Biomedical Engineering Reference
In-Depth Information
a fi ber mesh or a three-dimensional patterned
structure with variable pore size by weaving or
knitting. These mesh constructs have a large
surface area with high porosity, which induces
greater cell attachment and better nutrient dif-
fusion and waste removal [
polymer membrane [
]. One consider-
able advantage of this technique is that biomol-
ecules can be incorporated into the scaffold
without exposure to harsh chemical or thermal
conditions. In addition, changes in the polymer
composition, polymer concentration, and
solvent-to-nonsolvent ratio can be utilized to
augment the scaffold structure. However, the
effect of these modifi cations may be diffi cult to
predict. Phase separation has been used to
create scaffolds of poly(L)lactic acid (PLLA),
PLGA, and PLA [
59
,
70
,
97
]. However, the
increased porosity of these scaffolds causes
them to be mechanically unstable. To alleviate
this diffi culty, a fi ber-bonding technique has
been evolved to dissolve poly(lactic acid) (PLA)
in a solvent and to cast it over a PGA mesh that
is aligned in the desired shape [
97
]. Heating
the construct above the melting temperature of
PGA evaporates the solvent. The PGA mesh
becomes connected at fi ber cross-points when
the construct is cooled and PLA is redissolved.
Fiber bonding has also been used to fabricate
scaffolds from poly(
64
,
97
40
,
69
,
70
,
101
].
6.2.3.4 Melt Molding
Melt molding combines a polymer powder and
microspheres to form a scaffold [
]. This tech-
nique has been used with a fi ne PLGA powder
and gelatin microspheres heated in a Tefl on
mold [
97
].
Although this technique allows for greater
structural stability, it has some disadvantages.
The porosity varies and cannot be fi nely
controlled. In addition, the solvent used to
dissolve the polymer may be harmful to an
incorporated cell population and the sur-
rounding tissue.
ε
-caprolactone) (PCL) [
31
]. Heating the polymer above the
glass transition temperature allows the polymer
powder to melt. The molded polymer is then
removed and placed in water, where the
entrapped microspheres are removed; this
results in a three-dimensional porous struc-
ture. There are several advantages to this tech-
nique. Pore size is directly related to the
microsphere diameter, and changing the
polymer-to-gelatin ratio modifi es the porosity.
Furthermore, biomolecules can be incorpo-
rated into the scaffold, since this process occurs
under moderate conditions without the use of
organic solvents. Also, by changing the shape
of the mold, a defi ned construct shape can be
developed. However, this technique may involve
very high temperatures if semicrystalline poly-
mers are heated beyond their glass transition
temperature [
90
,
97
6.2.3.2 Solvent-Casting Particulate Leaching
Solvent-casting particulate leaching is a tech-
nique by which dispersed particles such as
sodium chloride, tartrate, citrate, or saccha-
rose are mixed in solution with a polymer and
cast in a mold [
]. Casting or freeze drying
is performed to evaporate the solvent. The
dispersed particles are leached out of the scaf-
fold, leaving void spaces that form a porous
and highly interconnected structure. This
process allows the independent control of
porosity and pore size [
67
,
97
97
].
]. This technique has
been used to form constructs with PLA,
poly(D,L-lactic acid-co-glycolic acid) (PLGA),
and poly(propylene fumarate) (PPF) [
97
6.2.3.5 Freeze Drying
Freeze drying uses temperature change to
create a porous structure [
12
,
67
,
73
,
]. In this tech-
nique, synthetic polymers, such as PLGA, are
dissolved in cold solvents, glacial acetic acid, or
benzene, and water is added to create an emul-
sion [
39
87
,
95
].
6.2.3.3 Phase Separation
Phase separation is used to isolate components
of a heterogeneous mixture. In this process the
polymer is fi rst dissolved in a solvent such as
molten phenol, naphthalene, or dioxane [
]. The emulsion is quick-frozen
and the resulting ice crystals are sublimed by
the freeze-drying technique. This leaves a
highly connected porous matrix. With this
technique, pore size can be controlled by alter-
ing the freezing rate; in general, faster freezing
leads to smaller pores [
39
,
78
,
94
40
,
97
]. The polymer solvent solution is cooled,
causing liquid-liquid or solid-liquid phase
separation, with the polymer in a separate
phase from the solvent. The solvent is then
evaporated, leading to the formation of a porous
]. However, it is diffi -
cult to control pore structure with freeze dry-
ing alone. Better results can be achieved by
78
