Squid Beaks Use Chemical Trick to Keep From Tearing Off

Anne Minard
for National Geographic News
March 27, 2008
Researchers have figured out how a jumbo squid's hard, razor-sharp beak can slice through the soft tissue of its prey—without tearing off from the stress.

The work solves a longstanding mystery over a problem akin to anchoring a knife blade in Jell-O, according to the authors of the new study.

Squid beaks are one of the hardest organic materials known. They're also powerful: With a single notch through a captured fish's back, a squid beak can sever the fish's nerve cord and cause paralysis.

(Related: "Jumbo Squid Invading Eastern Pacific" [March 30, 2007].)

But each blow sends powerful forces toward the soft tissues at the beak's base, and no one had ever managed to figure out how the beak dissipates those forces to prevent damage.

So Ali Miserez and his colleagues at the University of California, Santa Barbara, painstakingly sampled bits of beak all the way from tip to base and discovered changing ratios of chitin (a tough sugar chain), water, and proteins in a matrix that spans the length of the structure.

The beak exhibited an overall stiffness gradient that differs a hundredfold from beak tip to base. Though rigid at its cutting end, the beak gradually becomes softer and more flexible as it approached the soft muscle tissue.

The findings offer a potential solution for the longtime engineering struggle to attach mechanically mismatched materials.

"This could really revolutionize the way engineers think about attaching materials together in all sorts of applications," Frank Zok, a study co-author, said.

The paper appears in this week's issue of the journal Science.

All Organic

Miserez and his team mapped out the specific chemical composition of each section of beak and matched it to the mechanical properties at that point. (See a photo of a jumbo squid, aka Humboldt squid or red devil.)

"What we find responsible for the high hardness and stiffness of the beak tip is a high density of strong chemical bonds between the proteins in the beak," he said.

He added that the beak's tip is stronger and stiffer than any synthetic polymers.

That strength and stiffness gradually fades until reaching the soft, pliable tissues of the beak's anchoring point.

It's remarkable that the squid beak comprises strictly organic materials, Miserez said. In contrast, mammalian teeth contain up to 90 percent minerals.

Miserez and his colleagues also wrote that the importance of water content in defining the stiffness gradient is "notable."

But just as key is the role of the protein matrix. In it, the protein is enriched with the amino acid histidine and contains another promising, gluey amino acid called Dopa.

Researchers have found that Dopa is the powerful glue behind some of nature's most powerful adhesive systems, including the dentino-enamel junction of mammalian teeth, the arthropod exoskeleton, geckos' sticky feet, and mussel byssal threads.

In a commentary accompanying the new study, Phillip Messersmith of Northwestern University, in Evanston, Illinois, touted Dopa's chemical versatility.

It has a high affinity for certain metals and "strongly adheres to both organic and inorganic surfaces," he wrote. "Dopa has captured the interest of scientists and engineers seeking to exploit its unique properties."

Materials scientists see potential for Dopa as a wet/dry adhesive and as an ingredient in polymer coatings.

Sticky Gift

Bill Kier, a biologist at the University of North Carolina at Chapel Hill who was not involved in the new paper, called it a "thorough and careful study" that provides new insight into how chemistry and hydration affect the mechanical properties of the squid beak.

"The paper will be of interest to researchers studying biomaterials and their integration and perhaps also to engineers interested in biologically inspired materials and structures," he said.

Materials scientists currently struggle to join dissimilar materials without damaging the weaker substance. Joints, adhesives, nuts, and bolts are humans' best answers.

"But these approaches have their limitations," study co-author Zok said. "If we could reproduce the property gradients that we find in the squid beak, it would open new possibilities for joining materials."

For example, he said, engineers could create a robust bond if they made a graded adhesive with properties to match one material on one side and a different material on the other side.

Lead study author Miserez said there's also an environmental angle to borrowing designs from animals like the squid.

"Biological materials are 'made' by animals at the temperature of oceans and using naturally occurring chemicals," he pointed out.

"If we can fully understand the chemistry and copy it, then that could lead to a generation of synthetic materials that are less harsh to the environment and made at a lower energetic cost."

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