Scientists try to imitate nature to concoct toughness

January 01, 2013

THE ASAHI SHIMBUN

Japanese scientists and engineers are trying to draw on nature’s elegance and simplicity to develop some of the toughest materials on the planet.

Take, for example, spider silk that is only five-1,000th of a millimeter in diameter, or one-10th to one-20th the thickness of a human hair. The silk seldom tears even when being whipped up by typhoon-class winds.

The world has an estimated 40,000 spider species, including 1,500 in Japan alone, and all of them spin silk, said Takahide Kamura, a professor of zoological taxonomy at Otemon Gakuin University.

The key to spider webs’ durability is a combination of softness and rigidity.

Spiders store liquid material that turns into silk when it is ejected from silk glands on their abdomens. The silk strands come in different strengths and properties depending on their function.

They include radial filaments that make up the skeleton of webs, spiral filaments to catch prey, and dragline filaments that support spiders when they dangle from heights.

Spiral filaments can stretch much further than radial filaments before they finally snap.

Silk strands spun from an identical material may have different properties depending on the chemical reactions when exposed to air. Radial and dragline filaments abound in sheet-like crystal structures, called "beta sheets," of amino acids. These strands can stretch and shrink to a certain extent because they also contain helical structures. But beta sheets, which account for the bulk, are basically both tough and rigid.

Spiral filaments, by contrast, contain no beta sheets but abound in helical structures, whose extensibility disperses the impact of prey and wind to prevent tears. Sticky knots along the filaments also help to hold on to prey.

"Spiders are master artisans who use different silk types for different purposes," said Masao Nakagaki, a professor of molecular entomology at Shinshu University who is exploring genetic ways to use silkworms to mass-produce spider silk. "To produce strong threads, humans use heat and a lot of chemical agents, and thereby place a burden on the environment. But spiders can produce strong silk strands so matter-of-factly."

The design of the spider web is also a key to its strength.

Ko Okumura, a professor of soft matter physics at Ochanomizu University, said he and a colleague three years ago simulated how spider webs withstand the impact and weight of objects, such as prey, that get stuck.

He said they identified three major advantages about the fact that spiral filaments are more pliable than radial filaments.

For one thing, there is less of an impact on radial filaments, which helps prevent the web from being totally destroyed when prey struggles to get free in one area.

Second, a few snapped spiral filaments hardly affect the overall stress distribution on the web, unlike most materials that tend to fail from stress concentrated on one spot.

Third, slight changes in the number of radial and spiral filaments in a web due to partial repairs rarely damages the robustness of the web as a whole.

"The toughness is a product of a combination of strong and rigid radial filaments and soft and extensible spiral filaments," Okumura said.

A team of U.S. and Italian scientists reached similar conclusions in February 2012 from numerical simulations and tests involving real webs. They found that the silk stretched pliantly under stress from a breeze or small prey.

For larger disruptions, the silk initially extended, stiffened and snapped when it could stretch no further. But the web was designed so that localized holes would not lead to catastrophic tears, the researchers said.

Spiders are not the only creatures that concoct toughness through a combination of soft and rigid structures.

The surface of pearls has a stratified structure consisting of alternating hard and soft layers.

A joint study by Okumura and a French scientist, published in 2001, said that when an external impact exceeds the tolerance limit of the harder portion and damages the softer layer, the latter acts like a cushion to disperse the stress and prevent total failure from the damage.

The claws of peacock mantis shrimp, which inhabit warm seas in Asia and elsewhere, also have structures that dampen the impact. Their claws, also known as dactyl clubs, are folded under their abdomens at normal times but are swung out like hammers on approaching prey and predators.

Their strikes are strong enough to smash through small fish heads and shells. But the clubs can remain intact even after 50,000 repetitive high-velocity, high-energy strikes.

A research article published in June 2012 by a U.S.-based team of scientists said the clubs are made of hard layers of hydroxyapatite, an ingredient of human teeth, while soft layers of calcium carbonate and chitin compose the inner region.

The composite structure constitutes lines of defense to prevent impact-induced fractures at layer boundaries and prevent them from causing catastrophic failures.

Lobsters and scarab beetles provide further examples of nature's way to mitigate impact by combining hard and soft materials.

For researchers, the applications from such physical processes in natural creatures could be used in a number of ways to protect human lives.

Some spider silk is thought to be tougher than Kevlar used in bulletproof vests.

The mantis shrimp clubs are drawing attention for their potential use in lightweight but strong materials for vehicles, aircraft and military equipment.

(This article was written by Ryoko Takeishi and Akira Hatano.)

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