Biomedical Engineering Reference
In-Depth Information
through metabolism and biosynthesis of new tissue scaffolding to the activa-
tion of cytokines (Laquerriere et al. , 2003) and other biochemical pathways
regulating or modulating cellular morphology and function. Indeed, certain
properties of metallic implants have been implicated in thrombosis, adverse
cellular responses and the release of different types of growth factors, all
circumstances thought to trigger the mechanism of fi bromuscular prolifera-
tion and the formation of vessel ISR (Al-Lamee, 2000). In the quest to
improve stent biocompatibility and reduce ISR, multidisciplinary research
has been conducted into stent coatings to reduce the extent of thrombosis
and the hyperproliferative cellular response leading to ISR (Van Der
Giessen et al ., 1992; Bertrand & Sipehia, 1998; Gutensohn et al. , 2000). It
is clear, therefore, that surface modifi cation and surface treatment tech-
niques are a major area of interest in the development of the optimal stent
(Mauri et al. , 2005; Tamai et al. , 2000).
7.4
Surface modifi cations and coatings
Metal corrosion and limited biocompatibility have prompted research in
the development of surface treatments and coatings able to reduce the host
response towards the implant. In addition, prevention of ISR has been
pursued through the delivery of cytostatic or anti-infl ammatory drugs, the
release of which has been achieved through their encapsulation in poly-
meric coatings.
A number of different materials can be used to cover a stent surface
using an array of techniques including dipping, plasma spraying, plating,
sputtering or surface-induced mineralisation (Van Beusekom & van der
Giessen, 2000). Some materials have been used as coatings per se, while
others have been specifi cally developed as a means of local drug delivery
(Mauri et al. , 2005). These have been categorised according to their com-
position into inorganic materials, synthetic and biodegradable polymers,
immoblised or adsorbed drugs and drug eluting and/or degradable
matrices.
￿ ￿ ￿ ￿ ￿
7.4.1 Metal coatings
The reactions that metal surfaces undergo with oxygen are both variable
and complex (Chilton, 1968). Oxygen can form a protective oxide fi lm,
in a process known as passivation. Stainless steel, titanium, tantalum and
zirconium all form a spontaneous protective oxide fi lm (as discussed in
Section 7.3.1) which prevents their corrosion to varying degrees. However,
the long-term stability of these oxides also varies; some are soluble in the
natural environment and others form more stable and inert passive fi lms
(Su, 1998).
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