Biocompatibility of cardiovascular devices - Biomaterials and Devices for the Circulatory System - page 62

The extent of this injury response can be dependent on the specifi c bio-
materials. An examination of the histologic characteristics of stainless steel,
Ni/Ti, and polyvinylalcohol devices used for closure of a patent ductus
arteriosis in lambs demonstrated that the remodeling of thrombotic lesions
by mesenchymal cellular infi ltration into the thrombus occurred at different
rates, with infi ltration of stainless steel thrombi occurring several months
after the earlier infi ltration seen in polyvinylalcohol and nickel/titanium
thrombi. Of note, no differences in endothelial coverage or infl ammation
were apparent among any of the groups (Sigler et al. , 2000). Direct com-
parisons of PET and metallic self-expandable stents have documented sig-
nifi cantly greater thrombogenicity, worse patency, and increased intralumi-
nal stenosis in stents made of PET (Wilczek et al. , 1996). In a study compar-
ing the biocompatibility of ePTFE/nitinol, polyester/nitinol, polycarbonate
urethane/cobalt (Co)-alloy, or bare Ni-Co-Ti-steel stents implanted in
arteries of sheep, signifi cantly less stenosis and infl ammation was seen in
the bare metallic stent in comparison to all other stent grafts. Interestingly,
endothelialization on only ePTFE/nitinol stent-grafts was incomplete com-
pared with the apparently complete endothelialization of the other devices,
which may once again highlight the complicated relationship of endothelial
growth and function to the development of myointimal hyperplasia (Cejna
et al. , 2001, 2002).
In addition to the intimal hyperplastic response, devices composed of
synthetic materials are prone to thrombogenicity, due in part to intrinsic
properties of the biomaterial, the lack of a functional endothelium, and the
amount and nature of protein adsorption which occurs on the surface.
Fiberoptic analyses have demonstrated wide variability in platelet adher-
ence to several synthetic and metallic biomaterials, with woven polymeric
PET demonstrating the greatest amount after fi ve minutes in an ex vivo
perfusion system (Schaub et al. , 2000), consistent with histologic evidence
of the apparent thrombogenicity of PET (De Scheerder et al. , 1995). Genetic
expression patterns of platelets circulating in ex vivo fl ow systems contain-
ing tantalum stents demonstrate increased expression of CD62p (P-
selectin) and CD63 expression within a few minutes of perfusion compared
with platelets circulating in systems without stents, point to the universality
of this problem to most biomaterials (Gutensohn et al. , 1997).

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Toxicity

The ability of a material to support viable cellularity and minimize sur-
rounding cell necrosis and apoptosis is an important contributor to the
durability of devices. Metallic stents, especially, often have toxicities related
to the release of by-products of metallic corrosion. Nickel-titanium, while
an advantageous biomaterial for various applications in cardiovascular