Gecko Inspired Micro-Fibrillar Adhesives for Wall Climbing Robots

Gecko Inspired Micro-Fibrillar Adhesives for Wall Climbing Robots on
Micro/Nanoscale Rough Surfaces
Burak Aksak, Michael P. Murphy, and Metin Sitti
Abstract— This paper presents the fabrication, characterization and testing of bio-inspired synthetic dry adhesive fiber
arrays. Fibers were fabricated via micromolding followed
by spatula tip formation via dipping. Arrays of fibers with
diameters between 28 µm and 57 µm and a height of 114 µm
were fabricated with high uniformity on 6.25 cm2 areas with up
to 95% yield. Adaptation to uneven surfaces was observed with
fiber elongations over 6 times the original height of the fiber.
The unstructured sample exhibited 1.6 times as much adhesion
as the fiber array sample for the flat punch indenter. However,
fibrillar samples demonstrated up to 5.3 times as much adhesion
as the unstructured sample for the hemispherical indenter. The
fibrillar adhesive sample was implemented on a wall climbing
robot which was able to carry itself and climb a distance on a
painted wall and wood door.
I. INTRODUCTION
Wall climbing robots have the ability to reach areas and
perform tasks that ground based and flying robots cannot.
A wide variety of climbing robots have been developed in
the past decades. In general, climbing robots use one of
four types of attachment mechanisms; vacuum suction [1]–
[7], magnetic attraction [8], gripping with claws or grasping
mechanisms [9], [10], or adhesives [11]–[15]. For very rough
surfaces such as brick, gripping has been demonstrated as
an effective method [9], and for smooth glass-like surfaces,
adhesive attachment mechanisms have been successfully implemented. However, many surfaces that might be desirable
for scaling do not fall into either of these two categories
(very rough or very smooth), painted structures and walls
for example. To investigate the possibilities for climbing such
structures we look to nature, where this problem has been
solved many times over.
Various insects and lizards, notably the Gecko lizard, have
specialized footpads which enable them to climb on a wide
variety of surfaces of any orientation using dry adhesion. Arrays of micro- or nano-scale hairs create large contact areas
with climbing surfaces. Recently there has been significant
research progress in developing synthetic micro- and nanoscale mimics of these adhesive systems. Researchers are
characterizing biological samples [16]–[18] and fabricating
synthetic adhesive arrays from polymers [19], [20], carbon
nanotubes [21], [22], using methods such as micro/nanomolding [12], [20], [23]–[28], nano-embossing [29], carbon
nanotube growth [21], [22], fiber drawing [30], and lithography [19], [31], [32].
In a previous study we fabricated spatula tipped fibers
and studied their adhesion to hemispherical surfaces [28].
All authors are with the Department of Mechanical Engineering, Carnegie
Mellon University, Pittsburgh, Pennsylvania, [email protected]
In this paper we investigate the rough surface adaptation
properties of these fibers by examining their adhesion against
flat and curved surfaces and comparing the results with flat
unstructured control sample. To demonstrate the advantages
of spatula tipped fibers over the flat unstructured sample
for rough surface adhesion, we implemented these adhesives as footpads in a wall-climbing robot which climbs
on non-smooth surfaces. In this paper, Section II describes
the advantages of fibrillar adhesives. Fabrication processes,
experimental apparatus and procedures, and samples are
described in Section III. Results are presented and discussed
in Section IV. Initial tests and application scenarios are
described in Section V. Finally, conclusions and future
directions are reported in Section VI.
II. F IBRILLAR A DHESIVES
One advantage of fibrillar adhesives over flat unstructured
adhesives is that each fiber deforms independently, which
allows them to access deeper recessions to make contact.
Even with the reduced total area due to the spaces between
the fibers, the actual contact area can be greater than that
of a flat adhesive in contact with a rough surface (Fig. 1).
When a flat adhesive contacts a rough surface, contact is only
made at the highest asperities, and deformation of the bulk
layer is relatively small. Because of their structures, fibrillar
adhesives have a much lower effective Young’s modulus,
and can deflect more while conforming to surface roughness.
This allows larger surface roughness asperities to be tolerated
as well as some forms of contamination. Details of roughness
adaptation can be found in [28], [31], [33]. Although the
contact area at each tip can be small, the summation of the
contact areas of all of the fibers in contact can be quite
significant, particularly if the fibers can stretch or deflect
and remain in contact for large extensions.
Fig. 1. The contact area of a flat polymer against a rough surface (a) can
be less than the contact area of a fibrillar adhesive against the same surface
(b).
Another advantage of fibrillar surfaces is their ability to
enhance adhesion by contact splitting [34]. If contact is
split into many finer independent contacts, adhesive strength
increases due to load sharing. However, adhesive force is
directly proportional to both adhesive strength and contact
area. To exploit the advantage from fibrillar adhesives, the
enhancement from contact splitting must compensate for the
reduction in contact area due to the area missing between
the fiber tips.
III. M ATERIALS AND M ETHODS
A. Sample Preparation
A two step fabrication method is used to form the fiber arrays with spatula tips. Initially, vertical or angled cylindrical
fibers were created using a fabrication technique detailed in
previous work [31]. In the second fabrication step, detailed
in previous work [28], a dipping technique was used to form
spatula tips on the existing cylindrical fibers. The techniques
described in [28], [31] were modified for better uniformity
and larger areas by carefully controlling the backing layer
thickness (to approximately 1.1mm). Using this fabrication
method, yield as high as 95% is achievable for fiber patches
as large as 2.5 cm by 2.5 cm. The dimensions for the
resulting fibers used in the following experiments are 57 µm
stem diameter, 114 µm tip diameter, 113 µm length, and 13◦
angle with respect to vertical. The center-to-center spacing
between the square packed fibers was 160 µm. In addition to
fiber array samples, a flat unstructured sample was included
for comparison.
Fig. 2. Scanning Electron Microscope images of 57 µm diameter angled
fiber array sample illustrating a) High uniformity, and b) spatula tips and a
13◦ angle with respect to vertical.
B. Apparatus
A custom adhesion characterization system was used
to perform adhesion measurements. The system was built
on an inverted optical microscope (Eclipse TE200; Nikon,
Melville, NY, USA) to enable the observation of contact
between the indenter and the sample. The indenter was
connected to a high resolution load cell (GSO-25; Transducer
Techniques Inc. Temecula, CA, USA) which was attached
to a high precision stage (MFA-CC; Newport, Irvine, CA,
USA). The stage was placed on a two-axis goniometer
(GON40-U; Newport, Irvine, CA, USA) to facilitate alignment between the indenter and the sample by observing
the contact. The fiber sample arrays were placed on the
microscope stage fibers facing up toward the indenter. The
stage, controlled by the custom real-time software, lowers
the indenter at a fixed speed (1 µm/s) until a pre-specified
preload (Pp ) is reached. Then the indenter is retracted at
the same speed until detachment occurs. The maximum
separation force recorded during retraction is called adhesion
(Pa ). Slow approach and retraction speeds were chosen
in order to minimize viscoelastic effects. By varying the
preload, performance curves were created for each sample
by plotting the adhesion (Pa ) vs. the preload (Pp ).
Two different indenters, a 1 mm diameter silicon disk and
a 6 mm diameter glass hemisphere, were used to estimate the
adhesion enhancement and degradation of fabricated fibers
over unstructured samples. The glass hemisphere indenter
represents a rough surface which has a well-defined height
distribution and a wavelength larger than the size of the
fibers. Therefore the experiments performed with the hemisphere provide insight into the adhesion performance against
a rough surface. On the other hand, the atomically smooth
flat silicon disk is relevant to the adhesion performance
on smooth flat surfaces. In addition to the adhesion measurement system, a profile view system [31] was used to
qualitatively observe the interaction of fiber arrays in contact
with a surface.
Fig. 3. Video still frames of 28 µm diameter fibrillar structures contacting a
6 mm glass hemisphere. a) The two surfaces before contact; b) The surfaces
are moved into contact; c) The fibers stretch to remain in contact as the
surfaces are moved away from each other and shifted laterally; d) Fibers
stretch to more than 6 times their initial length.
Fiber arrays were placed into contact with surfaces and
a)
100
120
80
100
60
80
Adhesion [ kPa ]
Adhesion [ mN ]
observed under the profile view system. The 6 mm diameter
glass sphere used in the adhesion measurements was used as
the indenter in these observations. In the profile view system
it was observed that the fibers stretched up to 6.6 times
their initial length before pull-off occurred (Fig. 3d). After
pull-off, the fibers returned to their initial configurations
(Fig. 3a) and were not visibly damaged by the deformation.
A video of this interaction is available online [35]. The fibers
were also stretched laterally (Fig. 3c) to observe behavior
under combined shear and normal loading conditions that
are common in climbing animals and robots.
60
40
40
20
00
20
1
2
3
4
5
6
7
Cycle #
8
9
0
10
b) 70
90
80
60
Adhesion [ mN ]
60
40
Adhesion [ kPa ]
70
50
50
40
30
Fiber Array
Unstructured Surface
20
30
20
Another surface that was used to examine the interaction
of fibrillar adhesives with uneven surfaces was a silicon
wafer with SU8 photoresist low aspect ratio structures which
made uneven steps on the surface of the wafer. The fibers
were able to attach in the recessed areas and stretch to
remain in contact during retraction as seen in Fig. 4c. These
observations suggest that these fiber arrays exhibit enhanced
adhesion against rough surfaces.
IV. R ESULTS AND D ISCUSSION
The repeatability of adhesion of the fiber array was investigated by performing adhesion tests at 8 mN preload
for ten cycles on the same area (Fig. 5a). After two cycles
the adhesion dropped from approximately 95 mN to 40 mN
for the remainder of the experiments. The subsequent data
in the flat punch adhesion vs. preload figure were obtained
after the repeatability tests were preformed without moving
the sample. The reduction in adhesion after the first two
measurements may be due to damage to the fibers from
excessive stretching or surface degradation. After the dropoff in adhesion following the first two measurements, there
appeared to be no significant degradation.
The adhesion vs. preload results for the silicon flat punch
indenter tests are plotted in Fig. 5b. For both samples the
variation of adhesion with increasing preload was minimal.
The flat sample had a approximately constant adhesion value
of 65 mN whereas the fiber sample exhibited approximately
10
0
0
c) 80
70
5
10
15
20
Preload Force [ mN ]
25
0
Fiber Array
Unstructured Surface
60
Adhesion [ mN ]
Fig. 4. Video still frames of 28 µm diameter fibrillar structures contacting
a surface with varying height profile which is overlayed by a white line for
clarity. a) The two surfaces before contact, b) The surfaces are moved into
full contact where the fiber tips touch the recessed areas, c) The surfaces
are moved away from each other and the fibers stretch to remain in contact.
The fibers in the recessed areas and on the asperities stretch and remain in
contact at the same time, d) the fibers in the recessed areas pull off and
return to their initial configurations.
10
50
40
30
20
10
0
0
2
4
6
8
10
12
Preload Force [ mN ]
14
16
18
Fig. 5. a) Repeatability test data for the 1 mm flat punch adhesion tests
with a constant 8 mN preload. Adhesion vs. preload data for the 1 mm
diameter flat punch (b) and 6 mm diameter hemisphere (c) indenters taken
at a speed of 1 µm/s on arrays of 57 µm diameter angled spatula tip fibers.
40 mN of adhesion. As discussed in Section II, one possible
mechanism for adhesion enhancement is contact splitting.
Since the silicon punch is completely flat, rough surface
adaptation enhancements are not seen and the experimental
results reveal the effect of contact splitting only. In this
case, the increased adhesion due to contact splitting was
not great enough to overcome the reduction in total contact
area. The spatula tips cover only 36% of the total area of
the punch, the rest of the area is the space between the
fibers. However, considering only the actual contact area, the
fibrillar sample exhibits higher adhesion per unit area than
the control sample, indicating that contact splitting does have
a significant effect.
In contrast to the flat punch indenter experimental results,
in the glass hemisphere experiments, the fibrillar adhesive
sample exhibited significantly higher adhesion than the unstructured surface for every preload tested and the difference
grew with increasing preload (Fig. 5c). For low preloads,
the fibrillar sample exhibited 2.5 times (25 mN:10 mN) as
much adhesion as the unstructured sample. At the highest preload tested, the fibrillar sample exhibited 5.3 times
(79 mN:15 mN) as much adhesion as the unstructured control
sample.
This relative change in behavior is attributed to the ability of the fibers to stretch over large distances, allowing
fibers with different stretched lengths to remain in contact
simultaneously as seen in Figures 3d and 4c, in other words,
roughness adaptation. This evidence of roughness adaptation
suggests that the fibrillar adhesives will exhibit superior
performance to the unstructured sample on rough surfaces, as
long as the wavelength of the rough surface is larger than the
tip size. Considering that the fibrillar adhesion values in the
flat punch experiments were fairly close to the unstructured
results, the findings suggest that the fibrillar sample could
be a more versatile attachment pad. For climbing on rougher
surfaces, which is significantly more challenging, the fibrillar
adhesives are far superior to unstructured samples, while
sharing similar performance on smooth surfaces.
V. A PPLICATIONS
perfectly smooth, so the adhesives will not support as much
load on these surfaces. With the fibrillar adhesive footpads
the robot could carry its own weight (78 g) on a vertical
painted interior wall (Fig. 6) and a wooden door (Fig. 7),
both with visible surface roughness. The surface roughness
was high enough to prevent the robot from sticking to the
wall at all with flat unstructured adhesive pads. Using the
fibrillar footpads, the robot was able to climb a few steps up
the vertical wall and door before detaching from the surface
(Fig. 7). A video of this climbing trial is available online [36].
In this trial, with freshly fabricated adhesive pads, the robot
climbed successfully four steps and then lost contact with
the surface. Since the robot climbed four steps and has three
footpads per side, this means that while the robot had fresh
footpads, it climbed successfully, but when the footpads that
had been on the wall previously were carrying the weight
of the robot, they were unable to support the load and the
robot fell. In subsequent attempts, the robot took successively
fewer steps each time before falling, and soon was unable to
adhere to the surfaces at all. These observations suggest that
contamination of the fibrillar footpads severely degraded the
performance. Contamination was observed by eye and optical
microscopy after the tests. Another possible explanation for
the decreasing performance of the pads is the degradation of
the fibrillar pads as seen in Figure 5a.
When the robot was placed on vertical acrylic, smooth
painted steel, and laminated wood surfaces, even with fouled
footpads, the fibrillar adhesives adhered so thoroughly that
the actuators could not peel them away to climb.
Fig. 6.
Photo of Waalbot climbing a painted interior wall using
polyurethane fiber arrays as attachment pads.
To demonstrate the effectiveness of the fibrillar adhesives,
they were implemented as footpads on a tetherless wall
climbing robot called Waalbot [11], named after the van der
Waal’s forces which it dominantly uses to climb. Waalbot
has previously used unstructured elastomer adhesives for
attachment and was able to climb smooth surfaces such as
acrylic and glass.
Extrapolating the results in Figure 5b, two of the robot’s
19 mm diameter footpads should be able to support 2.89 kg
of load in the normal direction if climbing on a perfectly
smooth surface. However, in reality, most surfaces are not
Fig. 7. Video frames of Waalbot climbing vertically up a wood door. (a)
shows the starting position and (b) shows the final step before detachment.
The overlayed lines illustrate the position of the robot after each of the four
steps.
The initial performance of Waalbot with the fibrillar adhesive footpads suggests that these fibrillar adhesives show
promise as repeatable adhesives for non-smooth surfaces, if
the degradation issue can be solved.
VI. C ONCLUSIONS
A fabrication method was developed and used to form
polyurethane adhesive fiber arrays with areas of 6.25 cm2
with spatula tips and excellent uniformity. These samples
were characterized using a silicon flat punch indenter and
a glass hemisphere. The results indicate that the fibrillar
samples exhibit up to 5.3 times as much adhesion as the
unstructured sample for the hemisphere indenter (smooth
curved surface) whereas the unstructured sample yielded
higher adhesion for flat punch experiments (smooth flat
surface). The findings suggest that the fibrillar adhesives
perform better against uneven (low frequency roughness)
surfaces than flat unstructured adhesive pads.
A profile view system was utilized to observe fibers
conforming to uneven surfaces via their ability to stretch
as much as 6 times their initial length and be compressed
without losing tip contact. These fibrillar adhesives were
also observed to adhere to recessed regions and asperities
of uneven surfaces simultaneously. Fibrillar adhesive samples
were implemented as foot pads for a tetherless wall climbing
robot which was able to carry its own weight on a vertical
wall whose surface was slightly rough (painted) and to which
the unstructured samples would not stick. The robot was able
to climb a few steps before detaching from the climbing
surfaces.
We have recently developed several new modifications to
the tips of the fiber arrays including angled spatula tips for
directional adhesion and friction (Fig. 8a) and hierarchical
fibers (Fig. 8b). Future work includes increasing yield for
such processes and modeling and characterizing these adhesive systems, and implementing them into the Waalbot
footpads.
Other future work includes improving the design of Waalbot to suit the performance characteristics of the fibrillar
adhesive footpads rather than the unstructured pads to improve efficiency and overall performance. It is evident that
research into self-cleaning or contamination resistance is also
required.
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