Patent Application
20120328822
The present invention relates to dry adhesives, and methods for making dry adhesives including, for example, fibrillar microfibers and nanofibers.
Nature provides endless inspiration for solutions to engineering challenges. Particularly at the small (sub-millimeter) scale, millions of years of evolution has resulted in fascinating structures with unique, sometimes non-intuitive properties. In the case of small agile climbing animals, fibrillar foot-pads as a solution for gripping surfaces has evolved many times. Similar structures are present in animals of different phyla, including arthropods (spiders, insects), and chordates (lizards), suggesting independent evolution. There is also evidence that these structures evolved independently within different types of lizards (Geckos, Anoles, and Skinks), with slightly different resulting structures [D. Irschick, A. Herrel, and B. Vanhooydonck, “Whole-organism studies of adhesion in pad-bearing lizards: creative evolutionary solutions to functional problems,” Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology, vol. 192, no. 11, pp. 1169-1177, 2006].
There exist a wide variety of fibrillar adhesives across the wide variety of animals, which utilize these structures. Some insects have fibrillar foot pads which secrete oily fluids which aid in adhesion, while others have completely dry structures. Adhesive pads which do not utilize secretions are called “dry adhesives,” as they leave no residue on the surfaces to which they adhere. Dry adhesives exhibit many unique adhesive properties. They act similar to a pressure sensitive adhesive such as tape, but are highly repeatable with long lifetimes, do not require cleaning, and, often in combination with small claws, adhere to surfaces which are anywhere from atomically smooth silicon to extremely textured rock. Furthermore, they exhibit directional properties, adhering in one direction, and easily releasing from the surface when loathed in another. Adhesion pressures as high as 200 kPa have been demonstrated for gecko subdigital toepads and single fiber (seta) measurements exhibited adhesion pressures greater than 500 kPa (50 N/cm2) [K. Autumn, “Biological Adhesives,” Springer Berlin Heidelberg, 2006]. Using advanced fibrillar adhesives, several gecko species are capable of carrying up to 250% of their own body weight up a smooth vertical surface. Dry fibrillar adhesives are also quite power efficient. They can be attached and detached from surfaces with very low forces by means of special loading and peeling motions. Once adhered to a surface, they require no power to maintain contact, and resist detachment for long periods of time.
Interestingly, and against intuition, the material that makes lip these high performance adhesive footpads is not sticky at all. The fibers are made from a β-keratin, much like bird claws and feathers. It is the small size-scale and geometrical structure, which allows this material to act as a powerful and versatile attachment mechanism.
Mechanics of Fibrillar Adhesion
The hair-like structures of gecko footpads have fascinated scientists for well over a century, with various hypotheses about the mode of attachment. In 1884, Simmermacher proposed the hypothesis that gecko lizards might adhere to surfaces using micro-suction cups [G. Simmermacher, “Untersuchungen ber haftapparate an tarsalgliedem von insekten,” Zeitschr. Wiss. Zool, vol. 40, pp. 481-556, 1884]. Fifty years later, Dellit carried out experiments in a vacuum winch demonstrated that suction is not the dominant attachment mechanism in geckos [W. D. Dellit, “Zur anatomie and physiologie der geckozehe,” Jena. Z. Naturw, vol. 68, pp. 613-656, 1934]. Similarly, electrostatic adhesion and micro-interlocking were ruled out. It was not until the advent of the Scanning Electron Microscope that scientists were able to investigate the true structure of these microscopic features. What they observed is a forest of microscale fibers, each branching into finer and finer hairs, ending in spatula-like tips. It is this structure that turns the stiff keratin into a capable adhesive.
Conventional pressure sensitive adhesives such as adhesive tapes, gels, and soft elastomers function by deforming into the shape of the contacting surface when pressed into contact. Materials with very low Young's modulus (stiffness) conform to surfaces to create large contact areas and do not store enough elastic energy to induce separation from the surface after the loading is removed. However, due to their low modulus, these materials tend to pick up contaminants from the surface, and are typically not re-usable.
Stiffer materials do not easily conform to surface roughness, and if deformed into intimate contact through high loading, store enough elastic energy to return to their original shape, peeling away from the surface in the process of relaxation. Bulk stiff materials generally do not exhibit tackiness or adhesion due to this self-peeling behavior.
The structures found in the attachment pads of the animals described above consist of arrays of thousands or millions of hair-like fibers, which stanch vertically or at an angle from the pad surface. Each fiber acts independently and generally has a specialized tip structure. The hairs in these fibrillar adhesives conform to the roughness of the climbing surface to increase the real contact area much like the deformation of soft adhesive tape, resulting in high adhesion by surface forces [K. Autumn et al., “Adhesive force of a single gecko foot-hair,” Nature, vol. 405, pp. 681-685, 2000]. This adhesion, called dry adhesion, is argued to arise from molecular surface forces such as van der Waals forces [K. Autumn et al., “Evidence for van der Waals adhesion in gecko setae,” Proceedings of the National Academy of Sciences USA, vol. 99, pp. 12252-12256, 2002], possibly in combination with capillary forces [G. Huber et al., “Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements,” Proceedings of the National Academy of Sciences USA, vol. 102, pp. 16293-16296, 2005]. Although the total potential contact area of a surface broken up into fibers is less than the area of a flat surface because of the gaps between the fibers, the ability for each fiber to bend and conform to the surface roughness allows thousands, millions or billions of fibers no make small individual contacts, which add up to a large surface area. In comparison, a flat surface only makes contact with the asperities of a surface, and since the deformations of bulk material are typically small, the total contact area is much less than in the fibrillar case. An illustration of this comparison can be seen in
Because of the high aspect ratio (height to diameter) of the fibers in
Animals with very low mass, such as insects, generally have a simply micro-fiber structure with specialized tips. In large lizards such as the Tokay gecko the fibers take on a complicated branched structure with microscale (4-5 μm) diameter base fibers which branch down to sub-micron (200 nm) diameter terminal fibers. At the end of these terminal fibers are specialized tips.
The most advanced fibrillar dry adhesives are found in the heaviest animals such as the Tokay and New Caledonia Giant Gecko gecko which can weigh up to 300 grams. Gecko toes have been shown to adhere with high interfacial shear strength to smooth surfaces (88−200 kPa). These animals have adhesive pads with many levels of compliance including their toes, foot tissue, lamellae, and fibers. This multi-level hierarchy allows the adhesives pads to conform to surface roughness with various frequency and wavelength scales. The fibers are angled with respect to the animals' toes, and the branched tips are also oriented with respect to the base of the fiber. The result is that the gecko pad exhibits a high level of directional dependence, high adhesion while dragging the toe inwards, and no adhesion in the opposite direction. This directionality is sometimes referred to as frictional anisotropy or, more appropriately, directional adhesion.
Studies of gecko footpads have revealed that due to their asymmetric angled structure, they are non-adhesive in their resting state, and a dragging motion is required to induce adhesive behavior [K. Autumn et al., “Frictional adhesion: a new angle on gecko attachment,” Journal of Experimental Biology, vol. 209, pp. 3569-3579, 2006]. Reversing the direction of this dragging motion removes the fibers from the surface with very little force.
Motivation for Fabrication of Dry Adhesives
The dry fibrillar adhesive structures found in nature exhibit properties, which may be highly desirable in synthetic materials. The mechanics which gives rise to the adhesion in these structures does not rely on liquids or pressure differentials, therefore fibrillar dry adhesives are uniquely suited for a variety of uses. Since dry adhesives leave no residue and can grip over large areas, they could be used as grippers for delicate parts for transfer and assembly of anything from computer chips in a clean-room to very large porous carbon-fiber panels for vehicle construction.
If manufactured inexpensively, synthetic dry adhesives could also find uses in daily life as a general adhesive tape for hanging items, fastening clothing, or as a grip enhancement in athletic activities such as gloves, shoes, and grips.
Man-made dry adhesives might be used for temporary attachment of structures during assembly, or allow astronauts to grip the smooth outer surfaces of spacecraft during extravehicular missions.
Since biological dry adhesives allow animals to climb on smooth surfaces, synthetic dry adhesives should enable robots to do the same. Robots with dry adhesive grippers may be used for inspection and repair of spacecraft hulls, or terrestrial structures. Since the adhesives require no power to remain attached, climbing robots could perch for days, weeks, or months with very little power usage. Also, due to the power efficient attachment and detachment, robots might move as easily up a wall as they currently traverse the ground. Similarly, one day, gloves covered in synthetic dry adhesives might allow humans to easily scale smooth vertical surfaces.
There are potential applications for fibrillar adhesives in the field of medicine as well. Safe, non-destructive temporary tissue adhesives could assist in surgical procedures. Capsule endoscopes might use fibrillar adhesives to anchor to intestine walls without damaging the tissue in order to closely examine or biopsy an area of interest. Fibrillar adhesives may also be designed for attachment to skin as an alternative to conventional adhesive bandages and patches.
In 2000, when Autumn et al. published work measuring the adhesion of a single gecko seta, suggesting that it is the van der Waals intermolecular forces dominantly, which allow geckos to climb, it spawned a field of research into understanding and modeling the underlying principles of fibrillar adhesion, and fabricating synthetic mimics. Soon after, Autumn et al. demonstrated van der Waals forces and a unique geometry are primarily responsible for the adhesion. Sitti and Fearing created the first synthetic fibrillar adhesives by silicone rubber micromolding in the same year [M. Sitti and R. S. Fearing, “Nanomolding based fabrication of synthetic gecko foot-hairs,” In Proceedings of the IEEE Nanotechnology Conference, pp. 137-140, 2002].
In the years since then, there has been a flurry of research, with more than 50 publications on the topic in 2007 alone. Autumn continues to test biological specimens which provide insights into the mechanisms of adhesion, self-cleaning [W. R. Hansen and K. Autumn, “Evidence for self-cleaning in gecko setae,” Proceedings of the National Academy of Sciences USA, vol. 102, no. 2, pp. 385-389, 2005], and the directional properties of real gecko setae.
Huber and Sun demonstrated evidence that suggests that capillary forces of ambient water layers on surfaces play a significant role in fibrillar adhesion. Contact mechanics researchers such as Persson, Crosby, and Hui have investigated the crack trapping nature of patterned and fibrillar surfaces, which they have shown to increase the adhesion and toughness of the interfaces. In addition, Hui studied the bending and buckling nature of fibrillar surfaces, and the effects of this behavior on the adhesion of simple pillars. Arzt has investigated the effects of scale and shape of natural fibrillar adhesives, concluding that tip shape has less importance at smaller size scales. Several groups have demonstrated an inverse correlation between animal size and fibril tip dimension, with the heaviest animals having the finest fiber structures [E. Arzt, S. Gorb, and R. Spolenak, “From micro to nano contacts in biological attachment devices,” Proceedings of the National Academy of Sciences USA, vol. 100, no. 19, pp. 10603-10606, 2003].
The mechanics of fiber to fiber interactions have been studied and modeled to determine the proper spacing and patterning for a high density of fibers without clumping. Fibers will clump together if the adhesion energy between neighboring fibers is greater than the stored elastic energy of the fibers-bending into contact. The resulting equations can be used to calculate the closest spacing without permanent collapse.
The effects of crack trapping on increasing the toughness and adhesion of fibrillar surfaces have been studied on the macro-scale as well as the micro-scale. Several structures have been tested, and show enhancement over non-fibrillated structures.
The roughness adaptation of gecko pads has also been investigated through testing and modeling. The mechanics of fiber deformation and buckling reveals that fibrillar structures can decrease the effective modulus of the surface by several orders of magnitude, allowing conformation to various rough and curved surfaces.
In addition to research to understand and model the mechanics of adhesion, several research groups have developed fabrication techniques to create synthetic fibrillar arrays. Since van der Waal's forces are universal, a wide variety of materials and techniques may be used to construct the fibers. Initially, simple vertical fiber arrays were fabricated from various materials such as polymers. Methods such as electron-beam lithography, micro/nanomolding, nanodrawing, and self-assembly are employed to fabricate fibers from polymers, polymer organorods, and multi-walled carbon nanotubes.
Generally, arrays of simple pillar structures were not effective in increasing the adhesion of surfaces. Significant adhesion enhancement was demonstrated only when the flat tips of the structures were fabricated to have higher radii for increased contact area. Gorb et al. fabricated polyvinylsiloxane fibers with thin plate flat mushroom tips which demonstrated adhesion enhancement as well as contamination resistance [S. Gorb et al., “Biomimetic mushroom-shaped fibrillar adhesive microstructure,” Journal of The Royal Society Interface, vol. 4, pp. 271-275, 2007]. Similarly, Del Campo et al. developed techniques for forming flat mushroom tips as well as more complex 3D geometries, including asymmetric tips, by dipping [A. Del Campo et al., “Patterned surfaces with pillars with controlled and 3d tip geometry mimicking bioattachment devices,” Advanced Materials, vol. 19, pp. 1973-1977, 2007]. Kim et al. developed fabrication methods to form microscale fibers with flat mushroom tips by exploiting the champagne glass effect during Deep Reactive Ion Etching to form negative templates in silicon on oxide wafers [S. Kim and M. Sitti, “Biologically inspired polymer microfibers with spatulate tips as repeatable fibrillar adhesives,” Applied Physics Letters, vol. 89, no. 26, pp. 261911, 2006]. In addition, Kim demonstrated the importance of controlling the thickness of the backing layer in order to reduce coupling between fibers.
Glassmaker et al. fabricated polymer fibers topped with a terminal film which exhibited adhesion enhancement over tipless pillars and unstructured surfaces [Nicholas J. Glassmaker et al., “Biologically inspired crack trapping for enhanced adhesion,” Proceedings of the National Academy of Sciences, vol. 104, pp. 10786-10791, 2007]. Angled pillars with a terminal film have also been fabricated with directional properties [H. Yao et al., “Adhesion and sliding response of a biologically inspired fibrillar surface: experimental observations,” Journal of The Royal Society Interface, vol. 5 no. 24, pp. 723-733 2007]. By angling the pillars beneath the terminal film, the resultant structure exhibits anisotropic adhesion. In addition to stern angle, the angle of the surface of the tip with respect to the stem is as crucial in terms of controlling the anisotropic behavior in adhesion and friction. Kim et al. [S. Kim et al., “Smooth Vertical Surface Climbing With Directional Adhesion,” IEEE Transactions on Robotics, vol. 24, no. 1, pp. 1-10, 2008] fabricated synthetic sub-millimeter wedges with the stern and tip surface of each individual wedge oriented at an angle with respect to the backing layer of the wedge array. These structures exhibited anisotropic friction much-like the biological counterparts. While the magnitude of friction was an order of magnitude less than the biological gecko footpads, adhesion in normal direction was negligible. Later Asbeck et al. [A. Asbeck et al., “Climbing rough vertical surfaces with hierarchical directional adhesion,” IEEE International Conference on Robotics and Automation, Kobe, Japan, 2009] fabricated similarly shaped wedges that are an order of magnitude smaller which showed similar adhesion performance to the sub-millimeter wedges. Adhesion improvement, still low compared to the biological gecko footpad, occurred when they topped sub-millimeter wedges with a terminal film comprised of micro-wedges.
Higher modulus synthetic fibrillar adhesives have been developed on the sub-micron diameter scale. These fibers, made from stiffer materials (E>1 GPa) such as polypropylene, polyimide, and nickel, carbon nanofibers and carbon nanotubes. Although these stiffer fibers do not adhere well in the normal direction, and require high preloads to make intimate contact, shear adhesion pressures of up to 36 N/cm2, which is higher than the adhesion strength of the gecko, have been demonstrated.
To more closely mimic the structure of the gecko's foot hairs, work has also been done to fabricate hierarchical fibers with multi-level structures. Ge et al. bundled carbon nanotubes into pillars which deform together but have individually exposed tips. [L. Ge et al., “Carbon nanotube-based synthetic gecko tapes,” Proceedings of the National Academy of Sciences, vol. 104, no. 26, pp. 10792-10795, 2007]. Photolithography has been used to fabricate simple micro-pillars on top of base pillars [A. Del Campo and E. Arzt, “Design parameters and current fabrication approaches for developing bioinspired dry adhesives,” Macromolecular Bioscience, vol. 7, no. 2, pp. 118-127, 2007]. On the millimeter scale, Shape Deposition Manufacturing has been used to fabricate hierarchical structures in thin polymer plates, which are stacked into arrays [M. Lanzetta and M. R. Cutkosky, “Shape deposition manufacturing of biologically inspired hierarchical microstructures,” CIRP Annals—Manufacturing Technology, vol. 57, pp. 231-234, 2008]. Kustandi et al. demonstrated a fabrication technique to use nanomolding in combination with micromolding to create a hierarchical structure with superhydrophobic properties.
In addition to dry adhesives, other work is being conducted on synthetic fibers with oily coatings, inspired by beetle adhesion, which exhibit increased adhesion over uncoated structures.
The microfiber fabrication methods described above are very expensive for producing commercial quantities of adhesive materials. Moreover, they cannot efficiently and controllably produce angled fibers with specialized tips or hierarchical structures with specialized tips. Accordingly, there is a need for improved dry adhesives and improved methods for making dry adhesives. In particular, there is a need for dry adhesives having greater adhesive forces and improved durability. In addition, there is a need for methods of making dry adhesives with lower costs of production. Those and other advantages of the present invention will be described in more detail hereinbelow.
The present invention includes adhesives, methods for making adhesives, and fibers made according to those methods. Many embodiments are possible with the present invention. For example, the present invention provides methods to fabricate fibrillar structure which have specialized tips that increase adhesion, and provide directionality to adhesion. Methods are described to fabricate fibrillar structures with angled tips. Methods are also provided to fabricate hierarchical fibrillar structures.
The present invention provides methods for fabrication of vertical and angled micro-, and nanofibers with adhesive qualities. The present invention further provides methods for the fabrication of micro- and nanofibers that have specialized tips or are hierarchically structured with specialized tips. Polymer micro- and nanofiber arrays are fabricated through a micro molding process which duplicates lithographically formed master template structures with a desired fiber material. This technique enables fabrication of fiber arrays inexpensively and with high yields, and enables the fabrication of fibers with controlled angles. In the present invention, the fiber ends are then dipped in a polymer solution, prior to further processing which create specialized and hierarchical tips to the fibers.
In one embodiment, after the dipping in a polymer solution, the assembly is then pressed against a surface at a pre-determined angle to fabricate flattened tips at the ends of the fibers.
In another embodiment, fibers are fabricated using the methods of the present invention in different sizes, for example microfibers and nanofibers, and the smaller fibers are attached to the tips of the larger fibers by making contact with the liquid polymer at the end of the larger fibers to create hierarchical structures.
In another embodiment, fibers are fabricated according to the methods herein, dipped in a polymer solution, which is in turn molded to fowl hierarchical structures with smaller fiber structures attached to the tips of the larger fibers.
In another embodiment, the methods described herein are used to fabricate three-level hierarchical fiber structures.
In another embodiment, fibers are fabricated using the methods of the present invention, the fibers are then dipped in a polymer solution, and the assembly is pressed against an array of smaller fibers, such as carbon nanotubes, to form hierarchical structures.
There are several unique aspects to the fiber design described in this application. One is an enlarged and oriented terminal end or tip of the fiber. The enlarged tip increases the contact area of the fiber thus enhancing the interfacial resistance to separation between the fiber and the adhering surface. This shape also allows for more uniform distribution of the applied stress over the fiber tip surface [A. V. Spuskanyuk et al., “The effect of shape on the adhesion of fibrillar surfaces,” Acta Biomaterialia, vol. 4, no. 6, pp. 1669-1676, 2008]. Another design aspect is the incorporation of sharp edges at the perimeter of the tip. The detachment of a single fiber usually starts from the edge as an edge crack followed by the propagation of this edge crack along the entire interface, which results in complete separation. The crack starts at the edge due to the fact that the edge acts as a stress concentrator and creates a singular stress state. For instance, when a soft fiber is in contact with a relatively rigid smooth surface, the stress at the edge of the fiber (o′e) is singular and has the fowl:
o′
e
=Ao′c
−a (1)
In equation (1), u is the applied stress far from contact, A is a constant determined by the shape of the fiber, c is the distance from the edge of the fiber, and a is the order of stress singularity determined by the angle at the edge of contact [D. B. Bogy, “Two edge-bonded elastic wedges of different materials and wedge angles under surface traction,” Journal of Applied Mechanics, vol. 38, pp. 377-386, 1971]. Note that at the edge of contact, c=0, stress is infinite. According to (1), it is possible to reduce the severity of stress singularity and as such improve detachment resistance by reducing A and a. Enlarged tip shape featured in our fiber design allows for lower A values and reduces the severity of stress at the edge. In addition, sharper edges at the perimeter of the tip lower the order of stress singularity a adding another dimension of stress reduction at the contact edge. According to the work by Bogy, it is also possible to eliminate the stress singularity via sharper contact edges. While the oriented fashion of the stem and the base provides us with the directional properties, enlarged tip with sharper edges improve performance in both gripping and releasing direction. Furthermore, we obtain high adhesion in normal direction, which is not achievable with wedge designs [S. Kim et al., “Smooth Vertical Surface Climbing With Directional Adhesion,” IEEE Transactions on Robotics, vol. 24, no. 1, pp. 1-10, 2008; A. Asbeck et al., “Climbing rough vertical surfaces with hierarchical directional adhesion,” IEEE International Conference on Robotics and Automation, Kobe, Japan, 2009].
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:
a and 1b illustrate the contact area of a flat material and a fibrillar material against a rough surface;
e provide data and microphotographs of force vs. time data for double level vertical fibrillar materials fabricated according to the present invention; and
Gecko toes have been shown to adhere with high interfacial shear strength to smooth surfaces (88-200 kPa), using microscale angled fiber structures on their feet. However, even with such large adhesion pressures, the detachment forces measured during climbing are nearly nonexistent. The gecko is able to release its adhesive toes without overcoming the large adhesion forces, which it relies on to climb and cling to surfaces. These animals are able to control the amount of adhesion of its footpads during climbing by controlled motions during detachment.
Autumn et al. demonstrated that natural gecko setae exhibit extreme frictional anisotropy, with significant adhesive friction when dragged along their natural curvature (‘gripping’ or ‘with’ direction), and only Coulomb friction in the ‘releasing’ or ‘against’ direction. When loaded in the ‘releasing’ direction, the adhesive pads are easily peeled from the surface. We fabricated angled fibers with un-oriented mushroom tips to mimic this directional behavior, but no significant anisotropy was observed. Yao et al. observed directional adhesion and shear interface strength in angled sub-millimeter diameter PDMS stalks with a terminal film. Kim et al. have demonstrated sub-millimeter diameter angled polymer stalk arrays with angled ends, for use in a climbing robot, which exhibit desirable anisotropic shear forces. However, both of these larger-scale structures demonstrate significantly lower interfacial shear and adhesion strength than the gecko or microscale polymer fibers with mushroom tips.
Fibrillar structures have also been fabricated to increase (or decrease) friction. In addition, fiber surfaces have been created which provide shear adhesion using vertical arrays of single and multi-walled carbon nanotubes. Unfortunately, these fibrillar structures require very high preloads in order to provide interfacial shear strength. Stiff polypropylene sub-micron diameter fibers have been shown to exhibit shear adhesion without requiring high preloading. Polyurethane micro-fibrillar structures have demonstrated interfacial shear strength of over 400 kPa, but due to these high forces, irreversible damage occurs during detachment.
In this invention, we describe fabrication methods and structures that combine the high interfacial strength of mushroom tipped micron-scale fibers with the directionality of fiber structures with both angled stalks and tip endings. In Section 1.2, the fabrication techniques are detailed for single level structures. Experimental results are presented in Section 1.3, including investigation of adhesion anisotropy and adhesion control. In Section 1.4.2, the fabrication techniques for multi-level structures are detailed. Experimental results for the multi-level structures are provided in Section 1.4.6.
1.1 The Structure.
The present invention includes a variety of structures for dry adhesives.
The stems 22 are attached to the backing layer 20. The illustrated embodiment shows a dry adhesive 10 having six stems 22, although a dry adhesive according to the present invention may have more or fewer than six stems 22. It is possible that a dry adhesive 10 could have a single stem 22 although in most applications the dry adhesive 10 will likely have many stems 22.
The stems 22 have first 24 and second 26 ends on opposite sides of the stem 22. The first end 24 of the stem 22 is connected to the backing layer 20, and the second end 26 of the stem 22 is connected to the tip 28.
The tip 28 includes an expanded surface 30 which is generally away from the stem. The expanded surface 30 is larger than the stem 22. In other words, the expanded surface 30 has an area that is greater than a cross sectional area of the second end 26 of the stem 22, when the cross-sectional area of the second end of the stem 22 is measured in a plane parallel to the expanded surface 30. The expanded surface 30 may be planar or it may be non-planar. For example, the expanded surface 30 may be concave or convex or it may have other features such as recesses and projections. If the expanded area 30 is non-planar, the cross-sectional area of the stem 22 can be measured parallel to a plane that most closely approximates the expanded surface 30.
In the illustrated embodiment the expanded surface 30 is not parallel to the backing layer 20. This orientation has been found to provide superior results with dry adhesives 10, although it is not required that the expanded surface 30 be non-parallel to the backing layer 20. For example, the present invention may also include tips 28 with an expanded surface 30 that is parallel to the backing layer 20.
The relationship between the backing layer 20, stem 22, and tip 28 can vary in different embodiments of the present invention. In the illustrated embodiment, the stem forms an angle 0 relative to a line perpendicular to the backing layer 20. Similarly, the expanded surface 30 forms an angle β—relative to a plane parallel to the backing layer 20. The angle 13 can be defined during the fabrications process, as will be described in more detail hereinbelow.
Typically, the angles θ and β are between zero and ninety degrees. However, it is possible for those angles to be greater than ninety degrees. For example, if the backing layer 20 is non-planar, if it contains recesses into which the stem 22 can be bent, or if it otherwise makes allowances for the stem 20 to adopt such an orientation, then the angle θ may be greater than ninety degrees. Other variations are also possible, such as a J-shaped stem 22, which allows θ to be greater than ninety degrees. Similarly, it is also possible for β to be greater than ninety degrees, such as if the stem 22 takes a different shape or orientation from that illustrated herein. For example, a J-shaped stem 22 may allow for the expanded surface 30 to be rotated more than ninety degrees.
1.2 Fabrication of Specialized Tips on Single Level Structures
The fabrication process for creating directional adhesives with specialized tips 28 begins with the fabrication of an array of cylindrical base fibers. Angled or vertical base fiber arrays are fabricated through a micromolding process which duplicates lithographically formed master template structures with a desired fiber material. This method for fabrication of the fiber arrays is described in U.S. patent application Ser. No. 12/448,242, by the same inventors, which is incorporated herein by reference.
a-3d illustrates one embodiment of a fabrication process according to the present invention. In that embodiment, the fabrication process is used for adding angled mushroom tips 28 to fibers 22. In
In one embodiment of the present invention, 1 in2 fiber arrays are molded from polyurethane with a ≈1 mm thick backing layer using a thin spacer to define the thickness and ensure uniformity. A thin film of liquid ST-1060 polyurethane is spun onto a polystyrene substrate for 45 seconds at 4,000 rpm. The fiber array is placed on the film of liquid polyurethane (
The construct of the fiber array with the specialized tip material are then cured by methods known to those skilled in the art.
The fiber arrays are then peeled from the substrate, and the fibers return to their initial angle (θ), tilting the tips to an angle of (β−θ) as shown in
In one embodiment of the present invention, the microfibers have diameters of ˜35 μm and lengths of ˜100 μm, with base fiber angles from 0° to 33° from horizontal. The fibers are arranged in a square grid pattern with a center-to-center spacing of 120 μm. The fiber arrays are fabricated from a polyurethane elastorner with a Young's modulus of ˜3 MPa, chosen for its high tear strength and high strain before failure.
This process has been implemented to form the first synthetic fibers with angled spatular tips. By varying the fiber geometry or the load during curing, the tip angle can be fabricated anywhere from 0 (no tip angle) to 90° (tips parallel to the fiber stem, see
a-4d are scanning electron microscope images of arrays of 35 μm diameter angled polyurethane microfibers with angled mushroom tips which were constructed according to one embodiment of the present invention. Tip orientation can be controlled to form tips with varying angles: (a) 34°; (b) 90°; (c,d) 23°. Details of the tip can be seen in (d).
Step 100 includes applying a liquid polymer 40 to the second end 26 of the stem 22 as described above, for example, with reference to
Step 102 includes contacting the liquid polymer on the stem with a tip shaping surface. See, for example,
Step 104 includes bending the stem relative to the backing layer while contacting the liquid polymer on the stem with the tip shaping surface. See, for example,
Step 106 includes curing the liquid polymer to form a tip on the second end of the stem while bending the stem relative to the backing layer and while contacting the liquid polymer on the stem with the tip shaping surface. See, for example,
Step 108 includes removing the tip from the tip shaping surface after the liquid polymer cures. See, for example,
Many variations and modifications are possible with this method. Some of those variations and modifications will be described below.
For example, step 100 may include dipping the second end 26 of the stem 22 in a liquid polymer 40 followed by removing the second end 26 of the stem 22 from the liquid polymer 40 after the liquid polymer 40 is applied to the second end 26 of the stem 22. In other embodiments, the liquid polymer 40 maybe applied by methods other than dipping, such as by spraying or otherwise applying the liquid polymer 40. In such cases, the step of removing the second end 26 from the liquid polymer 40 may not be needed in some embodiments.
Step 102, contacting the liquid polymer 40 on the stem 22 with a tip shaping surface 42, may include forming the expanded surface 30 in the liquid polymer 40 as described above. This may also include forming a planar surface in the liquid polymer 40 where the liquid polymer 40 contacts the tip shaping surface 42. The planar surface may be formed, for example, by using a tip forming surface 42 that is planar. However, other tip forming surfaces 42 may be used to form other expanded surfaces 30 on the tip 28. For example, concave or convex tip forming surfaces 42 may be used, as well as tip forming surfaces 42 having recesses, bumps, or other features that can be used to shape the expanded surface 30 of the tip 28.
As described above, the expanded surface 30 of the tip 28 may have an area that is greater than a cross-sectional area of the second end 26 of the stem 22 in a plane parallel to the expanded surface 30 of the tip 28.
Step 104, bending the stem, may include applying a load to the backing layer. This is one way of being the stem 22, although other ways may also be used with the present invention.
Step 104, bending the stem, may also include bending the stem 22 in a direction away from a perpendicular orientation with the backing layer 20. In other words, a non-parallel stem 22 may be bent in such a way as to exaggerate or increase the extent to which the stem 22 is nonparallel with the backing layer 20.
Step 104, bending the stein 22 relative to the backing layer 20 while contacting the liquid polymer 40 with the tip shaping surface 42, may include bending the stem to form an angle β relative to an imaginary line perpendicular to the backing layer, wherein β is greater than 0 and less than ninety degrees, as described above.
Step 106, curing the liquid polymer cures to form a tip 28, may include forming an expanded surface 30 in the tip 28 where the tip 28 contacts the tip shaping surface 42, and wherein after removing the tip 28 from the tip shaping surface 42 the expanded surface 30 of the tip 28 forms an angle β−θ relative to an imaginary plane parallel to the backing layer 20.
Step 106, curing the liquid polymer 40 to form a tip 28, may include forming the expanded surface 30 on the tip 28 where the tip 28 contacts the tip shaping surface 42. In other words, the shape of the expanded surface 30 may be formed during the curing step 106, when the liquid polymer 40 on the stern 22 changes from liquid form to cured or solid form and retains the general shape at the time of curing. As a result, curing the liquid polymer 40 forms an expanded surface 30 indicative of the tip shaping surface 42.
After step 108, removing the tip 28 from the tip shaping surface 42, the expanded surface 30 of the tip 28 may be planar and not parallel to the backing layer 20, as described in more detail herein.
After step 108, removing the tip from the tip shaping surface, the method of the present invention may result in a stem 22 that forms an angle θ relative to an imaginary line perpendicular to the backing layer 20, wherein θ is greater than zero degrees and less than ninety degrees. As described above, it is also possible for θ to be greater than ninety degrees. Other values for θ are also possible with the present invention. For example, θ may be zero degrees if the stems 22 are perpendicular to the backing layer 20.
Many other variations and modifications are also possible. For example, the method may also include maintaining the backing layer 20 parallel to the tip shaping surface 42 during step 104, when bending the stem 22 relative to the backing layer 104. The method may also include maintaining the backing layer 20 parallel to the tip shaping surface 30 during step 106, curing the liquid polymer 106. In other embodiments, the backing layer 20 may be maintained non-parallel to the tip shaping surface 42.
Step 200 includes applying a liquid polymer 40 to the second end 26 of the stem 28.
Step 202 includes contacting the liquid polymer 40 on the stem 22 with a tip shaping surface 42.
Step 204 includes forming an expanded, planar surface in the liquid polymer 40 where the liquid polymer contacts the tip shaping surface 42.
Step 206 includes bending the stem 22 relative to the backing layer 20 while contacting the liquid polymer 40 on the stem 22 with the tip shaping surface 42, wherein bending the stem 22 includes bending the stem 22 in a direction away from a perpendicular orientation with the backing layer 20.
Step 208 includes maintaining the backing layer 20 parallel to the tip shaping surface 42 when bending the stem 22 relative to the backing layer 22.
Step 210 includes curing the liquid polymer 40 to form a tip 28 on the second end 26 of the stem 22 while bending the stern 22 relative to the backing layer 20 and while contacting the liquid polymer 40 on the stem 22 with the tip shaping surface 42, wherein the expanded surface of the liquid polymer forms an expanded surface 30 of the tip 28 during curing, wherein the expanded surface 30 of the tip 28 has an area that is greater than a cross-sectional area of the second end 26 of the stem 22 in a plane parallel to the expanded surface 30 of the tin 28.
Step 212 includes maintaining the backing layer 20 parallel to the tip shaping surface 42 when curing the liquid polymer 40.
Step 214 includes removing the tip 28 from the tip shaping surface 42 after the liquid polymer 40 cures, wherein the expanded surface 30 of the tip 28 is not parallel to the backing layer 20 after removing the tip 28 from the tip shaping surface 42.
Many variations and modifications are possible according to the present invention. For example, after removing the tip 28 from the tip shaping surface 42, the method of the present invention may result in a stem 22 that forms an angle θ relative to an imaginary line perpendicular to the backing layer, wherein θ is greater than zero degrees and less than ninety degrees. Other values for θ are also possible with the present invention.
Step 206, bending the stem 22 relative to the backing layer 20 while contacting the liquid polymer 40 with the tip shaping surface 42, may include bending the stem 22 to form an angler. relative to an imaginary line perpendicular to the backing layer 20, wherein 13 is greater than θ and less than ninety degrees. Other values for θ and β are also possible with the present invention.
Step 210, curing the liquid polymer 40 form a tip, may include forming an expanded surface 30 in the tip 28 where the tip 28 contacts the tip shaping surface 42. Also, after step 214, removing the tip 28 from the tip shaping surface 42, the expanded surface 30 of the tip 28 may form an angle (β−θ) relative to an imaginary plane parallel to the backing layer 20.
Other variations and modifications are also possible with the present invention.
1.3 Friction Anisotropy: Results
Shear and normal adhesion of our angled fiber arrays with specialized tips fabricated with the methods of the present invention as described above were measured in a variety of ways to investigate interfacial shear strength, directionality, and the controllability of adhesion. In one measurement method, normal and shear forces were measured during a fixed shear displacement of 500 μm between a 6 mm diameter glass spherical indenter and the fiber array. In a second set of experiments with the same indenter, we measured the effect of varying shear displacements on the resulting shear and normal forces.
A custom adhesion characterization system, described previously [B. Aksak, M. P. Murphy, and M. Sitti, “Adhesion of biologically inspired vertical and angled polymer microfiber arrays,” Langmuir, vol. 23, no. 6, pp. 3322-3332, 2007], was used for the adhesion and shear experiments.
The experiments will now be described in more detail. In the fixed displacement experiments, an indenter was pressed into contact with the fibers to a specified preload value of 5 mN (
The fibers with no tip angle (
In contrast, the results from the angled tip fiber sample (
The measured anisotropic characteristics of the angled tip samples from
Although the asymmetric geometry of fiber tips can result in asymmetric stress distributions at the edges of the contact interface as described by Bogy, we hypothesize that the observed anisotropic behavior arises primarily due to the stresses caused by the moment created when the tip is sheared. This can be understood by qualitative analysis of the rotation of the tip during shear loading in each direction (
Any rotation angle of the tip 28 with respect to its original orientation causes a peeling moment, which, in combination with shear and tensile stresses at the interface, can cause an edge of the tip 28 to detach. When a fiber is compressed into intimate contact with a surface, the tip angle rotates from its original angle Øo(
Three samples with varying geometry were tested using the same experimental setup outlined above. The resulting data from three representative samples are plotted together along with SEM images of the samples in
Maximum shear force was not found to have any direct dependence on base fiber angle (51-78°), tip angle (0-34°), base fiber diameter (32-45 μm), or base fiber length (74-105 μm) within the variations between the samples. However, a strong correlation was seen between maximum shear force and tip area, as illustrated in
1.3.1 Adhesion Control
It has been shown that a shear displacement is required before biological gecko foot-hairs (setae) can provide adhesion to a surface. To demonstrate the ability to control adhesion of our microfiber arrays via shear displacement, a separate set of experiments was performed.
The results in
c-e) shows profile views of angled mushroom tip fibers at the instant before final detachment after varying shear displacements. Any shear displacement in the ‘releasing’ direction resulted in negligible fiber extension and very low adhesion as the fibers slid out of contact with the indenter (
As a demonstration of the macroscale adhesion of the directional microfiber array, a small area (1 cm2) of a sample with 14° tip angle was attached to a glass-slide 70 which supported a hanging weight of 1 kg in pure shear in the ‘gripping’ direction, an interfacial shear strength of ˜100 kPa (
1.3.2 Summary
We have described embodiments of the present invention in which fiber array constructs are created by dipping an angled fiber array into a thin film of liquid polymer and then pressed against a substrate to form specialized tips with controllable orientation to the fibers. These constructs exhibit similar shear adhesive strength to the gecko lizard's feet on smooth surfaces, as demonstrated with macro-scale support of significant loads (1 kg/cm2). These adhesives exhibit directional characteristics, gripping when loaded in one direction, and releasing when loaded in the opposite shear direction. We have shown that the adhesion can be controlled by varying the shear displacement before loading in the normal direction. The angled tips of the fibers create a larger contact area and are responsible for the observed shear anisotropy. We have identified tip area as a main design parameter for the magnitude of the interfacial shear strength, and the tip angle as a design parameter to control the anisotropy ratio. The fabrication methods described in this invention can be easily extended to smaller size scales and stiffer materials to more closely mimic the gecko's adhesive structures. The high magnitude anisotropic adhesion of these materials may enable efficient gripping and releasing of structures. Additional embodiments of the invention will now be described.
1.4 Multi-Level Hierarchical Fibers
In other embodiments of the present invention, the fiber arrays fabricated according to the methods described above, are again placed into a thin film or liquid polymer. In these embodiments, however, instead of then pressing the wet polymer at the tip of the fibers onto a flat surface, the wet polymer is pressed onto either an array of smaller scale fibers, or onto a mold to create an array of smaller scale fibers on the tips of the fiber array. These methods result in a hierarchical fiber array construct, as described in further detail below. These structures provide improved adhesive characteristics for adherence to uneven and rough surfaces, and mimic the hierarchical fiber structures observed in nature.
The motivation for the creation of hierarchical structures is to provide greater adhesion to uneven and rough surfaces. Adaptation to uneven and rough surfaces is a major feature of biological fibrillar adhesives. Most natural and man-made surfaces are not perfectly smooth, and traditional adhesives are typically less effective on rougher surfaces. Fibrillar adhesive materials with large areas and high uniformity can be fabricated according to the methods of the present invention described below. We also describe experimental results, which characterize the adhesive performance of the hierarchical materials against a smooth flat punch and a smooth curved surface. The performance results are compared to a flat control sample. Furthermore, we describe observations that 21 were made about the interaction of fibrillar adhesives with uneven surfaces by viewing these interactions from the side with a microscope.
One advantage of fibrillar adhesives over flat unstructured adhesives is that each fiber deforms independently, which allows them to access deeper recessions to make contact. Even with the reduced total area due to the spaces between the fibers, the actual contact area can be greater than that of a flat adhesive in contact with a rough surface (
Another advantage of fibrillar surfaces is their ability to enhance adhesion by contact splitting. If contact is split into many finer independent contacts, adhesive strength increases due to load sharing. However, adhesive force is directly proportional to both adhesive strength and total contact area. To exploit the advantage from fibrillar adhesives, the enhancement from contact splitting must compensate for the reduction in contact area due to the lost area between the fibers.
1.4.1 Hierarchical Structures in Nature
In nature, the most advanced fibrillar dry adhesives are found in the heaviest animals such as the tokay gecko which can weigh up to 300 grams. In comparison to the insects whose bodies are much lighter and do not require high performance adhesion, these animals have more complex adhesive pads with many levels of compliance including their toes, foot tissue, lamellae, and fibers. Additionally, these fibers branch from a micron-scale diameter to sub-micron diameter tip fibers. The fiber structure is similar to a branching tree or a broom. This multi-level hierarchy allows the adhesives pads to conform to surface roughness with various frequency and wavelength scales. The toes and tissue conform to large-scale (mm scale) roughness, and each subsequent level conforms to roughness at its corresponding size-scale. Finally, the sub-micron tip fibers can access the smallest surface valleys.
This type of multi-level structure is desirable for synthetic fibrillar adhesives as well. In this section, we disclose several fabrication techniques for creating hierarchical synthetic fibers according to the present invention. These methods result in hierarchies from the millimeter scale to sub-micron scale. Fabrication results are also demonstrated and described. Finally, hemispherical indenter tests are used to examine the effect of hierarchy on adhesion and interface toughness.
1.4.2 Fabrication
The present invention includes several embodiments to fabricate fibrillar structures with multiple levels of hierarchy. These methods span the size scales from millimeter scale molding to nanoscale carbon nanofiber embedding. The following sections detail the fabrication processes and provide experimental 23 results of these techniques.
1.4.3 Nanoscale Hierarchy
In order to reach into the smallest recesses of a surface, the distal fibers of an adhesive pad should have sub-micron diameters, as seen in the gecko's setae. It is possible to create synthetic fibrillar surfaces with nanoscale diameter tip fibers by embedding vertical arrays of carbon nanotubes or carbon nanofibers into the tips of base fibers.
In one embodiment of the present invention, the mushroom tip fabrication process detailed previously is altered to enable the embedding of smaller scale fibers 60, such as carbon nanofibers or carbon nanotubes, into the tips 28 of polyurethane fibers.
The process illustrated in
Vertical arrays of carbon nanofibers 60 were used in one embodiment of the present invention. Those skilled in the art will recognize that other small scale or nanofiber arrays could be used. Carbon nanofibers have sufficient stiffness to prevent lateral collapse and are able to be closely spaced. Although carbon nanofibers have high stiffness, they also can be grown to high aspect ratios, allowing them to be compliant in the vertical direction. Another advantage of carbon nanofibers for this process is that the weakest part of the structure is at the base where the fiber meets the carrier wafer, due to a widening of the cone-shaped carbon sheet structure near the interface (
Initial results (
In another embodiment of the present invention, methods are provided to fabricate hierarchical structures with specialized tips on the smaller scale fibers. As we have shown previously as well as observed in natural fibrillar adhesives, widened tips provide a significant increase in adhesion. We have developed a tip fabrication process that allows the tip fiber shape to be controlled by micro-molding.
In this process, after the previously detailed dipping of base fibers 22 in a liquid polymer layer 40, the fibers 22 are placed onto an etched silicon wafer. This wafer has micron-scale diameter cylindrical holes with a widened tip formed by Deep Reactive Ion Etching. These silicon-on-oxide negative templates can be fabricated according to the methods described in U.S. patent application Ser. No. 12/448,243, which is incorporated herein by reference.
One embodiment of the process illustrated in
Material constructs fabricated with this process can be seen in
One advantage of this fabrication method is that there is no constraint on the scale of the tip fibers or second layer stems 60. For example, nanoscale tip fibers 60 may be integrated into microscale base fibers 22 and tips 28 with this technique.
An overview of the methods of making dry adhesives according to one embodiment of the present invention will now be provided starting with
Step 300 includes applying a liquid polymer to the second end 26 of the stem 22.
Step 302 includes molding the liquid polymer 40 on the stem 22 in a mold 50, wherein the mold 50 includes a negative features 52 having a cross-sectional area that is less than a cross-sectional area of the second end 26 of the stem 22.
Step 304 includes curing the liquid polymer 40 in the mold 50 to form a tip 28 at the second end 26 of the stem 22, wherein the tip 28 includes a second layer stem 60, corresponding to the negative features 52 in the mold 50; and
Step 306 includes removing the tip 28 from the mold 50 after the liquid polymer 40 cures.
Many variations and modifications are possible with the present invention. Some of those variations and modifications. For example, the stem 22 may be perpendicular to the backing layer 20, or the stem 22 may be non-perpendicular to the backing layer 20. Other examples are provided below.
Step 306, removing the tip from the mold, can include etching the mold from the tip. For example, the tip 28 can be removed from the mold 50 by etching the mold 50 as opposed to, for example, pulling the tip 28 out of the mold 50. Other variations are also possible. If the mold 50 is etching from the tip 28, the method may also include covering the stern 22 with a protective polymer layer, such as polyurethane, before etching the mold 50. This may be done, for example, to protect the stem 22 from the etching processes.
The method of the present invention can be used to make many variations of dry adhesives. In one embodiment, the stem 22 is microscale and the second layer stem 60 is nanoscale. Other variations are also possible. For example, the present invention also includes microscale second layer stems 60 on milliscale stems 22, smaller microscale stems 60 on larger microscale stems 22, and other variations. The present invention can also be used to make dry adhesives with more than two levels of stems 22, 60. For example, the present invention may be used to make dry adhesives with three levels of stems, four levels of stems, or more.
Step 308, molding the liquid polymer on the stem, may include filling the negative features 52 in the mold 50 with the liquid polymer 40 via capillary forces.
Step 300, applying a liquid polymer 40 to the second end 26 of the stem 22, includes dipping the second end 26 of the stein 22 in the liquid polymer 40 and removing the second end 26 of the stern 22 from the liquid polymer 40 after the liquid polymer 40 is applied to the second end 26 of the stem 22.
The present invention may also include bending the stem 22 relative to the backing layer 20 while molding 302 the liquid polymer 40 on the stem 22 in the mold 50. The present invention may also include bending the stem 22 relative to the backing layer 20 while curing the liquid polymer 40 in the mold 40. If the stem 22 is bent while the liquid polymer 40 cures, the tip 28 can be made to take on different shapes, depending on the extent to which the stem 22 is bent, as described herein. The bending of the stem may include applying a load to the backing layer 20. Furthermore, the stem may be bent in a direction away from a perpendicular orientation with the backing layer, as described herein.
Step 310 includes applying a liquid polymer to the second layer stem.
Step 312 includes molding the liquid polymer on the second layer stem with a second mold, wherein the second mold includes a recess having a cross-sectional area that is less than a cross-sectional area of the second layer stem.
Step 314 includes curing the liquid polymer in the mold to form a tip on the second layer stem, wherein the tip on the second layer stem includes a third layer stern, and wherein the third layer stem corresponds to the recess in the second mold.
Step 316 includes removing the tip on the second layer stem from the second mold.
Step 320 includes applying a liquid polymer 40 to the second layer stem 60.
Step 322 includes inserting a plurality of fibers 80 into the liquid polymer 40 on the second layer stem 60, wherein the plurality of fibers 80 have a cross-sectional area which is less than a cross-sectional area of the second layer stem 60.
Step 324 includes curing the liquid polymer 40 on the second layer stem 60 with the plurality of fibers 80 in the liquid polymer 40.
Many variations and modifications are possible with this embodiment of the present invention. For example, the fibers may be nanotubes, nanowires, nanofibers, or other materials.
Step 330 includes applying a liquid polymer 40 to the second end 26 of the stem 22.
Step 332 includes inserting a plurality of fibers 80 into the liquid polymer 40 on the second end of the stem, wherein the plurality of fibers have a cross-sectional area which is less than a cross-sectional area of the second end of the stem.
Step 334 includes curing the liquid polymer with the plurality of fibers in the liquid polymer.
Many variations and modifications are possible with this embodiment of the present invention. For example, the fibers may be nanotube, nanofiber, nanowire arrays, or other structures. Also, the stem 22 may be perpendicular or non-perpendicular to the backing layer 20.
Step 332, inserting a plurality of fibers into the liquid polymer 40, may include inserting a plurality of fibers connected to a base. Also, after step 334, curing the liquid polymer 40, the present invention may include separating the plurality of fibers from the base.
1.4.4 Macroscale Hierarchy
The previously described techniques are intended to add tip fibers onto molded base fibers to create a multi-layer fibrillar adhesive. Another method to create a fibrillar structure is to create compliance in the backing layer at a larger scale than the base fibers. Even simple slits in an otherwise unstructured material has been demonstrated to increase the average fracture energy of flat elastomers by an order of magnitude, due to inhibited crack propagation. This is seen in the feet of geckos, where the base fibers are attached to thin plate-like structures with spaces in between called lamellae. These lamellae increase macro-scale compliance and prevent crack propagation. For synthetic adhesives, backing layer patterning can be integrated with one of the tip fiber methods described above to create a two-level hierarchy. Like the biological lamellae, these fibers act to arrest cracks and increase compliance.
Fabrication of macro-micro scale hierarchical structures is accomplished using a technique similar to that described above in
Fabrication of the base fibers 22 is accomplished by using a rapid prototyping system (Invision HR, 3D Systems) to print plastic master templates of the desired structures. It is possible to create fibers with diameters as small as 250 μm with this hardware, but the technique is not limited to any particular size scale. Non-cylindrical geometries are possible using this technique as well. The master template is molded with silicone rubber (HS II, Dow Corning) to create a negative mold. After separation from the master template, the negative mold is used to replicate the base structures from polymers such as polyurethane. Wide flat mushroom tips 28 are optionally added to these base fibers 22 in the same way as described for micro-scale fibers. The liquid polyurethane 40 of
The larger length-scale of these dual-level hierarchical fibers, the roughness adaptation to larger amplitude rough surfaces should be significantly increased. This effect will be investigated in detail in Section 1.4.6.
1.4.5 Three-Level Hierarchy
In another embodiment of the present invention, the macroscale hierarchy fabrication technique are combined with the microscale hierarchy technique to fabricate three-level hierarchical fibers, each level having mushroom shaped tips for increased area. Combining the processes is relatively straightforward, but does require several steps to complete. (
Many variations are possible. For example, it is possible to simplify this process by doing some or all of the steps illustrated in
Initial results of three-level hierarchical fiber fabrication are promising.
Polyurethane structures (
1.4.6 Experiments
Four samples were fabricated from polyurethane (ST-1060; BJB Enterprises), an unstructured control sample, a single level fiber sample, a double level vertical sample, and a double level angled sample. The double level samples were fabricated using the techniques described in Section 1.4.4. The details of the samples can be seen in Table 1.
All of the samples, other than the unstructured sample, have identical terminal fibers, with 50 μm diameter stems, 100 μm height, and 110 μm diameter mushroom tips with 160 μm center-to-center spacing. The unstructured sample was molded against the same substrate so that it has the same surface properties as the fiber samples. Since the terminal fibers are identical between the three fiber samples, the only difference between them is the contact area fraction, and the structure beneath the terminal fibers. In the Single Level case, this structure is a solid backing layer of the polyurethane. In the hierarchical samples, this structure is an array of larger base fibers. The base fibers are intended to make the sample effectively more compliant. However, along with the increased compliance, the contact area fraction (area−open space between fibers) is significantly reduced. The total contact area fraction for the Double Level samples is the product of the contact area fraction of the terminal layer (36%) and the contact area fraction of the base layer. The contact area fraction of the unstructured sample is 100%.
The four samples were tested using a 12 mm hemispherical smooth glass indenter. Because the extension length of the Double Level samples is high (mm scale), a retraction speed of 200 μm/s was chosen to minimize the duration of the experiments. Similarly, the approach speed was set to 50 μm/s. Although viscoelastic effects are present due to the relatively high strain rate, these experiments are intended to compare the hierarchical structures to the previously characterized Single Level Fibers and unstructured samples in a relative manner, not to determine their quantitative adhesive characteristics. Five experiments were performed on the same area of each sample at each specified preload between 2 mN and 400 mN.
The resulting performance curves are plotted together in
Results from the experiments in
Since a hemispherical indenter represents a special case of a rough surface, these result suggest that the Double Level Vertical fibers provide higher adhesion against surfaces with high amplitude (mm scale) roughness.
To examine the sample-indenter interaction in more detail, the Force-Distance data for the four samples are plotted together in
Using the indentation depths of from these data, it is possible to estimate the size of the contact zone using the geometrical equations for a spherical cap. The contact zone area ac, is found as
a
cz=πΔp(2R−Δp) (2)
where Δp is the indentation depth and R is the radius of the hemispherical indenter. The contact zone areas for these tests were found to be 2.7 mm2, 3.5 mm2, 12.8 mm2, and 11.2 mm2 for the Unstructured, Single Level Fiber, Double Level Angled, and Double Level Vertical samples, respectively. Multiplying the estimated contact zone areas by the contact area fraction for each sample results in an estimate for the real contact area. For the four samples, the real contact areas were found to be 2.7 mm2, 1.26 mm2, 1.28 mm2, and 2.18 mm2 for the Unstructured, Single Level Fiber, Double Level Angled, and Double Level Vertical samples, respectively. Therefore, the enhancements due to contact splitting and load sharing for the Double Level Vertical sample increased the adhesion.
The adhesions, maximum negative force, for the samples are compared in
The Force-Distance data can be used to calculate the energy dissipated during detachment for each of the samples, which indicates the toughness of an interface. This energy is seen in
The advantage of hierarchical fibers does not only appear at large preloads, it is also evident at low preloads as well.
To examine the behavior of a hierarchical sample interacting with an uneven surface, an experiment was run while recording a video of the side view of the sample. The test data (force vs. time) is illustrated in
1.5 Repeatability
Although the present invention has generally been described in terms of specific embodiments and implementations, the present invention is applicable to other methods, apparatuses, systems, and technologies. The examples provided herein are illustrative and not limiting, and other variations and modifications of the present invention are contemplated. Those and other variations and modifications of the present invention are possible and contemplated, and it is intended that the foregoing specification and the following claims cover such modifications and variations.
This application is a Divisional Application of U.S. Non-provisional application Ser. No. 12/562,683, filed Sep. 18, 2009, which claims priority from U.S. Provisional Application 61/192,482, filed Sep. 18, 2008, both are incorporated by reference herein. This application is related to U.S. patent application Ser. No. 12/448,242, filed Jun. 12, 2009, and U.S. patent application Ser. No. 12/448,243, filed Jun. 12, 2009.
This invention was made, in part, with government support under Grant Number CMMI-0900408 awarded by the National Science Foundation. The United States government may have certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61192482 | Sep 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12562683 | Sep 2009 | US |
| Child | 13533386 | US |
