Cardiovascular stents - Biomaterials and Devices for the Circulatory System - page 186

carbon for the treatment of acute coronary syndromes and concluded that
their use was associated with a low restenosis rate within six months of
follow-up (Wöhrle et al. , 2009).

7.4.5 Ceramic coatings

The rationale for utilising ceramic biomaterials was originally founded
upon their inert chemical and biochemical nature. Subsequently, ceramic
biomaterials have been utilised in a range of applications including bulk
implants such as joint and bone replacement coatings that act as an inert
barrier between non-self materials and tissues (Lemons, 1996). Ceramics
offer a number of advantages, including the fact that they may be produced
from elements that are normal to human biological environments. In addi-
tion they can provide controlled structures that infl uence local interactions,
including attachment along device interfaces and act as a barrier between
foreign materials and tissues (Hubler, 1999; Lemons, 1996).
Since they offer so many potential advantages in biomaterial-based appli-
cations, ceramic coatings have attracted interest as a method of improving
the biocompatibility of cardiac stents. One such coating is hydroxyapatite
(HA), a non-toxic, non-allergenic non-infl ammatory crystalline derivative
of calcium phosphate (Rajtar et al. , 2006). Two types of HA coating are
possible. Firstly, ultra-thin fi lms that comprise 0.1-0.2 µm of high density
non-porous HA which provide a more biocompatible surface. These ultra-
thin coatings are deposited onto stainless steel or Co-Cr alloys using sol-gel
heated to 450 °C for improved structural and mechanical properties (Rajtar
et al. , 2006). These ultra-thin layers can also act as a nucleation layer for
additional micro-thin (0.3-1.0 µm) layers of electrochemically deposited
highly porous HA with the potential for drug delivery (Rajtar et al. , 2006),
thus avoiding the safety issues surrounding durable polymer coated drug
delivery stents described later in this chapter. These polymer-free, drug
eluting stents (VESTAsync eluting stents produced by MIV Therapeutics)
have recently undergone clinical trials (VESTASYNC1 trial) which have
demonstrated that the HA coated stent was effective in reducing in-stent
late lumen loss and neointimal hyperplasia at both four and nine month
follow-ups (Costa et al. , 2009). Van der Giessen and coworkers have also
recently demonstrated that reducing the dose of sirolimus eluting from an
HA coated stent reduces the delayed vascular healing seen with polymer-
based stents (Cypher) without affecting neointimal hyperplasia (van der
Giessen et al. , 2009).
Iridium oxide (IROX), a biocompatible, inert ceramic is reported to act
both as a barrier to metal ions and promote the conversion of hydrogen
peroxide that is produced by leucocytes in response to damage to the vessel
upon implantation into water and oxygen. This catalytic activity is suggested

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