Biomedical Engineering Reference
In-Depth Information
scaffolds because they are currently FDA-
approved for use as suture materials and as
drug-delivery systems. PGA can be highly crys-
talline (
tion of the cyclic monomer
-caprolactone and
is degraded by bulk hydrolysis. This material
has a slower degradation rate than PLA and is
easily copolymerized with other polymers [
ε
%), depending on its prepara-
tion method, and is hydrophilic in nature. Its
high crystallinity makes it nonsoluble in many
organic solvents except for those that are highly
halogenated. PGA is mainly synthesized by
methods employing ring-opening polymeriza-
tion, and, like all polyesters, is degraded pri-
marily by bulk hydrolysis of ester linkages at
random sites. PGA crystallinity has a large
impact on material degradation rate, because
the more crystalline portions retard water
entry and thus hydrolytic cleavage [
46
%-
50
3
,
70
-caprolactone) was used to
fabricate three-dimensional nanofi brous scaf-
folds, allowing for in vitro chondrogenesis of
seeded mesenchymal stem cells over
]. Recently, poly(
ε
3
weeks
[
63
].
4.5.2.3 Poly(Orthoesters)
Poly(orthoesters) (POEs) are hydrophobic poly-
mers that are degraded by surface erosion. Dif-
ferent degradation rates can be achieved by the
addition of lactide groups, because carboxylic
acids released by the degradation of the lactide
segments facilitate the degradation of the
orthoester [
].
PLA is another type of biodegradable and
biocompatible poly(
8
,
71
-ester). It is also synthe-
sized by ring opening polymerization and has
two isomeric forms, D(
α
]. An in vivo comparison between
POE and PLGA scaffolds for bone tissue engi-
neering found that POE scaffolds maintained
their structural integrity after
32
). Like PGA,
it is degraded by bulk hydrolysis of the ester
linkage catalyzed by the presence of the degra-
dation product, lactic acid [
−
) and L(
+
weeks,
whereas PLGA scaffolds partially collapsed
after
6
and
12
]. PLA can also
occur in crystalline forms, with the degree of
crystallinity ranging as high as
65
6
weeks [
6
].
%. It is more
hydrophobic than PGA and therefore has a
slower degradation rate and a higher modulus
[
37
4.5.2.4 Poly(Anhydrides)
Poly(anhydrides) are prepared by a melt con-
densation reaction of diacid molecules. They
degrade by surface erosion and thus have been
widely investigated as vehicles for biocompati-
ble controlled release [
]. This high mechanical strength makes it
a desirable material for orthopedic fi xation
devices [
8
,
72
]; however, the release of degrading
crystal-like particles can be problematic.
Lactic acid and glycolic acid are often
copolymerized at various ratios yielding
poly(lactic-co-glycolic acid) (PLGA), with dif-
ferent properties from those of either of the
homopolymers. The major difference is that
the copolymer is amorphous within a wide
range of copolymer ratios because of the dis-
ruption of the crystalline phases and therefore
has a faster degradation rate and lower elastic
modulus than PGA or PLA alone[
19
]. Poly(anhydrides),
however, are not strong enough to be used as
orthopedic materials, so photocross-linking or
combination with other polymers such as poly-
imides has been used to improve the overall
mechanical properties of implants [
90
32
].
4.5.2.5 Poly(Ethylene Glycol)-Based
Materials
Poly(ethylene glycol) (PEG) is a hydrophilic,
highly biocompatible polymer with a variety of
biomedical applications. Many different types
of PEG-based materials have been developed
as hydrogel scaffolds, including poly(ethylene
glycol)-diacrylate (PEG-DA) and poly(ethylene
glycol)-dimethacrylate (PEG-DM) [
]. A
study using two-dimensional and three-
dimensional PLGA scaffolds impregnated with
recombinant human bone morphogenetic
protein
8
,
42
,
76
) and seeded with rabbit
bone marrow stromal cells has reported in
vitro osteogenic differentiation and ECM pro-
duction over
2
(rhBMP-
2
2
months [
44
].
].
Work with PEG-DM has demonstrated that it
could encourage cartilage-like ECM production
from encapsulated bovine chondrocytes over
23
,
67
,
102
4.5.2.2 Poly(
e
-Caprolactone)
Poly(
-caprolactone) (PCL) is a semicrystalline
polymer with a melting temperature of
ε
4
59
to
weeks in vitro [
]. Although these deriv-
atives often have limitations as scaffold materi-
als because of their lack of degradability, PEG of
12
,
23
,
67
64
C. PCL is
also synthesized by ring-opening polymeriza-
C and a glass temperature of
−
60
