Biological Adhesion for Locomotion on Rough Surfaces: Basic Principles and A Theorist's View

MRS Bulletin 05/2007; 32(06):486 - 490. DOI: 10.1557/mrs2007.82

ABSTRACT Surface roughness is the main reason why macroscopic solids usually do not adhere to each other with any measurable strength, and a root-mean-square roughness amplitude of only 1 μm is enough to completely remove the adhesion between normal rubber (with an elastic modulus E ≈ 1 MPa) and a hard, nominally flat substrate. Biological adhesion systems used by insects and geckos for locomotion are built from a relatively stiff material (keratin or chitin–protein composite with E ≈ 1 GPa). Nevertheless, strong adhesion is possible even to very rough substrate surfaces by using noncompact solid structures consisting of fibers (setae) and plates (spatulae). Biological systems use a hierarchical building principle, where the thickness of the fibers or plates decreases as one approaches the outer surface of the attachment pad, to optimize the binding to rough surfaces while simultaneously avoiding elastic instabilities, for example, lateral bundling of fibers.

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    ABSTRACT: The contact of adhesive structures to rough surfaces has been difficult to investigate as rough surfaces are usually irregular and opaque. Here we use transparent, microstructured surfaces to investigate the performance of tarsal euplantulae in cockroaches (Nauphoeta cinerea). These pads are mainly used for generating pushing forces away from the body. Despite this biological function, shear stress (force per unit area) measurements in immobilized pads showed no significant difference between pushing and pulling on smooth surfaces and on 1-microm high microstructured substrates, where pads made full contact. In contrast, on 4-mum high microstructured substrates, where pads made contact only to the top of the microstructures, shear stress was maximal during a push. This specific direction dependence is explained by the interlocking of the microstructures with nanometre-sized "friction ridges" on the euplantulae. Scanning electron microscopy and atomic force microscopy revealed that these ridges are anisotropic, with steep slopes facing distally and shallow slopes proximally. The absence of a significant direction dependence on smooth and 1-microm high microstructured surfaces suggests the effect of interlocking is masked by the stronger influence of adhesion on friction, which acts equally in both directions. Our findings show that cockroach euplantulae generate friction using both interlocking and adhesion.
    Journal of Comparative Physiology 08/2009; 195(9):805-14. · 1.86 Impact Factor
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    ABSTRACT: The exceptionally adhesive foot of the gecko remains clean in dirty environments by shedding contaminants with each step. Synthetic gecko-inspired adhesives have achieved similar attachment strengths to the gecko on smooth surfaces, but the process of contact self-cleaning has yet to be effectively demonstrated. Here, we present the first gecko-inspired adhesive that has matched both the attachment strength and the contact self-cleaning performance of the gecko's foot on a smooth surface. Contact self-cleaning experiments were performed with three different sizes of mushroom-shaped elastomer microfibres and five different sizes of spherical silica contaminants. Using a load-drag-unload dry contact cleaning process similar to the loads acting on the gecko foot during locomotion, our fully contaminated synthetic gecko adhesives could recover lost adhesion at a rate comparable to that of the gecko. We observed that the relative size of contaminants to the characteristic size of the microfibres in the synthetic adhesive strongly determined how and to what degree the adhesive recovered from contamination. Our approximate model and experimental results show that the dominant mechanism of contact self-cleaning is particle rolling during the drag process. Embedding of particles between adjacent fibres was observed for particles with diameter smaller than the fibre tips, and further studied as a temporary cleaning mechanism. By incorporating contact self-cleaning capabilities, real-world applications of synthetic gecko adhesives, such as reusable tapes, clothing closures and medical adhesives, would become feasible.
    Journal of The Royal Society Interface 01/2014; 11(94):20131205. · 4.91 Impact Factor
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