Mechanical Properties of Excised Human Skin
A. Ní Annaidh1, M. Ottenio2, K. Bruyère2, M. Destrade1 and M.D. Gilchrist1,3
1
2
School of Electrical, Electronic & Mechanical Engineering, University College Dublin, Belfield, Dublin 4, Ireland
Laboratoire de Biomécanique et Mécanique des Chocs, Institut National de Recherche sur les Transports et leur Sécurité, 25 Francois
Mitterand, 69675, Bron Cédex, France
3
School of Human Kinetics, University of Ottawa, Ontario K1N 6N5, Canada
Abstract—In this study we have investigated in influence of
location, gender and orientation on the deformation characteristics of 55 samples of human excised skin. Uniaxial tensile
tests were carried out at a strain rate of 0.012s-1 on excised
human skin from the back. The deformation characteristics of
skin (Ultimate Tensile Strength (P<0.0001), Failure Strain
(P=0.0177), Young’s Modulus (P<0.0076), Initial Slope
(P=0.0375) and Strain Energy (P=0.0101)) were found to be
dependent upon the orientation of specimens with respect to
the Langer’s Lines. The location of specimens on the back was
also found to have a significant effect on the Ultimate Tensile
Strength (P =0.0002), the Young’s Modulus (P=0.0017) and the
Strain Energy (P=0.005).
sections at right angles to the lines showed a marked preponderance of connective and elastic tissue fibres cut transversely, while those cut parallel show the fibres running
longitudinally. Cox however, failed to describe the extent of
the microscopical investigation.
Early tensile tests carried out by Ridge and Wright[4] in
1966 suggest that the deformation characteristics of skin are
dependent upon specimen orientation with respect to these
Langer lines. Recent work conducted by Liang and Boppart5
using a technique known as Optical Coherence Elastography indicates a large difference between the Young’s
Modulus of skin parallel and orthogonal to Langer’s Lines.
Keywords— Skin, Langer’s Lines, Histology
I. INTRODUCTION
Skin is an anisotropic material. That is, its mechanical
behaviour is not the same in every direction. This phenomenon has been noted as far back as the 19th century by Karl
Langer[1] who mapped the natural lines of tension which
occur within the skin. The lines are created by puncturing
the skin with a circular device. The wounds then assume an
elliptical shape and by joining the major axes of the ellipses
a system of tension lines can be drawn as shown in Figure
1. These lines are known as Langer’s Lines or Cleavage
Lines. In 1892 Kocher[2] first realized the surgical significance of the Langer’s Lines and observed that incisions
made along these lines will cause little or no scarring,
whereas incisions transverse to them will gape and lead to
unsightly scars. In 1941 Cox re-investigated the cleavage
lines of the skin[3]. This was the first piece of original work
on the Langer’s Lines since their discovery. Cox identified
differences between the cleavage lines he observed and
those of Langer’s. Later these lines would be distinguished
from the Langer’s Lines as the Cox lines. Unlike Langer,
Cox investigated the cleavage line pattern for excised human skin also. It was found that the line pattern remained
unaltered leading Cox to conclude that the lines of increased
tension must produce intrinsic structural changes to the
skin. Skin sections were made in two planes, one parallel to
the cleavage line and one at right angles. Cox claims those
Fig. 1 Langer’s Lines of the back
II.
MATERIALS AND METHODS
A. Specimen Preparation
Tensile tests and specimen preparation were carried out
in INRETS, Lyon, France. Skin was excised from seven
cadavers (three male, four female). The average age of the
subjects was 89 ± 6 years and none had any related skin
diseases. Only the skin from the back of the body was available for use. This was excised from the body with a scalpel.
Each sample of skin was cut into a dog bone shape specimen using a custom made die and any underlying adipose
pattern with black spray paint. This random speckled pattern is required for the system to track the deformation.
C. Optimization
Data fitting of our experimental data was performed to
assist in the selection of a suitable constitutive model for
human skin. A MATLAB routine available in the Optimization Toolbox called lsqcurvefit was used for this purpose.
Two isotropic non-linear elastic constitutive models were
fitted to the stress-stretch curves obtained through the tensile tests and were assessed individually. The Fung
model[7] is shown in Equation 1 and the Gent model[8] is
shown in Equation 2.
Fig. 2 Orientation of samples procured from the back
tissue was then carefully removed with a scalpel. The
thickness of the skin after removal of adipose tissue was
measured using Venier Callipers and the mean thickness
was 2.56±0.39mm. It was observed that skin removed from
the lumber area of the back contained more adipose tissue
than other areas. Consistent with other investigations it was
observed that both shrinkage and expansion of the specimens occurred [3, 4, 6]. It is believed that this is due to the
release of the resting tension within the skin. Specimens
were obtained in various orientations, shown in Figure 1, to
attempt to correlate them with the direction of Langer lines.
Each test specimen was sprayed with black spray paint so as
to create a random speckled pattern, required for the use of
Digital Image Correlation. The skin was stored in moistened
paper and refrigerated at 4oC until it was ready to be tested.
A total of 55 samples were successfully tested.
B. Tensile Tests
Tensile tests were performed using a Universal Tensile
Test machine. Samples were clamped using specially designed anti-slip clamps due to the tendency of samples to
slip in grips. The strain was calculated using a cable actuated position sensor. The velocity of the cross-head was
50mm/minand the strain rate was 0.012 s-1.Tensile load was
measured with a 1 kN piezoelectric load cell. Each tensile
test was videoed two Dalsa Falcon 1.4M100 digital video
cameras at twenty frames/second. This was to record any
abnormal behaviour during the experiments and also for the
use of Digital Image Correlation (DIC). DIC was incorporated as an alternative method to measure deformations and
was performed using Vic2D ® Software (Version 2009 Correlated Solution, Inc.). A grey base layer of spray paint
was applied to the specimen followed by a random speckled
2
−1
s11F = µe b (λ1 + 2 λ1 −3 ) (λ1 − λ1−2 )
(1)
µJ m
(
λ1 − λ1−2 )
2
−1
J m − λ1 − 2λ1
(2)
s11G =
Where S is the nominal stress, !1 is the stretch and " is
the initial shear modulus. b is a parameter associated with
the strain stiffening effect whereas Jm is a parameter associated with limited chain extensibility.
D. Statistical Analysis
A statistical analysis of the experimental results was carried out using the General Linear Model procedure in SAS
9.1 (SAS Institute Inc., USA). A multivariate analysis of
variance followed by the Tukey-Kramer post-hoc test were
utilized to determine the influence of gender, orientation
and location of the various test samples. Significance levels
for all tests were set to p < 0.05. When performing this test
a normal distribution is assumed. To ensure our data set
followed a normal distribution a Lilliefors test was performed using the lillietest function in MATLAB R2007b.
III.
RESULTS
For each tensile test performed a force-displacement
curve was obtained. The nominal stress was then calculated
by dividing the force by the undeformed cross sectional area
of the specimen. The stretch ratio was calculated by dividing the deformed length of the specimen by the original
length. In this way Nominal Stress Vs Stretch Ratio graphs
were plotted for each specimen. A number of characteristics
from these curves were identified as descriptive parameters.
They are illustrated in Figure 3.
A multiway analysis of variance found that orientation
with respect to the Langer’s Lines has a significant effect on
the Ultimate Tensile Strength (UTS) (P<0.0001), the Strain
Energy (P=0.0101), the Failure Stretch (P=0.0177), the
Elastic Modulus (P<0.0076) and the Initial Slope
(P=0.0375). A further simple effects analysis for orientation
revealed the Estimated Marginal Means for each of the
parameters listed above. These results are tabulated in Table
1 below.
Table 1 Estimated Marginal Means of orientation with .respect to Langer’s
Lines
Fig. 3 Typical stress-stretch graph for experiments. The ultimate tensile
strength is the maximum stress until failure and is indicated by A. The
Elastic Modulus is the slope of the linear portion of the curve shown by B.
The failure stretch is the maximum stretch obtained before failure, shown
by C. The initial slope is the slope of the curve at infinitesimal strains and
is shown by D. The strain energy is the energy per unit volume consumed
by the material during the experiment and is represented by the area under
the curve.
Parallel
22.67
45o
23.94
Perpendicular
12.56
Strain Energy (MJ/m3)
3.27
4.13
2.43
Failure Stretch
1.63
1.49
1.55
Elastic Modulus (MPa)
84.76
93.28
58.20
Initial Slope (MPa)
1.63
1.31
0.98
Orientation
UTS (MPa)
A. Influence of Orientation
B. Influence of Location & Gender
The influence of specimen orientation on the mechanical
behaviour of samples was investigated. It was found that the
global orientation (that is, whether the samples were vertical, horizontal or at 45o) of the specimens did not have a
significant effect on the mechanical behaviour. However,
the influence of orientation with respect to the Langer Lines
was found to have a significant effect. Specimens were
classed as being parallel, perpendicular or at 45o to the
Langer’s Lines by comparing specimen orientation to
known generic orientations of Langer Lines.
Figure 4 is an example of three stress-stretch curves representing adjacent samples in three different orientations.
This graph depicts the anisotropic behaviour of skin very
well. The different responses of the three different specimens can be clearly identified.
When results were grouped into three locations (Upper
Back, Middle Back and Lower Back), the multiway analysis
of variance also found the location of specimens to have a
significant effect on the UTS (P=0.0002), the Strain Energy
(P=0.005) and the Elastic Modulus (P=0.0017), but not on
the Failure Stretch nor the Initial Slope.
The influence of gender on the aforementioned parameters was also investigated. It was found to be a significant
factor for the Failure Stretch (P=0.0077) and for the Initial
Slope (P=0.0026) only.
C. Optimization
It was found that the Fung model performed better under
tensile conditions with an average R2 value of 97.5% as
compared to 95.8% for the Gent model. The average values
obtained through curve fitting for the Fung model were " =
8.1±3.9 MPa, b = 3.2±0.9. The average values obtained for
the Gent model were " = 10.1±4.5 MPa, Jm = 0.7±0.2.
It was found that the orientation with respect to the
Langer’s Lines had a significant effect on the value of " for
both the Fung (P<0.0001) and the Gent (P<0.0001) model.
IV.
Fig. 4 Typical stress-stretch graph for experiments
DISCUSSION
As discussed by Holzapfel[9] there are a number of issues involved with tensile tests. Despite these short comings
it was decided to perform in vitro tensile tests over in vivo
testing for a number of reasons. Firstly, in vitro tests provide simple stress strain relationships that can be modeled
and quantified easily. Secondly, in vitro testing allows histology to be carried out, which was also carried out as part
of this study. Lastly, for the application of the this work in a
further study into the mechanics of stabbing incidents, we
are interested in failure of skin, and in vitro testing makes it
possible to test until failure.
It has been shown that the UTS, strain energy and Elastic
Modulus vary depending on location. However only skin
from the back was available for testing and so these results
only cover a small region. Although previous studies have
also shown that the properties of skin are dependent upon
specimen location[10, 11].
All subjects were between the ages of 81-97. This meant
that no comparisons based on the age of subjects could be
performed. Other studies have however investigated the
influence of age on the mechanical properties of skin and it
has been well documented that the extensibility of skin
decreases with age[10-12].
It is important to note that both the Fung and Gent models are isotropic models and that they performed well considering their simplicity. It was shown that the parameters
are dependent upon the orientation of the specimen. This
illustrates that to fully describe the mechanical behaviour of
skin anisotropic models must be used.
Thus far, it has only been possible to examine our data
with respect to the perceived orientation of Langer Lines.
However, there is variability in the directions of Langer
Lines between subjects and no universal pattern of maximum tensions exist[13]. Currently, the orientation of the
Langer Lines cannot be identified with certainty unless the
skin of a whole cadaver is punctured. With histological data
it would be possible to assess the experimental data with
respect to the orientation of collagen fibres. This would be
vital information for those wishing to model skin as an
anisotropic material. With this information it would also be
possible to confirm whether or not the Langer’s Lines have
an anatomical basis.
V. CONCLUSIONS
Some mechanical properties of excised human skin have
been evaluated through tensile testing. The mean Ultimate
tensile strength was 21.5±8.4MPa. The mean Strain Energy
was 3.6±1.6MJ/m3. The mean Elastic Modulus was
83.3±38.9 MPa. The mean initial slope was 1.18±0.88 MPa
and the mean Failure Stretch was 54±17%. It has been
shown that the orientation with respect to Langer Lines has
a significant effect on the deformation characteristics of
skin, including the Ultimate tensile strength (P<0.0001),
strain energy (P=0.0101), failure strain (P=0.0177), initial
slope (P=0.0375), and Young’s Modulus (P=0.0101).
Through curve-fitting values were obtained for the parameters associated with the Fung and Gent models. For the
Fung model; " = 8.1±3.9 MPa, b = 3.2±0.9 and for the Gent
model " = 10.1±4.5 MPa, Jm = 0.7±0.2.
ACKNOWLEDGMENT
Funding for this project has been provided by the Irish
Research Council for Science and Engineering Technology
and the Irish Department of Justice Equality and Law Reform.
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Prof. Michael Gilchrist
University College Dublin
Belfield
Dublin
Ireland
michael.gilchrist@ucd.ie