Page 339 - Biodegradable Polyesters

12.7 Conclusions and Outlook 317

PLLA/PGA MFCs still hold some potential – additional drawing may increase
the orientation of the PGA molecules further, perhaps enough to enhance their
creep resistance. It may also be possible to modify PGA to increase its T ,thus
g
enhancing its creep resistance. It is indeed quite challenging to find a polymer
with all of the listed characteristics but perhaps it is worthwhile searching beyond
polymers which are normally considered biodegradable in the body, such as
PLLA and PGA. For example, studies on explanted vascular graft devices made of
poly(ethylene terephthalate) (PET) have shown that some degree of degradation
occurred in vivo [48] and Rudakova et al. [49] report that PET is completely
degraded in dogs and humans in 30 ± 7 years. This information invokes thinking
more laterally about solutions – PET is just an example of a polymer which might
be suitable if it is acceptable for it to remain in the vessel wall for many years
after complete degradation of the stent matrix material. This is something which
is yet another unknown: what is acceptable with regard to stent reinforcements?
May they be biostable or very slow-degrading materials if the effect of their
remaining in the vessel wall after the stent has degraded is neutral? Collaboration
and formation of interdisciplinary teams are needed to answer these types of
questions to guide effective research in this area in the future.
Adding to this, there may be alternative polymer combinations – the matrix
does not have to be PLLA. In fact, there may even be biodegradable polymer
combinations for MFC in which, for example, PLLA is the reinforcing component
surrounded by a ductile matrix. Furthermore, additional drawing steps could be
added to maximize the orientation of the reinforcing polymer as well as decrease
fibril diameters. A very interesting overlap of MFCs and particulate composites
provides even more potential for improvement and scope for future work because,
as mentioned before, nanoparticles could be used to modify the reinforcing poly-
mer’s thermal properties to enhance creep resistance.

12.7
Conclusions and Outlook
MFCs based on PLLA/PGA were successfully produced via cold-drawing of
PLLA/PGA wire with excellent fibril formation in the case of PLLA/PGA MFC
(70/30 by wt%). The MFC technique’s benefits are clear from the tensile test
results – the PLLA/PGA MFCs are tougher, stiffer, and stronger than the simple
PLLA/PGA blends from which they were derived. PLLA/PGA MFC (70/30 by
∘
wt%) is 35% stiffer and 84% stronger than neat PLLA in tension at 37 C. Relax-
ation tests at the same temperature indicate, however, that the creep resistance
of PLLA/PGA MFCs is poorer than that of neat PLLA. DMTA results reveal that
it is the onset of glass transition of PGA which is the underlying cause for the
creep propensity of PLLA/PGA MFCs. For load-bearing implants, this highlights
creep failure as a significant concern. Nevertheless, these fully biodegradable
MFCs exhibit properties which may be favorable for applications beyond medical
implants. Furthermore, there is still scope for development of the MFCs such