Multi-Objective Design of a Biodegradable Coronary Artery Stent - Cardiovascular and Cardiac Therapeutic Devices - page 2

1 Introduction
Now that metallic stents are approaching a mature stage of evolution and advances
in the development of biodegradable scaffolds are supporting the realization of a
fourth revolution in interventional cardiology [ 1 ], it is timely to consider what
lessons have been learnt from the design of permanent metallic devices, and how
best practice might be applied to the development of a growing number of poly-
meric scaffolds, in particular.
Interventional cardiology was first revolutionized in the 1970s when Andreas
Gruntzig and colleagues performed the first human coronary balloon angioplasty
in 1977. The second revolution occurred in the mid 1980s with the introduction of
bare metal stents; even though the Palmaz-Schatz stent was not approved by the
Food and Drug Administration (FDA) in the United States until 1994. Nearly a
decade later, FDA approval was granted for Johnson & Johnson's Cypher drug
eluting stent (DES), signifying a key milestone in the third revolution in inter-
ventional cardiology. DESs represent the current state of the art and a useful
review of DESs is available in [ 2 ]. Whilst most widely available stents are
deployed using balloon inflation [ 3 ], there is also a range of self-expanding devices
which also emerged in the 1990s [ 4 ]. Three of the most popular DESs are listed in
Table 1 along with the Cypher platform and Abbott Vascular's second generation
bioresorbable vascular scaffold, BVS-B.TheBVS-B device is one of the major
forerunners in the fourth revolution which started in the early to mid-1990s with
tests of non-biodegradable and degradable polymers in porcine animal models
[ 5 , 6 ] and with the first-in-man procedure of the fully biodegradable Igaki-Tamai
coronary stent in 1998. Early results were presented by Tamai, Igaki and others in
2000 [ 7 ].
Some key questions are inevitably posed when considering the evolution of
stent design and the implications for fully biodegradable devices. For example:
(i) is there sufficient justification for stents that perform the necessary function and
then disappear (within approximately 2 years)?; and (ii) will it be possible to
develop strong enough polymeric stents, capable of treating the wide range of
challenging disease states routinely encountered in the cath-lab? With respect to
the first question, there is an increasing body of evidence to support the positive
claims of Serruys [ 1 ] and others [ 8 ], including the long-term evidence (based on a
follow-up study lasting more than 10 years) of the first-in-man Igaki-Tamai stents
for safe and effective treatment using bioresorbable scaffolds [ 9 ], and the recent
identification of late positive remodeling and late lumen gain [ 10 ], and the
implications for the recovery of normal vasomotion. Indeed, there are a number of
compelling reasons for favoring the biodegradable scaffold concept over that of the
metallic cage.
Despite these encouraging findings, the second question above brings a major
issue into focus concerning the material properties of currently available polymers
and the geometric constraints associated with them. Whilst platinum-chromium
has an elastic modulus, K = 203 GPa, the value reported for the polymer used in