Tissue Engineering: Focus on the Musculoskeletal System - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 209

becomes a volumic pixel (or voxel ) [80]. In fabricating a scaffolding micro-
structure, a SLS platform scans the monomer with the focused laser beam,
moving the focal point along a three-dimensional route and creating con-
tinuous voxels for the microstructure. The minimum feature that a CW-SLS
platform can develop equals the size of a voxel, which is about 100-500
microns. The scanning of the focal point, relative to the monomer bath, can
be created by either moving the monomer bath by a three-dimensional stage
or moving the focal point by motorized optics.
SLS by Femtosecond Pulsed (fs) Laser
The minimum features created by using SLS with fs-laser are drastically
finer than with CW laser; the typical size of voxels created by fs-SLS is 200
nm (0.0002 mm) [81]. The mechanism behind the performance of fs-laser
is named by “two-photon absorption,” a non-linear effect that takes place
under sufficiently strong irradiation, as in the case of using fs-laser [82]. In
two-photon absorption, a light-sensitive molecule, such as photoinitiators,
becomes sensitive to photons half the energy level at which the absorption
of a single photon takes place. For example, given that a molecule absorbs a
wavelength only around 380 nm in single-photon absorption, in two-photon
absorption it absorbs 760 nm light. Red to near-infrared (640-800 nm)
femtosecond lasers are frequently used to create voxels in monomers that
polymerize in ultraviolet (320-400 nm) light. Because the probability that
two-photon absorption takes place in a molecule is in proportion to the
square of light intensity [83], under focused fs irradiation the monomers
polymerize in a very narrow region, in the order of 100 nm, around the
focal point.
Projection-Printing Stereolithography
Projection-printing stereolithography (PPS) develops a microstructure by
forming a sequence of cross-sectional slices in a photocurable monomer [84].
A typical PPS platform includes a sequence of photomasks to pattern the
cross-sections of microstructure, a light source to illuminate the photomask
patterns, and an optical lens to project the illuminated patterns onto the pho-
tocurable monomer and cure the monomer according to the patterns; the sur-
face of monomer turns into a thin cross-sectional slice upon the projection.
After the formation of each cross-sectional slice, the microstructure is repo-
sitioned (downward) by the stage to prepare for creating another slice; this
procedure is repeated until every cross-sectional slice for a microstructure
is built. PPS differs from SLS in that the former generates a one-dimensional
scanning (to stack the cross-sectional slices) to fabricate a three-dimensional
structure, while the latter generates a three-dimensional scanning (by the
3-D motion of a motorized optics) to create continuous, small voxels in three-
dimensional space. This gives PPS a much higher fabrication speed, espe-
cially for making larger tissue-engineering scaffolds. The resolution of PPS