Page 329 - Biodegradable Polyesters

12.4 Biodegradable Stents 307

Table 12.2 Comparison of biodegradable stents [25].

Stent Material Drug Drug Radial Absorption
eluting? eluted support time
period

Igaki-Tamai PLLA No — 6 mo 2 yr
BVS PLLA Yes Everolimus Cohort A-<1month 2 yr
Cohort B-
REVA Tyrosine-derived No — 3–6 mo 2 yr
polycarbonate
Magnesium Magnesium alloy No — Days or weeks 4 mo
stent
BTI Salicylate linker Yes Sirolimus and 3mo 6 mo
salicylic acid

into the bloodstream [20]. The implication of this is that endothelialization of the
stent should occur before any fragments begin to break off (Table 12.1).
Because of the array of demands of biodegradable stent materials, it is not sur-
prising that researchers are experimenting with a wide range of materials with
a broad spectrum of properties. To illustrate this, a selection of biodegradable
stents, reviewed by Ormiston and Serruys [25], is shown in Table 12.2 – note that
these stents have been through clinical trials. Images of various biodegradable
stents can be found in [25, 27–29]. The radial support period of these stents varies
from days (in the case of a magnesium stent) through to 6 months for a poly(L-
lactic acid) (PLLA) stent.
To demonstrate examples of the pros and cons of different materials, the use of
magnesium and polymers for biodegradable stents is compared. The magnesium
stenthas thedistinctdisadvantageofashort period of vessel support becauseit
degrades fairly quickly. However, an advantage it has over polymeric stents is that
magnesium has far superior elastic modulus so it can be designed to have thinner,
narrower struts than polymeric stents while still offering good vessel support.
Another advantage of magnesium is the short time for total absorption – just
4 months versus 2 years for a PLLA stent – but again, this comes at the cost of a
short vessel-support period. However, there are many biodegradable polymers
available and more are being synthesized as polymer chemistry continues to
develop. Table 12.3 shows selected properties of some biodegradable polymers to
illustrate this.
Magnesium is not the only metal which has been considered – iron stents
have been under investigation too. Iron takes longer to degrade in the body
than magnesium and it has far superior mechanical properties to polymers
but it seems that researchers have steered away from it because of toxicity
concerns [39].