Biomedical Engineering Reference
In-Depth Information
with minimal regurgitation. All preseeded constructs were covered with tissue
while the unseeded control showed no tissue formation and an inappropriate
cell number and collagen content compared with native tissue. A limitation of
the preseeded scaffolds is the lack of formation of a confluent endothelial cell
layer, although no thrombi have been observed. Tensile tests showed a continu-
ous decrease in both the scaffold modulus and strength during the implantation
period. Based on the mechanical testing it was concluded that the scaffold is no
longer necessary and should have been degraded at 17 weeks after implanta-
tion [280]. More recently, the application of rapid prototyping techniques to
fabricate porous P3HO-3HH heart valve scaffolds has been described [269].
Vascular Grafts
Tubular scaffolds made from a polyglactin/PGA mesh composite were success-
fully tested to replace a pulmonary artery in sheep [288]. However, preliminary
studies using this material in the high pressure systemic circulation resulted
in aneurysm formation due to the fast polymer resorption within 6-8 weeks.
Therefore, P3HO-3HH was introduced as an alternative material with a much
longer degradation time to withstand the systemic pressure [40]. Vascular
grafts made from a PGA mesh as inner layer, to promote cell attachment and
tissue formation, and three outer layers of nonporous P3HO-3HH films, to pro-
vide mechanical support, were tested in the aortic position in sheep. Scaffolds
preseeded with a mixture of endothelial cells, smooth muscle cells, and fibrob-
lasts remained without aneurysm formation during the 5-month implantation
period. In contrast, all unseeded control grafts became occluded, which was in
part caused by the thrombogenic PGA mesh. A very thin and loose tissue was
found on the unseeded materials. Increased cell density and collagen formation
as well as endothelial cell layering has been found on the preseeded materials.
However, metabolic activities tended to increase significantly over time [40].
As described above, this might result in excessive ECM production [286], which
has to be clarified in a long-term implantation study.
It is worth noting that P3HO-3HH has also been tested as a sealant of
Dacron vascular prostheses. Although the polymer impregnation contributed
to the impermeabilization of the graft wall the P3HO-3HH impregnated grafts
showed a delayed healing process in comparison to preclotted nonimpreg-
nated grafts when tested as an infrarenal aortic substitute in dogs. This was
explained by the slow resorption time of P3HO-3HH limiting collagenous tis-
sue formation and infiltration into the graft [53].
7
Outlook
The development of biomaterials is one of the most promising fields in
biomedical research. Although an increasing number of biodegradable poly-
