Cardiovascular stents - Biomaterials and Devices for the Circulatory System - page 197

7.7.1 Polymeric biodegradable stents
Polymeric biodegradable stents have the potential to remain in situ for a
predicted period of time keeping the vessel wall patent before degrading
into non-toxic substances. Efforts to develop a fully biodegradable stent
have been slow due to mechanical complexities and currently there are no
commercially available bioresorbable stents. However, a number of trials
are currently in progress. Polymers that have been exploited for use as stent
materials include poly-L-lactic acid (PLLA) including the Igaki-Tamai ®
stent which is a PLLA monofi lament with a zigzag helical design which has
undergone a feasibility study in 2000 patients. Tyrosine-derived polycarbon-
ate bioresorbable stents (REVA stents) with drug delivery matrix capabili-
ties are also in human clinical trials due to reach their fi ve year follow-up
in 2012 (the REVA Endovascular Study of a Bioresorbable Coronary Stent
- RESORB). The BVS everolimus-eluting stent has also been studied in
the ABSORB trial. This stent consists of a polylactic acid backbone with a
polymeric coating of poly-D,L-lactide and everolimus, an anti-proliferative
and anti-infl ammatory drug that is loaded onto the polymer. An excellent
review on the state of biodegradable stents in 2009 and the outcome of
relevant clinical trials has recently been published by Raoul Bonan and
Anita Asgar (2009).

7.7.2 Metallic biodegradable stents
Magnesium has been utilised due to its biocompatibility, biocorrosion prop-
erties and invisibility to X-rays, which is useful in reducing artefacts during
imaging with systems such as computed tomography (CT) or magnetic
resonance imaging (MRI) (Bonan & Asgar, 2009). Heublein and colleagues
(2003) have demonstrated that magnesium stents placed in the coronary
arteries of pigs lost mechanical integrity within 35-56 days with no evidence
of thromboembolic events.
Human trials (the Clinical Performance and Angiographic Results of
Coronary Stenting with Absorbable Metal Stents - PROGRESS-AMS
trial) have been undertaken on Lekton Magic bioabsorbable coronary
stents produced by Biotronik, Bulach of Switzerland from a magnesium
alloy (WE43) containing zirconium (

￿ ￿ ￿ ￿ ￿

< 5%), yttrium ( < 5%) and rare earths

( 5%). The study demonstrated that the stents were both safe and effi ca-

cious but also that rates of restenosis and revascularisation were higher than
desired and comparable with results seen with non-stented balloon angio-
plasty (Bonan & Asgar, 2009).
Iron has also been investigated as a corrodible stent material in a limited
number of small (rabbit) and large (minipig and domestic pigs) animal
models (Peuster et al. , 2001, 2006; Waksman et al. , 2008). Despite concerns

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