Biomimetics

Nature has gone through evolution over the 3.8 G year since life is estimated to have appeared on the Earth [1]. Nature has evolved objects with high performance using commonly found materials. These function on the macroscale to the nanoscale.

The understanding of the functions provided by objects and processes found in nature can guide us to imitate and produce nanomaterials, nanodevices and processes. On nanoscale, many of the atoms are still located on the surface, or one layer removed from the surface, as opposed to the interior. Thus, different properties are observed on this scale due to the interface that is not observed in the bulk or individual atoms.

Since the properties depend on the size of the structure, instead of the nature of the material, reliable and continual change can be achieved using a single material [2]. As to nanoscale structure, nature is the best teacher giving from God. Biologically inspired design or adaptation or derivation from nature is referred to as ‗biomimetics‘.

It means mimicking biology or nature and is defined as ‗the study of the formation, structure or function of biologically produced substances and materials and biological mechanisms and processes especially for the purpose of synthesizing similar products by artificial mechanisms which mimic natural ones‘. Nature uses commonly found materials, and properties of the materials and surfaces result from a complex interplay between the surface structure and the morphology and physical and chemical properties. Many materials, surfaces and devices provide multifunctionality.

3

Molecular-scale devices, superhydrophobicity, self-cleaning, drag reduction in fluid flow, energy conversion and conservation, high adhesion, reversible adhesion, aerodynamic lift, materials and fibers with high mechanical strength, biological self-assembly, antireflection, structural coloration, thermal insulation, self-healing and sensoryaid mechanisms are some of the examples found in nature that are of commercial interest.

There are three areas had grab the eyes from academic and commercial field include cleaning surface, optics and adhesion. The most familiar object within the surface cleaning area for us is Lotus absolutely. The surface of lotus leaves has two levels of microscopic roughness (Fig. 1.1). This hierarchical roughness along with a hydrophobic wax coating makes the lotus leaves superhydrophobic [3-4]. A water droplet forms a large contact angle with low contact angle hysteresis. This results in the water droplets rolling off the surface, leaving the surface clean.

Figure 1.1 Water on the surface of a lotus leaf and the microscopic image of the surface of a lotus leaf.

Moth eyes are the elite in optics area. Bernhard & Miller discovered that the outer surface of the facet lenses in moth-eyes consists of an array of cuticular protuberances termed corneal nipples. Moths use hexagonal arrays of nonclose-packed (NCP) nipples as antireflection coatings (ARCs) to reduce reflectivity from their compound eyes [5-6, 8] (Fig. 1.2). The outer surface of the corneal lenses of moths consists of

4

NCP arrays of conical protuberances, termed corneal nipples, typically of sub-300nm height and spacing. These arrays of subwavelength nipples generate a graded transition of refractive index, leading to minimized reflection over a broad range of wavelengths and angles of incidence [7]. Accordingly, it increases the transmittance, and therefore the initial interpretation of the nipple array was that it helps to enhance the light sensitivity of the light-craving moths.

Figure 1.2 SEM images of (a) HCP micro hemispheres and (b), hexagonally NCP nanonipples covering an ommatidial surface.

A gecko is the largest animal that can produce high (dry) adhesion to support its weight with a high factor of safety. The secret of the gecko‘s adhesive properties lies in the microstructure and nanostructure of gecko feet [9-10]. Microscopy shows that gecko feet are covered with millions of small hairs called setae, which further divide into hundreds of smaller spatulas (Fig. 1.3) [11]. When such a structure is placed against any surface, hairs adapt and allow a very large area of contact with the surface.

The van der Waals interaction between approximately millions setae and the substrate after contact is sufficient for the gecko to adhere and allow them to climb vertical surfaces at speeds of over 1 m, with the capability to attach or detach their toes in milliseconds. It has been suggested that this same hairy carpet on the gecko feet also

5

plays an important role in self-cleaning [12].

Figure 1.3 (a) a gecko toe. Each toe contains hundreds of thousands of setae and each seta contains hundreds of spatula. SEM micrographs (at different magnifications) of (b) the setae (ST) and (c) the spatula (SP).

Gecko foot-hair, moth eyes and lotus surface mimicking structure were reported in ―Nature‖ and ―Science‖ in the past several times which we will have review next chapter. that shows a way to manufacture a prototype such as gecko-tape made by microfabrication of dense arrays of flexible plastic pillars with self-cleaning, re-attachable, the geometry of which is optimized to ensure their collective adhesion as shown in Fig. 1.3 which proves that the re-attachable dry adhesives based on the gecko principle can find a variety of applications.

The emerging field of biomimetics is already gaining a foothold in the scientific and technical arena. It is clear that nature has evolved and optimized a large number of materials and structured surfaces with rather unique characteristics. As we understand the underlying mechanisms, we can begin to exploit them for commercial applications.

6