US20140004291A1 - Dry adhesives and methods for making dry adhesives - Google Patents
Dry adhesives and methods for making dry adhesives Download PDFInfo
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- US20140004291A1 US20140004291A1 US14/016,651 US201314016651A US2014004291A1 US 20140004291 A1 US20140004291 A1 US 20140004291A1 US 201314016651 A US201314016651 A US 201314016651A US 2014004291 A1 US2014004291 A1 US 2014004291A1
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- stem
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- tip
- fiber
- dry adhesive
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
- C09J7/00—Adhesives in the form of films or foils
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- C09J7/02—
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- A—HUMAN NECESSITIES
- A44—HABERDASHERY; JEWELLERY
- A44B—BUTTONS, PINS, BUCKLES, SLIDE FASTENERS, OR THE LIKE
- A44B18/00—Fasteners of the touch-and-close type; Making such fasteners
- A44B18/0046—Fasteners made integrally of plastics
- A44B18/0049—Fasteners made integrally of plastics obtained by moulding processes
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C33/00—Moulds or cores; Details thereof or accessories therefor
- B29C33/38—Moulds or cores; Details thereof or accessories therefor characterised by the material or the manufacturing process
- B29C33/3842—Manufacturing moulds, e.g. shaping the mould surface by machining
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C33/00—Moulds or cores; Details thereof or accessories therefor
- B29C33/42—Moulds or cores; Details thereof or accessories therefor characterised by the shape of the moulding surface, e.g. ribs or grooves
- B29C33/424—Moulding surfaces provided with means for marking or patterning
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C33/00—Moulds or cores; Details thereof or accessories therefor
- B29C33/44—Moulds or cores; Details thereof or accessories therefor with means for, or specially constructed to facilitate, the removal of articles, e.g. of undercut articles
- B29C33/52—Moulds or cores; Details thereof or accessories therefor with means for, or specially constructed to facilitate, the removal of articles, e.g. of undercut articles soluble or fusible
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C39/00—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor
- B29C39/22—Component parts, details or accessories; Auxiliary operations
- B29C39/26—Moulds or cores
- B29C39/34—Moulds or cores for undercut articles
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C39/00—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor
- B29C39/22—Component parts, details or accessories; Auxiliary operations
- B29C39/36—Removing moulded articles
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C39/00—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor
- B29C39/22—Component parts, details or accessories; Auxiliary operations
- B29C39/42—Casting under special conditions, e.g. vacuum
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C99/00—Subject matter not provided for in other groups of this subclass
- B81C99/0075—Manufacture of substrate-free structures
- B81C99/0085—Manufacture of substrate-free structures using moulds and master templates, e.g. for hot-embossing
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
- C09J9/00—Adhesives characterised by their physical nature or the effects produced, e.g. glue sticks
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29L—INDEXING SCHEME ASSOCIATED WITH SUBCLASS B29C, RELATING TO PARTICULAR ARTICLES
- B29L2031/00—Other particular articles
- B29L2031/756—Microarticles, nanoarticles
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B2203/00—Basic microelectromechanical structures
- B81B2203/03—Static structures
- B81B2203/0361—Tips, pillars
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C2201/00—Manufacture or treatment of microstructural devices or systems
- B81C2201/03—Processes for manufacturing substrate-free structures
- B81C2201/034—Moulding
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
- C09J2301/00—Additional features of adhesives in the form of films or foils
- C09J2301/30—Additional features of adhesives in the form of films or foils characterized by the chemical, physicochemical or physical properties of the adhesive or the carrier
- C09J2301/302—Additional features of adhesives in the form of films or foils characterized by the chemical, physicochemical or physical properties of the adhesive or the carrier the adhesive being pressure-sensitive, i.e. tacky at temperatures inferior to 30°C
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
- C09J2301/00—Additional features of adhesives in the form of films or foils
- C09J2301/30—Additional features of adhesives in the form of films or foils characterized by the chemical, physicochemical or physical properties of the adhesive or the carrier
- C09J2301/31—Additional features of adhesives in the form of films or foils characterized by the chemical, physicochemical or physical properties of the adhesive or the carrier the adhesive effect being based on a Gecko structure
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/23907—Pile or nap type surface or component
- Y10T428/23957—Particular shape or structure of pile
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
- Y10T428/2913—Rod, strand, filament or fiber
- Y10T428/2933—Coated or with bond, impregnation or core
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
- Y10T428/2913—Rod, strand, filament or fiber
- Y10T428/2933—Coated or with bond, impregnation or core
- Y10T428/2935—Discontinuous or tubular or cellular core
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
- Y10T428/2913—Rod, strand, filament or fiber
- Y10T428/2973—Particular cross section
- Y10T428/2976—Longitudinally varying
Definitions
- the present invention is a Divisional application of U.S. Non-provisional application Ser. No. 12/448,243 entitled “DRY ADHESIVES AND METHODS FOR MAKING DRY ADHESIVES” filed on Jun. 12, 2009, which is a national stage entry of Patent Cooperation Treaty international application serial number PCT/US2007/025684, entitled “DRY ADHESIVES AND METHODS FOR MAKING DRY ADHESIVES” filed on Dec. 14, 2007, which claims the benefit of U.S. Provisional Application Ser. No. 60/874,787, filed on Dec. 14, 2006, all are incorporated herein by reference.
- This invention relates to dry adhesives, and methods for making dry adhesives including, for example, microfibers and nanofibers.
- U.S. Pat. No. 6,722,026 discloses a method of removably adhering a semiconductor substrate with microfibers which possess spatulate tips, but does not disclose a method for fabrication of such spatulate tips.
- U.S. patent application Ser. No. 11/281,768 (published as US 2006-0202355 A1) discloses a variety of formulae for enhancing friction of fibers and mentions fibers with T-shaped ends, but does not describe a method of fabrication for such T-shaped ends.
- U.S. Pat. No. 6,737,160 and U.S. patent application Ser. Nos. 10/039,574 (published as US 2003 0124312 A1), 10/655,271 (published as US 2005 0072509 A1), 10/747,923 (published as US 2005 0148984 A1) and 11/030,752 (published as US 2005 0151385 A1) describe the use of microfibers as dry adhesives, and describe several methods for the fabrication of spatulate tips on such microfibers.
- One method uses an oxide/nitride semiconductor process to fabricate the shafts, the ends of which are then roughened to produce spatula. This method cannot make spatulate tips.
- Another described method uses a pipette, through which a liquid polymer is extruded until a hemispherical drop forms at the end of the pipette, which could then be flattened against a smooth surface to create a flat spatulate tip.
- This method can allow only micrometer scale fibers due to the diameter limitation of micro-pipettes.
- Another described method to fabricate spatulate tips is lithographically induced self-construction. This technique uses electrostatic attraction to pull liquid through a mask, to thereby ‘sprout’ spatulae. This could enable micro/nano-meter scale low aspect ratio fibers with no tips.
- a third method described to fabricate spatulate tips involves the use of a nano-imprinting roller.
- a final method uses a two-layer photoresist process to make fibers with tips. This method can only make fibers with tips from photoresist polymers, which are very brittle.
- U.S. patent application Ser. Nos. 10/863,129 (published as US 2005-0271869-A1) and 10/982,324 (published as US-2005-0271870-A1) disclose a method for forming hierarchical structures of microfibers with smaller microfibrils attached to the end.
- these applications describe a method to fabricate microstructures with broader tips on narrow shafts that could be considered to be spatulate tips.
- This method uses a time multiplexed deep etching process, such as the Bosch process to etch wells in a substrate. Through alternation of etching and passivation, the process can produce an array of microfibers with large heads on top of narrow shafts. This process can make microfibers with flat tips from only silicon type of stiff and brittle materials that can be etched in the Bosch process.
- microfiber fabrication methods described above are very expensive for producing commercial quantities of adhesive materials. Therefore there is a need for better methods for economically producing microfiber-based dry adhesives. Moreover, fibers with flat tips and diameters of hundreds of nanometer cannot be reliably fabricated from wide range of polymer materials in the above processes.
- the present invention is directed to dry adhesives and methods for making dry adhesives.
- Prior art efforts to produce microfiber-based dry adhesives have not produced adhesive forces of magnitudes equal to those produced by gecko adhesion, have not shown acceptable durability, and have suffered from very high costs of production, making them commercially infeasible.
- the present invention provides dry adhesives and methods for fabrication of dry adhesives which provide superior adhesive qualities, and does so in a manner which is reproducible, scalable and cost effective.
- the invention also includes: apparatus (e.g., Kraus vibratory doser) to add alkaline material (such as, pulverized calcite limestone) directly to the tank assembly when additional alkalinity is needed to complete the ferrous iron oxidation and precipitation reactions; and a separate container assembly to thicken iron oxides produced by the treatment process.
- apparatus e.g., Kraus vibratory doser
- alkaline material such as, pulverized calcite limestone
- the invention has the capacity to discharge substantially iron free water with circumneutral pH.
- the present invention provides improved dry adhesive materials and methods for fabrication of dry adhesive materials.
- dry adhesive materials may be, for example, micro- and nanofiber-based materials.
- the present invention provides methods for the fabrication of such fibers which will produce flat tips on the fibers. These tips are sometimes described herein as being “spatulate” tips, although the present invention may be used to form flattened tips of many shapes and is not limited to only forming spatulate tips.
- the present invention includes a method of forming a dry adhesive fiber in a structure including an etch layer and a barrier layer, wherein the etch layer and the barrier layer are adjacent to each other and are made from different materials.
- the method includes forming an opening through the etch layer and to the barrier layer, expanding the opening in the etch layer at the barrier layer, filling the opening with a material, removing the barrier layer from the material in the opening, and removing the etch layer from the material in the opening.
- the method includes forming a plurality of dry adhesive fibers including forming a plurality of openings through the etch layer and to the barrier layer, expanding the openings in the etch layer at the barrier layer, filling the openings with a material, forming a backing layer over the openings and on a surface of the etch layer opposite the barrier layer wherein the material in a plurality of the openings is connected via the backing layer, removing the barrier layer from the material in the openings, and removing the etch layer from the material in the opening and from the backing layer.
- the present invention includes a dry adhesive fiber.
- the fiber includes a tip having a flat surface, a layer of fluorocarbon on the flat surface of the tip, a base, a stem connecting the tip and the base wherein the stem has a surface, and a hydrophobic and low surface energy layer on the surface of the stem.
- the present invention includes a dry adhesive including a plurality of fibers, with each of the fibers including a tip having a flat surface, a layer of fluorocarbon on the flat surface of the tip, a base, a stem connecting the tip and the base wherein the stem has a surface, and a hydrophobic and low surface energy layer on the surface of the stem.
- the dry adhesive includes a backing layer connected to the bases of the plurality of fibers.
- the present includes a dry adhesive fiber array including two or more layers of dry adhesive fibers.
- fibers may be formed by molding a master template fabricated using deep reactive ion etching and the notching effect in the following steps: (a) A silicon-on-insulator wafer top surface is patterned using optical lithography; (b) A negative fiber array template is formed using a two-step deep reactive ion etching process: At first, isotropic etching is conducted for forming circular supporting base of each fiber; then, DRIE is followed for the vertical walls, and the notching effect on the oxide layer forms a template for molding fibers with spatulate flat tips; (c) The template is filled with a liquid polymer under vacuum and the polymer is cured, or it is filled by gas phase deposition of a polymer; and (d) Polymer fiber arrays with spatulate tips and a backing layer are released from the template: The bottom silicon layer is etched away by XeF 2 dry etching; thin oxide layer is removed by buffered oxide etching; top silicon layer is
- FIG. 1 illustrates one embodiment of a dry adhesive fiber according to the present invention
- FIG. 2 illustrates one embodiment of a dry adhesive including a plurality of fibers attached to a backing layer
- FIGS. 3A-D is a Schematic illustration of process flow steps for the fabrication of microfiber arrays according to one embodiment of the present invention.
- FIGS. 4 a - 4 h illustrate one embodiment of a method of forming a two layer fiber array 30 according to the present invention
- FIG. 5 is a scanning electron microscope image of the isometric view of a polyurethane elastomer microfiber array illustrated in FIG. 3 with 4.5 ⁇ m fiber diameter, nine ⁇ m tip diameter, 20 ⁇ m length, and 44% fiber density (Scale bar: 50 ⁇ m);
- FIG. 6 is a graph of macroscale adhesion (upper plot) and overall work of adhesion (lower plot) of polyurethane microfibers and flat polyurethane flat control surface on a six mm diameter glass hemisphere for varying preloads;
- FIG. 7( a ) illustrates a scanning electron microscope image of the profile view of a polyurethane elastomer microfiber array with spatulate tips with around 5 ⁇ m fiber stem diameter, 9 ⁇ m tip diameter, 20 ⁇ m length, and 44% fiber density;
- FIG. 7( b ) illustrates experimental pull-off force (adhesion) (F ad ) data for elastomer microfiber arrays on a silicon disk with 0.43 mm radius with four different normalized backing layer thicknesses (h/a) for a 10 mN preload (four data points were plotted for each backing thickness on different fiber array locations);
- FIGS. 8 a and 8 b illustrates schematics of the force analysis in two different backing layer thickness cases under constant displacement: (a) fibers on an infinitesimal thick backing layer; (b) fibers on a very thick backing layer;
- FIG. 9 illustrates the stem-fiber angle and the tip wedge angle
- FIG. 10 illustrates the stem being angled obligue to the backing layer and tip
- FIGS. 11A and 11B illustrate where the cross sectional area of the stem of the plurality of fibers includes a plurality of cross sectional areas along the longitudinal length;
- FIG. 12 illustrates the stem being angled oblique to the backing layer and tip
- FIG. 13 illustrates the second plurality of fibers having oblique angles relative to the tip of the first and second plurality of fibers.
- dry adhesive refers generally to individual dry adhesive fibers and also to materials including a plurality of dry adhesive fibers connected together.
- the present invention will also be described in terms of micro- and nanofibers, although the present invention is applicable to a wide variety of sizes and is not necessarily limited to a particular size range.
- FIG. 1 illustrates one embodiment of a dry adhesive fiber 10 according to the present invention.
- the fiber 10 includes a tip 12 , a base 14 , and a stem 16 .
- the shape of the fiber 10 and the relative size of the various parts of the fiber 10 can vary from that shown in the figure. Different applications and materials, for example, can make different shapes and sizes desirable, and such variations are within the scope of the present invention.
- the tip 12 includes a flat surface 20 .
- a layer of fluorocarbon 22 may be on the flat surface 20 , as described hereinbelow.
- the layer of fluorocarbon 22 may be of varying thickness, and is not necessarily shown to scale in FIG. 1 . In this particular illustration, the layer of fluorocarbon 22 is shown as being relatively thick for the purpose of illustration.
- the base 14 is opposite the tip 12 and is often attached to a backing layer, as is described in more detail with respect to FIG. 2 .
- the base 14 is illustrated as being thicker than the stem 16 and is also illustrated as including a curved surface 24 extending outward from the stem 16 to the edge of the base 14 .
- the relatively large, curved shape of the base 14 allows the base 14 to better withstand strain causes by the bending of the fiber 10 , thereby providing for greater durability of the fiber 10 .
- the base 14 may have different shapes and sizes.
- the base 14 may be thinner than the stem or of the same thickness as the stem 16 .
- the base 14 may be indistinguishable from the stem 16 in size and shape, although it is still the base 14 because it represents the end of the fiber 10 that is typically attached to another structure, such as a backing layer.
- the stem 16 connects the tip 12 and the base 14 .
- the stem 16 may also include a hydrophobic layer 26 as will be described in more detail hereinbelow.
- the stem 16 is shown as being at right angles to both the tip 12 and the base 14 in FIG. 2 . However, in other embodiments the stem 16 may be oriented differently, such as at an oblique angle (acute or obtuse) with respect to one or both of the tip 12 and the base 14 (see FIGS. 10 , 12 , and 13 ).
- the stem 16 is illustrated as being straight, although the stem 16 may be made to be curved or to have other non-straight characteristics.
- FIG. 13 also illustrates the second layer of fibers 110 at an oblique angle (acute or obtuse) with respect to one or both of the tip 112 and the base 114 (not shown) or tip 12 (when there is no base 114 ).
- FIGS. 10 , 12 , and 13 are illustrations and are not meant to limit the invention with regards to uniform and non-uniform cross-sectional areas, and orientation of fibers 16 , 110 relative to base 14 or tip 12 or tip 112 .
- FIG. 2 illustrates one embodiment of a dry adhesive structure 30 including a plurality of fibers 10 attached to a backing layer 32 .
- the combination of a plurality of fibers 10 connected to a backing layer 32 will sometimes also be referred to herein as an array 30 or a fiber array 30 .
- the backing layer 32 is connected to the bases 14 of the fibers 10 .
- the dry adhesive structure 30 is not limited to the specifics illustrated in FIG. 2 .
- a dry adhesive 30 according to the present invention may be made in different sizes, with different numbers of fibers 10 having different spacings and different shapes and dimensions.
- the number of fibers 10 , their spacing, dimensions, and other characteristics of the dry adhesive 30 may be uniform or may vary (non-uniform).
- the dry adhesive 30 may have alternating patterns of fiber sizes and spacings, or the shape of the fibers 10 may be different in different parts of the dry adhesive 30 . Many other variations are also possible.
- the backing layer 32 may be the same as the material used to make the fibers 10 , or the backing layer 32 may be made from a different material.
- the thickness of the backing layer 32 can have a significant effect on the performance of the fiber array 30 , and this is discussed in more detail hereinbelow.
- FIGS. 3 a - 3 d illustrate a method of making a dry adhesive according to one embodiment of the present invention.
- FIG. 3 a illustrates the basic structure in which a dry adhesive is formed.
- the structure includes an etch layer 40 and a barrier layer 42 , as well as a substrate 44 on which the etch 40 and barrier 42 layers are formed.
- the etch layer 40 and the barrier layer 42 are adjacent to each other and are made from different materials.
- FIG. 3 a is not necessarily to scale, and the dimensions and shapes of the various parts of the structure may vary depending on the particular application.
- the etch layer 42 may be a single, homogenous layer, or it may be formed from and include more than one layer of the same or different materials.
- etch layer 40 means one or more layers which are etched or from which material is otherwise removed so as to form the structure described herein. In the illustrated embodiment, the etch layer 40 is changed so as to form a mold for use in manufacturing the dry adhesive fibers 10 and the dry adhesive fiber array 30 .
- the barrier layer 42 may also be made from one or several layers of the same or different materials.
- the barrier layer 42 is made from a different material having different properties than the etch layer 40 . Unlike the etch layer 40 , the barrier layer 42 acts as a stop and does not dramatically change its shape during the manufacturing process.
- the barrier layer 42 maintains a relatively constant-flat shape that is used to form the flat surfaces 20 on the tips 12 of the fibers 10 .
- barrier layer 42 will lose some material during the formation of the openings, which are described below. However, this loss of material is very small compared to that of the etch layer 40 , and the concept of barrier materials is well understood in the art.
- barrier layer 42 generally means the one or more layers which form a border of the openings 50 (described below) but which are not significantly etched or from which significant material is not otherwise removed during the formation of the openings 50 , which are described below.
- the barrier layer 42 is generally described as having a flat shape, although in other embodiments the barrier layer 42 may be formed otherwise so as to form different shapes for the tips 12 of the fibers 10 .
- the barrier layer 42 may be formed with a curved shape, or with a surface having other features such as recesses or protrusions.
- the substrate 44 may be used in connection with the etch 40 and barrier 42 layers. However, in methods where the substrate 44 is not required to form and/or to support the etch 40 and barrier 42 layers, the substrate layer 44 may be omitted.
- openings 50 are formed in the etch layer 40 .
- the openings 50 are formed through the etch layer 40 and to the barrier layer 42 .
- the openings 50 may be created, for example, by forming a patterned layer 52 on the surface of the etch layer 40 and then exposing the structure to an etching process or to other processes for removing the exposed material from the etch layer 40 .
- the patterned layer 52 may be photoresist, a contact mask, or some other material used to selectively expose portions of the etch layer 40 .
- the patterned layer 52 may be omitted and the openings may be formed directly, such as through e-beam processes.
- the openings 50 are expanded in the etch layer 40 at the barrier layer 42 to form expanded openings 54 which will shape the tip 12 of the fibers 10 .
- the process of forming the expanded openings 54 will be described in more detail hereinbelow.
- the openings 50 may also be expanded 56 near the top surface 58 of the etch layer 40 , on the surface 58 opposite the barrier layer 42 .
- This expanded opening 56 will shape the base 14 of the fibers 10 .
- the base 14 will be thicker than the stem 16 .
- the expanded opening 56 is given a rounded shape, it will cause the base 14 to be formed with a rounded shape, as described herein.
- the formation of this expanded opening 56 may, for example, be performed with an isotropic etch prior to the formation of the opening 50 and may be performed in the portions of the etch layer 40 not covered by the patterning layer 52 .
- the process of forming the openings 50 may, under some processes, form a layer of hydrophobic material 60 on the side walls of the openings.
- a fluorocarbon layer 62 may be formed on the barrier layer 42 where the flat surface 20 of the tip 12 will be formed. This fluorocarbon layer 62 has been found to cause increased adhesion at the flat surface 20 of the tip 12 .
- the hydrophobic layer 60 is a smooth, non-stick surface which is also advantageous. The formation of these layers 60 and 62 will be described in more detail hereinbelow.
- the photoresist or other patterned layer 52 may be removed.
- the openings 50 are filled with a material 70 .
- the material 70 may be a polymer, a metal, or any other material from which the fibers 10 can be made.
- the backing layer 32 may be formed over the openings 50 and on the surface 58 of the etch layer 40 opposite the barrier layer 42 .
- the material 70 in the openings 50 is connected together via the backing layer 32 , forming a single structure including a plurality of fibers 10 and the backing layer 32 .
- the backing layer 32 may be formed in several ways.
- the backing layer 32 may be formed separately from the material 70 filling the openings 50 and applied over the openings 50 and on the surface 58 of the etch layer 40 .
- the backing layer 32 may be formed from the same material 70 as that used in the openings 50 , in which case, for example, the process of filling the openings 50 may be allowed to continue after the openings 50 are filled, so that the material 70 fills the surface 58 over the openings 50 to form the backing layer 32 .
- a mold (not shown) may be formed on top 58 of the etching layer 40 to contain the extra material 70 used to form the backing layer 32 .
- a step of compressing or squeezing the material forming the backing layer 32 may also be performed so that the backing layer 32 is formed to a desired thickness.
- Other methods of controlling the thickness of the backing layer 32 are also possible, such as by trimming or cutting excessive backing layer 32 material.
- the thickness of the backing layer 32 can have a significant effect on the performance of the fiber array 30 , and will be discussed in more detail hereinbelow
- the etch layer 40 and barrier layer 42 may be removed from the material 70 , leaving the molded dry adhesive fibers 10 and backing later 32 .
- the substrate layer 44 may also be removed if it was used.
- wet etch processes for the removal of the etch layer 40 tend to cause “clumping” of, or an attraction between, adjacent fibers 10 . This is caused by hydrostatic pressure from the liquid remaining from the wet etching. It has been found that dry etch processes are particularly advantageous for removing the etch layer 40 and reducing or eliminating the clumping or lateral or vertical collapsing of fibers. However, the use of a dry etch process is not required with the present invention, and other processes may be used.
- supercritical carbon dioxide (CO 2 ) drying can be used to release the fibers without clumping or collapsing issues where after wet etching of the etch layer, released fibers can be soaked in liquid CO 2 , heated and applied pressure over the supercritical point to remove CO 2 .
- CO 2 supercritical carbon dioxide
- the present invention may also be used to make a single fiber 10 , or to make a plurality of fibers 10 that are not connected to a backing layer 32 .
- the present invention may also include two or more layers of fibers 10 or fiber arrays 30 having two or more layers of fibers 10 .
- FIGS. 4 a - 4 h illustrate one embodiment of a method of forming a two layer fiber array 30 according to the present invention.
- the two layers of fibers are different sizes.
- the smaller fibers 110 shown in FIGS. 4 g and 4 h ) may be smaller in volume, or smaller in their dimensions.
- FIGS. 4 a and 4 b are analogous to FIGS. 3 a and 3 b and they illustrate the first structure in which the openings 50 are created and in which the first layer of fibers 10 will be formed.
- FIGS. 4 c and 4 d illustrate the second structure in which the second layer of fibers 110 is formed.
- the second structure includes an etch layer 140 and a barrier layer 142 , as well as a substrate 144 on which the etch 140 and barrier 142 layers are formed.
- the etch layer 140 and the barrier layer 142 are adjacent to each other and are made from different materials.
- the second structure may include the same or different elements as those described hereinabove with respect to FIGS. 3 a and 3 b .
- the openings 150 in the second structure may have the same features as the openings 50 in the first structure, although they differ in their size.
- FIG. 4 a is not necessarily to scale, and the particular dimensions of the various parts of the structure may vary depending on the particular application.
- FIGS. 4 c and 4 d are similar to FIGS. 4 a and 4 b , except that the fibers 110 to be formed in the process of FIGS. 4 c and 4 d are smaller than those formed in the process of FIGS. 4 a and 4 b .
- FIG. 4 a may represent a process with a twenty ⁇ m thick etch layer 40 , three ⁇ m diameter openings 50 , and twelve ⁇ m spacing between opening 50 centers.
- FIG. 4 c may represent a process with a two ⁇ m thick etch layer 140 , a 300 nm diameter openings 150 , and 800 nm spacing between opening 150 centers.
- the etching process of both FIGS. 4 b and 4 d may be, for example, a dry etching process. That process may include isotropic etching, followed by deep reactive ion etching, and then expanding the opening 50 , 150 near the barrier layer 42 , 142 to form a desired shape for the base 14 , 114 of the fibers 10 , 110 .
- the second fibers 110 may be larger than the first fibers 10 .
- FIG. 4 e illustrates the structures of FIGS. 4 b and 4 d being connected together.
- the dashed line illustrates where the two structures are joined together.
- the structures are oriented so that the openings 50 , 150 correspond to or engage with each other.
- the exposed openings 50 in the first structure corresponding to or engage with exposed openings 150 in FIG. 4 d .
- This orientation is accomplished in FIG. 4 e by rotating the top structure 180 degrees before connecting the two together.
- connection may be formed, for example, by silicon fusion bonding or by other processes.
- the bonding is accomplished with ten minutes of piranha cleaning to remove photoresist and anti-reflective coating.
- ten minutes of oxygen plasma etching to remove the film that is natively generated in the openings 50 , 150 during deep reactive ion etching.
- ten minutes of piranha cleaning for final surface cleaning. After the final cleaning, the two structures are pressed to each other and annealed at 1,000 degrees Celsius in a quartz furnace to bond them.
- FIG. 4 f illustrates the bonded structure after the top layers (the barrier layer 42 and substrate 44 ) are removed. This may be done, for example, with a buffered oxide etching process. The resulting structure is a two layer mold open at the top where the barrier layer 42 and substrate 44 used to be.
- FIG. 4 g illustrates the two layer structure after being filled with a material 70 .
- the material 70 may by any of many different materials, and this process may be, but is not required to be, performed in a vacuum chamber.
- FIG. 4 g also illustrates that not all of the second openings 150 are necessarily utilized.
- one of the second openings 180 is not connected to the first openings 50 and, therefore, that opening 180 is not filled with material 70 .
- FIG. 4 h illustrates the resulting two layer fiber array 30 after the surrounding structure or mold (the remaining etch layers 40 , 140 , the second barrier layer 142 , and the second substrate 144 ) has been removed.
- the removal may be performed, for example, with a XeF 2 etch to remove the bottom silicon layer and a buffered oxide etch to remove the oxide layer, as described above with respect to FIGS. 3 a - 3 d .
- a further XeF 2 etch may be used to remove the upper silicon layer.
- the present invention has been described in terms of a two layer fiber array 30 , the present invention may also be used to produce a fiber array 30 having more than two layers. Furthermore, the present invention is not limited to multilayer fiber arrays 30 , and it may also be used, for example, to make multilayer individual fibers and to make other structures.
- FIG. 3 a - d illustrates one embodiment of the fabrication process of polymer microfibers with flat and larger diameter spatulate tips 12 .
- FIG. 3 a illustrates a silicon-on-insulator (SOI) wafer (purchased from Addison Engineering) which is used as a substrate 44 which has 20 ⁇ M thick top silicon layer as the etch layer 40 and 0.5 ⁇ m thick SiO 2 layer as the barrier layer 42 .
- SOI silicon-on-insulator
- the negative fiber array template is formed in FIG. 3( b ).
- the required fiber profile shape is obtained in two steps. At first, isotropic etching is used for forming the circular supporting shape 56 of the base 14 of each fiber 10 to reduce the stress concentration for preventing the fracture of fibers 10 at their bases 14 . Next, deep reactive ion etching (DRIE) is followed for forming vertical high aspect ratio microchannels 50 .
- DRIE deep reactive ion etching
- Isotropic etching and DRIE were carried out consecutively in STS Multiplex ICP RIE with 20 mT pressure, 130 sccm SF 6 , 20 sccm O 2 , 600 W coil power, and 120 W platen power.
- the vertical etching reaches to the silicon oxide layer 42 , it cannot proceed vertically anymore and then starts to expand laterally 54 on the oxide interface between the etch layer 40 and the barrier layer 42 .
- This effect is called the notching effect [M. E. McNie, D. O. King, V. Nayar, M. C. L. Ward, J. S. Burdess, C. Quinn, and S. Blackstone, Proc. of IEEE International SOI Conference, 60 (1997)].
- the spatulate tip 12 diameter is determined. Then, in FIG. 3( c ), the template is filled with a liquid polymer 70 under vacuum to remove not only the trapped air but also the native gas in the liquid polymer 70 , and then the polymer 70 is cured.
- any polymer 70 e.g. Parylene® C
- any other fiber material 70 can be also gas phase deposited inside to the template 50 .
- the polymer fiber array 30 with a backing layer 32 is released by three step etching: At first, the bottom silicon layer 44 is etched away by XeF 2 dry etching; The thin oxide layer 42 is removed by buffered oxide etching (BOE); Finally, the 20 ⁇ m thick top silicon layer 40 is etched away by XeF 2 etching in around 30 minutes to release the fibers 10 with a backing layer 32 .
- the final etching step is particularly important. Since using a wet etching technique such as KOH etching would result in clumped fibers 10 due to capillary forces during drying, XeF 2 dry etching is used to prevent any clumping issues.
- each released polymer fiber 10 is expected to be coated with this hydrophobic and low surface energy thin film 26 on their side walls 16 (not at the spatulate tip 12 surface 20 ).
- the film 60 on the side walls of the openings 50 and the fibers 10 can be characterized as a passivation film or a fluorocarbon thin film. This very low surface energy coating 26 could reduce the cohesion of microfibers 10 significantly, and thus it could minimize any clumping during the mechanical contact of neighboring fibers 10 .
- Fiber material can be fabricated from any polymer 70 which can be in a liquid solution form or can be gas phase deposited;
- Array 30 of fibers 10 can be fabricated in large areas up to 8 inch wafer size cost effectively using a single mask; (3) The yield is almost 100%;
- This method can be extended to the fabrication of hundreds of nanometer diameter fibers 10 easily by using a higher resolution lithography step in FIG. 3 a by using phase masks or interference lithography.
- FIG. 5 illustrates a scanning electron microscope (SEM) (Hitachi 2460N) image of the resulting microfiber array 30 with 4.5 ⁇ m fiber diameter, 9 ⁇ m tip diameter, 4.5 ⁇ m base supporter diameter, 20 ⁇ m length, and 12 ⁇ m spacing between each fiber center (44% fiber tip area density). These geometries are held by 80 sec isotropic etching for the fiber base supporter structures and by 21 min 20 sec vertical etching for the fiber and spatulate tips 12 .
- SEM scanning electron microscope
- Performance of a fibrillar adhesive is characterized by its macroscale adhesion (P) and overall work of adhesion (W).
- P macroscale adhesion
- W overall work of adhesion
- a custom tensile macroscale adhesion measurement setup was built.
- a glass hemisphere instead of a flat glass surface was selected as the test surface in order to have no alignment errors during the measurements.
- the hemisphere was contacted to and retracted from the fiber array sample with a pre-specified preload force and a very slow speed (1 ⁇ m/s) to minimize any viscoelastic effects.
- the maximum tensile force during the glass hemisphere and fiber array separation gave the adhesion, and the hysteresis area between the loading and unloading curves gave the dissipated energy between the loading and unloading of the fiber array. Dividing this dissipated energy by the maximum circular contact area during loading gave W 10 .
- an inverted microscope (Nikon Eclipse TE200) is used to measure the real circular maximum contact area between the hemisphere and the fiber array 30 .
- Adhesion and overall work of adhesion of 15 ⁇ 15 mm 2 area and one mm thick ST-1060 polyurethane fiber array 30 samples and a 1 mm thick flat and smooth ST-1060 surface were measured on the glass hemisphere using the above setup.
- the flat polyurethane surface was used as a control substrate to show the relative enhancement of P and W by structuring the same material as a cylindrical microfiber with flat spatulate tips. Since ST-1060 is also etched slightly during the final XeF 2 dry etching step in FIG. 1( d ), flat sample was also exposed to XeF 2 for about 30 minutes to have the same surface roughness with the flat spatulate tip surface.
- the fiber array 30 and the glass hemisphere interface adhesion and overall work of adhesion are measured as shown in FIG. 6 .
- Plots show the error bars measured from the force-distance data at two different locations on the fiber array 30 for preloads up to 25 mN.
- Adhesion values saturate as preload increases, and the array of fibers has around 4 times higher adhesion than the flat surface.
- Dividing the adhesion to the optically measured maximum circular contact area during loading, maximum adhesion pressure for the fiber array 30 can be computed as 18 N/cm 2 at a preload pressure of 12 N/cm 2 .
- Overall work of adhesion of the fibers 10 is five times higher than the one from the flat elastomer surface.
- This energy dissipation enhancement is due to the lost energy during separating the elastic and highly stretched fibers 10 from the adhered glass surface as given in (2).
- Macroscale adhesion data from the fiber array 30 in this work are compared with the previous works as given in Table 1.
- Table 1 is a comparison of adhesive strength among various natural and synthetic gecko inspired micro/nanofibers [Y. Zhao, T. Tong, L. Delzeit, A. Kashani, M. Meyyappan, and A. Majumdar, J. Vac. Sci. Techno. B, 24(1), 331 (2006)].
- the polymer fibers 10 with spatulate tips 12 show better adhesion pressure than other synthetic gecko inspired fibrillar adhesives with no spatulate tips 12 although the single fiber 10 in this work is over 20 times thicker than the single fibers 10 which were fabricated in other works.
- microfibers with tips 12 can be scaled down to hundreds of nanometers in diameter using phase mask type of sub-micron lithography techniques.
- N times self-similar scaling down in fiber diameter will generate ⁇ square root over (N) ⁇ times higher adhesion [E. Art, S. Gorb, and R. Spolenak, PNAS, 100, 10603 (2003)], and smaller fibers will need less preload than larger fibers to obtain the same adhesion.
- the thickness of the backing layer 32 has a greater effect on the performance of dry adhesives than was previously known.
- the effect of the backing layer 32 thickness on adhesion was investigated for single-level elastomer fibrillar adhesives 30 .
- Polyurethane microfiber arrays 30 with spatulated tips 12 on a 160 ⁇ m thick backing layer 32 show nine times greater adhesion strength (around 22 N/cm 2 ) than those with a 1120 ⁇ m thick backing 32 .
- a theoretical model is proposed to explain this difference in which very thin backing layers 32 promote equal load sharing, maximizing adhesion, while very thick backings can lead to reduced adhesion due to edge stress concentration. Therefore, backing layer 32 thickness should be considered as a significant parameter for design of high performance fibrillar adhesives.
- the backing layer 32 thickness effect on adhesion of elastomeric single-level microfiber structures 30 will now be described. Although a thick backing layer 32 improves the roughness adaptation and fiber 10 contact abilities due to increased effective compliance, this study shows that a thick backing layer 32 could reduce the macroscale adhesion of the fibers 10 on smooth surfaces significantly.
- All fiber arrays 30 in the samples have a stem 16 diameter of around five ⁇ m and a tip 12 and base 14 support diameter of nine ⁇ m.
- the total length of a fiber 10 is 20 ⁇ m and the minimum spacing between fiber centers is 12 ⁇ m as displayed in FIG. 7( a ).
- a custom tensile setup was built to measure the adhesion of the samples. The measure of adhesion in this work is the pull-off load.
- a silicon disk with 0.43 mm radius and nanometer scale surface roughness was fabricated by patterning a polished silicon (100) wafer using optical lithography and deep reactive ion etching.
- the silicon disk was attached to a load cell (Transducer Techniques, GSO-25), and moved vertically by a motorized stage (Newport, MFA-CC) with 100 nm resolution.
- the disk was contacted to and retracted from the fiber array sample with specified preload forces and a slow speed (1 ⁇ m/s) to minimize viscoelastic effects.
- the maximum pull-off force was recorded.
- the contact area and the deformation of fiber array 32 during loading and unloading was recorded using a camera (Dage-MT1, DC330) attached to an inverted optical microscope (Nikon, Eclipse TE200).
- the spacing of the fibers 10 are typically very small in comparison with the contact radius a and the thickness of the elastic layer h.
- the maximum pull-off force occurs in the equal load sharing (ELS) regime, where all the fibers in adhesive contact with the indenter bear the same load.
- ELS equal load sharing
- the maximum pull-off force F max is directly proportional to the contact area
- the ELS limit is strictly valid if the backing layer thickness h is very small compared to a.
- Another limit is a very thick backing layer with very stiff fibers, that is, when h/a ⁇ and ⁇ ka/2G is very large where G is the shear modulus.
- the interfacial displacement is dominated by the deformation of the elastic layer and the stress distribution is given by the classical solution of a rigid punch in contact with a half space [K. L. Johnson, Contact Mechanics, Cambridge University Press (1985)].
- the normal stress at the punch edge has a square root singularity characteristic of an opening crack. For ⁇ >>1 and h/a>>1, the pull-off force F ad in this limit can be derived as
- polyurethane microfiber arrays 30 with spatulated tips 12 on 160 ⁇ m thick backing layer 32 show adhesion strength (around 22 N/cm 2 ), nine times greater than fiber arrays 30 with thickness of 1120 ⁇ m.
- a theoretical model is proposed to explain this difference in which very thin backing layers 32 promote equal load sharing, maximizing adhesion.
- very thick backings 32 can lead to reduced adhesion, because of edge stress concentration similar to a rigid punch in adhesive contact with a half space. This work shows the significance of backing layer 32 thickness on equal load sharing of single-level microfiber arrays 30 on smooth surfaces.
- the present invention describes a method for fabrication of polyurethane elastomer microfiber arrays with flat spatulate tips. For a preload pressure of around 12 N/cm 2 , adhesion pressures up to 18 N/cm 2 and overall work of adhesion up to 11 J/m 2 are demonstrated for polyurethane fibers with 4.5 ⁇ m fiber diameter, 9 ⁇ m tip diameter, 20 ⁇ m length, and 44% fiber tip area density on a 6 mm diameter glass hemisphere. These repeatable fibrillar adhesives would have wide range of applications as space, biomedical, sports, etc. adhesives.
- the present invention has generally been described in general terms and in terms of specific embodiments and implementations, the present invention is applicable to other methods, apparatuses, systems, and technologies.
- the present invention can be used with a variety of materials, such as metals, ceramics, other polymers, Paralyne, carbon, crystals, liquid crystals, Teflon, semiconductors, piezoelectric materials, conductive polymers, shape memory alloy materials, and organic materials, and these and other materials could be deposited or molded inside the etch layer 40 as described hereinabove.
- the base 14 of the fibers 10 may not be necessary in some cases, in which case the base may be omitted or it may be considered to be the part of the fiber 10 attached to another structure.
- the spatulate tip fibers 10 with or without a hydrophobic surface coating can be used as a superhydrophobic surface where the water contact angle could be increased more due to spatulate tip geometry and fiber spacing.
- etching if the etching time is long enough the tip endings could combine and fibers with a continuous flat thin-film can be formed as another type of fiber based adhesives or materials.
- Different fiber cross-section geometry square, ellipsoid, triangle, etc.), base geometry (pyramid, etc.), tip diameter, fiber packing geometry (hexagonal or square), high or low aspect ratio, and constant or variable fiber density is possible with the present invention.
- the spatulate fibers 10 can be used as static friction enhancing materials in addition to enhanced adhesion materials.
- Micro or nanoscale patterning methods such as interference lithography, electron-beam lithography, nanoimprinting, directed self-assembly, dip pen lithography, laser micro- or nano-machining, micro/nano-milling, and extreme UV lithography can be used to pattern the etch layer for fabricating micro- or nanoscale fibers with spatulate tips.
- FIG. 9 illustrates the stem-fiber angle ⁇ and the tip wedge angle ⁇ .
- the stem-fiber angle ⁇ and the tip wedge angle ⁇ are controlled angles during the silicon removal process. The range of these angles is 1-180 degrees and 1-179 degrees, respectively.
- Stem-fiber angle ⁇ is the angle between the stem outer surface 16 b and the tip outer surface 12 b , which varies depending on the location of the point along the tip outer surface 12 b .
- Tip wedge angle ⁇ is the angle between the tip outer surface 12 b and the tip flat surface 20 , which varies depending on the location of the point along the tip outer surface 12 b .
- the three illustrations of the fiber 10 show tips 12 with different tip heights (h 1 , h 2 , h 3 ) and tip diameters or cross sections (d 2 , d 3 , d 4 ) relative to stems 16 with the same stem diameter or cross section (d 1 ). Any ratio of diameters to height are acceptable.
- the tips 12 can have a curvature (as shown in the figures) or a straight line (not shown) between the tip flat surface edge (or outer perimeter of the tip diameter) 20 a and the junction 16 a of tip 12 and stem 16 .
- the angles are controlled during the silicon removal process. After the vertical removal process reaches to the silicon oxide (barrier) layer, these angles could be controlled by specifically tuning and controlling the silicon removal step duration, removal ion type, and removal ion energy level parameters.
- FIGS. 11A and 11B illustrate where the cross sectional area of the stem of the plurality of fibers includes a plurality of cross sectional areas along the longitudinal length, where the plurality of cross sectional areas comprise a first cross sectional area larger than a second cross sectional area to form a non-uniform stem diameter along the longitudinal length.
- FIG. 11B illustrates the first cross sectional area is adjacent to the backing layer 14 and the second cross sectional area is adjacent to the tip 12 of the each fiber 16 of the plurality of fibers.
- FIG. 11A illustrates the first cross sectional area is adjacent to the tip 12 of the each fiber 16 of the plurality of fibers and the second cross sectional area is adjacent to the backing layer 14 .
- FIGS. 11A illustrates the first cross sectional area is adjacent to the tip 12 of the each fiber 16 of the plurality of fibers and the second cross sectional area is adjacent to the backing layer 14 .
- 11A and 11B provide examples and are not meant to limit the present invention.
- the non-uniform cross-sectional areas can also be applied to single-level fiber embodiments (shown in FIGS. 1 , 2 , 8 , and 10 ), and to second layer smaller fibers 110 .
- FIG. 13 illustrates the second plurality of fibers having oblique angles relative to the tip including an angle between the tip of the each fiber of the secondary plurality of fibers and the stem of the each fiber of the secondary plurality of fibers not being equal to 90 degrees.
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Abstract
A dry adhesive and a method of forming a dry adhesive. The method includes forming an opening through an etch layer and to a barrier layer, expanding the opening in the etch layer at the barrier layer, filling the opening with a material, removing the barrier layer from the material in the opening, and removing the etch layer from the material in the opening.
Description
- The present invention is a Divisional application of U.S. Non-provisional application Ser. No. 12/448,243 entitled “DRY ADHESIVES AND METHODS FOR MAKING DRY ADHESIVES” filed on Jun. 12, 2009, which is a national stage entry of Patent Cooperation Treaty international application serial number PCT/US2007/025684, entitled “DRY ADHESIVES AND METHODS FOR MAKING DRY ADHESIVES” filed on Dec. 14, 2007, which claims the benefit of U.S. Provisional Application Ser. No. 60/874,787, filed on Dec. 14, 2006, all are incorporated herein by reference.
- This invention relates to dry adhesives, and methods for making dry adhesives including, for example, microfibers and nanofibers.
- There have been recent findings on the mechanisms by which geckos adhere to and climb smooth vertical surfaces. Geckos are exceptional in their ability to climb up smooth vertical surfaces because their hierarchical micro/nanoscale foot-hairs with their spatulate tips can attach to almost any smooth or micro/nanoscale rough surface repeatedly with a controllable adhesion pressure up to around 10 N/cm2 (100 kPa) [K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full, Nature, 405, 681 (2000)]. Recent findings have shown that van der Waals and possibly capillary forces play a dominant role in their fibrillar adhesion [K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full, Nature, 405, 681 (2000)] [K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full, PNAS, 99, 12252 (2002)] [G. Huber, H. Mantz, R. Spolenak, K. Mecke, K. Jacobs, S, N. Grob, and E. Artz, PNAS, 102(45), 16293 (2005)] [G. Huber, S, N. Grob, R. Spolenak, and E Artz, Biology Letters, 1, 2 (2005)].
- Many adhesion and contact mechanics models for the microfibrillar interfaces have been developed [H. Gao and H. Yao, PNAS, 101, 7851 (2004)] [T. Tang, C. Hui, and N. J. Glassmaker, J. Roy. Soc. Interface, 2, 505 (2005)] [N. J. Glassmaker, A. Jagota, C. Hui, and J. Kim, J. Roy. Soc. Interface, 1, 1 (2004)] [C. Hui, N. J. Glassmaker, T. Tang, and A. Jagota, J. Roy. Soc. Interface, 1, 35 (2004)] [B. N. J. Persson, J. Chemical Physics, 118, 7614 (2003)] [A. J. Crosby, M. Hageman, and A. Duncan, Langmuir, 21, 11738 (2005)] and synthetic fibrillar adhesives have been attempted to be fabricated. Fabrication methods for recent micro/nanoscale synthetic dry adhesives consist of electron-beam lithography [A. K. Geim, S. V. Dubnos, I. V. Grigorieva, K. S, Novoselov, A. A. Zhukov, and S. Y. Shapoval, Nature Materials, 2, 461 (2003)], replication of templates using molding or casting [D. Campolo, S. Jones, and R. S. Fearing, Proc. of the IEEE Nanotechnology Conf., 12 (2003)], drawing [H. E. Jeong, S. H. Lee, P. Kim, and K. Y. Suh, Nano Letters, 6, 1508 (2004)], printing [M. Sitti and R. S. Fearing, J. Adhesion Science and Technology, 17(5), 1055 (2003)], growing [Y. Zhao, T. Tong, L. Delzeit, A. Kashani, M. Meyyappan, and A. Majumdar, J. Vac. Sci. Techno. B, 24(1), 331 (2006)], and more complex microfabrication combined with self-assembly [M. T. Northen and K. L. Turner, Nanotechnology, 16, 1159 (2005)]. These works focused on fabricating micro/nanoscale high aspect ratio and high density polymer or carbon nanotube fibers on a flat substrate.
- Some researchers have attempted to recreate the spatulate tips that occur naturally on gecko hairs as such broadened tips on fibers provide additional surface area, promoting adherence, while acting to prevent clumping of the fibers.
- U.S. Pat. No. 6,722,026 discloses a method of removably adhering a semiconductor substrate with microfibers which possess spatulate tips, but does not disclose a method for fabrication of such spatulate tips. U.S. patent application Ser. No. 11/281,768 (published as US 2006-0202355 A1) discloses a variety of formulae for enhancing friction of fibers and mentions fibers with T-shaped ends, but does not describe a method of fabrication for such T-shaped ends.
- U.S. Pat. No. 6,737,160 and U.S. patent application Ser. Nos. 10/039,574 (published as US 2003 0124312 A1), 10/655,271 (published as US 2005 0072509 A1), 10/747,923 (published as US 2005 0148984 A1) and 11/030,752 (published as US 2005 0151385 A1) describe the use of microfibers as dry adhesives, and describe several methods for the fabrication of spatulate tips on such microfibers. One method uses an oxide/nitride semiconductor process to fabricate the shafts, the ends of which are then roughened to produce spatula. This method cannot make spatulate tips. Another described method uses a pipette, through which a liquid polymer is extruded until a hemispherical drop forms at the end of the pipette, which could then be flattened against a smooth surface to create a flat spatulate tip. This method can allow only micrometer scale fibers due to the diameter limitation of micro-pipettes. Another described method to fabricate spatulate tips is lithographically induced self-construction. This technique uses electrostatic attraction to pull liquid through a mask, to thereby ‘sprout’ spatulae. This could enable micro/nano-meter scale low aspect ratio fibers with no tips. A third method described to fabricate spatulate tips involves the use of a nano-imprinting roller. This could also enable micro/nano-meter scale low aspect ratio fibers with no tips. A final method uses a two-layer photoresist process to make fibers with tips. This method can only make fibers with tips from photoresist polymers, which are very brittle.
- U.S. patent application Ser. Nos. 10/863,129 (published as US 2005-0271869-A1) and 10/982,324 (published as US-2005-0271870-A1) disclose a method for forming hierarchical structures of microfibers with smaller microfibrils attached to the end. In one embodiment, these applications describe a method to fabricate microstructures with broader tips on narrow shafts that could be considered to be spatulate tips. This method uses a time multiplexed deep etching process, such as the Bosch process to etch wells in a substrate. Through alternation of etching and passivation, the process can produce an array of microfibers with large heads on top of narrow shafts. This process can make microfibers with flat tips from only silicon type of stiff and brittle materials that can be etched in the Bosch process.
- The microfiber fabrication methods described above are very expensive for producing commercial quantities of adhesive materials. Therefore there is a need for better methods for economically producing microfiber-based dry adhesives. Moreover, fibers with flat tips and diameters of hundreds of nanometer cannot be reliably fabricated from wide range of polymer materials in the above processes.
- 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.
- The present invention is directed to dry adhesives and methods for making dry adhesives. Prior art efforts to produce microfiber-based dry adhesives have not produced adhesive forces of magnitudes equal to those produced by gecko adhesion, have not shown acceptable durability, and have suffered from very high costs of production, making them commercially infeasible. The present invention provides dry adhesives and methods for fabrication of dry adhesives which provide superior adhesive qualities, and does so in a manner which is reproducible, scalable and cost effective. The invention also includes: apparatus (e.g., Kraus vibratory doser) to add alkaline material (such as, pulverized calcite limestone) directly to the tank assembly when additional alkalinity is needed to complete the ferrous iron oxidation and precipitation reactions; and a separate container assembly to thicken iron oxides produced by the treatment process. The invention has the capacity to discharge substantially iron free water with circumneutral pH.
- The present invention provides improved dry adhesive materials and methods for fabrication of dry adhesive materials. These dry adhesive materials may be, for example, micro- and nanofiber-based materials. In particular, the present invention provides methods for the fabrication of such fibers which will produce flat tips on the fibers. These tips are sometimes described herein as being “spatulate” tips, although the present invention may be used to form flattened tips of many shapes and is not limited to only forming spatulate tips.
- According to one embodiment, the present invention includes a method of forming a dry adhesive fiber in a structure including an etch layer and a barrier layer, wherein the etch layer and the barrier layer are adjacent to each other and are made from different materials. The method includes forming an opening through the etch layer and to the barrier layer, expanding the opening in the etch layer at the barrier layer, filling the opening with a material, removing the barrier layer from the material in the opening, and removing the etch layer from the material in the opening.
- According to another embodiment, the method includes forming a plurality of dry adhesive fibers including forming a plurality of openings through the etch layer and to the barrier layer, expanding the openings in the etch layer at the barrier layer, filling the openings with a material, forming a backing layer over the openings and on a surface of the etch layer opposite the barrier layer wherein the material in a plurality of the openings is connected via the backing layer, removing the barrier layer from the material in the openings, and removing the etch layer from the material in the opening and from the backing layer.
- According to another embodiment, the present invention includes a dry adhesive fiber. The fiber includes a tip having a flat surface, a layer of fluorocarbon on the flat surface of the tip, a base, a stem connecting the tip and the base wherein the stem has a surface, and a hydrophobic and low surface energy layer on the surface of the stem.
- According to another embodiment, the present invention includes a dry adhesive including a plurality of fibers, with each of the fibers including a tip having a flat surface, a layer of fluorocarbon on the flat surface of the tip, a base, a stem connecting the tip and the base wherein the stem has a surface, and a hydrophobic and low surface energy layer on the surface of the stem. In addition, the dry adhesive includes a backing layer connected to the bases of the plurality of fibers.
- According to another embodiment, the present includes a dry adhesive fiber array including two or more layers of dry adhesive fibers.
- Many variations are possible with the method and fiber according to the present invention. For example, fibers may be formed by molding a master template fabricated using deep reactive ion etching and the notching effect in the following steps: (a) A silicon-on-insulator wafer top surface is patterned using optical lithography; (b) A negative fiber array template is formed using a two-step deep reactive ion etching process: At first, isotropic etching is conducted for forming circular supporting base of each fiber; then, DRIE is followed for the vertical walls, and the notching effect on the oxide layer forms a template for molding fibers with spatulate flat tips; (c) The template is filled with a liquid polymer under vacuum and the polymer is cured, or it is filled by gas phase deposition of a polymer; and (d) Polymer fiber arrays with spatulate tips and a backing layer are released from the template: The bottom silicon layer is etched away by XeF2 dry etching; thin oxide layer is removed by buffered oxide etching; top silicon layer is etched away by XeF2 etching.
- 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.
- 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:
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FIG. 1 illustrates one embodiment of a dry adhesive fiber according to the present invention; -
FIG. 2 illustrates one embodiment of a dry adhesive including a plurality of fibers attached to a backing layer; -
FIGS. 3A-D is a Schematic illustration of process flow steps for the fabrication of microfiber arrays according to one embodiment of the present invention; -
FIGS. 4 a-4 h illustrate one embodiment of a method of forming a twolayer fiber array 30 according to the present invention; -
FIG. 5 . is a scanning electron microscope image of the isometric view of a polyurethane elastomer microfiber array illustrated inFIG. 3 with 4.5 μm fiber diameter, nine μm tip diameter, 20 μm length, and 44% fiber density (Scale bar: 50 μm); -
FIG. 6 is a graph of macroscale adhesion (upper plot) and overall work of adhesion (lower plot) of polyurethane microfibers and flat polyurethane flat control surface on a six mm diameter glass hemisphere for varying preloads; -
FIG. 7( a) illustrates a scanning electron microscope image of the profile view of a polyurethane elastomer microfiber array with spatulate tips with around 5 μm fiber stem diameter, 9 μm tip diameter, 20 μm length, and 44% fiber density; -
FIG. 7( b) illustrates experimental pull-off force (adhesion) (Fad) data for elastomer microfiber arrays on a silicon disk with 0.43 mm radius with four different normalized backing layer thicknesses (h/a) for a 10 mN preload (four data points were plotted for each backing thickness on different fiber array locations); -
FIGS. 8 a and 8 b illustrates schematics of the force analysis in two different backing layer thickness cases under constant displacement: (a) fibers on an infinitesimal thick backing layer; (b) fibers on a very thick backing layer; -
FIG. 9 illustrates the stem-fiber angle and the tip wedge angle; -
FIG. 10 illustrates the stem being angled obligue to the backing layer and tip; -
FIGS. 11A and 11B illustrate where the cross sectional area of the stem of the plurality of fibers includes a plurality of cross sectional areas along the longitudinal length; -
FIG. 12 illustrates the stem being angled oblique to the backing layer and tip; and -
FIG. 13 illustrates the second plurality of fibers having oblique angles relative to the tip of the first and second plurality of fibers. - The present invention is directed to dry adhesives and methods for making dry adhesives. The term “dry adhesive”, as used herein, refers generally to individual dry adhesive fibers and also to materials including a plurality of dry adhesive fibers connected together. The present invention will also be described in terms of micro- and nanofibers, although the present invention is applicable to a wide variety of sizes and is not necessarily limited to a particular size range.
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FIG. 1 illustrates one embodiment of a dryadhesive fiber 10 according to the present invention. In that embodiment, thefiber 10 includes atip 12, abase 14, and astem 16. The shape of thefiber 10 and the relative size of the various parts of thefiber 10 can vary from that shown in the figure. Different applications and materials, for example, can make different shapes and sizes desirable, and such variations are within the scope of the present invention. - The
tip 12 includes aflat surface 20. A layer offluorocarbon 22 may be on theflat surface 20, as described hereinbelow. The layer offluorocarbon 22 may be of varying thickness, and is not necessarily shown to scale inFIG. 1 . In this particular illustration, the layer offluorocarbon 22 is shown as being relatively thick for the purpose of illustration. - The
base 14 is opposite thetip 12 and is often attached to a backing layer, as is described in more detail with respect toFIG. 2 . Thebase 14 is illustrated as being thicker than thestem 16 and is also illustrated as including acurved surface 24 extending outward from thestem 16 to the edge of thebase 14. The relatively large, curved shape of thebase 14 allows the base 14 to better withstand strain causes by the bending of thefiber 10, thereby providing for greater durability of thefiber 10. However, in other embodiments thebase 14 may have different shapes and sizes. For example, thebase 14 may be thinner than the stem or of the same thickness as thestem 16. For example, thebase 14 may be indistinguishable from thestem 16 in size and shape, although it is still the base 14 because it represents the end of thefiber 10 that is typically attached to another structure, such as a backing layer. - The
stem 16 connects thetip 12 and thebase 14. Thestem 16 may also include ahydrophobic layer 26 as will be described in more detail hereinbelow. Thestem 16 is shown as being at right angles to both thetip 12 and the base 14 inFIG. 2 . However, in other embodiments thestem 16 may be oriented differently, such as at an oblique angle (acute or obtuse) with respect to one or both of thetip 12 and the base 14 (seeFIGS. 10 , 12, and 13). In addition, thestem 16 is illustrated as being straight, although thestem 16 may be made to be curved or to have other non-straight characteristics. The angular orientation of theend 16 a of thefiber 16 withtip 12 is tangent with a point along the curvature oftip 12.FIG. 13 also illustrates the second layer offibers 110 at an oblique angle (acute or obtuse) with respect to one or both of thetip 112 and the base 114 (not shown) or tip 12 (when there is no base 114).FIGS. 10 , 12, and 13 are illustrations and are not meant to limit the invention with regards to uniform and non-uniform cross-sectional areas, and orientation offibers tip 12 ortip 112. -
FIG. 2 illustrates one embodiment of adry adhesive structure 30 including a plurality offibers 10 attached to abacking layer 32. The combination of a plurality offibers 10 connected to abacking layer 32 will sometimes also be referred to herein as anarray 30 or afiber array 30. Thebacking layer 32 is connected to thebases 14 of thefibers 10. Thedry adhesive structure 30 is not limited to the specifics illustrated inFIG. 2 . On the contrary, adry adhesive 30 according to the present invention may be made in different sizes, with different numbers offibers 10 having different spacings and different shapes and dimensions. The number offibers 10, their spacing, dimensions, and other characteristics of thedry adhesive 30 may be uniform or may vary (non-uniform). For example, thedry adhesive 30 may have alternating patterns of fiber sizes and spacings, or the shape of thefibers 10 may be different in different parts of thedry adhesive 30. Many other variations are also possible. - The
backing layer 32 may be the same as the material used to make thefibers 10, or thebacking layer 32 may be made from a different material. The thickness of thebacking layer 32 can have a significant effect on the performance of thefiber array 30, and this is discussed in more detail hereinbelow. -
FIGS. 3 a-3 d illustrate a method of making a dry adhesive according to one embodiment of the present invention. -
FIG. 3 a illustrates the basic structure in which a dry adhesive is formed. The structure includes anetch layer 40 and abarrier layer 42, as well as asubstrate 44 on which theetch 40 andbarrier 42 layers are formed. Theetch layer 40 and thebarrier layer 42 are adjacent to each other and are made from different materials.FIG. 3 a is not necessarily to scale, and the dimensions and shapes of the various parts of the structure may vary depending on the particular application. - The
etch layer 42 may be a single, homogenous layer, or it may be formed from and include more than one layer of the same or different materials. As used herein,etch layer 40 means one or more layers which are etched or from which material is otherwise removed so as to form the structure described herein. In the illustrated embodiment, theetch layer 40 is changed so as to form a mold for use in manufacturing the dryadhesive fibers 10 and the dryadhesive fiber array 30. - The
barrier layer 42 may also be made from one or several layers of the same or different materials. Thebarrier layer 42 is made from a different material having different properties than theetch layer 40. Unlike theetch layer 40, thebarrier layer 42 acts as a stop and does not dramatically change its shape during the manufacturing process. Thebarrier layer 42 maintains a relatively constant-flat shape that is used to form theflat surfaces 20 on thetips 12 of thefibers 10. - In practice, the
barrier layer 42 will lose some material during the formation of the openings, which are described below. However, this loss of material is very small compared to that of theetch layer 40, and the concept of barrier materials is well understood in the art. As used herein,barrier layer 42 generally means the one or more layers which form a border of the openings 50 (described below) but which are not significantly etched or from which significant material is not otherwise removed during the formation of theopenings 50, which are described below. - The
barrier layer 42 is generally described as having a flat shape, although in other embodiments thebarrier layer 42 may be formed otherwise so as to form different shapes for thetips 12 of thefibers 10. For example, thebarrier layer 42 may be formed with a curved shape, or with a surface having other features such as recesses or protrusions. - The
substrate 44 may be used in connection with theetch 40 andbarrier 42 layers. However, in methods where thesubstrate 44 is not required to form and/or to support theetch 40 andbarrier 42 layers, thesubstrate layer 44 may be omitted. - With reference to
FIG. 3 b,openings 50 are formed in theetch layer 40. Theopenings 50 are formed through theetch layer 40 and to thebarrier layer 42. Theopenings 50 may be created, for example, by forming a patternedlayer 52 on the surface of theetch layer 40 and then exposing the structure to an etching process or to other processes for removing the exposed material from theetch layer 40. The patternedlayer 52 may be photoresist, a contact mask, or some other material used to selectively expose portions of theetch layer 40. In other embodiments, the patternedlayer 52 may be omitted and the openings may be formed directly, such as through e-beam processes. - After the
openings 50 are formed, theopenings 50 are expanded in theetch layer 40 at thebarrier layer 42 to form expandedopenings 54 which will shape thetip 12 of thefibers 10. The process of forming the expandedopenings 54 will be described in more detail hereinbelow. - The
openings 50 may also be expanded 56 near thetop surface 58 of theetch layer 40, on thesurface 58 opposite thebarrier layer 42. This expandedopening 56 will shape thebase 14 of thefibers 10. By expanding 56 theopening 50, thebase 14 will be thicker than thestem 16. Furthermore, if the expandedopening 56 is given a rounded shape, it will cause the base 14 to be formed with a rounded shape, as described herein. The formation of this expandedopening 56 may, for example, be performed with an isotropic etch prior to the formation of theopening 50 and may be performed in the portions of theetch layer 40 not covered by thepatterning layer 52. - The process of forming the
openings 50 may, under some processes, form a layer ofhydrophobic material 60 on the side walls of the openings. Similarly, under some processes, afluorocarbon layer 62 may be formed on thebarrier layer 42 where theflat surface 20 of thetip 12 will be formed. Thisfluorocarbon layer 62 has been found to cause increased adhesion at theflat surface 20 of thetip 12. Thehydrophobic layer 60 is a smooth, non-stick surface which is also advantageous. The formation of theselayers - After the
openings layer 52 may be removed. - With reference to
FIG. 3 c, after theopenings 50 are formed, they are filled with amaterial 70. Thematerial 70 may be a polymer, a metal, or any other material from which thefibers 10 can be made. After theopenings 50 are filled with thematerial 70, thebacking layer 32 may be formed over theopenings 50 and on thesurface 58 of theetch layer 40 opposite thebarrier layer 42. The material 70 in theopenings 50 is connected together via thebacking layer 32, forming a single structure including a plurality offibers 10 and thebacking layer 32. - The
backing layer 32 may be formed in several ways. Thebacking layer 32 may be formed separately from thematerial 70 filling theopenings 50 and applied over theopenings 50 and on thesurface 58 of theetch layer 40. Alternatively, thebacking layer 32 may be formed from thesame material 70 as that used in theopenings 50, in which case, for example, the process of filling theopenings 50 may be allowed to continue after theopenings 50 are filled, so that thematerial 70 fills thesurface 58 over theopenings 50 to form thebacking layer 32. A mold (not shown) may be formed ontop 58 of theetching layer 40 to contain theextra material 70 used to form thebacking layer 32. In addition, a step of compressing or squeezing the material forming thebacking layer 32 may also be performed so that thebacking layer 32 is formed to a desired thickness. Other methods of controlling the thickness of thebacking layer 32 are also possible, such as by trimming or cuttingexcessive backing layer 32 material. The thickness of thebacking layer 32 can have a significant effect on the performance of thefiber array 30, and will be discussed in more detail hereinbelow - With reference to
FIG. 3 d, after the material 70 in theopenings 50 has cured or otherwise achieves a desired state, theetch layer 40 andbarrier layer 42 may be removed from thematerial 70, leaving the molded dryadhesive fibers 10 and backing later 32. In addition, thesubstrate layer 44 may also be removed if it was used. - The use of wet etch processes for the removal of the
etch layer 40 tend to cause “clumping” of, or an attraction between,adjacent fibers 10. This is caused by hydrostatic pressure from the liquid remaining from the wet etching. It has been found that dry etch processes are particularly advantageous for removing theetch layer 40 and reducing or eliminating the clumping or lateral or vertical collapsing of fibers. However, the use of a dry etch process is not required with the present invention, and other processes may be used. For example, supercritical carbon dioxide (CO2) drying can be used to release the fibers without clumping or collapsing issues where after wet etching of the etch layer, released fibers can be soaked in liquid CO2, heated and applied pressure over the supercritical point to remove CO2. - Although this embodiment of the method has been described in terms of making
several fibers 10 attached to abacking layer 32, the present invention may also be used to make asingle fiber 10, or to make a plurality offibers 10 that are not connected to abacking layer 32. Furthermore, the present invention may also include two or more layers offibers 10 orfiber arrays 30 having two or more layers offibers 10. -
FIGS. 4 a-4 h illustrate one embodiment of a method of forming a twolayer fiber array 30 according to the present invention. In this embodiment, the two layers of fibers are different sizes. The smaller fibers 110 (shown inFIGS. 4 g and 4 h) may be smaller in volume, or smaller in their dimensions. Alternatively, it is also possible to form amulti-layer fiber array 30 in which the different layers of fibers are the same size. -
FIGS. 4 a and 4 b are analogous toFIGS. 3 a and 3 b and they illustrate the first structure in which theopenings 50 are created and in which the first layer offibers 10 will be formed. -
FIGS. 4 c and 4 d illustrate the second structure in which the second layer offibers 110 is formed. In the illustrated embodiment, the second structure includes anetch layer 140 and abarrier layer 142, as well as asubstrate 144 on which theetch 140 andbarrier 142 layers are formed. Theetch layer 140 and thebarrier layer 142 are adjacent to each other and are made from different materials. The second structure may include the same or different elements as those described hereinabove with respect toFIGS. 3 a and 3 b. In other words, theopenings 150 in the second structure may have the same features as theopenings 50 in the first structure, although they differ in their size.FIG. 4 a is not necessarily to scale, and the particular dimensions of the various parts of the structure may vary depending on the particular application. -
FIGS. 4 c and 4 d are similar toFIGS. 4 a and 4 b, except that thefibers 110 to be formed in the process ofFIGS. 4 c and 4 d are smaller than those formed in the process ofFIGS. 4 a and 4 b. For example,FIG. 4 a may represent a process with a twenty μmthick etch layer 40, threeμm diameter openings 50, and twelve μm spacing betweenopening 50 centers. In contrast,FIG. 4 c may represent a process with a two μmthick etch layer 140, a 300nm diameter openings 150, and 800 nm spacing betweenopening 150 centers. - Similarly, the etching process of both
FIGS. 4 b and 4 d may be, for example, a dry etching process. That process may include isotropic etching, followed by deep reactive ion etching, and then expanding theopening barrier layer base fibers - However, variations are also possible with this aspects of the present invention and, for example, the
second fibers 110 may be larger than thefirst fibers 10. -
FIG. 4 e illustrates the structures ofFIGS. 4 b and 4 d being connected together. The dashed line illustrates where the two structures are joined together. When connected, the structures are oriented so that theopenings openings 50 in the first structure corresponding to or engage with exposedopenings 150 inFIG. 4 d. This orientation is accomplished inFIG. 4 e by rotating thetop structure 180 degrees before connecting the two together. - This connection may be formed, for example, by silicon fusion bonding or by other processes. In one embodiment, the bonding is accomplished with ten minutes of piranha cleaning to remove photoresist and anti-reflective coating. Followed by ten minutes of oxygen plasma etching to remove the film that is natively generated in the
openings -
FIG. 4 f illustrates the bonded structure after the top layers (thebarrier layer 42 and substrate 44) are removed. This may be done, for example, with a buffered oxide etching process. The resulting structure is a two layer mold open at the top where thebarrier layer 42 andsubstrate 44 used to be. -
FIG. 4 g illustrates the two layer structure after being filled with amaterial 70. As described above, thematerial 70 may by any of many different materials, and this process may be, but is not required to be, performed in a vacuum chamber. -
FIG. 4 g also illustrates that not all of thesecond openings 150 are necessarily utilized. In this embodiment, one of thesecond openings 180 is not connected to thefirst openings 50 and, therefore, thatopening 180 is not filled withmaterial 70. -
FIG. 4 h illustrates the resulting twolayer fiber array 30 after the surrounding structure or mold (the remaining etch layers 40, 140, thesecond barrier layer 142, and the second substrate 144) has been removed. The removal may be performed, for example, with a XeF2 etch to remove the bottom silicon layer and a buffered oxide etch to remove the oxide layer, as described above with respect toFIGS. 3 a-3 d. A further XeF2 etch may be used to remove the upper silicon layer. - Although the present invention has been described in terms of a two
layer fiber array 30, the present invention may also be used to produce afiber array 30 having more than two layers. Furthermore, the present invention is not limited tomultilayer fiber arrays 30, and it may also be used, for example, to make multilayer individual fibers and to make other structures. - The theory related to the present invention will now be presented. Flat and larger diameter (also referred to as cross-sectional area)
spatulate tips 12 are postulated to enhance the adhesion and work of adhesion significantly due to the increased tip contact area at the fiber-surface interface [H. Gao and H. Yao, PNAS, 101, 7851 (2004)]. In order to model the work of adhesion enhancement approximately, asingle polymer fiber 10 is assumed to be stretched while its volume is conserved. In addition, if pull-off of eachfiber tip 12 is assumed to happen simultaneously where overall pull-off force per unit area is a constant value (c1) and the elastic deformation is assumed to happen at thefiber stem 16 only where the polymer Young's modulus (E) is assumed to be constant. Then, the maximum stretched length (xc) and work of adhesion (W) of asingle fiber 10 during separation can be computed as -
- where x0 is the
initial stem 16 length, D is the fiberspatulate tip 12 diameter, and d0 is thefiber stem 16 diameter. From (1) and (2),elastomer fibers 10 withlarger diameter tips 12 elongate and dissipate energy significantly, and thus the work of adhesion perfiber 10 is increased. Moreover, adhesion is also increased by afiber array 30 with larger flatspatulate tips 12 since: (1) The fracture mechanics of the microfibers is flaw insensitive [M. Murphy, B. Aksak, and M. Sitti, Langmuir, under review (2006)] (the stress at the interface is uniform and equal to the intrinsic adhesion strength at the instant of pull-off) and thus enables the maximum possible adhesion pressure; (2) Flat and compliantspatulate tips 12 enable easier contact to a smooth surface with almost no alignment problem; (3) Fiber stretching enables larger number offibers 10 staying in contact with a smooth surface during pull-off. Therefore, this invention is focused on fabrication ofpolymer microfibers 10 with flat and largerspatulate tips 12 for fibrillar adhesives with improved adhesion capability. - One embodiment of the fabrication process according to the present invention will now be provided. The present invention is not limited to the specifics details of this embodiment, and these details are illustrative of the present invention, and not limiting.
FIG. 3 a-d illustrates one embodiment of the fabrication process of polymer microfibers with flat and larger diameterspatulate tips 12.FIG. 3 a illustrates a silicon-on-insulator (SOI) wafer (purchased from Addison Engineering) which is used as asubstrate 44 which has 20 μM thick top silicon layer as theetch layer 40 and 0.5 μm thick SiO2 layer as thebarrier layer 42. After an optical lithography step forms a desiredpattern 52 on the top 58 of theetch layer 40, the negative fiber array template is formed inFIG. 3( b). The required fiber profile shape is obtained in two steps. At first, isotropic etching is used for forming the circular supportingshape 56 of thebase 14 of eachfiber 10 to reduce the stress concentration for preventing the fracture offibers 10 at theirbases 14. Next, deep reactive ion etching (DRIE) is followed for forming vertical high aspect ratio microchannels 50. Isotropic etching and DRIE were carried out consecutively in STS Multiplex ICP RIE with 20 mT pressure, 130 sccm SF6, 20 sccm O2, 600 W coil power, and 120 W platen power. When the vertical etching reaches to thesilicon oxide layer 42, it cannot proceed vertically anymore and then starts to expand laterally 54 on the oxide interface between theetch layer 40 and thebarrier layer 42. This effect is called the notching effect [M. E. McNie, D. O. King, V. Nayar, M. C. L. Ward, J. S. Burdess, C. Quinn, and S. Blackstone, Proc. of IEEE International SOI Conference, 60 (1997)]. By controlling thelateral etching 54 time, thespatulate tip 12 diameter is determined. Then, inFIG. 3( c), the template is filled with aliquid polymer 70 under vacuum to remove not only the trapped air but also the native gas in theliquid polymer 70, and then thepolymer 70 is cured. Here, any polymer 70 (e.g. Parylene® C) or anyother fiber material 70 can be also gas phase deposited inside to thetemplate 50. InFIG. 3( d), thepolymer fiber array 30 with abacking layer 32 is released by three step etching: At first, thebottom silicon layer 44 is etched away by XeF2 dry etching; Thethin oxide layer 42 is removed by buffered oxide etching (BOE); Finally, the 20 μm thicktop silicon layer 40 is etched away by XeF2 etching in around 30 minutes to release thefibers 10 with abacking layer 32. Here, the final etching step is particularly important. Since using a wet etching technique such as KOH etching would result in clumpedfibers 10 due to capillary forces during drying, XeF2 dry etching is used to prevent any clumping issues. Moreover, since the DRIE process natively creates a hydrophobicthin submicron film 60 on thetemplate microchannel 50 side walls during the process [A. A. Ayon, R. Braff, C. C. Lin, H. H. Sawin, and M. A. Schmidt, J. of Electrochemical Society, 146(1), 339 (1999)], each releasedpolymer fiber 10 is expected to be coated with this hydrophobic and low surface energythin film 26 on their side walls 16 (not at thespatulate tip 12 surface 20). Thefilm 60 on the side walls of theopenings 50 and thefibers 10 can be characterized as a passivation film or a fluorocarbon thin film. This very lowsurface energy coating 26 could reduce the cohesion ofmicrofibers 10 significantly, and thus it could minimize any clumping during the mechanical contact of neighboringfibers 10. - Besides forming the flat
spatulate tips 12, the above fabrication process has other advantages with respect to previous fibrillar adhesive fabrication methods: (1) Fiber material can be fabricated from anypolymer 70 which can be in a liquid solution form or can be gas phase deposited; (2)Array 30 offibers 10 can be fabricated in large areas up to 8 inch wafer size cost effectively using a single mask; (3) The yield is almost 100%; (4) This method can be extended to the fabrication of hundreds ofnanometer diameter fibers 10 easily by using a higher resolution lithography step inFIG. 3 a by using phase masks or interference lithography. - High tensile strength elastomer polyurethane (ST-1060, BJB Enterprise) with Young's modulus of around 3 MPa was selected as the fiber adhesive material.
FIG. 5 illustrates a scanning electron microscope (SEM) (Hitachi 2460N) image of the resultingmicrofiber array 30 with 4.5 μm fiber diameter, 9 μm tip diameter, 4.5 μm base supporter diameter, 20 μm length, and 12 μm spacing between each fiber center (44% fiber tip area density). These geometries are held by 80 sec isotropic etching for the fiber base supporter structures and by 21min 20 sec vertical etching for the fiber andspatulate tips 12. - Performance of a fibrillar adhesive is characterized by its macroscale adhesion (P) and overall work of adhesion (W). To characterize these parameters for the fabricated
fiber arrays 30 during adhering to a glass hemisphere, a custom tensile macroscale adhesion measurement setup was built. A glass hemisphere instead of a flat glass surface was selected as the test surface in order to have no alignment errors during the measurements. A 6 mm diameter very smooth glass hemisphere (ISP Optics, QU-HS-6) attached to a load cell (Transducer Techniques, GSO-25) was moved vertically by a motorized stage (Newport, MFA-CC) with 100 nm resolution. The hemisphere was contacted to and retracted from the fiber array sample with a pre-specified preload force and a very slow speed (1 μm/s) to minimize any viscoelastic effects. The maximum tensile force during the glass hemisphere and fiber array separation (pull-off force) gave the adhesion, and the hysteresis area between the loading and unloading curves gave the dissipated energy between the loading and unloading of the fiber array. Dividing this dissipated energy by the maximum circular contact area during loading gave W10. During the force measurements, an inverted microscope (Nikon Eclipse TE200) is used to measure the real circular maximum contact area between the hemisphere and thefiber array 30. - Adhesion and overall work of adhesion of 15×15 mm2 area and one mm thick ST-1060
polyurethane fiber array 30 samples and a 1 mm thick flat and smooth ST-1060 surface were measured on the glass hemisphere using the above setup. The flat polyurethane surface was used as a control substrate to show the relative enhancement of P and W by structuring the same material as a cylindrical microfiber with flat spatulate tips. Since ST-1060 is also etched slightly during the final XeF2 dry etching step inFIG. 1( d), flat sample was also exposed to XeF2 for about 30 minutes to have the same surface roughness with the flat spatulate tip surface. - Using the above setup, the
fiber array 30 and the glass hemisphere interface adhesion and overall work of adhesion are measured as shown inFIG. 6 . Plots show the error bars measured from the force-distance data at two different locations on thefiber array 30 for preloads up to 25 mN. Adhesion values saturate as preload increases, and the array of fibers has around 4 times higher adhesion than the flat surface. Dividing the adhesion to the optically measured maximum circular contact area during loading, maximum adhesion pressure for thefiber array 30 can be computed as 18 N/cm2 at a preload pressure of 12 N/cm2. Overall work of adhesion of thefibers 10 is five times higher than the one from the flat elastomer surface. This energy dissipation enhancement is due to the lost energy during separating the elastic and highly stretchedfibers 10 from the adhered glass surface as given in (2). ST-1060 fibers stretched up to 500% strain in the experiments (observed by profile view optical imaging) which would show the reason of the enhancement of the elastic energy loss during unloading of thefibers 10. - Macroscale adhesion data from the
fiber array 30 in this work are compared with the previous works as given in Table 1. Table 1 is a comparison of adhesive strength among various natural and synthetic gecko inspired micro/nanofibers [Y. Zhao, T. Tong, L. Delzeit, A. Kashani, M. Meyyappan, and A. Majumdar, J. Vac. Sci. Techno. B, 24(1), 331 (2006)]. Thepolymer fibers 10 withspatulate tips 12 show better adhesion pressure than other synthetic gecko inspired fibrillar adhesives with nospatulate tips 12 although thesingle fiber 10 in this work is over 20 times thicker than thesingle fibers 10 which were fabricated in other works. In order to even increase the adhesion performance in this work, microfibers withtips 12 can be scaled down to hundreds of nanometers in diameter using phase mask type of sub-micron lithography techniques. In addition, N times self-similar scaling down in fiber diameter will generate {square root over (N)} times higher adhesion [E. Arzt, S. Gorb, and R. Spolenak, PNAS, 100, 10603 (2003)], and smaller fibers will need less preload than larger fibers to obtain the same adhesion. -
TABLE 1 Materials & Structures Macroscale Adhesion (N/cm2) Gecko (Tokay) foot-hairs with 10 [Autumn, Nature, 405, 681 spatulate tips (2000)] Silicone rubber fibers with 60 μm 0.003 [Sitti, J. Adhesion Science length and 200 nm diameter and Technology, 17(5), 1055 (2003) Polyurethane fibers 20-60 μm 0.5 [D. Campolo, S. Jones, and R. S. length and 200 nm diameter Fearing, Proc. of the IEEE Nanotechnology Conf., 12 (2003)] Polyimide fibers with 2 μm 3 [A. K. Geim, S. V. Dubnos, I. V. length and 500 nm diameter Grigorieva, K. S. Novoselov, A. A. Zhukov, and S. Y. Shapoval, Nature Materials, 2, 461 (2003)] Polyurethane fibers with 100 μm 3.8 [M. Murphy, B. Aksak, and M. length and 25 μm diameter Sitti, Langmuir, under review (2006)] Multi-walled carbon nanotubes 11.7 [Y. Zhao, T. Tong, L. Delzeit, with 40 μm length and 20-30 A. Kashani, M. Meyyappan, and A. nm diameter Majumdar, J. Vac. Sci. Techno. B, 24(1), 331 (2006)] Polyurethane fibers with 20 μm 18 length, 4.5 μm diameter, and 9 μm spatulate tips - The Effect of the Backing Layer Thickness.
- It has also be found that the thickness of the
backing layer 32 has a greater effect on the performance of dry adhesives than was previously known. The effect of thebacking layer 32 thickness on adhesion was investigated for single-levelelastomer fibrillar adhesives 30.Polyurethane microfiber arrays 30 withspatulated tips 12 on a 160 μmthick backing layer 32 show nine times greater adhesion strength (around 22 N/cm2) than those with a 1120 μmthick backing 32. A theoretical model is proposed to explain this difference in which very thin backing layers 32 promote equal load sharing, maximizing adhesion, while very thick backings can lead to reduced adhesion due to edge stress concentration. Therefore, backinglayer 32 thickness should be considered as a significant parameter for design of high performance fibrillar adhesives. - As discussed above, the adhesion of biologically inspired fibrillar dry adhesive has been studied extensively in combination with developments of various fabrication methods. Based on dominant forces of van der Waals [K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full, PNAS, 99, 12252 (2002)] and possibly capillary [G. Huber, H. Mantz, R. Spolenak, K. Mecke, K. Jacobs, S, N. Grob, and E. Artz, PNAS, 102(45), 16293 (2005)] forces, vertical cylindrical micro/nanofiber arrays [A. K. Geim, S. V. Dubnos, I. V. Grigorieva, K. S, Novoselov, A. A. Zhukov, and S. Y. Shapoval, Nature Materials, 2, 461 (2003)] were proposed as fibrillar adhesives at first. Design parameters for these fibers were proposed as the fiber radius, aspect ratio [C. Greiner, A. del Compo, and E. Arzt, Langmuir, 23, 3495 (2007)], tip shape [H. Gao and H. Yao, PNAS, 101, 7851 (2004)], and material properties [K. Autumn, C. Majidi, R. E. Groff, A. Dittmore, and R. Fearing, J. Exp. Biol., 209, 3558 (2006)]. Inspired by footpads of various animals in nature such as insects and geckos, spatulated tips on single-level cylindrical [S. Kim and M. Sitti, Applied Physics Letters, 89, 261922 (2006)][N. J. Glassmaker, A. Jagota, C-Y. Hui, & J. Kim, J. R. Soc. Interface, 1, 22 (2004)], angled [B. Aksak, M. P. Murphy, and M. Sitti, Langmuir, 23, 3322 (2007)] and hierarchical [N. J. Glassmaker, A. Jagota, C—Y. Hui, W. L. Noderer, M. K. Chaudhury, PNAS, 104, 10786 (2007)][A. del Campo and E. Arzt, Molecular Bioscience, 7(2), 118 (2007)] fibers were introduced for developing high performance fibrillar adhesives. In addition, one of the recent findings demonstrates that the real contact perimeter is a more important geometrical factor governing adhesion than the real contact area [M. Varenberg, A. Peressadko, S. Gorb, and E. Arzt, Applied Physics Letters, 89, 121905 (2006)]. However, the role of
backing layer 32 thickness on adhesion has not been investigated in detail so far. - The
backing layer 32 thickness effect on adhesion of elastomeric single-level microfiber structures 30 will now be described. Although athick backing layer 32 improves the roughness adaptation andfiber 10 contact abilities due to increased effective compliance, this study shows that athick backing layer 32 could reduce the macroscale adhesion of thefibers 10 on smooth surfaces significantly. - We measured the pull-off force of single-level
elastomer fiber array 30 samples withdifferent backing layer 32 thicknesses and developed a theoretical model to explain the observed results. Polyurethane (ST-1060, BJB Enterprise)fiber array 30 samples withspatulated tips 12 are fabricated using the procedure reported in S. Kim and M. Sitti, Applied Physics Letters, 89, 261922 (2006). Briefly, we first fabricate negative silicon fiber array templates using deep reactive ion etching as described hereinabove.Liquid polyurethane 70 is filled into these siliconnegative templates 50 and cured. Thesilicon templates 50 are then etched using XeF2, and thefibers 10 are released. Thefinal backing layer 32 thickness of each sample is determined by regulating the gap between the negative template and a glass slide on it. - All
fiber arrays 30 in the samples have astem 16 diameter of around five μm and atip 12 andbase 14 support diameter of nine μm. The total length of afiber 10 is 20 μm and the minimum spacing between fiber centers is 12 μm as displayed inFIG. 7( a). A custom tensile setup was built to measure the adhesion of the samples. The measure of adhesion in this work is the pull-off load. A silicon disk with 0.43 mm radius and nanometer scale surface roughness was fabricated by patterning a polished silicon (100) wafer using optical lithography and deep reactive ion etching. The silicon disk was attached to a load cell (Transducer Techniques, GSO-25), and moved vertically by a motorized stage (Newport, MFA-CC) with 100 nm resolution. The disk was contacted to and retracted from the fiber array sample with specified preload forces and a slow speed (1 μm/s) to minimize viscoelastic effects. The maximum pull-off force was recorded. In addition, the contact area and the deformation offiber array 32 during loading and unloading was recorded using a camera (Dage-MT1, DC330) attached to an inverted optical microscope (Nikon, Eclipse TE200). - Adhesion of four samples with 160, 280, 630, and 1120
μm backing layer 32 thicknesses was measured and is shown inFIG. 7( b). Pull-off forces were measured at five different locations on eachfiber array 30 for a preload of ten mN. The sample with the thinnest backing layer 32 (160 μm) showed the highest adhesion, with average pull-off force about nine times greater than that of the 1120μm layer 32. - Our interpretation of this surprising finding, that reduced compliance enhances adhesion, lies in the idea that a
thinner backing layer 32 promotes equal sharing of the load by thefibers 10. As shown schematically inFIG. 8( a), if adisplacement 8 were applied to threefibers 10 on an infinitesimallythin backing layer 32, each one would experience the same vertical tension, Fy1. If the fiber pulled-off at a characteristic force Ff=kfδf where kf is the stiffness of thefiber 10 and δf is thefiber 10 elongation at pull-off, then the pull-off force of the system would be 3Ff. If thethin layer 32 were replaced by a very thick backing 32 (FIG. 8( b)), the y-direction forces would be Fy1=kfδ and Fy2=kfδ+FB where FB is the additional force required to keep thetip 12 attached to the disk due to non-uniform deformation of thebacking layer 32. Because thebacking layer 32 is elastic, FB=αkfδ for some positive α. The twofibers 10 on the side will pull-off when kfδ+FB=Ff and the total force at pull-off would be (3-α)Ff. - To quantify this idea for a large number of
fibers 10 in contact, we note that the spacing of thefibers 10 are typically very small in comparison with the contact radius a and the thickness of the elastic layer h. Hence, we can treat thesefibers 10 as a foundation consisting of elastic springs between the rigid indenter and thebacking layer 32. The foundation can support a normal stress σ, which is related to the displacement of foundation, d by σ=kd where k is the stiffness of the foundation. Note that d is the difference in normal displacement between the surface of the indenter and thebacking layer 32. The stiffness can be determined by assuming that thefibers 10 are bars with height L and effective cross-sectional area Aeff, k=ρEAeff/L where ρ is the number of fibrils per unit area. From the known geometry and stiffness of the fibers, k=2.37×10−10 N/m3 - where ρ=1/(12×10−6)2 fibers/m2, E=3 MPa, Aeff=πr2, r=2.5 μm, and L=14 μm.
- The maximum pull-off force occurs in the equal load sharing (ELS) regime, where all the fibers in adhesive contact with the indenter bear the same load. To see how ELS depends on the backing layer thickness and the contact area, assume that all the fibers in contact are in this regime, so at pull-off, we have
-
σf kδ f (3) - In the ELS limit, the maximum pull-off force Fmax is directly proportional to the contact area,
-
F max=πα2σf (4) - where a is the radius of the disk. The ELS limit is strictly valid if the backing layer thickness h is very small compared to a. Another limit is a very thick backing layer with very stiff fibers, that is, when h/aΘ∞ and α≡ka/2G is very large where G is the shear modulus. In this limit, the interfacial displacement is dominated by the deformation of the elastic layer and the stress distribution is given by the classical solution of a rigid punch in contact with a half space [K. L. Johnson, Contact Mechanics, Cambridge University Press (1985)]. The normal stress at the punch edge has a square root singularity characteristic of an opening crack. For α>>1 and h/a>>1, the pull-off force Fad in this limit can be derived as
-
F ad=4F max/(2πα)1/2 (5) - This equation shows that, given Fmax, the maximum extent of strength reduction can be predicted. The data in
FIG. 7( b) show a decrease in pull-off force with increasing thickness. Theoretically, in the limit of very small thickness, pull-off force should asymptotically approach the ELS limit, which depends on the unknown fiber pull off stress, σf. In this limit, the pull-off load is significantly affected by variability in σf. This is reflected in the scatter of the pull-off force data associated with the sample with the smallest h/a. - The theoretical problem of determining pull-off forces as a function of α and h/a is more involved and will be addressed in a future work.
- In summary,
polyurethane microfiber arrays 30 withspatulated tips 12 on 160 μmthick backing layer 32 show adhesion strength (around 22 N/cm2), nine times greater thanfiber arrays 30 with thickness of 1120 μm. A theoretical model is proposed to explain this difference in which very thin backing layers 32 promote equal load sharing, maximizing adhesion. In the other extreme, verythick backings 32 can lead to reduced adhesion, because of edge stress concentration similar to a rigid punch in adhesive contact with a half space. This work shows the significance ofbacking layer 32 thickness on equal load sharing of single-level microfiber arrays 30 on smooth surfaces. - The present invention describes a method for fabrication of polyurethane elastomer microfiber arrays with flat spatulate tips. For a preload pressure of around 12 N/cm2, adhesion pressures up to 18 N/cm2 and overall work of adhesion up to 11 J/m2 are demonstrated for polyurethane fibers with 4.5 μm fiber diameter, 9 μm tip diameter, 20 μm length, and 44% fiber tip area density on a 6 mm diameter glass hemisphere. These repeatable fibrillar adhesives would have wide range of applications as space, biomedical, sports, etc. adhesives.
- Although the present invention has generally been described in general terms and in terms of specific embodiments and implementations, the present invention is applicable to other methods, apparatuses, systems, and technologies. For example, the present invention can be used with a variety of materials, such as metals, ceramics, other polymers, Paralyne, carbon, crystals, liquid crystals, Teflon, semiconductors, piezoelectric materials, conductive polymers, shape memory alloy materials, and organic materials, and these and other materials could be deposited or molded inside the
etch layer 40 as described hereinabove. Furthermore, thebase 14 of thefibers 10 may not be necessary in some cases, in which case the base may be omitted or it may be considered to be the part of thefiber 10 attached to another structure. Thespatulate tip fibers 10 with or without a hydrophobic surface coating can be used as a superhydrophobic surface where the water contact angle could be increased more due to spatulate tip geometry and fiber spacing. During the fiber tip formation (etching), if the etching time is long enough the tip endings could combine and fibers with a continuous flat thin-film can be formed as another type of fiber based adhesives or materials. Different fiber cross-section geometry (square, ellipsoid, triangle, etc.), base geometry (pyramid, etc.), tip diameter, fiber packing geometry (hexagonal or square), high or low aspect ratio, and constant or variable fiber density is possible with the present invention. Although the etch layer removal has generally been described in terms of a dry etch process, it is still possible to use a wet etching method according to the present invention. Thespatulate fibers 10 can be used as static friction enhancing materials in addition to enhanced adhesion materials. Micro or nanoscale patterning methods such as interference lithography, electron-beam lithography, nanoimprinting, directed self-assembly, dip pen lithography, laser micro- or nano-machining, micro/nano-milling, and extreme UV lithography can be used to pattern the etch layer for fabricating micro- or nanoscale fibers with spatulate tips. -
FIG. 9 illustrates the stem-fiber angle δ and the tip wedge angle α. The stem-fiber angle δ and the tip wedge angle α are controlled angles during the silicon removal process. The range of these angles is 1-180 degrees and 1-179 degrees, respectively. Stem-fiber angle δ is the angle between the stemouter surface 16 b and the tipouter surface 12 b, which varies depending on the location of the point along the tipouter surface 12 b. Tip wedge angle α is the angle between the tipouter surface 12 b and the tipflat surface 20, which varies depending on the location of the point along the tipouter surface 12 b. The three illustrations of thefiber 10show tips 12 with different tip heights (h1, h2, h3) and tip diameters or cross sections (d2, d3, d4) relative to stems 16 with the same stem diameter or cross section (d1). Any ratio of diameters to height are acceptable. Thetips 12 can have a curvature (as shown in the figures) or a straight line (not shown) between the tip flat surface edge (or outer perimeter of the tip diameter) 20 a and thejunction 16 a oftip 12 andstem 16. The angles are controlled during the silicon removal process. After the vertical removal process reaches to the silicon oxide (barrier) layer, these angles could be controlled by specifically tuning and controlling the silicon removal step duration, removal ion type, and removal ion energy level parameters. -
FIGS. 11A and 11B illustrate where the cross sectional area of the stem of the plurality of fibers includes a plurality of cross sectional areas along the longitudinal length, where the plurality of cross sectional areas comprise a first cross sectional area larger than a second cross sectional area to form a non-uniform stem diameter along the longitudinal length.FIG. 11B illustrates the first cross sectional area is adjacent to thebacking layer 14 and the second cross sectional area is adjacent to thetip 12 of the eachfiber 16 of the plurality of fibers.FIG. 11A illustrates the first cross sectional area is adjacent to thetip 12 of the eachfiber 16 of the plurality of fibers and the second cross sectional area is adjacent to thebacking layer 14.FIGS. 11A and 11B provide examples and are not meant to limit the present invention. The non-uniform cross-sectional areas can also be applied to single-level fiber embodiments (shown inFIGS. 1 , 2, 8, and 10), and to second layersmaller fibers 110. -
FIG. 13 illustrates the second plurality of fibers having oblique angles relative to the tip including an angle between the tip of the each fiber of the secondary plurality of fibers and the stem of the each fiber of the secondary plurality of fibers not being equal to 90 degrees. - 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.
Claims (39)
1. A dry adhesive fiber structure, comprising:
a dry adhesive fiber including:
a tip having a surface and an end opposite the surface; and
a stem having (i) a distal end connected to the end of the tip at a tip point of connection, (ii) a proximal end opposite the distal end, (iii) a longitudinal length from the distal end to the proximal end, and (iv) a plurality of cross-sectional areas along the longitudinal length.
2. The dry adhesive fiber structure according to claim 1 , wherein the surface further comprises a flat surface.
3. The dry adhesive fiber structure according to claim 1 ,
wherein the tip further comprises a plurality of tip cross-sectional areas;
wherein the plurality of tip cross-sectional areas increase from the tip point of connection to the surface of the tip;
wherein the stem further comprises a stem cross-sectional area of the plurality of cross-sectional areas at the tip point of connection; and
wherein the stem cross-sectional area of the stem at the tip point of connection is equal to or less than a tip cross-sectional area of the plurality of tip cross-sectional areas of the tip at the tip point of connection.
4. The dry adhesive fiber structure according to claim 1 , wherein the plurality of cross sectional areas of the stem have same cross sectional areas to form a uniform stem cross-sectional area along the longitudinal length.
5. The dry adhesive fiber structure according to claim 1 , wherein the plurality of cross sectional areas of the stem have different cross sectional areas to form a non-uniform stem cross-sectional area along the longitudinal length.
6. The dry adhesive fiber structure according to claim 1 , further comprising a backing layer connected to the proximal end of the stem.
7. The adhesive fiber array according to claim 6 , further comprising an angle between the stem and the backing layer being equal to 90 degrees.
8. The adhesive fiber array according to claim 6 , further comprising an angle between the stem and the backing layer not being equal to 90 degrees.
9. The adhesive fiber array according to claim 1 , further comprising an angle between the tip and the stem being equal to 90 degrees.
10. The adhesive fiber array according to claim 1 , further comprising an angle between the tip and the stem not being equal to 90 degrees.
11. The dry adhesive fiber structure according to claim 1 , wherein the surface of the tip comprises a layer of fluorocarbon.
12. The dry adhesive fiber structure according to claim 1 ,
further comprising a base having a flat surface, an end opposite of the flat surface, and a plurality of base cross-sectional areas,
wherein the end of the base is connected to the proximal end of the stem at a base point of connection,
wherein the plurality of base cross-sectional areas increase from the base point of connection to the flat surface of the base;
wherein the stem further comprises a stem cross-sectional area at the base point of connection; and
wherein the stem cross-sectional area at the base point of connection is equal to or less than a base cross-sectional area of the plurality of base cross-sectional areas at the base point of connection.
13. The dry adhesive fiber structure according to claim 1 , wherein the stem further comprises a surface having a hydrophobic and low surface energy layer on the surface of the stem.
14. The dry adhesive fiber structure according to claim 1 , wherein the tip and the stem are made of a material selected from the group consisting of liquid polymer and gas phase polymer.
15. The dry adhesive fiber structure according to claim 12 , wherein the base is made of a material selected from the group consisting of liquid polymer and gas phase polymer.
16. The dry adhesive fiber structure according to claim 1 ,
wherein the tip further comprises a tip longitudinal axis;
wherein the stem further comprises a stem longitudinal axis; and
wherein the tip longitudinal axis and stem longitudinal axis are coaxial, thereby forming a symmetric orientation between the tip and the stem.
17. The dry adhesive fiber structure according to claim 12 , further comprising a backing layer connected to the flat surface of the base.
18. The dry adhesive fiber structure according to claim 1 ,
wherein the plurality of cross sectional areas of the stem further comprises at least two stem cross-sectional areas.
19. The dry adhesive fiber structure according to claim 1 , further comprising a plurality of second layer fibers connected to the surface of the tip of the dry adhesive fiber.
20. The dry adhesive fiber structure according to claim 1 , further comprising two or more dry adhesive fibers connected to a backing layer to form a dry adhesive fiber array.
21. The dry adhesive fiber structure according to claim 20 , further comprising a plurality of second layer fibers connected to at least one dry adhesive fiber of the dry adhesive fiber array.
22. The dry adhesive fiber structure according to claim 1 , further comprising a second layer fiber connected to the tip of the dry adhesive fiber, wherein the second layer fiber comprises:
a second layer fiber tip having a surface and an end opposite the surface; and
a second layer fiber stem having (i) a distal end connected to the end of the second layer fiber tip at a second layer tip point of connection, and (ii) a proximal end opposite the distal end.
23. The dry adhesive fiber structure according to claim 22 , wherein the surface of the second layer fiber tip further comprises a flat surface.
24. The dry adhesive fiber structure according to claim 22 ,
wherein the second layer fiber tip further comprises a plurality of second layer fiber tip cross-sectional areas;
wherein the plurality of second layer fiber tip cross-sectional areas increase from the second layer fiber tip point of connection to the surface of the second layer fiber tip;
wherein the second layer fiber stem further comprises a second layer fiber stem cross-sectional area at the second layer tip point of connection; and
wherein the second layer fiber stem cross-sectional area of the second layer fiber stem at the second layer tip point of connection is equal to or less than a second layer fiber tip cross-sectional area of the plurality of second layer fiber tip cross-sectional areas at the second layer fiber tip point of connection.
25. The dry adhesive fiber structure according to claim 22 , wherein the surface of the second layer fiber tip comprises a layer of fluorocarbon.
26. The dry adhesive fiber structure according to claim 22 ,
further comprising a second layer fiber base having a flat surface, an end opposite the flat surface, and a plurality of second layer fiber base cross-sectional areas,
wherein the end of the second layer fiber base is connected to the proximal end of the second layer fiber stem at a second layer fiber base point of connection,
wherein the plurality of second layer fiber base cross-sectional areas increase from the second layer fiber base point of connection to the flat surface of the second layer fiber base,
wherein the second layer fiber stem further comprises a cross-sectional area at the second layer fiber base point of connection, and
wherein the cross-sectional area of the second layer fiber stem at the second layer fiber base point of connection is equal to or less than a second layer fiber base cross-sectional area of the plurality of second layer fiber base cross-sectional areas at the second layer fiber base point of connection.
27. The dry adhesive fiber structure according to claim 22 , wherein the second layer fiber stem further comprises a surface having a hydrophobic and low surface energy layer on the surface of the second layer fiber stem.
28. The dry adhesive fiber structure according to claim 22 , wherein the second layer fiber tip and the second layer fiber stem are made of a material selected from the group consisting of liquid polymer and gas phase polymer.
29. The dry adhesive fiber structure according to claim 26 , wherein the second layer fiber base is made of a material selected from the group consisting of liquid polymer and gas phase polymer.
30. The dry adhesive fiber structure according to claim 22 ,
wherein the second layer fiber tip further comprises a second layer fiber tip longitudinal axis;
wherein the second layer fiber stem further comprises a second layer fiber stem longitudinal axis; and
wherein the second layer fiber tip longitudinal axis and the second layer fiber stem longitudinal axis are coaxial, thereby forming a symmetric orientation between the second layer fiber tip and the second layer fiber stem.
31. The dry adhesive fiber structure according to claim 22 ,
wherein the second layer fiber tip further comprises a second layer fiber tip longitudinal axis;
wherein the second layer fiber stem further comprises a second layer fiber stem longitudinal axis; and
wherein the second layer fiber tip longitudinal axis and the second layer fiber stem longitudinal axis are not coaxial, thereby forming an asymmetric orientation between the second layer fiber tip and the second layer fiber stem.
32. The dry adhesive fiber structure according to claim 22 , wherein the second layer fiber stem further comprises a uniform cross-sectional area along a longitudinal length of the stem.
33. The dry adhesive fiber structure according to claim 22 , wherein the second layer fiber stem further comprises a non-uniform cross-sectional area along a longitudinal length of the stem.
34. The dry adhesive fiber structure according to claim 22 ,
wherein the second layer fiber stem further comprises a plurality of second layer fiber stem cross-sectional areas; and
wherein the plurality of second layer fiber stem cross-sectional areas increase from the distal end of the second layer fiber stem to the proximal end of the second layer fiber stem.
35. The dry adhesive fiber structure according to claim 22 ,
wherein the second layer fiber stem further comprises a plurality of second layer fiber stem cross-sectional areas; and
wherein the plurality of second layer fiber stem cross-sectional areas decrease from the distal end of the stem to the proximal end of the second layer fiber stem.
36. The dry adhesive fiber structure of claim 1 , further comprising a base having a longitudinal length, wherein the base is connected to the proximal end of the stem.
36. The dry adhesive fiber structure of claim 1 , further comprising a base having a longitudinal length, wherein the base is connected to the proximal end of the stem.
37. The dry adhesive fiber structure of claim 1 , further comprising a stem-fiber angle δ that ranges between 1-180 degrees.
38. The dry adhesive fiber structure of claim 1 , further comprising a tip wedge angle δ that ranges between 1-179 degrees.
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US8524092B2 (en) * | 2006-12-14 | 2013-09-03 | Carnegie Mellon University | Dry adhesives and methods for making dry adhesives |
US8206631B1 (en) * | 2008-09-18 | 2012-06-26 | Carnegie Mellon University | Methods of making dry adhesives |
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WO2008076391A2 (en) | 2008-06-26 |
US8524092B2 (en) | 2013-09-03 |
US20140010988A1 (en) | 2014-01-09 |
US20140065347A1 (en) | 2014-03-06 |
US10774246B2 (en) | 2020-09-15 |
US20100136281A1 (en) | 2010-06-03 |
WO2008076391A3 (en) | 2008-10-09 |
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