Microstructure and nanomechanical properties of the exoskeleton of an ironclad beetle (Zopherus haldemani)

Abstract

This study examined natural composite structures within the remarkably strong exoskeleton of the southwestern ironclad beetle (Z. haldemani). Structural and nanomechanical analyses revealed that the exoskeleton’s extraordinary resistance to external forces is provided by its exceptional thickness and multi-layered structure, in which each layer performed a distinct function. In detail, the epicuticle, the outmost layer, comprised 3%–5% of the overall thickness with reduced Young’s moduli of 2.2–3.2 GPa, in which polygonal-shaped walls (2–3 μm in diameter) were observed on the surface. The next layer, the exocuticle, consisted of 17%–20% of the total thickness and exhibited the greatest Young’s moduli (∼15 GPa) and hardness (∼800 MPa) values. As such, this layer provided the bulk of the mechanical strength for the exoskeleton. While the endocuticle spanned 70%–75% of the total thickness, it contained lower moduli (∼8–10 GPa) and hardness (∼400 MPa) values than the exocuticle. Instead, this layer may provide flexibility through its specifically organized chitin fiber layers, known as Bouligand structures. Nanoindentation testing further reiterated that the various fibrous layer orientations resulted in different elastic moduli throughout the endocuticle’s cross-section. Additionally, this exoskeleton prevented delamination within the composite materials by overlapping approximately 5%–19% of each fibrous stack with neighboring layers. Finally, the innermost layer, the epidermis contributing 5%–7 % of the total thickness, contains attachment sites for muscle and soft tissue that connect the exoskeleton to the beetle. As such, it is the softest region with reduced Young’s modulus of ∼2–3 GPa and hardness values of ∼290 MPa. These findings can be applied to the development of innovative, fiber-reinforced composite materials.