南台科技大學
化學工程與材料工程系所
碩士論文
GECKO BIOMIMETIC FOR
REVERSIBLE ADHESION
研究生：施幸華
Silvia Adriana Estrada Alvarez
指導教授：林鴻儒 教授
中華民國 103 年
ii
ABSTRACT
i
ACKNOWLEDGMENTS
ii
TABLE OF CONTENTS
ABSTRACT .................................................................................................................................. I
ACKNOWLEDGMENTS............................................................................................................II
TABLE OF CONTENTS........................................................................................................... III
LIST OF TABLES ..................................................................................................................... IV
LIST OF FIGURES.................................................................................................................... IV
CHAPTER I ................................................................................................................................. 1
INTRODUCTION ...................................................................................................................................... 1
1. Background .................................................................................................................................. 1
2. Research Motivation................................................................................................................. 2
CHAPTER II ................................................................................................................................ 4
LITERATURE REVIEW ........................................................................................................................... 4
1. THE TOE ADHESIVE FORCE .................................................................................................. 4
2. ADHESION ..................................................................................................................................... 5
3. CONTACT ANGLE ....................................................................................................................... 5
4. ADEQUATE MATERIALS ......................................................................................................... 7
CHAPTER III .............................................................................................................................. 8
RESEARCH METHOD.............................................................................................................................. 8
1. Chemicals ....................................................................................................................................... 8
2. Glassware and Equipment ..................................................................................................... 8
3. Methodology................................................................................................................................. 8
4. Scanning electron microscope ............................................................................................. 8
5. Contact Angle............................................................................................................................... 8
CHAPTER IV............................................................................................................................... 9
RESULTS AND DISCUSSION ................................................................................................................... 9
1. Scanning electron microscope ............................................................................................. 9
2. Contact Angle............................................................................................................................... 9
CHAPTER V ..............................................................................................................................10
CONCLUSION ........................................................................................................................................ 10
REFERENCES ...........................................................................................................................11
iii
LIST OF TABLES
Table 1 Selected physical and mechanical properties of common biocompatible
polymerrs. ................................................................................................................................. 7
LIST OF FIGURES
Figure 1 The hierarchical structures of Gekko gecko. ........................................................... 2
iv
CHAPTER I
INTRODUCTION
1. BACKGROUND
GECKOS AND THEIR TOES STRUCTURE
Geckos are a type of reptile belonging to the infraorder Gekkota. They are found
in different sizes from 1.6 to 60cm, with different weights. They are common in warm
regions of the world, where several species of geckos make their home inside human
residences, but they are harmless to human healthy. Unlike most lizards, geckos are
usually nocturnal and because they are great climbers they include in their diet insect
pest, such as roaches. Many species of them posses the amazing ability to run up and
down, wander, race, sprint and even fight on smooth wall and ceilings with ease using
their remarkable toes. Which are specialized enabling them to climb smooth and
vertical surfaces and even indoor ceilings. Gecko toes have special adaptations that
allow them to adhere to most surfaces without the use of liquids or surface tension.
About 60% of gecko species have adhesive toe pads; such pads have been gained and
lost repeatedly over the course of gecko evolution. [Gamble et al. 2012]
Comparative studies of hundreds of insects and other animal species revealed
that biological attachment systems basically converge to two principal designs: a
‘‘hairy’’ system consisting of finely structured protruding hairs with size ranging
from a few hundred nanometers to a few microns, dependent upon the animal species,
and a ‘‘smooth’’ system with relatively smooth surface covering a fine tissue
microstructure [Scherge and Gorb, 2001; Niederegger et al., 2002; Gorb et al., 2000].
Gecko toes are not sticky in the same manner as traditional pressure-sensitive
adhesives (PSAs), such as adhesive tape. Instead, the toes are covered with a ‘‘smart”
adhesive of micro- and nano-structures of b-keratin that adhere strongly yet are nonfouling, non-tacky and detach easily [Autumn K., 2000; Autumn K. et al., 2006].
Each toe bears a hierarchical structure composed of a series of ridges bearing
hair-like shafts (setae) which divide into hundreds of flat tips called spatula [Autumn
K. et al., 2006], shown in Fig. 1. A gecko is found to have hundreds of thousands of
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keratinous hairs or setae on its foot; each seta is 30–130 lm long and contains
hundreds of protruding submicron structures called spatulae (Fig. 1). Possible
mechanisms of biological attachment include mechanical surface interlocking, fluid
secretion (capillarity and viscosity) and molecular adhesion (van der Waals
interaction). If we consider the animal hairy systems, the density of setae strongly
increases with the body weight of the animal, and geckos have the highest hair density
among all animal species that have been studied (Scherge and Gorb, 2001).
Figure 1 The hierarchical structures of Gekko gecko.
A toe of gecko contains hundred of thousands of setae and each seta contains hundreds of spatulae. (a)
and (b): scanning electron micrographs of rows of setae at different magnifications and (c): spatulae, the
finest terminal branches of seta. ST: seta; SP: spatula; BR: branch. [H. Gao et al. 2004]
2. RESEARCH MOTIVATION
Surfaces with reversible adhesion are of interest in various applications,
such as self-cleaning windows and display screens for electronic devices,
exterior paints for building, navigation ships, textiles, and solar panels. Because
these type of surfaces exhibit wear resistance and self-cleaning, they can also
detach via peeling action to provide reversible adhesion. Attempts are being
made to develop climbing robots using gecko inspired structures [Cutkosky, M.,
et al., 2009].
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Due to the highly optimized and efficient properties of living nature
organism surfaces and their potential applications, researchers have studied
their mechanisms and exploited them for commercial applications [Bailey, L.W.
et al., 1975, Bhushan, B., 2012., Bhushan, B. et al. 2012, Choi. K.M., 2003]. In order
to get the Lotus effect, hierarchical structures using wax structures, nanotubes,
and nanoparticles have been fabricated by a large number of investigators. In
order to obtain the gecko effect, a high density of nanofibers is required. Hard
materials such as carbon nanotubes have been used to fabricate gecko-like
structures to get high fiber density [Bhushan B. 2010, Cutkosky M 2009]. The
hard materials provide high resistance to wear and surface contamination. To
provide higher adhesion and adaptability to mating surfaces, soft materials such
as polymers have been used to fabricate a one-level fibril structure [DeLassus
P.T. et al. 2009, Kiat-ammuay S. et al. 2010, Lee, J. 2008, Lee, H. et al. 2012, Ruibal
R. 1965]. The fabrication processes used are complicated and generally provide a
low aspect ratio of length (height) to diameter of fibers, and the diameter of the
fibers is generally on the microscale.
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CHAPTER II
LITERATURE REVIEW
1. THE TOE ADHESIVE FORCE
The high-adhesion mechanism of geckos is based on so-called division of
contacts [Arzt, et al., 2003. Autumn, 2007., Bhushan, 2007. Bhushan, 2010].
Cumulative van der Waals attraction results in strong adhesion. In addition to
strong adhesion, because of a three level hierarchical structure, they have the
ability to adapt to a variety of surfaces. They exhibit wear resistance and selfcleaning.
The total clinging force produced by a Tokay gecko can be more than 20 N
[Irschick, D. J. et al., 1996], a strong force for an animal with a body weight of
about 43 grams and an average 227 mm2 of footpad area. The gecko footpad
areas are covered with hundreds of thousands of setae with a density of
5300/mm2 [Ruibal, R. et al 1965]. Each seta is branched into hundreds of
spatulas with dimensions of about 100 nm, as shown in Fig. 1. This configuration
allows the spatulas to follow the surface topography. Assuming that all spatulas
are in contact with the surface, the adhesion force contribution of individual seta
and spatula is around 20 mN and tens of nanonewtons, respectively. Numerous
attempts have been made to study and understand the nature of the adhesive
force between the spatulas and the surface [Autumn, K. et al., 2002] as well as
the effect of spatula orientation [Autumn, K. et al. 2000] but the complex
structure of a gecko seta has made it difficult to determine how many spatulas
are in instantaneous physical contact with the sensor. Nevertheless it is crucial to
understand the nature of gecko adhesion force to manufacture gecko mimicking
devices [Geim, A. K. et al. 2003]. In general, the total force between two surfaces
in close proximity consists of many components [Israelachvili, J. N. 1992.]
including the van der Waals, dipole, and capillary forces. However,
discrimination among the individual force components presents a considerable
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challenge especially for weak surface interaction forces such as the van der
Waals force, because it is typically accompanied by stronger forces such as the
capillary force in air or a dipole force in water. Due to the fact that in any natural
habitat the relative humidity is always at least 10%, it is possible that the
capillary force plays a role in the gecko’s consistently impressive adhesion.
2. ADHESION
Adhesion can be defined as the tendency of different types of particles or
surfaces to bind to one another. There are several forces that can cause adhesion,
between these ones there is dispersive adhesion. In this type of adhesion two
materials are held together by van der Waals forces, that are the attraction
between two molecules, each of which has a region of slight positive and
negative charge. In a system with solid-liquid-gas (such as a melting ice cube
surrounded by air) the contact angle is used to evaluate adhesiveness indirectly.
Generally, cases where the contact angle is low are considered of higher
adhesion per unit area. This approach assumes that the lower contact angle
corresponds to a higher surface energy. [Israelachvili, J. N. 1992.]
3. CONTACT ANGLE
The primary parameter that characterizes wetting is the static contact angle,
which is defined as the angle that a liquid makes with a solid. The contact angle
depends on several factors, such as surface energy, surface roughness, and its
cleanliness [Nosonovsky, et al., 2008.; Adamson,1990.; Bhushan,1999.; Bhushan,
2002.]. Surfaces with contact angles in the 0° ≤ 𝜃 ≤ 90° and 90° ≤ 𝜃 ≤ 180°
ranges are hydrophilic and hydrophobic, respectively. In particular, surfaces
with contact angles between 150° and 180° are called superhydrophobic. Water
contact angle hysteresis (CAH) is another property of interest to reduce drag in
fluid flow. CAH occurs due to surface roughness and heterogeneity. Contact angle
hysteresis is a measure of energy dissipation during the flow of a droplet along a
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solid surface. A liquid droplet on a solid surface removes contaminant particles
by rolling in addition to sliding at low CAH. Surfaces with low CAH (<10°) are
generally referred to as self-cleaning [Nosonovsky, et al., 2008]. The contact
angle can be determined by minimizing the net surface free energy of the system
of a liquid droplet on a solid surface.
Superhydrophobic surfaces exhibit extreme water repellent properties.
Certain plant leaves, notably Lotus leaves, are known to be superhydrophobic
and self-cleaning with low adhesion, known as the Lotus effect [Nosonovsky, et
al., 2008.; Adamson,1990.; Bhushan,1999.; Bhushan, 2002.; Bhushan, 2012.].
These properties are achieved by having a hydrophobic surface and a
hierarchical structure with both micro- and nanoscale dimensions.
The explanation for the adhesion properties of the gecko feet can be found
in the morphology of the skin on the toes of the gecko. On the Lotus surface, the
papillose epidermal cells form asperities and provide microscale roughness. A
range of waxes made from a mixture of long chain hydrocarbon compounds
which are not easily wetted are usually present on the Lotus surface. A
microscale roughness surface is covered by sub-microscale asperities of threedimensional epicuticular waxes, creating a hierarchical structure. The
hierarchical structure of the Lotus surface has low adhesion due to the low
density of the sub-microscale asperities. Nanoscale roughness allows water
droplets to sit easily on the apex of the nanostructures, because air pockets occur
in the valleys of the structure under the droplet, resulting in high contact angle
and
low
CAH.
Therefore,
the
Lotus
leaves
have
low
adhesion,
superhydrophobicity, and self-cleaning.
The toe skin of the gecko is also comprised of a complex hierarchical
structure of lamellae, setae, branches, and spatulae [R. Ruibal, et al. 1965] The
division of contacts serves as a means for increasing adhesion [Autumn, K.,2002].
The surface energy approach can be used to calculate adhesion force in a dry
environment in order to calculate the effect of division of contacts.
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4. ADEQUATE MATERIALS
Polypropylene and polyethylene, biocompatible polymers, were used in
earlier studies on the fabrication of hierarchical structures, where it was found
that either the gecko effect or the lotus effect (low adhesion, superhydrophobic
surface) can be attained by varying the size and the number of fibers per unit
area [Bhushan B. et al. 2012, Lee H, et al. 2012]. Polymers of choice for
prostheses include silicone-based polymers such as polydimethylsiloxane (PDMS)
[Wolfaardt, J.F. , et al., 1996] and polyethylene [Kiat-amnuay S., et al. 2010].
Table 1 provides a summary of selected physical (such as the melting
temperature, Tm, and glass transition temperature, Tg) and mechanical
properties (such as the tensile modulus and tensile strength) of polypropylene,
polyethylene and polysiloxanes.
Conventional polysiloxane-based prostheses require the use of adhesives,
which may cause irritation to the patient [Wolfaardt, J.F. 1996]. Therefore, there
is a need to identify other types of adhesives.
Table 1 Selected physical and mechanical properties of common
biocompatible polymerrs.
a [Zhu,
L. 199]
F. et al. 1999]
c Thermoset polymer
d [Andrews, R.J., et al., 1999]
e [Choi ,K.M. et al., 2003]
f [DeLassus, P.T., et al., 1999]
b [Bai,
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CHAPTER III
RESEARCH METHOD
1. CHEMICALS
2. GLASSWARE AND EQUIPMENT
3. METHODOLOGY
4. SCANNING ELECTRON MICROSCOPE
5. CONTACT ANGLE
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CHAPTER IV
RESULTS AND DISCUSSION
1. SCANNING ELECTRON MICROSCOPE
2. CONTACT ANGLE
9
CHAPTER V
CONCLUSION
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