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«UNIVERSITY OF CALIFORNIA Santa Barbara Design and Characterization of Fibrillar Adhesives A Dissertation submitted in partial satisfaction of the ...»

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Revisions to the microfabrication processes are another option for creating better adhesives. A sticky default state was present with the semicircular fibers because the mold was smooth on the top of the fiber. Using a thicker polymer layer for the angled lithography, such as polymethylglutarimide (PMGI) SF-19, would eliminate reaching the glass wafer underneath. Taller fibers could also create higher shear and adhesion forces by making contact with the opposing surface over greater lengths, as well as offer greater compliance to surface roughness. The maximum angle from vertical achievable with the angled lithography was found to be 20◦. Dry etching techniques using a Faraday cage have been used to create angled fibers angled 30–60◦ from vertical [47]. Such a process would allow for a greater tilt angle and a larger range of angles for better understanding of the influence of tilt. An addition benefit of the angled dry etching technique is the mold material would be made of silicon which has been show to have greater reusability than the polymer molds.

Chapter 7. Conclusions and Future Work The pressure values of the developed adhesives have yet to surpass those achieved by the gecko.

To create higher forces, the fiber density should be able to be increased for larger areas of contact. No fiber-to-fiber adhesion was seen for the rectangular flap or semicircular adhesives, suggesting that the fibers can be placed closer to one another. An alternative approach to higher force values involves optimizing the fiber. The use of materials other than PDMS allows for greater freedom when selecting geometric features such as aspect ratio and minimum fiber size. A different material will also allow specific selection of tensile strength, elongation to break, toughness, and Young’s modulus. Material selection will be limited by the ease of which it can be integrated with the fabrication process. Materials which can’t be separated from the mold or have a tendency to break can’t be used. Depending on the selections made, anisotropy could be further increased from the present values.

7.2.2 Integration with Climbing Robots An active area of research and potential application for the gecko-like adhesives being developed is integration with climbing robots. Climbing robots can be used for remote sensing applications or exploration of dangerous environments without risking human life. Current robots have successfully climbed a variety of surfaces, but are not yet practical to use in their envisioned environments. For Chapter 7. Conclusions and Future Work the adhesives presented here to be integrated with climbing robotic platforms, the microfabrication techniques must be expanded to larger areas, testing must be increased to centimeter dimensions, and environmental characterization must be performed.

The current microfabrication processes can reliably create adhesive patches on the centimeter scale. However, climbing robots can weigh up to 4 kg [39] and will require large patch sizes to support the robot during inverted movement and vertical climbing. A step-and-repeat approach using a small area of the photolithography mask was chosen in the past for characterization of multiple fiber designs using a single mask. Future designs will need to use modeling to optimize fiber shape, dimensions, and density so that single designs can be patterned over an entire 4-inch wafer. An alternative to full wafer patterning involves designing processes to join smaller samples together over a large area. The microfabrication of the angled flaps and semicircular fibers was only possible over a maximum area of 2 cm2. When larger areas were attempted, delamination was seen to occur between the the PMGI and glass wafer due to stress mismatches between the mold material and wafer. A repeatable method to join these smaller pieces would allow for sizes relevant to climbing robots to be created.

Although the microfabrication process currently creates patch areas greater than those needed by the 4 mm diameter glass puck, there is not a method to Chapter 7. Conclusions and Future Work characterize them. Modifications to the Bio-F test apparatus would allow for large test areas by replacing the glass puck with a glass slide or wafer. In this manner, the size of the adhesive would limit the test area. With larger patch sizes, the glass leveling attachment would likely need to be modified. Replacing the screws with micrometers would reduce the smallest incremental motion and improve alignment when leveling the two surfaces. A second option for less precise measurements would be to test the adhesives without precise alignment using load-pull and load-drag tests on the macroscale test station. The macroscale test station requires placing the adhesive by hand and uses weights to load the adhesive sample. The lack of alignment procedures leveling the two surfaces is similar to adhesive placement when used with a climbing robot and offers insight into the scalability of the adhesive.

The surfaces that a climbing robot would encounter are much different than those used in testing the adhesive. Surfaces are likely to be rough on multiple length scales and dirty. The self-cleaning property has been discussed in Section 7.2.1, but the results have great relevance to climbing robots. Tests of synthetic adhesives have been performed against a variety of rough surfaces. The rough substrate has been patterned in a regular manner [47] and in some cases, the pattern wavelength of the rough surface has matched the wavelength of the adhesive structures [63]. Other testing has used common surfaces with large gaps in roughChapter 7. Conclusions and Future Work ness [4] or randomly rough surfaces with variations in roughness only extending over slightly more than an order of magnitude [118]. Adhesive characterization over all relevant roughness scales, as well as the ability to adapt to these variations in roughness, will be needed for truly mobile robots.





A final area of work will be to characterize the adhesive in different environmental conditions. The velocities used for the adhesive testing of rectangular flaps and semicircular fibers has been very low to minimize any rate-dependent effects.

However, the retraction speed has been shown to influence the adhesion of fibrillar structures [1] and viscoelastic effects have been used for pick-and-place operations of microplatelets and glass cover slips [73]. Similar characterization of rate effects on future adhesives will need to be performed on the Bio-F tester. The Bio-F test apparatus does not have the ability to control the humidity, which has been shown to influence the adhesive properties of gecko spatulae at the nanoscale [43].

Existing collaboration with Jacob Israelachvili’s lab would allow the adhesives to be tested in different humidities using the surface forces apparatus (SFA).

Clearly, there is much work needed to fully develop a gecko-inspired adhesive.

Adhesive fiber hierarchy, self-cleaning properties, and higher force values all must be introduced into the design of future adhesives to have gecko-like behavior.

Increasing the adhesive area, characterizing these larger patches on a variety of surfaces, and characterization of other effects such as testing speed and humidity Chapter 7. Conclusions and Future Work all must be performed before the adhesives can be integrated on climbing robots.

The potential applications of a true gecko-like adhesive are limited only by one’s imagination which makes future research in this area an exciting endeavor.

Bibliography [1] U. Abusomwan and M. Sitti, Effect of retraction speed on adhesion of elastomer fibrillar structures, Applied Physics Letters, 101 (2012), p. 211907.

[2] B. Aksak, M. P. Murphy, and M. Sitti, Adhesion of biologically inspired vertical and angled polymer microfiber arrays, Langmuir, 23 (2007), pp. 3322–3332.

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[5] K. Autumn, Properties, principles, and parameters of the gecko adhesive system, in Biological Adhesives, A. Smith and J. Callow, eds., Springer Berlin Heidelberg, 2006, pp. 225–256.

[6] K. Autumn, Gecko adhesion: Structure, function, and applications, Mrs Bulletin, 32 (2007), pp. 473–478.

[7] K. Autumn, A. Dittmore, D. Santos, M. Spenko, and M. Cutkosky, Frictional adhesion: a new angle on gecko attachment, Journal of Experimental Biology, 209 (2006), pp. 3569–3579.

[8] K. Autumn and N. Gravish, Gecko adhesion: evolutionary nanotechnology, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 366 (2008), pp. 1575–1590.

Bibliography [9] K. Autumn, S. T. Hsieh, D. M. Dudek, J. Chen, C. Chitaphan, and R. J. Full, Dynamics of geckos running vertically, J Exp Biol, 209 (2006), pp. 260–272.

[10] K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full, Adhesive force of a single gecko foot-hair, Nature, 405 (2000), pp. 681–685.

[11] K. Autumn and A. M. Peattie, Mechanisms of adhesion in geckos, Integrative and Comparative Biology, 42 (2002), pp. 1081–1090.

[12] K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full, Evidence for van der waals adhesion in gecko setae, Proceedings of the National Academy of Sciences of the United States of America, 99 (2002), pp. 12252–12256.

[13] M. D. Bartlett, A. B. Croll, D. R. King, B. M. Paret, D. J.

Irschick, and A. J. Crosby, Looking beyond fibrillar features to scale gecko-like adhesion, Advanced Materials, 24 (2012), pp. 1078–1083.

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McMeeking, Peeling of a tape with large deformations and frictional sliding, Journal of the Mechanics and Physics of Solids, 61 (2013), pp. 1265 – 1279.

[15] B. Bhushan, A. G. Peressadko, and T.-W. Kim, Adhesion analysis of two-level hierarchical morphology in natural attachment systems for ’smart adhesion’, Journal of Adhesion Science and Technology, 20 (2006), pp. 1475– 1491.

[16] A. Bietsch and B. Michel, Conformal contact and pattern stability of stamps used for soft lithography, Journal of Applied Physics, 88 (2000), pp. 4310–4318.

[17] F. P. Bowden and D. Tabor, The friction and lubrication of solids, vol. 1, Clarendon Press, Oxford, 2001.

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Marshall, L. Pruitt, and C. M. Puttlitz, Nanoindentation of polydimethylsiloxane elastomers: Effect of crosslinking, work of adhesion, and fluid environment on elastic modulus, Journal of Materials Research, 20 (2005), pp. 2820–2830.

Bibliography [19] S. Chary, J. Tamelier, and K. Turner, A microfabricated geckoinspired controllable and reusable dry adhesive, Smart Materials and Structures, 22 (2013), p. 025013.

[20] M. K. Chaudhury and G. M. Whitesides, Direct measurement of interfacial interactions between semispherical lenses and flat sheets of poly(dimethylsiloxane) and their chemical derivatives, Langmuir, 7 (1991), pp. 1013–1025.

[21] B. Chen, P. Wu, and H. Gao, Pre-tension generates strongly reversible adhesion of a spatula pad on substrate, Journal of The Royal Society Interface, 6 (2009), pp. 529–537.

[22] K. M. Choi and J. A. Rogers, A photocurable poly(dimethylsiloxane) chemistry designed for soft lithographic molding and printing in the nanometer regime, Journal of the American Chemical Society, 125 (2003), pp. 4060– 4061.

[23] J. Y. Chung and M. K. Chaudhury, Roles of discontinuities in bioinspired adhesive pads, Journal of The Royal Society Interface, 2 (2005), pp. 55–61.

[24] C. A. Dahlquist, Pressure-sensitive adhesives, in Treatise on Adhesion and Adhesives, R. Patrick, ed., vol. 2, Marcel Dekker, New York, 1969, pp. 219–260.

[25] J. Davies, S. Haq, T. Hawke, and J. Sargent, A practical approach to the development of a synthetic gecko tape, International Journal of Adhesion and Adhesives, 29 (2009), pp. 380 – 390.

[26] P. Day, M. Cutkosky, R. Greco, and A. McLaughlin, Effects of He++ ion irradiation on adhesion of polymer microstructure-based dry adhesives, Nuclear Science and Engineering, 167 (2011), pp. 242–247.

[27] A. delCampo, C. Greiner, I. lvarez, and E. Arzt, Patterned surfaces with pillars with controlled 3D tip geometry mimicking bioattachment devices, Advanced Materials, 19 (2007), pp. 1973–1977.

[28] B. V. Derjaguin, V. M. Muller, and Y. P. Toporov, Effect of contact deformations on the adhesion of particles, Journal of Colloid and Interface Science, 53 (1975), pp. 314–326.

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[33] S. Gorb, M. Varenberg, A. Peressadko, and J. Tuma, Biomimetic mushroom-shaped fibrillar adhesive microstructure, Journal of The Royal Society Interface, 4 (2007), pp. 271–275.

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