EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

In addition to the scent release and the protection from evaporation, the peel of citrus fruits provides mechanical protection. Ripe citrus fruits may drop from heights of up to 20 m, as in pomelo trees, and have to withstand enormous impacts. Within the framework of livMatS, we analyze fruit peels of different citrus species as inspiration for damping technical materials systems. The peel can be subdivided into the inner albedo (mesocarp) and the outer flavedo (exocarp) and consists of parenchymatous cells forming a spongy structure with varying density from the outside to the inside of the peel. In the peel a smooth structural transition from albedo to flavedo occurs and the two tissue types do not show a clear boundary. Additionally, mainly radial running vascular bundles appear within the whole peel and oil glands are found in the flavedo. The inner albedo cells are smaller and less densely arranged than the outer flavedo cells and have significantly larger intercellular spaces, resulting in a density gradient in the spongy peel tissue, which represents a biological composite. The mechanical properties of the peel were characterized by compression and drop weight tests. The latter reveal that the peel of all tested citrus species dissipates about 90% of the kinetic energy and that in particular, the comparatively thin peel of the lemon has an energy dissipation of over 93%. The transverse contraction of the peel was characterized by compression tests and shows very low Poisson’s ratios for lemon and citron, and even negative Poisson’s ratios for pomelo, which are an indicator for auxetic properties. Transverse contraction in response to compression impacts is highly interesting for technical damping systems as protection helmets. Hence, the high-energy dissipation and low Poisson’s ratio of Citrus peels represent promising concept generators for bioinspired materials systems.