Biomedical Engineering Reference
In-Depth Information
right ventricle, with blood pressures of about 40 to 60mmHg. These grafts are not
recommended for use with the left ventricle, which has blood pressures of up to
240mmHg. Ruptures are likely due to the insufficient strength of those biological
grafts, especially in the early stages after implantation. Some additional degradable
implants could help to mechanically support the grafts until sufficient strength is
obtained through in vivo remodeling processes.
Different degradable materials are used in clinical research. A few ceramics can
degrade, for instance the group of calcium phosphates [
14
]. However, ceramics
present the problem of high strength in combination with low ductility. For the
intended implants, high ductility is a necessary characteristic. Some polymers de-
grade into nontoxic substances in the physiological environment, but polymers have
low strength and are limited to low stress applications due to their creep behavior
[
15
,
16
]. Furthermore, inflammation was observed in soft tissues after implantation
of polymeric grafts [
17
]. Metal implants appear as an alternative. Magnesium and
its alloys have been the particular focus of recent research [
18
]. Their mechanical
properties are preferable to those of ceramics and polymers. Various alloys were
developed as implant materials [
19
]; furthermore, it seems possible to customize
degradation performance for specific applications [
20
]. Since magnesium is part of
the physiological metabolism, it can be seen as nontoxic in physiological concentra-
tions [
21
]. Magnesium alloy scaffolds can support biological grafts until they achieve
sufficient strength [
22
]. Still, the biocompatibility of such alloys needs to be tested
in order to investigate the influence of alloying elements and corrosion products on
the host's tissue [
23
,
24
].
To this end, scaffolds made from magnesium alloy LA63 were implanted epicar-
dially and tested for in vivo behavior in a porcine model. All investigations performed
demonstrated high biocompatibility. Nevertheless, fractures in the scaffolds were de-
tected one month after implantation [
24
]. Flat scaffolds were used, which the surgeon
manually deformed to match the heart curvature shape before sewing them onto the
myocardium. It is very likely that the material suffered damage due to high strains
during the deformation process. It was assumed that preoperative adaptation of the
scaffold to the heart curvature would extend its durability. In vitro tests showed a
53% increase in the lifetime of preformed scaffolds as compared to the flat scaffolds
[
25
]. Presently, in vivo tests of the preformed scaffolds have not been carried out.
Based on the assumption that biological grafts develop sufficient strength after about
three months, we still see potential to improve our scaffold designs for an extended
durability.
The manual development of scaffold designs using a 'trial and error' method—
manufacturing specimens and testing them in vitro and in vivo—is very expensive.
Instead, the use of standard engineering tools like CAE
1
is recommended to improve
the development process and therefore save time and costs.
A finite element model is developed to mimic the heart's left ventricular move-
ment. Using this, a designed scaffold is tested and highly stressed and strained regions
are identified. Based on that knowledge, the scaffold shape is improved and new shape
1
Computer Aided Engineering.
