自然界的材料都是经过数百万年进化至今，只有最成功的才能够留存下来。所以天然的材料往往具有良好的机械性能。柚子的抗冲击性能就是一个很好的例子。柚子是最大的柑橘类的水果，它的直径在15~25cm之间，重量可以达到6kg。即使从10m的高处落下，柚子的外表面也几乎看不出明显的伤痕。
柚子皮的独特结构使它可以承受高达数千牛顿的冲击力，并且吸收大量的能量。它主要由两种不同的生物组织构成：含有皮脂腺(oil glands)的外表皮(epidermis/exocarp)，以及大部分的白色的海绵状中间皮(mesocarp)。如果能够把这样的组织结构特征移植到工程材料当中，就可以制备轻的、并具有高抗振性能(damping capability)的更加安全的工程部件。
在一项德国铸造研究院(German Foundry-Institute RWTH Aachen)同弗莱堡大学生物系以及柏林技术学院合作的项目中，研究者们就致力于把柚子皮中的结构特征移植到工程材料上。
对柚子皮的结构分析发现从柚子外皮部分到中间皮部分其密度是一个逐渐变化的过程。我们无法把致密的外皮与蜂窝状的中间皮清楚地区分开。这种渐变的组织也就避免了在组织成分、结构以及机械性能上的突变，从而降低了在收到冲击时发生组织撕裂(delamination)的可能性。
因为金属具有良好的加工性能及机械性能，所以成为研究者们首选的实验材料。对材料的加工必须一方面要获得均匀的多孔结构，而且还要保证给与铸造工艺一定的自由度，从而将不同级别的结构有机地结合起来，例如：分级的空洞、二次空洞、纤维以及复合结构。通过对熔模铸造工艺 (investment casting process)进行一定的改造，研究者们比较理想地达到了上述的各项要求。
铸造实验表明所选择的合金材料可以很好地制备成为仿生结构。总体来讲，该项目的成果为仿生多级结构的发展提供了一个良好的基础。(生物谷Bioon.com)
生物谷推荐英文摘要：
Advanced Engineering Materials DOI: 10.1002/adem.201080065
Pummelos as Concept Generators for Biomimetically Inspired Low Weight Structures with Excellent Damping Properties
Sebastian F. Fischer M.Sc.1,*, Marc Thielen2, Ruth R. Loprang M.Sc.3, Robin Seidel2, Claudia Fleck3, Thomas Speck2, Andreas Bührig-Polaczek1
Natural materials often exhibit excellent mechanical properties. An example of outstanding impact resistance is the pummelo fruit (Citrus maxima) which can drop from heights of 10?m and more without showing significant outer damage. Our data suggest that this impact resistance is due to the hierarchical organization of the fruit peel, called pericarp. The project presented in the current paper aims at transferring structural features from the pummelo pericarp to engineering materials, in our case metal foams, produced by the investment casting process. The transfer necessitates a detailed structural and mechanical analysis of the biological model on the one hand, and the identification and development of adequate materials and processes on the other hand. based on this analysis, engineering composite foam structures are developed and processed which show enhanced damping and impact properties. The modified investment casting process and the model alloy Bi57Sn43 proved to be excellent candidates to make these bio-inspired structures. Mechanical testing of both the natural and the engineering structures has to consider the necessity to evaluate the impact of the different hierarchical features. Therefore, specimens of largely varying sizes have to be tested and size effects cannot be ignored, especially as the engineering structures might be upscaled in comparison with the natural role model. All in all, the present results are very promising: the basis for a transfer of bio-inspired structural hierarchical levels has been set.
