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First published online December 26, 2008
Journal of Experimental Biology 212, 155-162 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.019232

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Ultrastructure and physical properties of an adhesive surface, the toe pad epithelium of the tree frog, Litoria caerulea White

Ingo Scholz, W. Jon P. Barnes^*, Joanna M. Smith and Werner Baumgartner

Department of Cellular Neurobionics, Institute of Biology 2, RWTH-Aachen, Kopernikusstrasse 16, 52056 Aachen, Germany

^* Author for correspondence at present address: Centre for Cell Engineering, Joseph Black Building, Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK (e-mail: j.barnes{at}bio.gla.ac.uk)

Accepted 10 November 2008

Knowledge of both surface structure and physical properties such as stiffness and elasticity are essential to understanding any adhesive system. In this study of an adhesion surface in the tree frog, Litoria caerulea White, a variety of techniques including atomic force microscopy were used to investigate the microstructure and properties of an epithelium that adheres through wet adhesion. Litoria toe pads consist of a hexagonal array of flat-topped epithelial cells, separated by mucus-filled channels. Under an atomic force microscope, this `flat' surface is highly structured at the nanoscale, consisting of a tightly packed array of columnar nanopillars (described as hemidesmosomes by previous authors), 326±84 nm in diameter, each of which possesses a central dimple 8±4 nm in depth. In fixed tissue (transmission electron microscopy), the nanopillars are approximately as tall as they are broad. At the gross anatomical level, larger toe pads may be subdivided into medial and lateral parts by two large grooves. Although the whole toe pad is soft and easily deformable, the epithelium itself has an effective elastic modulus equivalent to silicon rubber (mean E_eff=14.4±20.9 MPa; median E_eff=5.7 MPa), as measured by the atomic force microscope in nanoindentation mode. The functions of these structures are discussed in terms of maximising adhesive and frictional forces by conforming closely to surface irregularities at different length scales and maintaining an extremely thin fluid layer between pad and substrate. The biomimetic implications of these findings are reviewed.

Key words: tree frog, adhesion, electron microscopy, atomic force microscopy, effective elastic modulus, tribology

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