Bioinspired composites design: mechanical and optical characterization of the Hercules beetle elytra

Abstract

Reproducing the structure of biological materials has rendered synthetic materials with remarkable properties. Here we studied the features of a rhinoceros beetle, Dynastes Hercules, aiming to understand its structural components and relate them to its properties. This beetle changes color, from yellow-green to completely black in diverse humidity conditions due to a porous layer that absorbs water. The objectives of the work included the characterization of the reflectance spectra of the porous layer in the visible and shortwave infrared regions, and, for the first time, determine its mechanical properties. This work provides the foundation for a longer-term objective: design and generate synthetic materials that mimic the functionality of biological materials. In this particular case, the new multifunctional material could find application as lightweight camouflage effective under visible and shortwave infrared conditions. Elytra samples taken from dead specimens of Dynastes Hercules were sectioned and prepared for optical property characterization and mechanical testing. Spectral reflectance measurements were taken under wet and dry conditions which showed a similar signature to foliage. Optical and Scanning Electron Microscopy along with Focus Ion Beam cross-sectional analysis were used to examine the pores and tubular structures that allow water into the sponge-like layer that promotes the color changes. Tensile tests were used to determine Young’s Modulus and tensile strength of the elytra. Hardness and density measurements are also included. The measurements rendered specific modulus and specific strength values similar to wood. In addition, attempts to mimic the porous layer using anodizing techniques to render tubular inorganic structures are reported.