Functional Surfaces in Biology: Mechanisms and Applications - Biomimetics: Biologically Inspired Technologies - page 382

Figure 15.1 Diagram of functions of cuticular microstructures (a) aerodynamically active surfaces, (b) grooming,
(c) sound generation, (d) food grinding, (e) filtration devices, (f) hydrodynamically active surfaces, (g) air retention,
(h) thermoregulation, and (i) body coloration pattern. (With permission of Springer Science þ Business Media
B.V.)

point of view of biomimetics. It is also important to mention here their rather special properties as
living structures, such as growth without interruption of function, ability to adjust to changing
environment, and ability of self-repair. These functions are still unavailable to engineers using
nonliving materials, but clearly represent a challenge for the future developments.
In the present chapter, we discuss some functions of biological surfaces (Figure 15.1) that are
potentially interesting for biomimetics, and demonstrate several examples of materials and systems
that were developed based on inspirations from biology.

15.2 SURFACES OF JOINTS AND SKIN: ANTIFRICTION AND DRAG REDUCTION

One of the challenges in designing moving parts of microelectromechanical systems (MEMS) is
fabrication of joints allowing precise motion of parts about one rotational axis or multiple axes. One
problem is the high friction, stiction, and wear rate of joints (Scherge et al., 1999; Komvopoulos,
2003). Wear of the interacting surfaces is a consequence of friction, affecting the material's
contact points by becoming deformed or being torn away. Friction and wear are strongly correlated
processes by which the points of the surfaces in contact change their topography continuously.
Capillary adhesion, due to the presence of a water layer in contact, can account for a great part of
the measured friction or lead to the stiction between a contact pair (Scherge et al., 1999). These are
critical issues limiting the operational lifetime and negatively influencing the technological poten-
tial of MEMS. Conventional methods of lubrication cannot always be used, especially in devices
with medical applications. Friction reduction in some man-made mechanical systems is based on
the different hardnesses of elements in contact (Miyoshi, 2001; Li et al., 2004), the use of
hydrophobized surfaces, and the application of surface texture, which minimizes the real contact
area between two solid surfaces. Research on optimization of surface texture has been done using a
''trial and error'' approach (Scherge and Schaefer, 1998; Etsion, 2004). However, ideas from the
studies of surface properties of biological micro-joints might represent a shortcut towards a
solution.
The biological world is part of the physical world, and therefore, the rules of mechanics also
apply to living systems. Living creatures move on land, in air, and in water. Complex motions
inside their bodies provide fluid circulation or generate forces for locomotion. The resistance
against motion mediated by surrounding media and by the mechanical contact with various
substrates was an evolutionary factor, which contributed to the appearance of many surfaces
adapted to reduce such resistance. But, one always needs friction to generate force to move on a
substrate or to overcome the drag caused by friction elsewhere. A living motion system becomes
optimized when it is capable of minimizing friction at one end of the system while maximizing it at
the other end (Radhakrishnan, 1998). In other words, a living device needs a combination of