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Recent biological research suggests that reliable, agile climbing on smooth vertical surfaces requires controllable adhesion. In nature, geckos control adhesion by properly loading the compliant adhesive structures on their toes. These strongly anisotropic dry adhesive structures produce large frictional and adhesive forces when subjected to certain force/motion trajectories. Smooth detachment is obtained by simply reversing these trajectories. Each toe's hierarchical structure facilitates intimate conformation to the climbing surface resulting in a balanced stress distribution across the entire adhesive area. By controlling the internal forces among feet, the gecko can achieve the loading conditions necessary to generate the desired amount of adhesion. The same principles have been applied to the design and manufacture of feet for a climbing robot. The manufacturing process of these directional polymer stalks is detailed along with test results comparing them to conventional adhesives
Directional AdhesiveStructures for Controlled Climbing
on Smooth Vertical Surfaces
Daniel Santos, Sangbae Kim, MatthewSpenko, Aaron Parness, Mark Cutkosky
Center for Design Research
Stanford University
Stanford, CA 94305-2232, USA
AbstractRecent biological research suggests that reliable,
agile climbing on smooth vertical surfaces requirescontrol-
lable adhesion. In nature, geckos control adhesion by prop-
erly loading the compliant adhesivestructures ontheir toes.
These strongly anisotropic dry adhesivestructures produce
large frictional and adhesiveforces when subjected to certain
force/motion trajectories. Smooth detachmentis obtained by
simply reversing these trajectories. Each toe’shierarchical
structurefacilitates intimate conformation to the climbing
surface resulting in abalanced stress distribution across the
entireadhesivearea. By controlling the internal forces among
feet, the gecko can achievethe loading conditions necessary to
generate the desired amount of adhesion. The same principles
havebeen applied to the design and manufactureof feet fora
climbing robot. The manufacturing process of these Directional
Polymer Stalks is detailed along with test results comparing
them to conventional adhesives.
As mobile robots extend their range of traversable terrain,
interest in mobility on vertical surfaces has increased. Pre-
vious methods of climbing include using suction [18], [19],
magnets [5], [29], and avortex[28] to adhere to avariety
of smooth, flat, vertical surfaces. Although these solutions
havehad some success on non-smooth surfaces, in general,
the variety of climbable surfaces is limited. Taking cues
from climbing insects, researchers havedesigned robots that
employlarge numbers of small (10µmtip radius) spines
that cling to surface asperities [1], [22]. This approach works
well for surfaces such as concrete or brick but cannot be used
for smooth surfaces likeglass.
Recently,robots havebeen demonstrated that use adhe-
sives for climbing. Early approaches used pressure-sensitive
adhesives (PSAs) to climb smooth surfaces [9], [26], while
more recent approaches used elastomeric pads [8], [27].
PSAs tend to foul quickly,which prevents repeated use,
and also require relatively high energy for attachment and
detachment. Elastomer pads are less prone to fouling, but
generate lower levels of adhesion.
In an effort to create an adhesivethat does not foul over
time, there has been research on “dry” or “self-cleaning”
adhesives that utilize stiffmaterials in combination with
microstructured geometries to conform to surfaces. Fig. I
shows arange ofadhesivesolutions ordered in terms of
feature size and effectivemodulus. Amaterial is considered
tackywhen the effectivemodulus is less than 100kPa[2],
Fat PMMA Rubber Epoxy
Young’s Modulus (Pa)
β−keratin gecko
setal array
1mm 100um 1um 10nm
tacky nontacky
Shape sensitivity LowHigh
polymer stalks) β−keratin
Feature Size
Fig. 1. Shape sensitivity of different structures and modulus of elasticity of
various materials. Microstructured geometries can lower the overall stiffness
of bulk materials so that theybecome tacky.This principle allows geckos
to use β-keratin for their adhesivestructures.
[7]. Since adhesion is primarily aresult of van der Waals
forces, which decrease as 1/d3where dis the distance
between the materials, it is crucial to conform to the surface
over all relevant length scales.
Dry adhesives, such as the geckohierarchyof microstruc-
tures consisting of lamellae, setae, and spatulae, conform to
the surface despite having bulk material stiffnesses that are
relatively high (approximately 2GPafor β-keratin) [2]. The
hierarchical geometry lowers the effectivestiffness to make
the system function like a tackymaterial.
Synthetic dry adhesives havebeen under development
for several years. Examples include arrays of vertically
oriented multiwall carbon nanotubes [31], [32] and polymer
fibers [11], [15], [20], [25]. These adhesives employstiff,
hydrophobic materials and therefore havethe potential tobe
self-cleaning. In anumber of cases, useful levels of adhesion
havebeen obtained, but only with careful surface preparation
and high preloads. As the performance of these synthetic
arrays improves, their effectivestiffnesses could approach
the 100kPa“tack criterion”.
Adifferent approach usesstructured arrays of moderately
soft elastomeric materials with abulk stiffness less than
2007 IEEE International Conference on
Robotics and Automation
Roma, Italy, 10-14 April 2007
1-4244-0602-1/07/$20.00 ©2007 IEEE. 1262
3MPa.Because these materials are softer to begin with,
theyconform to surfaces using feature sizes on the order of
100µm.One example is amicrostructured elastomeric tape
[8], [21]. Because the material is notvery stiff, it attracts dirt.
However,in contrast to PSAs, it can be cleaned and reused.
The microstructured adhesivepatches described in Section
III, termed Directional Polymer Stalks (DPS), also employ
an elastomer but are designed to exhibit adhesiononly when
loaded in aparticular direction.
In addition to stiffness, feature size and shape of the struc-
ture is important in creating adhesion. Asdiscussed in [10],
[11], [16], [30], the available adhesiveforce is afunction
of the shape and loading of the micro-structured elements.
The importance of optimizing tip shape increases as feature
size increases. For extremely small elements such as carbon
nanotubes, the distal geometry is relatively unimportant,
but for larger features (O(100µm))tip geometry drastically
affects adhesion. At these sizes, the optimal tip geometry,
where stress is uniformly distributed along the contact area,
has atheoretical pulloffforce of more than 50-100 times [10]
that of apoor tip geometry.In Section III we describe the
processes wehavedeveloped to obtain desired shapes at the
smallest sizes our current manufacturing procedures allow,
and in Section IV we present experimental results obtained
with these shapes.
At present, no synthetic solution has replicated the adhe-
sion properties of geckofeet. However the main obstacle
to robust climbing is not moreadhesion but controllable
adhesion. Stickytape is sufficiently adhesivefor alight-
weight climbing robot, but its adhesiveforces are difficult
to control. Geckos control theiradhesion with anisotropic
microstructures, consisting of arrays of setal stalks with spat-
ular tips. Instead of applying high normal preloads, geckos
increase theirmaximum adhesion by increasing tangential
force, pulling from the distal toward the proximal ends of
their toes [3]. In conjunction with their hierarchical struc-
tures, this provides geckos with acoefficient of adhesion,
µ=Fa/Fp,between 8and 16 [2] depending on conditions,
where Fais the maximum normal pulloffforce and Fpis
the maximum normal preload force.
A. Description of Contact Models
The frictional-adhesionmodel (Fig. 2) is used to describe
the geckoadhesion system [3]. When pulling along the adhe-
sivedirection (B, positivetangential), the maximum adhesive
force is directly proportional to the applied tangential force:
FNFTtan α(1)
where FNis the normal force, FTis the tangential force
(positivewhen pulling from distal to proximal), and αis the
angle of abest fit line for test data obtained with individual
setae, setal arrays, and geckotoes [3]. When pulling against
the adhesivedirection (A), the behavior is described by
Coulomb friction. An upper limit is placed on the maximum
−100 0 100 200 300
Tangential Force (%Body Weight)
Normal Force (%Body Weight)
Fig. 2. Comparison of frictional-adhesion and JKR contact models. Both
models havebeen scaled to allowa50g geckoor robot to cling to an
inverted surface. Parameters and overlayed data for the anisotropic frictional-
adhesion model are from [3] for geckosetae, setal arrays, and toes. The
isotropic JKR model is based on parameters in [21], [23].
tangential force in the adhesivedirection (C), which is a
function of limb and materialstrength.
Fig. 2also compares frictional-adhesion and the Johnson-
Kendall-Roberts (JKR) model [12], [13], an isotropic adhe-
sion model based on spherical elastic asperities in contact
with aflat substrate. This model predicts that maximum
adhesion occurs at zero tangential force. Increasing tangential
force decreases the contact area, thereby decreasing the
overall adhesion. For positivevalues of normal force FT
2/3[24]. The models havebeen scaled to givecomparable
values of adhesion and tangential force limits, and the curves
represent the maximum normal and tangential force at which
acontact will fail.
The anisotropic model shows howmaximum adhesion can
be controlled simply by modulating the tangential force at the
contact. Its intersection with the origin allows for contact ter-
mination with negligible forces, whereas the isotropic model,
which does not intersect the origin, predicts large force
discontinuities at contact termination. This feature makes the
anisotropic model better-suited for vertical climbing than the
isotropic model. If the anisotropyis aligned properly,then
gravity passively loads the contact to increase adhesion.
B. Implications for control ofcontact forces
In general, both anisotropic and isotropic adhesives may
provide adhesion comparable to the body weight of agecko
or arobot; however,the models lead to different approaches
for controlling contact forces during climbing. Asimplified
planar model of aclimbing geckoor robot (Fig. 3) is used
for studying the implications of different contact models.
Work in dexterous manipulation [14] is adapted to study the
static stability of the model on inclined surfaces. There are
four unknowns and three equilibrium constraints, leaving one
degree of freedom: the balance of tangential force between
the front (FT1)and rear (FT2)foot (i.e. the internal force),
Fig. 3. 2-Dimensional model of ageckowith twofeet in contact with aflat
inclined plane. Foot-substrate interactions are modeled as point contacts.
Anisotropic Adhesive
Isotropic Adhesive
Fig. 4. Schematic of optimal tangential forces for isotropic and anisotropic
adhesion at different inclinations. Arrowdirections and magnitudes shown
in proportion to optimal tangential forces (dot represents zero tangential
FInt =FT1FT2.The maximum tangential force for each
foot is limited by the contact model.
The stability of the system can be used to determine
howbest to distribute contact forces between thefeet. The
stability margin is the minimum distance, in force-space, over
all feet, that anyfoot is from violating the contact constraints.
It defines the maximum perturbation force that the system
can withstand without failure of anyfoot contacts.
Let Fi=[FTi,FNi]be the contact force at the ith foot.
The contact model can be defined by aparametric convex
curveR(x, y),with points F=[FT,FN]lying inside the
curvebeing stable contacts. The distance any particular foot
is from violating acontact constraint is then:
x,y (||FiR(x, y)||).(2)
For amodel with two feet in contact with the surface, the
overall stability margin becomes d=min(d1,d2),where d1
represents the front foot and d2represents the rear foot.
The 2-D model’sextra degree of freedom canbe used to
maximize the stability margin. This produces different force
control strategies using the anisotropic or isotropic models at
different surface inclines (Fig. 4). On avertical surface the
front foot must generate adhesion. The anisotropic model
predicts the front foot should bear more of the gravity load,
since increasing tangential force increases available adhesion.
The isotropic model predicts the opposite, namely that the
rear foot should bear more of the gravity load, because
tangential forces on the front foot decrease its available
adhesion. On an inverted surface, the isotropic model predicts
zero tangential forces for maximum stability since gravity
is pulling along the normal. Alternatively,the anisotropic
model cannot generate adhesion without tangential forces
and this model must rotate the rear foot and pull inward to
Fig. 5. Stickybot experimental climbing robot for testing directional
adhesives. Each limb has twotrajectory degrees of freedom (fore-aft and
in-out of the wall) and one toe-peeling degree of freedom. The entire robot
weighs 370 grams.
generate tangential forces that will produce enough adhesion
for stability.Interestingly,the anisotropic model predicts that
reversing the rear foot and pulling inward is also optimal on
level ground, which would increase the maximum pertur-
bation force that could be withstood. Thepredictions of the
anisotropic model qualitatively match observations of geckos
running on walls and ceilings and reorienting their feet as
theyclimb in different directions [4].
The utility of anisotropic adhesion has been demonstrated
on anewexperimental robot, Stickybot (Fig. 5). Details of
Stickybot design and control are covered in acompanion
paper [17]. In this section we explain the DPS manufacturing
process, and in the nextsection we present test results
comparing the DPS to isotropic stalks of equivalent size and
The anisotropic stalks used on the bottom of Stickybot’s
feet are fabricated from apolyurethane (InnovativePolymers,
IE-20 AH Polyurethane, Shore-20A hardness, E300kPa).
Custom miniature tooling was used to create amold from
which the DPS were fabricated (Fig. 6). After aprocess
of trial and error,ageometry was found that produced
reasonable results for climbing. The stalks are cylindrical and
tilted with respect to the backing. The upper stalk is cropped
at an oblique angle thatcreates asharp tip. The cylinders
are 380µmin diameter and approximately 1.0mm long from
base to tip. Cylinder axes are inclined 20and slanted tips
are inclined 45,bothwith respect to the vertical. The shape
of the stalks is defined by the intersections of slanted circular
holes with narrow Vee-shaped grooves.First, the grooves are
cut into the mold using acustom 45degree slitting saw.This
angle dictates the angleof the tip. Slanted circular holes are
then drilled into the grooves such that the opening resides
entirely on the 45face.
Asilicone (TAP Plastics, Silicone RTV Fast Cure Mold-
Making Compound) form-fitting capis molded from the
Vee grooves before holes are created. Liquid polymer is
poured into the mold and capillary action fills the holes. The
form-fitting cap is pressed down into the grooves, forcing
excess polymer out the sides (Fig. 6). An SEMphoto of
Filling liquid
with top mold
Fig. 6. Molding process used to fabricate anisotropic patches. Mold is
manufactured out of hard wax and then filled with liquid urethane polymer.
Acap eliminates contact with air and creates final tip geometry.
Fig. 7. 380µmφanisotropic stalks oriented at 20with stalk faces oriented
at 45,both with respect to normal.
the stalks created using this process isshown in Fig. 7. The
process yields asharp, thin tip (10 30µmthickness). When
the stalksfirst contact asurface, this tip adheres and the
tangential force required to engage the remaining area of the
DPS face is very low.Fig. 7shows the geometry of the stalks
in both the unloaded and loaded states.
Specimens of the anisotropic material were tested under
avariety of tangential and normal loading conditions to
characterize their adhesiveproperties. For comparison, an
array of isotropic cylinders (vertical cylinders of the same
diameter with flat tops) made from the same polymer was
also tested.
Both the anisotropic and isotropic patches were approxi-
mately elliptical in shape with atotal area of 3.54cm2,
corresponding to one toe of Stickybot. The anisotropic
specimens contained 500 individual stalks while isotropic
specimens contained 250 stalks. Specimens were prepared
by washing with soap and water and then blowing dry with
compressed air.Theywere mounted using thin double-sided
tape to aflat aluminum backing.
The specimens and aluminum backing were fixed on a
two-axis linear positioning stage (VelmexMAXY4009W2-
S4) driven under servocontrol at 1kHz.Specimens were
brought into contact with astationary glass plate affixed to a
Fig. 8. Adhesion forces as afunction of pulloffangle for anisotropic
(700µmpreload) and isotropic (150µmpreload) patches. For anisotropic
patches, adhesion is maximum at shallowpulloffangles in the adhesive
direction and drops steadily as the angle becomes normal to the surface,
becoming negligible at shallowangles in the non-adhesivedirection.
6-axis force/torque sensor (ATI Gamma Transducer). The po-
sitioning stage is astiff, screwdriven device with atrajectory
accuracyof approximately ±20µmwhile in motion at speeds
of 1mm/s.The sensor resolution is approximately 25mN
and 0.5mNm for forces and torques, respectively.Force and
torque data were sampled at 1kHz and filtered at 10Hz using
a3rd orderButterworth filter.
Following aprocedure used to measure geckosetal array
adhesion forces [3], synthetic patches were moved along a
controlled trajectory in the normal and tangential directions
while measuring resulting forces. Specimens were brought
into contact with the glass substrate and preloaded to a
specified depthin the normal axis. The approach angle for the
anisotropic patches was 45,moving with the stalk angle (i.e.
loading the stalks in the preferred direction for adhesion), and
for the isotropic patch was 90,along the normal direction.
The patches were then pulled away from the glass substrate at
departure angles between 15(mostly parallel to the surface,
with the angle of the anisotropic stalks) and 165(mostly
parallel to the surface, against the angle of the anisotropic
stalks). Velocity was maintained at 1mm/s,which provided
afavorable tradeoffbetween avoiding dynamic forces and
minimizing viscoelastic effects.
Fig. 8illustrates the performance of the stalks as afunction
of pulloffangle. The anisotropic patches produce maximum
adhesion when loaded in the positivetangential direction, as
arobot would load them when clinging to avertical wall.
At angles less than 30,the maximum adhesion force is
approximately 2.3mN/stalk (1.2N for the entire patch), and
the corresponding value of µwas approximately 4.5. Pulling
offin the normal direction generates adhesion of about 2/3
the peak value, and whenpulling offagainst the angle of
the stalks the adhesion drops to less than 10% the peak
value. The work required to load an unload and adhesive
material (Work of Adhesion) has also been used as ameasure
of adhesion performance [6]. At apreload of 700µm,the
maximum work loop is approximately 5.2J/m2at a15
pulloffangle and the minimum work loop is 0.3J/m2at
-4-2 0 2 4
Tangential Force (mN/stalk)
Normal Force (mN/stalk)
100µm preload depth
150µm preload depth
300µm preload depth
-2 0 2 4 6
Tangential Force (mN/stalk)
500µm preload depth
600µm preload depth
700µm preload depth
Fig. 9. Experimental limit curves for isotropic and anisotropic patches at different preload depths. Data points correspond to maximum forces at pulloff.
Three series havebeen plotted to showthe dependence of limit curves on the preload.
a120pulloffangle. For the isotropic patch, maximum
adhesion is obtained when pulling offin the purely normal
direction, dropping to zero for pulloffangles slightly over
45with respect to the normal. The isotropic patch has a
maximum adhesiveforce nearly as high as the anisotropic
patches, but requires ahigher preload force, resulting in aµ
of approximately 0.5.
The anisotropic patches were also tested on machined
granite todetermine howsurface roughness affects adhesion.
The surface roughness (Ra)of glass is typically less than
10nm and the surface roughness of the graniteis about
10µm.At apreload depth of 700µm,maximum adhesion
force on polished graniteis 1.0mN/stalk (0.5N for the
entire patch) resulting in an approximately 60% decrease in
adhesion force compared to glass.
Fig. 9summarizes the results for the maximum tangential
and normal forces of the different patches over arange
of preload depths and pulloffangles. The results can be
compared directly with the models in Fig. 2. As expected, the
isotropic specimen shows abehavior similar to that predicted
by the JKR model: The limit curveis symmetric about the
vertical axis. Maximum adhesion is obtained when pulling
in the purely normal direction. Under positivenormal forces
Coulomb friction is observed.
The anisotropic patches behavesimilarly to geckosetae.
Fitting aline to the data for positivevalues of tangential
force results in an α35(compared to approximately
30for the gecko[3]). When loaded against their preferred
direction (FT<0)theyexhibit amoderate coefficient of
friction; between these two modes, the data intersects the
origin. Thus, likethe geckosetae, the synthetic patches can
easily be detached by controlling internal forces toreduce
the tangential force at the contact. However,unlikethe gecko
setae, the synthetic stalks start to lose adhesion at high levels
of tangential force, at which point the contact faces of the
stalks start to slip.
Fig. 9also shows that forces for isotropic and anisotropic
patches scale with increasing preload. For the isotropic
patches, maximum adhesion is obtained when the specimen
00.5 11.5 22.5 33.5 4
Typical Isotropic Force Profile
Normal Force (N)
00.5 11.5 22.5 33.5 4
Typical Anisotropic Force Profile
Time (s)
Normal Force (N)
Fig. 10. Comparison of normal force profiles of anisotropic and isotropic
patches on aclimbing robot. Point Aon the curves refers to the preloading
phase of the cycle. Point Bhighlights when the foot is in the adhesive
regime during astroke. Points Cand Dare when the foot is unloaded and
detached, causing large normal forces in the case of the isotropic patch.
is preloaded to 300µmafter initial contact, resulting in
anormal preload of 14.3mN/stalk.For the anisotropic
patches, a700µmpreload depth provided maximum adhe-
sion, which corresponds to apreload of 0.5mN/stalk.
Larger preloads resulted in no further significant increase
in adhesion; smaller preloadsproduced less adhesion.
Given the foregoing results, anisotropic and isotropic
specimens can be expected to produce rather different effects
when used on arobot. Fig. 10 showstypical force plots for
anisotropic and isotropic toepatches on the Stickybot robot.
The data for three successive cycles are plotted to show
overall variability.In each case, the robot cycled asingle
legthrough an attach/load/detach cycle on the same 6-axis
force sensor in the previous tests nowmounted into avertical
wall. The other three limbs remained attached to the wall
throughout the experiment. In this test, the isotropic patches
consisted of vertical cylinders with athin upper membrane
bridging the gaps between the cylinders, which increased
the contact area. In each case, legtrajectories were tuned
empirically to provide best results for either the isotropic or
anisotropic patches.
As the plots show,the isotropic patches required alarger
normal force (A) to produce comparable amounts of com-
bined tangential force and adhesion for climbing (B). The
unloading step for the anisotropic patches (C, D) is accom-
plished rapidly and results in negligible detachment force as
the legis removed. In contrast, the isotropic patch requires a
longer peeling phase (C) and produces alarge pulloffforce
(D) as the legis withdrawn. This large detachment force was
the main limitation of the isotropic patches, producing large
disturbances that frequently caused the other feet to slip.
This paper describes the design and manufacture of novel
adhesives and presents experimental evidence that empha-
sizes theimportance of controllable, directional adhesion for
aclimbing robot. Amodel of geckoadhesion is presented
and compared to acommonly used isotropic model from the
literature, the JKR model. It is shown that the anisotropic
nature ofthe frictional-adhesion model, combined with the
fact that at zero tangential force there is zero adhesive
force, allows arobot to smoothly load and unload afoot.
Current work entails scaling down the size of the anisotropic
stalks in order to utilize harder materialsand climb rougher
surfaces. This will allowfor feet that are easier to clean,
yet still conform and adhere well to surfaces. Future work
includes using analytical or numerical methods to understand
howthe patch geometry will affect adhesion performance
on different surfaces and extending our understanding of
anisotropic adhesion to 3D. This may better predict and
explain the behavior of geckos and guide the design and
control of climbing robots.
Wethank Kellar Autumn and hisstudents for discussions
on anisotropic adhesion and testing procedure. This work
was supported through the DARPABioDynotics Program,
the Intelligence Community Postdoctoral FellowProgram,
and the Stanford-NIH Biotechnology Training Grant.
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... 22 In addition to reversibility, controllability of an engineered adhesive is critical for robotic applications. 23,24 Control can be achieved by including an actuator to engage with and detach from a surface or by morphologically programming the struc-ture of the adhesive. Active actuation requires an energetic input to achieve attachment and detachment, thereby adding complexity to a design that could potentially reduce the scalability of a design to smaller sizes or larger number of adhesion points. ...
... 33 The morphological computation exhibited by these microstructures therefore creates controllable adhesion without necessitating additional methods of actuation to attach to and detach from a surface. 23 Directional microstructured adhesives which enable novel robotic locomotion are limited by the environment in which they are applied. Dry adhesives are most effective for adhering in dry environments and are significantly less effective in a wet environment. ...
Reversible adhesion provides robotic systems with unique capabilities, including wall climbing and walking underwater, and yet the control of adhesion continues to pose a challenge. Directional adhesives have begun to address this limitation by providing adhesion when loaded in one direction and releasing easily when loaded in the opposite direction. However, previous work has focused on directional adhesives for dry environments. In this work, we sought to address this need for directional adhesives for use in a wet environment by tuning the morphology of suction discs to achieve anisotropic adhesion. We developed a suction disc that exhibited significant directional preference in attachment and detachment without requiring active control. The suction discs exhibited morphological computation-that is, they were programmed based on their geometry and material properties to detach under specific angles of loading. We investigated two design parameters-disc symmetry and slits within the disc margin-as mechanisms to yield anisotropic adhesion, and through experimental characterizations, we determined that an asymmetric suction disc most consistently provided directional adhesion. We performed a parametric sweep of material stiffness to optimize for directional adhesion and found that the material composition of the suction disc demonstrated the ability to override the effect of body asymmetry on achieving anisotropic adhesion. We modeled the stress distributions within the different suction disc symmetries using finite element analysis, yielding insights into the differences in contact pressures between the variants. We experimentally demonstrated the utility of the suction discs in a simulated walking gait using linear actuators as one potential application of the directional suction disc.
... In addition, the microfibers with high length diameter ratio are prone to breakage during demolding. In the same year, the team led by Mark Cutkosky used the machining method with multi-process to create an anisotropic microstructure called directional polymer stalk, which has the inclined feature at both the base and the end face (as shown in Fig. 4b) [62,66]. The corresponding adhesive tests showed that the microstructure exhibits the characteristic of friction adhesion similar to that of gecko's foot, that is, the corresponding normal adhesion can be adjusted by the tangential friction. ...
... At the same time, the double-sided adhesive arrays were further obtained by antisymmetric curing and bonding of the two single-layer adhesive arrays. Compared with the larger scale microstructures, the adhesive capacity of the microfibers with the few microns in size obtained in Fig. 4 The gecko-like dry adhesive surface based on the mechanism of shear induction: a cylindrical polyurethane microfibers with the tilt angle of 72° [65]; b anisotropic microstructures called directional polymer stalk [66]; c inclined fiber with a spatula-shaped tip feature [70]; d inclined fiber structure with the inclined mushroomshaped tip [72]; e angled nano-hairs with a bulged flat top [64]; f double-inclined microfiber structure [73]; g inclined fiber structures with square caps [74]; h vertical microstructure array with equilateral triangle tip [75]; i vertical asymmetrical microstructure with a spatula-shaped tip feature [76]; j micro-pillar with large overhanging cap [77]; k vertical wedge-shaped microstructure [26]; l inclined wedgeshaped microstructure [79]; m inclined semi-cylindrical microfiber structure [83]; n double-layer inclined microstructure with tri-prism base [85] this study is indeed improved due to the smaller size. In addition, the research results also confirmed that both the monolayer adhesive arrays and the antisymmetric bilateral adhesive arrays obtained by the study exhibited obvious anisotropic adhesive characteristic and controllable adhesive characteristic, as well as their adhesive state can be controlled by switching the gripping direction. ...
Full-text available
Gecko has the ability to climb flexibly on various natural surfaces because of its fine layered adhesion system of foot, which has motivated researchers to carry out a lot of researches on it. Significant progresses have been made in the gecko-like dry adhesive surfaces in the past 2 decades, such as the mechanical measurement of adhesive characteristics, the theoretical modeling of adhesive mechanism and the production of synthetic dry adhesive surfaces. Relevant application researches have been carried out as well. This paper focuses on the investigations made in recent years on the gecko-like dry adhesive surfaces, so as to lay the foundation for further research breakthroughs. First, the adhesion system of gecko’s foot and its excellent adhesive characteristics are reviewed, and the adhesive models describing the gecko adhesion are summarily reviewed according to the different contact modes. Then, some gecko-like dry adhesive surfaces with outstanding adhesive characteristics are presented. Next, some application researches based on the gecko-like dry adhesive surfaces are introduced. Finally, the full text is summarized and the problems to be solved on the gecko-like dry adhesive surfaces are prospected.
... It has a weight of about 50Kg and can carry a maximum load of 10Kg for inspection tasks of large concrete buildings. The NDT wallclimbing robot developed by South Bank University in London, UK, is used for inspection of weld seams [11,12]. The main structure is two-stage articulation, which has a fast moving speed and good obstacle-obstacle ability. ...
Full-text available
Aiming at the problem of low wall adsorption capacity of the wall-climbing robot, this paper designs a negative-pressure climbing wall tracing robot; the structure of the robot is designed, and a double-wheel differential control driving mode is determined; The key technical parameters are analyzed theoretically, and the calculation method of the minimum pressure difference and the minimum distance of the chassis from the ground is obtained. The numerical simulation of the negative pressure generator based on Fluent verified the rationality of the structure of the negative pressure generator. The distribution characteristics of the pressure in the negative pressure generator are displayed and analyzed. Finally, the physical model of the robot is built and the feasibility of the design is verified.
... The advantage of electrostatic adhesion is that it generates an adhesive force on a wide variety of surfaces ranging from glass and steel to rougher surfaces such as wood and concrete. This is in contrast to other adhesion methods such as micro-spines, suction, electromagnetic, and others techniques that can be very surface specific [6][7][8]. The disadvantages of electrostatic adhesives are that the adhesion level is relatively weak and is highly dependent on the separation gap between the adhesive and target substrate. ...
Conference Paper
Full-text available
USC’s Space Engineering Research Center (SERC) and the Jet Propulsion Laboratory (JPL) have created a unique “octopus” tentacle end effector for robotic systems that uses formable electro-adhesion (EA) and Gecko adhesion capture cloth material. REACCH, or Reactive Electro-Adhesive Capture ClotH, supports soft capture of objects of any size, shape, surface finish or material on-orbit. To-date SERC/JPL have developed initial tentacles and backing spines that can control the EA/G material. Prototypes with two tentacles have been demonstrated and tested on a 3-DOF air bearing device in 1g in one plane of grip. The results show promise to further develop this new type of grappling mechanism able to make first soft contact with an object, with a technology that merges compliance and control elements for future on-orbit servicing and assembly missions. This paper presents the initial design and test results on this type of system.
... For example, in soft grippers, adhesion has been explored for preventing or controlling slip and the handling of complex objects with large curvatures or requiring new grasp configurations [204]. Climbing applications also require fast and selective adhesion on surfaces for traction (adhesion 'on') and movement (adhesion 'off') of autonomous devices [205][206][207]. Many physical mechanisms can lead to adhesion: mechanical interlocks at different scales (e.g. ...
... Another known method is dry adhesion which does not need external energy to stick onto a surface, because of Van der Waals force. This method cannot be applied when using rough surface or when the surface is covered in dust and other pollutants [11][12][13]. ...
In this paper, extensive testing on an electrostatic wall-climbing robot is carried out to examine the electrostatic adhesion force when using the Free Flapping Foils (FFF) method. This method is successfully applied to an electrostatic robot to enable it to stick more firmly, by about three times, on walls with different materials, including wood, plaster, granite, glass and climb wall with a continuous movement. The innovation in this paper is that successful tests were carried out, when using thin flexible electrodes made from aluminium foil and free movement of those electrodes. The FFF electrodes are flexible enough to get the shape and roughness of the wall and increases the contact surface. Thus, they increase the adhesion force and would also improve performance of the robot in rough surfaces. The experimental parameters including charging voltage, charging time, number of electrodes, area of electrodes, wall material (permittivity) and surface finishing of wall that affect the electrostatic carrying force were investigated. The robot could cross obstacles with different heights. The final robot was 1.27 Kg and able to convey 14 N extra load when sticking and climbing vertical walls.
The adhesive performance of biomimetic controllable adhesive based on wedge-shaped microstructures is affected by some relevant control parameters in the process of loading and unloading. An appropriate selection of these control parameters is of great significance for its effective application. However, few researches have concentrative and comprehensive explored these control parameters. In order to make up for the shortcoming, this study systematically explored the macroscopic adhesive performance of the self-developed wedge-shaped microstructures under different loading and unloading control parameters. The results show that preloading depth and tangential dragging distance have a positive effect on the adhesive performance, while preloading angle and peeling angle have a negative effect on the adhesive performance. Specifically, a low preloading angle can weaken the normal preloading force under same preloading depth, thereby improving the preloading benefit; the application of tangential dragging distance can also induce the normal preloading force generated in the preloading stage to change to the adhesion, so as to stimulate more adhesion. Based on the interactive analysis of these control parameters, it can be sure that applying a moderate normal preloading force and a larger tangential dragging distance to the wedge-shaped microstructures at low preloading angle not only can make the wedge-shaped microstructures possess better adhesive capacity, but also can obtain a good preloading benefit. In addition, the promotion effect of a low peeling angle on the adhesive performance also implies that a higher peeling angle should be used to realize the easy detachment of the adhesive interface. The first concentrative and comprehensive investigation of the relevant control parameters of wedge-shaped microstructures lays the foundation for designing climbing robot or adhesive gripper based on the wedge-shaped microstructures, and also provides guidance for formulating the corresponding control strategies.
The final goal of this work is the development of functional rubber sheets with micro rubber structures such as friction free, adhesion, and impact adsorption rubbers, etc. We report a micro rubber structure that can successfully perform flexible passive walking with 3 V-shaped units consisting of 4 legs to achieve very low friction. We show how to miniaturize and integrate this structure to produce, by means of a micro rubber molding process using the stereo lithography method, a rubber sheet with 64 legs. The prototype is designed, fabricated, and tested. Under certain conditions, the 64-legged rubber sheet successfully realizes flexible passive walking down an incline.
Electroadhesion device allows one to pick up almost all the objects regardless of their shape or types of materials by means of the electrostatic Maxwell force, which is developed by the dielectric induced polarization on the subject surface. In this study, we propose the modelling methodology and its experimental verification that could maximize the lifting shear force of the electroadhesive device to reach well over the human-finger grip force, say, ca. 8.9 kPa, which has not been achieved yet in this device system. In this study, we maximized the lifting force up to 33.05 kPa for paper objects by scaling down the electrode pitches in the scale of micrometers while avoiding the voltage breakdown using the boundary-edge-length modeling methodology.1 The developed model equation expressed adhesion lifting force as a function of the boundary edge length, applied voltage, and impedance, demonstrating that the model equation agreed well with the experimental output of our device and allowed the lifting force well over the human-finger grip. The in-situ charge transfer resistance measurement value of the impedance analysis (RCT), indicating the amount of polarization, was decreased in the order of paper and glass, and it was clearly related to the enhanced lifting force of two types of object (23.9 and 50.0 kPa, respectively). Hence, the impedance analysis could quantify the magnitude of polarizations and amount of induced charges of objects while in contact with the device.
Full-text available
This paper discusses the influence of surface energy on the contact between elastic solids. Equations are derived for its effect upon the contact size and the force of adhesion between two lightly loaded spherical solid surfaces. The theory is supported by experiments carried out on the contact of rubber and gelatine spheres.
Conference Paper
Full-text available
The RiSE robot is a biologically inspired, six legged climbing robot, designed for general mobility in scansorial (vertical walls, horizontal ledges, ground level) environments. It exhibits ground reaction forces that are similar to animal climbers and does not rely on suction, magnets or other surface-dependent specializations to achieve adhesion and shear force. We describe RiSE's body and leg design as well as its electromechanical, communications and computational infrastructure. We review design iterations that enable RiSE to climb 90° carpeted, cork covered and (a growing range of) stucco surfaces in the quasi-static regime.
Pressure-sensitive adhesives (PSA) are nonmetallic materials that are used to bond other materials on their surfaces through adhesion and cohesion. These adhesives are used in self-adhesive tapes, labels, sign and marking films and protective films, along with dermal dosage systems for pharmaceutical applications, in biomedical electrodes, the assembly of automotive parts, toys, and electronic circuits and keyboards. The design and formulation for the adhesive and cohesive properties is influenced by the physical state of the formulating components and the PSA technology includes solvent-borne, water-borne, and solvent-free systems. Solvent-borne, water-borne, and solvent-free acrylic PSAs are manufactured by polymerization from a wide range of acrylic, methacrylic and other monomers, with low levels of monomers having pendant functional groups in a refluxing organic solvent in the presence of an initiator, such as organic peroxides or azo compounds.
This paper describes a study of adhesion between elastic solids and in particular the effect of a tangential force upon the size of the contact area. In the first part of the paper, the relation between the stress intensity factor of the normally loaded contact and the overall energy balance approach is discussed. In the second and main part of the paper, an analysis is given for the influence of a tangential force on the adhesive contact. The equation derived to describe its effect on the contact size has been verified by experiments carried out on rubber hemispheres pressed against a glass flat. The experimental results show qualitatively a clear reduction in contact area when a tangential force acts and quantitatively a reasonable agreement with theory within the limits of experimental error.
The connexion between heat-seal and pressure-sensitive adhesives, including delayed-tack heat-seal adhesives, is illustrated in chemical terms.
Vertically aligned multiwalled carbon nanotube (MWCNT) arrays can mimic the hairs on a gecko’s foot and act as a dry adhesive. We demonstrate the van der Waals interactions originated dry adhesion between MWCNT array surfaces and various target surfaces over millimeter-sized contact areas. The adhesive strengths were measured over 10 N/cm2 in the normal direction and about 8 N/cm2 in the shear direction with glass surface. The adhesion strength over repeated cycles is limited by the relatively poor adhesion of MWCNTs to their growth substrate, which was improved significantly by adding molybdenum to the catalyst underlayer. We also measured the interfacial work of adhesion as a fundamental adhesion property at the interface. Our measured values of a few tens of mJ/m2, which falls in the range of typical van der Waals interactions energies, provide a direct proof of the van der Waals dry adhesion mechanism. Furthermore, in contrast to other dry adhesives, we show that MWCNT adhesives are electrically and thermally conducting, which makes them a unique interfacial material.
Most recent data on hairy systems demonstrated their excellent adhesion and high reliability of contact. In contrast to smooth systems, some hairy systems seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs, pins). The size of single points gets smaller and their density gets higher as the body mass increases. Previous authors explained this general trend by applying the JKR theory, according to which splitting up the contact into finer subcontacts increases adhesion. Fundamental importance of contact splitting for adhesion on smooth and rough substrata has been previously explained by a very small effective elastic modulus of the fibre array. This article provides the first experimental evidence of adhesion enhancement by division of contact area. A patterned surface made out of polyvinylsiloxane (PVS) has significantly higher adhesion on a glass surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata. An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts.
This paper discusses three fundamental problems relating to grasping and manipulating objects within an articulated, multifingered hand: determining how hard to squeeze an ob ject in order to ensure a secure grasp, determining the finger- joint motions required to produce a desired motion of the object, and determining the workspace of the hand. Squeezing the object, or the application of internal grasp forces, is reduced to a linear programming problem which considers friction and joint torque limit constraints. The relationship between the finger-joint motions and the motion of the object, for the case of pure rolling between the finger tips and the object, is formulated as a set of differential equa tions. The total workspace for a hand is determinedfor spe cial cases of planar and spatial hands.