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.