AFM Identification of Beetle Exocuticle: Bouligand Structure and Nanofiber Anisotropic Elastic Properties

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (1 of 8) 1603993 performed on abalone nacre revealed that the material exhibits a toughness similar to aluminum even when it contains 95% calcite, a ceramic.[1] Espinosa and co-workers showed that such toughness arises from a millimeter size process zone consisting of tablets sliding without pullout.[2,3] Recent developments in advanced manufacturing such as freeze casting and 3D printing have made possible the direct translation of the elegant architectures from natural materials to artificial materials and structures. More importantly, the new technologies have pushed the sizes of building elements to an even smaller scale for biomimicry.[4] These advances put further requirements on the characterizations of the structural and material properties of natural materials, which has always been challenging since most natural materials are anisotropic and composed of complex hierarchies across several length scales. The exoskeletons of crustaceans and beetles are of significant interest due to their multifunctional properties, such as supporting the animal’s body weight and resisting predator attacks and environmental damage while enabling locomotion.[5–7] The core region inside the exoskeletons is composed of chitin fibrils arranged in a helicoidal stacking, which is also known as Bouligand type or twisted plywood structures.[8] Characterization of the Bouligand structure along with the constituent fiber properties is essential for understanding the intriguing structure-function relationship. For example, using diffraction contrast transmission electron microscopy, Giraudguille et al. characterized the Bouligand structure in the cuticle of a crab and discussed the morphology of the crystalline chitin and its surrounded protein.[9] In other studies, Raabe and co-workers presented the plywood arrangements of chitin-protein fibers in lobster samples by analyzing X-ray diffraction patterns.[10,11] Compared to crustacean cuticles in which the fibrous structures are mineralized, beetle cuticle is almost purely polymeric and the Bouligand structure inside generally appears at a smaller length scale. Beetle cuticle has been extensively investigated to gain insight into its various functions.[6] Such studies showed that the mechanical properties of its constituents play key roles in the remarkable mechanical performance of the system. An example is the efficient wing attachment, consisting of elastic hinges and wear resistant articulations,[12] that affect aerodynamic performance[13] and fatigue life.[14] Hence, characterization of cuticle structure, as an inspiring structural composite AFM Identification of Beetle Exocuticle: Bouligand Structure and Nanofiber Anisotropic Elastic Properties