Biomedical Engineering Reference
In-Depth Information
currently mainly used to make calcium phosphate-based scaffolds for bone engineering. The main
drawback of SLS is that incorporation of sensitive biomolecules is diffi cult because of the need to
locally heat the powder layer so as to sinter it. Antonov et al. produced 3-D bioactive, biodegrad-
able scaffolds utilizing SLS using a different approach to conventional techniques. Instead of using
infrared radiation (
λ
=
10.6 µm), which leads to polymer particles melting and fusing together, they
used near-infrared (
λ
0.97 µm) laser radiation, which is not absorbed by particles. This meant the
melting process was limited to just the polymer surface, which could render the polymer particles
capable of encapsulating delicate bioactive species thus circumventing this important problem asso-
ciated with traditional SLS.
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=
2.3.1.3
Solid Ground Curing
Besides the classical laser-based SL process, alternative processes using digital mask generators
(e.g., liquid crystal displays or digital mirror devices [DMDs]) have been used successfully to build
structures out of polymers and ceramics. In the RP literature this process is also termed solid
ground curing (SGC) or digital light processing (DLP). In contrast to traditional UV-laser-based
SL machines, DLP systems are signifi cantly cheaper and therefore more versatile in respect to
material modifi cations. At the same time DLP machines can expose a whole layer at once, whereas
laser-based systems have to scan the contour of the object sequentially. DLP systems are based on
a digital micromirror device. By projecting a bitmap onto the photosensitive resin, the liquid resin
can be solidifi ed selectively. Theoretically, DLP systems can be used to fabricate scaffolds with
high resolution and geometric complexity. However, a prerequisite, and consequent limitation, is
the availability of a light-curable biocompatible and bioresorbable polymer material. The wider
application of SGC in designing scaffolds is mainly driven by developments of photochemically
driven gelation technology of biomacromolecules that are chemically modifi ed with photodimeriz-
able groups.
2.3.2 T
HREE
-D
IMENSIONAL
P
RINTING
The three-dimensional printing (3-DP) technology was developed at the MIT.
25
Three-dimensional
printing is used to create a solid object by ink-jet printing a binder into selected areas of sequen-
tially deposited layers of powder. Each layer is created by spreading a thin layer of powder over the
surface of a powder bed. The powder bed is supported by a piston, which descends upon powder
spreading and printing of each layer (or, conversely, the ink-jets and spreader are raised after print-
ing of each layer, and the bed remains stationary). Instructions for each layer are derived directly
from a computer-aided design (CAD) representation of the component. The area to be printed is
obtained by computing the area of intersection between the desired plane and the CAD representa-
tion of the object. The individual sliced segments or layers are joined to form the 3-D structure.
The unbound powder supports temporarily unconnected portions of the component as the scaffold
is built but is removed after completion of the printing.
The solvent drying rate is an important variable in the production of scaffolds by 3-DP. Very
rapid drying of the solvent tends to cause warping of the printed plotting of dots in 3-D with or with-
out incorporation of cells. Much, if not all, of the warping can be eliminated by choosing a solvent
with a low vapor pressure. It has been found that it is often an advantage to combine solvents to
achieve minimal warping and adequate bonding between the biomaterial particles. Thus, an aggres-
sive solvent can be mixed in small proportions with a solvent that has lower vapor pressure. After
the binder has dried in the powder bed, the fi nished component can be retrieved, and the unbound
powder is removed for postprocessing, if necessary.
The 3-DP process is capable of overcoming the limitations of some SFF techniques, such as
those associated with manufacturing certain designs, for example, the overhanging structures. The
solution lies in the layering of powders. As the layers are spread, there is always a supporting plat-
form of powder for printing and binding to take place. Thus, as long as the parts are connected
