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The secret of the ‘superhero’ spider leads advances in field of biomimetics

by Rowan Hooper

Three thousand years ago a bunch of Chinese silkworm farmers got fed up with their job. Instead of carrying out the tedious task of harvesting hundreds of silkworm cocoons for their silk, the farmers wondered if there wasn’t an easier way they could make the stuff artificially. There was, but the techniques wouldn’t be available for another few millennia.

At about the same time, over in ancient Greece, the architect/inventor Daedalus was also inspired by nature. According to myth, Daedalus was languishing in the labyrinth of King Minos with his impetuous son. Inspired by the views from the labyrinth of the birds flying overhead, he made artificial wings from feathers held together by wax, and flew from his prison to freedom (his son Icarus wasn’t so lucky, but that’s not the part of the story we’re interested in today).

In a better documented, if more mundane example, the Swiss engineer Georges de Mestral invented Velcro in the 1950s when he realized how the hooks of burrs clung to his dog’s fur.

These three stories are early examples of a rapidly emerging field: Biomimetics, the study of the engineering and design solutions that evolve from natural selection. While human ingenuity is remarkable, and the discoveries and inventions we have made are often breathtaking, we usually achieve only poor imitations of solutions found in nature. This is not surprising, since natural selection has had millions of years longer than we’ve had to work on honing ways to live.

But more and more, scientists are replicating in the lab what evolution has optimized in nature. There is, for example (those Chinese silkworm farmers would be pleased to know), a genetically engineered goat and its milk contains the protein for spider’s silk. On Tuesday, scientists from Penn State University presented their plans for “morphing airplane wings” — wings that change shape according to the speed and duration of flight.

Project leader George Lesieutre, professor of aerospace engineering at Penn State, said at the Structures, Structural Dynamics and Materials Conference in Palm Springs, Calif., that flying efficiently at high speed requires small, swept wings. On the other hand, flying slowly requires long narrow wings. So his team designed morphing wings that can change both wing area and cross-section shape to accommodate different flight speeds. And in the engineering equivalent of a mixed metaphor, the wing is covered with a segmented outer skin resembling the scales of a fish. Since the underlying structure of the wing undergoes radical change, the overlaying skin must be able to change with it. Lesieutre said he thinks the segmented skin idea holds great promise. The skin is composed of overlapping plates and is similar to the conveyers on airport baggage carousels.

The skeleton of the wing is composed of repeating diamond-shaped units made from straight metal members connected at angles with bend able or “compliant” shape-memory alloys. The tendons in each unit, like the ropes that shape a tent, can pull the units into new configurations that can spring back to their original shapes when the tendon tension is released.

So far, the design team has built a tabletop model of the compliant cellular truss structure and a computer model of the wing structure. Not surprisingly, like the spider-silk-in-goat-milk project (which promises light-weight bulletproof fabric), the Penn team is supported by grants from NASA and the Defense Advanced Research Projects Agency.

Such agencies are excited by the potential of modern biomimetics and will be encouraged by another development revealed this week: The secret of how spiders stick to ceilings.

A couple of years ago researchers discovered how geckos are able to defy gravity and stick to ceilings; spiders do it in essentially the same way. The base of each of the spider’s feet is covered with tiny hairs, and each individual hair is itself covered in even more hairs. These smaller hairs are called setules, and they are what makes the spider stick.

German and Swiss scientists used a technique called Atomic Force Microscopy to measure the electrical force between the tiny hairs and the surface the spiders stick to. Weak on its own, the combined force exerted by many thousands of the tiny hairs gives the spider incredible clinging power.

Andrew Martin, from the Institute of Technical Zoology and Bionics in Germany, said, “We found out that when all 600,000 tips are in contact with an underlying surface the spider can produce an adhesive force of 170 times its own weight. That’s like Spiderman clinging to the flat surface of a window by his fingertips and toes only, whilst rescuing 170 adults who are hanging on to his back.”

Because the attractive force — known as Van der Waals force — isn’t affected by the surrounding environment, it would be possible to develop materials that stick on wet or greasy surfaces. All-weather Post-it notes, for example. Less prosaically, said Antonia Kesel, head of the research group in Bremen, “you could also imagine astronauts using spacesuits that help them stick to the walls of a spacecraft — just like a spider on the ceiling.”

Kesel said the research was aimed at uncovering the tricks spiders had acquired through natural selection.

“We found that it was all down to a microscopic force between molecules,” she said. “We now hope that this basic research will lead the way to new and innovative technology.”

Certainly, biomimetics is promising more than at any stage in its long history, and it’s definitely looking more like Peter Parker than Icarus.