Biomedical Engineering Reference
In-Depth Information
more hydrophobic PLA into the polymeric structure limits the water uptake
and reduces the rate of backbone hydrolysis.
Lactic acid is a chiral molecule (i.e. a molecule that is not superimposable
on its mirror image), it exists in two stereoisomeric forms that give rise to
four morphologically different polymers:
d
-PLA,
l
-PLA,
d l
-PLA and meso-
PLA. the crystalline properties differ from polymer to polymer. Both
d
and
l
polymers are semicrystalline materials, whereas the optically inactive
d
L-
PLA is always amorphous. For this reason,
l
-PLA is preferred for application
where high mechanical strength and toughness are needed.
Other biodegradable polymers
Polyphosphazenes are at the interface between inorganic and organic polymers
as their backbone consists of nitrogen-phosphorus bonds. the main applications
of polyphosphazenes are in the field of drug delivery and in skeletal tissue
regeneration.
50, 51
Polydioxanone (Pds) is a poly(ether ester) with interest
for the medical field owing to its degradation to low toxicity products. It
was the first polymer to be used as monofilament suture.
in view of the importance of PGA, PLA and their copolymers poly(lactic-
co-glycolic acid) (PLG) in the field of biomaterials, a more detailed description
of their properties will be given.
8.4.2 Synthesis of poly(glycolic acid), poly(lactic acid) and
copolymers poly(lactic-co-glycolic acid)(PLG)
these polymers are synthesized by ring opening polymerization (roP)
of the corresponding monomers or mixtures of the monomers in different
ratios (Fig. 8.6).
the use of roP of glycolide and/or lactide instead of direct polymerization
of the corresponding acid (glycolic acid and lactic acid, respectively) is
because the latter leads to low molecular weight polymers and therefore poor
mechanical properties. For example, the molecular weight of PLA, obtained
via direct polymerization, is approximately 10 000-20 000 g mol
-1
, while via
ring opening polymerization it rises to 500 000 g mol
-1
or even higher.
8.4.3 Physical properties of poly(glycolic acid), poly(lactic
acid) and copolymers PLG
PGA is highly crystalline (45-55%) with a high melting point (220-225ºc)
and a glass transition temperature of 35-40ºc.
52
PGA polymers exhibit high
tensile strength and low elongation and consequently have a high modulus
(table 8.2). the homopolymer of
l
l-lactide (
l
(l-PLA) is a semicrystalline polymer
