"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.
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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