A sticky puzzle is solved
Gecko ‘glue’ provides a model for new synthetic dry adhesives

By Robert Sanders, Public Affairs

The gecko’s ability to adhere to smooth surfaces is now understood. M. Moffett photo

04 September 2002 | Scientists have discovered the secret behind geckos’ ability to walk up walls and dangle from ceilings, and to prove it, they have synthesized the sticky tips of the reptiles’ toe hairs.

The minute artificial hair tips, though rudimentary, work nearly as well as the millions that line the geckos’ own feet, showing that it is possible to mimic nature and build nano-scale structures that stick to many different surfaces, as well as in environments where gluelike adhesives fail.

“This is the first step, the pathway to synthesizing the first self-cleaning dry adhesive,” says Robert Full, professor of integrative biology. “It’s going to take a lot of work to figure out the best way to make a biologically inspired adhesive, but this demonstrates that the path to manufacturing them is there.”

A self-cleaning dry adhesive would have many uses, such as moving semiconductors around in a vacuum chamber, it could also stick to surfaces under water or in space.

Full and his colleagues at Lewis & Clark College in Portland, Oregon, UC Santa Barbara, and Stanford University reported their findings in mid-August in the online edition of the Proceedings of the National Academy of Sciences.

“We have solved the puzzle of how geckos use millions of tiny foot hairs to adhere to even molecularly smooth surfaces, such as polished glass,” says Kellar Autumn, one of Full’s former students, now an assistant professor of biology at Lewis & Clark College and lead author of the study. “Our new data prove once and for all how geckos stick.”

The proof is in the pudding, though. Engineers at Berkeley created synthetic hair tips that stick almost as well as the geckos’ own.

“We’ve synthesized the smallest part of the entire foot, but now we’ve got to make billions of them to get significant adhesive force,” says Ron Fearing, a professor of electrical engineering and computer science. “We don’t have a Post-it yet.”

Full and Kellar reported two years ago the secret of getting gecko toe hairs to stick without the use of suction, glue, or static electricity; it was the angle created by the hair touching a surface. Hundreds or thousands of pads at the end of each hair interact on a molecular level with the surface to generate a combined attraction that keeps the gecko attached. If, for example, the gecko used all of its pads at once, the strength of the combined adhesive force would allow it to support 280 pounds of weight.

“We can apply those underlying principles and create a similar adhesive by breaking a surface into small bumps,” Fearing says. “These preliminary physical models provide proof that humans can fabricate gecko glue.”

For information, see www.lclark.edu/~autumn/PNAS.