A Biological Metamaterial—Probing the Bending Mechanics of Stingray Skeletons

Stingrays achieve highly efficient swimming locomotion through slow, graceful undulations of their enlarged pectoral fins. While a non-expert might assume these fins to be comprised solely of soft tissue, in reality, their internal structure consists of an intricate tessellation of hundreds of rigid cartilaginous rods strung together into parallel chains by hinge joints—all embedded in a soft muscular matrix. It has been proposed that this fascinating anatomy serves to modulate the bending rigidity of the fins to optimize locomotion. To date, however, limited efforts have been made to quantitatively explain the relationships between the underlying anatomical geometry and the resulting mechanical properties of the fins. In this work, we use a combination of experimental and numerical techniques to study this system, probing the structure-property relationships of this class of structure under shell-bending conditions. We relate these mechanical properties to the resulting hydrodynamic behavior of the fins, proposing a more rigorous explanation for the arrangement of cartilage observed in nature. In the future, we hope that this work can lay the foundations for more efficient artificial underwater remotely-operated vehicles, drawing inspiration from these biological examples.