Biomedical Engineering Reference
In-Depth Information
low molecular weight can readily be excreted by
humans and therefore can be copolymerized
with other polymers such as PLA and PPF to
be used as a biodegradable scaffold material
[
for the delivery of chondrocytes for articular
cartilage replacement in tissue engineering
[
].
Oligo(poly(ethylene glycol) fumarate) (OPF)
is yet another novel biodegradable fumarate-
based polymer. It is synthesized by the combi-
nation of PEG and fumaryl chloride in the
presence of triethylamine [
26
26
,
83
].
4.5.2.6 Poly(Amino Acids)
Poly(amino acids) have been considered as
promising materials for biomedical applica-
tions because of their composition. However,
the polymerization of pure poly(amino acids)
is hard to control precisely. Furthermore,
depending on the combination of amino acids,
these materials can evoke an immune response
in vivo [
]. Both in vitro
and in vivo studies using this material demon-
strated good biocompatibility, with a minimal
infl ammatory response observed after implan-
tation for
51
12
weeks in cranial defects in rats and
14
weeks in osteochondral defects in rabbits
[
]. High water absorption and mild in
situ cross-linking conditions enable OPF to
encapsulate living cells or bioactive growth
factors for orthopedic tissue regeneration [
38
,
86
,
95
]. For these reasons, synthetic
pseudo poly(amino acids), such as tyrosine-
based polycarbonate, have been investigated
recently. The polycarbonate not only exhibits
good biocompatibility, but also supports the
attachment of osteoblasts and osteoprogenitor
cells. In addition, by varying the structure of
the repeating unit, this material is easily modi-
fi ed to exhibit a range of mechanical proper-
ties, degradation rates, and bioactivity [
34
,
76
41
,
94
]. Recently, OPF has been explored as a cell
carrier for marrow stromal cells. After
weeks
of culture in vitro, cells remained alive. Evi-
dence of osteoblastic differentiation, including
calcifi ed ECM production throughout the
hydrogel, was observed (Fig.
4
4
.
3
) [
93
].
81
].
4.5.2.7 Fumarate-Based Polymers
Poly(propylene fumarate) (PPF) is a biodegrad-
able poly(ester) whose degradation generates
1
-propanediol and fumaric acid, the latter of
which is a naturally-occurring material pro-
duced in the Krebs cycle [
,
2
]. A number
of methods can be used to synthesize PPF, and
each produces polymers with unique physical
properties [
2
,
25
,
27
]. The backbone of this
polymer contains double bonds, which lead to
the formation of a three-dimensional network
either by photocross-linking with bis(
25
,
32
-
trimethylbenzoyl) phenylphosphine oxide
(BAPO) or by thermal cross-linking with
benzoyl peroxide [
2
,
4
,
6
]. PPF has been inves-
tigated for use in injectable orthopedic implants
because it possesses, in its cross-linked form,
mechanical properties similar to those of can-
cellous bone [
25
,
98
]. Its mechanical properties
can be further improved by the alteration of
cross-linking agents or by the incorporation of
a nanophase or microphase [
98
Figure 4.3.
Histology of oligo(poly(ethylene glycol) fuma-
rate) hydrogels containing rat marrow stromal cells after 7 (A),
21(B), and 28(C) days of in vitro culture with media supple-
mented with dexamethasone. Polymer is labeled P, mineral-
ized matrix is labeled M, and arrows indicate the location of
some of the cells found throughout the hydrogel. Reproduced
with permission from Temenoff et al. [93]. Copyright 2004,
American Chemical Society.
]. In an in vivo
study using rabbits, photocross-linked PPF
scaffolds with different pore sizes and porosi-
ties exhibited good biocompatibility [
43
].
Additionally, P(PF-co-EG) has been evaluated
for use as a thermoreversible hydrogel scaffold
27
