Evaluation of 3D Printing and Its Potential Impact on
Biotechnology and the Chemical Sciences
PART 2
The concept of inkjet printing was initially described in 1878 by
Lord Rayleigh, and in 1951, Siemens patented the first two-dimensional (2D)
inkjet type printer called a Rayleigh break-up inkjet device. Since its
advent, inkjet printing has served much use in commercial industry, mostly for
printing inks on paper. However, as far back as 2001, inkjet printing has also
been used for printing structures out of sol–gel, conductive polymers, ceramic,
metal, and nucleic acid or protein materials, as described in reviews.
The two main types of inkjet printing are continuous and
drop-on-demand (DOD). The continuous inkjet printing technique, which was
developed by Sweet at Stanford University in the mid 1960s, requires
electrostatic plates in the printer head to direct ink droplets onto paper for
printing or into a waste compartment to be recycled and reused. The
droplet size and spacing are controlled by application of a pressure wave
pattern. In the DOD technique, which was pioneered by Zoltan and Kyser and
Sears a voltage and pressure pulse directs the ink droplet, eliminating
the need for the separation of waste ink from the printer head. Since the
advent of the two main inkjet printing methods, there has been significant
development of new printing techniques in both categories.
3D inkjet printing is mainly a powder-based method where layers of
solid particles, typically 200 μm in height with particle sizes ranging between
50 and 100 μm, are bound together by a printed liquid material to generate
a 3D model. Specifically, a first layer of powder is distributed evenly on the
top of a support stage, e.g., by a roller, after which an inkjet printer head
prints droplets of liquid binding material onto the powder layer at desired
areas of solidification. After the first layer is completed, the stage drops
and a second powder layer is distributed and selectively combined with printed
binding material. These steps are repeated until a 3D model is generated, after
which the model is usually heat-treated to enhance the binding of the powders
at desired regions. Unbound powder serves as support material during the
process and is removed after fabrication.
As a powder-based 3D printing technique, inkjet printing does not
require photopolymerizable materials or liquids with modified viscosities.
Powders from polymer, ceramic, or glass materials can be combined with liquid
binding materials to generate a 3D model, which has significantly expanded
the technique’s application in areas like art design and industrial
modeling. Biological scaffold fabrication, which will be discussed in
more detail in later sections particularly with respect to drug delivery
studies, was impacted by the aforementioned technique. However, one caveat for
the method is that the printed liquid’s chemical and physical properties will
dominate those of the printed device. For example, many polymer glues are
biologically toxic and thus cannot be used for tissue scaffold
fabrication. Another limitation of inkjet printing is the optical
transparency of finished devices; incomplete interaction of binding liquid with
powder particles can cause a material to have reduced transparency due to light
scattering, which would be a severe limitation for microscopy studies. Unbound
particles can also result in significant porosity of finished materials and
surface roughness, which, depending on the application, can be an advantage or
limitation. However, nonpowder based (usually polymer-based) inkjet methods do
exist, e.g., Stratasys PolyJet technology, which can print 16 μm
photopolymer layers.
Selective
Laser Sintering (SLS)
Developed by Carl Deckard and Joseph Beaman in the Mechanical
Engineering Department at the University of Texas-Austin in the
mid-1980s, SLS is another powder-based 3D model fabrication method.
Although SLS is similar to inkjet printing, SLS uses a high power laser, e.g,
CO2 and Nd:YAG, to sinter polymer powders to generate a 3D
model, rather than using liquid binding materials to glue powder particles
together. In the SLS process, a first layer of powder is distributed evenly
onto a stage by a roller and is then heated to a temperature just below the powder’s
melting point. Following the cross-sectional profiles designated in the .STL
file, a laser beam is selectively scanned over the powder to raise the local
temperature to the powder’s melting point to fuse powder particles together.
After the first layer is completed, a second layer of powder is added, leveled,
and sintered in the desired areas. These steps are repeated to create a 3D
model. The powders that are not sintered by the laser serve as support material
during the process and are removed after fabrication. Figure 3B
provides a schematic of the process of SLS. A more detailed discussion of SLS
basics such as laser types and binding mechanisms can be found in the
literature.
One advantage of SLS is that a wide range of materials can be
used, from polymers such as polycarbonate (PC), polyvinyl chloride (PVC),
acrylonitrile butadiene styrene (ABS), nylon, resin, and polyester to
metal and ceramic powders. Also, a binding liquid material is not required
in SLS. However, SLS printed models suffer shrinkage or deformation due to
thermal heating from the laser and subsequent cooling, which has been under
investigation in the literature for some time. Achievable resolutions in
SLS techniques are dependent on multiple parameters including laser power and
focusing as well as the size of the powder material. Work is ongoing to push
fabricated feature sizes using SLS below that of inkjet printing techniques,
roughly below 50 μm.
Fused
Deposition Modeling (FDM)
Developed by Scott Crump of Stratasys, FDM is one of the most
widely used manufacturing technologies for rapid prototyping today. FDM
fabricates a 3D model by extruding thermoplastic materials and depositing the
semimolten materials onto a stage layer by layerThermoplastic filaments, the
material used to build 3D models, are moved by two rollers down to the nozzle
tip of the extruder of a print head, where they are heated by temperature
control units to a semimolten state. As the print head traces the design of
each defined cross-sectional layer horizontally, the semimolten materials are
extruded out of the nozzle and solidified in the desired areas. The stage then
lowers and another layer is deposited in the same way. These steps are repeated
to fabricate a 3D structure in a layer-by-layer manner. The outline of the part
is usually printed first, with the internal structures (2D plane) printed layer
by layer. Surface defects from this particular process include staircase and
chordal effects resulting from the nature of the slicing software and .STL file
format. Internal defects can result from heterogeneities in the filament feed
diameter and density, as these can effect how the material is extruded from the
printer nozzle.
A notable advantage of FDM is that it can create objects
fabricated from multiple material types by printing and subsequently changing
the print material, which enables more user control over device fabrication for
experimental use. Besides conventional materials such as PC, polystyrene (PS),
and ABS, FDM can also print 3D models out of glass reinforced
polymers, metal, ceramics, and bioresorbable
materials. However, a binder is typically mixed with ceramic or metal powders,
enabling the material to be used in a filament form.
Laminated
Object Manufacturing (LOM)
LOM, developed by Helisys, generates a 3D model by stacking
layers of defined sheet materials such as paper, plastic, and metal. As
shown in the schematics in Figure 5, after
the first layer of a sheet material is loaded onto a stage, a laser (CO2 lasers
have been used) or razor traces the
designed cross-section to define the pattern on the layer. After the
excess material of the sheet is removed, a second layer covers the previous
layer and the laser or knife tracing will define the next pattern based on
information in the .STL file. Adjacent layers are combined by use of adhesives
or welding, e.g., for paper or metal, respectively. These steps are
repeated to generate a layered 3D model.
The LOM process does require a heating step during production,
either on the support stage or on the roller, to ensure that the adhesive acts
to bond the sheets together. The effects of this step on the nonuniformities in
the fabricated part are, in general, minimal compared to defects that arise in
other techniques, such as FDM. However, if the local temperature of either
the roller or the stage is not controlled well enough, the part could become
delaminated due to inefficient adhesive heating or the part can suffer
structural damage if the temperature is high enough to damage the adhesive.(66) Another limitation of LOM is that the materials applicable
for method use are limited by their ability to be formed into sheets and to be
integrated with adhesive.
Each additive manufacturing technique described above has its own
limitations and advantages in producing prototype models. For a better
understanding, numerous comparisons of these rapid prototyping techniques have
been reported in detail.
Department of Chemistry, Michigan State University, 578 South
Shaw Lane, East Lansing, Michigan 48824, United States
Anal. Chem., 2014, 86 (7), pp
3240–3253
CONTINUES
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