U.S. patent application number 13/845702 was filed with the patent office on 2013-09-26 for dry adhesives and methods of making dry adhesives.
The applicant listed for this patent is Burak Aksak, Michael Murphy, Metin Sitti. Invention is credited to Burak Aksak, Michael Murphy, Metin Sitti.
Application Number | 20130251937 13/845702 |
Document ID | / |
Family ID | 46272867 |
Filed Date | 2013-09-26 |
United States Patent
Application |
20130251937 |
Kind Code |
A1 |
Sitti; Metin ; et
al. |
September 26, 2013 |
DRY ADHESIVES AND METHODS OF MAKING DRY ADHESIVES
Abstract
Dry adhesives and methods of making dry adhesives including a
method of making a dry adhesive including applying a liquid polymer
to the second end of the stem, molding the liquid polymer on the
stem in a mold, wherein the mold includes a recess having a
cross-sectional area that is less than a cross-sectional area of
the second end of the stem, curing the liquid polymer in the mold
to form a tip at the second end of the stem, wherein the tip
includes a second layer stem; corresponding to the recess in the
mold, and removing the tip from the mold after the liquid polymer
cures.
Inventors: |
Sitti; Metin; (Pittsburgh,
PA) ; Murphy; Michael; (Waltham, MA) ; Aksak;
Burak; (Lubbock, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sitti; Metin
Murphy; Michael
Aksak; Burak |
Pittsburgh
Waltham
Lubbock |
PA
MA
TX |
US
US
US |
|
|
Family ID: |
46272867 |
Appl. No.: |
13/845702 |
Filed: |
March 18, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12562643 |
Sep 18, 2009 |
8398909 |
|
|
13845702 |
|
|
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|
61192482 |
Sep 18, 2008 |
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Current U.S.
Class: |
428/92 ;
428/156 |
Current CPC
Class: |
Y10T 428/23929 20150401;
B29K 2105/0097 20130101; B29C 43/18 20130101; B29C 2043/185
20130101; B29K 2075/00 20130101; B29K 2105/20 20130101; C09J 7/20
20180101; C09J 2301/31 20200801; B29C 43/021 20130101; C09J 175/04
20130101; B29C 39/10 20130101; B29C 39/123 20130101; B29C 41/20
20130101; B29C 39/24 20130101; B82Y 30/00 20130101; C09J 2475/00
20130101; B29C 39/42 20130101; B29K 2105/0058 20130101; B05D 1/005
20130101; Y10T 428/23957 20150401; Y10T 428/24479 20150115 |
Class at
Publication: |
428/92 ;
428/156 |
International
Class: |
C09J 7/02 20060101
C09J007/02 |
Goverment Interests
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND
DEVELOPMENT
[0002] This invention was made, in part, with government support
under Grant Number CMMI-0900408 awarded by the National Science
Foundation. The United States government may have certain rights in
this invention.
Claims
1. A dry adhesive, comprising: a backing layer; a stem having first
and second ends on opposite ends of the stem, wherein the first end
of the stem is connected to the backing layer, and wherein the stem
is not perpendicular to the backing layer; and a tip connected to
the second end of the stem, wherein the tip includes an expanded
surface, wherein the expanded surface is planar, wherein the
expanded surface is not parallel to the backing layer, and wherein
the expanded surface has an area that is greater than a
cross-sectional area of the second end of the stem in a plane
parallel to the expanded surface.
2. The dry adhesive of claim 1, including: a plurality of stems,
each stem having first and second ends at opposite ends of the
stem, wherein the first end of each of the stems is connected to
the backing layer, and wherein each of the stems is not
perpendicular with the backing layer; and each of the stems
includes a tip connected to the second end of a corresponding stem,
wherein each tip includes an expanded surface, wherein each
expanded surface is planar, and wherein each expanded surface is
not parallel to the backing layer.
3. The dry adhesive of claim 2, wherein: the stem forms an angle
.theta. relative to a line perpendicular to the backing layer,
wherein .theta. is greater than zero degrees and less than ninety
degrees; and the expanded surface of the tip forms an angle
.beta.-.theta. relative to a plane parallel to the backing layer,
wherein .beta. is greater than .theta. and less than ninety
degrees.
4. A dry adhesive structure comprising: a backing layer; and a
non-perpendicular stem, wherein the stem includes first and second
ends on opposite sides of the stem, wherein the first end of the
stem is connected to the backing layer, wherein the stem forms an
angle .theta. relative to an imaginary line perpendicular to the
backing layer, wherein the angle .theta. is greater than zero and
less than ninety degrees, wherein the stem forms an angle .beta.
relative to the imaginary line perpendicular to the backing layer,
wherein the angle .beta. is greater than the angle .theta. and less
than ninety degrees, wherein the tip of the second end of the stem
comprises a cured polymer having an expanded surface, and wherein
the expanded surface of the tip forms an angle .beta.-.theta.
relative to an imaginary plane parallel to the backing layer.
5. The dry adhesive structure according to claim 4, wherein the
expanded surface of the tip has an area that is greater than a
cross-sectional area of the second end of the stem in a plane
parallel to the expanded surface of the tip.
6. The dry adhesive structure according to claim 4, wherein the
expanded surface of the tip is planar; and the expended surface of
the tip is not parallel to the backing layer after removing the tip
from the tip shaping surface.
7. The dry adhesive structure according to claim 4, further
comprising a plurality of non-perpendicular stems, wherein each
non-perpendicular stem of the plurality of non-perpendicular stems
comprises first and second ends on opposite sides of the stem, and
wherein the first end of the each non-perpendicular stem is
connected to the backing layer.
8. The dry adhesive structure according to claim 7, wherein the
each non-perpendicular stem of the plurality of non-perpendicular
stems further comprises an expanded surface on the second end.
9. The dry adhesive structure according to claim 8, wherein the
each non-perpendicular stem of the plurality of non-perpendicular
stems further comprises: an angle .theta. relative to an imaginary
line perpendicular to the backing layer, wherein the angle .theta.
is greater than zero and less than ninety degrees; and an angle
.beta. relative to the imaginary line perpendicular to the backing
layer, wherein the angle .beta. is greater than the angle .theta.
and less than ninety degrees.
10. The dry adhesive structure according to claim 8, wherein the
expanded surface comprises a cured polymer.
11. The dry adhesive structure according to claim 8, wherein the
expanded surface comprises an angle .beta.-.theta. relative to the
imaginary plane parallel to the backing layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional application of U.S. patent
application Ser. No. 12/562,643 (now U.S. Pat. No. 8,398,909),
which claims the benefit if U.S. Provisional Application Ser. No.
61/192,482, filed Sep. 18, 2008, which are both incorporated by
reference herein. This application is related to U.S. patent
application Ser. No. 12/448,242, filed Jun. 12, 2009, and U.S.
patent application Ser. No. 12/448,243, filed Jun. 12, 2009.
FIELD OF THE INVENTION
[0003] The present invention relates to dry adhesives, and methods
for making dry adhesives including, for example, fibrillar
microfibers and nanofibers.
BACKGROUND OF THE INVENTION
[0004] Fibrillar Adhesives in. Nature
[0005] Nature provides endless inspiration for solutions to
engineering challenges. Particularly at the small (sub-millimeter)
scale, millions of years of evolution has resulted in fascinating
structures with unique, sometimes non-intuitive properties. In the
case of small agile climbing animals, fibrillar foot-pads as a
solution for gripping surfaces has evolved many times. Similar
structures are present in animals of different phyla, including
arthropods (spiders, insects), and chordates (lizards), suggesting
independent evolution. There is also evidence that these structures
evolved independently within different types of lizards (Geckos,
Anoles, and Skinks), with slightly different resulting structures
[D. Irschick, A. Herrel, and B. Vanhooydonck, "Whole-organism
studies of adhesion in pad-bearing lizards: creative evolutionary
solutions to functional problems," Journal of Comparative
Physiology A: Neuroethology, Sensory, Neural, and Behavioral
Physiology, vol. 192, no. 11, pp. 1169-1177, 2006].
[0006] There exist a wide variety of fibrillar adhesives across the
wide variety of animals, which utilize these structures. Some
insects have fibrillar foot pads which secrete oily fluids which
aid in adhesion, while others have completely dry structures.
Adhesive pads which do not utilize secretions are called "dry
adhesives," as they leave no residue on the surfaces to which they
adhere. Dry adhesives exhibit many unique adhesive properties. They
act similar to a pressure sensitive adhesive such as tape, but are
highly repeatable with long lifetimes, do not require cleaning,
and, often in combination with small claws, adhere to surfaces
which are anywhere from atomically smooth silicon to extremely
textured rock. Furthermore, they exhibit directional properties,
adhering in one direction, and easily releasing from the surface
when loathed in another. Adhesion pressures as high as 200 kPa have
been demonstrated for gecko subdigital toepads and single fiber
(seta) measurements exhibited adhesion pressures greater than 500
kPa (50N/cm.sup.2) [K. Autumn, "Biological Adhesives," Springer
Berlin Heidelberg, 2006]. Using advanced fibrillar adhesives,
several gecko species are capable of carrying up to 250% of their
own body weight up a smooth vertical surface. Dry fibrillar
adhesives are also quite power efficient. They can be attached and
detached from surfaces with very low forces by means of special
loading and peeling motions. Once adhered to a surface, they
require no power to maintain contact, and resist detachment for
long periods of time.
[0007] Interestingly, and against intuition, the material that
makes lip these high performance adhesive footpads is not sticky at
all. The fibers are made from a .beta.-keratin, much like bird
claws and feathers. It is the small size-scale and geometrical
structure, which allows this material to act as a powerful and
versatile attachment mechanism.
[0008] Mechanics of Fibrillar Adhesion
[0009] The hairlike structures of gecko footpads have fascinated
scientists for well over a century, with various hypotheses about
the mode of attachment. In 1884, Simmermacher proposed the
hypothesis that gecko lizards might adhere to surfaces using
micro-suction cups [G. Simmermacher, "Untersuchungen ber
haftapparate an tarsalgliedem von insekten," Zeitschr. Wiss. Zool,
vol. 40, pp. 481-556, 1884]. Fifty years later, Dellit carried out
experiments in a vacuum winch demonstrated that suction is not the
dominant attachment mechanism in geckos [W. D. Dellit, "Zur
anatomie and physiologie der geckozehe," Jena. Z. Naturw, vol. 68,
pp. 613-656, 1934]. Similarly, electrostatic adhesion and
micro-interlocking were ruled out. It was not until the advent of
the Scanning Electron Microscope that scientists were able to
investigate the true structure of these microscopic features. What
they observed is a forest of microscale fibers, each branching into
finer and finer hairs, ending in spatula-like tips. It is this
structure that turns the stiff keratin into a capable adhesive.
[0010] Conventional pressure sensitive adhesives such as adhesive
tapes, gels, and soft elastomers function by deforming into the
shape of the contacting surface when pressed into contact.
Materials with very low Young's modulus (stiffness) conform to
surfaces to create large contact areas and do not store enough
elastic energy to induce separation from the surface after the
loading is removed. However, due to their low modulus, these
materials tend to pick up contaminants from the surface, and are
typically not re-usable.
[0011] Stiffer materials do not easily conform to surface
roughness, and if deformed into intimate contact through high
loading, store enough elastic energy to return to their original
shape, peeling away from the surface in the process of relaxation.
Bulk stiff materials generally do not exhibit tackiness or adhesion
due to this self-peeling behavior.
[0012] The structures found in the attachment pads of the animals
described above consist of arrays of thousands or millions of
hair-like fibers, which stanch vertically or at an angle from the
pad surface. Each fiber acts independently and generally has a
specialized tip structure. The hairs in these fibrillar adhesives
conform to the roughness of the climbing surface to increase the
real contact area much like the deformation of soft adhesive tape,
resulting in high adhesion by surface forces [K. Autumn et al.,
"Adhesive force of a single gecko foot-hair," Nature, vol. 405, pp.
681-685, 2000]. This adhesion, called dry adhesion, is argued to
arise from molecular surface forces such as van der Waals forces
[K. Autumn et al., "Evidence for van der waals adhesion in gecko
setae," Proceedings of the National Academy of Sciences USA, vol.
99, pp. 12252-12256, 2002], possibly in combination with capillary
forces [G. Huber et al., "Evidence for capillarity contributions to
gecko adhesion from single spatula nanomechanical measurements,"
Proceedings of the National Academy of Sciences USA, vol. 102, pp.
16293-16296, 2005]. Although the total potential contact area of a
surface broken up into fibers is less than the area of a flat
surface because of the gaps between the fibers, the ability for
each fiber to bend and conform to the surface roughness allows
thousands, millions or billions of fibers no make small individual
contacts, which add up to a large surface area. In comparison, a
flat surface only makes contact with the asperities of a surface,
and since the deformations of bulk material are typically small,
the total contact area is much less than in the fibrillar case. An
illustration of this comparison can be seen in FIG. 1, which
illustrates the contact area of a flat stiff material 2 against a
rough surface 4 (FIG. 1a) can be less than the contact area of a
fibrillar adhesive 6 against the same surface 4 (FIG. 1b) despite
the area lost between the fibers.
[0013] Because of the high aspect ratio (height to diameter) of the
fibers in FIG. 1b, the fibrillar surface's effective modulus is low
despite the material modulus typically being quite high. The
keratinous materials found in geckos' fibers are estimated to have
a Young's modulus of approximately 1-2.5 GPa. However, due to their
hairy structure, the effective modulus is closer to 100 kPa, much
like a soft tacky elastomer.
[0014] Animals with very low mass, such as insects, generally have
a simply micro-fiber structure with specialized tips. In large
lizards such as the Tokay gecko the fibers take on a complicated
branched structure with microscale (4-5 .mu.m) diameter base fibers
which branch down to sub-micron (200 nm) diameter terminal fibers.
At the end of these terminal fibers are specialized tips.
[0015] The most advanced fibrillar dry adhesives are found in the
heaviest animals such as the Tokay and New Caledonia Giant Gecko
gecko which can weigh up to 300 grams. Gecko toes have been shown
to adhere with high interfacial shear strength to smooth surfaces
(88-200 kPa). These animals have adhesive pads with many levels of
compliance including their toes, foot tissue, lamellae, and fibers.
This multi-level hierarchy allows the adhesives pads to conform to
surface roughness with various frequency and wavelength scales. The
fibers are angled with respect to the animals' toes, and the
branched tips are also oriented with respect to the base of the
fiber. The result is that the gecko pad exhibits a high level of
directional dependence, high adhesion while dragging the toe
inwards, and no adhesion in the opposite direction. This
directionality is sometimes referred to as frictional anisotropy
or, more appropriately, directional adhesion.
[0016] Studies of gecko footpads have revealed that due to their
asymmetric angled structure, they are non-adhesive in their resting
state, and a dragging motion is required to induce adhesive
behavior [K. Autumn et al., "Frictional adhesion: a new angle on
gecko attachment," Journal of Experimental Biology, vol. 209, pp.
3569-3579, 2006]. Reversing the direction of this dragging motion
removes the fibers from the surface with very little force.
[0017] Motivation for Fabrication of Dry Adhesives
[0018] The dry fibrillar adhesive structures found in nature
exhibit properties, which may be highly desirable in synthetic
materials. The mechanics which gives rise to the adhesion in these
structures does not rely on liquids or pressure differentials,
therefore fibrillar dry adhesives are uniquely suited for a variety
of uses. Since dry adhesives leave no residue and can grip over
large areas, they could be used as grippers for delicate parts for
transfer and assembly of anything from computer chips in a
clean-room to very large porous carbon-fiber panels for vehicle
construction.
[0019] If manufactured inexpensively, synthetic dry adhesives could
also find uses in daily life as a general adhesive tape for hanging
items, fastening clothing, or as a grip enhancement in athletic
activities such as gloves, shoes, and grips.
[0020] Man-made dry adhesives might be used for temporary
attachment of structures during assembly, or allow astronauts to
grip the smooth outer surfaces of spacecraft during extravehicular
missions.
[0021] Since biological dry adhesives allow animals to climb on
smooth surfaces, synthetic dry adhesives should enable robots to do
the same. Robots with dry adhesive grippers may be used for
inspection and repair of spacecraft hulls, or terrestrial
structures. Since the adhesives require no power to remain
attached, climbing robots could perch for days, weeks, or months
with very little power usage. Also, due to the power efficient
attachment and detachment, robots might move as easily up a wall as
they currently traverse the ground. Similarly, one day, gloves
covered in synthetic dry adhesives might allow humans to easily
scale smooth vertical surfaces.
[0022] There are potential applications for fibrillar adhesives in
the field of medicine as well. Safe, non-destructive temporary
tissue adhesives could assist in surgical procedures. Capsule
endoscopes might use fibrillar adhesives to anchor to intestine
walls without damaging the tissue in order to closely examine or
biopsy an area of interest. Fibrillar adhesives may also be
designed for attachment to skin as an alternative to conventional
adhesive bandages and patches.
PRIOR ART
[0023] Synthetic Fibrillar Adhesives
[0024] In 2000, when Autumn et al. published work measuring the
adhesion of a single gecko seta, suggesting that it is the van der
Waals intermolecular forces dominantly, which allow geckos to
climb, it spawned a field of research into understanding and
modeling the underlying principles of fibrillar adhesion, and
fabricating synthetic mimics. Soon after, Autumn et al.
demonstrated van der Waals forces and a unique geometry are
primarily responsible for the adhesion. Sitti and Fearing created
the first synthetic fibrillar adhesives by silicone rubber
micromolding in the same year [M. Sitti and R. S. Fearing,
"Nanomolding based fabrication of synthetic gecko foothairs," In
Proceedings of the IEEE Nanotechnology Conference, pp. 137-140,
2002].
[0025] In the years since then, there has been a flurry of
research, with more than 50 publications on the topic in 2007
alone. Autumn continues to test biological specimens which provide
insights into the mechanisms of adhesion, self-cleaning [W. R.
Hansen and K. Autumn, "Evidence for self-cleaning in gecko setae,"
Proceedings of the National Academy of Sciences USA, vol. 102, no.
2, pp. 385-389, 2005], and the directional properties of real gecko
setae.
[0026] Huber and Sun demonstrated evidence that suggests that
capillary forces of ambient water layers on surfaces play a
significant role in fibrillar adhesion. Contact mechanics
researchers such as Persson, Crosby, and Hui have investigated the
crack trapping nature of patterned and fibrillar surfaces, which
they have shown to increase the adhesion and toughness of the
interfaces. In addition, Hui studied the bending and buckling
nature of fibrillar surfaces, and the effects of this behavior on
the adhesion of simple pillars. Arzt has investigated the effects
of scale and shape of natural fibrillar adhesives, concluding that
tip shape has less importance at smaller size scales. Several
groups have demonstrated an inverse correlation between animal size
and fibril tip dimension, with the heaviest animals having the
finest fiber structures [E. Arzt, S. Gorb, and R. Spolenak, From
micro to nano contacts in biological attachment devices,"
Proceedings of the National Academy of Sciences USA, vol. 100, no.
19, pp. 10603-10606, 2003].
[0027] The mechanics of fiber to fiber interactions have been
studied and modeled to determine the proper spacing and patterning
for a high density of fibers without clumping. Fibers will clump
together if the adhesion energy between neighboring fibers is
greater than the stored elastic energy of the fibers-bending into
contact. The resulting equations can be used to calculate the
closest spacing without permanent collapse.
[0028] The effects of crack trapping on increasing the toughness
and adhesion of fibrillar surfaces have been studied on the
macro-scale as well as the micro-scale. Several structures have
been tested, and show enhancement over non-fibrillated
structures.
[0029] The roughness adaptation of gecko pads has also been
investigated through testing and modeling. The mechanics of fiber
deformation and buckling reveals that fibrillar structures can
decrease the effective modulus of the surface by several orders of
magnitude, allowing conformation to various rough and curved
surfaces.
[0030] In addition to research to understand and model the
mechanics of adhesion, several research groups have developed
fabrication techniques to create synthetic fibrillar arrays. Since
van der Waal's forces are universal, a wide variety of materials
and techniques may be used to construct the fibers. Initially,
simple vertical fiber arrays were fabricated from various materials
such as polymers. Methods such as electron-beam lithography,
micro/nanomolding, nanodrawing, and self-assembly are employed to
fabricate fibers from polymers, polymer organorods, and
multi-walled carbon nanotubes.
[0031] Generally, arrays of simple pillar structures were not
effective in increasing the adhesion of surfaces. Significant
adhesion enhancement was demonstrated only when the flat tips of
the structures were fabricated to have higher radii for increased
contact area. Gorb et al. fabricated polyvinylsiloxane fibers with
thin plate flat mushroom tips which demonstrated adhesion
enhancement as well as contamination resistance [S. Gorb et al.,
"Biomimetic mushroom-shaped fibrillar adhesive microstructure,"
Journal of The Royal Society Interface, vol. 4, pp. 271-275, 2007].
Similarly, Del Campo et al. developed techniques for forming flat
mushroom tips as well as more complex 3D geometries, including
asymmetric tips, by dipping [A. Del Campo et al., "Patterned
surfaces with pillars with controlled and 3d tip geometry mimicking
bioattachment devices," Advanced Materials, vol. 19, pp. 1973-1977,
2007]. Kim et al. developed fabrication methods to form microscale
fibers with flat mushroom tips by exploiting the champagne glass
effect during Deep Reactive Ion Etching to form negative templates
in silicon on oxide wafers [S. Kim and M. Sitti, "Biologically
inspired polymer microfibers with spatulate tips as repeatable
fibrillar adhesives," Applied Physics Letters, vol. 89, no. 26, pp.
261911, 2006]. In addition, Kim demonstrated the importance of
controlling the thickness of the backing layer in order to reduce
coupling between fibers.
[0032] Glassmaker et al. fabricated polymer fibers topped with a
terminal film which exhibited adhesion enhancement over tipless
pillars and unstructured surfaces [Nicholas J. Glassmaker et al.,
"Biologically inspired crack trapping for enhanced adhesion,"
Proceedings of the National Academy of Sciences, vol. 104, pp.
10786-10791, 2007]. Angled pillars with a terminal film have also
been fabricated with directional properties [H. Yao et al.,
"Adhesion and sliding response of a biologically inspired fibrillar
surface: experimental observations," Journal of The Royal Society
Interface, vol. 5 no. 24, pp. 723-733 2007]. By angling the pillars
beneath the terminal film, the resultant structure exhibits
anisotropic adhesion. In addition to stem angle, the angle of the
surface of the tip with respect to the stem is as crucial in terms
of controlling the anisotropic behavior in adhesion and friction.
Kim et al. [S. Kim et al., "Smooth Vertical Surface Climbing With
Directional Adhesion," IEEE Transactions on Robotics, vol. 24, no.
1, pp. 1-10, 2008] fabricated synthetic sub-millimeter wedges with
the stem and tip surface of each individual wedge oriented at an
angle with respect to the backing layer of the wedge array. These
structures exhibited anisotropic friction much-like the biological
counterparts. While the magnitude of friction was an order of
magnitude less than the biological gecko footpads, adhesion in
normal direction was negligible. Later Asbeck et al. [A. Asbeck et
al., "Climbing rough vertical surfaces with hierarchical
directional adhesion," IEEE International Conference on Robotics
and Automation, Kobe, Japan, 2009] fabricated similarly shaped
wedges that are an order of magnitude smaller which showed similar
adhesion performance to the sub-millimeter wedges. Adhesion
improvement, still low compared to the biological gecko footpad,
occurred when they topped sub-millimeter wedges with a terminal
film comprised of micro-wedges.
[0033] Higher modulus synthetic fibrillar adhesives have been
developed on the sub-micron diameter scale. These fibers, made from
stiffer materials (E.gtoreq.1 GPa) such as polypropylene,
polyimide, and nickel, carbon nanofibers and carbon nanotubes.
Although these stiffer fibers do not adhere well in the normal
direction, and require high preloads to make intimate contact,
shear adhesion pressures of up to 36 N/cm.sup.2, which is higher
than the adhesion strength of the gecko, have been
demonstrated.
[0034] To more closely mimic the structure of the gecko's foot
hairs, work has also been done to fabricate hierarchical fibers
with multi-level structures. Ge et al. bundled carbon nanotubes
into pillars which deform together but have individually exposed
tips. [L. Ge et al., "Carbon nanotube-based synthetic gecko tapes,"
Proceedings of the National Academy of Sciences, vol. 104, no. 26,
pp. 10792-10795, 2007]. Photolithography has been used to fabricate
simple micro-pillars on top of base pillars [A. Del Campo and E.
Arzt, "Design parameters and current fabrication approaches for
developing bioinspired dry adhesives," Macromolecular Bioscience,
vol. 7, no. 2, pp. 118-127, 2007]. On the millimeter scale, Shape
Deposition Manufacturing has been used to fabricate hierarchical
structures in thin polymer plates, which are stacked into arrays
[M. Lanzetta and M. R. Cutkosky, "Shape deposition manufacturing of
biologically inspired hierarchical microstructures," CIRP
Annals--Manufacturing Technology, vol. 57, pp. 231-234, 2008].
Kustandi et al. demonstrated a fabrication technique to use
nanomolding in combination with micromolding to create a
hierarchical structure with superhydrophobic properties.
[0035] In addition to dry adhesives, other work is being conducted
on synthetic fibers with oily coatings, inspired by beetle
adhesion, which exhibit increased adhesion over uncoated
structures.
[0036] The microfiber fabrication methods described above are very
expensive for producing commercial quantities of adhesive
materials. Moreover, they cannot efficiently and controllably
produce angled fibers with specialized tips or hierarchical
structures with specialized tips. Accordingly, there is a need for
improved dry adhesives and improved methods for making dry
adhesives. In particular, there is a need for dry adhesives having
greater adhesive forces and improved durability. In addition, there
is a need for methods of making dry adhesives with lower costs of
production. Those and other advantages of the present invention
will be described in more detail hereinbelow.
BRIEF SUMMARY OF THE INVENTION
[0037] The present invention includes adhesives, methods for making
adhesives, and fibers made according to those methods. Many
embodiments are possible with the present invention. For example,
the present invention provides methods to fabricate fibrillar
structure which have specialized tips that increase adhesion, and
provide directionality to adhesion. Methods are described to
fabricate fibrillar structures with angled tips. Methods are also
provided to fabricate hierarchical fibrillar structures.
[0038] The present invention provides methods for fabrication of
vertical and angled micro-. and nanofibers with adhesive qualities.
The present invention further provides methods for the fabrication
of micro- and nanofibers that have specialized tips or are
hierarchically structured with specialized tips. Polymer micro- and
nanofiber arrays are fabricated through a micro molding process
which duplicates lithographically formed master template structures
with a desired fiber material. This technique enables fabrication
of fiber arrays inexpensively and with high yields, and enables the
fabrication of fibers with controlled angles. In the present
invention, the fiber ends are then dipped in a polymer solution,
prior to further processing which create specialized and
hierarchical tips to the fibers.
[0039] In one embodiment, after the dipping in a polymer solution,
the assembly is then pressed against a surface at a pre-determined
angle to fabricate flattened tips at the ends of the fibers.
[0040] In another embodiment, fibers are fabricated using the
methods of the present invention in different sizes, for example
microfibers and nanofibers, and the smaller fibers are attached to
the tips of the larger fibers by making contact with the liquid
polymer at the end of the larger fibers to create hierarchical
structures.
[0041] In another embodiment, fibers are fabricated according to
the methods herein, dipped in a polymer solution, which is in turn
molded to fowl hierarchical structures with smaller fiber
structures attached to the tips of the larger fibers.
[0042] In another embodiment, the methods described herein are used
to fabricate three-level hierarchical fiber structures.
[0043] In another embodiment, fibers are fabricated using the
methods of the present invention, the fibers are then dipped in a
polymer solution, and the assembly is pressed against an array of
smaller fibers, such as carbon nanotubes, to form hierarchical
structures.
[0044] There are several unique aspects to the fiber design
described in this application. One is an enlarged and oriented
terminal end or tip of the fiber. The enlarged tip increases the
contact area of the fiber thus enhancing the interfacial resistance
to separation between the fiber and the adhering surface. This
shape also allows for more uniform distribution of the applied
stress over the fiber tip surface [A. V. Spuskanyuk et al., "The
effect of shape on the adhesion of fibrillar surfaces," Acta
Biomaterialia, vol. 4, no. 6, pp. 1669-1676, 2008]. Another design
aspect is the incorporation of sharp edges at the perimeter of the
tip. The detachment of a single fiber usually starts from the edge
as an edge crack followed by the propagation of this edge crack
along the entire interface, which results in complete separation.
The crack starts at the edge due to the fact that the edge acts as
a stress concentrator and creates a singular stress state. For
instance, when a soft fiber is in contact with a relatively rigid
smooth surface, the stress at the edge of the fiber (o) is singular
and has the fowl:
.sigma..sub.e,=A.sigma.c.sup.-.alpha. (1)
[0045] In equation (1), .sigma. is the applied stress far from
contact, A is a constant determined by the shape of the fiber, c is
the distance from the edge of the fiber, and .alpha. is the order
of stress singularity determined by the angle at the edge of
contact [D. B. Bogy, "Two edge-bonded elastic wedges of different
materials and wedge angles under surface traction," Journal of
Applied Mechanics, vol. 38, pp. 377-386, 1971]. Note that at the
edge of contact, c=0, stress is infinite. According to (1), it is
possible to reduce the severity of stress singularity and as such
improve detachment resistance by reducing A and .alpha.. Enlarged
tip shape featured in our fiber design allows for lower A values
and reduces the severity of stress at the edge. In addition,
sharper edges at the perimeter of the tip lower the order of stress
singularity a adding another dimension of stress reduction at the
contact edge. According to the work by Bogy, it is also possible to
eliminate the stress singularity via sharper contact edges. While
the oriented fashion of the stem and the base provides us with the
directional properties, enlarged tip with sharper edges improve
performance in both gripping and releasing direction. Furthermore,
we obtain high adhesion in normal direction, which is not
achievable with wedge designs [S. Kim et al., "Smooth Vertical
Surface Climbing With Directional Adhesion," IEEE Transactions on
Robotics, vol. 24, no. 1, pp. 1-10, 2008; A. Asbeck et al.,
"Climbing rough vertical surfaces with hierarchical directional
adhesion," IEEE International Conference on Robotics and
Automation, Kobe, Japan, 2009].
[0046] Many other variations are possible with the present
invention. For example, different materials may be used to make the
fibers and the dry adhesive, and the geometry and structure of the
fibers and the dry adhesive may vary. In addition, different types
of etching and other material removal processes, as well as
different deposition and other fabrication processes may also be
used. These and other teachings, variations, and advantages of the
present invention will become apparent from the following detailed
description of the invention.
BRIEF DESCRIPTION OF THE DRAWING
[0047] Embodiments of the present invention will now be described,
by way of example only, with reference to the accompanying drawings
for the purpose of illustrating the embodiments and not for
purposes of limiting the invention, wherein:
[0048] FIGS. 1a and 1b illustrate the contact area of a flat
material and a fibrillar material against a rough surface;
[0049] FIG. 2 illustrates one embodiment of a dry adhesive
according to the present invention;
[0050] FIGS. 3a-d illustrates the fabrication process for angled
fibers with specialized tips fabricated according to the present
invention;
[0051] FIGS. 4a-d provides SEM images of angled fibers with
specialized tips fabricated according to the present invention;
[0052] FIGS. 5-7 illustrate methods of making dry adhesives
according to the present invention;
[0053] FIGS. 8a-c provides data on gripping and releasing
properties of materials formed according to the present
invention;
[0054] FIGS. 9a-d illustrates fiber tip behavior under various
loading conditions;
[0055] FIG. 10 provides data and associated SEM images for various
fiber and tip geometries fabricated according to the present
invention;
[0056] FIG. 11 provides data on the relationship between fiber tip
area and lateral force;
[0057] FIGS. 12a-e provides data and microphotographs indicating
the shear displacement of materials fabricated according to the
present invention;
[0058] FIGS. 13a-b provides photographs illustrating the
directionality of the shear force capacity of materials fabricated
according to the present invention;
[0059] FIG. 14 illustrates the interaction between hierarchical
fibrillar structures and a rough surface;
[0060] FIGS. 15a-e illustrates the fabrication process for
embedding carbon nanotubes or nanofibers into the tips of base
fibers according to the present invention;
[0061] FIGS. 16a-b provides SEM images of carbon nanofibers
embedded into the tips of base fibers fabricated according to the
present invention;
[0062] FIGS. 17a-d illustrates the fabrication process for molding
hierarchical fibrillar structures according to the present
invention;
[0063] FIGS. 18a-b provides SEM images of molded hierarchical
fibrillar structures fabricated according to the present
invention;
[0064] FIGS. 19-22 illustrate methods of making dry adhesives
according to the present invention;
[0065] FIG. 23 illustrates the fabrication process for molding
macro-micro hierarchical structures according to the present
invention;
[0066] FIG. 24 provides SEM images of molded macro-micro
hierarchical structures according to the present invention;
[0067] FIG. 25 illustrates the fabrication process for making
three-level hierarchical fibers according to the present
invention;
[0068] FIGS. 26a-d provides SEM images of three-level hierarchical
fibers according to the present invention;
[0069] FIGS. 27a-b provides SEM images of two embodiments of double
level hierarchical fibers fabricated according to the present
invention;
[0070] FIG. 28 provides comparison data on adhesion of
unstructured, single level, double level angled, and double level
vertical fibrillar materials;
[0071] FIGS. 29a-d provides comparison data on force-distance of
unstructured, single level, double level angled, and double level
vertical fibrillar materials;
[0072] FIG. 30 provides comparison data on force-distance of
unstructured, single level, and double level vertical fibrillar
materials;
[0073] FIGS. 31-31e provide data and microphotographs of force vs.
time data for double level vertical fibrillar materials fabricated
according to the present invention; and
[0074] FIG. 32 illustrates data indicative of the repeatability of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
1 Introduction
[0075] Gecko toes have been shown to adhere with high interfacial
shear strength to smooth surfaces (88-200 kPa), using microscale
angled fiber structures on their feet. However, even with such
large adhesion pressures, the detachment forces measured during
climbing are nearly nonexistent. The gecko is able to release its
adhesive toes without overcoming the large adhesion forces, which
it relies on to climb and cling to surfaces. These animals are able
to control the amount of adhesion of its footpads during climbing
by controlled motions during detachment.
[0076] Autumn et al. demonstrated that natural gecko setae exhibit
extreme frictional anisotropy, with significant adhesive friction
when dragged along their natural curvature (`gripping` or `with`
direction), and only Coulomb friction in the `releasing` or
`against` direction. When loaded in the `releasing` direction, the
adhesive pads are easily peeled from the surface. We fabricated
angled fibers with un-oriented mushroom tips to mimic this
directional behavior, but no significant anisotropy was observed.
Yao et al. observed directional adhesion and shear interface
strength in angled sub-millimeter diameter PDMS stalks with a
terminal film. Kim et al. have demonstrated sub-millimeter diameter
angled polymer stalk arrays with angled ends, for use in a climbing
robot, which exhibit desirable anisotropic shear forces. However,
both of these larger-scale structures demonstrate significantly
lower interfacial shear and adhesion strength than the gecko or
microscale polymer fibers with mushroom tips.
[0077] Fibrillar structures have also been fabricated to increase
(or decrease) friction. In addition, fiber surfaces have been
created which provide shear adhesion using vertical arrays of
single and multi-walled carbon nanotubes. Unfortunately, these
fibrillar structures require very high preloads in order to provide
interfacial shear strength. Stiff polypropylene sub-micron diameter
fibers have been shown to exhibit shear adhesion without requiring
high preloading. Polyurethane micro-fibrillar structures have
demonstrated interfacial shear strength of over 400 kPa, but due to
these high forces, irreversible damage occurs during
detachment.
[0078] In this invention, we describe fabrication methods and
structures that combine the high interfacial strength of mushroom
tipped micron-scale fibers with the directionality of fiber
structures with both angled stalks and tip endings. In Section 1.2,
the fabrication techniques are detailed for single level
structures. Experimental results are presented in Section 1.3,
including investigation of adhesion anisotropy and adhesion
control. In Section 1.4.2, the fabrication techniques for
multi-level structures are detailed. Experimental results for the
multi-level structures are provided in Section 1.4.6.
[0079] 1.1 The Structure.
[0080] The present invention includes a variety of structures for
dry adhesives. FIG. 2 illustrates one embodiment of a dry adhesive
10 according to the present invention. In that embodiment, the dry
adhesive structure 10 includes a backing layer 20, a plurality of
stems 22, and a tip 28. The term "fiber" will sometimes be used to
refer to the stem 22 and tip 28 together. The term "fiber" will
also sometimes be used to refer to the stem 22.
[0081] The stems 22 are attached to the backing layer 20. The
illustrated embodiment shows a dry adhesive 10 having six stems 22,
although a dry adhesive according to the present invention may have
more or fewer than six stems 22. It is possible that a dry adhesive
10 could have a single stem 22 although in most applications the
dry adhesive 10 will likely have many stems 22.
[0082] The stems 22 have first 24 and second 26 ends on opposite
sides of the stem 22. The first end 24 of the stem 22 is connected
to the backing layer 20, and the second end 26 of the stem 22 is
connected to the tip 28.
[0083] The tip 28 includes an expanded surface 30 which is
generally away from the stem. The expanded surface 30 is larger
than the stem 22. In other words, the expanded surface 30 has an
area that is greater than a cross sectional area of the second end
26 of the stem 22, when the cross-sectional area of the second end
of the stem 22 is measured in a plane parallel to the expanded
surface 30. The expanded surface 30 may be planar or it may be
non-planar. For example, the expanded surface 30 may be concave or
convex or it may have other features such as recesses and
projections. If the expanded area 30 is non-planar, the
cross-sectional area of the stem 22 can be measured parallel to a
plane that most closely approximates the expanded surface 30.
[0084] In the illustrated embodiment the expanded surface 30 is not
parallel to the backing layer 20. This orientation has been found
to provide superior results with dry adhesives 10, although it is
not required that the expanded surface 30 be non-parallel to the
backing layer 20. For example, the present invention may also
include tips 28 with an expanded surface 30 that is parallel to the
backing layer 20.
[0085] The relationship between the backing layer 20, stem 22, and
tip 28 can vary in different embodiments of the present invention.
In the illustrated embodiment, the stem forms an angle .theta.
relative to a line perpendicular to the backing layer 20.
Similarly, the expanded surface 30 forms an angle .beta.-.theta.
relative to a plane parallel to the backing layer 20. The angle 13
can be defined during the fabrications process, as will be
described in more detail hereinbelow.
[0086] Typically, the angles .theta. and .beta. are between zero
and ninety degrees. However, it is possible for those angles to be
greater than ninety degrees. For example, if the backing layer 20
is non-planar, if it contains recesses into which the stem 22 can
be bent, or if it otherwise makes allowances for the stem 20 to
adopt such an orientation, then the angle .theta. may be greater
than ninety degrees. Other variations are also possible, such as a
J-shaped stem 22, which allows .theta. to be greater than ninety
degrees. Similarly, it is also possible for .beta. to be greater
than ninety degrees, such as if the stem 22 takes a different shape
or orientation from that illustrated herein. For example, a
J-shaped stem 22 may allow for the expanded surface 30 to be
rotated more than ninety degrees.
[0087] 1.2 Fabrication of Specialized Tips on Single Level
Structures
[0088] The fabrication process for creating directional adhesives
with specialized tips 28 begins with the fabrication of an array of
cylindrical base fibers. Angled or vertical base fiber arrays are
fabricated through a micromolding process which duplicates
lithographically formed master template structures with a desired
fiber material. This method for fabrication of the fiber arrays is
described in U.S. patent application Ser. No. 12/448,242, by the
same inventors, which is incorporated herein by reference.
[0089] FIGS. 3a-3d illustrates one embodiment of a fabrication
process according to the present invention. In that embodiment, the
fabrication process is used for adding angled mushroom tips 28 to
fibers 22. In FIG. 3a, bare fibers 22 with angle .theta. are
aligned with a layer of liquid polymer 40. The liquid polymer 40
may be carried on a substrate or some other surface 44 for holding
the liquid polymer 40. In FIG. 3b, the fibers are dipped into the
liquid 40 and retracted, retaining some polymer 40 at the tips 28.
In FIG. 3c, the fibers 28 are brought into contact with a
tip-shaping surface 42, such as a substrate, and pressed with a
constant load during curing, bending the fibers 22 to angle .beta..
In FIG. 3d, the sample is peeled from the substrate 42 and the
fibers 22 return to their original angle .theta., resulting in tip
angle (.beta.-.theta.).
[0090] In one embodiment of the present invention, 1 in.sup.2 fiber
arrays are molded from polyurethane with a .apprxeq.1 mm thick
backing layer using a thin spacer to define the thickness and
ensure uniformity. A thin film of liquid ST-1060 polyurethane is
spun onto a polystyrene substrate for 45 seconds at 4,000 rpm. The
fiber array is placed on the film of liquid polyurethane (FIG. 3a).
The liquid polyurethane wets the tips of the fibers, and then the
fiber arrays are separated from the liquid film (FIG. 3b). Next,
the fiber arrays are placed onto a low surface energy substrate and
a weight, preferably (50-200 g) is placed onto the backing layer,
which bends the base fibers to desired angle .beta. (FIG. 3c) and
forms a specialized expanded tip with desired orientation to the
fibers. A variety of orientations of the tip to the fiber can be
fabricated by adjusting the angle at which the fiber arrays are
pressed onto the substrate to achieve desired adhesion and release
characteristics.
[0091] The construct of the fiber array with the specialized tip
material are then cured by methods known to those skilled in the
art.
[0092] The fiber arrays are then peeled from the substrate, and the
fibers return to their initial angle (.theta.), tilting the tips to
an angle of (.beta.-.theta.) as shown in FIG. 3d.
[0093] In one embodiment of the present invention, the microfibers
have diameters of .about.35 .mu.m and lengths of .about.100 .mu.m,
with base fiber angles from 0.degree. to 33.degree. from
horizontal. The fibers are arranged in a square grid pattern with a
center-to-center spacing of 120 .mu.m. The fiber arrays are
fabricated from a polyurethane elastomer with a Young's modulus of
.about.3 MPa, chosen for its high tear strength and high strain
before failure.
[0094] This process has been implemented to form the first
synthetic fibers with angled spatular tips. By varying the fiber
geometry or the load during curing, the tip angle can be fabricated
anywhere from 0 (no tip angle) to 90.degree. (tips parallel to the
fiber stem, see FIG. 4b below).
[0095] FIGS. 4a-4d are scanning electron microscope images of
arrays of 35 .mu.m diameter angled polyurethane microfibers with
angled mushroom tips which were constructed according to one
embodiment of the present invention. Tip orientation can be
controlled to form tips with varying angles: (a) 34.degree.; (b)
90.degree.; (c,d) 23.degree.. Details of the tip can be seen in
(d).
[0096] FIG. 5 illustrates one embodiment of a method of fabricating
dry adhesive structures according to the present invention. The
process may include forming a dry adhesive 10 with a structure
including a backing layer 20 and stem 22 as described above. The
stem 22 may be either perpendicular to the backing layer 20 or
non-perpendicular to the backing layer 20. As described above, the
stem includes first 24 and second 26 ends on opposite sides of the
stem 22, and wherein the first end 24 of the stem 22 is connected
to the backing layer 20 and the second end 26 of the stem 22 is
connected to the tip 28.
[0097] Step 100 includes applying a liquid polymer 40 to the second
end 26 of the stem 22 as described above, for example, with
reference to FIG. 3b. Although the present invention will generally
be described in terms of using a liquid polymer 40, it is possible
that other materials may also be used in place of liquid polymer
40.
[0098] Step 102 includes contacting the liquid polymer on the stem
with a tip shaping surface. See, for example, FIG. 3c above.
[0099] Step 104 includes bending the stem relative to the backing
layer while contacting the liquid polymer on the stem with the tip
shaping surface. See, for example, FIG. 3c above.
[0100] Step 106 includes curing the liquid polymer to form a tip on
the second end of the stem while bending the stem relative to the
backing layer and while contacting the liquid polymer on the stem
with the tip shaping surface. See, for example, FIG. 3c above.
[0101] Step 108 includes removing the tip from the tip shaping
surface after the liquid polymer cures. See, for example, FIG. 3d
above.
[0102] Many variations and modifications are possible with this
method. Some of those variations and modifications will be
described below.
[0103] For example, step 100 may include dipping the second end 26
of the stem 22 in a liquid polymer 40 followed by removing the
second end 26 of the stem 22 from the liquid polymer 40 after the
liquid polymer 40 is applied to the second end 26 of the stem 22.
In other embodiments, the liquid polymer 40 maybe applied by
methods other than dipping, such as by spraying or otherwise
applying the liquid polymer 40. In such cases, the step of removing
the second end 26 from the liquid polymer 40 may not be needed in
some embodiments.
[0104] Step 102, contacting the liquid polymer 40 on the stem 22
with a tip shaping surface 42, may include forming the expanded
surface 30 in the liquid polymer 40 as described above. This may
also include forming a planar surface in the liquid polymer 40
where the liquid polymer 40 contacts the tip shaping surface 42.
The planar surface may be formed, for example, by using a tip
forming surface 42 that is planar. However, other tip forming
surfaces 42 may be used to form other expanded surfaces 30 on the
tip 28. For example, concave or convex tip forming surfaces 42 may
be used, as well as tip forming surfaces 42 having recesses, bumps,
or other features that can be used to shape the expanded surface 30
of the tip 28.
[0105] As described above, the expanded surface 30 of the tip 28
may have an area that is greater than a cross-sectional area of the
second end 26 of the stem 22 in a plane parallel to the expanded
surface 30 of the tip 28.
[0106] Step 104, bending the stem, may include applying a load to
the backing layer. This is one way of being the stem 22, although
other ways may also be used with the present invention.
[0107] Step 104, bending the stem, may also include bending the
stem 22 in a direction away from a perpendicular orientation with
the backing layer 20. In other words, a non-parallel stem 22 may be
bent in such a way as to exaggerate or increase the extent to which
the stem 22 is nonparallel with the backing layer 20.
[0108] Step 104, bending the stein 22 relative to the backing layer
20 while contacting the liquid polymer 40 with the tip shaping
surface 42, may include bending the stem to form an angle .beta.
relative to an imaginary line perpendicular to the backing layer,
wherein .beta. is greater than .theta. and less than ninety
degrees, as described above.
[0109] Step 106, curing the liquid polymer cures to form a tip 28,
may include forming an expanded surface 30 in the tip 28 where the
tip 28 contacts the tip shaping surface 42, and wherein after
removing the tip 28 from the tip shaping surface 42 the expanded
surface 30 of the tip 28 forms an angle .beta.-.theta. relative to
an imaginary plane parallel to the backing layer 20.
[0110] Step 106, curing the liquid polymer 40 to form a tip 28, may
include forming the expanded surface 30 on the tip 28 where the tip
28 contacts the tip shaping surface 42. In other words, the shape
of the expanded surface 30 may be formed during the curing step
106, when the liquid polymer 40 on the stem 22 changes from liquid
form to cured or solid form and retains the general shape at the
time of curing. As a result, curing the liquid polymer 40 forms an
expanded surface 30 indicative of the tip shaping surface 42.
[0111] After step 108, removing the tip 28 from the tip shaping
surface 42, the expanded surface 30 of the tip 28 may be planar and
not parallel to the backing layer 20, as described in more detail
herein.
[0112] After step 108, removing the tip from the tip shaping
surface, the method of the present invention may result in a stem
22 that forms an angle .theta. relative to an imaginary line
perpendicular to the backing layer 20, wherein .theta. is greater
than zero degrees and less than ninety degrees. As described above,
it is also possible for .theta. to be greater than ninety degrees.
Other values for .theta. are also possible with the present
invention. For example, .theta. may be zero degrees if the stems 22
are perpendicular to the backing layer 20.
[0113] Many other variations and modifications are also possible.
For example, the method may also include maintaining the backing
layer 20 parallel to the tip shaping surface 42 during step 104,
when bending the stem 22 relative to the backing layer 104. The
method may also include maintaining the backing layer 20 parallel
to the tip shaping surface 30 during step 106, curing the liquid
polymer 106. In other embodiments, the backing layer 20 may be
maintained non-parallel to the tip shaping surface 42.
[0114] FIG. 6 illustrates another embodiment of the method
according to the present invention. That method includes forming a
dry adhesive 10 with a structure including a backing layer 20 and a
non-perpendicular stem 22, wherein the stem 22 includes first 24
and second 26 ends on opposite sides of the stem 22, and wherein
the first end 24 of the stem 22 is connected to the backing layer
20.
[0115] Step 200 includes applying a liquid polymer 40 to the second
end 26 of the stem 28.
[0116] Step 202 includes contacting the liquid polymer 40 on the
stem 22 with a tip shaping surface 42.
[0117] Step 204 includes forming an expanded, planar surface in the
liquid polymer 40 where the liquid polymer contacts the tip shaping
surface 42.
[0118] Step 206 includes bending the stem 22 relative to the
backing layer 20 while contacting the liquid polymer 40 on the stem
22 with the tip shaping surface 42, wherein bending the stem 22
includes bending the stem 22 in a direction away from a
perpendicular orientation with the backing layer 20.
[0119] Step 208 includes maintaining the backing layer 20 parallel
to the tip shaping surface 42 when bending the stem 22 relative to
the backing layer 22.
[0120] Step 210 includes curing the liquid polymer 40 to form a tip
28 on the second end 26 of the stem 22 while bending the stem 22
relative to the backing layer 20 and while contacting the liquid
polymer 40 on the stem 22 with the tip shaping surface 42, wherein
the expanded surface of the liquid polymer forms an expanded
surface 30 of the tip 28 during curing, wherein the expanded
surface 30 of the tip 28 has an area that is greater than a
cross-sectional area of the second end 26 of the stem 22 in a plane
parallel to the expanded surface 30 of the tin 28.
[0121] Step 212 includes maintaining the backing layer 20 parallel
to the tip shaping surface 42 when curing the liquid polymer
40.
[0122] Step 214 includes removing the tip 28 from the tip shaping
surface 42 after the liquid polymer 40 cures, wherein the expanded
surface 30 of the tip 28 is not parallel to the backing layer 20
after removing the tip 28 from the tip shaping surface 42.
[0123] Many variations and modifications are possible according to
the present invention. For example, after removing the tip 28 from
the tip shaping surface 42, the method of the present invention may
result in a stem 22 that forms an angle .theta. relative to an
imaginary line perpendicular to the backing layer, wherein .theta.
is greater than zero degrees and less than ninety degrees. Other
values for .theta. are also possible with the present
invention.
[0124] Step 206, bending the stem 22 relative to the backing layer
20 while contacting the liquid polymer 40 with the tip shaping
surface 42, may include bending the stem 22 to form an angle .beta.
relative to an imaginary line perpendicular to the backing layer
20, wherein .beta. is greater than .theta. and less than ninety
degrees. Other values for .theta. and .beta. are also possible with
the present invention.
[0125] Step 210, curing the liquid polymer 40 form a tip, may
include forming an expanded surface 30 in the tip 28 where the tip
28 contacts the tip shaping surface 42. Also, after step 214,
removing the tip 28 from the tip shaping surface 42, the expanded
surface 30 of the tip 28 may form an angle (.beta.-.theta.)
relative to an imaginary plane parallel to the backing layer
20.
[0126] Other variations and modifications are also possible with
the present invention.
[0127] 1.3 Friction Anisotropy: Results
[0128] Shear and normal adhesion of our angled fiber arrays with
specialized tips fabricated with the methods of the present
invention as described above were measured in a variety of ways to
investigate interfacial shear strength, directionality, and the
controllability of adhesion. In one measurement method, normal and
shear forces were measured during a fixed shear displacement of 500
.mu.m between a 6 mm diameter glass spherical indenter and the
fiber array. In a second set of experiments with the same indenter,
we measured the effect of varying shear displacements on the
resulting shear and normal forces.
[0129] A custom adhesion characterization system, described
previously [B. Aksak, M. P. Murphy, and M. Sitti, "Adhesion of
biologically inspired vertical and angled polymer microfiber
arrays," Langmuir, vol. 23, no. 6, pp. 3322-3332, 2007], was used
for the adhesion and shear experiments. FIGS. 8a-c illustrate those
experiments. FIG. 8a is an illustration of the displacements in the
experiments. An initial vertical preload (1) is followed by a shear
displacement (2) in either the `gripping` direction or the
`releasing` direction. FIGS. 8b and 8c illustrate shear and normal
forces during shear displacements after a 5 mN preload. Positive
normal force values indicate compression, and negative values
indicate adhesion. Positive shear displacements represent motion in
the `gripping` direction, and negative shear displacements
represent displacement in the `releasing` direction. Fibers with no
tip angle (FIG. 8b) show nearly isotropic shear behavior. For
samples with 28.degree. angled tips (FIG. 8c) the shear forces
during displacements in the `gripping` direction are significantly
higher than those seen in the `releasing` direction, and are
accompanied by adhesive force in the normal direction.
[0130] The experiments will now be described in more detail. In the
fixed displacement experiments, an indenter was pressed into
contact with the fibers to a specified preload value of 5 mN (FIG.
8a). When the preload is complete, approximately 30 fibers were in
contact with the indenter. Next a shear displacement between the
fibers and sphere was applied at a speed of 25 .mu.m/s for 500
.mu.m in either the `gripping` direction or in the opposite
`releasing` direction while the vertical indentation depth was held
constant. Data from these experiments for fibers with no tip angle,
and tip angle samples are shown in FIGS. 8b, c, respectively. All
data in each plot were taken at the same spot, and the close
spacing of the data illustrate the repeatability of the
adhesion.
[0131] The fibers with no tip angle (FIG. 8b) exhibit similar
magnitudes of the shear forces in both directions, although the
behaviors are not identical due to the non-vertical angle of the
base fiber. Fiber arrays with no tip angle were found to have shear
force anisotropy ratios (the ratio of the maximum shear force in
the `gripping` direction to the maximum shear force in the
`releasing` direction) as low as 1.07:1. During these trials, the
fiber tips were observed to adhere to the indenter and stretch when
sheared in either direction, resulting in similar adhesive
characteristics.
[0132] In contrast, the results from the angled tip fiber sample
(FIG. 8c) indicate highly anisotropic behavior. The mean maximum
shear force in the `gripping` direction is 5.6 times greater than
the one observed in the `releasing`-direction (a 5.6:1 shear force
anisotropy ratio). Also, the compressive normal force in the
releasing direction tests indicates that the shear forces observed
were due to classical Coulomb friction. In the `gripping` direction
experiments, the normal force is adhesive, meaning that the mode of
shear force generation cannot be Coulomb friction, which requires a
compressive normal force. Rather, it is the shear component of the
attached fibers under tension. Furthermore, visual observations of
these tests reveal that the fiber tips adhere and stretch when
displaced in the `gripping` direction, whereas the tips flip over
and slide when displaced in the `releasing` direction. This sliding
behavior suggests that the fibers quickly detach and cannot support
normal loading. In other words, they may be easily separated after
being displaced in this direction.
[0133] The measured anisotropic characteristics of the angled tip
samples from FIG. 8c are quite similar to the characteristics of
real gecko setae as measured by Autumn et al. The gecko setae
exhibit a similar shear force anisotropy ratio of .about.4.5:1, and
similar normal force characteristics.
[0134] Although the asymmetric geometry of fiber tips can result in
asymmetric stress distributions at the edges of the contact
interface as described by Bogy, we hypothesize that the observed
anisotropic behavior arises primarily due to the stresses caused by
the moment created when the tip is sheared. This can be understood
by qualitative analysis of the rotation of the tip during shear
loading in each direction (FIGS. 9a-d).
[0135] FIGS. 9a-d illustrate tip behavior under various loading
conditions. Tip angle o is illustrated beneath each side view
illustration with respect to o.sub.o. FIG. 9a illustrates original
unloaded geometry. FIG. 9b illustrates a fiber under preload
compression. FIG. 9c illustrates shearing the fiber in the
`releasing` direction creates large tip rotation, FIG. 9d
illustrates shearing the fiber in the `gripping` direction reduces
the tip rotation, returning to the original o.sub.o before
increasing the tip 28 rotation in the opposite direction.
[0136] Any rotation angle of the tip 28 with respect to its
original orientation causes a peeling moment, which, in combination
with shear and tensile stresses at the interface, can cause an edge
of the tip 28 to detach. When a fiber is compressed into intimate
contact with a surface, the tip angle rotates from its original
angle o.sub.o, (FIG. 9a) to a larger angle o.sub.o, (FIG. 9b). The
change in angle, .DELTA.o, introduces a moment which is relative to
the magnitude of the angle change from its undeformed state. The
peeling moment is increased if the fiber is sheared in the
releasing direction because it increases the already present tip 28
rotation to a larger angle (o.sub.r), increasing .DELTA.o as seen
in FIG. 9c. This increased moment peels the leading edge (A),
eventually detaching and overturning the fiber tip 28, as seen in
previous studies of mushroom shaped fibers. However, when sheared
in the `gripping` direction the fiber tip 28 begins to return to
its original angle, reducing the moment to zero (FIG. 9d). When the
magnitude of the moment is near zero, the normal stress
distribution at the interface is more evenly distributed, reducing
the chances of detachment. After this point, if the shearing in the
`gripping` direction is continued, .DELTA.o changes sign and begins
to increase in magnitude, eventually causing the leading edge (B)
to detach. The initial decrease in moment for shearing in the
`gripping` direction increases the allowable displacement before
detachment occurs, in contrast to the `releasing` direction where
the moment increases immediately. The increased displacement in the
`gripping` direction allows the fibers to stretch and maintain
contact, leading to high interfacial shear strength and
anisotropy.
[0137] Three samples with varying geometry were tested using the
same experimental setup outlined above. The resulting data from
three representative samples are plotted together along with SEM
images of the samples in FIG. 10. In particular, FIG. 10
illustrates anisotropy test data for three samples in which columns
from left to right illustrate: normal and shear forces in the
`releasing` direction and gripping direction, SEM images of samples
in profile view. Fibers with higher tip angle exhibit higher
anisotropy.
[0138] Maximum shear force was not found to have any direct
dependence on base fiber angle (51-78.degree.), tip angle
(0-34.degree.), base fiber diameter (32-45 .mu.m), or base fiber
length (74-105 .mu.m) within the variations between the samples.
However, a strong correlation was seen between maximum shear force
and tip area, as illustrated in FIG. 11.
[0139] FIG. 11 illustrates maximum measured lateral force has
direct dependence on tip area. This relationship confirms that
mushroom tips with large contact areas are beneficial for creating
high shear adhesion, similar to the dependence of normal adhesion
on tip area investigated previously. Also, the degree of anisotropy
was seen to be correlated with tip angle, where larger tip angles
resulted in larger differences between the shear resistances in the
`releasing` and `gripping` directions, which is consistent with the
expectations from the above analysis. These results indicate that
tip area can be used as a design parameter to control the level of
adhesion, while tip angle can be used to design for desired levels
of anisotropy.
[0140] 1.3.1 Adhesion Control
[0141] It has been shown that a shear displacement is required
before biological gecko foot-hairs (setae) can provide adhesion to
a surface. To demonstrate the ability to control adhesion of our
microfiber arrays via shear displacement, a separate set of
experiments was performed.
[0142] FIGS. 12a-e illustrate how adhesion is controllable by
varying the shear displacement of the fibers during loading. In
summary, FIG. 12a is an illustration of the displacements in the
Load-Drag-Pull experiments. FIG. 12b illustrates experimental data
of maximum adhesion values during normal direction separation
following varying shear displacements, as well as maximum shear
forces during shear displacement. FIGS. 12c-12e illustrate maximum
vertical stretching of fibers before detachment, following varying
shear displacements of: (c) 100 .mu.m (releasing direction); (d) 50
.mu.m (gripping direction), (e) 75 .mu.m (gripping direction),
(Scale bar: 100 .mu.m).
[0143] FIG. 12a will now be described in more detail. FIG. 12a
illustrates the displacements of the indenter in the `adhesion
control` experiments. First the indenter was moved into contact
with the fibers to apply a 5 mN preload force (step 1). Next, a
variable shear displacement was applied between the surfaces in
either the `gripping` or `releasing` direction. Finally, the
indenter was retracted away from the fibers. This type of
experiment is sometimes referred to as Load-Drag-Pull (LDP) [K.
Autumn et al., "Frictional adhesion: a new angle on gecko
attachment," Journal of Experimental Biology, vol. 209, pp.
3569-3579, 2006]. The maximum shear force during the shear
displacement phase (step. 2) and the maximum adhesion measured
during the shear displacement or retraction phase, whichever is
higher, (step 3) are plotted for varying shear displacements in
FIG. 12b.
[0144] The results in FIG. 12b confirm that, similar to gecko
setae, adhesion can be controlled by lateral displacement during
initial contact. Experiments with zero shear displacement, or
displacement in the `releasing` direction of any magnitude, result
in negligible adhesion and low shear forces. This is the same
behavior observed by Autumn et al in the natural gecko setae.
However, displacements in the `gripping` direction resulted in
large detachment forces in the normal direction, and generated
significantly higher shear forces during shear displacement as
well. For our samples, the adhesion value is maximized at
approximately 75 .mu.M of shear displacement before retraction.
After 75 .mu.m of shear displacement, the fibers were observed to
begin to contact each other, resulting in premature detachment,
which results in lower adhesion during retraction. Another reason
for the decrease in adhesion for experiments with higher shear
displacements is that many of the fibers begin to detach from the
indenter during the shear displacement due to high extension. When
the fibers detach during the shear displacement phase, they do not
contribute to the adhesion during the retraction phase, and the
resulting adhesion is low. The significant difference in the
adhesion in the `gripping` and `releasing` directions suggests
that, like the gecko's footpads, the angled tip microfiber
adhesives can provide controlled levels of adhesion to a surface
via loading in the `gripping` direction, and can be easily
separated from a surface via shear motion in the `releasing`
direction.
[0145] FIGS. 12(c-e) show profile views of angled mushroom tip
fibers at the instant before final detachment after varying shear
displacements. Any shear displacement in the `releasing` direction
resulted in negligible fiber extension and very low adhesion as the
fibers slid out of contact with the indenter (FIG. 12c). In the
`gripping` direction, the fibers stretched further before detaching
when displaced 75 .mu.m in shear (FIG. 12e) compared to the
detachment after a shear displacement of 50 .mu.m (FIG. 12d), which
is expected from the results in FIG. 12b. These images demonstrate
the significant difference in contact behavior for displacements in
the `gripping` and `releasing` directions. The profile view also
allowed direct observations of the fiber-fiber collisions which
often resulted in immediate detachment. Although close fiber
spacing can increase the number of fibers in contact with a surface
for a given area, it limits the maximum size of the tips (the tips
can merge during fabrication) and prevents long-range independent
motion of the fibers. Increasing fiber spacing, altering the fiber
angle orientation, or arranging the fibers in different patterns
may increase the adhesive performance of the fibers by increasing
the distance that fibers can extend before encountering a
neighboring fiber.
[0146] As a demonstration of the macroscale adhesion of the
directional microfiber array, a small area (1 cm.sup.2) of a sample
with 14.degree. tip angle was attached to a glass-slide 70 which
supported a hanging weight of 1 kg in pure shear in the `gripping`
direction, an interfacial shear strength of .about.100 kPa (FIG.
13a), which is within the range of measured interfacial shear
strength of gecko toes on smooth surfaces (88-200 kPa). When
reversed to the `releasing` direction, the same sample was able to
support only 200 g (.about.20 kPa) as illustrated in FIG. 13b.
However, for both of these experiments, the fiber sample could only
sustain the load for tens of seconds before detaching. The highest
sustained loading over five minutes was 500 g (.about.50 kPa) in
the `gripping` direction. The sample was a directional polyurethane
microfiber array with 14.degree. angled tips adhering to smooth
glass can support.
[0147] 1.3.2 Summary
[0148] We have described embodiments of the present invention in
which fiber array constructs are created by dipping an angled fiber
array into a thin film of liquid polymer and then pressed against a
substrate to form specialized tips with controllable orientation to
the fibers. These constructs exhibit similar shear adhesive
strength to the gecko lizard's feet on smooth surfaces, as
demonstrated with macro-scale support of significant loads (1
kg/cm.sup.2). These adhesives exhibit directional characteristics,
gripping when loaded in one direction, and releasing when loaded in
the opposite shear direction. We have shown that the adhesion can
be controlled by varying the shear displacement before loading in
the normal direction. The angled tips of the fibers create a larger
contact area and are responsible for the observed shear anisotropy.
We have identified tip area as a main design parameter for the
magnitude of the interfacial shear strength, and the tip angle as a
design parameter to control the anisotropy ratio. The fabrication
methods described in this invention can be easily extended to
smaller size scales and stiffer materials to more closely mimic the
gecko's adhesive structures. The high magnitude anisotropic
adhesion of these materials may enable efficient gripping and
releasing of structures. Additional embodiments of the invention
will now be described.
[0149] 1.4 Multi-Level Hierarchical Fibers
[0150] In other embodiments of the present invention, the fiber
arrays fabricated according to the methods described above, are
again placed into a thin film or liquid polymer. In these
embodiments, however, instead of then pressing the wet polymer at
the tip of the fibers onto a flat surface, the wet polymer is
pressed onto either an array of smaller scale fibers, or onto a
mold to create an array of smaller scale fibers on the tips of the
fiber array. These methods result in a hierarchical fiber array
construct, as described in further detail below. These structures
provide improved adhesive characteristics for adherence to uneven
and rough surfaces, and mimic the hierarchical fiber structures
observed in nature.
[0151] The motivation for the creation of hierarchical structures
is to provide greater adhesion to uneven and rough surfaces.
Adaptation to uneven and rough surfaces is a major feature of
biological fibrillar adhesives. Most natural and man-made surfaces
are not perfectly smooth, and traditional adhesives are typically
less effective on rougher surfaces. Fibrillar adhesive materials
with large areas and high uniformity can be fabricated according to
the methods of the present invention described below. We also
describe experimental results, which characterize the adhesive
performance of the hierarchical materials against a smooth flat
punch and a smooth curved surface. The performance results are
compared to a flat control sample. Furthermore, we describe
observations that 21 were made about the interaction of fibrillar
adhesives with uneven surfaces by viewing these interactions from
the side with a microscope.
[0152] 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
deformations of the bulk layer is relatively small. This leads to
an overall low contact area. Because of their structures, fibrillar
adhesives have a much lower effective Young's modulus, and can
deflect more to conform to surface roughness. In addition, the low
effective modulus prevents the material from attempting to return
to its original shape from stored elastic energy while attached to
a surface, effectively peeling itself away from the surface as seen
in unstructured polymers. This allows larger surface roughness
asperities to be tolerated as well as some forms of contamination.
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.
[0153] Another advantage of fibrillar surfaces is their ability to
enhance adhesion by contact splitting. 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 total 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 lost area between the fibers.
[0154] 1.4.1 Hierarchical Structures in Nature
[0155] In nature, the most advanced fibrillar dry adhesives are
found in the heaviest animals such as the tokay gecko which can
weigh up to 300 grams. In comparison to the insects whose bodies
are much lighter and do not require high performance adhesion,
these animals have more complex adhesive pads with many levels of
compliance including their toes, foot tissue, lamellae, and fibers.
Additionally, these fibers branch from a micron-scale diameter to
sub-micron diameter tip fibers. The fiber structure is similar to a
branching tree or a broom. This multi-level hierarchy allows the
adhesives pads to conform to surface roughness with various
frequency and wavelength scales. The toes and tissue conform to
large-scale (mm scale) roughness, and each subsequent level
conforms to roughness at its corresponding size-scale. Finally, the
sub-micron tip fibers can access the smallest surface valleys.
[0156] FIG. 14 illustrates a hierarchical structure that allows
roughness adaptation to small and large wavelength and amplitude of
surface roughness. In particular; a two level hierarchy is
illustrated in FIG. 14 where the large base fibers or stem 22
conform to the low-frequency, high amplitude roughness, while the
tip fibers 60 conform to the high frequency, low amplitude
roughness. Furthermore, the smaller tip fibers 60 have small
endings, which are more likely to lie flat against the adhering
surface due to their size scale. Where a large fiber tip may
encounter roughness underneath the tip, the surface may appear
locally flat at the length scale of the smaller fibers' 60 tips.
The smaller fibers 60 may be formed of the same material as the
large base fibers 22 and may be, for example, another layer of
stems, such as second layer stems. The smaller fibers 60 may be
formed, for example, with a molding process as described herein or
by other processes. The smaller fibers 60 may also be made from a
different materials than the large base fibers 22, such as with
carbon nanofibers of other materials, as described herein. Although
this embodiment of the invention has been illustrated with two
layers or hierarchies of stems, the present invention also includes
dry adhesives with more than two layers of stems.
[0157] This type of multi-level structure is desirable for
synthetic fibrillar adhesives as well. In this section, we disclose
several fabrication techniques for creating hierarchical synthetic
fibers according to the present invention. These methods result in
hierarchies from the millimeter scale to sub-micron scale.
Fabrication results are also demonstrated and described. Finally,
hemispherical indenter tests are used to examine the effect of
hierarchy on adhesion and interface toughness.
[0158] 1.4.2 Fabrication
[0159] The present invention includes several embodiments to
fabricate fibrillar structures with multiple levels of hierarchy.
These methods span the size scales from millimeter scale molding to
nanoscale carbon nanofiber embedding. The following sections detail
the fabrication processes and provide experimental 23 results of
these techniques.
[0160] 1.4.3 Nanoscale Hierarchy
[0161] In order to reach into the smallest recesses of a surface,
the distal fibers of an adhesive pad should have sub-micron
diameters, as seen in the gecko's setae. It is possible to create
synthetic fibrillar surfaces with nanoscale diameter tip fibers by
embedding vertical arrays of carbon nanotubes or carbon nanofibers
into the tips of base fibers.
[0162] In one embodiment of the present invention, the mushroom tip
fabrication process detailed previously is altered to enable the
embedding of smaller scale fibers 60, such as carbon nanofibers or
carbon nanotubes, into the tips 28 of polyurethane fibers.
[0163] FIGS. 15a-e illustrates one embodiment of that process
according to the present invention. In general, the process is for
embedding carbon nanotubes or carbon nanofibers, or other structure
60 into the tips 28 of base fibers to form a hierarchy. In FIG.
15a, fibers or stems 22 are dipped into a liquid polyurethane layer
40. In FIG. 15b, the ends of the fibers are coated with liquid
polyurethane 40. In FIG. 15c, the fiber array is placed into
contact with a vertical array of nanofibers or nanotubes or other
structures 80 which will form the second layer stem 60. In FIG.
15d, the fiber array is peeled from the surface, retaining the
embedded nanofibers 80 as a second layer stems 60. FIG. 15e, is an
illustration of the stacked conical structure of Carbon Nanofibers
that may be used with the present invention. The widening of the
conical structure near the base of the fibers 80 makes them most
likely to fracture at this point.
[0164] The process illustrated in FIGS. 15a-e will now be discussed
in more detail. The process utilizes an array of smaller fibers 80,
such as carbon nanofibers or carbon nanotubes or other structures
on, for example, a carrier wafer or chip 82. In the process, a base
fiber 22 array is dipped into liquid polyurethane 40 (FIG. 15a) and
picks up a layer of the liquid 40 on the tips of each fiber (FIG.
15b). After waiting some time to allow the liquid to partially
cure, increasing it viscosity, the material is then placed onto the
top of the vertical nanofiber 80 array (FIG. 15c). At this point,
the liquid polyurethane 40 is pulled into the nanofiber 80 array by
capillary forces. These forces are extremely strong, due to the
small spacing and large surface area between the fibers 80, so low
viscosity liquid polyurethane 40 would be completely absorbed. With
proper viscosity, the liquid polyurethane layer is partially
absorbed, resulting in a branch-like structure (see FIG. 16b).
After curing, the material construct is mechanically peeled from
the carrier wafer 82, breaking off the nanofibers 80 at their
bases. The final structure is a hierarchical fiber with an
extremely robust embedding of nanoscale diameter fibers 60 at the
tips.
[0165] Vertical arrays of carbon nanofibers 60 were used in one
embodiment of the present invention. Those skilled in the art will
recognize that other small scale or nanofiber arrays could be used.
Carbon nanofibers have sufficient stiffness to prevent lateral
collapse and are able to be closely spaced. Although carbon
nanofibers have high stiffness, they also can be grown to high
aspect ratios, allowing them to be compliant in the vertical
direction. Another advantage of carbon nanofibers for this process
is that the weakest part of the structure is at the base where the
fiber meets the carrier wafer, due to a widening of the cone-shaped
carbon sheet structure near the interface (FIG. 15e). This weakness
ensures that the fibers will break at this point when mechanically
peeled, resulting in a uniform height for all of the fibers.
[0166] Initial results (FIGS. 16a-b) confirm that embedding
nanofibers at the tips of polyurethane base fibers using the above
process is feasible. FIG. 16a illustrates a Scanning Electron
Micrograph of carbon nanofibers embedded into the tips of
polyurethane base fibers to form a hierarchical fiber structure.
FIG. 16b illustrates a detailed view of the branching structure and
uniform height of the carbon nanofibers.
[0167] In another embodiment of the present invention, methods are
provided to fabricate hierarchical structures with specialized tips
on the smaller scale fibers. As we have shown previously as well as
observed in natural fibrillar adhesives, widened tips provide a
significant increase in adhesion. We have developed a tip
fabrication process that allows the tip fiber shape to be
controlled by micro-molding.
[0168] In this process, after the previously detailed dipping of
base fibers 22 in a liquid polymer layer 40, the fibers 22 are
placed onto an etched silicon wafer. This wafer has micron-scale
diameter cylindrical holes with a widened tip formed by Deep
Reactive Ion Etching. These silicon-on-oxide negative templates can
be fabricated according to the methods described in U.S. patent
application Ser. No. 12/448,243, which is incorporated herein by
reference.
[0169] FIGS. 17a-d illustrate one embodiment of the process for
fabricating hierarchical fibrillar adhesives with controlled tip
fiber shape. FIG. 17a illustrates base fibers 22 with mushroom tips
28 that are dipped into a liquid polyurethane layer 40. The liquid
polyurethane 40 may be, for example, on a carrier surface. FIG. 17b
illustrates that some of the liquid polymer 40 is retained by the
tips 28. FIG. 17c illustrates the fiber 22/28 array placed onto an
etched silicon mold 50, where the liquid 40 from the tips 28 is
drawn into the negative features 52. FIG. 17d illustrates that
after the polyurethane 40 has cured, the silicon mold 50 is etched
away with, for example, a dry etching process. The polymer tips 40
may be removed mechanically by peeling from the mold 50, which is
preferred when the mold 50 is made from a compliant material such
as silicone rubber.
[0170] One embodiment of the process illustrated in FIG. 17 will
now be described in more detail. The process begins with an array
of base fibers 22 with flat tips 28, which are dipped (FIG. 17a)
into a thin liquid polyurethane layer 40 and then placed onto the
negative silicon master template 50 (FIG. 17c). Capillary forces
draw the liquid polymer into the cavities 52, which become filled
beneath the base fibers 22/28. The material is then cured according
to methods known to those skilled in the art, and the cured
material becomes second layer stems 60 formed in the mold 50. The
mold 50 is removed using, for example, Xe.sub.F2 dry etching to
expose the second layer stems 60. Since the etching process occurs
over several hours, the base fibers 22/28 must be protected from
the etching gases, as they are damaged by the prolonged exposure.
To prevent this, in one embodiment, the material construct is
encapsulated in protective polymer layer (not shown), such as
polyurethane, which seals the edges and does not allow the etching
gases to reach the base fibers 22/28. When etching is complete, the
final hierarchical structures remain (FIG. 17d).
[0171] Material constructs fabricated with this process can be seen
in FIGS. 18a-b. In particular, FIGS. 18a-b illustrate Scanning
Electron Micrograph of polyurethane hierarchical fibers with
mushroom tips. The base fibers have approximately 50 .mu.m diameter
and the tip fibers have 3 .mu.m diameter stems with 5 .mu.m
diameter tips.
[0172] One advantage of this fabrication method is that there is no
constraint on the scale of the tip fibers or second layer stems 60.
For example, nanoscale tip fibers 60 may be integrated into
microscale base fibers 22 and tips 28 with this technique.
[0173] An overview of the methods of making dry adhesives according
to one embodiment of the present invention will now be provided
starting with FIG. 19.
[0174] FIG. 19 illustrates one embodiment of a method of forming a
dry adhesive 10 with a structure including a backing layer 20 and a
stem 22, wherein the stem 22 includes first 24 and second 26 ends
on opposite sides of the stem 22, and wherein the first end 24 of
the stem 22 is connected to the backing layer 20. The second end 26
of the stem 22 is connected to the tip 28. An example of such as
structure is shown in FIGS. 17a-d.
[0175] Step 300 includes applying a liquid polymer to the second
end 26 of the stem 22.
[0176] Step 302 includes molding the liquid polymer 40 on the stem
22 in a mold 50, wherein the mold 50 includes a recess 52 having a
cross-sectional area that is less than a cross-sectional area of
the second end 26 of the stem 22.
[0177] Step 304 includes curing the liquid polymer 40 in the mold
50 to form a tip 28 at the second end 26 of the stem 22, wherein
the tip 28 includes a second layer stem 60, corresponding to the
recess 52 in the mold 50; and
[0178] Step 306 includes removing the tip 28 from the mold 50 after
the liquid polymer 40 cures.
[0179] Many variations and modifications are possible with the
present invention. Some of those variations and modifications. For
example, the stem 22 may be perpendicular to the backing layer 20,
or the stem 22 may be non-perpendicular to the backing layer 20.
Other examples are-provided below.
[0180] Step 306, removing the tip from the mold, can include
etching the mold from the tip. For example, the tip 28 can be
removed from the mold 50 by etching the mold 50 as opposed to, for
example, pulling the tip 28 out of the mold 50. Other variations
are also possible. If the mold 50 is etching from the tip 28, the
method may also include covering the stem 22 with a protective
polymer layer, such as polyurethane, before etching the mold 50.
This may be done, for example, to protect the stem 22 from the
etching processes.
[0181] The method of the present invention can be used to make many
variations of dry adhesives. In one embodiment, the stem 22 is
microscale and the second layer stem 60 is nanoscale. Other
variations are also possible. For example, the present invention
also includes microscale second layer stems 60 on milliscale stems
22, smaller microscale stems 60 on larger microscale stems 22, and
other variations. The present invention can also be used to make
dry adhesives with more than two levels of stems 22, 60. For
example, the present invention may be used to make dry adhesives
with three levels of stems, four levels of stems, or more.
[0182] Step 308, molding the liquid polymer on the stem, may
include filling the recess 52 in the mold 50 with the liquid
polymer 40 via capillary forces.
[0183] Step 300, applying a liquid polymer 40 to the second end 26
of the stem 22, includes dipping the second end 26 of the stein 22
in the liquid polymer 40 and removing the second end 26 of the stem
22 from the liquid polymer 40 after the liquid polymer 40 is
applied to the second end 26 of the stem 22.
[0184] The present invention may also include bending the stem 22
relative to the backing layer 20 while molding 302 the liquid
polymer 40 on the stem 22 in the mold 50. The present invention may
also include bending the stem 22 relative to the backing layer 20
while curing the liquid polymer 40 in the mold 40. If the stem 22
is bent while the liquid polymer 40 cures, the tip 28 can be made
to take on different shapes, depending on the extent to which the
stem 22 is bent, as described herein. The bending of the stem may
include applying a load to the backing layer 20. Furthermore, the
stem may be bent in a direction away from a perpendicular
orientation with the backing layer, as described herein.
[0185] FIG. 20 illustrates another embodiment of the present
invention in which further steps are performed after those
described with reference to FIG. 19.
[0186] Step 310 includes applying a liquid polymer to the second
layer stem.
[0187] Step 312 includes molding the liquid polymer on the second
layer stem with a second mold, wherein the second mold includes a
recess having a cross-sectional area that is less than a
cross-sectional area of the second layer stem.
[0188] Step 314 includes curing the liquid polymer in the mold to
form a tip on the second layer stem, wherein the tip on the second
layer stem includes a third layer stem, and wherein the third layer
stem corresponds to the recess in the second mold.
[0189] Step 316 includes removing the tip on the second layer stem
from the second mold.
[0190] FIG. 21 illustrates another embodiment of the present
invention. This embodiment of the method will be described with
reference to FIG. 19, although it may also be performed after the
steps of FIG. 20.
[0191] Step 320 includes applying a liquid polymer 40 to the second
layer stem 60.
[0192] Step 322 includes inserting a plurality of fibers 80 into
the liquid polymer 40 on the second layer stem 60, wherein the
plurality of fibers 80 have a cross-sectional area which is less
than a cross-sectional area of the second layer stem 60.
[0193] Step 324 includes curing the liquid polymer 40 on the second
layer stem 60 with the plurality of fibers 80 in the liquid polymer
40.
[0194] Many variations and modifications are possible with this
embodiment of the present invention. For example, the fibers may be
nanotubes, nanowires, nanofibers, or other materials.
[0195] FIG. 22 illustrates another embodiment of the present
invention including a method of forming a dry adhesive 10 with a
structure including a backing layer 20 and a stem 22, wherein the
stem 22 includes first 24 and second 26 ends on opposite sides of
the stem 22, and wherein the first end 24 of the stem 22 is
connected to the backing layer 20 and the second end 26 of the stem
22 is connect to the tip 28.
[0196] Step 330 includes applying a liquid polymer 40 to the second
end 26 of the stem 22.
[0197] Step 332 includes inserting a plurality of fibers 80 into
the liquid polymer 40 on the second end of the stem, wherein the
plurality of fibers have a cross-sectional area which is less than
a cross-sectional area of the second end of the stem.
[0198] Step 334 includes curing the liquid polymer with the
plurality of fibers in the liquid polymer.
[0199] Many variations and modifications are possible with this
embodiment of the present invention. For example, the fibers may be
nanotube, nanofiber, nanowire arrays, or other structures. Also,
the stem 22 may be perpendicular or non-perpendicular to the
backing layer 20.
[0200] Step 332, inserting a plurality of fibers into the liquid
polymer 40, may include inserting a plurality of fibers connected
to a base. Also, after step 334, curing the liquid polymer 40, the
present invention may include separating the plurality of fibers
from the base.
[0201] 1.4.4 Macroscale Hierarchy
[0202] The previously described techniques are intended to add tip
fibers onto molded base fibers to create a multi-layer fibrillar
adhesive. Another method to create a fibrillar structure is to
create compliance in the backing layer at a larger scale than the
base fibers. Even simple slits in an otherwise unstructured
material has been demonstrated to increase the average fracture
energy of flat elastomers by an order of magnitude, due to
inhibited crack propagation. This is seen in the feet of geckos,
where the base fibers are attached to thin plate-like structures
with spaces in between called lamellae. These lamellae increase
macro-scale compliance and prevent crack propagation. For synthetic
adhesives, backing layer patterning can be integrated with one of
the tip fiber methods described above to create a two-level
hierarchy. Like the biological lamellae, these fibers act to arrest
cracks and increase compliance.
[0203] Fabrication of macro-micro scale hierarchical structures is
accomplished using a technique similar to that described above in
FIGS. 17a-d. FIG. 23 illustrates one embodiment of a fabrication
process for macro-micro hierarchical structures
[0204] Fabrication of the base fibers 22 is accomplished by using a
rapid prototyping system (Invision HR, 3D Systems) to print plastic
master templates of the desired structures. It is possible to
create fibers with diameters as small as 250 .mu.m with this
hardware, but the technique is not limited to any particular size
scale. Non-cylindrical geometries are possible using this technique
as well. The master template is molded with silicone rubber (HS II,
Dow Corning) to create a negative mold. After separation from the
master template, the negative mold is used to replicate the base
structures from polymers such as polyurethane. Wide flat mushroom
tips 28 are added to these base fibers 22 in the same way as
described for micro-scale fibers. Instead of using the etched
silicon mold as in the previous Section, a soft silicone elastomer
mold 50 is used to create the tip fibers 60 (FIG. 23f), and a
subsequent dipping step (FIG. 23g-j) to add mushroom tips to these
fibers 60. The final two-level structure is illustrated in FIG.
23k.
[0205] FIG. 24 illustrates a Scanning Electron Micrograph of a two
level polyurethane fiber structure, with 50 .mu.m diameter mushroom
tipped fibers atop curved 400 .mu.m diameter base fibers with 1 mm
diameter mushroom tips.
[0206] FIG. 24 illustrates a typical two-level polyurethane fiber
structure that can be fabricated using this method. This sample is
comprised of 50 .mu.m diameter fibers with 100 .mu.m diameter
mushroom tips atop 400 .mu.m diameter base fibers with 1 mm
diameter mushroom tips. The curved base fibers demonstrate the
feasibility of creating complex shapes. The roughness of the base
fibers is due to the relatively low resolution of the rapid
prototype master template.
[0207] [The larger length-scale of these dual-level hierarchical
fibers, the roughness adaptation to larger amplitude rough surfaces
should be significantly increased. This effect will be investigated
in detail in Section 1.4.6.
[0208] 1.4.5 Three-Level Hierarchy
[0209] In another embodiment of the present invention, the
macroscale hierarchy fabrication technique are combined with the
micro scale hierarchy technique to fabricate three-level
hierarchical fibers, each level having mushroom shaped tips for
increased area. Combining the processes is relatively
straightforward, but does require several steps to complete. (FIG.
25). FIG. 25 illustrated one embodiment of the process flow for
fabricating three-level hierarchical fibers according to the
present invention. In this embodiment, smaller third level fibers
are added by the same method taught in the two-level description
above. The illustrated embodiment has first 22, second 60, and
third 90 levels.
[0210] Many variations are possible. For example, it is possible to
simplify this process by doing several of the steps (FIG. 25a-j)
and then using the resulting structure as a master template.
Forming a negative compliant silicone rubber mold at this step
allows fabrication of steps (FIG. 25k-m) following a single molding
step, rather than the many steps it would require otherwise. In
this way, the first steps must only be completed once to form the
master 2-level structures.
[0211] Initial results of three-level hierarchical fiber
fabrication are promising. FIGS. 26a-d illustrate Scanning Electron
Micrographs of 3-level hierarchical polyurethane fibers. FIG. 26a
illustrates curved base fibers with 400 .mu.m diameter. FIG. 26b
illustrates base fiber tip with mid-level 50 .mu.m diameter fibers.
FIG. 26c illustrates mid-level fibers in detail. FIG. 26d
illustrates terminal third level fibers at the tip of the mid-level
fibers are 3 .mu.m in diameter, 20 .mu.m tall, and have 5 .mu.m
diameter mushroom tips.
[0212] Polyurethane structures (FIGS. 26a-d) exhibit good
uniformity with the exception of the terminal tip fibers. Some of
the microscale tip fibers are collapsed due to their small
diameters and high aspect ratios, in addition to the large stresses
from the final release step in fabrication. Smaller scale fibers
benefit from stiffer materials, so it is likely beneficial to use
different materials for each of the hierarchical levels. This is
easily accomplished using the same fabrication process, simply by
dipping with alternate compatible materials for each level of
hierarchy.
[0213] 1.4.6 Experiments
[0214] Four samples were fabricated from polyurethane (ST-1060; BJB
Enterprises), an unstructured control sample, a single level fiber
sample, a double level vertical sample, and a double level angled
sample. The double level samples were fabricated using the
techniques described in Section 1.4.4. The details of the samples
can be seen in Table 1.
TABLE-US-00001 TABLE 1 Sample specifications. Total Base Base Base
Contact Fiber Fiber Fiber Area Sample Type Material Height Diameter
Angle Fraction Unstructured ST-1060 NA NA NA 100% Single Level
ST-1060 NA NA NA 36% Double Level ST-1060 1.75 mm 425 .mu.m
20.degree. 10% Angled Double Level St-1060 1.2 mm 300 .mu.m
0.degree. 19.5%.sup. Vertical
[0215] All of the samples, other than the unstructured sample, have
identical terminal fibers, with 50 .mu.m diameter stems, 100 .mu.m
height, and 110 .mu.m diameter mushroom tips with 160 .mu.m
center-to-center spacing. The unstructured sample was molded
against the same substrate so that it has the same surface
properties as the fiber samples. Since the terminal fibers are
identical between the three fiber samples, the only difference
between them is the contact area fraction, and the structure
beneath the terminal fibers. In the Single Level case, this
structure is a solid backing layer of the polyurethane. In the
hierarchical samples, this structure is an array of larger base
fibers. The base fibers are intended to make the sample effectively
more compliant. However, along with the increased compliance, the
contact area fraction (area-open space between fibers) is
significantly reduced. The total contact area fraction for the
Double Level samples is the product of the contact area fraction of
the terminal layer (36%) and the contact area fraction of the base
layer. The contact area fraction of the unstructured sample is
100%.
[0216] FIGS. 27a-b illustrate Scanning Electron Micrographs of
Double Level hierarchical fiber samples. FIG. 27a illustrates
Double Level Vertical, and FIG. 27b illustrates Double Level
Angled. The large areas between the fiber tips significantly reduce
the total contact area fraction.
[0217] The four samples were tested using a 12 mm hemispherical
smooth glass indenter. Because the extension length of the Double
Level samples is high (mm scale), a retraction speed of 200 .mu.m/s
was chosen to minimize the duration of the experiments Similarly,
the approach speed was set to 50 .mu.m/s. Although viscoelastic
effects are present due to the relatively high strain rate, these
experiments are intended to compare the hierarchical structures to
the previously characterized Single Level Fibers and unstructured
samples in a relative manner, not to determine their quantitative
adhesive characteristics. Five experiments were performed on the
same area of each sample at each specified preload between 2 mN and
400 mN.
[0218] The resulting performance curves are plotted together in
FIG. 28. In particular, FIG. 28 illustrates the performance curves
for unstructured, single level, double level angled, and double
level vertical samples against a 12 mm diameter glass hemisphere.
Error bars represent standard deviations. The double level vertical
fibers generally exhibit the highest adhesion.
[0219] Results from the experiments in FIG. 28 indicate that for
low preloads, the four samples exhibit similar adhesion. However,
for larger preloads, the adhesion of the two vertical fiber samples
(Single Level and Double Level Vertical) increase at a faster rate
than the increase for the unstructured sample. The reason for this
increase is that as the indenter is pressed further into contact
with the fibers with increasing preloads, the fibers deform and
allow neighboring fibers to come into contact with the indenter.
This is true for all of the fiber samples, especially the Double
Level samples, which have highly increased compliance. The contact
zone of the indenter on the unstructured sample does not increase
as much as in the case of the fiber samples, so the increase in
adhesion with increasing preloads is modest. The decrease in
adhesion for the Double Level Angled sample for preloads greater
than 128 mN is results from detachment during the preloading phase
when the indentation depth becomes too high. The angled fibers
detach under high preloads and do not contribute to the adhesion
during separation.
[0220] Since a hemispherical indenter represents a special case of
a rough surface, these result suggest that the Double Level
Vertical fibers provide higher adhesion against surfaces with high
amplitude (mm scale) roughness.
[0221] To examine the sample-indenter interaction in more detail,
the Force-Distance data for the four samples are plotted together
in FIGS. 29a-d. In particular, FIG. 29a illustrates Force-Distance
curves for the samples tested at a preload of 128 mN. FIG. 29b
illustrates Maximum adhesion. FIG. 29c illustrates Adhesion
pressures. FIG. 29d illustrates Dissipated energy. FIG. 29e
illustrates Total Work of Adhesion. The experimental parameters for
these tests were the same as above, and the preload was set to 128
mN. The unstructured sample saw a higher preload due to over shoot
during the indenting phase, its high stiffness and the small time
delay in stopping and retracting the indenter caused a higher
preload than for the other more compliant samples. The indentation
depth (maximum positive distance) of the indenter for the
unstructured sample and single level sample are similar (73 .mu.m
and 93 .mu.m, respectively), with the fiber sample being more
compliant. The Double Level Vertical sample is significantly more
compliant, with an indentation depth of 305 .mu.m, and the most
compliant sample was the Double Level Angled sample with an
indentation depth of 350 .mu.m.
[0222] Using the indentation depths of from these data, it is
possible to estimate the size of the contact zone using the
geometrical equations for a spherical cap. The contact zone area
ac, is found as
a.sub.cz=.pi..DELTA..sub.p(2R-.DELTA..sub.p) (2)
where .DELTA..sub.p is the indentation depth and R is the radius of
the hemispherical indenter. The contact zone areas for these tests
were found to be 2.7 mm.sup.2, 3.5 mm.sup.2, 12.8 mm.sup.2, and
11.2 mm.sup.2 for the Unstructured, Single Level Fiber, Double
Level Angled, and Double Level Vertical samples, respectively.
Multiplying the estimated contact zone areas by the contact area
fraction for each sample results in an estimate for the real
contact area. For the four samples, the real contact areas were
found to be 2.7 mm.sup.2, 1.26 mm.sup.2, 1.28 mm.sup.2, and 2.18
mm.sup.2 for the Unstructured, Single Level Fiber, Double Level
Angled, and Double Level Vertical samples, respectively. Therefore,
the enhancements due to contact splitting and load sharing for the
Double Level Vertical sample increased the adhesion.
[0223] The adhesions, maximum negative force, for the samples are
compared in FIG. 29b. The Double Level Vertical exhibited the
highest adhesion, followed by the Single Level Fiber, Double Level
Angled, and Unstructured samples, respectively. The adhesion
pressures, which are calculated by dividing the adhesion values by
the estimated contact zone areas are shown in FIG. 29c. The
adhesion pressures of the hierarchical samples are significantly
lower than the unstructured and single level fiber samples, likely
due to their significantly lower contact area fraction.
Furthermore, the small contact area of the Unstructured and Single
Level Fiber samples means that the contact area of the indenter was
relatively locally flat (less than 100 .mu.m of height change),
while the contact area of the Double Level samples contacted parts
of the indenter with over 300 .mu.m of height difference. Despite
the lower adhesion pressure of the Double Level Vertical sample,
due to its roughness adaptation characteristics, it was able to
adhere to the indenter with higher adhesion than the other samples.
The hierarchical structure was able to more than make up for a
contact area fraction of less than 20%, exhibiting the best
adhesion performance against the uneven geometry of the
indenter.
[0224] The Force-Distance data can be used to calculate the energy
dissipated during detachment for each of the samples, which
indicates the toughness of an interface. This energy is seen in
FIG. 29a as the area under the retraction curve for each sample.
The high retraction extension of the Double Level samples requires
a higher amount of energy to be expended during detachment. FIG.
29d shows the dissipated energy of each sample. Very little energy
is required to separate the Unstructured sample, while the Single
Level Fiber, 31 Double Level Angled, and Double Level Vertical each
require increasingly more energy, with the Double Level Vertical
sample requiring 39.4 times as much energy than the unstructured
sample. FIG. 29e shows the work of adhesion of each sample, a value
calculated by dividing the dissipated energy by the estimated
contact zone area. The hierarchy samples, even with much larger
contact zones, exhibited higher work of adhesion than the
unstructured sample, with the Double Level Vertical sample
exhibiting the highest work of adhesion, with 9.6 times as much as
the unstructured sample.
[0225] The advantage of hierarchical fibers does not only appear at
large preloads, it is also evident at low preloads as well. FIG. 30
illustrates Force-Distance curves for the samples tested at a
preload of 5 mN. In particular, FIG. 30 depicts Force-Distance data
for the Double Level Vertical sample along with the Single Level
Fibers and the unstructured sample tested at a preload of 5 mN. In
this test, hierarchical structures extended over 1.2 mm and adhered
with over 96 mN after being preloaded with only 5.5 mN, dissipating
10 times as much energy during detachment as the unstructured
sample, and 7.8 times as much energy as the Single Level Fiber
sample.
[0226] To examine the behavior of a hierarchical sample interacting
with an uneven surface, an experiment was run while recording a
video of the side view of the sample. The test data (force vs.
time) is illustrated in FIGS. 31a-e. Frames from associated still
side-view video images are shown below the data in the same figure
showing the approach (FIG. 31a), maximum preload condition (FIG.
31b), maximum adhesion (FIG. 31c), the last frame before final
detachment (FIG. 31d), and the fibers returned to their original
configuration (FIG. 31e). The edge of the sphere is outlined for
clarity. During retraction, both the terminal tip fibers and base
fibers are observed to stretch as the sample maintains contact with
the indenter for large extensions (FIGS. 31c and 31d).
[0227] 1.5 Repeatability
[0228] FIG. 32 illustrates data indicative of the repeatability of
the present invention. In particular, the normalized adhesion is
fairly constant with increasing numbers of experiments, indicating
that the adhesives retain their performance over many attachment
and detachment cycles.
[0229] Although the present invention has generally been described
in terms of specific embodiments and implementations, the present
invention is applicable to other methods, apparatuses, systems, and
technologies. The examples provided herein are illustrative and not
limiting, and other variations and modifications of the present
invention are contemplated. Those and other variations and
modifications of the present invention are possible and
contemplated, and it is intended that the foregoing specification
and the following claims cover such modifications and
variations.
* * * * *