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Functional Surfaces in Biology: Mechanisms and Applications 383

maximum friction required for acceleration, deceleration, and maneuvering combined with min-
imum friction in joints for economic energy expenditure. Adhesion phenomena can also contribute
to the functionality of such a system.
Vertebrate bones, that are joined with each other, are covered by cartilage, which is the gliding
surface of the joint. The coefficient of friction is very low (0.0026) (Fung, 1981). Cartilage is a
fibrous composite material of collagen fibers embedded in a highly hydrated proteoglycan gel
(Buckwalter, 1983; Aspden, 1994). The so-called white fibro-cartilage is responsible for joint
mobility. It provides lubrication of surfaces in contact (Ateshian, 1997) and serves as a kind of
damper under dynamic loads. Four theories explaining the cartilage lubrication mechanism have
been previously reviewed. These are fluid transport theory, lubrication layer theory, roller-bearing
theory, and cartilage material theory (Fung, 1981; Scherge and Gorb, 2001). In insect joints, which
work under lower loading forces, but much higher frequencies than vertebrate joints (Wootton and
Newman, 1979; Gronenberg, 1996), the joint surfaces are usually smooth or present a combination
of wavy and smooth counterparts (Figure 15.2). Underlying tissues are penetrated with canals,
which are presumably responsible for delivering lubricants in the contact area. The specialized
surface structures in the insect joints have been shown to confer friction-reducing properties in
certain insect surfaces (Perez Goodwyn and Gorb, 2004). The next step is to transfer the structural
and functional solutions found in biological joints to industrial systems.
Evolutionary processes have adapted swimming and flying organisms to interact efficiently with
the surrounding medium. Reduction of drag due to friction in the boundary-layer close to the body
surface is one of these adaptations. Skin secretion (mucus), compliant material of skin, scales,
riblets and the degree of roughness may influence the flow velocity gradient, the type of flow, and
the thickness of the boundary-layer around animals, and may seriously affect their drag in a positive
or negative way. Boundary-layer damping results from a combination of elastic and viscoelastic
structures in the skin of some animals. Dolphin skin has a very special design (Nachtigall, 1977). It
is very smooth and relatively soft. Under the pressure of microturbulence, the rubber-like outer

Figure 15.2 Examples of micro-joints in insects. (a) Lateral view of the wing double wave locking mechanism in
the bug Coreus marginatus (forewing part). This joint provides interlocking between both wings on the same side of
the body in the anterior direction allowing them to slide in the medial and lateral directions. (From Perez Goodwyn,
P.J. and S.N. Gorb (2004) J. Comp. Physiol. A 190: 575–580. With permission of Springer Verlag.) (b) Medial
aspect of the femoro-tibial joint (femoral part) of the leg in the beetle Melolontha melolontha. (c) Fracture of the
material of the joint in the beetle M. melolontha. (d) Diagram of the wing locking mechanism shown in (a).
(e) Diagram of the femoro-tibial joint shown in (b) and (c). Constructional principles and mechanical principles
found in such joints can be used to design joints in technical actuators (a)–(c).