Hierarchical structure and mechanical properties of snake (Naja atra) and turtle (Ocadia sinensis) eggshells

After hundreds of million years of evolution, natural armors have evolved in various organisms, and has manifested in diverse forms such as eggshells, abalone shells, alligator osteoderms, turtle shells, and fish scales. Eggshells serve as multifunctional shields for successful embryogenesis, such as protection, moisture control and thermal regulation. Unlike calcareous avian eggshells which are brittle and hard, reptilians have leathery eggshells that are tough and flexible. Reptilian eggshells can withstand collision damages when laid in holes and dropped onto each other, and reduce abrasion caused by buried sand. In this study, we investigate structure and mechanical properties of eggshells of Taiwan cobra snake (Naja atra) and Chinese striped-neck turtle (Ocadia sinensis). From Acid Fuchsin Orange G (AFOG) staining and ATR-FTIR examination, we found that both eggshells are mainly composed of keratin. The mechanical properties of demineralized snake and turtle eggshells were evaluated by tensile and fracture tests and show distinctly difference. Turtle eggshells are relatively stiff and rigid, while snake eggshells behave as elastomers, which are highly extensible and reversible. The exceptional deformability (110-230% tensile strain) and toughness of snake eggshells are contributed by the wavy and random arrangement of keratin fibers as well as collagen layers. Multi-scale toughening mechanisms of snake eggshells were observed and elucidated, including crack deflection and twisting, fibers reorientation, sliding and bridging, inter-laminar shear effect, as well as the α-β phase transition of keratin. Inspirations from the structural and mechanical designs of reptilian eggshells may lead to the synthesis of tough, extensible, lightweight composites which could be further applied in the flexible devices, packaging and bio-medical fields.

Amniotic eggshells serve as multifunctional shields for successful embryogenesis. The avian eggshells have been extensively studied while there are very few studies on reptilian eggshells and most of them focused on mineralization and embryotic development. For the first time, the hierarchical structure and mechanical properties of snake and turtle eggshells are comprehensively and comparatively studied. Both snake and turtle eggshells are multilayer, hierarchically-structured composites consisting mainly of keratin yet their mechanical behaviors are distinctly different. Turtle eggshells are stiff and rigid, while snake eggshells are highly extensible (>200%) and reversible due to multiple deformation stages, phase transition of keratin and various toughening mechanisms. We believe that this study will make positive scientific impact and interest the broad and multidisciplinary readership.