walls?" Ms. Turner asked rhetorically. To do that, she continued, you'd need "hands almost the size of your body. So, Spiderman gloves? Probably not."

e-mail: adavison@newspress.com