Vascular implants for peripheral arterial bypass and aortic aneurysm repair - Biomaterials and Devices for the Circulatory System - page 214

8.2.1 Drug eluting stents
Drug eluting stent technology based on a thin fi lm polymeric drug carrier
that slowly releases an anti-proliferative agent has been demonstrated to
be effective in reducing restenosis rates in coronary artery stenting.
However, there has been concern about the increased rate of late throm-
bosis over that seen in bare metal stents. Stent fracture has been reported
as a complication of drug-eluting coronary stenting, in particular sirolimus-
eluting stents (Lee et al. , 2007). When used in peripheral arteries, drug
eluting stents have not shown signifi cant improvements in restenosis rates
compared with bare metal stents (Duda et al. , 2006). It is thought that the
polymer used as the carrier for the drug delivery may provoke vessel irrita-
tion, endothelial dysfunction and chronic infl ammation, thereby inhibiting
the healing response. Current research is being carried out to develop novel
coatings with surface properties designed to promote selective adhesion
and growth of endothelial cells (EC) leading to rapid endothelialisation of
the arterial wall and stent (Nickson et al. , 2008).

8.2.2 Bioresorbable stents
Stents are designed to resist the compressive forces due to remodelling of
the arterial wall following angioplasty. Permanent metallic stents exert
elevated forces and potential mechanical trauma on the vessel wall over
long periods but it may only be required to provide radial support for a
limited time after deployment. Stents based on degradable polymers and
co-polymers of lactic acid (PLLA) and corrodible magnesium alloys have
been proposed as a means of providing the initial temporary support to the
vessel wall without the long-term complications of permanent metallic
stents. PLLA-based stents which can also be loaded with antiproliferative
drugs are currently under development (e.g. Abbott BVS everolimus-
eluting and Igaki-Tamai which are both self-expanding). However, because
of the relatively low elastic modulus of polymer materials, these stents tend
to have lower radial strength, higher immediate recoil and the stent struts
are more bulky than metallic stents. The Reva medical stent is made from
tyrosine-derived polycarbonate and has a unique construction that is
designed to prevent recoil during deployment. Magnesium alloy has similar
mechanical properties to stainless steel but will corrode completely over
time after implantation. Unlike other metallic materials, the corrosion prod-
ucts of magnesium stents are not harmful and may actually have cardiovas-
cular benefi ts (Bo and Pisu 2008). Short-term trials of Biotronik magnesium
balloon-expandable stents have been conducted in lower limb and coronary
arteries (Peeters et al. , 2005; Erbel et al. , 2007).

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