Page 324 - Biodegradable Polyesters

302 12 Biodegradable Polyesters Polymer–Polymer Composites

and were addressed as stents evolved to DESs. However, complications still per-
sisted but tend to occur later after implantation, for example, late stent thrombosis
(i.e., thrombosis occurring long after implantation). Naturally, we expect the
next evolutionary step to address this. It has been recognized that stented vessels
need support for only up to 6 months while they heal [15], so development of
stents which degrade and essentially disappear after their jobs are done began.
These stents are known as biodegradable,or bioabsorbable, stents. Biodegradable
stents offer the advantage of completely disappearing after their jobs are done,
which makes future intervention less complicated. Furthermore, there is no need
for rest-of-life pharmaceutical treatment as is the case for permanent stents
(aspirin is taken indefinitely after treatment) [12, 16]. Before biodegradable stent
development is discussed from a materials perspective, let us lay the foundation
for this by looking at stents from an engineering point of view.

12.3
Stents – an Engineering Point of View

In order to better understand the challenges of biodegradable stent development,
it is useful to first think about what a stent must endure during its lifetime. Thus
an engineering point of view is outlined in this section to distil some of the key
material requirements of biodegradable stents.

12.3.1
Stent Deployment: the Need for Ductility
As a starting point to describe what is required of stent materials, imagine that a
stent has been navigated to the target site of a vessel needing support. Now, the
stent is deployed via balloon expansion during which the balloon upon which the
stent is mounted is inflated, as shown in Figure 12.3a,b. Keep in mind that the stent
is expanded to a diameter greater than the desired final diameter. Once expansion
of the stent is complete, the balloon is deflated and withdrawn, Figure 12.3c. After
the balloon is deflated, the stent diameter reduces somewhat because the elastic
strains within the stent material are recovered, a phenomenon termed recoil.This
is why the stent has to be expanded beyond the desired final diameter. Once the
balloon has been deflated and withdrawn, the stent remains in place to support
the vessel, Figure 12.3d.
From a material point of view, it is important to note that the stent has to
undergo permanent deformation during the deployment process. Photos of a
stent prototype by Grabow et al. [17], taken before and after balloon-expansion,
clearly illustrate this (Figure 12.4). This permanent deformation leads to the first
of many material requirements: ductility. Ductile materials can be subjected to
large strains before breaking while brittle materials fracture at low strains instead
of deforming and stretching.