"Biomimetic Artificial Nanostructured Surfaces" In: Nanotechnologies For

379 11 Biomimetic Artifi cial Nanostructured Surfaces Emmanuel I. Stratakis and Vassilia Zorba 11.1 Introduction The study and simulation of biological systems with desired properties is popularly known as biomimetics (from the Greek bios , meaning life, and mimesis , meaning to imitate). This approach involves the transformation of the ideas, concepts and underlying principles that have been developed by Nature into man - made technol- ogy. Biological systems have, through almost four billion years, discovered unique solutions for complex problems which are smart, energy - effi cient, agile, adaptable, fault - tolerant, eco - friendly, and multifunctional. Such solutions emerged as a direct consequence of evolutionary pressure which, typically, forces natural species to become highly optimized and effi cient. The adaptation of methods and systems found in Nature into synthetic constructs is therefore desirable, and Nature provides a unique source of working solutions that can serve as models of inspiration for synthetic paradigms. The superior functions found in natural systems are often achieved through a sophisticated control of structural features at all length scales, starting from the macroscopic world down to the fi nest detail, right down to the level of the atom. Although the building blocks of bone, cartilage, cuticle, mucus, and silk can be relatively simple, they are organized in a rather complex, often hierarchical, manner. Such structural complexity is possible because the manufacture, deposi- tion, and secretion of biological entities are regulated at the cellular and subcellular (gene) level; thus, natural materials are not designed in their fi nal form, but rather are self - assembled. Although the concept of biomimetics emerged during the 1960s, it has been developing rapidly during the past decade due to advancements in nano - and biotechnologies. Currently, a large area of biomimetic research deals with functional micro - and nanostructures for nanoscale devices, water repellence, self - cleaning, drag reduction in fl uid fl ow, energy conversion and conservation, high adhesion, reversible adhesion, aerodynamic lift, materials and fi bers with high mechanical strength, antirefl ection, structural coloration, thermal insulation, self - healing and sensory aid mechanisms. All of these exceptional functionalities are Nanomaterials for the Life Sciences Vol.7: Biomimetic and Bioinspired Nanomaterials. Edited by Challa S. S. R. Kumar Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32167-4 380 11 Biomimetic Artifi cial Nanostructured Surfaces excellently demonstrated by natural systems, and are based on a variety of ingen- ious designs of biological surfaces. Similar to all natural materials, biological surfaces exhibit hierarchical structuring at both the micro - and nano - scales although, due to their structural and chemical complexity, the exact working mechanisms involved have been clarifi ed only for a few systems. In this respect, biological surfaces hide a virtually endless potential of technological ideas for the development of novel artifi cial materials and systems. Since all biological surfaces serve different functions simultaneously – that is, they are multifunctional – they become even more interesting from the point of view of biomimetics. It is also important to mention here their rather special properties as living structures, such as growth without any interruption of function, an ability to adjust to changing environments, and the capability of self - repair. Whilst these smart and responsive properties are still unavailable to scientists, they clearly represent a challenge for future developments. In this chapter, attention is focused on artifi cial nanostructured surfaces that have resulted from both mimicking, and being inspired by, Nature. (Note: in the following sections, the term “ nanostructures ” includes submicron - sized or smaller structures.) Representative examples among the huge variety of biological nanostructured surfaces which have been used – or which can potentially be used – for the development of artifi cial functional materials will be presented. In addition, a comprehensive review of the different approaches used to date for the fabrication of bioinspired artifi cial nanostructured surfaces is provided. Finally, an overview of technical applications that are either under development or available in the current marketplace, is followed by an outlook of future methods for the fabrication of novel artifi cial functional nanosurfaces. It is postulated that, the success of biologically inspired artifi cial surfaces is an indication that knowledge from Nature is an interminable source of inspiration to scientists and engineers in their quest for novel nanotechnological applications. It is also concluded that new, interdisci- plinary, strategies and routes should be required in the future in order to fully understand and accurately mimic the complex adaptive functionalities of biological nanostructures. Besides presenting recent advances achieved by these tech- niques, the chapter will also delineate existing limitations and discuss emerging possibilities and future prospects. 11.2 Learning from Nature: Properties of Natural Nanostructured Surfaces Nature offers a diverse wealth of functional surfaces, the properties of which are unmatched in today ’ s artifi cial materials. This is a consequence of the fact that biological surfaces provide multifunctional interfaces to their environment. There is a growing body of information describing natural surfaces with sophisticated design strategies, which lend the organisms and plants superior mechanical, self - cleaning, optical, adhesive, actuation, sensing and responsive capabilities. Nature develops biological objects by means of growth or biologically controlled self - 11.2 Learning from Nature: Properties of Natural Nanostructured Surfaces 381 assembly adapting to the environmental condition. Such adaptive and responsive self - assembly is provided by means of a hierarchical self - organization and optimi- zation of the biological material at each level of hierarchy, so as to yield outstanding performance [1] . Indeed, well - ordered, multiscale structures with dimensions of features ranging from the macroscale to the nanoscale are extremely common in natural materials. Additionally, the common feature of the largely unrelated natural surface designs is the use of high - aspect - ratio microstructures and nanostructures, with the desired functionality being achieved through a tailored synergy of surface morphology and chemistry. In the following subsections, the most prominent properties and different functionalities of natural nanostructured surfaces will be reviewed. The few case studies from fl ora and fauna presented here provide examples of the role that surface nanotexture plays in the functionality of biological materials. The important role of hierarchical multi - length scale roughness when tailoring the superior functional properties of surfaces, and which provides inspiration to scientists in their quest to design novel artifi cial materials, is also described. 11.2.1 Wetting Properties The special functionalities of certain organisms are usually not governed by the intrinsic property of materials, but are more likely related to the unique surface microstructures and nanostructures. This is especially the case for the special wetting characteristics that have been frequently observed in nature. Biological surfaces are quite diverse in their wettability 1) characteristics, some interesting examples of which are shown in Figure 11.1 a,b. In particular, plants with leaves that emerge from the water surface or grow on land are found to exhibit hydrophobic and water - repellent properties. In contrast, plants with fl oating leaves, submerged water - growing plants and some tropical and subtropical plants, dem- onstrate an opposite behavior as these are constructed for effi cient water absorp- tion through their surfaces [2] . Alternatively, the rough and sometimes piliferous surface epidermis of animals plays a similar role in maintaining remarkable wetting properties [3] . The outermost layer of the primary biological surface is known as the cuticle (in plants) or integument (in animals). Among the most important attributes of this layer is its wettability, that enables organisms to overcome the physical and 1) The wettability of a surface can be quantifi ed exhibits at least two remarkable wetting by measuring the macroscopic contact angle characteristics originating from a very high ( CA ) of a sessile water droplet deposited on CA (> 150 ° ) and a very small water roll - off it. A surface is termed hydrophilic or (sliding) angle ( < 5 ° ). The sliding angle hydrophobic when the CA is lower or higher denotes the lowest angle to which a surface than 90 ° , respectively, super - hydrophilic if the must be tilted for the roll - off of water droplets CA is less than 10 ° , and super - hydrophobic if it to occur. is above 150 ° . A water - repellent surface 382 11 Biomimetic Artifi cial Nanostructured Surfaces Figure 11.1 Multiscale structured surfaces in shells; (d) Optical properties: cicada wings, biology. Four types of superior properties can moth compound eyes, and sponge spur. In be found in hierarchical natural surfaces. each case the fi rst row shows a photograph of (a) Wetting and self - cleaning properties: lotus the biological surface, while the second and leaf, duck feather, and mosquito

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