The fracture properties and mechanical design of human fingernails
School of Biological Sciences, 3.614 Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT, UK
* Author for correspondence (e-mail: r.ennos{at}man.ac.uk)
Accepted 24 November 2003
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Summary |
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When nails were torn, cracks were always diverted transversely, parallel to the free edge of the nail. Cutting tests showed that this occurred because the energy to cut nails transversely, at approximately 3 kJ m2, was about half that needed (approx. 6 kJ m2) to cut them longitudinally. This anisotropy was imparted by the thick intermediate layer, which comprises long, narrow cells that are oriented transversely; the energy needed to cut this layer transversely was only a quarter of that needed to cut it longitudinally. In contrast the tile-like cells in the thinner dorsal and ventral layers showed isotropic behaviour. They probably act to increase the nail's bending strength, and as they wrap around the edge of the nail, they also help prevent cracks from forming. These results cast light on the mechanical behaviour and care of fingernails.
Key words: fingernails, mechanics, fracture
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Introduction |
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The structure of human nails has been quite well studied, both by
primatologists (Le Gros Clark,
1936; Soligo and Muller,
1998
) and by clinicians
(Lewis, 1954
;
Lewin, 1965
;
Achten, 1981
;
Dykyj, 1989
;
Ditre and Howe, 1992
). The nail
(Fig. 1A,B) is composed of
three layers of keratinous tissue, which are laid down by the matrix at the
base and below the nail (Fig.
1C). The narrow dorsal layer, which is composed of moderately hard
keratin, is laid down along the roof and at the very base of the nail bed,
though it is also laid down along the lateral fold. The harder intermediate
layer is laid down along the proximal nail bed up to the end of the lunula,
while the narrow ventral layer is made of soft keratin, and is laid down
towards the middle of the nail bed. Towards the tip of the nail bed the
protective hyponychium, which seals off the gap between the nail and finger,
is produced. The cells of the nail are produced continuously and become
keratinised, compacted and cemented together, before being forced down the
nail bed; the transition to mature tissue in the main intermediate layer
occurs at the end of the white lunula or `half moon'. The growth rate is
approximately 1 mm per week.
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Despite our extensive knowledge of the structure and development of nails,
little attempt has been made to relate their structure to their mechanical
function. It is generally agreed that nails serve as a stiff backing to the
soft terminal pads, preventing the skin at the ends of the finger and toes
from rolling backwards over the distal phalanx. They thereby help to improve
grip, sensitivity and manipulation of small objects. In addition, of course,
the nails can be used mechanically themselves: to prise open cracks, lever up
objects, and scratch and fight. During all of these activities, nails are
loaded from below and have to resist upward bending forces. In two ways they
are clearly well suited to doing this. First, they are convex in shape like
the channel veins of insects (Wootton,
1981), feather shafts (Purslow and Vincent, 1974), and petioles of
many plants (Vogel, 1988
;
Ennos et al., 2000
). Second,
nail, like hair, horn and hoof, is composed of a composite material, keratin,
in which long, slender
-keratin protein fibres are embedded in an
amorphous protein matrix (Fraser and
MacRae, 1980
), an arrangement which should make it both stiff and
tough. The properties of the nail, however, particularly its fracture
properties, will depend on the shapes of the cells of which it is composed and
the orientation of the keratin fibres within them. Unfortunately, though
previous authors state that the cells are flattened in the plane of the nail
(Achten, 1981
), they give
little information about the shape of the cells or orientation of keratin
within that plane. Baden (1970
)
deduced from x-ray diffraction that the keratin fibres are preferentially
oriented transversely across the nail, but as he failed to distinguish between
the three layers, it was impossible to say whether or not this occurs
throughout the nail's thickness.
From a mechanical viewpoint, the two obvious ways of orienting the fibres
in the nail both have advantages and disadvantages. The optimal way to produce
a nail that would resist upward bending would be for the cells and fibres to
be orientated longitudinally. This has the disadvantage, however, that such a
nail would also readily split longitudinally along its length, damaging the
delicate nail bed. An optimal design for preventing damage to the nail bed
would be to orientate the cells and fibres transversely, parallel to the edge
of the lunula in the manner deduced by Baden
(1970). Any cracks would
therefore tend to run across the nail. Such an orientation of fibres parallel
to the edge of the generative tissue is indeed seen in horses' hooves
(Bertram and Gosline, 1986
;
Kasapi and Gosline, 1997
),
where it helps to prevent cracks running up the hoof. This arrangement works
well in these thick structures, which are loaded mainly in compression.
However, in narrow nails, which are regularly subjected to bending forces,
cracks could readily be produced at the margins or surface of the nail, and
once produced would run across it, so that the nail would readily break.
This study was designed to investigate how the composite human fingernail combines such apparently mutually exclusive properties as the ability to resist bending, to resist cracks forming, and to prevent cracks, once formed, running longitudinally down the nail. To do this, we carried out a series of mechanical tests and microscopic studies. First, we investigated how cracks run through nail material. Next we carried out scissor cutting tests to determine how the toughness of nail material varies with the orientation of the crack driven through it; we also measured the toughness of the individual layers. Finally we investigated the fine structure of nails by taking scanning electron micrographs of the fracture surfaces of nails, and of their outer surface, and carried out polarizing light microscopy to determine the preferred orientations of keratin fibres.
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Materials and methods |
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Tear tests
20 nails were subjected to qualitative tear tests to determine if they tore
preferentially in any orientation. A 1 mm long notch was cut into each nail
section with a razor blade, at either the central point of the distal edge or
the side edge (Fig. 2). The two
`legs' of each nail were then held with tweezers and pulled in opposite
directions, out of the plane of the nail, to give a shearing force that would
cause the crack to propagate through the sample. The samples were then
examined using a Wild M8 dissecting microscope (Heerbrugg, Switzerland) to
determine the orientation of the free-running crack.
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Scissor cutting tests on whole nails
Each nail clipping was cut into four sections, giving two sections from the
central region of the nail and two from the outer edges. The comparative
toughness to fracture between inner and outer sections and between cuts across
and down each nail was then investigated by carrying out scissor cutting tests
on each section; sections from the left hand side of the nail were cut
transversely, while the two from the right hand side were cut
longitudinally.
The scissor cutting tests were similar to those devised by Lucas and
Pereira (1990), and widely
used since to investigate the toughness of leaves and other plant tissues
(Wright and Illius, 1995
;
Choong, 1996
; Lucas et al.,
1997
,
2000
). A pair of precision
scissors (Radio Spares model 539-609, Corby, UK) was mounted on the lower
platform of a model 4301 universal testing machine (Instron, High Wycombe,
UK), with its downward-pointing blade held rigid. The scissors were opened and
the crosshead of the Instron, with a 100 N load cell mounted on it, was then
lowered until a probe on the crosshead just touched the handle of the
scissors. A sample of nail was then removed from the water, dried with tissue
paper to remove surface water and attached, orientated in the correct way, to
the lower blade of the scissors using adhesive gum.
To perform the actual tests, the crosshead of the Instron was then lowered by a distance of 5 mm at a rate of 10 mm min1, causing the blades of the scissors to close and cut through the nail. The force required to do this was measured by the load cell, and an interfaced computer calculated the total energy required by integrating the force over distance. Finally the crosshead and scissors were returned to their original positions and the procedure was repeated, but without any nail. The net energy to cut through the nail was taken to be the difference between the energy required to close the scissors with and without the nail. Finally the toughness of the nail was calculated by dividing the energy required to cut through it by the cross sectional area of the cut surface; this was equal to the length of the cut, measured using calipers, multiplied by the mean thickness of the nail, measured using a micrometer screw gauge and taking three measurements per sample.
The mean and standard deviation of the toughness in the transverse and longitudinal planes of inner and outer nail section were calculated. Paired t-tests were used to determine if there was any difference between the toughness in the two directions and between inner and outer regions of the nail.
Scissor cutting tests on individual layers
Samples of the three nail layers were prepared by wetting clippings for 24
h and then peeling the individual layers apart with forceps. This process
produced good separation of the three layers, at least over small areas. Ten
nail clippings were prepared in this way, and cut in half to give two samples
of each layer either side of the centre of the nail. The samples were then
subjected to scissor cutting, pieces to the left of the clipping being
subjected to transverse cuts and those to the right to longitudinal cuts, and
values for toughness were calculated as described above, measuring the length
of cut and thickness of the sample in the same way. Paired t-tests
were used to determine whether there was a difference in transverse and
longitudinal toughness for each individual layer.
Scanning electron microscopy
Fracture surfaces of nails were prepared by the method used to carry out
the tearing tests. The nails were then mounted on scanning electron microscopy
(SEM) stubs, with the fracture surface upwards, coated in gold and examined
under a Cambridge 360 scanning electron microscope (Cambridge, UK).
Polarized light microscopy
Preferred fibre orientation was tested using polarized light microscopy,
using a method similar to that of Earland et al.
(1962). Sections of each of the
three layers of the nail were produced by placing lengths of nail on a
microscope slide and cutting and scraping away the other layers using a razor
blade. Sections were then observed between crossed Nichol prisms, and the
microscope stage rotated, while the brightness of the image was examined. If
the fibres have a preferential orientation the image should be bright or dark
according to the orientation of the fibres with respect to the prisms, the
image being brightest when the fibres are arranged at 45° to the prisms
and darkest when they are parallel or perpendicular to them. If there is no
preferred orientation the image will remain dark in all orientations.
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Results |
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In contrast, cracks readily propagated in the transverse orientation around the nail, following the curvature of the free edge and lunula (Fig. 2B). Cracks only deviated occasionally from this pathway in the dorsal layer of the nail, where the tear sometimes moved proximally, and the dorsal layer peeled off from the intermediate layer beneath.
Scissor cutting tests on whole nails
The results of the scissor cutting tests
(Fig. 3A) enable us to
understand why there were such differences in fracture properties in the two
directions. The central sections of the nails were almost twice as tough in
the longitudinal plane as in the transverse plane, a difference that the
paired t-test showed to be highly significant
(t13=57.3, P<0.001). The outer sections were
slightly less tough and showed a slightly lower (but still highly significant)
degree of anisotropy (t13=48.6, P<0.001).
|
Scissor cutting tests on individual layers
The results of the tests on individual layers
(Fig. 3B)localised the
anisotropy of the nail to the intermediate layer; it was almost four times as
tough in the longitudinal plane as in the transverse plane, a difference that
the paired t-test showed to be highly significant
(t8=27.7, P<0.001). In contrast both the
dorsal and ventral layer showed essentially isotropic behaviour with low
toughnesses in both the longitudinal and transverse planes. In neither were
differences in toughness between the directions significant.
Scanning electron microscopy
Scanning electron microscopy of the transverse fracture surfaces
(Fig. 4AD) clearly
revealed the three nail layers mentioned by other authors. They also showed
that the layers differed markedly in structure, a fact that could further
explain the results of the mechanical tests. The dorsal layer (seen most
clearly in Fig. 4C) appeared to
be composed of flat, overlapping slate-like sheets, which were oriented in the
plane of the nail. In contrast the thick intermediate layer was more fibrous,
with the fibres oriented transversely, parallel to the free edge of the nail.
Finally, the thin ventral layer was more similar to the dorsal layer. The only
difference in structure occurred at the edge of the nail
(Fig. 4D), where the dorsal and
ventral and bottom layers appeared to become thicker, producing a flat sheet,
which wrapped around the outside to produce a smooth outer edge. The fracture
surface itself was relatively smooth in the intermediate layer
(Fig. 4A,B), where the line of
failure followed the orientation of the fibres. In the dorsal and ventral
layers, in contrast, the fracture surface was more jagged, the fracture
surface being deflected around individual sheets. Sometimes the failure
surface in the dorsal layer moved proximally relative to the fracture of the
intermediate layer (Fig.
4B).
|
Polarized light microscopy
Preferred fibre orientation was only detected in the intermediate layer, in
which fibres were oriented in the transverse direction, parallel to lunula and
hence to the long axis of the cells of which it was composed.
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Discussion |
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Microscopy and toughness tests on the individual layers showed why this anisotropy occurred. The fracture properties of the nail are dominated by the thick intermediate layer, composed of long narrow cells, which are orientated laterally, parallel to the lunula and the free edge of the nail. Only around a quarter of the energy is needed to cut this layer transversely compared with longitudinally; a transverse cut just has to separate the cells whereas longitudinal cuts would have to cut through the cells. This pattern will therefore result in automatic trimming of the nail parallel to the lunula.
The anisotropy is somewhat reduced by the thin isotropic dorsal and ventral
layers that are composed of tile-like cells with randomly oriented keratin
fibres, particularly towards the edge of the nail where they become relatively
thicker. So what are the functions of these layers? One effect they might have
is to help increase the overall toughness of the nail by constraining the
cells of the intermediate layer so that more of the cells have to be cut
through rather than being separated during scissor tests. However, the reason
why the toughness of the complete nail is greater than the average for the
three layers may instead be due to a volume effect. Unlike in hair, cell walls
are very conspicuous in nail (Baden,
1970) and it is possible that cutting through nail could cause
them to buckle plastically, just like the cell walls of wood
(Lucas et al., 1997
). Thicker
samples of nail would consequently be tougher than thin ones because the size
of the plastic zone would be greater, just as in wood
(Lucas et al., 1997
).
The main purpose of the dorsal and ventral layers, however, is probably to
resist longitudinal bending. These two layers, away from the neutral axis of
the nail, are well placed to resist tensile and compressive forces in the
longitudinal direction, which would be set up when the nail is bent upwards,
and also to resist the transverse forces that might be set up by uneven
loading of the nail. The forces would be strongly resisted by their randomly
oriented keratin fibres. This interpretation is supported by Baden's findings
(Baden, 1970) about the
stiffness of human nail tissue. Using bending tests, he found that the bending
modulus of nail, approximately 35 GPa, was similar in the transverse
and longitudinal directions, a finding consistent with the idea that most of
the bending load is resisted by the two isotropic outer layers that are set
furthest from the nail's neutral axis. In contrast, measuring stiffness using
the speed of sound through nail, he found that the Young's modulus was approx.
twice as high (4.3±0.3 GPa) transversely as longitudinally
(2.1±0.3 GPa). This probably occurred because the tensile and
compressive properties of the nail would be dominated by the thick anisotropic
intermediate layer.
The arrangement of fibres in the outer layers into tile-like cells also has a further advantage that they provide a smooth waterproof covering, which can protect the fibrous intermediate layer. Finally, there is one extra level of sophistication in the design. The dorsal and ventral layers wrap around the lateral edge of the nail, producing a smooth covering that prevents potentially dangerous cracks from forming there.
The whole design of the human fingernail leads to only two potential drawbacks, with which most readers may be only too painfully familiar! Because of the structure of the dorsal layer, in which the cells are packed like overlapping tiles, there is no preferred orientation for cracks to propagate in this tissue. This is why when you tear or bite your nails, the dorsal tissue will often delaminate from the intermediate layer and cracks in it will run towards the base of the nail (Figs 3B, 4B). This problem becomes particularly acute towards the lateral edges of the nail where the outer layers are thicker. They start to dominate the fracture properties of the nail in this region and cracks can start running proximally `into the quick', causing damage to the soft skin along the lateral nail fold. Most people will be familiar with the bleeding around the edge of the nail, and the pain that results. The delamination of the tile-like dorsal surface also allows nails to chip relatively easily, spoiling their appearance and reducing their strength.
The principles of the mechanical design of fingernails also have important
implications for nail care. In traditional manicures, nails are filed into a
point rather than being cut parallel to the lunula. From a mechanical
viewpoint one might expect this to be disastrous, because towards the point,
the edge would not be protected by the wrapping round of the dorsal and
ventral layers; cracks would therefore more readily form, and the nail would
be more likely to break. One might also expect varnished nails to be more
prone to destructive cracking than unvarnished ones. The hard outer layer
would reduce the nail's toughness, just as an outer brittle lacquer greatly
reduces the toughness of polymers (Atkins
and Mai, 1985). The varnish, which is isotropic, might also
dominate the fracture properties of the nail and so cracks might be able to
run equally easily in all directions. Experimental work is needed to test
these ideas, but it is clear that our increased understanding of nail
mechanics could help to improve nail care and to develop novel nail care
products and nail prostheses.
Clearly, this preliminary study has only started to scratch the surface of
fingernail design. More needs to be known on the elastic properties of the
three layers and their ultrastructure, as well as their toughness. More also
needs to be known of inter-individual and interracial differences; the effects
of disease and mineral deficiency; and the differences between fingernails and
toenails. Finally, there is a need for interspecific comparisons, to
investigate the changes in structure that accompanied the evolutionary changes
in shape from the insectivore claw to the primate nail. The study of Baden
(1970) also suggests there are
substantial differences in fibre orientation between the nails of different
primates, depending on the shape and curvature of their nails, differences
which need to be investigated more thoroughly. Nonetheless this preliminary
study has shown that human fingernails are extremely sophisticated
structures, apparently well designed for their function, and that an
understanding of the comparative functional morphology of nails and claws is
well within our grasp.
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Acknowledgments |
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