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

Figure 2 Fragment of model and used FEM mesh
The FE modelling was started by data collection. Thickness of the top skin layers was obtained for a 25 year old man by
Optical Coherence Tomography (OCT). The OCT performs high-resolution, cross-section topographic imaging of the
integral microstructure in tissue. Fig 3 shows an OCT image of human skin on the inner forearm of healthy 25 year old man.
The OCT enables a measurement of the thickness of the outer layers such as epidermis and dermis. The different shades
represent different layers of skin (see fig. 3) (see (Podoleanu, 2005) for full details). The white line on the top of skin it is not
the stratum corneum, but a light reflection. The thickness of stratum corneum and deeper layers were obtained from literature
(C.Pailler-Mattéi, 2006) (F. M. Hendriks, 2003) (Geerligs, 2009). The thickness of the layers is shown in Table 2.
Figure 3 The OCT image of human skin on the inner forearm of healthy 25 years-old man.
A tentative validation of the FE model was carried out by comparing indentation depth of the probe for the real test and
the FE model. Unfortunately, Kwiatkowska et al. did not publish an indentation depth of the probe, but published a Young
modulus for skin that was obtained during different test conditions. The indentation depth for the different loads and
dimensions of the probe was calculated using Hertz theory. First the contact area was calculated between a smooth spherical
steel indenter and the flat human skin. The contact radius a (Fig. 6.) as a function of the normal load F n radius of ball R and
reduced modulus E*, can be described by equation 2 (Fischer-Cripps, 2002) (D.A. Spance, 1975) (C. Pailler-Mattéi, 2006).
Figure 4 Contact between a sphere and an elastic half-space.
where the equivalent elastic reduced modulus E* is given by the following expression (C. Pailler-Mattéi, 2006):
(
(
For a rigid sphere in the contact with human skin the contribution of the elastic properties of the probe is negligible, so
equation (3) can be reduced to (see (M. Kwiatkowska et al., 2009) for full details):
)
3
)
(2)
(3)