Page 326 - Biodegradable Polyesters

304 12 Biodegradable Polyesters Polymer–Polymer Composites

the first being that the majority of biodegradable stents are made of thermoplastic
polymers which are prone to creep deformation and the second being the effect
of degradation on material performance.
It is very challenging to design with creep in mind because of the numerous vari-
ables involved. For example, if a stent collapses slightly through creep deformation
and the vessel experiences a corresponding narrowing, the pressure exerted on the
stent decreases because the vessel is now less “stretched” than it was immediately
after stent implantation. Therefore the creep rate would slow down because the
lower the stresses in a material, the lower the creep rate. Furthermore, as a ves-
sel heals we should expect that the pressure it exerts on a stent decreases as it
gradually regains the ability to support itself. This implies that resistance to creep
deformation is most critical in the early stages after stent implantation but there
seems to be a gap in the knowledge of the healing profile of a stented vessel, from
an engineering point of view at least.
12.3.3
A Vessel Is Not Static: Material Fatigue Considerations

With every heartbeat, blood pressure in coronary arteries fluctuates [18] and some
branches also experience significant curvature changes as they bend and unbend
during each cardiac cycle [19]. Blood pressure fluctuations result in subsequent
vessel diameter fluctuations which would, in turn, cause the pressure exerted on
an implanted stent to fluctuate. Furthermore, a stent implanted in a region where
the vessel’s curvature changes significantly would be forced to bend and unbend
with each heartbeat. Both of these phenomena result in the presence of cyclic
stresses in the stent, which raises the concern of fatigue failure. In the case of
biodegradable stents, fatigue failure is destined to occur at some point when the
stent material integrity has diminished enough but care must be taken to prevent
premature fatigue failures.

12.3.4
Material Degradation: a Critical Variable

The material considerations discussed so far apply to both permanent stents and
biodegradable stents but a consideration unique to biodegradable stents is the
effect of degradation on the mechanical characteristics of the material. In order
to design a biodegradable stent, one first needs to understand how the properties
of a potential stent material will change with time as it degrades. It is a complex
challenge because there is still much to be learned about the rate at which support
required by a healing vessel decreases, whether or not it is linear, and how much
it varies from patient to patient depending on age, medical conditions, and so on.
Perhaps when there is more knowledge in this area, biodegradable stents could be
tailored to meet the needs of specific patients.