Arapaima Fish Scale: One of the Toughest Flexible Biological Materials

In order for fish scales to provide protection from predators without significantly compromising mobility, they have to be lightweight, flexible and tough. The Arapaima fish scale is a superb example of these properties, which enable survival in piranha-infested seasonal lakes of the Amazon. These elasmoid scales are composed of two layers: a laminate composite of parallel collagen fibrils arranged in a Bouligand-like pattern, and a hard, highly mineralized surface layer that prevents initial penetration damage. We measure here the J-integral based fracture toughness of the scale and find that the crack-growth toughness as high as ~200 kJ⋅m-2, representing a very high fracture resistance for a flexible biological material. This toughness is primarily the result of multiple mechanisms of deformation acting in concert in the twisted plywood structure of the scale, involving the collagen lamellae at varying orientations controlling crack advance through stretching, rotation, delamination and shear, and finally fracture. These toughening mechanisms operate in sequence at the crack tip, retarding its advance in a most effective manner. The toughness values obtained for the Arapaima scales indicate that they are among the toughest of nature’s flexible biological materials.