Biomedical Engineering Reference
In-Depth Information
PPF scaffolds with average pore sizes and pore shapes, that is, 600- and 900-µm pores, were
similar to those of CAD models, but they depended on directions in those with 300-µm pores.
Porosity and permeability of PPF scaffolds decreased as the number of closed pores in original
models increased, particularly when the pore size was 300 µm, as the result of low porosity and
pore occlusion. The authors conclude that their results show that 3-D printing in combination with
injection molding can be applied to crosslinkable polymers to fabricate 3-D porous scaffolds with
controlled pore structures, porosity, and permeability using their CAD models.
In the material science literature, another term for extrusion-based systems is used, namely,
direct-write techniques, which rely on the formulation of colloidal inks for a given deposition
scheme.
15
The techniques employed in direct writing are pertinent to many other fi elds besides
scaffold fabrication such as the capability of controlling small volumes of liquid accurately. Direct-
write techniques involving colloidal ink can be divided into two approaches: (1) droplet-based
approach including direct ink-jet printing and hot-melt printing and (2) continuous (or fi lamentary)
techniques.
The key to the versatile fabrication method of the SFF is its ability to literally build the model
from its respective basic raw materials. However, the major limitations also lie in its methodology
of building and bonding of raw materials together. For the SL technique the raw materials must be
in a liquid form and must be photopolymerizable. The raw materials for the SLS must be able to
melt and be severed cleanly, respectively; particles and layers must also be able to bond together
based on the energy supplied. The 3-DP powder and binder combination must be compatible and
must adhere the bulk material effectively. Finally, the FDM can only use a thermoplastic material.
With these limitations on the building materials, SFF further restricts the list and availability of
biomaterials that can be used for forming scaffolds or devices using this technology. Some research-
ers have begun to explore other options to exploit the macroscopic geometry and internal intrinsic
architecture attainable by the SFF, along with its convenience and accuracy of duplication from
medical imaging sources. An emerging method is to fabricate a negative mold based on the scaffold
design and cast the scaffold using the desired materials, which may not be usable in an SFF setting.
Indirect SFF routes rely on an additional molding step after fabricating the master pattern by RP.
In conclusion, indirect SFF adds further versatility and detail in scaffold design and fabrication.
The previous restriction on casting was the inability of molds to produce complex geometry and
internal architecture. But now with indirect SFF, traditional casting processes with these SFF molds
can meet the specifi c tissue engineering requirements, including mechanical integrity and custom-
ized shapes. A highlighted advantage of indirect SFF is cost savings as the materials required for
mold casting is substantially less and need not be processed into a dedicated form for any particular
SFF process, such as processing into a powder for SLS and 3-DP. In addition, indirect SFF allows
the usage of a wide range of materials or a combination of materials (composites or copolymers).
However, some drawbacks still exist, including the resolution of the SFF method, as the cast model
would inherit the errors and defects from the mold, such as cracks and dimensional changes. Also,
a mold removal method must be developed to remove the mold while preserving the as-cast scaffold
in an intact manner without disturbing its desired properties.
2.4 FUTUREDIRECTIONS
2.4.1 I
NTRODUCTION
The main challenge in preparing a useful TEC is to obtain a homogenous distribution of cells, and
hence new tissue, throughout the entire 3-D scaffold volume. Furthermore, there exist two possi-
bilities of incorporating cells into the scaffolds: (a) seeding of cells onto the surface of the scaffold
subsequent to scaffold fabrication and (b) the incorporation of cells into the scaffold during the
fabrication process. This second approach is of interest especially when incorporating cells into
the scaffold material. Hence, the dream of tissue engineers is to build a structural and functional
