自然界的材料都是經(jīng)過數(shù)百萬年進化至今，只有最成功的才能夠留存下來。所以天然的材料往往具有良好的機械性能。柚子的抗沖擊性能就是一個很好的例子。柚子是最大的柑橘類的水果，它的直徑在15~25cm之間，重量可以達到6kg。即使從10m的高處落下，柚子的外表面也幾乎看不出明顯的傷痕。

　　柚子皮的獨特結(jié)構(gòu)使它可以承受高達數(shù)千牛頓的沖擊力，并且吸收大量的能量。它主要由兩種不同的生物組織構(gòu)成：含有皮脂腺(oil glands)的外表皮(epidermis/exocarp)，以及大部分的白色的海綿狀中間皮(mesocarp)。如果能夠把這樣的組織結(jié)構(gòu)特征移植到工程材料當(dāng)中，就可以制備輕的、并具有高抗振性能(damping capability)的更加安全的工程部件。

　　在一項德國鑄造研究院(German Foundry-Institute RWTH Aachen)同弗萊堡大學(xué)生物系以及柏林技術(shù)學(xué)院合作的項目中，研究者們就致力于把柚子皮中的結(jié)構(gòu)特征移植到工程材料上。

　　對柚子皮的結(jié)構(gòu)分析發(fā)現(xiàn)從柚子外皮部分到中間皮部分其密度是一個逐漸變化的過程。我們無法把致密的外皮與蜂窩狀的中間皮清楚地區(qū)分開。這種漸變的組織也就避免了在組織成分、結(jié)構(gòu)以及機械性能上的突變，從而降低了在收到?jīng)_擊時發(fā)生組織撕裂(delamination)的可能性。

　　因為金屬具有良好的加工性能及機械性能，所以成為研究者們首選的實驗材料。對材料的加工必須一方面要獲得均勻的多孔結(jié)構(gòu)，而且還要保證給與鑄造工藝一定的自由度，從而將不同級別的結(jié)構(gòu)有機地結(jié)合起來，例如：分級的空洞、二次空洞、纖維以及復(fù)合結(jié)構(gòu)。通過對熔模鑄造工藝 (investment casting process)進行一定的改造，研究者們比較理想地達到了上述的各項要求。

　　鑄造實驗表明所選擇的合金材料可以很好地制備成為仿生結(jié)構(gòu)。總體來講，該項目的成果為仿生多級結(jié)構(gòu)的發(fā)展提供了一個良好的基礎(chǔ)。(生物谷Bioon.com)

　　生物谷推薦英文摘要：

　　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.