Biocompatibility of cardiovascular devices - Biomaterials and Devices for the Circulatory System - page 61

The biomechanical effects of circulatory fl ow are translated to the cel-
lular level according to the intrinsic biomechanical and viscoelastic
properties of cardiovascular devices themselves. Thus, the compliance and
elasticity of various biomaterials can signifi cantly impact the phenotypic
and proliferative properties of cardiovascular cells adherent to the surface
or involved in the healing response. In one study, two composite partially
bioresorbable PG910/polypropylene aortic grafts differing only in their
non-resorbable polypropylene elasticity properties were implanted in
rabbits. It was found that by one to two months, the PG910 component was
bioresorbed, and the differential mechanical properties of the explanted
prostheses/tissue complexes in each group were determined by the elastic
property of the remaining non-resorbed polypropylene. Cell proliferation
rates within the inner capsule of the aortic graft were signifi cantly higher
at later time points in the higher compliance grafts and this correlated with
increased inner capsule thickness. These results demonstrate the apparent
relationship between cellular activity and the intrinsic deformative proper-
ties of biomaterials implanted in circulatory fl ow conditions (Zenni et al. ,
1993).

3.3 Specifi c biocompatibility issues
While the above discussed issues affect all biomaterials and devices to
various extents, specifi c limitations can be attributed to synthetic, metallic,
and tissue biomaterials used for cardiovascular devices. The following sec-
tions highlight major biocompatibility limitations associated with these
groups of biomaterials.

3.3.1 Synthetic and metallic devices
Thrombosis and intimal hyperplasia

￿ ￿ ￿ ￿ ￿

Synthetic biomaterials such as ePTFE, PET, as well as metallic alloys of
titanium (Ti), nickel (Ni), and iron (stainless steel) are frequently used for
many cardiovascular devices, as alternatives to autologous tissue, as in the
case of bypass conduits, and as materials for valves, stents, and patches to
repair cardiac defects. However, the lack of a native functioning tissue often
makes these devices especially prone to thrombosis and myointimal hyper-
plasia. In the case of coronary revascularization by balloon angioplasty,
while the advent of stenting after angioplasty has partially mitigated the
mechanic elastic recoil which occurs after angioplasty-related injury, the
myointimal hyperplastic response has continued to hinder the long-term
durability of fi rst generation coronary stents (Nikkari and Clowes, 1994;
Nakatani et al. , 2003).