Page 170 - Biodegradable Polyesters

148 6 Shape Memory Systems with Biodegradable Polyesters

with the electrospun PCL nanofiber mat. Note that the semi-IPN version should
have self-healing activity, which was, however, not yet tested.

6.2.5.3 Interpenetrating Network
Full-IPN-structured thermosets are also suitable SMP systems. This was shown
by Zhang et al. [79]. The IPN structure was composed of cross-linked PU
(polylactide-glycolide-based (PLAGA) diol cross-linked by polyisocyanate) and
cross-linked PEG dimethacrylate (cured by UV irradiation). The content of the
latter was varied between 0 and 50 wt%. The IPNs were amorphous and exhibited
∘
a rather broad T range (T between −23 and 63 C) that served for T .Both
g g trans
R and R data were reported over 93%.
f r
6.3
Applications

Biodegradable polyester SMPs are mostly used in and developed for applications
in the human body. That is the reason many R&D works addressed the adjust-
ment of T trans ,related to T m or T , respectively, to the body temperature. The
g
other aspect, usually covered in the related research, was the investigation of the
in vitro biodegradability using phosphate-buffered saline solutions. The medical
applications of biodegradable polyesters are typically surgical sutures, catheters,
and stents. In many other medical applications, such as bone fixing, polymers
other than polyesters are used.
Absorbable polymer sutures for wound closure appeared in the 1970s. These
PGA-based sutures registered tremendous growth, a trend which continues even
now. Since the wound should be closed for healing, which can be easily triggered
by a transition from temporary to permanent shape, the interest turned to smart
sutures making use of the SM behavior. The attribute “smart” may be related to the
antimicrobial and healing activities, set by the incorporation of suitable additives
into the sutures [80, 81].
Nowadays, metallic stents, also from SM alloys, are widely accepted in the
surgery. Their use is, however, associated with some complications due to the
stiffness (Young’s moduli) mismatch between the metal and the tissue. The related
“tissue trauma” is repeated when the stent is removed surgically after the dis-
ease has been cured. Biodegradable polymer stents have been getting focused
attention due to the recognition that the stents are only temporarily needed in
the body and they may have similar flexibility and elasticity as the body tissues
[82, 83]. PLA (T linked with T )and PCL(T linked with T )were
trans g trans m
used first as stent materials. The related T and T values are much higher
g m
than the body temperature, and thus their expansion was supported by hot
liquid filled balloons. Instead of balloon-expandable stents, R&D works were
devoted to self-expandable stents with SM function. Note that they should have
sufficient elasticity to expand. Venkatraman et al. [83] used PLAGA copolymers
of various compositions and thus T values, to produce single- and bilayer stents.
g