Page 325 - Biodegradable Polyesters

12.3 Stents – an Engineering Point of View 303

Vessel wall
Balloon
catheter
Stent
(a) (b)

(c) (d)

Pressure exerted
Blood pressure on stent by vessel

(e)
Figure 12.3 Stent delivery via balloon-expansion (a-d) and a schematic of stent loading
conditions (e).

10 mm

Figure 12.4 A stent prototype made (bottom) balloon-expansion [17]. The stent
of a poly(L-lactic acid)(PLLA)/poly(4- diameter remains enlarged because of per-
hydroxybutyrate) blend. The stent manent deformation of the PLLA/P4HB dur-
is shown before (top) and after ing balloon expansion.
12.3.2
Importance of Creep after Implantation
Once the stent has been deployed it provides support to the healing vessel to
prevent it from collapsing. This implies that the vessel exerts an inward pressure on
the stent which must be supported until the vessel has healed sufficiently, mean-
ing that the stent needs to endure pressure for an extended period of time. With
long-term loading comes the possibility of creep, which is the gradual deforma-
tion of a material under load – even if the stresses caused by this load are well
below the maximum load the material can handle. In the case of a stent, creep
would cause a gradual collapse, resulting in narrowing of the vessel. Creep is an
especially important consideration for biodegradable stents for two main reasons,