Biomedical Engineering Reference
In-Depth Information
2.3
SOLID FREE-FORM FABRICATION
2.3.1 I
NTRODUCTION
The tissue engineering fraternities are a long way off matching Nature in her ability to grow func-
tioning tissues and organs. It is clear that much knowledge needs to be gained at all levels of tissue
engineering, but from an engineering point of view the past decade has seen great advances in
scaffold design. Starting with simple foams and fi bers, it was quickly realized that connectivity
between the pores, pore dimensions, and properties of the scaffold materials are of vital importance
in facilitating cell seeding, migration, proliferation, and the production of extracellular matrix. Sig-
nifi cant advances have also been made in incorporating bioactive molecules into scaffolds. As more
is learned about the behavior of cells on micro and nanopatterned topographies, and plasma and
chemically treated surfaces, future developments in scaffold fabrication technology are likely to be
better tailored to produce designs and techniques that are targeted toward specifi c cell and tissue/
organ types, and even individual patients. Hybrid scaffold fabrication techniques have already been
shown to be effective in pushing the limits of previously known techniques to meet the demands of
more complex biological structures. One of the most pressing issues in current scaffold fabrication
designs is the absence of vascularization; a strategy for addressing this issue is now of the utmost
importance. Greater control of scaffold parameters is probably going to be just as important as
vascularization in future scaffold designs. SFF has gained a lot of attention as it offers the ability to
overcome some of the control problems that are already mentioned above.
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SFF and rapid prototyping (RP) are used to fabricate complex-shaped scaffolds by selectively
adding materials, in a layer-by-layer, computer-generated process. Several SFF techniques are
depicted in Figure 2.5.
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Lewis
22
coined the term “direct ink writing” and the term describes fabrication methods that
employ a computer-controlled translation stage, which moves a pattern-generating device, that is, an
ink-deposition nozzle, to create materials with controlled architecture and composition. Lewis divides
them into fi lamentary-based approaches, such as robocasting (or robotic deposition) micropen writing
and fused deposition, and droplet-based approaches, such as ink-jet printing and hot-melt printing. Ink
designs that have been employed include highly shear thinning colloidal suspensions, colloidal gels,
polymer melts, dilute colloidal fl uids, waxes, and concentrated polyelectrolyte complexes. These inks
solidify either through liquid evaporation, gelation, or a temperature- or solvent-induced phase change.
However, reviewing the current biomaterials and tissue engineering literature, it is concluded
that the terms SFF and RP are mainly used to defi ne the technologies described below namely,
sterolithography, selective laser sintering, solid ground curing, three dimensional printing, extru-
sion and direct writing. Today, SFF provides a powerful instrument in the tissue engineer's toolbox
for the generation of scaffold technology platforms.
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One of the major benefi ts offered by SFF
technology is the fl exibility to create parts with highly reproducible architecture and compositional
variation across the entire matrix due to the computer-controlled fabrication process. The applica-
tion of SFF technologies in scaffold fabrication is wide and varied. Some of the more acknowledged
techniques are described in the following sections. Figure 2.6 summarizes these technologies with
regard to the dimensions of scaffolds, which can be fabricated using each respective technique.
2.3.1.1 Stereolithography
Stereolithography (SL) is one of the most commonly used rapid manufacturing and RP technolo-
gies. It is considered to provide high accuracy and good surface fi nish. SL is an additive fabrication
process utilizing a vat of ultraviolet (UV)-sensitive photopolymer and a laser to build parts of a
layer at a time. Each part is traced by the laser beam on the surface of the UV-sensitive photopoly-
mer solidifying it. SL is based on the use of a focused UV laser, which is vector-scanned over the
top of a liquid bath of a photopolymerizable material. The UV laser causes the bath to polymerize
