Biomedical Engineering Reference
In-Depth Information
combining with other techniques, such as the
above-described particulate-leaching method
[
three-dimensional cross-section of the scaffold
is built out of a roll of sheets that have been
lined with an adhesive [
17
].
]. The layers are cut
by a carbon dioxide laser and bonded by heat
and/or pressure. The technique does not allow
formation of small inner holes within the scaf-
fold, a disadvantage with respect to nutrient
and waste transport [
98
6.2.3.6 Gas Foaming
In gas foaming, scaffold pores are formed by
gases that are under pressure or undergoing a
chemical reaction [
98
].
]. It is the bubbles in the
polymer that cause pore formation in the con-
struct. Variations in gas volume and in the rates
of gas nucleation and diffusion modify the
porosity and pore structure of the scaffold. By
this method the scaffold is formed in a moder-
ate environment without the use of organic sol-
vents. The results of gas foaming, like those of
freeze drying, can be improved by combining it
with particulate leaching [
71
6.2.4.2 Three-Dimensional Printing
Three-dimensional printing forms sequential
powder layers of the scaffold by ink-jet printing
a binder [
]. In this technique, a computer
model is used to create a slicing algorithm that
defi nes the structure of each scaffold layer.
Powder is thinly layered over a bed, and binder
material is printed on top where the scaffold is
to be formed. A piston is lowered to allow the
next layer of powder to be spread and bonded.
In this technique, the packing density of the
powder particles can be used to control the
adhesive bonding of the material and thus
the resulting mechanical strength [
42
,
57
]. Gas foaming has
been used with PLLA, PLGA, and PLA to create
scaffolds for bone tissue [
82
61
,
71
,
82
].
6.2.4 Rapid Prototyping
(Solid Free-Form Fabrication)
]. This
technique has been used to create scaffolds
from polyethylene oxides (PEOs), PLA, PCL,
and PLGA [
42
All of the techniques described above are
limited in how they regulate scaffold parame-
ters such as pore size, pore shape, pore inter-
connectivity, and pore wall thickness. This lack
of fi ne control has led to the development of
new techniques to produce scaffolds directly
from a computer-aided design model. Rapid
prototyping, also known as solid free-form fab-
rication, has been used to guide surgical proce-
dures based on computerized topography of
the patient in question [
30
,
42
,
47
,
57
,
96
].
6.2.4.3 Laser Stereolithography
Laser stereolithography is another computer-
aided design method that allows for three-
dimensional scaffold formation. This method
is similar to the three-dimensional printing
described above, but utilizes a liquid polymer
to fabricate a scaffold [
]. These techniques
generally produce three-dimensional scaffolds
in a layer-by-layer fashion and can be designed
to form very specifi c shapes. These techniques
can be carried out at room temperature, thereby
allowing for cell encapsulation and biomole-
cule incorporation without signifi cantly affect-
ing viability. However, these methods involve
processes that alter some polymers limiting
their use in fabrication. Rapid prototyping
techniques include sheet lamination, three-
dimensional printing, laser stereolithography,
and fused deposition modeling [
98
]. The computer model
creates two-dimensional slices of the scaffold
model which modulate a platform submerged
in a liquid photopolymer. The liquid is then
exposed to a focused laser light, which cures
the polymer, forming a solid at specifi c points.
A signifi cant advantage of this technique is the
ability to produce complex internal architec-
ture. Furthermore, liquid solutions containing
different biomolecules can be used for each
scaffold layer [
57
]. When this technique was
used with cross-linking of diethyl fumarate
(DEF) and PPF, it resulted in pore sizes that
ranged from
57
20
,
42
,
57
,
98
].
150
to
800
ยต
m, with porosity as
6.2.4.1 Sheet Lamination
Sheet lamination is a technique that creates
scaffolds with a layer-by-layer approach. A
high as
]. Laser stereolithography has
also been used to create scaffolds with PEG
diacrylate [
90
% [
20
60
].
