|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online November 14, 2008
Journal of Experimental Biology 211, 3677-3681 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.023218
The effect of humidity on the fracture properties of human fingernails
1 Materials Science Centre, School of Materials, Grosvenor Street, University of
Manchester, M1 7HS, UK
2 Faculty of Life Sciences, Jackson's Mill, PO Box 88, Sackville Street,
Manchester, M60 1QD, UK
* Author for correspondence (e-mail: s.j.eichhorn{at}manchester.ac.uk)
Accepted 26 September 2008
 |
Summary |
|---|
 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Key words: fingernails, mechanics, moisture, fracture
|
INTRODUCTION |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
). Although the
structure of fingernails has been comprehensively reported in the literature
by primatologists and clinicians (Achten,
1981; Ditre and Howe,
1992; Dykyj, 1989;
Le Gros Clark, 1936;
Lewin, 1965;
Lewis, 1954;
Soligo and Muller, 1999),
little attempt has been made to understand how they perform mechanically under
changing environmental conditions.
The nail plate comprises three histological layers of keratinous tissue,
which are deposited by the nail matrix at the base of the nail unit beneath
the skin (Achten, 1981;
Caputo et al., 1982). These
layers are arranged in a sandwich-like structure as shown in
Fig. 1A. The intermediate
layer, in which the keratin fibres are oriented laterally, comprises
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 nail's
thickness, and wraps round the edge of the intermediate layer to join up with
the thinner ventral layer. In both of these outer layers the keratin fibres
show no preferred orientation.
|
), performed by carrying out cutting tests with instrumented
scissors, showed that this design is admirably suited to limit and control
fracture. The lateral orientation of the fibres in the thick intermediate
layer ensures that the work of fracture is greater proximally than laterally,
deflecting cracks laterally away from the nail bed and allowing self-trimming.
The outer layers of the sandwich, meanwhile, protect the intermediate layer
and prevent cracks forming at its edge. A disadvantage is that the nail layers
tend to peel apart at their boundaries, particularly when wet
(Farren et al., 2004). One of the problems with this research, however, is that cutting tests were carried out when the nails were saturated with water. In life, nails will have intermediate humidity as they are moistened by the nail bed, and this will affect both their moisture content and their mechanical properties.
Fraser and Macrae (Fraser and Macrae,
1980) highlighted six main factors that determine the mechanical
properties of filament–matrix composites such as the fingernail. These
are: (1) the mechanical properties of the filaments; (2) filament length; (3)
orientation and packing of the filaments; (4) the mechanical properties of the
matrix; (5) the volumetric ratio of matrix to filaments; and (6) the adhesion
of the matrix to the filaments. Most of these factors are determined by the
morphology of the nail, and would remain invariant with a changing
environment. Factors 4 and 6, however, may be expected to vary according to
the hydration status of the nail. Indeed, in horses' hooves it has been found
that the water content varies along the length of the hoof, resulting in
differences in the mechanical properties measured in different regions
(Bertram and Gosline, 1987).
These authors suggested that the hoof is more hydrated close to the growth
region, which allows it to be malleable, thus acting as a shock absorber.
Hydration was shown to decrease distally, terminating in the hoof tip, which
is stiffer than the growth region and also provides physical protection
against abrasion (Bertram and Gosline,
1987).
Nails are primarily composed of keratin, which is a complex coiled-coil
protein (Creighton, 1997).
Sequence studies of mammalian filament proteins have shown that they possess a
regular pattern of hydrophobic residues, which favours the formation of a
coiled-coil rope-like structure (Crick,
1953), composed of segments around 15 nm in length
(Crewther et al., 1978;
Gough et al., 1978). The
mechanical properties of the nail will depend on how these filaments are bound
to the matrix material. Analysis of these proteins has revealed a pentapeptide
repeat, which suggests that they are stabilised by disulphide bridges
(Dopheide, 1973;
Elleman et al., 1973). The
adhesion between the matrix and the keratin fibrils occurs in mammalian
material between high sulphur regions in the former and low sulphur regions in
the latter, where the net charges are of opposite sign
(Crewther and Dowling, 1960;
Gillespie and Simmonds, 1960)
and it is thought that this adhesion mechanism will be disrupted by the
presence of moisture. Disruption of the disulphide bridges is hypothesised to
alter the mechanics of the nails, particularly the ability to cut them along
the fibre direction.
This study was designed to investigate the effect of relative humidity (RH) on the moisture content and fracture properties of nails; effects that will have important implications for nail care and maintenance.
|
MATERIALS AND METHODS |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Moisture content control
The use of saturated salt solutions has been a common method for
controlling the RH of biological samples
(Winston and Bates, 1960).
Small containers must be used in order to store the samples (<1 litre
capacity), and a fixed physical barrier must be present to prevent contact
between the salt solution and the sample
(Winston and Bates, 1960).
The saturated salt solutions used for this study were prepared by adding powdered salts to boiling, distilled water until no more solid could dissolve. These solutions were then thoroughly mixed using a magnetic stirrer and allowed to cool before use. A small volume (100 ml) of each solution was then poured into a 0.25 l container. Fingernail samples were placed above the solutions on pieces of non-absorbent cotton wool. The range of solutions used to control humidity, along with the RH values obtained, are reported in Table 1.
|
The 100% RH samples are different to the immersed samples previously
studied (Farren et al., 2004)
in that the properties of the immersed samples depend on their ability to
absorb moisture. The samples that were immersed in water were generally
thicker than any of the samples analysed in our study, which suggests that
they took on much more water. Hence it was anticipated that the properties of
these nails would be different to those of the nails in our study. In order to
assess the moisture content of the nails, as a function of RH, 20 test samples
were clipped and weighed immediately, equilibrated sequentially using the
different RH environments, weighed again, and finally oven dried at 80°C
for a period of 5 days. The in vivo water content and the water
content at the different RH values were then calculated by comparing the mass
of freshly clipped nail specimens and the mass of nails exposed to varying
hydration conditions with that of the oven-dried samples. Tests were conducted
to verify that the nails had reached equilibrium by weighing the nails over
time. It was found that the mass of the nails did not change after they were
exposed to the humidity environments for longer than 48 h.
Clipper cutting tests
Cutting tests for estimating the fracture properties of plant and animal
tissue (Eichhorn et al., 2007;
Farren et al., 2004;
Lucas and Pereira, 1990;
Vincent, 1992) have usually
involved placing a pair of scissors between the platform and the crosshead of
a universal testing machine, and recording the work done to fracture or cut a
specimen. The main limitation of this technique is that the frictional forces
generated during the closing of the blades are not constant, thereby
increasing the variability of the data
(Bonser et al., 2004). In order
to overcome these difficulties a pair of Boots (Nottingham, UK) toenail
clippers were placed within the jaws of an Instron model 4301 Universal
Testing machine, with the lower blade held rigid, as described by Bonser and
colleagues (Bonser et al.,
2004). The clippers have approximately a 25 mm radius of
curvature. These clippers are normally operated by depressing a lever attached
to a spigot proximal to the blades. This was removed, so that the force could
be applied directly to the upper lever of the clippers by the compression
platen of the testing machine. In order to calibrate this set up, the blades
were first positioned approximately 1.4 mm apart, which was taken as the zero
gauge length. They were then gradually closed until they met, at which point
the load started to increase sharply. The machine was subsequently stopped,
and lower extension limits set at this point. The set up was then calibrated
to calculate the work required to close the blades without any sample in
place. The jaws of the clippers were opened, and the crosshead of the Instron,
with a 1 kN load cell mounted into it, was lowered at a speed of
8.3x10–5 ms–1, causing the blades to
close.
In preliminary experiments, load–displacement traces of scissors closed without a sample in place were found to vary from one experimental run to another. This was found not to be the case with clippers. The method of cutting is entirely different with clippers in that different fracture modes are likely to be generated compared with the test using scissors. This may result in differences in comparing mechanical data from scissors and clippers.
After calibration, the nails were tested by placing samples between the
blades and repeating the cutting procedure previously described. Test pieces
about 3 mmx3 mm were cut from the nail clippings using sharp scissors
(see Fig. 1B). It was not
possible to obtain larger sample sizes than this in the proximal direction. A
crosshead speed of 8.3x10–5 ms–1 was
found to be effective in making a clean cut through the samples. Each sample
was cut in both the lateral direction (perpendicular to the growth axis) and
the longitudinal/proximal direction (parallel to the growth axis). Thirty nail
samples were tested at each humidity level, and in each cut direction. Samples
were placed within the jaws of the clipper and tested within 4 min, after
which time significant moisture regain is reported to occur
(Schulz et al., 2002).
The force required to cut through the samples was recorded by the instrumentation software, and the energy was calculated as the integral of the force–displacement curve. The energy required to cut through the sample was then calculated as the total energy minus the energy required to close the clippers without a sample in place. In order to calculate the cross-sectional area of the cut, their lengths were measured using callipers, and then this value was multiplied by the mean thickness of the nail. The thickness of the nails was measured using a micrometre screw gauge calliper, taking three measurements per sample. The energy required to cut the sample was then divided by the cross-sectional area to give a value for the work of fracture.
Scanning electron microscopy
In order to observe the fracture surface of a torn sample of nail to see
the form of the fibrous structure, samples were mounted onto aluminium stubs,
with carbon tabs attached, and gold coated using an Edwards S150B Sputter
Coating Unit for a period of 20 s. Images of the torn surfaces were then
obtained using a Topcon 300 Series scanning electron microscope (SEM). Samples
were placed within the SEM chamber, and scanned using a beam of energy of 5
keV.
|
RESULTS |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
|
1 mm) was due to the clippers coming
together and being strained before the cut was made; the peak in force
occurred when the nail was being cut.
|
|
DISCUSSION |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
) who
showed that the moisture content ranged between 10% and 60%. The values are
also similar to the moisture content of other keratinous structures. Bertram
and Gosline showed that the moisture content of hoof wall varied considerably
at different RH levels (Bertram and
Gosline, 1987), but a study by Leach on hoof wall showed that it
had an in vivo moisture content of 17–24%
(Leach, 1980). Ostrich
feathers, another predominantly keratinous material, have been shown to
contain 11% moisture at 50% RH, but claw material from the same species had
only 4.5% moisture at the same RH (Taylor
et al., 2004). Differences between our data and those of other
workers are probably due to morphological differences between samples. The non-linear relationship between the moisture content of nails and RH may be due to diffusion characteristics; the data are of a typically Fickian nature – a greater amount of water is taken up by the nails as the RH is increased. This may reflect the transition from nails containing mainly tightly bound water to containing both tightly and loosely bound or surface water. It may be the case that the keratin fibres expand with increasing RH, giving rise to an increase in the available surface area to which the water can bind. Saturation may also occur, whereby the internal structure is fully saturated and therefore the surface of the nail takes on more water. The data indicate that the RH levels of 55–80% are close to what one might expect in vivo; a moisture content of greater than 63% is only likely to be reached if the nails are fully immersed in water.
The clipper test that we used provides a reliable technique for estimating
the work of fracture, albeit in a cutting mode rather than generating
controlled crack growth. Preliminary tests have shown that the amount of
energy required to close the clippers is consistent from test to test, and
this is therefore an improvement on scissor cutting used in previous studies
(Eichhorn et al., 2007;
Farren et al., 2004;
Lucas and Pereira, 1990;
Vincent, 1992) in which
friction between the scissor blades during closure may have resulted in
inconsistency and experimental error. The test proved useful for nails
conditioned in the range 33–100% RH. When testing nails conditioned at
0% RH it was found that the mode of failure was extremely rapid, and was
therefore difficult to detect using the equipment. It was possible to obtain
reliable data in the proximal direction at 0% RH, although there was a
different fracture process; brittle fracture occurred leaving a powder rather
than cleanly cut surfaces. The most likely reason for the brittle fracture of
the nails at 0% RH is a desiccation of the protein matrix, leading to crack
growth along many interfaces where typically water was present prior to
drying.
The force–displacement trace (Fig. 3A) itself shows some small steps during the fracture of the nail. These are due to the low sampling rates used to collect the data. It can be seen that the steps occur during the initial loading period, before the sample is being tested, and so are nothing to do with the fracture of the specimen itself.
The work of fracture values determined in this study (11–22 kJ
m–2) are higher than those previously published by Farren and
colleagues of 3–6 kJ m–2, which were found using a
scissor apparatus (Farren et al.,
2004). These differences may be due to the fact that Farren and
colleagues kept their samples wet until testing took place
(Farren et al., 2004), and so
it would be expected that lower work of fracture results would be obtained.
The data are, however, in reasonable agreement with values obtained by Pereira
and colleagues of 6–17 kJ m–2
(Pereira et al., 1997). It is
interesting to note that Pereira and colleagues found that fingernails were
`tougher' (higher work of fracture) in the lateral direction
(Pereira et al., 1997), which
is the opposite of what we have found in our study. It seems counter-intuitive
that it would take more energy to cut between fibres, in the lateral
direction, than across fibres in the proximal direction.
Samples soaked in water were tested using the clippers and a value of
8.35±1.95 kJ m–2 (N=7) was found for their
proximal work of fracture. This value is slightly larger than values obtained
using scissors on fully immersed samples
(Farren et al., 2004), but
well below the value found using clippers at 100% RH (about 13 kJ
m–2). This suggests that the results are broadly similar to
those obtained using scissors, but a different value was expected given that
the fracture mechanisms of the nail are thought to be different with clippers
than with scissors. The value obtained for soaked nails is, however, similar
to those of Pereira and colleagues, who used sharp scissors to cut their
tissue (Pereira et al.,
1997).
The present data show that humidity influences the work of fracture of
nails, particularly when cut in the proximal direction. Nails were found to be
much more resistant to cutting in the proximal direction at all humidity
levels (F5,54=98.66, P<0.001), particularly at
55% RH, where the work of fracture was found to be nearly twice that in the
lateral direction. A SEM image of a nail torn (using a trouser tear test)
laterally is shown in Fig. 4.
The fibres are predominantly oriented in the lateral direction, and so it is
conceivably easier to fracture the sample in this direction. In contrast,
cutting proximally requires fibre fracture, which takes more energy than
breaking the interfaces between fibres. The mechanism by which fracture occurs
more readily at 55% RH in the lateral direction may be plasticisation of the
matrix material that binds the fibres together. This matrix material is known
to contain a large number of sulphur bridges, which connect the matrix to the
fibres (Crewther and Dowling,
1960; Gillespie and Simmonds,
1960). It is therefore thought that a breakdown of these sulphur
bridges may lead to a plasticisation of the matrix. A study of the relative
sulphur content of nails between mammalian species may therefore prove
insightful as to their relative ability to resist fracture and their habitat
and behavioural characteristics (e.g. foraging, etc.). The large fracture
anisotropy at 55% RH suggests that cracks are very unlikely to propagate
longitudinally, towards the nail bed. This is advantageous in terms of
preventing irreversible damage to the nail unit, and shows that at this RH
(which is within the in vivo region) the nails behave in a
mechanically optimal manner.
|
|
Acknowledgments |
|---|
|
References |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Achten, G. (1981). Histpathology of the nail. In The Nail (ed. M. Pierre), pp.1 -14. Edinburgh: Churchill Livingstone.
Bertram, J. E. A. and Gosline, J. M. (1987).
Functional design of horse hoof keratin: the modulation of
mechanical-properties through hydration effects. J. Exp.
Biol. 130,121
-136.
Bonser, R. H. C., Saker, L. and Jeronimidis, G. (2004). Toughness anisotropy in feather keratin. J. Mater. Sci. 39,2895 -2896.[CrossRef]
Caputo, R., Gasparini, G. and Contini, D. (1982). A freeze-fracture study of the human nail plate. Arch. Dermatol. Res. 272,117 -125.[CrossRef][Medline]
Creighton, T. E. (1997). Proteins: Structures and Molecular Properties. New York: W. H. Freeman.
Crewther, W. G. and Dowling, L. M. (1960). Effects of chemical modifications on the physical properties of wool: a model of the wool fibre. J. Text. Inst. 51,775 -791.
Crewther, W. G., Inglis, A. S. and McKern, N. M. (1978). Amino-acid sequences of alpha-helical segments from s-carboxymethylkeratin-a-complete sequence of a type-ii segment. Biochem. J. 173,365 -371.[Medline]
Crick, F. H. C. (1953). The packing of alpha-helices-simple coiled-coils. Acta Crystallogr. 6, 689-697.[CrossRef]
Ditre, C. M. and Howe, N. R. (1992). Surgical anatomy of the nail unit. J. Dermatol. Surg. Oncol. 18,665 -671.[Medline]
Dopheide, T. A. (1973). Primary structure of a protein, component 0.62, rich in glycine and aromatic residues, obtained from wool keratin. Eur. J. Biochem. 34,120 -124.[CrossRef][Medline]
Dykyj, D. (1989). Anatomy of the nail. Clin. Podiatr. Med. Surg. 6, 215-228.[Medline]
Eichhorn, M. P., Fagan, K. C., Compton, S. G., Dent, D. H. and Hartley, S. E. (2007). Explaining leaf herbivory rates on tree seedlings in a Malaysian rain forest. Biotropica 39,416 -421.[CrossRef]
Elleman, T. C., Lindley, H. and Rowlands, R. J. (1973). Periodicity in high-sulfur proteins from wool. Nature 246,530 -531.[CrossRef][Medline]
Farren, L., Shayler, S. and Ennos, A. R.
(2004). The fracture properties and mechanical design of human
fingernails. J. Exp. Biol.
207,735
-741.
Fraser, R. D. B. and MacRae, T. P. (1980). Molecular structure and mechanical properties of keratins. In The Mechanical Properties of Biological Materials (ed. J. F. V. Vincent and J. D. Currey), Symposium of the Society for Experimental Biology XXXIV, pp. 211-246. Cambridge: Cambridge University Press.
Gillespie, J. M. and Simmonds, D. H. (1960). Amino acid composition of a sulphur-rich protein from wool. Biochim. Biophys. Acta 39,538 -539.[Medline]
Gough, K. H., Inglis, A. S. and Crewther, W. G. (1978). Amino-acid sequences of alpha-helical segments from s-carboxymethylkeratin-a-complete sequence of a type-i segment. Biochem. J. 173,373 -385.[Medline]
Hamrick, M. W. (1998). Functional and adaptive significance of primate pads and claws: evidence from New World anthropoids. Am. J. Phys. Anthropol. 106,113 -127.[CrossRef][Medline]
Leach, D. H. (1980). Gait fatigue – function of the hoof wall. Mod. Vet. Pract. 61, 417.
Le Gros Clark, W. E. (1936). The problem of the claw in primates. Proc. Zool. Soc. Lond. 1936, 1-24.
Lewin, K. (1965). Normal finger nail. Br. J. Dermatol. 77,421 -430.[CrossRef][Medline]
Lewis, B. L. (1954). Microscopic studies of fetal and mature nail and surrounding soft tissue. AMA Arch. Derm. Syphilol. 70,732 -747.[Medline]
Lucas, P. W. and Pereira, B. (1990). Estimation of the fracture-toughness of leaves. Funct. Ecol. 4, 819-822.[CrossRef]
Pereira, B. P., Lucas, P. W. and SweeHin, T. (1997). Ranking the fracture toughness of thin mammalian soft tissues using the scissors cutting test. J. Biomech. 30, 91-94.[CrossRef][Medline]
Schulz, B., Chan, D., Backstrom, J., Rubhausen, M., Wittern, K. P., Wessel, S., Wepf, R. and Williams, S. (2002). Hydration dynamics of human fingernails: an ellipsometric study. Phys. Rev. E 65,061913 .[CrossRef]
Soligo, C. and Muller, A. E. (1999). Nails and claws in primate evolution. J. Hum. Evol. 36, 97-114.[CrossRef][Medline]
Taylor, A. M., Bonser, R. H. C. and Farrent, J. W. (2004). The influence of hydration on the tensile and compressive properties of avian keratinous tissues. J. Mater. Sci. 39,939 -942.[CrossRef]
Vincent, J. F. V. (1992). Biomechanics-Materials. Oxford: IRL Press.
Winston, P. W. and Bates, D. H. (1960). Saturated solutions for the control of humidity in biological-research. Ecology 41,232 -237.[CrossRef]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
