Tissue Engineering: Focus on the Musculoskeletal System - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 210

microfabrication depends on the quality of the projecting lens; normally, the
minimum feature that PPS can fabricate is thinner than 10 micrometers, or
at a cellular scale.
The photomasks for PPS can be fabricated like the ones for electronics pho-
tolithography [85]; however, it is more convenient and cost-efficient to use
a dynamic photomask instead. The DLP chipsets (from Texas Instruments)
for digital micromirror display are widely used digital photomasks [86, 87].
A DLP chip has on its surface more than a million digitally controllable
micromirrors; each micromirror can be electrostatically tilted to an “on”
or an “off” state. Upon an illumination, the micromirrors at the “on” state
reflect the incident light toward the projecting lens and become the bright
pixels at the projected curing image. Using this dynamic mask, therefore, the
photomask pattern for each cross-sectional slice of a microstructure becomes
programmable, making scaffold microfabrication faster and easier.
Syringe-Pump Microfabrication
In syringe-pump microfabrication (SPM), monomers for microstructure are
extruded at a stable rate from a microsyringe, which is actuated by a syringe
pump. The monomer squeezed from the needle is immediately cured to form
a continuous, micron-scaled line to construct microstructures; the method
to cure the monomer depends on which type of crosslinkable material the
monomer is. To cure alginic acid, for example, the needle tip is submersed in
a calcium chloride solution; the extruded alginic acid is thus crosslinked by
calcium ions through electrostatic binding [88]. As well, to cure a photocross-
linkable monomer, the tip of the needle can be continuously exposed to a
curing wavelength [89]. The minimum feature (diameter of the line) created
by this method is about 1 micron. Similar to SLS microfabrication, SPM is a
3-D scanning method and includes a three-dimensional motorized system
to continuously change the relative position between the syringe tip and the
fabricated microstructure.
Selecting a Microfabrication Platform
In making microstructures for tissue engineering, it is important to select
a fabrication platform to comply with the material properties of monomers,
the solidifying mechanism, and the geometry of fabricated scaffolds. SLS
and PPS platforms, for instance, are limited for photopolymerizable mono-
mers. On the other hand, microsyringe stereolithography has the advantage
that it is applicable to every type of the aforementioned crosslinking mecha-
nism, including ionic and catalyzed crosslinking. Manufacturing resolution
is another important issue to consider. For example, SLS with femtosecond
laser offers sub-micron resolution and is suitable for patterning subcellu-
lar-scaled microstructures, while the other platforms are more suitable for
making cellular-scaled microstructures. The time for creating scaffolds is