A predatory sea worm known as Perinereis cultrifera possesses jaws that do not fit neatly into either biological or metallic categories. These jaws consist of structural proteins reinforced with metal ions, giving them mechanical properties unlike those of either conventional metals or ordinary biological tissue.
This unique combination of composition and mechanical behavior led researchers from TU Wien and the University of Vienna to propose an entirely new material classification. In a recent paper published in Biophysics Reviews, the team suggests that the creature’s jaws represent a distinct class of material, which they term bio-metal.
The jaws of bristle worms such as P. cultrifera are formed by linking structural proteins with metal ions, producing a material tough enough to crush and bite prey. Previous studies called these tissues metallike biomaterials, meaning they share some properties with metals, like strength or conductivity. The new study argues that this term is too broad. According to the researchers, a true bio-metal must meet three criteria: it must be hard, display specific strain mechanics, and have a well-defined ion-protein structure.
Hardness With a Metal Signature
To test if the jaw met these criteria, the researchers used nanoindentation, which means pressing a tiny probe into the tissue to measure its properties. They also conducted chemical tests and imaging at the same time. The results confirmed earlier findings that the jaw tips contain higher concentrations of metal ions than the center, likely making the tips harder.
When the probe pressed into the jaw at different depths, the researchers observed something called the Nix-Gao size effect. This effect, seen before in metals like copper and silver, means that smaller areas resist deformation more than larger ones. The reason is that strain gradients become larger in smaller regions, increasing resistance to deformation. Finding this metal-like behavior in worm jaws is one reason scientists have compared the material to metals.
Where the Metal Comparison Breaks Down
However, the jaw does not behave exactly like metals such as copper. Its elasticity changes depending on the size of the area tested, which is not seen in crystalline metals.
“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 Christian Hellmich, one of the paper’s authors. Copper and silver can show a hardness size effect, but neither exhibits the size-dependent elasticity that the team measured in the worm jaw. That distinction is one reason the researchers argue that these materials deserve their own category.
To explain this property, the researchers built a mathematical model showing how the jaw’s atomic structure leads to the elastic size effect. This model helps explain the effect and gives a basis for future research.
“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,” Hellmich said. “All this comes with true excitement about the beauty, elegance, and refinement found in and produced by nature.”
Next, the researchers plan to explore how genetic changes influence the development of these materials. If successful, the work could eventually help engineers develop new biomimetic materials inspired by the ion-protein architecture found in these worms.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.