Biomedical Engineering Reference
In-Depth Information
Fig. 11.8 a
Comparison of experimental andmodel force-displacement response of forehead region
of one volunteer,
b
force at 1.1mm displacement in different in-plane directions for all central cheek
area of all volunteers. From Ref. [
25
]. Copyright 2013 by Elsevier. Adapted with permission
stress was applied representing the in vivo tension inherent in human skin. After
the pre-stress was applied the domain was remeshed such that the diameters of the
large and small partitions were 37.5 and 4mm, respectively. For the second step of
the analysis, all nodes outside the 37.5mm diameter partition were fixed. The nodes
inside the probe area were moved according to the protocol in the experiment. The
total sum of the nodal reaction forces in the probe region was calculated.
The measured force-displacement response for all tests was non-linear,
anisotropic, and viscoelastic (Fig.
11.8
). There was a large inter-subject variation
in the skin stiffness of the central cheek area and also a large intra-subject variation
in the skin stiffness at different facial locations. The direction in which the force-
displacement response was stiffest at each location corresponded to the reported
direction of Relaxed Skin Tension Lines (RSTL) [
33
] at that location. The one ex-
ception to this was the forehead region, where the direction of stiffest response was
orthogonal to the RSTL direction.
The finite element model simulated the non-linear, anisotropic, and viscoelastic
behaviour of the skin observed in the experiments (Fig.
11.8
). The error-of-fit between
the model and experiments ranged from 12 to 23%. The in vivo stresses ranged from
15.9 to 89.4kPa.
11.2.2.3 Muscle Materials
To represent muscle mechanics in the FE face and tongue models we used a
transverse-isotropic muscle material based on the constitutive equation proposed
by Blemker et al. [
27
]. This type of material has stiffness properties in the direc-
tion along the muscle fiber that differ from properties in the directions orthogonal to
