Biomedical Engineering Reference
In-Depth Information
pendant group) with reactive and hydrophilic groups (hydroxyl, amino,
carboxyl, thiol and so on). The physical properties such as crystallinity,
T
m
,
T
g
,
hydrophobicity, and mechanical properties can be affected by such modifications to
give somewhat controllable degradation rates.
PGA was one of the very first degradable polymers ever investigated for bio-
medical use. PGA found favor as a degradable suture, and has been actively used
since 1970 [
45
-
47
]. Because PGA is poorly soluble in many common solvents,
limited research has been conducted with PGA-based drug delivery devices.
Instead, most recent research has focused on short-term tissue engineering scaffolds.
PGA is often fabricated into a mesh network and has been used as a scaffold for bone
[
48
-
51
], cartilage [
52
-
54
], tendon [
55
,
56
], and tooth [
57
].
PCL is a semicrystalline polyester with great solubility in common organic
solvents, a low
T
m
(55-60
C), and low
T
g
(
54
C) [
58
]. Because of PCL's very
low in vivo degradation rate and high drug permeability, it has found favor as a long-
term implant delivery device. PCL has low tensile strength (
23 MPa), but very
high elongation at breakage (4,700%), making it a very good elastic biomaterial [
59
].
PCL and PCL composites have also been used as tissue engineering scaffolds for the
regeneration of bone [
60
-
62
], cartilage [
63
], skin [
64
], nerve [
65
], and other tissues.
These representative aliphatic polyesters are often used in copolymerized form
in various combinations, for example, poly(lactide-
co
-glycolide) (PLGA) [
66
-
68
]
and poly(lactide-
co
-caprolactone) [
69
-
73
], to improve degradation rates, mechanical
properties, processability, and solubility by reducing crystallinity. Other monomers
such as 1,4-dioxepan-5-one (DXO) [
74
-
76
], 1,4-dioxane-2-one [
77
], and trimethylene
carbonate (TMC) [
28
] (Fig.
2
) have also been used as comonomers to improve
the hydrophobicity of the aliphatic polyesters as well as their degradability and
mechanical properties.
<
2.1.2 Polyesters Having Reactive (Hydrophilic) Side-Chain Groups
To design amphiphilic and/or reactive copolymers containing aliphatic polyesters,
one of the most promising approaches is copolymerization with functional
monomers having protected reactive side-chain groups. Some kinds of monomers
having reactive (hydrophilic) side-chain groups have been reported (Fig.
3
).
Recently, the synthesis of various types of functional polyesters has been reviewed
[
15
-
19
].
Malic acid (MA) has two carboxylic acid groups and one hydroxyl group, and can
be metabolized by the tricarboxylic acid (TCA) cycle. There are three different types
of poly(malic acid)s (PMAs), poly(
a
-malic acid) (
a
-PMA), poly(
b
-malic acid) (
b
-
PMA), and poly(
a
,
b
-malic acid) (
a
,
b
-PMA).
a
-PMA can be synthesized by ROP of
the protected cyclic dimer malide dibenzyl ester (MDBE), and subsequent
deprotection [
78
]. Although the molecular weight of the
a
-PMA homopolymer is
less than 3,800 Da because of its low reactivity, copolymerization of MDBE with
lactide was possible to give a higher molecular weight copolymer having reactive
carboxylic acid side-chain groups [
78
]. The use of
a
-PMA as a water-soluble carrier
