Biomedical Engineering Reference
In-Depth Information
to exactly predict the stress and strain situation of the loaded scaffolds, but tenden-
cies can be observed. A consistent modeling technique was introduced, allowing
comparisons and assessment of different scaffold shapes.
It was shown that flat scaffolds fail too early and are therefore not regarded any-
more. Preforming of the scaffolds seems to be a promising improvement concept
and was applied to all designs. According to the used modeling technique, scaffolds
of 0.5mm thickness appear to be inappropriate because they are not stiff enough
to constrain a bulging graft. Design changes to the standard scaffold brought minor
improvements, but extensive design changes were needed. The method of tensile
triangles was introduced and tested [
30
]. Tensile triangles are a promising tool and
were applied to scaffolds where it seemed appropriate. Complex structures some-
times needed several iterations of shape adjustments. Also, the quantity of stress
reduction depends on the available space to enlarge tensile triangles. Furthermore,
new scaffold designs were introduced. A few of them appear promising. For shape
6 and shape 9, maximal von Mises stress was reduced by about 45 and 27%, re-
spectively, in comparison with the standard scaffold. The benefit of the improved
scaffolds needs to be tested in vitro.
Plastic strains in a structure under cyclic loading can lead to very fast fatigue.
Therefore, our goal was to design scaffolds that are strained in the elastic range.
However, stresses in elastic range must still be monitored. Stresses can accelerate
corrosion [
26
]. For this reason, highly stressed areas of a specimen are attacked
faster. Further, stress corrosion cracking (SCC) can lead to rapid fracture [
35
]. Many
magnesium alloys have a threshold stress for SCC on the order of 50% of the yield
stress [
36
]. The yield point of LA63 is at 137MPa, but all developed scaffolds were
loaded above 70MPa. Furthermore, fatigue is also related to stresses in the elastic
range. Presently, no endurance tests have been performed on the in-house alloy LA63.
However, for several aluminum containing magnesium alloys, the endurance limit
was below 70MPa for 10
7
cycles [
37
,
38
]. Consequently, the superposition of both
effects—fatigue and corrosion—can lead to a premature material failure.
Future work will include enhancing the simulation by including a bulging graft.
With this, the most promising scaffolds will be assessed again. Results will be com-
pared with in vitro tests. If the stresses are still too high, then the concept of a
one-piece scaffold must be reconsidered. An alternative scaffold consisting of seg-
ments was already presented. This implant is expected to have significantly lower
stresses. Therefore, the risk of fatigue and also of accelerated corrosion due to high
stresses is decreased. One disadvantage of this scaffold type is the laborious handling
in surgery required for the movable segments.
5 Conclusion
A finite element model was developed to simulate myocardial scaffolds that are de-
formed through cardiac motion. Incidents that were already observed in vitro and in
vivo were predictably replicated in this simulation. Hence, flat scaffolds should be
