ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

formula can be written as a linear matrix inversion problem. Fung et al. performed a lot of experiments on the biological
tissue and they observed that the stress increases much faster than the strain as soon as the small deformation context is not
applicable (Y.C. Fung, 1993). This sudden increase of the stiffness cannot be described using a linear constitutive law such as
the Hook‟s law. Because the skin is a viscoelastic homogenous material, the best way to describe its properties is the
Mooney-Rivlin theory by describing an energy function W (M. Mooney, 1940) (Hung, et al., 2009) (E. Keeve, et al., 1998)
(Gladilin, et al., 2003) (Zachow, et al., 2000):
( ) ( ) ( ) ( ) ( )( ) (1)
where I 1 and I 2 are the first and second invariant of the deformation tensor E.
Although, as noted earlier, skin has different properties in difference body locations, in this work focus was placed on the
forearm skin as this was used in the experimental work to be modelled (see (M. Kwiatkowska et al., 2009)).
3. PREVIOUS EXPERIMENTAL STUDY
The FE modelling of human skin described in this paper is based on experiments with reciprocating sliding of a smooth
steel ball against human forearm skin in vivo (see (M. Kwiatkowska et al., 2009)) for full details). The aim was to study skin
friction and deformation.
The experiment consists of three steps. The first step was the indentation part when load was applied. Before the
indentation a reference point was marked (fig. 1(a)), located 8.8mm away from the point of indentation and in the plane of
the probe movement. The motion of this reference point was tracked using video image analysis for the lateral motion and an
LVDT sensor for the vertical motion. In the next step the probe was moved a further 8.8mm away from the indentation point,
initially deforming the skin, and then the friction test was started. The friction test consisted of a cyclical repeating movement
of the probe 17.6mm back and forth. Two diameters of probe (d 1=2mm and d 2=5mm) and two loads (0.19N and 0.5N) were
used in the study. The maximum probe velocity was 16mm/s. During the reciprocating sliding the forearm skin was observed
to deform forming a “bow wave” (as shown in Figure 1(b)), and “stick” and “slip” regions were observed between probe and
skin (M. Kwiatkowska et al., 2009). This behaviour can be attributed to friction between the skin and the probe and the
viscoelastic properties of the skin. The sizes of the “bow wave” effect and of the “stick” and “slip” regions were used for
validation of the FE model.
a)
b)
Figure 1 a) Method of deformation measurement of human skin during friction test (M. Kwiatkowska et al., 2009). b)
Deformation of human skin under spherical probe loading and lateral displacement (M. Kwiatkowska et al., 2009).
4. FE MODELLING
It is well-known that FE modelling is a compromise between the accuracy of the results and the time needed to achieve
them. As such it needs to be kept as simple as possible. In this case modelling of the human skin was broken up into two
steps: reproducing the skin geometry and attributing mechanical properties, boundary conditions.
In this study the deformation of the forearm skin was analysed using the FE code MSC-MARC. Figure 2 shows a small
section of the created 2D FE mesh. The structure of the human skin used is composed of four main layers with a muscle layer
added beneath.
2