Multi-Objective Design of a Biodegradable Coronary Artery Stent - Cardiovascular and Cardiac Therapeutic Devices - page 3

Table 1 Comparison of stent platforms

Platform Company Material Elastic modulus (GPa) Strut thickness (lm)

BVS-B Abbott Vascular PLLA 3.3 150

Cypher Johnson & Johnson 316L SS 193 140

Element Boston Scientific PtCr 203 81

Endeavor Medtronic CoCr 210-243 90

Xience Abbott Vascular CoCr 210-243 81

PLLA poly-l-lactic acid. SS stainless steel, PtCr platinum-chromium alloy, CoCr cobalt-

chromium alloy

the Abbott Vascular BVS-B device is K = 3.3 GPa. 316 L stainless steel has
K = 193 GPa.
Due to the low stiffness of polymers, all currently known scaffolds have sig-
nificantly thicker struts than modern metallic stents; for example, the BVS-B
scaffold has a strut thickness, t = 150 lm (0.150 mm) and, in trials, is being
restricted to relatively straightforward, non-calcified de novo lesions [ 11 ]. From the
thicknesses listed in Table 1 , it is clear that significant research is necessary to
bridge the gap between polymeric and metallic materials. For reference, the original
Igaki-Tamai stent had a thickness of 170 lm.
However, a further question comes to mind at this point: how will long term
performance compare between devices made from the different materials, bearing
in mind that the merits of biodegradable scaffolds may more than compensate the
geometric disadvantages (e.g. thicker and wider struts) necessitated by inferior
material properties? Additionally, the trend towards thinner, narrower struts has
recently led to a new problem of stent distortion [ 12 , 13 ], a phenomenon high-
lighting the need to carefully consider multiple measures of performance. In this
case, the drive towards thinner struts and greater flexibility has dominated all other
considerations and has resulted in devices with compromised longitudinal strength.
Indeed, the recognition that stent design involves a number of potentially con-
flicting performance metrics (or objectives) is allied in this chapter to the tools
(computational and experimental) that are available to both simulate device per-
formance and to steer the design process itself.
Section 2 focuses on the evolution in design of metallic stents, primarily with
respect to geometry. In Sect. 3 , currently known biodegradable scaffolds are
described and mainly geometric comparisons are made in terms of key defining
features. Design methodologies are considered in Sect. 4 , including how they
might be best exploited in the development of polymeric devices.

2 Evolution in Design

When considering the evolution of coronary stent design, it is immediately
apparent that two key geometric differences have emerged relative to the Palmaz-
Schatz device: (i) open cells and (ii) thinner struts. The original Palmaz-Schatz