Biomedical Engineering Reference
In-Depth Information
PLA may be synthesized from three isomers leading to: poly
(
L
-lactide) (PLLA), poly(
D
-lactide) (PDLA) or poly(
D
,
L
-lactide)
(PDLLA). PLLA is used frequently since its degradation products are
similar to naturally occurring
L
(+) lactic acid. PLLA is a relatively
hard semi-crystalline polymer (crystallinity around 37%) with a high
melting temperature, while PDLLA is an amorphous polymer with
only a glass transition temperature around 55°C and therefore lower
tensile strength. Moreover, PLLA degrades very slowly (5 months to 5
years) and PDLLA has an intermediate degradation rate (12-16
months) between PLLA and PGA [CHE 08]. As a consequence,
PLLA is generally used as screws or pins in orthopedic applications,
while PDLLA is employed in drug delivery systems [PAR 07]. To
modulate the mechanical properties and the degradation kinetics of
PGA and PLA, different ratios of PLGA copolymers have been
commercially developed and used in a wide range of biomedical
applications, such as suture reinforcements, drug delivery vehicles,
and skin replacement materials. PLGA with compositions between 25
and 70% GA monomer is amorphous. In these copolymers, the
degradation rate decreases when the ratio of LA/GA monomers
increases [NAI 07].
PCL is a semi-crystalline polyester with a low glass transition
temperature that makes it semi-rigid at room temperature. PCL has
low modulus and tensile strength combined with a high elongation to
break, as well as a good organic solvent solubility. As a consequence,
PCL may be processed to obtain various material shapes, such as
microspheres, fibers and porous materials, and therefore it is used as
wound closure staples, scaffolds or long-term drug delivery systems,
such as one-year implantable contraceptive [ULE 11]. Indeed, PCL
degrades slowly (2-3 years) in comparison with other polyesters. Its
hydrolysis leads to low-concentrated caproic acid that does not cause a
significant negative reaction in the surrounding tissue and that is
completely metabolized since caproic acid enters the citric acid cycle.
PCL also degrades through enzymatic attacks. Finally, due to its
unusual properties, PCL may be compatibly blended with a wide
range of other polymers and is also used as a soft block in
polyurethane formulations [EDL 02].
