Wen Yang¹, Haocheng Quan¹, Marc A. Meyers¹, Robert O. Ritchie²; ¹University of California San Diego, ²University of California, Berkeley

For fish scales to provide protection from predators without compromising mobility, they have to be lightweight, flexible and tough. The Arapaima fish scale is a superb example of these properties, which enables survival in piranha-infested seasonal lakes of the Amazon. The elasmoid scales comprise a laminate composite layer of parallel collagen fibrils arranged in a Bouligand-like pattern under a hard, highly-mineralized surface layer that prevents penetration damage. We measure here a J-based crack-growth fracture-toughness of the scale as high as ~200 kJ.m-2, representing an exceptional fracture resistance for a flexible biological material. This toughness is primarily the result of multiple deformation mechanisms acting in concert in the Bouligand-like structure of the scale, involving collagen lamellae at varying orientations controlling crack advance through a sequence stretching, rotation, delamination and shear at the crack-tip before fracture. Indeed, the toughness values measured for Arapaima scales are among the highest of Nature’s flexible materials.