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My goal is to control the haptic properties of hair-like polymer arrays modeled after gecko setae by tuning the design parameters of the constituent block copolymers. Geckos employ a digital adhesive system that enables them to sustain large normal and shear forces yet still ambulate quickly. The origin of this behavior lies in the unique structure of hairs located on the geckos’ toes. The development of gecko-like adhesives is of current interest in the design of climbing robots. Such robots will find applications in search and recovery, surgical procedures, and intelligence. I am working with an interdisciplinary team including biologists and electrical engineers to synthesize novel adhesives that can be applied to climbing robots. Gecko setae are micron-scale hairs, embedded in a soft toe pad, that branch into nanoscale stalks capped by a flattened spatula. (see Figure 1) The hierarchical structure of the setae promotes adhesion to surfaces over a broad range of length scales, enabling geckos to adjust to varying degrees of surface roughness. Nanoscale roughness requires the compliant stalks to deform into surface irregularities while larger scale roughness requires the entire seta to bend. Setal compliance is necessary to ensure contact between the spatulae and substrate. Van der Waals forces at this contact point are expected to be the principal mechanism of gecko adhesion. This assumption is drawn from a study that shows geckos are able to adhere equally well to hydrophobic and hydrophilic surfaces so long as the substrate is polarizable [1]. The Van der Waals model of adhesion emphasizes the geometry of the spatula over material composition, affording my experiment a greater degree of flexibility in the materials that I can use to model gecko hairs.
Figure 1 [2] I plan to model the adhesive system with arrays of oriented polystyrene (PS) cylinders on a silicon substrate. Diblock copolymers display a stable hexagonally packed cylindrical morphology across a specific range of minor phase volume fractions. I will take advantage of this equilibrium diblock separation to create cylindrical domains of polystyrene (PS) in a matrix of polymethylmethacrylate (PMMA). The polymer will be exposed to ultraviolet radiation which simultaneously crosslinks the PS and depolymerizes the PMMA. The PMMA matrix can then be rinsed away with acetic acid, leaving an array of hexagonally packed hairs [3]. The elastic modulus of cross-linked PS is 2 GPa, well within the 1-15 GPa range of elastic moduli observed for the stalks of gecko spatulae. The effective elastic modulus for an array of stalks is significantly less than the modulus for a single stalk because the hierarchical structure of the seta lends compliance to the overall array. I can reasonably expect to model a soft adhesive with stiffer polymer cylinders because the mechanical testing procedure mounts the stiff array on a compliant wire, simulating a gecko seta. This does not preclude my intentions to vary the compliance of stalks and other physical properties of the array. The compliance of the stalks is less important for adhesion, since geometry controls the Van der Waals forces, but becomes significant for minimizing the phenomena of clumping. Clusters of stalks form aggregates that reduce the surface free energy at the cost of straining the stalks. Stiffer stalks with greater spacing tend to minimize clumping, but this also reduces the density of the array. A high density of hairs enhances adhesion through the principle of contact splitting, whereby multiple surface contacts display a greater maximum pull-off force than a single contact of the same surface area. However, a high density of synthesized hairs results in clumping that is rarely seen in the animal. Gecko setae exhibit a tetragonal rather than hexagonal packing, and it is believed that this less energetically favorable configuration prevents clumping. Tetragonally packed diblocks have not been observed, but there is the possibility of triblock systems with this morphology. The adjustable parameters that govern compliance of the array are the orientation of cylinders on the substrate, the aspect ratio of the cylinders, their density, and geometric arrangement. The force with which the hair contacts a surface is directly proportional to the modulus and the radius, and inversely proportional to the length of the hair. While the modulus of PS is set, the compliance can be adjusted through the aspect ratio. The length of the hairs depends on the thickness of the spin-coated film; the diameter is controlled by the size of the PS block. I can only vary the block within the range of volume fractions that produce a cylindrical morphology, but the thickness of the film is more flexible to adjustment. The orientation of the hairs affects the angle with which they contact the surface, determining the strength of the bond formed. Controlling the orientation of the cylinders is the most difficult task that I anticipate during this project. For preliminary trials I will use two known methods of orientation: the first technique employs a substrate with equal affinity for both copolymers to stand the cylinders on end. I will create such a surface by coating the silicon substrate in a uniform monolayer of octadecyltrichlorosilane (OTS). The disadvantage of this method is that the orientation effect only propagates through several hundred nanometers of film thickness, whereas the setae I want to model are on the micron scale. The second orientating technique accommodates a thicker film. After spin coating the polymer on a silicon substrate, I will sandwich the film beneath an aluminum wafer, and anneal the polymer in the presence of an electrical field. This orients the cylinders normal to the substrate. In the future I would like to try sandwiching different polymer film thicknesses between two silanated substrates, combining the two techniques, and using different monolayers on the substrate. During the term of this project I will develop skills in synthesizing and characterizing polymers. To this end I will be trained in the use of AFM, SEM, and neutron scattering. I will design a set up that allows me to apply high voltage across two electrodes of known spacing. My most difficult objective will be to manipulate the orientation and spacing of polymer cylinders on a substrate. The final goal is to develop a bench scale procedure for creating gecko-like adhesives.
[1] Autumn, K. et al. PNAS, 2002, 99, 19, 12252. [2] Sitti, Metin. Journal of Adhesion Science and Tech. 2003, 18, 7, 1055. [3] Russel,T. et al. Science, 2000, 290, 2126. 4. A. J. Nedoma, M. L. Robertson, N. S. Wanakule, N. P. Balsara, "Measurements of the Composition and Molecular Weight Dependence of the Flory-Huggins Interaction Parameter." Macromolecules, In Press. 3. A. J. Nedoma, M. L. Robertson, N. S. Wanakule, N. P. Balsara, "Measurements of the Flory-Huggins Interaction Parameters using a series of Critical Binary Blends." Industrial and Engineering Chemistry Research, Accepted. 2. M. J. Park, A. J. Nedoma, P. L. Geissler, A. Jackson, D. Cookson, N. P. Balsara, “Humidity-Induced Phase Transitions in Ion-Containing Block Copolymer Membranes”. Macromolecules. 41(6), 2271-2277, 2008. 1. N. S. Wanakule, A. J. Nedoma, M. L. Robertson, Z. Fang, A. J. Jackson, B. A. Garetz, N. P. Balsara, “Characterization of micron-sized periodic structures in multicomponent polymer blends by ultra-small-angle neutron scattering and optical microscopy,” Macromolecules 41(2), 471-477, 2008.
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