Biomedical Engineering Reference
In-Depth Information
H
H
H
H
H
H
[
C(CH
2
)
5
O ]
n
CN
O
(CH
2
)
4
C N
(CH
2
)
4
N
C
O(CH
2
)
4
O
N C N (CH
2
)
4
N
m
O
O
O
O
Polyurethane (PU)
(
)
n
(
)
x
(
)
y
(
)
x
HO
CH
2
CH
2
O
H
HO
CH
2
CH
2
O
CH
2
CH
CH
3
O
CH
2
CH
2
O
H
Poly(ethylene glycol) (PEG)
Poly(ethylene oxide) (PEO)
Pluronic
®
(ethylene oxide-propylene oxide block copolymer)
O
O
NHR
1
N P
NHR
2
(
)
n
(
)
n
O
CH
2
CH
O
C
CH
CH
C
CH
3
Polyphosphazene
Poly(propylene fumarate) (PPF)
O
O
O
O
(
)
n
(
)
n
CH
2
CHCH
2
(CH
2
)
8
CO
CH
2
CH
NH
C
CH
2
CH
2
O
O
O C
C
C
O
CH
2
CH
3
OH
O
Poly(tyrosine isocyanate)
Poly(glycerol sebacate)
Fig. 7.2-11 Chemical structure of synthetic, absorbable polymers used for tissue engineering (except for
a
-hydroxyacid polymers).
these requirements, because these polymers have been
used for elastic devices such as indwelling catheters,
intra-aortic balloons, and left ventricular assist devices.
Segmented PUs are basically synthesized from
a polymer with a terminal hydroxyl group at both chain
ends (HO-P-OH) and an excess of diisocyanate (OCN-
R-NCO). Upon mixing them under an excess of diiso-
cyanate, a prepolymer having terminal isocyanate groups
is produced under formation of urea bonds between
-OH and -NCO. Addition of a diamine chain extender
to this prepolymer increases the chain length to form
segmented PU. The P portion associates to yield a soft
segment while the R portion forms a hard segment. If
the P portion comprises hydrolysable units, the resultant
PU exhibits bioabsorption.
cell scaffolding as well. Among them are hydroxyapatite
(HAp) [Ca
10
(PO
4
)
6
(OH)
2
] and b-tricalcium phosphate
(b-TCP) [b-Ca
3
(PO
4
)
2
]. Synthetic HAp is very slowly
absorbed in the body by attack of osteoblasts, whereas
b-TCP undergoes absorption probably both with and
without such biological assist. A precursor of biological
apatite in bone tissue is octacalcium phosphate (OCP)
[Ca
8
H
2
(PO
4
)
6
5H
2
O]. The OCP is absorbed at a higher
rate than HAp and b-TCP. A large advantage of HAp as
scaffold is that this is a natural component of bones and
bioactive (or bioconductive). The inherent brittleness of
calcium phosphates limits their use to the exclusive graft
material in non-load-bearing defects.
Coral with a chemical composition of calcium carbon-
ate (CaCO
3
) has an advantage of good communication
between pores and is readily converted to HAp, resulting
in a high affinity for cells with tissue-regenerating ability.
A porous material prepared from coral skeletons is an op-
timal scaffold for bone regeneration, but the mechanical
properties of the composite in the hydrate state are
much inferior to those of the natural bone. In addition,
coral plays an important role in the marine environment
and has now become difficult to obtain, because its use is
controlled by environmental regulations such as the
Washington Treaty.
Polyphosphazenes
Polyphosphazenes comprise a large class of macro-
molecules with alternating phosphorus and nitrogen
atoms in the backbone and a wide variety of organic,
inorganic, or organometallic side groups. Some linear
polyphosphazenes undergo rapid hydrolytic degrada-
tion, but exhibit poor mechanical properties.
7.2.2.3 Inorganic materials—calcium
phosphate
7.2.2.4 Composite materials
Biological apatite found naturally in bone comprises
a range of minerals and has osteoconductive and osteo-
philic properties. Inorganic minerals have been used for
Inorganic scaffolds are too brittle and not amenable to
trimming at operation table, when prefabricated as
