Biomedical Engineering Reference
In-Depth Information
7.3.2 Cobalt-chromium
Cobalt-chromium (Co-Cr) is a biocompatible material that has been
utilised in biomedical applications for some years (Donachie, 1998). Co-
Cr has the advantage of having a high corrosion resistance and higher
radial strength and radiopacity than stainless steel, allowing stents to be
manufactured with thinner struts but providing a similar radiological visi-
bility to its steel-based counterparts (Kereiakes
et al.
, 2003). As such
Co-Cr has been applied to the production of both self-expanding and
balloon expanding stents including the Wallstent
®
(Boston Scientifi c
Corp, Natick, MA), Guidant Multi-Link Vision
®
(Guidant Corporation)
which is produced from the alloy L650 (Co-20Cr-15W-10Ni), the
Driver
®
-stent (Medtronic) produced from the alloy MP35N (Co-20Cr-
35Ni-10Mo) and Costar-Sent
®
(Biotronic) whose high strength has
allowed the engineering of drug reservoirs within its struts (O'Brien &
Carroll, 2009).
7.3.3 Platinum-iridium alloys
Alloys containing 90% platinum and 10% iridium have also been
exploited for the their radiopacity (Hijazi
et al.
, 1995), which has been
shown in an animal model to give reduced artefacts when compared with
316L stainless steel, and excellent corrosion resistance (Teitelbaum,
et al.
,
1988; Park & Kim, 2003). Bhargava and co-workers compared Pt-Ir stents
(Touchstone, Bombay, India) with 316L stainless steel stents (Palmaz-
Schatz stents) in a porcine model and demonstrated the alloy to be bio-
compatible with a low infl ammatory response and with a comparatively
small neointima formation (Bhargava
et al.
, 2000). However, the mechani-
cal properties of this alloy are less impressive with 16% recoil compared
with 5% for the stainless steel implants. Nonetheless, in a human clinical
trial the Pt-Ir-based Angiostent (NuMED, Inc., Hopkinton, NY, and
Angiodynamics, Glens Falls, NY) was reported to be safe, effective and
easily deployed at the lesion site (Foti
et al.
, 1998). The presence of iridium
also provides the potential to be irradiated to produce a radioactive stent
(Bhargava
et al.
, 2000) as discussed in the radioactive stent section of this
chapter.
The ultimate goal of cell-material interface engineering is to control the
cellular response. In general, cells respond to three main classes of stimuli:
chemical, topographical and mechanical. These three distinct stimuli can
have very similar effects in infl uencing the cell's response often mediated
via biochemical signalling cascades which regulate a wide range of meta-
bolic processes (Volloch & Kaplan, 2002). The responses may range from
changes in cell surface adhesion and remodelling of the extracellular matrix
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