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Abstract

The scales of the Arapaimas, a large freshwater fish (up to 200 kg), are a superb example of a natural material that has evolved to provide protection against predators. The Arapaimas lives primarily in Amazon basin lakes that become infested with piranhas as the dry season advances and is covered with scales having up to 10 cm length. They have a collagen interior with a highly mineralized external layer. The collagen fibers form a cross-lamellar arrangement which produces a laminate composite. The internal collagenous layer provides the flexibility to the scales, which can undergo significant elastic deformation prior to failure, providing considerable toughness. The mineralized exterior provides the required protection against predation. The flexibility of the biomineral is enabled by two factors: (i) a corrugated configuration in which the tensile strains are minimized; (ii) a graded composition. We propose herein that the structure of the scales, a composite consisting of a mineralized external layer with surface corrugations and an internal flexible collagenous foundation, can serve as inspiration for the design of flexible composites having a hard ceramic surface. The piranha (Serrasalminae) is one of the principal predators in seasonal lakes, in which fish get trapped. Its teeth form triangular arrays creating a guillotine action that is highly effective in slicing through muscle. The bite force is calculated for a piranha with a mass of 1 kg from estimated maximum force values applied by three muscle groups connecting the jaw to the mandible: it is ≈20 N. Mechanical tests on the scales and teeth demonstrate that the cutting and puncturing ability of the piranha teeth cannot penetrate the Arapaimas scales.