Structure and Mechanical Performance of a “Modern” Fish Scale

Abstract

Protective materials and structures found in natural organisms may inspire new armors with improved resistance to penetration, flexibility, light weight, and other interesting properties such as transparency and breathability. All these attributes can be found in teleost fish scales, which are the most common types of scales in modern fish species. In this work, we have studied the structure and mechanics of fish scales from striped bass (Morone saxatilis). This scale is about 200–300 µm thick and consists of a hard outer bony layer supported by a softer cross‐ply of collagen fibrils. Perforation tests with a sharp needle indicated that a single fish scale provides a high resistance to penetration which is superior to polystyrene and polycarbonate, two engineering polymers that are typically used for light transparent packaging or protective equipment. Under puncture, the scale undergoes a sequence of two distinct failure events: First, the outer bony layer cracks following a well defined cross‐like pattern which generates four “flaps” of bony material. The deflection of the flaps by the needle is resisted by the collagen layer, which in biaxial tension acts as a retaining membrane. Remarkably this second stage of the penetration process is highly stable, so that an additional 50% penetration force is required to eventually puncture the collagen layer. The combination of a hard layer that can fail in a controlled fashion with a soft and extensible backing layer is the key to the resistance to penetration of individual scales.

Structure and mechanical properties of selected protective systems in marine organisms

“…Fish scales provide protection from a variety of predators by integrating highly ordered type I collagen fibers with hydroxyapatite biomineral [13,179,180]. Scales are composites that are composed of multiple layers.…”

“…These generally include mineralized collagen and enamel-like (or bony) layers (example shown in Fig. 25a) [13,[180][181][182]. When a collagen layer is formed, it often has a Bouligandlike structure in order to provide increased toughness and strength in many directions [44,183].…”

“…When a collagen layer is formed, it often has a Bouligandlike structure in order to provide increased toughness and strength in many directions [44,183]. Scales are most commonly arranged in overlapping sheets that allow for smooth motion of the body for locomotion while ensuring full coverage for protection from predators [13,180]. This overlapping pattern minimizes drag to ease swimming by regulating wave propagation about the body [184][185][186].…”

“…This overlapping pattern minimizes drag to ease swimming by regulating wave propagation about the body [184][185][186]. While scales can vary greatly in size, shape and arrangement from species to species, they have been classified into three relevant general groups: placoid, elasmoid (with two sub-groups: cycloid and ctenoid) and cosmoid [13,180]. A fourth group, ganoid scales, is rare and only reported in predominantly freshwater fish such as gars [187] and bichirs [181] and, as a Adapted from: (a) [180], (b) [13,188], (c) [13].…”

“…25c). They are found in teleosts, a major group of marine and freshwater fish that includes more than 26,000 species within 40 orders and 448 families; the majority of living fishes are members of this group, including the striped sea bass (Morone saxatilis) [180,190,191] and sea bream (Pagrus major) [64]. Cycloid scales, found mainly in soft fin ray fish, such as salmon, carp and eels, have a smooth margin.…”