Bhushan B. Nanotribology and Nanomechanics: An Introduction
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1072 B. Bhushan et al.
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20
Gecko Feet: Natural Hairy Attachment Systems
for Smart Adhesion – Mechanism, Modeling
and Development of Bio-Inspired Materials
Bharat Bhushan
Summary. Leg attachment pads of several creatures including many insects, spiders, and
lizards, are capable of attaching to a variety of surfaces and are used for locomotion. Geckos,
in particular, have the largest mass and have developed the most complex hairy attachment
structures capable of smart adhesion – the ability to cling to different smooth and rough sur-
faces and detach at will. These animals make use of about three million microscale hairs
(setae) (about 14,000/mm
2
) that branch off into hundreds of nanoscale spatulae (about a bil-
lion spatulae). This hierarchical surface construction gives the gecko the adaptability to cre-
ate a large real area of contact with surfaces. Modeling of the gecko attachment system as
a hierarchical spring model has provided insight into adhesion enhancement generated by
this system. Van der Waals forces are the primary mechanism utilized to adhere to surfaces,
and capillary forces are a secondary effect that can further increase adhesion force. Preload
applied to the setae increases adhesive force. Although a gecko is capable of producing on
the order of 20 N of adhesive force, it retains the ability to remove its feet from an attachment
surface at will. The adhesive strength of gecko setae is dependent on the orientation; maxi-
mum adhesion occurs at 30°. During walking a gecko is able to peel its foot from surfaces
by changing the angle at which its setae contact a surface. A man-made fibrillar structure
capable of replicating gecko adhesion has the potential for use in dry, superadhesive tapes
that would be of use in a wide range of applications. These adhesives could be created using
micro/nanofabrication techniques or self-assembly.
20.1 Introduction
Leg attachmentpads of several creatures including many insects, spiders and lizards
are capable of attaching to a variety of surfaces and are used for locomotion [36].
Biological evolution over a long period of time has led to optimization of their
leg attachment systems. The attachment pads have the ability to cling to different
smooth and rough surfaces and detach at will. The dynamic attachment ability will
be referred to as reversible adhesion or smart adhesion [20]. Many insects (e.g.,
beetles and flies) and spiders have been the subject of investigation. However, the
attachment pads of geckos have been the most widely studied due to the fact that
theyhavethe highestbody massand exhibitthe most versatileand effectiveadhesive
1074 B. Bhushan
known in nature. As a result, the vast majority of this chapter will be concerned with
gecko feet.
Although there are over 1000 species of geckos [39, 59] that have attachment
pads of varying morphology[73], the Tokay gecko (Gekko gecko) has been the main
focus of scientific research [4,42,48]. The Tokay gecko is the second largest gecko
species, attaining respective lengths of approximately 0.3–0.4mand0.2–0.3mfor
males and females. They have a distinctive blue or gray body with orange or red
spots and can weigh up to 300g [88]. These geckos have been the most widely
investigated species of gecko due to the availability and size of these creatures.
Almost 2500 years ago, the ability of the gecko to “run up and down a tree in
any way, even with the head downwards” was observed by Aristotle [2] (Book IX,
Part 9). Even though the adhesive ability of geckos has been known since the time
of Aristotle, little was understood about this phenomenon until the late nineteenth
century when microscopic hairs covering the toes of the gecko were first noted.
The development of electron microscopy in the 1950s enabled scientists to view
a complex hierarchical morphology that covers the skin on the gecko’s toes. Over
the past century and a half, scientific studies have been conducted to determine
the factors that allow the gecko to adhere to and detach from surfaces at will,
including surface structure [3, 5, 48, 73–75, 77, 93]; the mechanisms of adhesion
[6,7,11,25,33,42,44,47,73,78,80,85,90]; and adhesion strength [3,6,42,46–48].
Recent work in modeling the gecko attachment system as a system of springs
[20,55–58]has providedvaluableinsightinto adhesionenhancement.Van der Waals
forcesare widely accepted in literature as the dominant adhesivemechanism utilized
by hierarchical attachment systems. Capillary forces created by humidity naturally
present in the air can further increase the adhesive force generated by the spatulae.
Both experimental and theoretical work support these adhesive mechanisms.
There is great interest among the scientific community to further study the char-
acteristics of gecko feet in the hope that this information couldbe applied to the pro-
duction of micro/nanosurfaces capable of recreating the adhesion forces generated
by these lizards [18]. Commonman-made adhesivessuch as tape or glue involvethe
use of wet adhesives that permanently attach two surfaces. However, replication of
the characteristics of gecko feet would enable the development of a superadhesive
polymertapecapable ofclean, dryadhesion(e.g. [17,19,32,37,64,65,81,82,96,97]).
These reusable adhesives have the potential for use in everyday objects such as
tapes, fasteners, and toys and in high technology such as microelectronic and space
applications. Replication of the dynamic climbing and peeling ability of geckos
could find use in the treads of wall-climbing robots [8,63,83].
20.2 Hairy Attachment Systems
There are two kinds of attachment pads – relatively smooth and hairy types. Rela-
tively smooth pads, so-called arolia and euplantulae are soft deformable and are
found in tree frogs, cockroaches, grasshoppers and bugs. The hairy types consist of
long deformable setae and are found in many insects (e.g., beetles, flies), spiders
20 Gecko Feet: Natural Hairy Attachment Systems for Smart Adhesion 1075
and lizards. The microstructures utilized by beetles, flies, spiders and geckos have
similar structures and can be seen in Fig. 20.1a. As the size (mass) of the creature
increases, the radius of the terminal attachment elements decreases. This allows
a greater number of setae to be packed into an area, hence increasing the linear
dimension of contact and the adhesion strength. Arzt et al. [3] determined that the
density of the terminal attachment elements ρ
A
per m
−2
strongly increases with
increasing body mass m in kg. In fact, a master curve can be fit for all the different
species (Fig. 20.1b)
logρ
A
= 13.8+ 0.669logm . (20.1)
The correlation coefficient of the master curve is equal to 0.919. Beetles and
flies have the largest attachment pads and the lowest density of terminal attachment
elements. Spiders have highly refined attachment elements that cover the leg of the
spider. Geckos have both the highest body mass and greatest density of terminal ele-
ments (spatulae). Spiders and geckos can generate high dry adhesion whereas bee-
tles and flies increase adhesion by secreting liquid stored generally within a spongy
layer of cuticle and is delivered at the contacting surface through a system of porous
channels [3,54]. It should be noted that in the smooth attachment system discussed
earlier, the secretion is essential for attachment.
Figure 20.2 shows the scanning electron micrographs of the end of the legs of
two flies – fruit fly (drosophila melanogaster) and syrphid fly. The fruit fly uses
setae with flattened tips (spatulae) on the two hairy rods for attachment to smooth
surfaces and two front claws for the attachment to rough surfaces. The front claws
are also used for locomotion. The syrphid fly uses setae on the legs for attachment.
In both cases, fluid is secreted at the contacting surface to increase adhesion.
Fig. 20.1. (a) Terminal elements of the hairy attachment pads of a beetle, fly, spider, and
gecko shown at two different scales [3]
1076 B. Bhushan
Bugs
Mass (g)
1
b) Terminal elements (per 100 )m
2
μ
Dry adhesion,
setae branched
Lizards
Spiders
Beetles
Flies
()Evarcha arcuata
Wet adhesion,
setae (mostly)
unbranched
10
–5
10
–6
10
3
10
100
10
–1
10
–2
10
–3
10
–4
10
–3
10
–2
10
–1
11010
2
Fig. 20.1. (continued) (b) the dependence of terminal element density on body mass [29].
Data are from Arzt et al. [3] and Kesel et al. [54]
End of the leg of syrphid fly
End of the legs of fruit fly
(drosphila melanogaster)
Fig. 20.2. SEM micrographs of the end of the legs
of fruit fly (drosophila melanogaster) and syrphid
fly [36]
20 Gecko Feet: Natural Hairy Attachment Systems for Smart Adhesion 1077
20.3 Tokay Gecko
20.3.1 Construction of Tokay Gecko
The explanationfor the adhesiveproperties of gecko feet can be found in the surface
morphologyof the skin on the toes of the gecko. Theskin is comprised of a complex
hierarchical structure of lamellae, setae, branches, and spatulae [73]. Figure 20.3
shows various SEM micrographs of a gecko foot, showing the hierarchical structure
downto the nanoscale.Figure 20.4showsa schematicof thestructureand Table 20.1
summarizes the surface characteristics. The gecko attachment system consists of
an intricate hierarchy of structures beginning with lamellae, soft ridges, that are
1–2mm in length [73] that are located on the attachment pads (toes) that compress
easily so that contact can be made with rough bumpy surfaces. Tiny curved hairs
known as setae extend from the lamellae with a density of approximately 14,000 per
square millimeter (Schleich and Kästle, 1986).These setae are typically 30–130µm
in length and 5–10µm in diameter [42,73,74,93]and are composed primarily of β-
keratin [61,75] with some α-keratin component[72]. At the end of each seta, 100 to
1000 spatulae [42,73] with a diameter of 0.1–0.2µm [73] branch out and form the
points of contact with the surface. The tips of the spatulae are approximately 0.2–
0.3 µm in width [73], 0.5 µm inlength and 0.01µm in thickness[70] and garnertheir
name from their resemblance to a spatula.
The attachment pads on two feet of the Tokay gecko have an area of about
220mm
2
. About three million setae on their toes can produce a clinging ability
of about 20N (vertical force required to pull a lizard down a nearly vertical (85
◦
)
surface) [48] and allow them to climb vertical surfaces at speeds over 1m/s with
capability to attach and detach their toes in milliseconds. In isolated setae, a 2.5 µN
Table 20.1. Surface characteristics of Tokay gecko feet (Young’s modulus of surface material,
keratin = 1–20GPa
1,2
)
Component Size Density Adhesive force
Seta 30–130
3−6
/ 5–10
3−6
length/diameter (µm)
∼ 14,000
8,9
setae/mm
2
194 µN
10
(in shear)
∼ 20 µN
10
(normal)
Branch 20–30
3
/ 1–2
3
length/diameter (µm)
––
Spatula 2–5
3
/ 0.1–0.2
3,7
length/diameter (µm)
100–1000
3,4
spatulae per seta
–
Tip of spatula ∼ 0.5
3,7
/ 0.2–0.3
3,6
/ ∼ 0.01
7
–11nN
11
(normal)
length/width/thickness (µm)
1
[75];
2
[12];
3
[73];
4
[42];
5
[74];
6
[93];
7
[70];
8
[77];
9
[5];
10
[6];
11
[46]
1078 B. Bhushan
ST
5mμ
BR
ST
SP
BR
50 mμ20 mμ
a)
b) c)
d) e)
Fig. 20.3. (a)Tokay gecko [6]. The hierarchical structures of a gecko foot; (b) a gecko foot [6]
and (c) a gecko toe [4]. Each toe contains hundreds of thousands of setae and each seta
contains hundreds of spatulae. Scanning electron microscope (SEM) micrographs of (d) the
setae [31] and (e) the spatulae [31]. ST seta, SP spatula, BR branch
preload yielded adhesion of 20 to 40 µN and thus the adhesion coefficient, which
represents the strength of adhesion as a function of preload, ranges from 8 to 16 [7].
20.3.2 Adaptation to Rough Surfaces
Typical rough, rigid surfaces are only able to make intimate contact with a mating
surface only over a very small portion of the perceived apparent area of contact. In
20 Gecko Feet: Natural Hairy Attachment Systems for Smart Adhesion 1079
Lamellae
length 1-2 mm
Lamellae
total area of
two feet ~ 220 mm
2
To lamella
Upper level of seta
length 30-130 m
diameter 5-10 m
μ
μ
Attachment pads
~ 14 000 mm
2
Middle level (branches)
length 20-30 m
diameter 1-2 m
μ
μ
Lower level (spatulae)
length 2-5 m
diameter 0.1-0.2 m
μ
μ
Tips of spatulae
length 0.5 m
width 0.2-0.3 m
~
thickness ~ 0.01 m
μ
μ
μ
/seta 100-1000
Setae
Fig. 20.4. Schematic drawings of a Tokay gecko including the overall body, one foot, a cross-
sectional view of the lamellae, and an individual seta. ρ represents number of spatulae
fact, the real area of contact is typically two to six orders of magnitude less than the
apparent area of contact [14–16]. Autumn et al. [7] proposed that divided contacts
serve as a means for increasing adhesion. Surface energy approach can be used to
calculate adhesion force in the dry environment in order to calculate the effect of
division of contacts. If the tip of a spatula is considered as a hemisphere with radius
R adhesion force of a single contact F
ad
based on the so called JKR (Johnson–
Kendall–Roberts) theory [53], is given as
F
ad
=
3
2
πW
ad
R (20.2)
where W
ad
is the work of adhesion (units of energy per unit area). Equation (20.2)
shows that adhesionforce of a single contact is proportionalto a linear dimension of
the contact. For a constant area divided into a large number of contacts or setae, n,
the radius of a divided contact, R
1
,isgivenbyR
1
= R/
√
n (self-similar scaling) [3].
Therefore, the adhesion force of (20.2) can be modified for multiple contacts such
1080 B. Bhushan
that
F
ad
=
3
2
πW
ad
R
√
n
n =
√
nF
ad
(20.3)
where F
ad
is the total adhesion force from the divided contacts. Thus the total adhe-
sive force is simply the adhesion force of a single contact multiplied by the square
root of the number of contacts.
For a contact in the humid environment, the meniscus (or capillary) forces fur-
ther increase the adhesion force [14–16]. The attractive meniscus force (F
m
) con-
sists of a contributionby both Laplace pressure and surface tension, see Sect. 20.4.2
[15, 66]. Contribution by Laplace pressure is directly proportional to the menis-
cus area. The other contribution is from the vertical component of surface tension
around the circumference. This force is proportional to the circumference as is the
case for the work of adhesion [15]. Going through the analysis presented earlier,
one can show that the contribution from the component of surface tension increases
with splitting into a larger number of contacts. It increases linearly with the square
root of the number of contacts n (self-similar scaling) [18,23]
F
m
surface tension
=
√
n
(
F
m
)
surface tension
, (20.4a)
where F
m
is the force from the divided contacts and F
m
is the force of an individual
contact. This component of meniscus force is significant if the meniscus radius is
very small and the contact angles are relatively large.
During separation of two surfaces, the viscous force, F
v
, of divided contacts is
given as [23]
F
v
= F
v
/n , (20.4b)
where F
v
is the force from the divided contacts and F
v
is the force of an individual
contact.
The models just presented only consider contact with a flat surface. On natural
rough surfaces the compliance and adaptability of setae are the primary sources of
highadhesion. Intuitively, thehierarchicalstructure of geckosetae allows forgreater
contact with a natural rough surface than non-branched attachment system [82].
Modeling of the contact between gecko setae and rough surfaces is discussed in
detail in Sect. 20.6.
Material properties also play an important role in adhesion. A soft material is
able to achievegreater contactwith a mating surfacethan arigid material.Although,
geckoskin is primarily comprisedof β-keratin, a stiff material with a Young’smodu-
lus in the range of 1–20 GPa [12,75], the effective modulus of the setal arrays on
gecko feet is about 100kPa [4], which is approximately four orders of magnitude
lower than the bulk material. Young’s modulus of the gecko skin is compared with
that of various materials in Table 20.2. The surface of consumer adhesive tape is
selected to be very compliant to increase the contact area for high adhesion. Nature
has selected a relatively stiff material to avoid clinging to adjacent setae. Division
20 Gecko Feet: Natural Hairy Attachment Systems for Smart Adhesion 1081
Table 20.2. Young’s modulus of gecko skin and other materials for comparison
Material Young’s modulus
β-keratin, mostly present in gecko skin 1–20 GPa
Steel
Cross-linked rubber
Consumer adhesive tape (uncrosslinked rubber)
210 GPa
1MPa
1kPa
of contacts, as discussed earlier, provideshigh adhesion. By combiningoptimal sur-
face structure and material properties, mother nature has created an evolutionary
superadhesive.
20.3.3 Peeling
Although geckos are capable of producing large adhesion forces, they retain the
ability to remove their feet from an attachment surface at will by peeling action.
The orientation of the spatulae facilitates peeling. Autumn et al. [6] were the first
to experimentally show that adhesion force of gecko setae is dependent on the
three-dimensional orientation as well as the preload applied during attachment (see
Sect. 20.5.1). Due to this fact, geckos have developed a complex foot motion during
walking. First the toes are carefully uncurled during attachment. The maximum ad-
hesion occurs at an attachment angle of 30
◦
– the angle between a seta and mating
surface. The gecko is then able to peel its foot from surfaces one row of setae at
a time by changing the angle at which its setae contact a surface. At an attachment
angle greater than 30° the gecko will detach from the surface.
Shah and Sitti [79] determined the theoretical preload required for adhesion as
well as the adhesion force generated for setal orientations of 30°, 40°, 50°, and 60°.
We consider a solid material (elastic modulus, E, Poisson’s ratio, ν) to make contact
with the rough surface described by
f(x) = Hsin
2
πx
χ
(20.5)
where H is the amplitude and χ is the wavelength of the roughness profile. For
a solid adhesive block to achieve intimate contact with the rough surface neglecting
surface forces, it is necessary to apply a compressive stress σ
c
[52].
σ
c
=
πEH
2χ
1−ν
2
(20.6)
Equation (20.6) can be modified to account for fibers oriented at an angle, θ.The
preload required for contact is summarized in Fig. 20.5a. As the orientation angle
decreases, so does the required preload. Similarly, adhesion strength is influenced
by fiber orientation. As seen in Fig. 20.5b, the greatest adhesion force occurs at
θ = 30
◦
.