Imagine playing the game “20 Questions” with the first question being “Animal, Vegetable, or Mineral?”
In the case of the ancient sea insect Perinereis caltolifera, which lives to this day, the answer is surprisingly complex. Like other predatory caterpillars, this species has jaws made of structural proteins and ions. Animals use these hard mouthparts to chew, crush, and consume food.
The unusual configuration and behavior of these jaws has led researchers to use the new term “biometals” to describe them. This concept is currently becoming an emerging field of biophysical research.
What makes Biometal different?
Scientists have traditionally used expressions such as “metal-like biomaterials” or “biomaterials with metallic properties” to describe biological materials that resemble metals in strength and conductivity.
However, biometals are more specifically defined. Researchers classify them according to three main characteristics: hardness, how they respond to strain, and structures formed by ions and proteins.
In a study published in AIP Publishing’s Biophysics Reviews, scientists from the Vienna University of Technology and the University of Vienna investigated the metal-like behavior of sea worm jaws. Their goal was to clarify the differences between biometals and other natural materials.
Metal ions strengthen the chin
The researchers first measured the hardness of the jaw using nanoindentation, a technique that indents material on a microscopic scale. They combined these tests with chemical analysis and detailed image processing.
Their results confirmed earlier studies showing that metal ions are more concentrated at the tips of the jaws than in the central region. This higher concentration can make the tip harder and more suitable for chewing or crushing.
The researchers then tested the jaws using several indentation depths. They discovered an unexpected effect that is also seen in metals such as copper and silver.
This phenomenon, known as the Nix-Gao nanoindentation size effect, means that smaller areas of material are less likely to indent. In the worm jaw, rapid changes in strain over a small area appear to increase the interlocking of defects within the atomic structure, resulting in characteristic size effects.
Seaworm jaws don’t behave like regular metal
Although jaws share some characteristics with familiar metals, they also have mechanical properties that differ from them.
“The caterpillar jaws also showed size-dependent elasticity, which is a distinguishing feature of biometals when compared to standard crystalline metals such as copper and silver,” says author Christian Helmich.
The research team used a mathematical model to explain how these unusual elastic effects occur at the atomic level. But Helmich says researchers are just beginning to understand these natural materials.
Nature can create new materials
“We plan to refine our theoretical concepts, expand our experimental database by investigating additional species to perform dedicated calculations, and, perhaps most interestingly, investigate the link between genetic interventions and the corresponding materials design space,” he said. “All of this is accompanied by a real excitement for the beauty, elegance and sophistication found in and created by nature.”
