Biomedical Engineering Reference
In-Depth Information
6.2.4.4 Fused Deposition Modeling
In fused deposition modeling, the polymer is
deposited in thin layers on a base that solidifi es
as it attaches to the previous layer [
tinct from the two homopolymers [
]. PGA
and PLA are semicrystalline, whereas PLGA is
an amorphous solid. PGA degrades slowly,
whereas PLGA can degrade rapidly [
94
1
,
89
,
90
,
]. Initially
the technique was used only with nonresorb-
able materials, but it has recently been applied
to PCL and PCL/hyaluronic acid scaffolds [
42
97
]. The co-monomer ratio can be varied to
produce different mechanical, physical, and
degradation properties [
82
]. Degradation times
16
,
vary from
6
to
12
months when the monomer
42
]. As with the other computer tech-
niques, this process is highly reproducible.
Fused deposition modeling also supports
incorporation of pores into the scaffold to
modulate mechanical strength and molecular
diffusion.
,
77
,
100
ratio is
85
:
15
but only
1
to
2
months when the
ratio is
. The polymer can therefore be
readily engineered to an appropriate degrada-
tion rate [
50
:
50
]. Owing to its ester linkages, which
affect mechanical properties as PLGA degrades,
the polymer can also undergo bulk degrada-
tion (see Section
63
]. The degrada-
tion products include glycolic and lactic acids,
both of which can be removed via the body's
metabolic pathways [
6
.
3
.
4
below) [
63
6.2.5 Synthetic Polymers
for Scaffolds
].
The possibility of modulating PLGA proper-
ties signifi cantly, as well as the fact that it can
support a variety of cell types, has led to great
interest in this polymer for tissue engineering.
Osteoblasts attach to PLGA [
78
,
82
,
94
The molecular structure and properties of syn-
thetic polymers allow specifi c cell and tissue
processes to become part of engineered bone.
This is an advantage over natural polymers,
whose variable molecular structure makes for
less precise modifi cation. Synthetic polymers
are most often present in a semicrystalline or
an amorphous state. A semicrystalline polymer
contains dense chain regions randomly dis-
tributed throughout the material. These regions
act as physical cross-links and contribute to the
mechanical strength of the polymer network.
Amorphous polymers are similar to glass when
they are below their glass transition tempera-
ture and act like rubber when heated above that
temperature. The structure of amorphous poly-
mers can be altered by chemical bonding,
copolymerization, physical mixing, or blend-
ing [
], and extra-
cellular matrix (ECM) components, such as
osteopontin and osteonectin, along with colla-
gen, fi bronectin, vitronectin, and laminin, are
a b u n d a n t l y a s s o c i a t e d w i t h P L G A s c a f f o l d s [
26
,
44
].
These molecules are important for the extra-
cellular environment that osteoblasts require.
26
6.2.5.1.2 Poly(
ε
-Caprolactone)
Poly(
-caprolactone) (PCL) is an aliphatic poly-
ester with a repeating molecular structure of
fi ve nonpolar methylene groups and a single
polar ester group [
ε
]. A semicrystalline
polymer, PCL has a melting point of approxi-
mately
97
]. In their unmodifi ed form, synthetic
polymers lack biomolecules that can aid cell
attachment in some natural polymers. However,
synthetic polymer surfaces can be made to
include biomolecules that stimulate cell attach-
ment and proliferation [
63
°C and is formed by the ring-opening
polymerization of
60
]. PCL is
known to be highly water soluble and is hydro-
lyzed under physiologic conditions [
ε
-capolactone [
97
]. Degra-
dation to caproic acid occurs by either a bulk
or a surface mechanism. Caproic acid alters the
scaffold degradation rate, therefore the by-
product concentration should be kept low [
34
]. Common synthetic
polymers include polyesters, polyanhydrides,
polyphosphazenes,
92
polycarbonates.
and
].
PCL is known to degrade very slowly, with a
degradation time of approximately
97
poly(ethylene glycol).
].
To shorten the degradation rate for certain
applications, PCL has been copolymerized with
collagen, PGA, PLA and PEG [
2
years [
97
6.2.5.1 Polyesters
6.2.5.1.1 Poly(D,L-Lactic Acid-Co-Glycolic Acid)
Poly(D,L-lactic acid-co-glycolic acid) (PLGA),
is a copolymer of poly(lactic acid) (PLA) and
poly(glycolic acid) (PGA), with properties dis-
]. In addi-
tion, PCL may support load-bearing applica-
tions and can maintain mechanical strength
for an extended period of time [
9
,
22
,
74
1
].
