Cardiovascular stents - Biomaterials and Devices for the Circulatory System - page 181

1996). Nickel, chromium and molybdenum have all been found to elute
from stainless steel stents though a process attributed to the oxide fi lm
breakdown and local corrosion (Gutensohn et al. , 2000). The corrosion
resistance of titanium has been found to be due to the formation of a
surface metal oxide fi lm which retards corrosion. For metals such as chro-
mium and titanium this passivation has been found to be highly effective
(Bertrand & Sipehia, 1998). However, evidence of elevated metal concen-
trations in tissues, serum and urine has been described in patients bearing
titanium implants (Gotmann, 1997). Similar evidence has been found with
cobalt-chromium-based alloys which are supposed to have excellent corro-
sion resistance (Gotmann, 1997).

7.4.2 Synthetic polymer coatings

The notion of coating a stent with a biodegradable or less bioreactive
polymer has also been a major area of interest. These have included two
types of polymer coatings (i) biodegradable synthetic polymers and (ii)
non-biodegradable synthetic polymers (Van Beusekom & van der Giessen,
2000).
Synthetic polymer coatings have been the subject of study for vascular
applications for several decades. Materials including the biodegradable
polymers poly(glycolic acid)/poly(lactic acid) (PGLA), polycaprolactone
(PCL), poly(hydroxybutyrate valerate) (PHBV), polyorthoester (POE)
and poly(ethylene oxide)/poly(butylenes terephthalate) (PEO/PBTP)
and non-biodegradable polymers polyurethane (PU), silicone (SIL),
poly(ethylene terephthalate) (PETP) and poly(metacryloyl phosphorylcho-
line lauryl methacrylate) (PC) have been tested for both coating and replac-
ing metal stents. It has been found in all cases with the exception of PC
(Van Beusekom & van der Giessen, 2000) that these polymers evoke exten-
sive infl ammatory responses and fi brocellular proliferation (Bertrand &
Sipehia, 1998; Schwartz, 2001). Van der Giessen and colleagues (1992) com-
pared rates of thrombosis and neointimal formation using synthetic non-
biodegradable poly(methyl methacrylate) coated and uncoated stainless
steel stents, implanted into a pig coronary model. In agreement with
Schwartz (2001), they noted that all polymers elicited an intense infl amma-
tory response with the presence of multinucleated giant cells and macro-
phages surrounding proteinaceous debris and thrombus remnants. Overall,
no difference was found in neointimal proliferation.

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Biocompatible synthetic coatings

The BiodivYsio™ stent produced by Abbot Laboratories (Abbott
Park, IL) exploits the biocompatible coating derived from a cross-linked