Multi-scale Structural Characterization and Mechanical Evaluation of Protective Bio-composites: Inspirations from Cobra Snake and Chinese Striped-neck Turtle Eggshells

Abstract

Turtle eggshells and snake eggshells are both composed of keratin yet exhibit distinctly different mechanical properties. Under deformation, turtle eggshells behave as typical keratin which is stiff and strong while snake eggshells behave as elastin that is extensible and reversible. It is interesting to know how nature uses the same building block but mergers structure in different ways to achieve unique mechanical properties. In this study, the structural and mechanical design of eggshells of cobra snake (Naja atra) and Chinese striped-neck turtle (Ocadia sinensis) are investigated and compared. Hierarchical structure is observed by optical microscopy and FE-SEM. Crystalline minerals are analyzed by X-ray diffraction and compositions are confirmed by tissue staining methods and EDS. Results of tensile test, fracture test, and cyclic test are analyzed in combination with ATR-FTIR examination. Various toughening mechanisms are unveiled: Snake eggshell exhibits structural enhanced plasticity and extensibility while turtle eggshell is plywood structural promoted stiffness and strength. Dual functions are performed by both reptilian eggshells that protect hatchlings from outer attacks and enable them to break the eggshells easily from inside. However, the structure and mechanical responses of the two eggshells are so different that they evolve unique structures and strategies to break the eggshells. The directionally-dependent penetration resistance of the turtle eggshell and the sharp, cone-liked egg tooth of baby turtle successfully serve as protection from outside but enable effective penetration from inside. Plenty of scratch channels in the inner membrane of snake eggshell and the blade-liked egg tooth imply a repeated slicing strategy adopted by baby snake to break through the ductile eggshell. Inspirations from the structural and mechanical designs of two reptilian eggshells may lead to the novel synthesis of tough, extensible, lightweight polymer-based composites which can be applied in advanced flexible devices, packaging and bio-medical fields.