Arapaima Fish Scale: One of the Toughest Flexible Biological Materials

For fish scales to provide protection from predators without severely compromising mobility, they must be lightweight, flexible, and tough. The arapaima fish scale is a superb example of this, enabling its survival in piranha-infested lakes of the Amazon. These elasmoid scales comprise two layers: a laminate composite of parallel collagen fibrils arranged in a Bouligand-like pattern and a highly mineralized surface layer that prevents initial penetration damage. Here, we measure its J-integral fracture toughness and find that the crack-growth toughness is ∼100–200 kJ⋅m−2, representing a very high fracture resistance for a natural material. This toughness results from multiple deformation mechanisms acting in concert in the twisted plywood structure of the scale, involving the collagenous lamellae at varying orientations retarding crack advance through stretching, reorientation, delamination and shear, and fracture. The toughness values obtained for the arapaima scales indicate that they are among the toughest of nature's flexible biological materials.