Benefitting from its unique helically arranged fiber structures, fish skin is a superior biological protective tissue. Reported here is a study of the microscopic morphological characteristics and mechanical properties of the skin of three species of fish, that is, Taihu white fish (Erythroculter ilishaeformis), grouper (Cichlasoma managuense), and yellowfin seabream (Ditrema temminckii Bleeker), revealing the special protective mechanisms of fish skin. Experiments involving scanning electron microscopy show that the stratum compactum is the main part of fish skin and comprises helically stacked fibers, with the helical ply angles of the fibrous layers differing significantly for the different fish species and in different regions of their bodies. Tension and penetration experiments show that fish skin provides a fish's body with considerable mechanical protection from lacerations and bites inflicted by its natural enemies. Moreover, the mechanical tests show that fish skin has two different defensive mechanisms against tension and penetration loads, thereby offering a novel idea for designing body armor that is both flexible and tough.

The bamboo weevil beetle, Cyrtotrachelus buqueti, has evolved a particular flight pattern. When crawling, the beetle folds the flexible hind wings and stuffs under the rigid elytra. During flight, the hind wings are deployed through a series of deployment joints that are passively driven by flapping forces. When the hind wings are fully expanded, the unfolding joint realises self-locking. At this time, the hind wings act as a folded wing membrane and flap simultaneously with the elytra to generate aerodynamics. The functional characteristics of the elytra of the bamboo weevil beetle were investigated, including microscopic morphology, kinematic properties and aerodynamic forces of the elytra. In particular, the flapping kinematics of the elytra were measured using high-speed cameras and reconstructed using a modified direct linear transformation algorithm. Although the elytra are passively flapped by the flapping of the hind wings, the analysis shows that its flapping wing trajectory is a double figure-eight pattern with flapping amplitude and angle of attack. The results show that the passive flapping of elytra produces aerodynamic forces that cannot be ignored. The kinematics of the elytra suggest that this beetle may use well-known flapping mechanisms such as a delayed stall and clap and fling. K E Y W O R D S3D reverse reconstruction, aerodynamic force, bio-inspired structures, microstructure, wing kinematicsThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Fish skin is a biological material with high flexibility and compliance and can provide good mechanical protection against sharp punctures. This unusual structural function makes fish skin a potential biomimetic design model for flexible, protective, and locomotory systems. In this work, tensile fracture tests, bending tests, and calculation analyses were conducted to study the toughening mechanism of sturgeon fish skin, the bending response of the whole Chinese sturgeon, and the effect of bony plates on the flexural stiffness of the fish body. Morphological observations showed some placoid scales with drag-reduction functions on the skin surface of the Chinese sturgeon. The mechanical tests revealed that the sturgeon fish skin displayed good fracture toughness. Moreover, flexural stiffness decreased gradually from the anterior region to the posterior region of the fish body, which means that the posterior region (near the tail) had higher flexibility. Under large bending deformation, the bony plates had a specific inhibition effect on the bending deformation of the fish body, especially in the posterior region of the fish body. Furthermore, the test results of the dermis-cut samples showed that the sturgeon fish skin had a significant impact on flexural stiffness, and the fish skin could act as an external tendon to promote effective swimming motion.