Research on Morphology Specificity of Gecko Feet at Different Scales

Lei CAI1,2,a, Hao MEI1,2, Wenbo WANG1, Hao WANG1 and Zhendong DAI1,*,b

1Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, china 2College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, china aemail: caileitcl@163.com, bemail: zddai@nuaa.edu.cn

*corresponding author: Zhendong DAI

Keywords: Gekko gecko; foot; toes; setae lamella; area

Abstract: The fine-level structure of toes contributes to the extraordinary climbing ability of G.

gecko. Research on the morphology specificity of gecko feet at different scales provides anatomical

information for revealing gecko’s motor ability and provides theoretical basis and data which can

support the development of the robot gecko feet. During the research, we used KEYENCE

(VHX-600E) digital microscope system to observe and photograph the forefoot and hindfoot of G.

gecko at different scales. The image processing as well as calculation of foot area and toe area was

done using image processing software after the number of toe setae lamella was counted. Results:

there was no significant difference between the numbers of setae lamellae on the first toes in both

forefoot and hindfoot (p>0.3). The same situation also appeared in the second and forth toes.

The numbers of the third and the fifth toe setae lamella in forefoot are significantly different from

the corresponding ones in hindfoot (p<0.01), the difference between the whole area of the forefoot

and hindfoot was more significant (p<0.005). The area of the first toe in forefoot was different from

the corresponding one in hindfoot (p<0.05), while the areas of the other four toes were more

different from the corresponding ones in hindfoot (p<0.005). The results of the research suggest that

specific designs should be adopted in the devisal of forefoot and hindfoot as well as the

corresponding toes.

Introduction

The extraordinary climbing ability of geckos is considered a remarkable design of nature that is

attributed to the fine structure of its toes [1]

. The fine structure of toes [2-4]

as well as the mechanism

hidden below the adhesive capacity of setae array [5-8]

has greatly attracted researchers. The

meaningful research on the adhesion property of gecko setae conducted by Autumn has injected

new vitality to this field [6]

. Guo et al. has found the hierarchical innervations of G. gecko’s toes to

motion and reception [9]

. Meanwhile, the research on behavior and dynamics of gecko’s locomotion

provides a source of inspiration in mechanism design and gait planning of robot gecko [10-14]

. All

those work have improved the study of robot gecko rapidly. The researchers of Carnegie Mellon

University developed gecko robot “Geckobot” which can climb up to 85 ° stably on the Plexiglas

surfaces. The feet of “Geckobot” are made by eight Polydimethyl siloxane elastomers which

empower the excellent climbing ability [15]

. With the help of sticky feet, gecko robot “Stickybot”

developed by Stanford University can climb on smooth vertical and even overhanging surfaces [16]

.

Every foot of StickyBot contains four toes; each of them has a 4 cm2 patch of synthetic adhesive

termed directional polymer stalks [17]

. Researchers can regulate the adhesion and detachment

Applied Mechanics and Materials Vol. 461 (2014) pp 284-290Online available since 2013/Nov/21 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.461.284

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processes by controlling the hamstring of the toes. Although the performance of the robot gecko has

improved significantly, the robustness as well as the flexibility of movement of gecko robot is still

far behind the real gecko. When designed the gecko robot feet, researchers imitated the toe-like

structure and characteristics of the lamellae of the real gecko feet. However the differences between

the forefoot and hindfoot; morphological differences between the toes; local specificities such as

directions of setae in different locations of the toe had not been reflected in the design. Those robot

feet are lack of structural diversity and the stiffness of feet was too high which maybe limit the full

contact of the toe ventral viscous material with the contact surface. So those feet have a low

adaptability to different surface roughness. The design of robot gecko feet has become one of the

key constraints in the research of robot gecko. This study aims to providing theoretical basis and

guidance for the design of robot gecko feet by researching morphology specificity of the gecko feet

at different scales.

Materials and methods:

Animals:

Five geckos, (Gekko gecko (Linnaeus, 1785)), were used in this study (body weight: 55.1 ± 11.9

g, body length (snout to vent): 146.6 ± 5.7 mm, mean ± s.d.). All the geckos were purchased from

Nanning, Guangxi Zhuang Autonomous Region of China, and were raised in the animal room under

simulated the original environment with fresh water and live insects as food, and kept on a natural

light cycle.

Animal Anesthetize and Fix

G. gecko was anesthetized by intraperitoneal injection of pentobarbital sodium (30 mg/kg). After

the gecko was fully anesthetized (there was no grip reaction when touched the skin and the pain

reflection disappeared) then gecko was supine fixed, toes back were fixed to the dissecting dish

with a double-sided adhesive, gecko toes was naturally straight state.

Microscopic Observation and Area Calculation

The right forefoot and left hindfoot of G. gecko were observed and pictures of those parts were

taken using KEYENCE (VHX-600E) Digital Microscope System. G. gecko toe area was defined as

the entire toe area covered by setae lamella and the setae lamella was defined as the consecutive

setae coverage area where the major axis and the minor axis ratio ≥3. By magnifying 800 times of

the feet, we found that in the ventral part of gecko toes, besides the defined setae lamella there were

also setae in protuberance located in proximal end of the toe adjacent to setae lamella. To facilitate

the statistical analysis, the protuberances were not counted. The number of setae lamella was

counted and the whole feet, toes, toe locals were observed and taken pictures respectively to study

the specificity of fine structure in different positions of the feet. The rough contours of the feet as

well as the corresponding toes were then depicted and the areas of those parts were calculated. The

numbers and areas of setae lamellae in forefoot and hindfoot as well as in corresponding toes were

statistically analyzed using SPSS 10.0 and GraphPad Prism V5.01. All of the tested data were

presented as means ± standard deviations (mean ± s.d.).

Results:

Differences in Number and Morphology of Setae Lamella between Forefoot and Hindfoot

According to the definition of the gecko setae lamella, the number of setae lamella in the right

forefoot and the left hindfoot (n = 5) was counted, results shown in table 1. There is no claw in the

first toe of right forefoot (Fig. 1A) of gecko and there are 16.4 ± 0.55 setae lamellae on it. The

Applied Mechanics and Materials Vol. 461 285

numbers of the setae lamellae on second, third, forth, fifth toe are respectively 18.2 ± 1.10, 20.0 ±

0.71, 22.2 ± 0.84, and 18.2 ± 0.45. The directions of setae on different setae lamellae are different.

The setae on the proximal setae lamellae are almost perpendicular to the feet. In the telecentric setae

lamellae the setae grow to the center gradually. The setae in the toe tips are nearly parallel with

ventral part of the toe. In different toes, the numbers of the lamellae where setae grow centripetally

were very different (shown in Fig. 2).

Table 1 Lamella number of the right forefoot and left hindfoot as well as the correspond toes of

G. gecko (n = 5)

On the first toe of left hindfoot (Fig. 1B) there is no claw too and there are 16.6 ± 0.55 setae

lamellae. The second, third, forth, fifth toe respectively have 18.4 ± 0.89, 22.0 ± 1.00, 22.6 ± 0.55,

20.2 ± 0.84 setae lamellae. The statistic analysis of the setae lamellae number on different toes

suggests that there was no significant difference between the first toes in forefoot and hindfoot. The

same situation also happens in the second, fourth toe (p>0.3); the numbers of the third and the fifth

toe setae lamella in forefoot are significantly different from the corresponding ones in hindfoot

(p<0.01). The numbers of total setae lamellae in forefoot and hindfoot are different (p = 0.022).

Anima

number

Body

weight

[g]

Body

length

S-V

[mm]

Lamella number of toes of the

right forefoot

Total

number of

the right

forefoot

Lamella number of toes of the left

hindfoot Total number

of the left

hindfoot T1 T2 T3 T4 T5 T1 T2 T3 T4 T5

1 59.2 14.6 16 18 19 22 18 93 16 18 21 22 20 97

2 70.6 15.1 17 18 20 23 18 96 17 18 23 23 20 101

3 59.6 14.9 17 17 20 21 18 93 17 18 22 23 19 99

4 42.6 15.0 16 18 20 22 18 94 16 18 21 22 21 98

5 43.4 13.7 16 20 21 23 19 99 17 20 23 23 21 104

Fig. 1. The ventral view of the forefoot and hindfoot of G. gecko. T1, T2, T3, T4, T5 is the first

to fifth toes. A: Right forefoot of G. gecko; B: Left hindfoot of G. gecko.

286 Advances in Bionic Engineering

Differences in Areas between the Forefoot and Hindfoot as well as the Corresponding Toes

In accordance with the definition of the gecko toe area, the total areas and the toe areas of

forefoot and hindfoot of G. gecko (n = 5) are shown in table 2. The total area of right fore foot is

173.05 ± 15.48 mm2 and the toe areas of first to fifth toe are respectively 25.93 ± 3.68 mm

2, 31.66 ±

2.23 mm2, 37.46 ± 3.61 mm

2, 42.39 ± 3.95 mm

2, 35.61 ± 4.49 mm

2. Correspondingly, the total area

of left fore foot is 225.09 ± 18.07 mm2 and the toe areas of first to fifth toe are respectively 32.84 ±

2.83 mm2, 39.77 ± 3.41 mm

2, 51.21 ± 5.36 mm

2, 54.74 ± 4.64 mm

2, 46.53 ± 2.74 mm

2.

Table 2 Areas of the right forefoot and left hindfoot as well as the correspond toes of

Gekko gecko (n = 5)

The toe areas in forefoot and hindfoot were analyzed statistically. Besides the first toes of

forefoot and hindfoot which have significant difference, the other four toes and the total area of

forefoot are more different to the corresponding ones of hindfoot. There was significant difference

between the first toes in forefoot and hindfoot (p = 0.01), the second toes in forefoot and hindfoot

were more different, the same situation also happens in the third, fourth, fifth toe. The P values of

those toes are respectively 0.002, 0.001, 0.002, and 0.002. The difference between the areas of

Animal

number

Body

length

S-V

[mm]

Areas of toes of the right forefoot

[mm2]

Areas

of the

right

forefoot

[mm2]

Areas of toes of the left hindfoot

[mm2]

Areas of

the left

hindfoot

[mm2] T1 T2 T3 T4 T5 T1 T2 T3 T4 T5

1 14.6 30.01 34.67 34.48 41.00 38.78 178.94 34.28 40.23 52.76 53.60 47.00 227.87

2 15.1 28.23 31.82 39.31 44.53 35.43 179.32 35.46 41.61 55.05 57.63 46.65 236.40

3 14.9 26.60 31.95 41.98 46.99 39.49 187.01 34.03 43.46 56.26 60.15 48.17 242.07

4 15.0 24.25 31.47 38.39 42.92 36.19 173.22 32.22 39.17 48.96 54.43 48.92 223.70

5 13.7 20.55 28.39 33.15 36.51 28.18 146.78 28.22 34.40 43.00 47.89 41.90 195.41

Fig. 2. The differences between the directions of the setae. A: The fifth toe of the G. gecko 30×;

B: Setae at the tip of toe nearly parallel to the foot plane; C: Setae at the proximal end to toe

nearly perpendicular to the foot plane. Bar = 0.5 mm in B and C.

Applied Mechanics and Materials Vol. 461 287

forefoot and hindfoot was extremely significant (p = 0.001). The scatter diagram of the result is

shown in Fig. 3.

Discussion

Differences in the Areas of Setae Lamellae between Forefoot and Hindfoot

There is a significant difference in the morphological characteristics between the forefoot and

hindfoot of gecko, this kind of difference is mainly reflected by the corresponding toe contact areas

as well as the numbers of setae lamellae. The morphological specificity of forefoot and hindfoot of

gecko may be related to the different roles they play in motor function. The total area of the

hindfoot and the areas of toes in hindfoot are significantly bigger than the corresponding ones in

forefoot which indicate that the adhesive ability of hindfoot is stronger than the forefoot. The

research on the behavior and kinetics of G. gecko moving in different directions in the vertical plane

found that: during the upward and downward locomotion of G. gecko, the average velocity is

significantly different; the former is 0.33 ± 0.20 m/s when the latter one is 0.55 ± 0.29 m/s. The

force output of the forefoot and hindfoot is also very different. When moved upward, the forefoot

and hindfoot make an equivalent contribution to the driving force, in the downward process, the

hindfoot provide a driving force approximately 3-fold of the forefeet do [13]

. There are relatively

small differences in the number of setae lamellae in forefoot and hindfoot which shows that from

the perspective of flexible control of the movement there is no difference between the forefoot and

hindfoot. G. gecko primarily actives on steep inclined surface or near vertical surface and those

special living habits play a major impact on the evolutionary development of the morphological

structure of the limbs.

Morphological Differences between the Toes

The areas of five toes in one foot can be very different. Generally, the area of the first, second,

third, fourth toes in turn increases; the fourth toe area is the biggest in five ones while the fifth toe

area is between the second and third toe areas. During the locomotion of the G. gecko the fourth toe

Fig. 3. Scatter plot of the

toes areas between the

forefoot and the hindfoot

of G. gecko.

RF: right forefoot,

LH: left hindfoot,

RFT1: first toe of right

forefoot, LHT1: first toe

of left hindfoot.

*: with a significant

difference (p<0.05),

**: with an extremely

significant difference

(p<0.005).

288 Advances in Bionic Engineering

is the farthest away from the body; the third and the fifth toes located on both sides of the fourth toe;

The first toe is the nearest to the body. The toe which is the farthest away from the body has the

biggest area that can provide the greatest force output, while the nearest toe has the smallest area. In

addition, the first toes of forefoot and hindfoot of G. gecko have no claw. The reason for this is that

the first toe is centripetal and the disappearance of the claw can make the toe more fully in contact

with the contact surface and reduce the distance between the body and the contact surface which is

beneficial for adhering to the vertical plane or negative surface.

Differences in Direction Specificity and Length of the Setae

Setae on different setae lamellae which located in different positions of feet have distinct

directions. The setae which are distal to the body grow centripetally while proximal setae are

vertical to the feet. This particular way for setae growing is favor for solid adhesion of G. gecko on

different surfaces, when there is a relatively great variation in elevation between the contact

surfaces and the feet, G. gecko can ensure that there are as many as setae contact with the surfaces.

During the adhesion process on the same surface, the setae grow in different directions just form a

closed contact ring which can enhance the adhesion stability. The directions of setae are different in

different locations of the G. gecko toes, this can provide us a helpful inspiration in the setae array

arrangement when we design the robot gecko feet.

Acknowledgements

I would like to thank Tingting Liu for her help in experimental data processing. This work was

partially supported by the State Key Development Program for Basic Research of China (No.

2011CB302100), National Natural Science Foundation of China (No. 61175105).

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