Biomedical Engineering Reference
In-Depth Information
PCL has been used as a scaffold to support
osteoblast growth. A porous PCL scaffold facil-
itates osteoblast production of alkaline phos-
phatase, a marker of bone mineralization, and
favors attachment and proliferation of osteo-
blasts [
strates another method of degradation control
[
].
POE polymers are desirable for bone tissue
engineering because they undergo surface
degradation yet maintain mechanical stability.
They can therefore be used in load-bearing appli-
cations even while tissue formation is incom-
plete. Scaffolds constructed of POEs have been
implanted into calvarial defects and have been
shown to promote new bone formation [
34
,
63
]. PCL has also been combined with
hyaluronic acid to improve the compressive
strength associated with the polymer, thus
enhancing its application in bone tissue engi-
neering [
19
19
].
4
].
6.2.5.1.3 Poly(Propylene Fumarate)
Poly(propylene fumarate) (PPF) is an aliphatic
linear polyester composed of repeating units of
two ester groups and one central unsaturated
carbon-carbon double bond [
6.2.5.2 Other Synthetic Polymers
6.2.5.2.1 Polyanhydrides
Polyanhydrides have a polymer backbone con-
taining an anhydride bond [
]. The polymer
degrades by hydrolysis of an ester bond that
leads to formation of fumaric acid and propyl-
ene glycol [
29
]. They contain
bonds that easily react with water, causing deg-
radation via surface erosion [
48
]. Polyan-
hydrides are synthesized by a dehydration
reaction of diacids, and degrade into these non-
toxic diacid monomers, which are removed
from the body within weeks to months [
49
]. These by-products cause mild
and short infl ammation, and therefore the
polymer is likely to be biocompatible [
29
]. The
double bonds of PPF allow it to be covalently
cross-linked. Cross-linking in response to a
trigger allows scaffold fabrication in situ, thus
making PPF an injectable biomaterial [
29
].
The polyanhydride degradation rate can be
modifi ed by changing the monomer concentra-
tions: increasing hydrophobicity decreases
degradation rate. For example, polyanhydrides
synthesized with carboxyphenoxypropane
degrade over a period of
48
]. In
addition, the cured form of PPF has signifi cant
compressive and tensile strengths and may
therefore constitute scaffold material for bone
tissue engineering [
28
3
to
4
years. However,
].
PPF scaffolds with varying porosities and
pore sizes have been investigated to analyze
tissue response in cranial defects. In all cases
the scaffolds only induced a mild tissue
response and allowed for vascularization of the
area [
28
when synthesized with
79
% sebacic acid, the
construct degrades over
]. Further-
more, polyanhydride synthesis can be activated
by a trigger such as photocross-linking, and
therefore curing can occur in situ [
2
weeks [
49
].
Polyanhydrides were fi rst studied in an
attempt to regulate the release of bioactive mol-
ecules [
13
,
14
29
]. In addition, PPF scaffolds coated with
TGF-
induced signifi cant bone formation in
cranial defects [
β
1
]. Polyanhydrides have limited
mechanical stability and therefore are inappro-
priate for load-bearing applications. However,
when imides were incorporated to form cross-
linkable networks, the mechanical stability of
the construct was increased [
32
,
48
92
].
6.2.5.1.4 Polyorthoester
Polyorthoesters (POEs) are a family of biode-
gradable polymers [
]. This is
thought to be due to the rigidity of the aromatic
imide group [
1
,
91
]. They are formed through
a reaction of ketene acetals with hydroxy-
containing molecules, such as diols [
8
]. Specifi cally, scaffolds con-
taining succinic acid have shown compressive
strengths of
91
]. POEs
are hydrophobic substances and undergo
surface degradation [
24
MPa and were degraded by
hydrolysis of the anhydride bonds plus the
imide bonds [
50
to
60
]. However, the
properties of POEs can be modifi ed by copoly-
merization. For example, degradation of the
polymer can be adjusted to an appropriate rate
by incorporating short acid groups such as gly-
colic or lactic acid [
24
,
35
]. The mechanical proper-
ties of the polymer have been increased by
photocross-linking [
32
,
91
6
,
13
,
68
].
]. In addition, the
orthoester linkages present within POEs have
been found to be more susceptible to hydrolytic
cleavage in acids than bases, which demon-
4
,
35
6.2.5.2.2 Polyphosphazene
Polyphosphazene contains a backbone com-
posed of alternating nitrogen and phosphorus
