Biomedical Engineering Reference
In-Depth Information
size and the technique requires harsh solvents.
As previously mentioned, harsh solvents and
inadequate pore size could render the scaffold
ineffectual for tissue engineering purposes.
pore size is controlled in the horizontal plane.
For manipulation in the vertical plane, strands
are placed at varying angles to achieve specified
pore dimensions
[56]
.
A representative MDD method is the fused
deposition method (FDM). It begins with feed-
ing a strand of material into a liquefier. After
melting, the material is extruded as a series of
layers. The environment is controlled to main-
tain proper contact between layers. The subse-
quent scaffold has a honeycomb structure with
channel diameters in the hundreds of microm-
eters. FDM has also produced scaffolds with
polymeric and ceramic components for
mechanical strength and scaffolds that support
the growth and proliferation of various cell
types
[56]
.
A limitation of FDM is that the material being
fed into the liquefier has to be of a specific diam-
eter and possess certain material properties to
physically fit through the rollers and nozzle.
Most natural polymers cannot be used with the
FDM because of the high operating tempera-
tures required to melt them and produce strands.
The inability to incorporate natural polymers
limits the potential biomimicry of the scaffold.
Finally, the ability to achieve sufficient micropo-
rosity is inhibited by the deposition of dense
filaments.
To remedy these deficiencies, various modifi-
cations of the FDM have been employed. Newer
methods that eliminate the need for a precursor
filament produce scaffolds with sufficient poros-
ity and allow for the incorporation of biopoly-
mers and biomolecules through lower processing
temperatures
[56, 85]
. Moreover, methods have
been developed to significantly reduce the reso-
lution of the scaffold, producing filaments that
are in the tens of micrometers in diameter
[56]
.
Another development with deposition methods
is the ability to create hydrogel materials with
well-defined pore structures. These hydrogels
could provide the softer scaffolds needed for the
regeneration of soft tissues, along with other
scaffold properties
[57]
.
7.2.1.4 Rapid Prototyping/Solid Free-Form
Fabrication
Rapid prototyping
is another technique to cre-
ate porous 3D scaffolds. This method involves
using computer-aided design tools to pro-
duce a digital representation of a bioscaffold.
A depositor then layers polymeric or other
material types to exactly replicate the desired
shape. Layer-by-layer assembly, discussed
in Chapters 3 and 11, allows for the exact
control of morphological characteristics and
ultimately the mechanical properties of the
bioscaffold. The researcher easily controls
pore location and size as well as surface
characteristics of the scaffolds to ensure scaf-
fold success as a regeneration vehicle. More-
over, the predictability of the rapid prototyping
method allows for consistent reproduction of
various scaffold types with complex shape,
size, and other physical requirements
[50, 84]
.
There are several variants of the rapid proto-
typing method; however, all variants are catego-
rized by whether they use direct or indirect
fabrication of the scaffold and whether they
employ the melt-dissolution deposition (MDD)
technique or the particle-bonding technique
[56]
. The direct fabrication methods that use the
MDD technique are fused deposition method,
3D fiber-deposition technique, precision extru-
sion deposition, precise extrusion manufactur-
ing, low temperature deposition, multi-nozzle
deposition, pressure-assisted microsyringe, rob-
ocasting, 3D bioplotter, and rapid prototyping
robotic dispensing system
[56]
. In general, MDD
methods involve extruding a strand of material
through a small opening in a lateral direction
onto previous strands. The melted material
eventually solidifies and remains attached to the
preceding layer. By controlling the spacing
between adjacent strands of polymeric material,
