Biomedical Engineering Reference
In-Depth Information
Figure 6.9
The stress distribution for different
E
L
, the Young's modulus of the lump while the
Young's modulus of tissue was kept constant at
E
T
= 15 kPa. The applied load was
U
y
=1mm
and the lump was considered to be in the middle of the tissue
unknowns for this problem. Since the reported Young's modulus for lumps, such as in
breast cancer, is normally 10 times higher than that of the tissue [5], this variable can be
regarded as being a constant.
6.5 Experimental Validation
Validation is an important step to ensure that the numerical analysis was performed appro-
priately and that correct results were obtained. Nevertheless, performing experiments is
not always easy. For instance, preparing spherical lumps with exact dimensions on one
hand, and carving out spherical voids from bulk soft tissue that have exactly the same
dimensions is by no means a simple task. Any gaps between lumps and bulk tissue due
to inaccurate carving of the tissue yield erroneous results and pressure distribution. It is
also challenging to accurately adjust the center of the lump to the intended positions.
Creating identical boundary conditions in experiments and in the finite element model
is another challenging task. In addition, frictional forces could play an important role in
many experiments and are very difficult to measure and implement, both in the experi-
ments and the finite element analysis. Dimensional and geometrical differences between
the FEM and experiments are highly likely and could potentially be a source of error.
The utilized sensing elements are also important to the final result. In this study, an array
of the piezoelectric PVDF (polyvinylidene fluoride) sensing elements was used to register
the stress distribution over the contact area. Therefore, extreme care was used to ensure
that each prototype device contained an identical sensing element. However, after manu-
facturing, some discrepancies among the sensing elements were observed. Therefore, in
the initial stage, the elastomer without any lump was placed on the sensing array and
the outputs of the sensors were tested and calibrated. In this present study, due to the
difficulties just mentioned, only two different combinations of parameters were tested.
