A cross-section of a bristle worm jaw sample with the indentation testing sites labeled 1 to 11.
Credit: Zelaya-Lainez et al.
A sea worm’s metal-like jaws may represent an entirely new class of natural material.
In the classic guessing game “20 Questions,” imagine asking “animal, vegetable, or mineral?” to help narrow down the answer.
For the ancient sea worm Perinereis cultrifera (which is still around to this day), the answer is surprisingly complicated. This species and other predatory bristle worms have powerful jaws made from structural proteins combined with ions. They use these jaws to bite, crush, and consume food.
The jaws are so unusual in both composition and performance that some scientists have proposed a new name for materials like them: bio-metals. Their study is becoming an emerging area of biophysics.
What Makes a Material a Bio-Metal?
The term “bio-metal” describes more than a biological material that simply resembles metal. Scientific literature has previously used phrases such as “metallike biomaterials” or “biomaterials with metallike properties” for natural substances that approach metals in strength or electrical conductivity.
Bio-metals, however, are defined through a broader combination of characteristics. These include hardness, the way the material responds to strain, and its internal structure of proteins and ions.
Researchers from TU Wien (Vienna University of Technology) and the University of Vienna examined the metal-like behavior of the worm’s jaws in an effort to more clearly define this proposed category. Their findings were published in Biophysics Reviews, by AIP Publishing.
Metal Ions Strengthen the Jaw Tips
The team first measured the jaws’ hardness using nanoindentation, a technique that presses a microscopic probe into a material to determine how easily it can be dented. They combined these tests with chemical analysis and detailed imaging.
The results supported previous research showing that metal ions are more concentrated at the tips of the jaws than in their central regions. This greater concentration likely helps make the tips especially hard, which would be useful for biting and crushing prey.
The scientists then tested the jaws at several indentation depths. They found a surprising effect also observed in metals such as copper and silver, known as the Nix-Gao nanoindentation size effect.
At smaller scales, sections of the worm’s jaw became more difficult to dent. This occurs because strain changes more sharply across a smaller area, creating greater interlocking among disruptions in the material’s atomic structure. That behavior is a defining feature of the size effect.
A Property Ordinary Metals Do Not Share
Although the jaws resemble metals in several ways, they also possess mechanical qualities that make bio-metals distinct.
“Bristle worm jaws also showed size-dependent elasticity — this is a distinguishing feature of bio-metals when compared to standard crystalline metals like copper or silver,” said author Christian Hellmich.
In other words, the jaw’s ability to bend and return to its original shape also changes depending on the scale being examined. Conventional crystalline metals do not show the same behavior.
The researchers used mathematical models to explain how these unusual elastic effects could emerge from processes occurring at the atomic level. Hellmich said they are only scratching the surface of this research — pun intended.
Nature Could Inspire New Materials
The team now plans to examine more species, expand the available experimental evidence, and refine the theoretical framework behind bio-metals. The researchers are also interested in whether genetic changes could alter the structure and performance of these natural materials.
“We plan to extend the experimental database by investigating additional species to refine the theoretical concept and perform dedicated computations, and — perhaps most interestingly — to explore the link between genetic interventions and the corresponding material design space,” he said. “All this comes with true excitement about the beauty, elegance, and refinement found in and produced by nature.”
The Life of Earth
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