Page 327 - Biodegradable Polyesters

12.4 Biodegradable Stents 305

12.3.5
Engineering Solutions versus Clinical Implications

At first glance, some of the challenges of stent design may seem trivial to solve
but these trivial solutions may have negative clinical implications. For example,
making stent struts thicker and wider enhances the support provided to the vessel
but results in more restriction of blood flow as well as increased risk of small
side-branches of the vessel being blocked. Furthermore, endothelialization would
be more difficult in the case of thicker struts. To enhance stent performance
while minimizing negative impacts on clinical outcomes, stent materials must be
developed to have improved mechanical properties. The next sections will outline
biodegradable polymers used for medical applications, then focus on biodegrad-
able stent material development progress and compare the various materials
which have been, or are currently under, investigation for biodegradable stents.

12.4
Biodegradable Stents

12.4.1
Selection Criteria for Biodegradable Stent Materials

Engineering design work often involves careful selection of materials to suit the
application and the conditions under which the product or device will operate.
From Section 12.3, the requirements for mechanical function of stents are clear
but there are several more requirements arising from physiological and biolog-
ical considerations which have been outlined in several reviews [4, 11, 20–22].
Firstly, the material from which a stent is made, as well as the resulting degrada-
tion by-products, must be biocompatible [20–22]. In other words, they must not
cause harm to cells and organs and must not cause unfavorable reactions, such as
excessive inflammation.
Some of the requirements for stents are not exclusively material requirements
but also rely on the geometry of the stent. For one, the stent must be flexible
enough to be bent through curved vessels during navigation to the target site
of implantation [4, 22]. This implies that the material may have to experience
large strains but the bending stiffness of a stent is largely controlled by the
design geometry. Another requirement of a stent is that it must withstand a
minimum crush-pressure but the values stated in literature vary enormously
from 100 to 750 mmHg [20, 23, 24], sometimes quoted without explanation or
reasoning. In work by Lanzer [24] it is mentioned that 200 mmHg is the expected
pressure exerted by a vessel on a stent, assuming that the stent retains the vessel
at 10% overexpansion [24], but this figure does not take into account that blood
pressure opposes inward pressure from the vessel. Thus the pressure on the
stent is 200 mmHg – blood pressure. Blood pressure fluctuates between 80 and
160 mmHg with each cardiac cycle according to ASTM standard F2477, therefore
the stent would have to sustain a pressure of 80 mmHg, assuming a mean blood