生物仿生：植物表面结构和功能

本综述论文是德国Wilhelm Barthlott教授课题组针对植物表面的结构、功能及其演化进行了详细概述。文中不但结合了表面化学和功能结构，而且参考了可能的仿生应用。

35亿年来，1000万种生物物种的表面在环境的相互作用下演化成了高度复杂的多功能表面，生物有机体的层次结构及其功能的复杂性远远超过所有非生物的自然表面—即使生物体的超疏水性在自然界中受到制约，这也是3.5–4.5亿年前自植物和昆虫入侵陆地栖息地后的一个关键的进化步骤。应特别引起注意的是，全球环境变化使得物种急剧减少，生物作用急剧降低。地球上的生物控制群——植物，是具有大量多功能表面的生物有机体，表现出非常有趣的特征，其中的超亲水性和疏水性是本文讲述的重点特征。

我们估计超疏水性植物的叶子（例如：草）的总面积约2.5亿万平方公里，约占地球表面总面积的50%，此数据来自我们对近2万种物种所做的调查，进一步的数据参考了Barthlott等人的工作。水生无脉状植物和陆生脉状植物的基本区别是，后者呈现出一个特别有趣的表面化学和体系结构，本文详细描述了这种层次特征的多样性，首要的特征是表皮蜡质层叠加的聚合物表皮和单细胞到复杂多细胞结构的曲度，提出了这种多样性的描述性术语。

简单地说，植物表面的特征功能可分为六大类：

（1）机械特征

（2）对光谱辐射的反射和吸收

（3）减少水分流失或增加水分吸收

（4）黏附和无黏附（荷叶效应，昆虫诱捕）

（5）增加阻力和湍流，

（6）水下减阻气流或气体交换（Salvinia效应）

本文仅对仿生学史、现有和预期的仿生应用及令人印象深刻的频谱的简要回顾，并讨论了工程师和材料科学家面临的主要挑战及纳米涂层的耐久性和易脆性。

全文链接：http://dx.doi.org/10.1007/s40820-016-0125-1

文章引用信息：

Wilhelm Barthlott. Matthias Mail1. Bharat Bhushan. Kerstin Koch，Plant Surfaces: Structures and Functions for Biomimetic Innovations. Nano-Micro Lett. (2017) 9: 23. http://dx.doi.org/10.1007/s40820-016-0125-1.

Fig. 1 Hierarchical surface sculpturing of plants on the macroscopic scale. (a) The Saguaro (Carnegiea gigantea) is the largest cactus; it can grow up to 21 m tall. The stems are ribbed—even in the full sun of a desert in Arizona, large areas of the plant are shaded. At the same time, the ribs and elastic cuticle allow a rapid increase of the volume after sporadic rainfalls: the stems expand. Loss of water is a major problem for desert plants: the Saguaro is incrusted in a wax layer, but due to UV exposure the surfaces age and become wettable. Saguaros can live 150 years or more. (b) In contrast, the Giant Arum (Amorphophallus titanum) lives in the deepest shadows of the humid rain forest understories in Sumatra. Its flower opens for only one to two days; it reaches a height of three meter and is the largest blossom in the plant kingdom. The giant pleated ‘‘petal’’ (spathe) weights less than one kg: the largest light-weight construction amongst plants, possibly even in any organism. The riblets serve as mechanical stabilizers: when the spathe opens, its surface is hydrophobic to shed rain droplets. Very unusual wax crystals occur on the unpleasant smelling central column, which heats periodically to almost 40 C to generate a convection flow to attract insects

Fig. 2  Internal (‘‘non-cuticular’’) functional surfaces are usually hydrophilic, two examples from a squash or pumpkin (Cucurbita pepo) are illustrated: (a) pollen grain, its surface functions are connected with attachment and detachment to the pollinating insect and the stigma of the pumpkin flower, and possibly temperature control under insolation. In wind-dispersed pollen, these structures might also increase the Reynoldsnumbers, (b) a vessel-element of the same plant exhibiting complex spiral and perforated structures to transport water within the plant. The structural elements of internal surfaces fundamentally differ from the outer cuticular surfaces (compare, e.g., Figs. 3, 7, 14, and 15)

Fig. 3 SEM micrographs of two seed surfaces with concave cell sculpturing. The miniature seeds of both species: (a) indica and (b) Triphora trianthophora are optimized for seed dispersal by wind. They are hydrophobic and float for a short time in water (compare Aeginetia in Fig. 27): the concave sculpture of the non-living cells can be interpreted as a shrinkage deformation during seed maturing and drying. The bands which form an inner network in (a), and the surface pattern in (b) are built by cellulose. All these features are light-weight constructions and generate high Reynolds-numbers to prolong the floating time

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