Biomedical Engineering Reference
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also suggest possible strategies to develop novel adhesive materials for engineering
applications.
10.2 Hierarchical Attachment Structures of Gecko
Among hundreds of animal species for which adhesion plays an important role for
survival, the gecko stands out in terms of body weight and its extraordinary ability
to maneuver on vertical walls and ceilings [
1
]. Recent experimental measurements
[
2
-
5
] have provided evidence that the adhesion ability of gecko is primarily due to
van der Waals adhesion [
6
] between the contact surfaces (e.g., walls or ceilings) and
the gecko's feet, which are equipped with hundreds of thousands of keratinous hairs
called setae (Fig.
10.1
). Each seta is about 110
m long and branches near its tip
region into hundreds of thinner fibrils called spatulae, arranged in a fractal-like
hierarchical pattern (Fig.
10.1c
). While it is remarkable that gecko can make use of
the relatively weak van der Waals interactions to maneuver on unpredictable rough
surfaces under harsh environmental conditions, it may be even more impressive that
such robust adhesion appears to be easily releasable during animal locomotion.
What are the mechanics principles behind such robust and releasable adhesion
in biology?
Contact mechanics theories have been used to understand adhesion mechanisms
in both engineering and biology. The classical Hertz theory [
8
] assumes no adhe-
sive interactions between contacting objects. Johnson et al. [
9
] extended the Hertz
theory to contact between adhesive elastic spheres and developed the JKR (John-
son-Kendall-Roberts) model in which the contact area is determined via a balance
m
Fig. 10.1
The hierarchical adhesion structures of
Gekko gecko
. A toe of gecko contains hundreds
of thousands of setae and each seta branches near its tip region into hundreds of spatulae.
(
a
) Scanning electron micrographs of setae. (
b
) Spatulae, the finest terminal branches of seta.
ST
seta;
SP
spatula;
BR
branch (adapted, with permission, from: [
7
])
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