Adhesion of echinoderm tube feet to rough surfaces
1 Académie Universitaire Wallonie-Bruxelles, Université de
Mons-Hainaut, Laboratoire de Biologie Marine, 6 Avenue du Champ de Mars,
B-7000 Mons, Belgium
2 Max-Planck Institute for Metal Research, Department Arzt, Evolutionary
Biomaterials Group, 3 Heisenbergstraße, D-70569 Stuttgart,
Germany
* Author for correspondence (e-mail: romana_santos{at}yahoo.com)
Accepted 11 May 2005
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Summary |
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Key words: sea star, sea urchin, tube foot disc, tenacity, viscoelastic material, Paracentrotus lividus, Asterias rubens
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Introduction |
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During their activities, echinoderms have to cope with substrata of varying
degrees of roughness, as well as with changing hydrodynamic conditions, and
therefore their tube feet must adapt their attachment strength to these
environmental constraints. Although a few studies have evaluated the adhesive
capacities of echinoderm tube feet on different substrata
(Paine, 1926;
Thomas and Hermans, 1985
;
Flammang and Walker, 1997
;
Flammang et al., 2005
), none
of them investigated the influence of substratum roughness. Theoretically,
there are two ways by which tube feet can achieve an effective bonding of
their discs to a rough surface: either the disc remains flat and more adhesive
substances are secreted to fill the gaps between irregularities of the
substratum (Fig. 1A) or the
disc deforms to match the substratum profile and the adhesive is released as
an evenly thin film (Fig. 1B).
To address the question of how echinoderms attach to irregular surfaces, we
analysed tube foot tenacity (adhesion strength) and disc structural
deformation in response to substratum roughness and measured disc mechanical
properties in the tube feet of two common European echinoderm species, the sea
urchin Paracentrotus lividus and the sea star Asterias
rubens.
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Materials and methods |
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For scanning electron microscopy (SEM), the tube feet were dried by the critical point method, mounted on aluminium stubs and coated with gold in a sputter coater. They were observed with a JEOL JSM-6100 scanning electron microscope.
For light microscopy (LM), the tube feet were embedded in paraffin wax
(Paraplast; Sigma, Steinhem, Germany) using a routine method
(Gabe, 1968). They were
sectioned longitudinally at a thickness of 7 µm with a Microm HM 340E
microtome and the sections were collected on clean glass slides. They were
stained with Masson's trichrome or with azocarmine coupled with aniline blue
and orange G (Gabe, 1968
). The
sections were then observed and photographed with a Leitz Orthoplan light
microscope equipped with a Leica DC 300F digital camera.
Surface structure and tenacity of tube feet attached to substrata of differing roughness
Two pairs of polymer substrata were used in these experiments, each
including one smooth and one textured surface. The first pair of substrata,
consisting of polymethyl-methacrylate (PMMA), was manufactured in the
laboratory by the use of a two-step moulding technique. The `rough PMMA'
sample was prepared by curing liquid methyl-methacrylate on a negative
template shaped on polishing paper with 12 µm particle size. The `smooth
PMMA' sample was obtained in a similar way by using a negative template
moulded on a polished surface of polypropylene. The second pair of substrata,
made up of polypropylene (PP), was derived from laboratory supplies. The
`smooth PP' samples were prepared by cutting off square pieces (1
cm2) from the cover of a micropipette tip rack (MP Biomedicals,
Irvine, CA, USA), while the `rough PP' samples consisted of circular frits
(
1 cm in diameter) used in Model 422 Electro-Eluter (Bio-Rad, Hercules,
CA, USA).
The surface profile of the different substrata was examined in a scanning white light interferometer (Zygo NewView 5000; Zygo Corporation, Middlefield, CT, USA) at magnifications of 5 x and 50 x. The device included optics for imaging an object surface and a reference surface together onto a solid-state imaging array, resulting in an interference intensity pattern that was read electronically into a computer. A series of interferograms were generated as the objective was scanned perpendicular to the illuminated surface. They were then individually processed, and finally a complete 3-D image was constructed from the height data and corresponding image plane coordinates. From 3-D images, profilograms of sections and mean surface roughness parameters (mean roughness of the profile, Ra, and maximum height of the profile, Rz) and the profile length ratio (Lr) were obtained.
Tube feet of sea urchins and sea stars were allowed to adhere to clean pieces of the four types of substrata. When a tube foot remained firmly attached to the substratum it was cut off from the animal, fixed and processed for SEM as described above.
Adhesion force measurements of a single tube foot were performed with an
electronic dynamometer (AFG 10 N; Mecmesin, Horsham, UK) attached to a
Mecmesin-Versa Test motorized stand. This dynamometer measures forces up to 10
N with a precision of 0.002 N. Experiments were performed with sea urchins and
sea stars totally immersed in containers filled with seawater. Specimens were
put upside-down (to induce tube foot attachment), and a 1 cm2 piece
of substratum, connected to the dynamometer by a surgical thread, was
presented to the tube feet. When a single tube foot remained attached to the
substratum for at least 10 s, the dynamometer was moved upwards at a constant
speed of 15 mm min-1 in order to apply a force normal to the disc
until it detached, and the maximum adhesive force was recorded
(Flammang and Walker, 1997).
The piece of substratum was then immediately immersed for 1 min in a 0.05%
aqueous solution of the cationic dye Crystal Violet to stain the footprint
left by the tube foot after it had become detached
(Flammang et al., 1994
). This
footprint was measured with a graduated eyepiece mounted on a Leica Laborlux
light microscope. It was also photographed, and the digitized image was
analyzed with Semaphore® software (Jeol, Tokyo, Japan) to calculate the
surface area of the footprint.
The tenacity or adhesive strength (T) was then calculated by
dividing the measured adhesion force (Fa) by the
corresponding footprint surface area (S):
![]() | (1) |
![]() | (2) |
Tenacity measurements were carried out on tube feet from at least three different animals for each species. Results were statistically analysed with Statistica® software (Statsoft Inc., Tulsa, OK, USA) in order to reveal intraspecific differences in tube foot tenacity obtained on substrata of differing roughness. When necessary, logarithmic transformation was used to achieve homoscedasticity, followed by t-tests.
Mechanical properties of the tube foot disc
For both sea urchin and sea star tube feet, the mechanical properties of
the disc were measured with a micro-force tester (Tetra GmbH, Ilmenau,
Germany; see Scherge and Gorb,
2001 for details) composed of three main parts: a platform, a
glass spring (with a spring constant of 112 N m-1) and a fibre
optical sensor (Fig. 2). The
platform held the tube foot clamped by the stem and could be moved up and down
by a motorized stage. For each measurement, the platform was moved upwards
(loading period) and the tube foot disc brought into contact with a square
glass plate attached to the glass spring. Spring deflection was detected by
the fibre-optic sensor, whose signal was acquired by a computer. The tube foot
disc and the glass surface were kept in contact for 10 s (resting period) and
then the platform was moved downward (unloading period)
(Fig. 3A). Force
versus displacement data were continuously recorded by a computer
during loading, resting and unloading periods
(Fig. 3B). All experiments were
carried out at room temperature (22-24°C) and at a relative humidity of
47-56%.
|
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To investigate disc elasticity, the glass plate was brought as close as possible to the tube foot disc. Then, an upward displacement of 100 µm was programmed in order to induce a compression of the disc ranging from 50 to 100 µm. Measurements were performed on 15 randomly chosen tube feet of at least three individuals from each species. As both the disc and the glass spring deform simultaneously, the deflection of the glass spring, when pressed against a hard sample (glass slide), has to be subtracted from the tube foot force-displacement curve to calculate the true deformation of the disc (Fig. 4A). Using this procedure, force-deformation curves were recalculated for the tube feet of both species (Fig. 4B). The loading and unloading parts of the force-deformation curves were then used to obtain a loading and an unloading stiffness (expressed in N m-1), corresponding to the slope of the linear part of the curve. The modulus of elasticity (E-modulus, expressed in N m-2 or Pa) was also calculated as the ratio of stress to strain. Strain is a unitless parameter corresponding to the ratio between disc deformation and disc initial thickness. Stress was obtained by dividing the maximum applied force by the disc cross-sectional area and is expressed in Pa. For calculations, the disc was considered as a cylindrical structure, with a constant geometry of cross-sectional area. Mean values of disc thickness and diameter were obtained using 10 randomly chosen tube feet from at least three individuals of each species.
|
In each measurement, after loading the disc with a certain compression
force, there was a resting period, when the disc and the glass plate were kept
in contact without further loading or unloading
(Fig. 3). At the beginning of
the resting period, the measured force exponentially decreased. The force
decrease is due to the relaxation of the tube foot disc material. This
behaviour indicates viscoelastic properties of the disc material. To study the
viscous component of the disc material properties in detail, we designed a new
experiment, in which the disc was deformed by the glass plate up to a
pre-defined normal force of 1 mN. This procedure was repeated with four tube
feet of each species and data plotted as force-time curves
(Fig. 5A). Then, the relaxation
part (resting period) of the force-time curve was fitted with a standard
linear solid model (Wainwright et al.,
1976; Vincent,
1990
) in which the zero point of time corresponds to the maximal
load prior to relaxation (Fig.
5B). This model includes two elastic moduli
(E0 and E1), representing two springs
connected in parallel, and one time constant (
), representing a dashpot
serially connected to one of the springs. The formula used for fitting was:
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The micro-force tester used could not operate in seawater, thus all the experiments had to be performed in air. The tube feet tested were initially moist but became dry after a certain time. To minimize desiccation of the disc material, the dissected tube feet were taken out of seawater just before mechanical tests. To test the influence of evaporation, the time of each measurement was recorded. Measurements on every single disc were repeated at every minute of desiccation of the disc for 5 min. Fluctuations in disc thickness and diameter were also measured at every minute of desiccation up to 5 min.
Data were analysed with Statistica® software to search for differences in the mechanical properties of the tube feet between the two species studied, as well as for a possible effect of evaporation. When necessary, data were log-transformed followed by multi-factorial analysis of variance (ANOVA) and Tukey tests for multiple comparisons. The variability explained by each factor is derived from the sum of squares.
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Results |
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Discs of both species consist of two layers of approximately equal
thickness: a deeper supporting structure and a distal pad making contact with
the substratum (Fig. 7A,B). The
supporting structure consists mostly of a circular plate of connective tissue,
the so-called terminal plate, that is continuous on its proximal side with the
connective tissue sheath of the stem. The centre of the terminal plate or
diaphragm is very much thinner than its margin and caps the ambulacral lumen.
In P. lividus, the terminal plate (supporting structure) encloses a
calcified skeleton made up of four large ossicles arranged in a circle around
the ambulacral lumen. In A. rubens, on the other hand, the terminal
plate is composed of densely packed collagen fibres. In both species, the pad
is composed of a thick adhesive epidermis reinforced by bundles of collagen
fibres. Numerous branching connective tissue septa emerge from the distal
surface of the terminal plate and manoeuvre themselves between the epidermal
cells. The thinnest, distal branches of these septa attach apically to the
support cells of the epidermis. In P. lividus, these septa form an
irregular meshwork, whereas in A. rubens they are arranged as
well-defined radial lamellae [Fig.
7C,D; for a more detailed description of the disc epidermis of sea
stars and sea urchins tube feet, see Flammang et al.
(1994) and Flammang and Jangoux
(1993
)].
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Surface structure and tenacity of tube feet attached to substrata of different roughness
Table 1 shows the profile
parameters (Ra, Rz and
Lr) for the tested substratum surfaces. The two smooth
substrata did not present any important protuberance on their surfaces (Figs
8A,B,E,F,
9A). The profile parameters of
both smooth substrata (PMMA and PP) were clearly lower than those of their
rough equivalents (Table 1). The surface profile parameters measured for the smooth PMMA samples were very
similar to those from the smooth PP samples
(Table 1). On the other hand,
the mean roughness (Ra) and maximum height
(Rz) of the rough PP samples were 7-16 times larger in
comparison with the rough PMMA samples
(Table 1). Indeed, the surface
of the rough PMMA substratum was regularly micro-textured (Figs
8C,D,
9D) whereas the surface of the
rough PP substratum was very irregular, being made of aggregated particles
(Figs 8G,H,
9G). Furthermore, for both PMMA
and PP, the profile length ratio (Lr) was higher on rough
than on smooth substrata (Table
1). The larger the value of Lr, the sharper or
crisper the surface profile appears and the larger the true surface area of
the substratum is.
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The surface topography of tube foot discs attached to the various substrata was investigated by SEM. In the two species, the discs attached to smooth substrata presented flat and relatively smooth surfaces, whereas those attached to rough substrata had irregular surfaces (Fig. 9). Discs attached to rough PMMA were covered with evenly spaced slight indentations (Fig. 9E,F) whereas discs attached to rough PP showed larger and deeper indentations (Fig. 9H,I). The tube foot discs of P. lividus and A. rubens therefore seem to replicate the substratum profile (Fig. 9), although there is an important variability from one tube foot to another. This variability may be explained by the fact that no external pressure was applied on the tube feet. Therefore, the force with which the disc was pressed on the substratum was only due to their own natural movements and was probably quite variable.
Fig. 10 summarizes the results of adhesion measurements from the tube foot discs of the two species considered, when attached to substrata of different roughness. In sea urchins, mean tenacity was significantly influenced by the roughness of the substrata. On both PMMA and PP substrata, the tube feet of P. lividus produced higher tenacity on the rough substrata than on the smooth ones (0.34 and 0.47 MPa for PMMA; 0.14 and 0.28 MPa for PP; t-test, P<0.04). In sea stars, tube foot tenacity was also higher on the rough PMMA substratum than on its smooth counterpart (0.18 and 0.21 Mpa, respectively) but this difference was not significant (t-test, P=0.17). There are no tenacity data on PP because the tube feet of A. rubens did not adhere firmly enough to this substratum. In terms of corrected tenacity, however, roughness no longer influenced tube foot adhesion significantly (P. lividus - 0.34 and 0.42 MPa for PMMA, 0.14 and 0.09 MPa for PP; A. rubens - 0.18 and 0.19 MPa for PMMA; t-test, P>0.14). Between species, sea urchin tube feet produced significantly higher tenacities (T and Tc) than sea star tube feet on both smooth and rough PMMA substrata (t-test, P<0.003).
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Mechanical properties of the tube foot disc
In both species, there was a gradual deformation of tube foot disc under
the applied force during the loading process
(Fig. 4B). In freshly cut-off
tube feet, the mean deformation of the disc was 68.1±11.1 µm (mean
± S.D.) in P. lividus and 77.8±11.5 µm in
A. rubens, corresponding to a compression force of 1.5±1.2 mN
and 1.2±1.1 mN, respectively. Since the mean thickness of the disc was
224±34 µm for sea urchins and 455±32 µm for sea stars,
their deformations were approximately 30 and 17%, respectively.
Table 2 summarizes the results
for disc stiffness and elastic modulus measured each minute for 5 min on discs
from both species. Two-way ANOVA (species and evaporation time as independent
variables) revealed that loading and unloading stiffness as well as elastic
modulus of discs vary significantly between species
(Table 4). The three variables
were always higher in P. lividus than in A. rubens,
indicating that sea urchins have stiffer discs than those of sea star. An
additional two-way ANOVA (part of the curve and evaporation time as
independent variables) showed that, in both species, stiffness did not vary
significantly during loading and unloading of the discs at any time of
evaporation (P>0.05). In P. lividus, a significant
variability of the elastic modulus with time of evaporation was observed
(Tables 2,
4). However, no consistent
relationship was found between disc elasticity and time of desiccation, at
least up to 5 min.
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Table 3 presents the mean
values of the three parameters of the standard linear solid viscoelastic model
applied to measurements of the tube feet from both species. In order to search
for differences between the species studied as well as the times of
evaporation, a two-way ANOVA was performed for the three parameters of the
model (Table 4). The elastic
modulus E0 did not differ either between the two species
or between the different times of evaporation. The elastic modulus
E1 was slightly higher in P. lividus than in
A. rubens, but no effects were observed in terms of disc desiccation.
The time of relaxation () did not differ between the two species but was
significantly affected by the time of evaporation. Further analysis within
each species showed that, in A. rubens, the elastic modulus
E0 was significantly higher at 4 min of evaporation than
at 1 min and the time of relaxation was significantly higher at 2 min of
evaporation than at 0 min (Table
3). However, the differences observed are very variable and no
consistent pattern could be found.
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Discussion |
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Attachment strength of tube feet on rough substrata
Adhesion of echinoderm tube feet appears to be stronger on rough substrata
than on smooth ones. The tube foot discs of Paracentrotus lividus
showed a significantly higher tenacity on rough substrata in comparison with
smooth substrata. The increase in tenacity was of approximately 26% on PMMA
and 50% on PP. Tenacity values obtained for discs of Asterias rubens
attached to smooth and rough PMMA also showed an increase but, contrary to the
data obtained for echinoids, this increase was not significant. As for
corrected tenacity, it never varied significantly with substratum roughness.
The values of tenacity measured on the two polymer substrata were in the same
range as those reported in previous studies for single tube feet of A.
rubens (0.2 MPa; Flammang and Walker,
1997) and P. lividus (0.29 MPa;
Flammang et al., 2005
)
attached to smooth glass slides.
Influence of surface roughness on adhesion has only been investigated in
two other marine invertebrate taxa: barnacles and limpets. Barnacles attach
permanently to the substratum as adults but have a larval stage, the cyprid,
that uses temporary adhesion (Yule and
Walker, 1987). The cyprid larva of barnacles adheres temporarily
to the substratum by antennulary attachment discs while it explores a surface
during settlement. These discs demonstrate high deformability, allowing them
adaptation to the substratum profile, and produce an adhesive secretion.
Surface roughness influences the force required to detach the cyprids. An
increase in the roughness of a PMMA substratum from 0.03 to 1.0 µm
(Ra) results in an increase of antennule tenacity from
0.08 and 0.14 MPa (Yule and Walker,
1984
,
1987
). At metamorphosis, the
cyprid secretes a cement to attach itself permanently to the substratum. A
roughened surface also affords the newly metamorphosed barnacle better
adhesion than does a smooth surface; increasing the roughness of the PMMA from
0.03 to 1 µm significantly increases the tenacity from 0.16 to 0.5 MPa
(Yule and Walker, 1987
). In
limpets, the effect of different substrata on tenacity was investigated in
emersed animals attached to smooth glass, smooth and rough slate and smooth
PMMA (Grenon and Walker,
1981
). The highest tenacity values where obtained on glass and
rough slate (0.23 MPa), followed by smooth slate and PMMA (0.19 and 0.17 MPa,
respectively).
In sessile invertebrates attaching permanently to the substratum, the high
tenacity measured on rough surfaces is most likely due to mechanical
interlocking (Yule and Walker,
1987). The cement secreted by these organisms is initially fluid
and able to infiltrate deeply the pores and crevices of the substratum. After
polymerization, the cement becomes a material with high cohesive strength,
interlocking with the substratum (Cheung et
al., 1977
; Yule and Walker,
1987
). This is corroborated by observations made on the cement of
barnacles (Dougherty, 1990
),
tubeworms (Roscoe and Walker,
1995
) or mussels (Crisp et al.,
1985
), which forms a perfect cast of the surface features of the
substratum after the animal had been carefully detached. For invertebrates
using non-permanent adhesion, on the other hand, mechanical interaction
between the adhesive and the substratum surface is presumably not involved in
the increase of adhesive strength observed on rough substrata compared with
smooth ones. Several other explanations, which are not mutually exclusive,
have been proposed for this effect. Grenon and Walker
(1981
) suggested that, as
surface roughening alters the contact angle between water and the substratum
(Baier et al., 1968
), it would
presumably also modify the spreading of the adhesive secretion and
consequently the tenacity observed for certain organisms. However, according
to Kendall (2001
), roughening
does not modify the contact angle of water but rather retards the formation of
this wetting angle on surfaces (hysteresis effect). This may therefore modify
the speed of adhesion but not the strength of adhesion. Friction could also
explain the more important force required to detach organisms from rough
surfaces because the irregular surface introduces shear forces within the
adhesive layer even when normal pulls are applied, as in the present study and
in those on barnacle cyprids and limpets. A third explanation is the
retardation of crack propagation. Cracking is the mechanism by which an
adhesive material detaches from a surface
(Kendall, 2001
). On a rough
surface, crack propagation would have to follow a much longer, nonrectilinear
path at the interface (Baier et al.,
1968
), resulting in an increased adhesive strength. A final
explanation is the increase in geometrical area of contact as a result of
roughening, which leads to a more important adhesive force but not a higher
tenacity (both force and surface area increase together). However, as
substratum roughness is usually not taken into account, the contact area is
underestimated from the surface area of the adhesive organ (e.g. the surface
area of the foot in limpets; Grenon and
Walker, 1981
), hence the higher apparent tenacity on rough
substrata.
In our study, a corrected tenacity was calculated, which includes the actual profile length of the substratum and therefore the true surface area of contact. The fact that this corrected tenacity did not vary significantly with substratum roughness pleads in favour of the last hypothesis (i.e. adhesion force increase due to the increase in geometrical area of contact on rough surfaces). This corrected tenacity assumes that detachment occurs at the interface between the substratum and the adhesive. However, in the case of echinoderms, detachment generally occurs at the interface between the tube foot disc and the adhesive, which is left as a footprint on the substratum. Therefore, on irregular surfaces, the corrected tenacity is valid only if the tube foot disc deforms to match the substratum profile. Such a deformation was demonstrated by the SEM pictures taken on the disc surface of the tube feet attached to the rough polymers. It is possible due to the material properties of the disc.
Viscoelastic properties of the tube foot disc
Measurement of the mechanical properties of the tube foot disc in P.
lividus and A. rubens demonstrated that this structure is made
up of a very soft material with a mean elastic modulus ranging from 3 to 140
kPa. Loading and unloading stiffness, as well as the elastic modulus of the
disc, was higher in sea urchins than in sea stars
(Table 2). Higher stiffness of
sea urchin discs might be a consequence of the presence of a calcified
skeleton within their connective tissue. Furthermore, no significant
differences were found between loading and unloading stiffness and no
consistent desiccation effect could be established. Regarding the
stress-relaxation experiments, a standard linear solid viscoelastic model was
used to describe disc behaviour during the contact with the substratum. This
model comprises two elastic moduli (E0 and
E1), representing two springs (simulating the elastic
behaviour of solids whose resistance to deformation is a function of applied
force), and one time constant (), representing a dashpot (simulating the
viscous behaviour of fluids whose resistance to deformation depends on the
rate at which they are deformed). The elastic modulus E0
(4.1 kPa for sea urchins and 2.0 kPa for sea stars at 0 min) and the time of
relaxation
(5.1 s for sea urchins and 7.5 s for sea stars at 0 min) did
not differ between the two species whereas the elastic modulus
E1 was slightly higher (P=0.049) in P.
lividus (5.4 kPa at 0 min) than in A. rubens (3.4 kPa at 0 min).
None of these variables were consistently influenced by the desiccation of the
disc. These results indicate that echinoderm tube foot discs behave like a
viscoelastic material; i.e. they deform elastically under rapidly applied
forces and behave viscously under slowly acting forces.
To the best of our knowledge, no other studies have been published on the
material properties of adhesive surfaces from marine invertebrates. However,
such studies have been made on adhesive pads of insect legs. Some insects
possess smooth flexible pads whose function is to maximize contact area with
the substratum, regardless of the surface micro-texture, and to secrete a
lipid-like substance that is delivered to the contact area, constituting an
important component of attachment (Scherge
and Gorb, 2001). Like echinoderm tube foot discs, smooth insect
pads demonstrate viscoelastic properties of the material. The smooth pad of
the grasshopper Tettigonia viridissima attaches through a combination
of an adhesive secretion on the pad surface and a highly deformable pad
material. Gorb et al. (2000
)
reported that pads pressed against a structured silicon surface showed surface
indentation patterns that replicated the pattern of the silicon surface. Under
high loads, indentation corresponded to the height of silicon structures, and
under lower loads very weak deformations occurred. The smooth pad of T.
viridissima possesses an elastic modulus of 27.2 kPa, which is not very
different from that of the echinoderm tube foot disc. Although the relaxation
behaviour is different from that of the echinoderm tube foot disc, elastic
moduli from both attachment systems are in the same range.
The deformability and viscoelastic properties of the grasshopper attachment
pad have been related to its fibrous composite nature
(Gorb et al., 2000). Indeed,
the cuticle constituting the pad is made up of uniformly distributed fibres,
orientated perpendicularly to the pad surface. In the vicinity of the cuticle
surface, these fibres branch into numerous smaller fibres. Such an
organization presumably provides flexibility at two levels: (1) the local
level, when preferably branched fibres deform, and (2) the global level, when
the main fibres also deform. The first level of deformation is responsible for
adapting the pad surface to the substratum micro-roughness, whereas the second
one can fit the pad to its macro-roughness
(Gorb et al., 2000
).
Interestingly, the echinoderm tube foot disc is strikingly similar to the
insect smooth attachment pad in its organization, though in this case the
fibres are made up of collagen and manoeuvre themselves between the epidermal
cells. It may be suggested that the fine collagen branches provide
adaptability of the disc surface to the small irregularities of the
substratum, while the main fibres or lamellae fit the disc to the
macrosculpture of natural substrata.
A model for the adhesion of tube feet on rough substrata
The echinoderm tube foot disc shows both elastic and viscous behaviours
under load. To enable strong attachment between the disc material and the
substratum, a close proximity between opposite surfaces is required, which can
be achieved through high flexibility of at least one of the materials.
Echinoderms' discs proved to be highly deformable; their viscous behaviour
under slow self-imposed forces enables them to replicate surface profiles,
leading to an increase of the contact area between the disc and the
substratum. Then, the adhesive is deposited as a film, whose thinness is
advantageous for generation of strong adhesion, as shown for other glue-based
systems (Kendall, 2001).
Therefore, echinoderms are able to adapt to the substratum roughness by means
of disc flexibility. Very small surface irregularities, in the nanometre
range, can presumably be filled out with the adhesive secretion. When attached
strongly to the substratum, echinoderms are also exposed to strong external
forces. It can be hypothesized that, under short pulses of wave-generated
forces, attached discs behave elastically, distributing the stress along the
entire contact area. This would avoid crack generation and thus precludes disc
peeling and tube foot detachment.
List of symbols
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Acknowledgments |
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References |
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