Abstract— In this paper, we present an adhesion mechanism

that can be used for underwater climbing robots and

manipulators. The proposed design is inspired from tube feet of

sea urchins, which use a combination of suction by extra soft

sucker pads with assist of chemical adhesive materials. An

experimental analysis of the bioinspired adhesive suckers (in

single and array fashion) was performed and the obtained

results show a repeatable increase in generated forces by single

soft suckers, as compared with commercial sucker clamps. The

main advantage of this sucker is its capability of adapting to the

rough surfaces and to perform attachment. The other features

of the sucker are its simple design, ease of manufacturing, and

the capability of miniaturization for miniature robotic

applications.

I. INTRODUCTION

NDERWATER biological adhesion mechanisms are

strong sources of inspiration for robotic researchers for

the development of innovative underwater robots and

manipulators. When endowing robots with adhesion

capability, they can be able to climb surfaces, grasp and

manipulate objects, and in swimming robots that need to

stay in a fixed position for a special task, adhesion helps

them to withstand the underwater hydrodynamic forces.

Several investigations were addressed in order to understand

underwater adhesion principles and also, in order to apply

them in robotics. Anatomical studies and 3D reconstruction

of octopus suckers [2] were recently addressed in order to

understand their adhesion strategy, while simulation of

octopus suckers were reported [1]. Moreover, a climbing

robot based on sucker principle by using SMA actuators was

reported [3]. Bellows-type sucker was also studied for

underwater application based on bio-inspired principles [4].

There are different techniques used by marine animals.

The well-known techniques can be classified in three groups.

First, suction based adhesions like octopus[5] or leech [6];

second, adhesion based on chemical adhesive, like

mussels[7]; and, third combination of chemical adhesives

and suction like limpets [8],[9].

Sea urchins (Fig. 1, left) are a wonderful group of marine

animals, that can, temporarily, attach to a wide range of

substrates, move, and manipulate food, by using numbers of

Manuscript received January 31, 2012.

A. Sadeghi is with The BioRobotics Institute, Scuola Superiore Sant’Anna,

Pisa, Italy and with the Center for Micro-BioRobotics@SSSA, Istituto

Italiano di Tecnologia, Pontedera, Italy (corresponding author: phone:

0039-050-883493; fax: 0039-050-883101; e-mail: ali.sadeghi@iit.it).

L. Beccai (e-mail: lucia.beccai@iit.it) and B. Mazzolai (e-mail:

barbara.mazzolai@iit.it) are with the Center for Micro-

BioRobotics@SSSA, Istituto Italiano di Tecnologia, Pontedera, Italy.

external, extra soft elements (Fig. 1, right), which are called

tube feet [10]. These small, spiny and spherical shape marine

animals which belong to the Echinoderms (Phylum

Echinodermata) inhabit the ocean at all depths. Their

adhesive tube feet allow sea urchins to anchor to sea

substrates resisting to hydrodynamic generated forces, which

are produced by ocean strong waves [10]. The attachment

strengths for a special kind of sea urchins are reported

between 23.4 (±2.0) N and 46.3 (± 2.4) N as function of

their body size [11]. Moreover, in [10] the attachment

strength of sea urchins is reported as up to 100N. Based on

the external morphology of tube feet, they are classified into

six types: disc-ending, penicillate, knob-ending, lamellate,

digitate, and ramified [10]. Methods of attachment and

detachment in disc-ending tube feet have been studied by

several groups and two main different models were

proposed. One model explains the mechanism of both

attachment and detachment based on the chemical reaction

of secretions injected by tube feet on the substratum. This

theory focuses on mucus glands and structure of foot print

[12], [13]. The adhesive material works like twin glues and

adhesion is produced by a combination of due gland

adhesives [10], [12]. Instead, detachment happens by adding

a de-adhesive secretion to the surface [10], [12], [14]. The

second model is based on negative pressure, and assisted by

mucus secretion, and is explained in detail in the next

section [15],[16],[17].

In this work, we have considered the intriguing adhesion

techniques of tube feet, specifically of the disc-ending type,

as a source of inspiration to design and develop innovative

adhesion mechanisms for underwater robotic application. In

particular, we report on the design and development of a

simple active sucker that produces negative pressure inspired

by tube feet. We focused attention on the role of soft

materials and artificial mucus necessary to produce the

required force for sucker adhesion. Our final goal is to

implement a group of suckers which work better than

Design and Development of Innovative Adhesive Suckers Inspired

by the Tube Feet of Sea Urchins

A. Sadeghi, L. Beccai, Member, IEEE, B. Mazzolai, Member, IEEE

U

Fig. 1 Left, sea urchin is climbing aquarium wall; right, adhesive or disc

ended tube feet.

The Fourth IEEE RAS/EMBS International Conferenceon Biomedical Robotics and BiomechatronicsRoma, Italy. June 24-27, 2012

978-1-4577-1198-5/12/$26.00 ©2012 IEEE 617

existing suckers in terms of behavior on rough and cracked

surfaces.

II. BIO-INSPIRATION AND DESIGN

A. Disc-Ending Tube Feet

Disc-ending tube feet provide the ability of temporary

attachment and subsequently, locomotion and climbing for

sea urchins [10]. A generalized and simplified section of

disc-ending tube feet by using the histological model in [15]

was reconstructed and it is showed in Fig. 2. Each tube foot

is made by a soft stretchable and hollow cylinder which is

called stem; the stem ends to a disc shape geometry which is

called the disc.

The stem can elongate by hydraulic pressure produced by

the Water Vascular System (WVS) that brings the disc part

of tube foot up to the surface to which is attaching. The

retraction muscles retract the elongated stem to the initial

form. A combination of these hydraulic system and four

separate columns of retraction muscles transfer the tube foot

to a very flexible soft continuum arm that can manipulate

food and particles and also moves the animal body in

complex space.

The disc is another important part of disc-ending tube foot

which is a very soft member with viscoelastic properties and

adapts its surface to the substratum profile [18]. It can attach

to the surfaces and easily detach. The histological studies on

tube feet show each disc consists of a ring of mucus glands,

calcareous skeleton, levator muscles, diaphragm muscles

and tissues. Also a ring of sensory system so called cilia is

positioned on the skin of disc [15], [16].

In the attachment process of tube foot (depicted in detail

in Fig. 3), the disc adapts to the surface roughness and

secretions provide an initial adhesion and sealing. Then, the

levator muscles reshape the diaphragm to a concave form

and produce negative pressure. The volume of concave form

of diaphragm enlarges by retraction of disc muscles and

increases the amount of negative pressure. For detachment,

diaphragm muscles and stem retractor muscles reshape the

diaphragm to a convex form and detachment occurs.

[16][17].

B. Design of the artificial sucker

The concept of producing vacuum by reshaping a

diaphragm is not a new idea and it already exists in

commercial sucker clamps. In these suckers the center of

diaphragm raises up by a solid shaft and usually locks there

by a cam shaped mechanism. For releasing vacuum the lock

should open, but usually that doesn’t happen immediately

after opening the lock. Instead, release needs either some

time or some lateral mechanical stimulation. This could be

problematic for robotic applications. Also, these suckers

work well on smooth surfaces, like glass, but they are not a

good solution for clamping parts with rough surfaces. Our

goal is to modify sucker clamps in terms of their behavior on

rough surfaces.

The first and basic idea was to develop a diaphragm and

an actuation mechanism able to reshape the membrane for

producing negative pressure. For this goal, a simple sucker

was designed, which has a hard core with a soft rubber skin.

Inside the core part, a cavity is embedded under the

diaphragm. Such cavity allows the diaphragm to reshape in a

concave form (when an external actuation is applied) (Fig.

4). The hard core part supports the flexible structure when it

is in contact with the surface that the sucker is adhering to

(like calcareous skeleton in tube feet). By reshaping the

diaphragm to the concave form, negative pressure will be

produced.

Because normally there is no temperature change inside

the sucker during the suction process, the enlargement of

volume between sucker and substrate can be seen as an

isothermal process, where �� and �� are, respectively,

Fig. 4. The CAD model of proposed single suckers, before actuation

(left) and after (right).

Fig. 3. A, sensing the surfaces by cilia, none of the muscles of the

tube-foot are contracting yet. B, the disk is in contact with the

substratum and a sticky mucous is injected around the edge of the

disk, C, concaving the diaphragm by contraction of levator muscles,

D, diaphragm muscles help levator muscles to enlarges the suction

cavity E, Detachment starts by relaxation in levator muscles while the

diaphragm muscles still are contracting, F, contraction of the

diaphragm muscles and stem retractors draw up the edges of the disk,

till detachment completes.

A B C E F D

Fig.2. Simplified anatomic model of disc-ended tube foot

Diaphragm Muscle

Water Channel

Stem retractor muscle

Levator Muscle

Mucous Gland

Cilia

Stem

Calcareous Skeleton

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pressure and volume of the cavity between membrane and

substrate. Such entities, after cavity enlargement, can be

named as ��� and ��� which are described as follows:

��� � ��� � ∆��

P�� � ��� � ∆��

When using the ideal gas law, the pressure difference ∆P�

generated by volume change ∆�� can be derived as

∆P� �P��∆V�

���� � ∆��

The produced force by this sucker, �� can be calculated as

follows

�� � �P� P� � A

where P� is the pressure of environment and A is the area of

the sucker cup.

The produced force by our suckers will increase when we

use them in deep water, because of a larger pressure

produced by the environment. But in a normal situation for

obtaining a higher force, we should produce a greater

volume enlargement. This enlargement in sucker tube feet

happens by the assist of levator and diaphragm muscles and

it can increase when the animal feels danger. We can follow

this phenomenon in our suckers by controlling the amount of

volume change and membrane reshaping. This could be

useful in a robotic system in which, for example, low energy

consumption is tackled when contact with smooth surfaces is

obtained, while energy can be saved for powering the

motion on rough substrates.

C. Actuation Mechanism

In order to actuate the membrane different kinds of

techniques can be considered; like magnetic, SMA or

mechanical actuators. In our case, we use fluidic actuation

produced by a simple cylinder-piston pump. This pump is

connected to the cavity of the sucker by a small hole at its

center (Fig. 4). Our choice is supported by the fact that

assembly and manufacturing processes for mechanical, SMA

or magnetic actuation techniques, comparing with fluidic

actuation, are more difficult, especially when they are

integrated in a very soft material like that required for our

membrane.

For reshaping the diaphragm, it is preferable to use fluidic

actuation instead of solid mechanical actuators. This is

because in the second case it is difficult to pull up the entire

membrane surface, which would result like that schematized

in Fig. 5 (left). In contrast, fluidic actuation allows for a

greater volume enlargement of the membrane, resulting in a

larger negative pressure. Moreover, fluidic actuation

simplifies the shift from negative to positive pressure

modality. The latter is preferred to reshape the membrane in

convex manner (like the shape of tube foot diaphragm

during detachment process). If we follow this strategy

detachment can be achieved quite easily.

D. Design of an array of suckers

The fluidic actuation strategy allowed us to design an

array of suckers, by using just one pump, with a simple final

design (Fig. 6, left). We designed an array of suckers like

shown in Fig. 6 (right) to target adhesion to cracked

surfaces. These suckers are connected to the fluid pump by a

channel which is embedded in the sucker pad. This sucker

could be used in a wall climbing robot to achieve

locomotion over cracks and holes, without losing all the

vacuum force. Actually, when we use a matrix of suckers

instead of a big single sucker, we will lose the active area

which is producing the vacuum, so the obtained force will be

smaller. However, the array design will lead to a sucker

which behaves safer on cracked surfaces, thus increasing

robot performance.

III. PROTOTYPING AND EVALUATION

A. Development of artificial suckers

We developed two different kinds of single and array

suckers. Each single sucker has a hard plastic core with a 70

mm outer diameter. The cavity size for each sucker is a hole

with 60 mm diameter and 5 mm depth. Each core is covered

by a membrane consisting of soft silicone rubber, like shown

in Fig. 7 (left). Both suckers were built by traditional casting

of Dragon Skin® (Smooth-On, Inc. USA), but the

membrane of one of them was coated by a layer, of 1 mm

thickness, of Ecoflex 00-30 (Smooth-On, Inc. USA).

Therefore, the final thickness in both cases was of 5 mm,

however in the second case we had 4mm of Dragon Skin®

plus 1 mm of Ecoflex 00-30. For producing the cavity inside

the suckers we used butter as a sacrificing material. The low

melting point of butter helped us to remove this part without

affecting the properties of the membrane.

With the same techniques we also made an array of sucker

cups integrating 19 single suckers. Each one had a core of 10

mm diameter and 12 mm outer diameter, after casting with

silicone rubber Fig. 7 (right).

Fig. 6. The mechanism of actuation by fluids (left) and CAD model of

arrahed sucker pads (right)

Fig. 5. A schematization of the volume enlargement inside the

membrane of sucker cups by using fluidic actuation, left, and

mechanical actuators right.

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For attachment, the membrane of these suckers can reshape

to a concave form by means of a negative fluidic pressure

and for ease of detachment they can reshape to a convex

shape with positive pressure of fluid (Fig. 8).

B. Experimental Evaluation

The goal of our evaluation was to monitor the

performance of the prototyped suckers on different kinds of

surfaces and to compare the results with commercial sucker

clamps. For this reason, the prototypes were built having a

70 mm outer diameter, which is equal to the diameter of a

commercial sucker cup clamp . Moreover, we also addressed

experiments by adding a kind of lubrication mimicking

natural mucus assistance to adhesion.

Electromechanical experiments were carried out by means

of INSTRON 4464 equipment (which is normally used to

test tension, compression and reverse stress). Maximum

exerted forces by both the experimental sucker prototypes

and commercial sucker were measured in water

environment.

The sucker prototypes were mechanically interfaced to the

load cell of the machine (which had with maximum 1000N

load and 0.01N resolution) while the testing substrate was

installed on an aluminum frame immersed in water (see Fig. 9, left).

The experimental procedure consisted in bringing the

sucker down to the surface of the substrate. When the

membrane of sucker was in contact with the substrate, we

reshaped the membrane by means of a syringe like a

cylinder-piston pump, and the movement of the piston was

fixed by using mechanical stoppers. Then normal strain was

applied to sucker, which was pulled up with the commanded

speed, and the adhesion force was measured until the sucker

completely detached from the substrate. The commanded

speeds used were of 1, 5, 10, 50, 100 and 500 mm/min.

In total two sucker prototypes, one arrayed sucker and a

commercial sucker were evaluated. We selected plexiglass

as a smooth substrate (Rz 0.622 µm); then a band sawed

PVC (Rz 43.581 µm), a rough marble plate (Rz 89.867 µm)

and a rough cement plate were used as rough substrates. We

cannot indicate here the roughness of the cement because it

was too high and out of the range of our contour measuring

instrument (ZEISS Surfcom 130A, with 800 µm measuring

range). When evaluating forces at the interface with cement

substrate, we also investigated the effect of mucus. The

mucus was the result of solved edible gelatin leaves in water,

which was rubbed soft to the membrane part of the suckers.

All the tests repeated at least five times for each sucker in

each speed.

IV. RESULTS AND DISCUSSION

The commercial sucker did not achieve attachment to any

of the rough substrates, so it was just tested on plexiglass. A

comparison of the obtained force for each sucker on smooth

surface is shown in Fig. 10. In most of the tests, the recorded

forces for the commercial sucker and other suckers increased

with increasing pulling speed. The maximum force when

using a speed of 1 mm/min, for the commercial, Ecoflex and

Dragon Skin suckers resulted of 222.8N, 260.2N and

337.6N, respectively. The largest maximum forces for

commercial sucker and Dragon Skin type were of 262.3N

and 350.1N which were captured at 500mm/min speed.

Instead, for the Ecoflex sucker the absolute maximum force

was obtained when vertically pulling the sucker at

100mm/min and it was 318.5N. In the case of arrayed

sucker, as expected, it can attach to the surfaces with cracks

(Fig. 9 right), The maximum force for arrayed suckers was

of 36.03N, in case of a 10 mm/min speed and the maximum

force obtained on the cement substrate resulted in 6.738 N in

case of 500 mm/min speed.

As regards the single suckers, the large thickness of soft

membrane in single suckers (5mm) is a feature which helps

them to reform on the waviness and roughness of the

substrates. Therefore they can attach to the rough surfaces

(Fig. 11). However, in the case of the arrayed sucker the

thickness of the soft membrane was much lower (1mm)

therefore we can think that this is the reason because a

proper sealing as good as in single suckers, could not be

provided.

Fig. 9. The arrayed sucker interfaced to INSTRON (left) for

electromechanical testing. The arrayed sucker is shown (right)

attached on the intersection of ceramic plates.

Fig. 8. The membrane part of sucker can reform in concave and

convex shape, to provide attachment force and easy detachment.

Fig. 7. Soft bio-inspired sucker prototypes; single (left) and arrayed

sucker (right).

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These results show how the single soft suckers had a

better performance than commercial suckers. In addition to

the bioinspired design, the main reason is that the amount of

volume change created by the fluidic actuation in the single

soft sucker prototypes, as previously explained (see Fig. 5),

is larger than the one created in commercial sucker when

performing adhesion. Therefore, the generated forces are

higher.

The results of tests on band-sawed substrate (with an

oriented roughness), marble and cement, respectively are

shown in Fig. 12, 13 and 14. As it could be expected, the

charts show that, soft suckers can produce negative pressure

and attachment force on rough surfaces. This capability is

provided, because the soft materials can adapt to the

roughness of substrate and decrease the speed of leakage of

water to the vacuum chamber. In particular, Fig. 14 shows

how, when mucus is used on top of the cement substrates,

the soft sucker prototypes (disregarding of the soft material

they are made of) implement higher adhesion forces.

During the tests of the Dragon Skin sucker, for all the

speeds except for 500 mm/min, the process of detachment

started with a slipping, from the peripheral region towards

the center. On a first analysis, one could thing that this

process could have unexpected effects depending on the

frictional characteristics of the sucker/substrate interface.

Instead, at 500mm/min speed, the detachment process

occurred suddenly by making a kind of explosion sound.

This same phenomenon was obtained also for the Ecoflex

sucker at all the speeds, and also for Dragon Skin as well,

after adding gelatin mucus to the attachment process. This

shows that the membrane in the Ecoflex type sucker (which

is more flexible than the Dragon Skin one), adapts better on

the substrate roughness and with the assist of gelatin mucus

provides a better sealing.

V. CONCLUSION

The process of attachment and detachment in adhesive

tube feet in sea urchins was investigated. Taking inspiration

from such natural mechanism, the concept of increasing the

softness of sucker pads and adding a thin layer of mucus

between sucker membrane and substrate for obtaining

stronger attachment, was proposed, implemented and tested.

The obtained results showed a repeatable increase in

generated forces by single soft suckers, as compared with

commercial sucker clamps. By adding mucus to the

0

50

100

150

200

250

300

350

1 5 10 50 100 500

Forc

e[N

]

Speed mm/min

Cement-Ecoflex

Cement-

Ecoflex+Mucus

Cement-Dragon

Cement-

Dragon+mucus

Fig 14. Results of pulling force of soft sucker prototypes interfaced to

cement substrate alone, and with artificial mucus on top.

0

50

100

150

200

250

300

350

400

1 5 10 50 100 500

Fo

rce

[N

]

Speed [mm/min]

Dragon Skin

Ecoflex

Fig. 13. The average values of maximum forces in each speed for

Dragon Skin and Ecoflex suckers on marble substrate

0

50

100

150

200

250

300

350

400

1 5 10 50 100 500

Fo

rce

[N]

speed[mm/min]

Dragon skin

Ecoflex

Fig. 12. The average values of maximum forces in each speed for

Dragon Skin and Ecoflex suckers on band sawed plastic substrate

Fig. 11. Sucker cup shown when lifting a rough brick.

0

100

200

300

400

1 5 10 50 100 500

Forc

e[N

]

Speed [mm/min]

Comercial Dragon Skin Ecoflex

Fig. 10. Results of pulling force of suckers interfaced to smooth

plexiglass as substrate.

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attachment process the obtained forces were the following.

As regards the Dragon Skin sucker type, the forces increased

from 6.6% (for a 500mm/min pulling speed) to 66.38% (for

a 1 mm/min pulling speed). As regards the Ecoflex sucker

type, the forces increased from 29.2% (for a 500mm/min

speed) to a 232% (for a 1mm/min speed). The proposed

sucker is simple for final design and manufacturing process

and it can be miniaturized. This potentiality is shown, for

example, in Fig. 15. Here we depict a miniature arrayed

sucker system, embodying the proposed bio-inspired sucker

mechanism, with suckers having millimetric diameter.

As future work, we plan to increase the flexibility of

whole the body in arrayed suckers, in order to increase the

ability of matching with waviness and roughness in the

substrates. Also a mechanism for controlled dispensing of

the secretion will be designed and integrated in the sucker

system. We also plan to investigate the application of this

adhesion and detachment mechanism when integrated in soft

continuum arms. Such potential application is shown by the

developed prototype shown in Fig. 16. Such preliminary

prototype embodies the elongation and retraction model of

tube feet. This soft arm can elongate by hydraulic pressure

and moves around by movement of tendons. The arm can

grasp external objects by using the sucker and it will be

experimented as underwater soft manipulator.

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Fig. 16. The soft continuum sucker ended arm is grasping and

manipulating a part inside the water.

Fig. 15. A miniature arrayed sucker system, embodying the

proposed bio-inspired sucker mechanism.

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