Enhanced Wet Adhesion and Shear of Elastomeric Micro-Fiber Arrays

Such materials may have potential value as repeatable adhesives for wet environments, such as for medical devices. ..... Adhesion results for this mat...
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Enhanced Wet Adhesion and Shear of Elastomeric Micro-Fiber Arrays with Mushroom Tip Geometry and a Photopolymerized p(DMA-co-MEA) Tip Coating Paul Glass,†, Hoyong Chung,‡, Newell R. Washburn,†,‡ and Metin Sitti*,†,§ )

† Department of Biomedical Engineering, ‡Department of Chemistry, and §Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States. Co-first authors.

Received July 22, 2010. Revised Manuscript Received September 12, 2010 Using principles inspired by the study of naturally occurring sticky systems such as the micro- and nanoscale fibers on the toes of geckos and the adhesive proteins secreted by marine animals such as mussels, this study describes the development and evaluation of a novel patterned and coated elastomeric microfibrillar material for enhanced repeatable adhesion and shear in wet environments. A multistep fabrication process consisting of optical lithography, micromolding, polymer synthesis, dipping, stamping, and photopolymerization is described to produce uniform arrays of polyurethane elastomeric microfibers with mushroom-shaped tips coated with a thin layer of lightly cross-linked p(DMA-co-MEA), an intrinsically adhesive synthetic polymer. Adhesion and shear force characterization of these arrays in contact with a glass hemisphere is demonstrated, and significant pull-off force, overall work of adhesion, and shear force enhancements in submerged aqueous environments are shown when compared to both unpatterned and uncoated samples, as well as previously evaluated patterned and coated arrays with differing geometry. Such materials may have potential value as repeatable adhesives for wet environments, such as for medical devices.

Introduction Developing materials capable of demonstrating robust, repeatable adhesion to wet substrates is a major challenge that, if accomplished, may solve problems in applications ranging from medical device implementation to mobile robotics. One approach for developing such adhesive materials is to investigate and attempt to duplicate the underlying adhesive mechanisms of sticky systems found in nature, such as the hierarchical micro and nanoscale structures on the toe-pads of geckos and the adhesive proteins secreted by the holdfasts of mussels. Indeed, extensive work has gone toward investigating the mechanics and intermolecular forces involved in gecko adhesion as well as developing synthetic uncoated materials which take advantage *To whom correspondence should be addressed. E-mail: [email protected]. (1) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Nature 2000, 405, 681–685. (2) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J. Proc. Natl. Acad. Sci. 2002, 99, 12252–12256. (3) Hansen, W. R.; Autumn, K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 385–389. (4) Huber, G.; Mantz, H.; Spolenak, R.; Mecke, K.; Jacobs, K.; Gorb, S. N.; Arzt, E. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 16293–16296. (5) Arzt, E.; Gorb, S.; Spolenak, R. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 10603–10606. (6) Persson, B. N. J. J. Chem. Phys. 2003, 118, 7614–7621. (7) Peressadko, A.; Gorb, S. J. Adhes. 2004, 80, 247–261. (8) Glassmaker, N. J.; Jagota, A.; Hui, C.-Y.; Kim, J. J. R. Soc., Interface 2004, 1, 23–33. (9) Crosby, A.; Hageman, M.; Duncan, A. Langmuir 2005, 21, 11738–11743. (10) Jagota, A.; Bennison, S. J. Integr. Compar. Biol. 2002, 42, 1140–1145. (11) Hui, C. Y.; Glassmaker, N. J.; Tang, T.; Jagota, A. J. R. Soc., Interface 2004, 1, 35–48. (12) Chung, J. Y.; Chaudhury, M. K. J. R. Soc., Interface 2005, 2, 55–61. (13) Tian, Y.; Pesika, N.; Zeng, H.; Rosenberg, K.; Zhao, B.; McGuiggan, P.; Autumn, K.; Israelachvili, J. Proc. Natl. Acad. Sci. 2006, 103, 19320–19325. (14) Autumn, K.; Dittmore, A.; Santos, D.; Spenko, M.; Cutkosky, M. J. Exper. Biol. 2006, 209, 3569–3579. (15) Aksak, B.; Murphy, M.; Sitti, M. Langmuir 2007, 23, 3322–3332. (16) Sitti, M.; Fearing, R. S. J. Adhes. Sci. Technol. 2003, 17, 1055–1073.

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of such forces.1-34 However, as these intermolecular principles become less effective under water, only one of these synthetic uncoated materials has demonstrated moderate adhesion performance in submerged conditions,24 and it did so with contributions from vacuum instead of intermolecular forces.35 Because these micropatterned materials tend not to work well in wet environments, coating such structures with an intrinsically adhesive polymeric material may help in the overall adhesive performance of the structure, and indeed, two other works have demonstrated both temporary36 and permanent37 adhesion of patterned, coated materials to wet substrates. (17) Geim, A. K.; Dubonos, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov, A. A.; Shapoval, S. Y. Nat. Mater. 2003, 2, 461–463. (18) Murphy, M.; Aksak, B.; Sitti, M. J. Adhes. Sci. Technol. 2007, 21, 1281– 1296. (19) Murphy, M. P.; Aksak, B.; Sitti, M. Small 2009, 5, 170–175. (20) Lee, J.; Fearing, R. S.; Komvopoulos, K. Appl. Phys. Lett. 2008, 93, 191910. (21) Santos, D.; Spenko, M.; Parness, A.; Kim, S.; Cutkosky, M. J. Adhes. Sci. Technol. 2007, 21, 1317–1341. (22) Spolenak, R.; Gorb, S.; Gao, H.; Arzt, E. Proc. R. Soc. London Ser. A 2005, 461, 305–319. (23) Kim, S.; Sitti, M. Appl. Phys. Lett. 2006, 89, 261911–26913. (24) Gorb, S.; Varenberg, M.; Peressadko, A.; Tuma, J. J. R. Soc. Interface 2007, 4, 271–275. (25) Gorb, S.; Varenberg, M. J. Adhes. Sci. Technol. 2007, 21, 1175–1183. (26) Davies, J.; Haq, S.; Hawke, T.; Sargent, J. P. Int. J. Adhes. Adhes. 2009, 29, 280–290. (27) Cheung, E.; Sitti, M. Langmuir 2009, 25, 6613–6616. (28) Spuskanyuka, A. V.; McMeeking, R. M.; Deshpandea, V. S.; Arzt, E. Acta Biomater. 2008, 4, 1669–1676. (29) del Campo, A.; Greiner, C.; Arzt, E. Langmuir 2007, 23, 10235–10243. (30) del Campo, A.; Greiner, C.; Alvarez, I.; Arzt, E. Adv. Mater. 2007, 19, 1973–1977. (31) Sameoto, D.; Menon, C. J. Micromech. Microeng. 2009, 19, 115002. (32) Greiner, C.; Arzt, E.; del Campo, A. Adv. Mater. 2008, 20, 1–4. (33) Murphy, M. P.; Kim, S.; Sitti, M. ACS Appl. Mater. Interfaces 2009, 1, 849–855. (34) Jeong, H. E.; Lee, J.-K.; Kim, H. N.; Moon, S. H.; Suh, K. Y. Proc. Natl. Acad. Sci. 2009, 106, 5639–5644. (35) Varenberg, M.; Gorb, S. J. R. Soc., Interface 2008, 5, 383–385. (36) Lee, H.; Lee, B. P.; Messersmith, P. B. Nature 2007, 448, 338–341. (37) Mahdavi, A.; et al. Proc. Natl. Acad. Sci. 2008, 105, 2307–2312.

Published on Web 09/29/2010

DOI: 10.1021/la1029245

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Developing and optimizing such coatings requires an interdisciplinary understanding of polymer synthesis, polymer viscoelasticitiy, and polymer physics.38 Some strategies for developing adhesive polymeric coatings include the introduction of polar functionality on the adhesive surface,39 surface modification methods such as treatment with organic solvent,40 introduction of chlorinated polyolefin,41 or plasma treatment to form oxygencontaining chemical functional groups.42 Polymer blending can also significantly increase the adhesive ability of a surface.43,44 Other possibile adhesive coatings are those inspired by the adhesive proteins secreted by marine organisms like mussels. The extraordinary adhesive strength of mussels is attributed to crosslinking between polymer chains in the proteins found in their holdfasts.45-49 This cross-linking of mussel proteins primarily requires a catecholic precursor (3,4-dihydroxyphenyl-L-alanine (DOPA)) and/or a catechol oxidase.45 The reactive oxidized form of DOPA and quinone are believed to provide the mussel’s waterresistant adhesive ability.46,47 Due to these water-resistant properties of DOPA residues which allow for rapid and strong adhesion, research has been conducted to mimic the chemistry of DOPA with synthetic polymers.46,50-54 It has been shown that by coating gecko-inspired vertical nanopillar surfaces with a synthetic mussel-inspired adhesive containing DOPA, repeatable adhesion to both organic and inorganic substrates can be generated, even in wet conditions.36 However, while these coated materials demonstrated adhesion enhancement at the nanoscale, no macroscale performance was tested and no systematic enhancement was reported when compared to unpatterned materials. A later study demonstrated that by stamping filmterminated polyurethane microfibrillar arrays with a DOPAcontaining polymer, macroscale adhesive performance enhancement could be achieved when compared to both unpatterned and uncoated surfaces when in contact with submerged substrates.55 However, there will likely be an additional adhesion enhancement in wet environments if a material could be fabricated that consisted of micropatterned arrays of fibers with mushroomshaped tips with a coating of DOPA-containing polymer on each tip. By using this type of discrete array of fibers instead of the previously evaluated continuous film-terminated structures,55 increased pull-off adhesion force is expected due to the increased mechanical independence of each fiber as well as intrinsic advantages of the mushroom tip geometry, which have previously been studied and experimentally evaluated.23,24,26,28-31 (38) Awaja, F.; Gilbert, M.; Kelly, G.; Fox, B.; Pigram, P. J. Prog. Polym. Sci. 2009, 34, 948–968. (39) Occhiello, E.; Morra, M.; Morini, G.; Garbassi, F.; Humphrey, P. J. Appl. Polym. Sci. 1991, 42, 551–559. (40) Chen, J.-H.; Ruckenstein, E. J. Colloid Interface Sci. 1990, 135, 496–507. (41) Ryntz, R. A. Prog. Org. Coat. 1994, 25, 73–83. (42) Nihlstrand, A.; Hjertberg, T.; Johansson, K. Polymer 1997, 38, 3581–3589. (43) Tomasetti, E.; Legras, R.; Henri-Mazeaud, B.; Nysten, B. Polymer 2000, 41, 6597–6602. (44) Kang, H. M.; Yoon, T. H.; Bump, M.; Riffle, J. S. J. Appl. Polym. Sci. 2001, 79, 1042–1053. (45) Waite, J. H. Compar. Biochem. Physiol. B, Compa. Biochem. 1990, 97, 19–29. (46) Yu, M.; Hwang, J.; Deming, T. J. J. Am. Chem. Soc. 1999, 121, 5825–5826. (47) Yu, M.; Deming, T. J. Macromolecules 1998, 31, 4739–4745. (48) Silverman, H.; Roberto, F. Marine Biotechnol. 2007, 9, 661–681. (49) Waite, J. H.; Andersen, N. H.; Jewhurst, S.; Sun, C. J. Adhes. 2005, 81, 297–317. (50) Sever, M. J.; Weisser, J. T.; Monahan, J.; Srinivasan, S.; Wilker, J. J. Angew. Chem., Int. Ed. 2004, 43, 448–450. (51) Sun, C.; Waite, J. H. J. Biol. Chem. 2005, 280, 39332–39336. (52) Huang, K.; Lee, B. P.; Ingram, D. R.; Messersmith, P. B. Biomacromolecules 2002, 3, 397–406. (53) Lee, B. P.; Chao, C.-Y.; Nunalee, F. N.; Motan, E.; Shull, K. R.; Messersmith, P. B. Macromolecules 2006, 39, 1740–1748. (54) Chung, H.; Glass, P.; Pothen, J. M.; Sitti, M.; Washburn, N. R. Biomacromolecules submitted. (55) Glass, P.; Chung, H.; Washburn, N. R.; Sitti, M. Langmuir 2009, 25, 6607– 6612.

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In this study, a new process is presented for coating the tips of previously developed polyurethane fibers with mushroom tip geometry with a photopolymerized DOPA-containing polymer. Experimental evaluation in a submerged environment using a glass hemisphere is described, and significant adhesion force, adhesion hysteresis, and shear force performance enhancements are demonstrated when compared to previously evaluated geometries.55 Adhesion performance enhancement from a combination of suction and intermolecular forces is demonstrated. The fabrication process presented in this study may also be generalized for a variety of different tip coating materials, allowing for the development of repeatable adhesives to specifically targeted substrates in different environmental conditions.

Results and Discussions Poly(dopamine methacrylate-co-2-methoxyetheyl acrylate) (p(DMA-co-MEA)) was chosen as the tip coating material since it is well characterized and contains the DOPA-containing monomer dopamine methacrylate (DMA). DMA not only has the adhesive functionality of DOPA, it also contains a vinyl group, which makes it easy to polymerize with other vinyl monomers via radical polymerization. Micropatterned arrays of fibers with mushroom tips coated with lightly cross-linked p(DMA-co-MEA) were fabricated first by preparing an optimized p(DMA-co-MEA) precursor solution and then by carefully designing and implementing a multistep dipping and stamping process to transfer a sufficient p(DMA-co-MEA) precursor solution volume onto the tips of micromolded polyurethane fibers, at which point the tip coating material was polymerized. DMA was first synthesized and characterized according to a previously described process.55 An optimized photopolymerizable p(DMA-co-MEA) precursor solution was then prepared by mixing 1 part DMA with 7.2 parts MEA (by mole ratio), 0.017 parts photoinitiator (Irgacure 819, Ciba Specialty Chemicals), crosslinker (ethyleneglycol dimethacrylate (EGDMA), Aldrich Chemical Co.), and dimethylformamide (DMF, Aldrich Chemical Co.) according to a previously described technique,54 which is illustrated in Scheme 1. In testing of bulk p(DMA-co-MEA) material samples with various cross-linker concentrations, samples with 0.001% EGDMA molarity were determined to have optimum adhesion performance in both wet and dry glass hemisphere indentation testing54 and thus was chosen as the coating material in this study. This optimum adhesion is partially attributed to a balance between the viscous liquid-like qualities of the polymer and its elastic solidlike properties arising from changes in the cross-linking concentration. Photopolymerization was used to prepare these bulk adhesives, which allowed for reactions to be completed in 30 min, a relatively short time. In particular, using the divinyl cross-linking agent EGDMA as a tool for controlling the degree of cross-linking was an effective approach due to its ease and reproducibility. The biggest difference between EGDMA and a metal-based crosslinker is the cross-linking point of the polymer chains. As shown in Scheme 1, individual polymer chains were connected through the covalent bond provided from the divinyl group-containing EGDMA. These covalent bonds are robust and maintain the material bulk properties until the bond is broken by extreme external heat or force. Therefore, cross-linked polymers which contain EGDMA may offer more stable physical and chemical properties than a polymer cross-linked by a metal-based coordination bond.50,56 The coating process begins with an array of cylindrical polyurethane elastomer(ST-1087, BJB Enterprises) pillars of approximately 40 μm in diameter and 100 μm in height produced by a previously (56) Monahan, J.; Wilker, J. J. Chem. Commun. 2003, 1672–1673.

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Figure 1. Process diagram for material fabrication. (a) Arrays of untreated pillars are brought into contact with a polystyrene surface coated with a thin layer of liquid polyurethane. (b) The array is retracted, leaving a small amount of polyurethane on each pillar tip. (c) The array is stamped onto a polypropylene surface and the tips are allowed to cure, resulting in a mushroom shape. (d) The pillars with mushroom tips are brought into contact with a glass surface coated with a thin liquid layer of p(DMA-co-MEA) polymer precursor mixture. (e) The array is retracted, leaving a small amount of the p(DMA-co-MEA) precursor solution on each mushroom tip. A brief wait period allows some solvent to evaporate. (f) The array is stamped onto a clean, transparent polypropylene surface and exposed to 365 nm wavelength UV light for twentyfive minutes to initiate photopolymerization. (g) The patterned, coated array is peeled from the surface, resulting in the finished material. Scheme 1. Illustration of the Synthesis of Lightly Crosslinked Poly(dopamine methacrylate-co-2-methoxyetheyl acrylate) (p(DMA-co-MEA)) using Ethyleneglycol Dimethacrylate (EGDMA) as a Crosslinking Agent

developed photolithographic and micromolding process.15 The complete fabrication process for the p(DMA-co-MEA)-coated array of fibers with mushroom-shaped tips is illustrated in Figure 1. Polyurethane mushroom tips are produced first by dipping the array into a thin layer of uncured polyurethane precursor spun onto a polystyrene plate (as shown in Figure 1a), retracting the array such that small droplets of uncured polyurethane are present on the tip of each fiber (as shown in Figure 1b), and placed onto a polypropylene surface and allowed to cure (Figure 1c). After the mushroom tips have cured, the array is peeled from the polypropylene and the tips are dipped into a thin liquid layer of the prepared unpolymerized p(DMA-co-MEA) precursor solution which has been spun onto a clean glass plate Langmuir 2010, 26(22), 17357–17362

(as shown in Figure 1d). The array is retracted from the plate, such that a small droplet of unpolymerized p(DMA-co-MEA) precursor solution is present on each tip and allowed to rest in air (Figure 1e) to allow for some solvent evaporation. The coated, mushroom-tipped array is then stamped onto a clean, transparent polypropylene surface and exposed with ultraviolet light (365 nm, UVGL-25, UVP LLC) for thirty minutes to initiate polymerization of the tip coating (Figure 1f). Finally, the array is peeled from the surface, resulting in the final patterned, coated material (as shown in Figure 1g). The resulting catecholic functional groupcontaining p(DMA-co-MEA) tip coatings using this process is extremely thin, which is advantageous because thinner coatings are subjected to reduced stress and relaxation, which is more DOI: 10.1021/la1029245

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Figure 2. Microscopy images of fabricated patterned, p(DMA-co-MEA)-coated arrays of fibers with mushroom tips. (a) Top view optical microscopy image of array of fibers with mushroom-shaped tips and thin p(DMA-co-MEA) tip coating. (b) SEM image of (a). (c) Backscattered top view SEM image of p(DMA-co-MEA)-coated fibers with mushroom tips. Here, 5 nm diameter gold particles (bright pixels) were mixed with the tip material before the dip-coating process to visualize whether material was successfully transferred to the fibers. (d and e) Close-ups of (c). Scale bar =200 μm in (a) and (b). Scale bar =2 mm in (c). Scale bar =400 μm in (d). Scale bar =100 μm in (e).

efficient at transferring stress to the substrate which results in enhanced adhesion.57 A relatively stiffer polyurethane (ST-1087, E = 9.8 MPa) was used as the fiber material when compared to a previous coated structure study55 which used a more compliant polyurethane elastomer (ST-1060, BJB Enterprises, E = 2.9 MPa), because the stiffer material reduced the number of fibers that collapsed and stuck together during the material fabrication, particularly after the peeling processes which occur after steps (c) and (f) in Figure 1. Attractive qualities of this polyurethane also include its high toughness and strain before fracture. Figure 2, panels a and b, show optical microscopy and scanning electron microscope (SEM) images of a p(DMA-co-MEA)coated array of polyurethane fibers with mushroom tips fabricated using this protocol, respectively. Because it is difficult to visualize the thin p(DMA-co-MEA) coating with either SEM or optical microscopy, separate specimens were prepared which included 5 nm diameter gold particles suspended in the p(DMA-coMEA) solution before the step illustrated in Figure 1d. During the dipping process, many of these particles were transferred to the fibers, allowing for visualization of the presence of p(DMAco-MEA) on the mushroom tips. Figure 2c shows the top view of a backscattered SEM image of approximately 1200 fibers with gold nanoparticles embedded in the p(DMA-co-MEA) tip coating. Unlike the image illustrated in Figure 2b, this sample was not sputtered with a metallic coating before imaging, which lets the viewer distinguish the gold nanoparticles from the rest of the polymeric structure. The presence of gold, and thus the p(DMAco-MEA) coating layer, is apparent by the white pixels in the image. The distribution of brightness is fairly consistent over all 1200 fibers, but the degree of brightness is somewhat less regular, as evidenced by the higher magnification images shown in (57) Basin, V. E. Prog. Org. Coat. 1984, 12, 213–250.

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Figure 2, panels d and e. This is because the gold nanoparticles were not homogeneously soluble in the p(DMA-co-MEA) solution, so there were areas of particle concentration on the fiber tips after the microprinting coating process. The height of all fibers is quite consistent, as it is determined by the spinning parameters during the optical lithography mold formation.15 There can be some variability in fiber tip diameter due to differences in the uncured polyurethane droplet volume on each fiber after the dipping step illustrated in Figure 2b. Before testing, five control samples were also fabricated to compare the adhesion results of our p(DMA-co-MEA)-coated mushroom tipped microfibers with both coated and uncoated materials with various geometries. These control materials consisted of (1) uncoated mushroom-tipped microfiber arrays with identical geometry to the coated arrays described above, (2) flat polyurethane samples coated with photopolymerized p(DMAco-MEA), (3) flat uncoated polyurethane samples, (4) filmterminated microfibrillar arrays coated with p(DMA-co-MEA) which had previously demonstrated large-scale adhesion enhancement in wet environments, and (5) uncoated film-terminated microfibrillar arrays. While these last two materials were developed and evaluated in a prior study,55 results for the adhesion performance of the film-terminated structures are included here so that a clear comparison of the newly developed materials with the previous ones is possible. Uncoated fibers and both p(DMAco-MEA)-coated and uncoated flat specimens were used as controls for wet shear force characterization. A custom-built tensile adhesion testing apparatus was used to evaluate and compare the adhesive ability of the developed materials when in contact with a smooth glass hemisphere. A glass hemisphere was used as the contacting surface because it represents a nonflat surface and eliminates alignment problems Langmuir 2010, 26(22), 17357–17362

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Figure 3. Experimentally determined wet adhesion of uncoated fibrillar and p(DMA-co-MEA)-stamped materials with a 6 mm glass hemisphere for increasing preloads. The p(DMA-co-MEA)coated fibers with mushroom tips demonstrate 2-3 times adhesion enhancement over the previously developed film-terminated microfibrillar structures with a p(DMA-co-MEA) coating.55

during testing. These samples were fixed to the bottom of a 2 mm deep tank on a glass slide mounted to an inverted optical microscope (TE200, Nikon). A contacting surface, a 6 mm diameter glass hemisphere was fixed to the stem of a load cell (GSO-50, Transducer Techniques), which was attached to a linear stage (MFA-CC, Newport Corporation). Custom software was written to control the motion of the glass hemisphere while collecting data from the load cell. The shallow tank was filled with deionized (DI) water, entirely submerging the sample and its interface with the glass hemisphere. This larger scale adhesion testing apparatus was chosen instead of previously used micro- or nanoscale atomic force microscopy (AFM)-based studies36 because it allows for the evaluation of tens to hundreds of fibers at a time, which may behave very differently than the few fibers that can be tested at one time through AFM analysis. Adhesion results for this material in contact with a glass hemisphere in fully submerged conditions for increasing preloads are shown in Figure 3. The wet adhesion results illustrate a clear enhancement for the p(DMA-co-MEA) coated fibers with mushroom tips over the other engineered materials. Over this preload range, an average 32-times adhesion enhancement over flat, uncoated polyurethane is observed. Interestingly, the uncoated mushroom tips are the only uncoated material to demonstrate a significant adhesion ability in submerged conditions, with a 9.2-times enhancement over flat, uncoated materials. This is likely due to a pressure difference between the water and a sealed cavity which forms at the center of each tip (resulting in a microsuction cup) during pulloff from the glass hemisphere. This phenomenon has been previously reported for one other developed fiber with a mushroom-shaped tip.35 The presence of p(DMA-co-MEA) on the surface of each tip further increases the maximum pull-off force during testing resulting in cumulative enhancements of both micropatterning and coating. Here, the flat, coated sample demonstrates a modest 22% average enhancement over the tested preload range when compared to the uncoated flat control. Figure 4 shows the resulting force-distance curve for a p(DMA-co-MEA)-coated array of microfibers with mushroom tips at 50 mN preload and compares it the force-distance curves from a flat, uncoated polyurethane sample as well as previously measured results for the film-terminated coated fiber array.55 By comparing the adhesion hysteresis, the integrated area between the approach and retract curves during a single test cycle, it is clear that the effective interface toughness for coated, mushroomtipped materials is significantly enhanced with respect to both Langmuir 2010, 26(22), 17357–17362

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Figure 4. Force-distance data at 50 mN preload for flat untreated, p(DMA-co-MEA) treated film-terminated samples, and p(DMA-co-MEA) treated fibers with mushroom tips in submerged conditions during adhesion testing with a glass hemisphere.

Figure 5. Photograph of the automated shear measurement setup. (A) Two linear stages are mounted perpendicularly to one another and control the motion of the hemisphere. (B) A fixed load cell measures the preload forces in the normal direction. (C) A second load cell is attached to the stages and measures the shear forces. (D) The glass hemisphere makes contact with the fiber adhesive or flat sample being evaluated (E).

flat, unpatterned materials and the previously developed filmterminated structures. Shear force evaluation in submerged conditions for 150 mN of normal loading was also performed for samples in contact with the same glass hemisphere using a custom two-axis testing system. A photograph of the shear force testing setup is shown in Figure 5. Here, two identical motorized linear stages (A, MFA-CC, Newport Corporation) were mounted perpendicularly to one another. One load cell (B, DWHS-20, Transducer Techniques) to measure preload forces was mounted to a stationary plate, whereas a second load cell (C, MLP-10, Transducer Techniques) was mounted to the motorized stages to measure shear forces. A 6 mm glass hemisphere (D) was mounted to the second load cell, whereas the fabricated patterned material being evaluated (E) was mounted to the bottom of a shallow fluid-filled tank on the fixed load cell. Custom software was written to control the motion of the stages while collecting force data from the load cells. During a single test cycle, the vertical stage was first actuated to press the hemisphere in contact with the fabricated material until a predefined 150 mN preload force was met. Next, while keeping the vertical DOI: 10.1021/la1029245

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Figure 6. Experimentally determined wet shear force of p(DMAco-MEA)-coated fibers with mushroom tip geometry in contact with a 6 mm glass hemisphere submerged in DI water subjected to 150 mN of normal loading is compared with unpatterned and uncoated control samples. Error bars represent standard deviations of 10 measurements per sample.

stage stationary, the horizontal stage was actuated to drag the hemisphere along the length of the material, measuring the shear force at the interface of the two surfaces during this dragging process. Finally, the two stages were retracted to their original position. This testing could also be automated so that the process could be automatically repeated for any predefined number of test cycles. Each sample was first tested 10 times, and Figure 6 shows the average shear force for each of the four tested samples (uncoated and p(DMA-co-MEA)-coated flat samples and uncoated and p(DMA-co-MEA)-coated fibers with mushroom tip geometry.) From these results we can clearly see that there is a shear force enhancement from both patterning and coating. The mushroomtipped, p(DMA-co-MEA)-coated fibers demonstrated the highest shear force of all samples, which represented a 3.8-times enhancement over flat, uncoated samples and a 2.6-times enhancement over untreated patterned fibers. To evaluate the repeatability of the performance of the patterned, coated samples when subjected to shear loading, the shear force evaluation protocol was repeated over 100 test cycles and these results are illustrated in Figure 7. It is clear that there is no deterioration of the shear performance of the material over this number of testing cycles, illustrating the robustness of both the p(DMA-co-MEA) coating when subjected to repeated testing and the ability of the fibers to resist collapse and breaking. Both the adhesion and shear results demonstrate a significant enhancement for the p(DMA-co-MEA) coated mushroom-tipped fibers in contact with a glass hemisphere when compared to unpatterned or uncoated materials. Detailed numerical models describing fiber geometry, material properties, loading conditions, and interfacial conditions with the test substrate could be developed, but this is left as a future work. Instead, we can compare these results

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Figure 7. Experimentally determined repeated wet shear data for 100 test cycles on p(DMA-co-MEA)-coated elastomer pillars with mushroom tip geometry.

with previous experimental studies of mushroom-tipped structures to qualitatively understand the source of this adhesion enhancement. Gorb et al. identified the improved ability of mushroom-tipped structures to resist crack propagation during fiber pull off and attributed this performance to thin lip around the perimeter of the fibers.24 Scaled up versions of mushroom-shaped fibers clearly illustrated this trend, and also clearly demonstrated that fiber pull-off originates at the center of the fiber, and radiates outward, resulting in the formation of a cavity at the fiber tip.25 This cavity formation can result in a pressure difference with respect to the testing environment, which means that suction forces can also contribute to the adhesion of mushroom-tipped fibers. Indeed, this appears to be the case with our uncoated mushroom-tipped structures, which still demonstrate considerable adhesive ability underwater where plain cylindrical fibers and uncoated polyurethanes do not. The patterned, coated materials developed and evaluated in this study demonstrate a clear wet adhesion performance enhancement over uncoated and unpatterned materials, either coated or identically patterned uncoated materials, and previously developed coated and patterned materials. These measured performance enhancements demonstrate the promise of this hybrid patterning and coating approach to adhesive material design for real-world applications. The multistep fabrication protocol can also be adapted for different fiber materials or tip coatings to optimize the functionality of the material for a targeted substrate or application. Future studies in this field should focus on evaluating material performance against actual substrates, such as tissue testing for potential medical device applications or evaluation on rough uneven surfaces for potential robotic applications. Acknowledgment. The authors thank the CMU NanoRobotics Laboratory members for their valuable feedback and suggestions and Rongchao Jin for providing gold nanoparticles for SEM imaging. This work was partially supported by National Science Foundation (CMMI-0800408).

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