#Off the scales: Fish armor both tough and flexible

The collagen that makes up human skin, on the other hand, is “all messed up like a bowl of spaghetti, but it can unravel and align to absorb energy, which makes skin incredibly resistant to tearing,” Ritchie said. The Bouligand structure in the carp scale is much more organized but still makes for a very effective toughening mechanism.

The other noteworthy characteristic of a carp scale is the gradient between the hard and soft layers. “If we were making that as armor, we would have an interface between the hard and soft material. The interface is invariably a location where cracks and failures start,” said Ritchie, an expert in how materials fail. “The way nature does it: Instead of having these interfaces where there’s discontinuity between one material and another, nature makes a perfect gradient from the hard to the soft (tougher) material.”
Working in collaboration with the researchers at UC San Diego, the team has previously studied the arapaima, an Amazonian freshwater fish whose scales are so tough they are impenetrable to piranha, as well as other species. For this study they chose the carp, a modern version of the ancient coelacanth fish, also known for having scales that act as armor.
Now that the deformation and failure mechanisms of carp scales have been characterized, trying to reproduce these properties in an engineering material is the next challenge. Ritchie noted that advances in 3-D printing could provide a way to produce gradients the way nature does, and thus make a material that is both hard and ductile.
“Once we get a better handle on how to manipulate 3-D printing, we can start to make more materials in the image of nature,” he said.