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Functional Surfaces in Biology: Mechanisms and Applications 389

Hairy and smooth leg attachment pads are promising candidates for biomimetics of robot soles
adapted for locomotion. Similar principles can be applied to the design of microgripper mechan-
isms with an ability to adapt to a variety of surface profiles. Most recent data on hairy systems
demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems,
some hairy systems seem to operate with dry adhesion, and do not require supplementary fluids in
the contact area. Contacting surfaces in such devices are subdivided into patterns of micro or
nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and
their density higher as the body mass increases (Scherge and Gorb, 2001). We have explained this
general trend by applying the Johnson-Kendall, Roberts (JKR) contact theory, according to which
splitting up the contact into finer subcontacts increases adhesion (Arzt et al., 2003). The funda-
mental importance of contact splitting for adhesion on smooth and rough substrata has been
explained by a very small effective elastic modulus of the fibre array (Persson, 2003). Adhesion
enhancement by division of the contact area has also been demonstrated experimentally (Peres-
sadko and Gorb, 2004). A patterned surface, made out of polyvinylsiloxane (PVS), has significantly
higher adhesion on a glass surface than a smooth sample made out of the same material (Figure
15.7). This effect is even more pronounced on curved substrata. An additional advantage of
patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance
to defects of individual contacts.
Recently, Continental has developed a winter tyre with honeycomb profiles similar to those
existing on the attachment pads of the grasshopper Tettigonia viridissima (Gorb et al., 2000)

Figure 15.7 Biomimetic attachment devices. (From Peressadko, A. and S. Gorb (2004) J. Adhes. 80: 1–15. With
permission of Taylor and Francis.) (a) Surface of the attachment organ in the fly Calliphora vicina (SEM micrograph),
(b) prototype made of polyvinylsiloxane (PVS), (c) dependence of tenacity (adhesion per unit area) of the structured
and flat samples on the prepressure, and (d, e) structured PVS surfaces used to hold a glass slide.