Biomedical Engineering Reference
In-Depth Information
O
O
O
O
O
O
O
O
O
O
O
O
O
O
L-lactide
D-lactide
glycolide
ε
-caprolactone
O
O
O
O
O
O
O
O
O
β
-propiolactone
γ
-butyrolactone
δ
-valerolactone
trimethylenecarbonate
O
O
N
R
2
O
R
1
:H, CH
3
O
O
O
O
O
R
1
cyclodepsipeptide
O
1, 4-dioxane-2-one
1, 4-dioxepane-5-one
Fig. 2 Lactides, glycolide, and other typical cyclic monomers
PLA is one of the most popular aliphatic polyesters, and is obtained by ring-
opening polymerization (ROP) of LA (the cyclic dimer of lactic acid, 2,6-dimethyl-
1,4-dioxane-2,5-dione) using a catalyst or by direct polycondensation of lactic acid.
The most popular catalyst for ROP of LA is organic Sn, typically Sn(Oct)
2
.Most
commercially available PLAs are usually prepared from the
L,L
-isomer of lactide
(LLA) to give poly(
L
-lactide) (PLLA), because the naturally occurring lactic acid is
the
L
-isomer produced from glucose by microorganisms. PLLA is a semicrystalline
polymer with relatively high melting temperature (
T
m
), glass transition temperature
(
T
g
), and mechanical strength. Because PLLA is FDA-approved, and can be degraded
under physiological conditions to give
L
-lactic acid, which can be metabolized in the
human body, it has been applied for implantable biomedical materials.
Since PLLA is a degradable polymer having relatively good biocompatibility,
low toxicity and immunogenicity, and excellent mechanical properties, it has been
used in a variety of applications in the pharmaceutical and biomedical fields, and as
a degradable plastic for disposable consumer products. Recently, biodegradable
cellular scaffolds for tissue regeneration using PLLA and other aliphatic polyesters
have also been extensively studied. However, PLLA also has some disadvantages
for biomedical use. It is a brittle and hard polymer with very low elongation to
break and low compatibility with soft tissues. It is a hydrophobic polymer without
reactive functional groups (besides its termini). The degradation of bulk semicrys-
talline PLLA takes a long time, and the control of degradation is not so easy. The
properties of PLLA-based polymers can be modified by copolymerization (random,
block, and graft), hybridization with other polymers, change in molecular architec-
ture (branched, star-shaped, or dendrimers), and functionalization (end group and
