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First published online January 27, 2004
Journal of Experimental Biology 207, 735-741 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00814
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The fracture properties and mechanical design of human fingernails

L. Farren, S. Shayler and A. R. Ennos*

School of Biological Sciences, 3.614 Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT, UK



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  		Fig. 1. Longitudinal section (A) and top view (B) of the human fingernail, showing its gross structure and relation to the surrounding structures, and (C) schematic longitudinal and transverse sections through the human finger, showing the sites of nail generation. After Lewis (1954Go).

 


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  		Fig. 2. Typical directions of crack propagation which occur when attempts are made to tear nails (A) in the longitudinal distal to proximal direction and (B) a transverse direction. Broken lines show failure along the entire nail. The dotted line shows how failure can occur in the dorsal layer. In A the fracture is deflected transversely.

 


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  		Fig. 3. Results of the cutting tests on nail clippings (A) giving the mean toughness (± S.D., N=15) of central and outer sections of the nail, when cut in the longitudinal and transverse directions and (B) showing the mean toughness (± S.D., N=10) of each of the three layers of the fingernail, when cut in the longitudinal and transverse directions.

 


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  		Fig. 4. Scanning electron micrographs of the fracture surfaces of torn nail clippings, with the main fracture surface running in the transverse direction. (A) In the central area of the nail, fracture follows the orientation of the keratin fibres within the thick intermediate layer. Fracture through the plate-like cells of the dorsal and ventral layers is more jagged. (B) Towards the edge of the nail (left), the failure through the dorsal layer often moves more proximally as it peels off the intermediate layer. (C) A close up of the plate-like dorsal layer and fibrous intermediate layer. (D) Lateral edge of the nail cut with scissors. The dorsal and ventral layers get thicker towards the edge and wrap around the end of the intermediate layer, helping prevent cracks forming. Scale bars, 200 µm (A,B), 100 µm (C,D).

 

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© The Company of Biologists Ltd 2004