Burak Aksak

Gecko‐Inspired Directional and Controllable Adhesion

The structure that allows gecko lizards and insects to climb vertical and inverted surfaces with ease has been studied extensively since the mechanism of attachment was shown to be due dominantly to intermolecular surfaces forces. 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. These structures exploit the weak van der Waals interaction forces at the tips of the branching keratinous fiber arrays through their conformation into intimate contact with climbing surfaces, creating a large overall adhesion through millions of sub-micrometer scale contact points. These many contacts also resist peeling by disrupting crack propagations at the interface. Since these discoveries, many attempts have been made to replicate the structures seen in animals using synthetic materials to create adhesives with similar adhesive characteristics. Applications for such adhesives include wall-climbing robots, tissue adhesives for medical applications, and grippers for manipulation. Autumn et al. molded the first synthetic mimics by creating templates using an sharp probe followed by nanomolding. This effort was followed by attempts using electron-beam lithography, carbon nanotubes, nanodrawing, and micro/nanomolding to form high aspect ratio fibrillar structures. Higher adhesion was seen in structures with wider flat mushroom tips, demonstrating that tip size is an important parameter for generating large forces. The wider flat tips increase the contact area and may eliminate the stress singularities along the edge of the interface, as described by Bogy. Adhesion strengths as high as 180 kPa have been demonstrated with vertically aligned mushroom tipped microfibers, although damage occurs to the structures during detachment. Fibrillar structures have also been fabricated to increase (or decrease) friction. In addition, fiber surfaces have been created that 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-micrometer diameter fibers have been shown to exhibit shear adhesion without requiring high preloading.

Adhesion and anisotropic friction enhancements of angled heterogeneous micro-fiber arrays with spherical and spatula tips

Angled polyurethane fiber arrays are modified by adding soft spherical and spatula shaped tips via dipping. These fibers are characterized for adhesion and friction and compared with unmodified fibers and flat material samples. Sphere and spatula tip fiber samples demonstrate increased adhesion, with 10 and 23 times the maximum adhesion of the unmodified fiber sample, respectively. The sphere and spatula tip fiber samples also show increased friction, with 1.6 and 4.7 times the maximum friction of the unmodified fiber sample, respectively. Friction and adhesion are simultaneously observed in a synthetic dry angled fibrillar adhesive sample (spatula tip fiber sample). The direction dependent friction of angled fibers is investigated. The adhesion and friction results reported in this paper suggest that fibers with negligible adhesion can be modified to exhibit both significant adhesion and friction enhancements by the proposed fiber tip modifications.

Gecko inspired micro-fibrillar adhesives for wall climbing robots on micro/nanoscale rough surfaces

This paper presents the fabrication, characterization and testing of bio-inspired synthetic dry adhesive fiber arrays. Fibers were fabricated via micromolding followed by spatula tip formation via dipping. Arrays of fibers with diameters between 28 mum and 57 mum and a height of 114 mum were fabricated with high uniformity on 6.25 cm2 areas with up to 95% yield. Adaptation to uneven surfaces was observed with fiber elongations over 6 times the original height of the fiber. The unstructured sample exhibited 1.6 times as much adhesion as the fiber array sample for the flat punch indenter. However, fibrillar samples demonstrated up to 5.3 times as much adhesion as the unstructured sample for the hemispherical indenter. The fibrillar adhesive sample was implemented on a wall climbing robot which was able to carry itself and climb a distance on a painted wall and wood door.

Enhanced friction of elastomer microfiber adhesives with spatulate tips

Previous studies have demonstrated that gecko foot-hair inspired elastomer microfibers with spatulate tips have significant adhesion enhancement compared to the flat elastomer surface. In this study, we report the friction enhancement of these highly adhesive fibers and analyze the relation between adhesion and friction of elastomer microfiber arrays with spatulate tips. Fabricated polyurethane fiber arrays with spatulate tips demonstrate macroscale static friction pressures up to 41N∕cm2 for a preload pressure of 1.5N∕cm2 on a 6mm diameter smooth glass hemisphere.

Friction of partially embedded vertically aligned carbon nanofibers inside elastomers

Vertically aligned carbon nanofibers partially embedded inside polyurethane (eVACNFs) are proposed as a robust high friction fibrillar material with a compliant backing. Carbon nanofibers with 50–150nm in diameter and 20–30μm in length are vertically grown on silicon and transferred completely inside an elastomer by vacuum molding. By using time controlled and selective oxygen plasma etching, fibers are partially released up to 5μm length. Macroscale friction experiments show that eVACNFs exhibit reproducible effective friction coefficients up to 1. Besides high friction, the proposed fabrication method improves fiber-substrate bond strength, and enables uniform height nanofibers with a compliant backing.

Thermoresponsive hydrogel scaffolds with tailored hydrophilic pores.

Thermoresponsive hydrogels with efficient water-release channels were prepared by incorporating star-shaped macromolecular pore precursors, with degradable disulfide crosslinked cores and hydrophilic poly(ethylene oxide) (PEO) arms, into the gel network. The gel framework exhibiting lower critical solution temperature (LCST) behavior was synthesized by atom transfer radical polymerization (ATRP) of 2-(2-methoxyethoxy)ethyl methacrylate and ethylene glycol dimethacrylate. The incorporation of degradable star macromolecules (dSM) was facilitated by growing the gel from ATRP initiator sites contained within their cores. Following the formation of the gel, the dSM cores were degraded, yielding uniform pores lined with hydrophilic PEO chains. The effect of hydrophilic pores on thermoresponsive hydrogel performances was studied by comparing hydrogels containing hydrophilic pores with analogous hydrogels with neutral pores or with pore-free controls. Dye absorption/release experiments pointed to the suitability of newly synthesized hydrogels as controlled-release media, for example, for drug delivery. Cell culture experiments confirmed their nontoxicity and biocompatibility (cell viability >98%).

Dangling Chain Elastomers as Repeatable Fibrillar Adhesives

This work reports on repeatable adhesive materials prepared by controlled grafting of dangling hetero chains from polymer elastomers. The dangling chain elastomer system was prepared by grafting poly(n-butyl acrylate) (PBA) chains from prefunctionalized polydimethylsiloxane (PDMS) elastomer networks using atom transfer radical polymerization. To study the effects of chain growth and network strain as they relate to network adhesion mechanics, various lengths of PBA chains with degree of polymerizations (DP) of 65, 281, 508, and 1200 were incorporated into the PDMS matrix. PBA chains with a DP value of 281 grafted from a flat PDMS substrate showed the highest (approximately 3.5-fold) enhancement of nano- and macroscale adhesion relative to a flat raw (ungrafted and not prefunctionalized) PDMS substrate. Moreover, to study the effect of PBA dangling chains on adhesion in fibrillar elastomer structures inspired by gecko foot hairs, a dip-transfer fabrication method was used to graft PBA chains with a DP value of 296 from the tip endings of mushroom-shaped PDMS micropillars. A PBA chain covered micropillar array showed macroscale adhesion enhancement up to approximately 7 times relative to the flat ungrafted prefunctionalized PDMS control substrate, showing additional nonoptimized approximately 2-fold adhesion enhancement due to fibrillar structuring and mushroom-shaped tip ending. These dangling hetero chains on elastomer micro-/nanofibrillar structures may provide a novel fabrication platform for multilength scale, repeatable, and high-strength fibrillar adhesives inspired by gecko foot hairs.

The optimal shape of elastomer mushroom-like fibers for high and robust adhesion.

Summary Over the last decade, significant effort has been put into mimicking the ability of the gecko lizard to strongly and reversibly cling to surfaces, by using synthetic structures. Among these structures, mushroom-like elastomer fiber arrays have demonstrated promising performance on smooth surfaces matching the adhesive strengths obtained with the natural gecko foot-pads. It is possible to improve the already impressive adhesive performance of mushroom-like fibers provided that the underlying adhesion mechanism is understood. Here, the adhesion mechanism of bio-inspired mushroom-like fibers is investigated by implementing the Dugdale–Barenblatt cohesive zone model into finite elements simulations. It is found that the magnitude of pull-off stress depends on the edge angle θ and the ratio of the tip radius to the stalk radius β of the mushroom-like fiber. Pull-off stress is also found to depend on a dimensionless parameter χ, the ratio of the fiber radius to a length-scale related to the dominance of adhesive stress. As an estimate, the optimal parameters are found to be β = 1.1 and θ = 45°. Further, the location of crack initiation is found to depend on χ for given β and θ. An analytical model for pull-off stress, which depends on the location of crack initiation as well as on θ and β, is proposed and found to agree with the simulation results. Results obtained in this work provide a geometrical guideline for designing robust bio-inspired dry fibrillar adhesives.

The effect of aspect ratio on adhesion and stiffness for soft elastic fibres

The effect of aspect ratio on the pull-off stress and stiffness of soft elastic fibres is studied using elasticity and numerical analysis. The adhesive interface between a soft fibre and a smooth rigid surface is modelled using the Dugdale–Barenblatt model. Numerical simulations show that, while pull-off stress increases with decreasing aspect ratio, fibres get stiffer. Also, for sufficiently low aspect ratio fibres, failure occurs via the growth of internal cracks and pull-off stress approaches the intrinsic adhesive strength. Experiments carried out with various aspect ratio polyurethane elastomer fibres are consistent with the numerical simulations.

An experimental analysis of elliptical adhesive contact

The elliptical adhesive contact is studied experimentally utilizing two hemicylinders of elastomeric poly(dimethylsiloxane) (PDMS). Experimental results are compared with the recent approximate Johnson–Kendall–Roberts (JKR) theory for elliptical contacts, and the deviation of the experiments from this theory is discussed in detail. To do this, the cylinders are placed with different skew angles with respect to each other in order to emulate the effect of orientation. The maximum adhesion force and the size of the contact zone are determined experimentally under the action of surface energy. The difference of the maximum adhesion force between experiments and theory is found to increase as the contact area goes from mildly elliptical to slim elliptical contact. Similarly, it is observed that the contact area can be approximated to have elliptical geometry for a wide range of skew angles while a deviation is observed for slim elliptical contacts. Moreover, the reduction in the contact area is observed to be...