Biomedical Engineering Reference
In-Depth Information
atoms with two side groups attached to each
atom [
fold [
]. This suggests that polycarbonate
scaffolds can be designed to refl ect bone tissue
morphology and thus can induce growth appro-
priate to the specifi c site. Further studies with
poly(DTE carbonate) have show that it elicits
more direct bone apposition than other poly-
carbonates. This may be due to the ethyl ester
pendant group in the polymer [
85
]. Polyphosphazene is hydrophobic and
degrades by surface degradation into phos-
phate and ammonium salt by-products. Varia-
tion in polyphosphazene constructs can be
achieved by adding various hydrolytically
labile substituents to the phosphorus atoms
[
2
]. The degradation rate of phosphazenes
cannot be altered signifi cantly. These polymers
generally degrade slowly in vivo [
75
]. The hydro-
lysis of these groups produces calcium chela-
tion sites on the polymer surface that appear to
be related to polymer-bone bonding [
45
].
Polyphosphazenes have been of interest
because they can be readily modifi ed. Their
slow degradation rate makes them attractive
for long-term controlled-release devices [
75
45
].
6.2.5.2.4 Poly(Ethylene Glycol)
Poly(ethylene glycol) (PEG) is a linear-chained
polymer with an oxygen-carbon-carbon repeat-
ing unit. By varying the number of units, the
length and molecular weight of the polymer
can be changed [
].
Polyphosphazenes have also been used in
orthopedic applications because of their high
strength and surface degradation properties
[
75
]. Osteoblast cells seeded onto three-
dimensional polyphosphazene scaffolds have
been shown to support proliferation and skele-
tal tissue formation [
52
]. PEG homopolymer is
nondegradable. However, it can be copolymer-
ized with degradable polymers to allow degra-
dation [
15
,
33
51
].
]. PEG is highly water soluble due to
the oxygen molecule present in the polymer
backbone. Copolymerization of PEG with other
materials causes the subsequent material to
become more hydrophilic. This has led to
investigation of its potential function as a
hydrogel. However, linear PEG chains are sus-
ceptible to rapid diffusion and also have low
mechanical stability [
11
6.2.5.2.3 Polycarbonate
Tyrosine-derived polycarbonate (P(DTR car-
bonate)) is an amorphous polycarbonate that is
modifi able due to the presence of alkyl ester
pendant groups located within its linear chain
[
]. P(DTR carbonate) contains three bonds
that can be hydrolytically degraded: amide,
carbonate, and ester [
88
]. Networks of PEG can
be formed by attaching functional groups to
the ends of PEG chains and initiating their
cross-linking [
81
]. The carbonate bonds
have been found to degrade faster than the ester
bonds, and the amide bond is stable to hydroly-
sis at body temperature [
88
].
PEG has low mechanical stability and is
therefore not often used in bone tissue engi-
neering for load-bearing applications. However,
because it can be cross-linked into a network
with other synthetic materials and thereby
affect degradation, it is attractive as a copoly-
mer to obtain controlled erosion and degrada-
tion rates. PEG has been copolymerized with
poly(lactic acid), combined with a hydroxyapa-
tite ceramic, and used to deliver bone morpho-
genetic protein, resulting in complete repair of
bone defects [
72
,
79
,
86
]. The ester bond
degrades into carboxylic acid and alcohol,
whereas the carbonate bond by-products
include two alcohols and carbon dioxide [
27
,
88
].
P(DTR carbonate) is thought to be a biocompat-
ible material because it is based on the natural
amino acid tyrosine and degrades mostly into
nonacidic by-products [
88
].
P(DTR carbonate) can be modifi ed to degrade
over months or years [
18
]. It has been investi-
gated as a bone scaffold and shown to elicit a
response of bone ingrowth at the bone-polymer
interface [
18
]. Similarly, PEG has been com-
bined with PLA and p-dioxanone and used to
deliver bone morphogenetic protein (BMP),
exhibiting osteoconductive capacity [
46
]. In addition, research has demon-
strated the ability of osteoblast cells to attach
onto the surface of P(DTR carbonate) and
maintain their phenotype [
18
]. PEG
hydrogels have also been modifi ed with cell
adhesion peptides and used in tissue engineer-
ing. These gels delivered growth factors, result-
ing in effi cient and highly localized bone
regeneration [
62
]. Other investi-
gations with poly(deasminotyrosyl-tyrosine
ethyl ester carbonate) (poly(DTE carbonate))
demonstrated that bone ingrowth occurred in
cranial defects and that the patterns of bone
formation mimicked the structure of the scaf-
54
]. In addition, PEG has been
copolymerized with PLGA to form a foam that
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