Microindentation and nanoindentation of human fingernails at varying relative humidity
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A. Roland Ennos Faculty of Life Sciences, University of Manchester, Manchester, M1 3NJ, United Kingdom
Stephen J. Eichhorna) Materials Science Centre, School of Materials, University of Manchester, Manchester M1 7HS, United Kingdom (Received 22 July 2008; accepted 30 October 2008)
The indentation properties of human fingernails at varying humidity are reported. The samples were indented using both microindentation, to obtain their Vickers hardness and also nanoindented using a Berkovich indenter tip. The relative humidity (RH) of the samples was controlled by using salt solutions with a sealed and enclosed environment surrounding the testing equipment. It was shown that the Vickers hardness of the samples is sensitive to RH, with recovery of the nail material more readily occurring for nails tested at >55% RH. This recovery mechanism is discussed in terms of the structure of the nails, and this approach is also suggested as a technique for following recovery mechanisms in natural materials under varying humidity. The hardness obtained by nanoindentation is similar to previously published data, but does not change with humidity. The modulus of the nails is also insensitive to relative humidity, but in the same range as the value derived from the microindentation tests.
I. INTRODUCTION
Fingernails are an anatomical feature of primates that are used for a variety of functions including foraging and gripping. The mechanics of nails, in particular fingernails, have not been studied to a large extent. However, recent studies have investigated their anatomy and fracture properties. The nail plate comprises three histological layers of keratinous tissue that are deposited down by the nail matrix at the base of the nail unit beneath the skin.1,2 These layers are arranged in a sandwichlike structure as shown in Fig. 1. The intermediate layer, in which the keratin fibers are oriented laterally, makes up approximately two-thirds of the thickness of the nail. It is enclosed by the thinner dorsal layer, which makes up approximately a quarter of the thickness of the nail, and the even thinner ventral layer that together wrap around the edge of the intermediate layer. In both of these outer layers the keratin fibers show no preferred orientation. The function of the three layers was enlightened by Farren et al.,3 who carried out cutting tests with instrumented scissors. They showed that this design is admirably suited to limit and control fracture. The lateral orientation of the fibers in the thick intermediate layer ensured that the work of fracture was greater proximally a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0105
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 3, Mar 2009 Downloaded: 28 Apr 2015
than laterally, deflecting cracks laterally away from the nail bed and allowing self-trimming. The outer layers of the sandwich meanwhile protected the intermediate layer and prevented cracks forming at its edge. A disadvantage was that the nail l
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