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386 Biomimetics: Biologically Inspired Technologies

Figure 15.4 SEM micrographs of the ventral skin surface of the rattlesnake.

different functional loads (Nachtigall, 1974; Gorb, 2001). There is no doubt that many functional
solutions have evolved independently in different lineages. Many species of animals and plants are
supplied with diverse attachment devices, the morphology depending on the species biology and the
particular function in which the attachment device is involved. Evolutionary background and
behaviour influence the specific composition of attachment systems in each particular species.
There are eight fundamental classes of attachment principles: (1) hooks, (2) lock or snap, (3) clamp,
(4) spacer, (5) suction, (6) expansion anchor, (7) adhesive secretions (glue), and (8) friction (Gorb,
2001). However, different combinations of these principles also occur in existing attachment
structures. Three types of adhesion at the organism level are known: (1) temporary adhesion
allowing an organism to attach strongly to the substrate and detach quickly when necessary (see
below the subsection about locomotory attachment devices); (2) transitory adhesion permitting
simultaneous attachment and movement along the substrate; (3) permanent adhesion involving the
secretion of a cement. These three types of adhesion do not have the same purpose and use different
adhesive systems.
The industry of adhesives presently follows three main goals (Hennemann, 2000): (1) an
increase in the reliability of glued contact; (2) mimicking of natural, environment-friendly glues;
(3) development of mechanisms for application of a minute amount of glue to the surface. An
additional challenge is the use of substances or mechanisms which allow multiple attachment and
detachment, and enable attachment to a variety of surfaces.
Many biological attachment devices correspond to some of these requirements. One such
example is the hairy surface of the leg pulvillus in flies. This system uses a secretion enabling
hairs to attach and detach to diverse substrata very quickly. The hair design includes a mechanism
that delivers the secretion, in extremely small amounts, directly to the contact area, and only then
when contact to the substrate is achieved (Figure 15.5).
Walking machines usually use suckers to hold onto vertical and overhanging surfaces. A primary
disadvantage of this attachment principle is that a very smooth substrate surface is required. The
future goal should be to make robots that are able to walk on a variety of surfaces. Insects can walk
rather well on smooth and structured substrata, on inclines, vertical surfaces, and some of them even
on the ceiling.
In their evolution, animals have developed two distinctly different mechanisms to attach
themselves to a variety of substrates: with smooth pads or with setose, or hairy surfaces. Due to