具体实施方式：
[0091]Aspects and embodiments disclosed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. Aspects and embodiments disclosed herein are capable of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,”“comprising,”“having,”“containing,”“involving,” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
Micro-Scale Dry Adhesive Structures
[0092]Aspects and embodiments disclosed herein are generally directed to the formation of novel synthetic “dry adhesive” structures (the term dry adhesive comprising both adhesive and/or friction enhancing structures) and methods and apparatus for making same. Adhesive and/or friction enhancing structures disclosed herein may include micro-scale elements, for example, elements having characteristic dimensions of less than about 100 μm, and are thus referred to herein as micro-scale dry adhesive structures. An example of an embodiment of a micro-scale dry adhesive structure including a pattern of micro-elements is illustrated in FIG. 1A. The micro-scale dry adhesive structure 1 includes a plurality of micro-elements, microwedges 10, disposed on a backing 15. The microwedges 10 may have heights h of about between about 80 μm and about 120 μm and bases b with widths of between about 20 μm and about 40 μm, and length of between about 120 μm and about 160 μm. As illustrated in FIG. 1A, the microwedges may include leading edges 10l angled at an angle Γ of between about 20 degrees and about 65 degrees from a line or plane p defined by an upper surface 15s of the backing 15b or the bases of the microwedges. The microwedges may include trailing edges 10t angled at an angle α of between about 35 degrees and about 85 degrees from line or plane p. The microwedges may include centerlines 1 that bisect the microwedges and that are angled at an angle β of between about 30 degrees and about 70 degrees from line or plane p.
[0093]The microwedges 10 may have asymmetric tapers about their center lines 1. Tips t of the microwedges 10 may extend over the leading edges 10l of adjacent microwedges 10 and adjacent microwedges may define re-entrant spaces 10r defined below a trailing edge 10t of a first microwedge and above a leading edge 10l of a second microwedge 10 adjacent the first microwedge 10. These dimensions and angular ranges are examples, and aspects and embodiments disclosed herein are not limited to microwedge structures having these particular dimensions or angles.
[0094]Embodiments of the micro-scale dry adhesive structures disclosed herein may be formed from a polymer, for example, polydimethylsiloxane (PDMS), another silicone, polyurethane, or another polymeric material. Specific examples of polyurethanes that embodiments of the adhesive structures disclosed herein may be formed include M-3160 A/B polyurethane and L-3560 A/B polyurethane, available from BJB Enterprises. In some embodiments, the material from which embodiments of the micro-scale dry adhesive structures disclosed herein may be formed exhibit a Shore A hardness of between about 40 and about 60.
[0095]In some embodiments, the microwedges 10 of the micro-scale dry adhesive structure 1 may include an extra layer of cured material on the tips of the microwedges forming an adhesion and/or friction enhancing layer 20 (hereinafter “enhancement layer 20”), as illustrated in FIG. 2A, FIG. 2B and in the micrograph of FIG. 3. In some embodiments, the enhancement layers 20 have smoother surfaces than the microwedges 10 and may be added to the microwedges to increase the smoothness of portions of the microwedges proximate tips t of the microwedges 10. The enhancement layers 20 may be formed of an elastomeric material. The enhancement layers 20 may be formed from the same material as the remainder of the microwedges 10, but in some embodiments, may be formed of a different material that that of the remainder of the microwedges 10. The enhancement layers 20 may have smooth surfaces, as illustrated in FIG. 2A, FIG. 2B, and FIG. 3, but in other embodiments, may be patterned, for example, with ridges, columns, or other patterns. In some embodiments, the enhancement layers 20 may be present on only portions of a leading edges 10l of the microwedges 10, or in other embodiments may be present on both trailing edges 10t and leading edges 10l of the microwedges 10. (FIG. 2B.) The enhancement layers 20 may terminate at lips at the intersection of the enhancement layers 20 and the microwedges 10, for example, at the step illustrated in FIG. 3 at the bottom of enhancement layer 20 as it transitions to microwedge 10.
[0096]In some embodiments, the bases b of individual microwedges 10 may be spaced from one another, as illustrated in FIG. 1A, for example, by between about 0 μm and about 30 μm, and in other embodiments, for example, as illustrated in FIG. 2B, the leading edge 10l of a first microwedge may intersect a trailing edge 10t of a second microwedge 10 adjacent to the first microwedge 10 at bases b of the microwedges 10.
[0097]In some embodiments, the micro-scale dry adhesive structure may be mounted on a rigid base substrate, for example, a substrate including layers of carbon fibers and plywood, to provide the micro-scale dry adhesive structure with enhanced mechanical stiffness and/or to maintain the microwedges 10 in a substantially same plane.
[0098]In some embodiments, micro-scale dry adhesive structures as illustrated in FIGS. 1-3 may be formed by a micromachining process, for example, by cutting material from a surface of a support or other substrate to form the microwedges. Due to the large number of microwedges that may be included in some embodiments of micro-scale dry adhesive structures (from thousands to millions), serial micromachining processes may be too slow to be practical for the production of large numbers of micro-scale dry adhesive structures. In other embodiments, micro-scale dry adhesive structures as illustrated in FIGS. 1-3 may be formed using microlithography and etching techniques as known in the semiconductor industry. Such microlithography and etching techniques, however, are often complex and costly and may have difficulty fabricating microwedge arrays with re-entrant profiles as desired in some implementations. Accordingly, processes that involve forming micro-scale dry adhesive structures by molding have been developed.
Wax Molds for Micro-Scale Dry Adhesive Structures
[0099]In some embodiments, wax molds may be utilized for the production of micro-scale dry adhesive structures. In such embodiments, a wax mold base is formed into a desired size and shape, for example, by casting in a metallic, e.g., aluminum, mold, and a negative micro-element pattern is formed in the upper surface of the wax mold using, for example, a microtome blade or other micromachining tool. A material from which a micro-scale dry adhesive structure is desired to be formed, for example, a PDMS, silicone, or urethane material, is applied to the mold and allowed to cure. After curing, the cured material is removed from the mold with a positive micro-element pattern formed on the surface of the material that was in contact with the mold.
[0100]An example of a wax mold and supporting structure for casting micro-scale dry adhesive structures is illustrated in FIGS. 4A and 4B. The wax mold includes a wax section 25 formed as a frustum, for example, a truncated pyramid with a trapezoidal cross section. The wax section of the wax mold 25 may be formed from machinists wax or another material suitable for a particular implementation. Machinists wax, also known as machining wax or machinable wax, is a hard wax with a high melt point that has been formulated to deliver exceptional machining properties with high resolution detail. In some embodiments, epoxies or polyurethanes can be poured directly onto the surface of a mold formed from machinable wax without the need for sealers or release agents to provide for the epoxies or polyurethanes to release from the wax mold upon curing. Some formulations of machinable wax may have a hardness rating ranging from about Shore D 45 to about Shore D 58. In some embodiments, machinable wax is formulated from paraffin, polyethylene, and optionally, one or more additional components with relative amounts of the various components adjusted depending on desired properties, for example, hardness of the machinable wax. In other embodiments, the wax section 25 may be formed in the shape of a truncated cone or any other shape having one or more tapered sidewalls.
[0101]The wax mold further includes a base plate 30 to which wax section 25 is secured with a window frame shaped retainer 35. The retainer 35 has a tapered surface 40 with a taper corresponding to the taper of sides of the wax section 25 of the wax mold. The retainer 35 is secured to the base plate with one or more fasteners 45, for example, bolts, screws, or other fasteners known in the art. The retainer 35 interfaces with side walls of the wax section 25 of the wax mold to secure the wax section 25 of the wax mold to the base plate 30 for machining and casting of micro-scale dry adhesive structures.
[0102]In some embodiments, the wax section 25 is deposited in liquid form directly onto the base plate 30 and surrounded by a mold having a similar shape as retainer 35, but having a height extending to the level of the upper surface 55 of the wax section 25 and allowed to cool and harden. The mold is then removed and retainer 35 attached to the base plate 30 to hold the wax section 25 in place on the base plate.
[0103]In other embodiments, the wax section 25 may be formed in a mold not in contact with the base plate 30 and the lower surface 50 of the wax section 25 of the wax mold may be smoothed or planarized prior to mounting on the base plate 30 to minimize or eliminate gaps between lower surface 50 and the base plate 30 that might provide for the wax section 25 of the wax mold to deform as a pattern is cut into the upper surface 55 of the wax section 25 of the wax mold and/or during casting of a micro-scale dry adhesive structure on the wax section 25 of the wax mold. Additionally or alternatively, a layer of liquid wax may be provided on the lower surface 50 of wax section 25 of the wax mold and/or on the base plate 30 as the wax section 25 of the wax mold is being mounted to the base plate 30 to fill in any gaps or surface roughness on the lower surface 50 of the wax section 25 of the wax mold.
[0104]Once the wax section 25 of the wax mold is secured to the base plate 30, a negative micro-element array pattern, for example, a pattern for an array of microwedges may be formed on the top surface 55 using a microtome or other micromachining tool. A friction reducing or lubricating agent, for example, a mixture of detergent and water (e.g., a mixture of Ajax® liquid dish soap, available from Colgate-Palmolive and water) may be applied to the top surface 55 of the wax section 25 of the wax mold during formation of the micro-element array pattern to aid in insertion of the microtome or other micromachining tool into the wax section 25 of the wax mold and/or removal of the microtome or other micromachining tool from the wax section 25 of the wax mold. The friction reducing or lubricating agent reduces friction between a micromachining tool used to machine the wax section 25 and reduces surface roughness of machined features as compared to features machined without the use of the friction reducing or lubricating agent.
[0105]In some embodiments, the micro-element array pattern is machined in the wax section 25 of the wax mold along with features that provide for standoffs to be formed in micro-scale dry adhesive structures formed on the wax mold. An example of this is illustrated in FIGS. 5A-5C. FIGS. 5A-5C and 6A and 6B illustrate a mold with a wax section 25 having substantially vertical sides, however, it should be appreciated that a mold with a wax section 25 with tapered sides, such as that illustrated in FIGS. 4A and 4B may alternatively be used. In FIG. 5A, the unmachined wax section 25 of the wax mold is illustrated disposed on a base plate 30 or frame. In a first machining act, a generally centralized region 55c of the top surface 55 of the wax section 25 of the wax mold is machined to include a first recess 60 to accommodate the backing 15 of the micro-scale dry adhesive structures to be formed from the wax mold and one or more deeper recesses 65 to form standoffs on the microwedge adhesive structures to be formed from the wax mold. Patterns 70 for the micro-elements (e.g., microwedges) are then formed in a second centralized region 55d (within the first centralized region 55c) of the top surface 55 of the wax mold. The patterns 70 extend more deeply into the wax section 25 of the wax mold than the recesses 65. The patterns 70 may be negative microwedge patterns having the same or similar dimensions and angles as the positive microwedges 10 discussed above with reference to FIG. 1B.
[0106]After formation of the micro-element array pattern 70 on the top surface 55 of the wax section 25 of the wax mold, the wax mold may be cleaned to remove residual surfactant. In one embodiment of a mold cleaning operation, as shown in FIG. 6A, the base plate 30 including the machined wax section 25 of the wax mold is placed in a wash tub 75 in a sink 80 at an angle. Deionized (DI) water 85 which could be at room temperature or could be heated below the melting point of the wax is flowed into the wash tub 75 to rinse the wax mold. The DI water 85 overflows out of the wash tub 75 into the sink 80 during the rinse of the wax mold. The wax mold is rinsed with the DI water for about 20 minutes or until it is verified that the water runoff is clear with no surfactant bubbles. Once the rinse is complete, the wax mold is removed from the water bath and the water is drained from the wash tub 75 and sink 80. The wax mold is then returned to the wash tub 75. The wax mold should be placed so that it is perched on one end so it is almost standing up, as opposed to laying flat down inside the tub 75. As shown in FIG. 5B, isopropanol 90, for example, about 500 mL of isopropanol is flushed across the wax mold. The wax mold is blown dry with low pressure N2, keeping N2 directed at an angle across the wax surface (along the micro-element or microwedge direction).
[0107]After machining and/or washing of the wax mold, the machined upper surface 55 of the wax mold may be coated with a release agent 57 (illustrated in FIG. 5C) that will facilitate release of cured micro-scale dry adhesive structures from the wax mold. The release agent may be, for example, REPEL-SILANE™ (a 2% solution of dimethyldichlorosilane dissolved in octamethylcyclooctasilane, available from multiple vendors) release agent, or another release agent known in the art.
Polymer/Epoxy Molds for Micro-Scale Dry Adhesive Structures
[0108]In some implementations, it may be desirable to provide molds for microwedge adhesive structures that are more durable than molds formed from wax as described above. The use of durable molds for casting micro-scale dry adhesive structures may facilitate higher volume manufacturing than the use of wax molds due to a higher number of micro-scale dry adhesive structures that may be cast from a durable mold as compared to a wax mold prior to the mold beginning to show signs of deterioration and require reconditioning or replacement. Accordingly, some aspects disclosed herein include durable molds for micro-scale dry adhesive structure casting that are formed of materials stronger than machinists wax and methods of formation of same.
[0109]In some embodiments, molds for the casting of micro-scale dry adhesive structures as described herein may comprise or consist of a hard polymer, for example, an epoxy. In one particular embodiment, molds for the casting of micro-scale dry adhesive structures may comprise or consist of CONATHANE® epoxy and/or CONAPDXY® epoxy, low-shrinkage epoxies available from, for example, Cytec, Inc.
[0110]To form an epoxy mold for micro-scale dry adhesive structure casting, a known good micro-scale dry adhesive structure 1, for example, a micro-scale dry adhesive structure 1 formed in a wax mold as described above, is adhered to a rigid plate 120, for example a glass plate or other form of rigid flat plate. As illustrated in FIGS. 7A and 7B, a roller 125 including a rigid tube 130 covered with a compliant layer 135, for example, neoprene may be used to apply the micro-scale dry adhesive structure 1 to the rigid plate 120, squeezing the micro-scale dry adhesive structure 1 as it is applied to the rigid plate 120 to minimize the formation of air bubbles between the micro-scale dry adhesive structure 1 and the rigid plate 120. The micro-scale dry adhesive structure 1 may adhere to the rigid plate 120 by static electrical attraction, van der Waals forces, or by use of a temporary adhesive, for example, REVALPHA™ thermal release tape.
[0111]After the known good micro-scale dry adhesive structure 1 is adhered to a rigid plate 120, a dam 200 of material, for example, silicone or PDMS, is formed about the micro-scale dry adhesive structure 1 on the rigid plate 120 (FIG. 8). Optionally, REVALPHA™ thermal release tape or another release agent is disposed on the upper surface of the rigid plate 120 prior to forming or adhering the dam 200 to the rigid plate 120 to aid in removal of the dam 200 and/or epoxy mold from the rigid plate 120 after forming the epoxy mold. In some embodiments, the back surface (the surface not including the micro-element pattern) of the known good micro-scale dry adhesive structure 1 is cleaned, for example, in an O2 plasma prior to being adhered to the rigid plate 120. Optionally, a release layer, for example, REPEL-SILANE™ or vapor deposited trichlorosilane may be deposited on the surface of the rigid plate 120 and the micro-scale dry adhesive structure 1 within the boundaries of the dam 200. A mold material, for example, a polymer or epoxy 205 (e.g., CONATHANE® epoxy and/or CONAPDXY® epoxy) is then poured onto the surface of the rigid plate 120 and the micro-scale dry adhesive structure 1 within the boundaries of the dam 200 (and over the optional release layer, if used) (FIG. 9). In some embodiments, the polymer or epoxy 205 is degassed in a vacuum before and/or after pouring onto the surface of the rigid plate 120 and the micro-scale dry adhesive structure 1 within the boundaries of the dam 200 to facilitate a reduction or elimination of air bubbles in the polymer or epoxy 205. Additionally or alternatively, pressure may be applied to the polymer or epoxy 205, for example, by placing the structure illustrated in FIG. 9 in a high pressure environment, during curing of the polymer or epoxy 205 to facilitate a reduction or elimination of air bubbles in the cured polymer or epoxy 205.
[0112]In some embodiments, as illustrated in FIG. 10, a rigid plate 210, for example a glass plate or other flat surface, optionally wrapped in a release layer 215 is placed on top of the poured polymer or epoxy 205. The release layer 215 is selected to adhere less strongly to the polymer or epoxy 205, when cured, than the rigid plate 210 would adhere to the cured polymer or epoxy 205. In other embodiments, the release layer 215 is selected to not adhere to the rigid plate 210 and to be flexible when not adhered to the rigid plate 210 so that it may be more easily peeled from the cured polymer or epoxy 205 than the rigid plate 205 may be removed from the cured polymer or epoxy 205. The release layer 215 may comprise or consist of, for example, a polymer sheet (e.g., a sheet of polyvinyl chloride (PVC) or low density polyethylene (LDPE)), Saran™ plastic wrap, or aluminum foil. In some embodiments, the rigid plate 210 is cleaned, for example, with isopropanol, a plasma clean (e.g., in a CF4/O2 plasma), or a wet clean (e.g., in a H2O2/H2SO4 solution), prior to being wrapped in the release layer 215. In embodiments in which the rigid plate 210 is transparent or translucent, the rigid plate 210 may be pushed down on until the dam 200 is visible through the rigid plate 210. In some embodiments, a roller or other device is utilized to eliminate any air bubbles between the release layer 215 and the rigid plate 210 prior to placing the rigid plate 210 on the poured polymer or epoxy 205. A weight, for example, a pair of five pound weights 220, is placed on top of the rigid plate 210 (FIG. 11). The weights 220 may apply pressure to the poured polymer or epoxy 205 to aid in reducing or eliminating the formation of air bubbles in the poured polymer or epoxy 205 during curing. The polymer or epoxy 205 is left to cure. In some embodiments, heat may be applied to the polymer or epoxy 205 to accelerate curing, for example by placing the structure shown in FIG. 11 in an oven at about 80° C. for about 16 hours, or for a time and temperature suitable for the material and dimensions of the polymer or epoxy 205 layer. In other embodiments, the polymer or epoxy 205 is left to cure at room temperature. In some embodiments, the structure shown in FIG. 11 is placed in a pressure chamber at a pressure of, for example, between about 30 psi and 50 psi, while the polymer or epoxy 205 cures to aid in reducing or eliminating the formation of air bubbles in the poured polymer or epoxy 205 during curing.
[0113]After the polymer or epoxy 205 has cured, the weight(s) 220 are removed, the release layer 215 is unwrapped from the rigid plate 210, and the rigid plate 210, the release layer 215, and the dam 200 is removed from the rigid plate 120 (FIG. 12). The rigid plate 120 is removed, for example, by applying sufficient heat to cause a thermal release tape applied between to the rigid plate 120 and the micro-scale dry adhesive structure 1 to release, and the micro-scale dry adhesive structure 1 is peeled out of the cured polymer or epoxy 205 to leave a formed polymer or epoxy mold 220 (FIG. 13). The formed polymer or epoxy mold 220 may include negative microwedge patterns 70 having the same or similar dimensions and angles as the positive microwedges 10 discussed above with reference to FIG. 1B. The polymer or epoxy mold 220 be used to cast micro-scale dry adhesive structures.
Metal Molds for Micro-Scale Dry Adhesive Structures
[0114]In accordance with a further aspect a mold for casting micro-scale dry adhesive structures that is more durable than a polymer or epoxy mold may be formed from a metal or metal alloy. In some embodiments, the metal mold may be formed by electroforming, micromachining, or a combination of the two.
[0115]A process for electroforming a metal mold for casting micro-scale dry adhesive structures is illustrated beginning at FIG. 14. As illustrated in FIG. 14, a known good micro-scale dry adhesive structure 1, for example, a micro-scale dry adhesive structure 1 formed in a wax mold as described in U.S. patent application Ser. No. 13/451,713, and optionally mounted on a backing substrate 225, is secured to and/or in a plating fixture 230. In some embodiments, a cavity 235 is formed in the plating fixture to receive the backing substrate 225.
[0116]In other embodiments, where the micro-scale dry adhesive structure 1 is not mounted on a backing substrate 225, the micro-scale dry adhesive structure 1 may be directly adhered to a flat upper surface 240 of the plating fixture 230 using any of a variety of adhesives known in the art, for example, double-stick tapes (e.g., REVALPHA™ thermal release tape, Nitto Denko Corporation) or glues (e.g., Sil-Poxy® silicone rubber adhesive, Smooth-On Inc.). A roller including a rigid tube covered with a compliant layer, for example, neoprene may be used to apply the micro-scale dry adhesive structure 1 to the plating fixture 230, squeezing the micro-scale dry adhesive structure 1 as it is applied to the plating fixture 230 to minimize the formation of air bubbles between the micro-scale dry adhesive structure 1 and the plating fixture 230.
[0117]The plating fixture 230 may comprise steel or any other rigid, and optionally, conductive, material. In some embodiments, the backing 15 of the micro-scale dry adhesive structure 1 may extend above the upper surface 240 of the plating fixture 230, for example, by about 0.027 inches (about 0.06 cm) to set a uniform 0.027 inch recess into the finished metal mold to form the backing 15 of additional micro-scale dry adhesive structures 1 from the finished metal mold.
[0118]A fillet 245, for example, an epoxy fillet, may be formed at the interface 250 between side walls of the backing 15 of the micro-scale dry adhesive structure 1 and the plating fixture 230. The epoxy fillet 245 is used to fill any gaps that might be present between the micro-scale dry adhesive structure 1 and the cavity 235 of the plating fixture 230 to prevent metal from being electroformed in any such gaps and forming undesired features on an electroformed mold or that may make it difficult to release the completed electroformed mold from the plating fixture 230.
[0119]As illustrated in FIG. 15, the micro-elements 10 of the micro-scale dry adhesive structure 1 may be coated with a release layer 250 that will aid in releasing a metal mold electroformed on the micro-scale dry adhesive structure 1 from the micro-scale dry adhesive structure 1. In some embodiments, an adhesion layer 255 is first deposited on the micro-scale dry adhesive structure 1 to facilitate adhesion of the release layer 250 to the micro-scale dry adhesive structure 1. In some embodiments, the release layer 250 may include or consist of polytetrafluoroethylene (PTFE) or REPEL-SILANE™ and the adhesion layer 255 may include or consist of chromium and/or titanium. The adhesion layer 255 may be deposited on the micro-scale dry adhesive structure 1 by, for example, sputtering. The release layer 250 may be deposited on the adhesion layer 255 and/or micro-scale dry adhesive structure 1 by, for example, initiated chemical vapor deposition (iCVD) for PTFE, or vapor deposition for REPEL-SILANE™.
[0120]A seed metal layer 260, for example, a layer of molybdenum or copper, is deposited onto the release layer 250 or micro-scale dry adhesive structure 1 (FIG. 16, release layer 250 and adhesion layer 255 not shown for clarity) and the body 265 of the metal mold is formed on the seed layer 260, for example, by electroplating (FIG. 17, seed layer not visible). The body 265 of the metal mold may be the same metal as that of the seed layer 260 or a different metal, for example, copper, aluminum, steel, or a metal alloy.
[0121]The metal mold is then removed from the micro-scale dry adhesive structure 1 and plating fixture, resulting in a completed metal mold 270 (FIG. 18). The metal mold 270 may be inspected and in some embodiments, micromachining, for example, with a diamond tool or other micromachining tool to remove defects, to smooth surfaces of the metal mold 270, or to otherwise finish the metal mold 270. In some embodiments, a release agent, for example, PTFE, REPEL-SILANE™, or trichlorosilane may be coated on surfaces of the metal mold 270. The machined metal mold 270 may include negative microwedge patterns 70 having the same or similar dimensions and angles as the positive microwedges 10 discussed above with reference to FIG. 1B.
[0122]In other embodiments, the metal mold 270 may be used as an injection mold insert. The metal mold 270 may be placed in an injection molding apparatus in an opposed position to a backing substrate 225. A polymer material may be injected into the space between the metal mold 27 and the backing substrate 225 to form a micro-scale dry adhesive structure mounted on a backing substrate 225 in a single injection molding operation.
[0123]In other embodiments, a metal mold 270 for casting micro-scale dry adhesive structures may be formed without the use of a pre-fabricated micro-scale dry adhesive structure by directly machining a metal block 275. For example, a metal block 275 may optionally be roughly machined by standard micromachining tools, for example, micro-milling bits made from tool steel or polycrystalline diamond stock (˜0.001″-0.010″ in diameter), to form an array of wedge stubs 280 with a desired orientation, wedge angle and pitch. In some embodiments, cutouts between adjacent wedges may have dimensions, for example widths, about 10 μm to about 20 μm less than the cutouts that will be used to mold microwedges in a finished mold. A diamond tool or other fine finishing tool (formed from, for example silicon carbide or tool steel) may be used to further process the metal block 275 to form finished microgrooves 285 and complete the metal mold 270 (FIG. 19). Additionally or alternatively, a 3D printer may be utilized to form the array of wedge stubs 280 on the metal block 275. Electroplating may be performed on the 3D printed array of wedge stubs 280 to fill in voids left by the 3D printing operation and/or to smooth the array of wedge stubs 280. A diamond tool or other fine finishing tool may be used to further process the metal block 275 to form finished microgrooves 285 from the 3D printed array of wedge stubs 280 and complete the metal mold 270. Alternatively, the diamond or other fine finishing tool may be used to directly form wedge cutouts in a metal layer without first forming stubs (with the potential for more wear on the tool).
Casting of Micro-Scale Dry Adhesive Structures
[0124]An embodiment of a method for casting a micro-scale dry adhesive structure from a mold, for example, any of a wax mold, a polymer mold, an epoxy mold, or a metal mold as disclosed above, is illustrated in FIGS. 20A-20F. These figures illustrate casting micro-scale dry adhesive structure on a wax portion 25 of a wax mold, but it should be understood that substantially similar acts would be performed when casting a micro-scale dry adhesive structure from a mold formed of a different material, for example, a polymer, epoxy, or metal. FIG. 20A illustrates a casting material 95, for example, a polymer in liquid form, from which a micro-scale dry adhesive structure is to be formed, deposited onto the top surface 55 of a wax portion 25 of a wax mold. In some embodiments, the polymer comprises or consists of PDMS. In other embodiments, the polymer may comprise or consist of a silicone, a urethane or another polymer selected for an intended implementation. It should be understood that in some embodiments in which a wax mold it utilized, the wax mold may configured as illustrated and described with reference to FIGS. 4A and 4B, rather than as shown in FIGS. 20A-20F.
[0125]As shown in FIG. 20B, a rigid plate 100, for example, a glass plate or other flat surface wrapped in a release layer 105 is placed on top of the poured casting material 95. The release layer 105 is selected to adhere less strongly to the polymer material 95, when cured, than the rigid plate 100 would adhere to the cured polymer material 95. In other embodiments, the release layer 105 is selected to not adhere to the rigid plate 100 and to be flexible when not adhered to the rigid plate 100 so that it may be more easily peeled from the cured casting material 95 than the rigid plate 100 may be removed from the cured casting material 95. The release layer 105 may comprise or consist of, for example, a polymer sheet (e.g., a sheet of polyvinyl chloride (PVC) or low density polyethylene (LDPE)), Saran™ plastic wrap, or aluminum foil. In some embodiments, the rigid plate 100 is cleaned, for example, with isopropanol, a plasma clean (e.g., in a CF4/O2 plasma), or a wet clean (e.g., in a H2O2/H2SO4 solution), prior to being wrapped in the release layer 105. The rigid plate 100 is pushed down on the wax mold. In implementations where the rigid plate 100 is transparent or translucent, the rigid plate 100 is pushed down on until the wax portion 25 of the wax mold is visible through the rigid plate 100. In some embodiments, a roller or other device is utilized to eliminate any air bubbles between the release layer 105 and the rigid plate 100 prior to placing the rigid plate 100 on the poured casting material 95. A weight 110, for example, a five pound weight, is placed on top of the rigid plate 100 (FIG. 20C). The casting ma