# Preliminary In Vivo Experiments on Adhesion of Geckos

Authors: E. Lepore; S. Brianza; F. Antoniolli; M. Buono; A. Carpinteri; N. Pugno
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

We performed preliminary experiments on the adhesion of a Tokay gecko on surfaces with different roughness, with or without particles with significant different granulometry, before/after or during the moult. The results were analyzed using the Weibull statistics.


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## Body

## 1. Introduction

The Tokay gecko (Gekko Gecko) ability to climb or hang upside down to relatively smooth surfaces (Figure 1) is well-known since the past [1]. With a weight up to
300 g these animals exhibit a versatile and effective dry adhesion. Many hypotheses
were formulated to explain this peculiarity [2–18]. Electron microscopy showed that the toes of this animal are covered by hierarchically
organized microscopic hairs, further characterized by sub-nanohairs. The
application of a similar principle to different fields, for the development of
superadhesive smart materials, is nowadays of great interest [19–21].Figure 1
Gecko natural downwards position on a plexiglass surface.The aim of this paper is to describe a preliminary series of experiments performed
in vivo on a Tokay gecko. We recorded
the gecko adhesive times on two surfaces
characterized by significantly different roughness with or without particles
with significant different granulometry, before/after or during the moult. We
analyzed the adhesive times using classical Weibull statistics.


## 2. Material and Method

The adhesion capability of a 50 g female
Tokay gecko of approximately 20 cm in length was tested gently segregating
the animal in a (50×50×50 cm3) box provided with several air inlets.
One wall of the box was made of glass and the rest were made of plexiglass.
Experiments were performed at room temperature (∼24°C). All the
tests were performed under the supervision of a certified veterinarian. The
animal did not show any particular discomfort being manipulated or segregated
in the box, except its well-known aggressiveness.


### 2.1. Characterization of the Surface

The roughness of the materials used to
build each face of the box was accurately characterized using a
three-dimensional (3D) optical profilometer, Talysurf CLI 1000 (Taylor Hobson)
with the CLA Confocal Gauge 300 HE (300μm range and 10 nm vertical
resolution). A surface area of 0.1 mm × 0.1 mm of each material was evaluated at
50 μm/s and 100 Hz sampling rate. The final resolution was 201 points/profile
and all parameters of interest were referred to a 25 μm cutoff. The measured roughness
parameters were the standard amplitude parameters Sa, Sq, Sp, Sv, Ssk, Sz, and
the hybrid parameters Ssk and Sdr. Sa represents the surface arithmetical average
roughness; Sq is the mean square roughness and represents the mean square
deviation of profile from the middle line; Sp and Sv are, respectively, the
height of the highest peak and the deepness of the deepest valley (absolute
value); Sz is the average distance between the five highest peaks and the five
deepest valleys detected in the analyzed area; the parameter Ssk describes the
surface skewness; Sdr is equal to the ratio between effective and nominal
surface areas minus one.


### 2.2. Characterization of the Toe

Four frozen and formaldehyde fixed samples
of foots retrieved from two geckos died naturally were unfrozen at room
temperature and scanned with a LEICA CLS 150 XE stereoscope and pictures of
fields of interest recorded. A LEICA STEREOSCAN 430i electron microscope was
used to measure the size of the setae.The sequence of
movements performed by the gecko during adhesion was recorded by  gr-dx77u JVC
digital video camera. The video camera was located axial to the gecko forearm
and perpendicular to the ceiling surface of the box, approximately 25 cm far
from the animal body. Single frames were then extracted using Nero Vision
software.


### 2.3. Characterization of the Particles

The adhesion test consisted in the progressive rotation of the box up to 180° with
respect to its original position and in the description of the effect of the granulometries
of different particles on the gecko adhesion ability. Calcium carbonate
particles (50 g) and 2.5 mm diameter plastic spheres (100 g) were
used to test two significant different granulometries. In order to test the long-term effect of small granulometry particles, the test using calcium carbonate
was repeated also deeply cleaning the box but leaving dirt the animal feet. Plastic
spheres were tested both lying freely on the bottom of the box or fixed, in
order to form a restrained layer.


### 2.4. Adhesion Experiments and Weibull Statistics

A
standard timer was used to measure the gecko failure times, defined as the
number of seconds the animal was able to keep the upside down position before
moving into another position (e.g., falling down). Tests were performed during
the moult period and during the non-moult period.The Weibull statistics, widely used for describing
the strength and fatigue life in solids, was used to analyze the gecko adhesion
ability. Thus, the loss of adhesion was treated as an interfacial failure.


## 2.1. Characterization of the Surface

The roughness of the materials used to
build each face of the box was accurately characterized using a
three-dimensional (3D) optical profilometer, Talysurf CLI 1000 (Taylor Hobson)
with the CLA Confocal Gauge 300 HE (300μm range and 10 nm vertical
resolution). A surface area of 0.1 mm × 0.1 mm of each material was evaluated at
50 μm/s and 100 Hz sampling rate. The final resolution was 201 points/profile
and all parameters of interest were referred to a 25 μm cutoff. The measured roughness
parameters were the standard amplitude parameters Sa, Sq, Sp, Sv, Ssk, Sz, and
the hybrid parameters Ssk and Sdr. Sa represents the surface arithmetical average
roughness; Sq is the mean square roughness and represents the mean square
deviation of profile from the middle line; Sp and Sv are, respectively, the
height of the highest peak and the deepness of the deepest valley (absolute
value); Sz is the average distance between the five highest peaks and the five
deepest valleys detected in the analyzed area; the parameter Ssk describes the
surface skewness; Sdr is equal to the ratio between effective and nominal
surface areas minus one.


## 2.2. Characterization of the Toe

Four frozen and formaldehyde fixed samples
of foots retrieved from two geckos died naturally were unfrozen at room
temperature and scanned with a LEICA CLS 150 XE stereoscope and pictures of
fields of interest recorded. A LEICA STEREOSCAN 430i electron microscope was
used to measure the size of the setae.The sequence of
movements performed by the gecko during adhesion was recorded by  gr-dx77u JVC
digital video camera. The video camera was located axial to the gecko forearm
and perpendicular to the ceiling surface of the box, approximately 25 cm far
from the animal body. Single frames were then extracted using Nero Vision
software.


## 2.3. Characterization of the Particles

The adhesion test consisted in the progressive rotation of the box up to 180° with
respect to its original position and in the description of the effect of the granulometries
of different particles on the gecko adhesion ability. Calcium carbonate
particles (50 g) and 2.5 mm diameter plastic spheres (100 g) were
used to test two significant different granulometries. In order to test the long-term effect of small granulometry particles, the test using calcium carbonate
was repeated also deeply cleaning the box but leaving dirt the animal feet. Plastic
spheres were tested both lying freely on the bottom of the box or fixed, in
order to form a restrained layer.


## 2.4. Adhesion Experiments and Weibull Statistics

A
standard timer was used to measure the gecko failure times, defined as the
number of seconds the animal was able to keep the upside down position before
moving into another position (e.g., falling down). Tests were performed during
the moult period and during the non-moult period.The Weibull statistics, widely used for describing
the strength and fatigue life in solids, was used to analyze the gecko adhesion
ability. Thus, the loss of adhesion was treated as an interfacial failure.


## 3. Results

### 3.1. Characterization of the Surface

Surface profiles of glass (Figure2) and plexiglass (Figure 3) showed two homogeneous surfaces without significant 3D alterations, apart from small isolate bubbles on the glass surface derived from the fabrication process (melting). Table 1 summarizes averaged roughness parameters for the materials of interest. The surface of glass had a larger number of
plateaus and several deep thin valleys in comparison to plexiglass.Table 1
Superficial roughness parameters of plexiglass and  glass.
PlexiglassGlassSa (μm)0.0330.031Sq (μm)0.0420.041Sp (μm)0.2520.366Sv (μm)0.2770.434Ssk−0.122−0.381Sz (μm)0.4320.609Sdr (%)0.4900.574Figure 2
Glass: surface of glass without any
superficial treatment.Figure 3
Plexiglass: surface of plexiglass
without any superficial treatment.


### 3.2. Characterization of the Toe

Multiscale observations confirmed classical
observations [22–31]: the foot of Tokay
gecko is characterized by a hierarchical structure starting with macroscopic
lamellae, composed by setae, each of them containing different  spatulae, representing the terminal contact units
(Figure 4). The typical hyperextension of the toes has been clearly
observed.Scanning electron microscopy
of  the hierarchical structures of a
gecko foot. Each toe contains hundreds of thousands of setae and each seta
contains hundreds of spatulae.


### 3.3. Characterization of the Particles

Small Granulometry
Immediately after feet soiling with calcium carbonate (Figure5) the ability of gecko to
adhere to the surfaces vanished. Repeating the test one hour later no
improvement in the adhesive ability of gecko has been observed. Self-cleaning
was not observed. The gecko did not try to clean licking or moving quickly its
feet and it seemed to be less aggressive, because of its perceived difficulty.Figure 5
A gecko’s dirty foot after treatment: the macroscopic
aspect of lamellae is as a compact covered lamella, where the calcium carbonate
fills all the empty free spaces between the setae.Large Granulometry
The unconstrained spheres shifted away under the feet, thus capable of adhering
to the underlying surfaces (plexiglass or glass). On the constrained layer made
by the same spheres gecko exhibits its typical adhesive properties up to 180
deg rotation (Figure6),
making use also of its claws (Figure 7).Figure 6
Adhesion on a layer of fixed spheres (2.5 mm in diameter).Figure 7
The nails on the tip of each toe.


### 3.4. Adhesion Experiments and Weibull Statistics

The measured times to failure were
treated with Weibull Statistics. Accordingly, the distribution of failure (F) is given by:
(1)F(t;m;t0)=1−e−(t/t0)m,
where t is the measured adhesion time, m is the shape parameter, and t0 is the scale parameter of the distribution of failure (F). The cumulative probability Fi(t) can be obtained experimentally as
(2)Fi(ti)=i−1/2N,where N is the total
number of performed tests and the measured times of failure, ti,…,tN,
are ranked in an ascending order. The Weibull statistics was found to be
appropriate for describing the adhesion times on different surfaces. For
example, the first series on plexiglass during the non moult period
(measurements are reported in Table 2, Test 1) showed a Weibull modulus ∼1.3
with a correlation coefficient of R2=0.95, see Figure 8. The scale parameter t0
was 413.86 seconds, approximately corresponding to 6 minutes and 53
seconds. The time measurements during the moult period (Table 2, Test 2), showed a
decrease in the adhesion ability. The scale parameter t0
was 200.79 seconds, approximately corresponding to 3 minutes and 20
seconds. The Weibull modulus during the moult increased up to ∼2.2,
with a correlation
R2=0.94
,
see Figure 9.Table 2
Measurement results of gecko’s time adhesion on plexiglass surface: first set  before the moult; second
set during the moult.
Test 1Test 2No.Time(s)Time(s)137592134104314510841601085197142621514872281908292192932321610369310115683801270013707141268Figure 8
First set of
measurements: Weibull statistics of gecko’s time adhesion before the moult (Table2, Test 1).Figure 9
Second set of
measurements: Weibull statistics of gecko’s time adhesion during the moult (Table2, Test 2).We have also observed an extraordinary increase in
adhesion’s time during the phase just following the moult, corresponding to adhesion
times of the order of 2-3 hours, both on plexiglass and
on glass.The adhesive
ability of gecko drastically decreases when the gecko moults. In the first
series of measurements of gecko’s time adhesion, the values are strongly
variable by spanning between two orders of magnitude, in the range 37−1268 seconds, while the
values of the second series obtained from the same gecko but during its moult
are much less dispersed, from 59 to 380 seconds. The more variable
the failure time, the lower the parameterm.
Note that the shape parameter m of
the first series (m = 1.36) is lower
than in the second case (m = 2.23). As the failure shape parameter suggests, the failure
during the moult becomes an almost deterministic process.


## 3.1. Characterization of the Surface

Surface profiles of glass (Figure2) and plexiglass (Figure 3) showed two homogeneous surfaces without significant 3D alterations, apart from small isolate bubbles on the glass surface derived from the fabrication process (melting). Table 1 summarizes averaged roughness parameters for the materials of interest. The surface of glass had a larger number of
plateaus and several deep thin valleys in comparison to plexiglass.Table 1
Superficial roughness parameters of plexiglass and  glass.
PlexiglassGlassSa (μm)0.0330.031Sq (μm)0.0420.041Sp (μm)0.2520.366Sv (μm)0.2770.434Ssk−0.122−0.381Sz (μm)0.4320.609Sdr (%)0.4900.574Figure 2
Glass: surface of glass without any
superficial treatment.Figure 3
Plexiglass: surface of plexiglass
without any superficial treatment.


## 3.2. Characterization of the Toe

Multiscale observations confirmed classical
observations [22–31]: the foot of Tokay
gecko is characterized by a hierarchical structure starting with macroscopic
lamellae, composed by setae, each of them containing different  spatulae, representing the terminal contact units
(Figure 4). The typical hyperextension of the toes has been clearly
observed.Scanning electron microscopy
of  the hierarchical structures of a
gecko foot. Each toe contains hundreds of thousands of setae and each seta
contains hundreds of spatulae.


## 3.3. Characterization of the Particles

Small Granulometry
Immediately after feet soiling with calcium carbonate (Figure5) the ability of gecko to
adhere to the surfaces vanished. Repeating the test one hour later no
improvement in the adhesive ability of gecko has been observed. Self-cleaning
was not observed. The gecko did not try to clean licking or moving quickly its
feet and it seemed to be less aggressive, because of its perceived difficulty.Figure 5
A gecko’s dirty foot after treatment: the macroscopic
aspect of lamellae is as a compact covered lamella, where the calcium carbonate
fills all the empty free spaces between the setae.Large Granulometry
The unconstrained spheres shifted away under the feet, thus capable of adhering
to the underlying surfaces (plexiglass or glass). On the constrained layer made
by the same spheres gecko exhibits its typical adhesive properties up to 180
deg rotation (Figure6),
making use also of its claws (Figure 7).Figure 6
Adhesion on a layer of fixed spheres (2.5 mm in diameter).Figure 7
The nails on the tip of each toe.


## 3.4. Adhesion Experiments and Weibull Statistics

The measured times to failure were
treated with Weibull Statistics. Accordingly, the distribution of failure (F) is given by:
(1)F(t;m;t0)=1−e−(t/t0)m,
where t is the measured adhesion time, m is the shape parameter, and t0 is the scale parameter of the distribution of failure (F). The cumulative probability Fi(t) can be obtained experimentally as
(2)Fi(ti)=i−1/2N,where N is the total
number of performed tests and the measured times of failure, ti,…,tN,
are ranked in an ascending order. The Weibull statistics was found to be
appropriate for describing the adhesion times on different surfaces. For
example, the first series on plexiglass during the non moult period
(measurements are reported in Table 2, Test 1) showed a Weibull modulus ∼1.3
with a correlation coefficient of R2=0.95, see Figure 8. The scale parameter t0
was 413.86 seconds, approximately corresponding to 6 minutes and 53
seconds. The time measurements during the moult period (Table 2, Test 2), showed a
decrease in the adhesion ability. The scale parameter t0
was 200.79 seconds, approximately corresponding to 3 minutes and 20
seconds. The Weibull modulus during the moult increased up to ∼2.2,
with a correlation
R2=0.94
,
see Figure 9.Table 2
Measurement results of gecko’s time adhesion on plexiglass surface: first set  before the moult; second
set during the moult.
Test 1Test 2No.Time(s)Time(s)137592134104314510841601085197142621514872281908292192932321610369310115683801270013707141268Figure 8
First set of
measurements: Weibull statistics of gecko’s time adhesion before the moult (Table2, Test 1).Figure 9
Second set of
measurements: Weibull statistics of gecko’s time adhesion during the moult (Table2, Test 2).We have also observed an extraordinary increase in
adhesion’s time during the phase just following the moult, corresponding to adhesion
times of the order of 2-3 hours, both on plexiglass and
on glass.The adhesive
ability of gecko drastically decreases when the gecko moults. In the first
series of measurements of gecko’s time adhesion, the values are strongly
variable by spanning between two orders of magnitude, in the range 37−1268 seconds, while the
values of the second series obtained from the same gecko but during its moult
are much less dispersed, from 59 to 380 seconds. The more variable
the failure time, the lower the parameterm.
Note that the shape parameter m of
the first series (m = 1.36) is lower
than in the second case (m = 2.23). As the failure shape parameter suggests, the failure
during the moult becomes an almost deterministic process.


## 4. Conclusions

We have showed that the gecko adhesive
ability is drastically reduced when particles characterized
by a “small” granulometry are interposed between toe and surface. Large
particles can be controlled during adhesion, similarly to the surface
roughness. The Weibull Statistics is found to be appropriate to describe the
adhesion times of geckos. As the parameterm decreases, the failure time becomes more variable, describing a more stochastic and
less deterministic process. This suggests that adhesion becomes a more deterministic process during the moult.



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*Source: 194524-2008-09-30.xml*