Biomedical Engineering Reference
In-Depth Information
2.1.5 Simulations
In the first simulation the response of a flat 1mm thick scaffold was investigated
that is deformed by a surgeon to match the shape of the heart curvature before it is
sewn onto the myocardium. For this the 'thumb'-model from Fig.
2
was used and
the myocardium was deformed into the preformed state. The second simulation
examined the behavior of a preoperative preformed scaffold (stress free) that is
deformed frompreformed to end diastolic to end systolic state. This simulation of one
heart movement cycle was performed for scaffolds of 0.5mm and 1mm thickness.
2.2 Modeling Results
The flat scaffold was deformed in two steps by thumbs and then kept in a position
equivalent to the preformed state. Relatively high stresses occurred in the scaffold,
as can be seen in Fig.
5
. The yield point of 137MPa was exceeded in several areas.
Furthermore, stresses above the ultimate tensile strength of 197MPawere determined
for a few locations. Stresses beyond the yield point indicate plastic deformations,
which are displayed in Fig.
5
. The plastic strain that corresponds to the ultimate tensile
strength was determined to be 2.6%. This value was used as scale limit; therefore,
strains exceeding it are plotted in red in Fig.
5
. Areas stressed and strained above
the point of ultimate tensile strength are susceptible to material overload, therefore
material failure is likely. Further, plastic strains during cyclic loading can lead to
fatigue.
The stress distribution of preformed scaffolds, deformed into end diastolic and into
end systolic state, is shown in Fig.
6
. Compared to the flat scaffold, calculated stresses
were much lower. Maximal stress in the 0.5mm thick scaffold was determined to be
26MPa and 59MPa for the end diastolic and end systolic states, respectively. For the
1mm thick scaffold maximal stress was higher: 45MPa in end diastolic and 96MPa
in end systolic state. No plastic strains were observed.
From these results, it can be concluded that it is advisable to use initially preformed
scaffolds instead of flat ones. Material stresses in flat scaffolds are quite high and
early failures due to fatigue are very likely. Therefore, only preformed scaffolds were
considered in following simulations.
