Evaluation
of 3D Printing and Its Potential Impact on Biotechnology and the Chemical
Sciences
PART 1
Nearing
30 years since its introduction, 3D printing technology is set to revolutionize
research and teaching laboratories. This feature encompasses the history of 3D
printing, reviews various printing methods, and presents current applications.
The authors offer an appraisal of the future direction and impact this
technology will have on laboratory settings as 3D printers become more
accessible.
Historical
Perspective
The conception of 3D printing, also referred to as additive
manufacturing (AM), rapid prototyping (RP), or solid-freeform technology (SFF),
was developed by Charles Hull. With a B.S. in engineering physics from the
University of Colorado, Hull started work on fabricating plastic devices from
photopolymers in the early 1980s at Ultra Violet Products in California. The
lengthy fabrication process (1–2 months) coupled with the high probability of
design imperfections, thereby, requiring several iterations to perfect,
provided Hull with the motivation to improve current methods in prototype
development. In 1986, Hull obtained the patent for stereolithography and
would go on to acquire countless more patents on the technology, including, but
not limited to, those cited in this article. In 1986, he established 3D Systems
and developed the .STL file format, which would “complete the electronic
‘handshake’ from computer aided design (CAD) software and transmit files for
the printing of 3D objects.” Hull and 3D Systems continued to develop the
first 3D printer termed the “Stereolithography Apparatus” as well as the first
commercial 3D printer available to the general public, the SLA-250. With Hull’s
work, in addition to the development and subsequent patenting of fused
deposition modeling (FDM) by Scott Crump at Stratasys in 1990, 3D printing
was poised to revolutionize manufacturing and research.
MIT professors Michael Cima and Emanuel Sachs patented the first
apparatus termed “3D printer” in 1993 to print plastic, metal, and ceramic
parts. Many other companies have developed 3D printers for commercial
applications, such as DTM Corporation and Z Corporation (which merged with 3D
Systems), and Solidscape and Objet Geometries (which merged with Stratasys).
Others include Helisys, Organovo, a company that prints objects from living
human tissue, and Ultimaker. Open source options such as RepRap, a desktop 3D
printer capable of replicating the majority of its own parts, have been
available since 2008.
3D printing technology has found industrial applications in the
automotive and aerospace industries for printing prototypes of car and airplane
parts, in the architectural world for printing structural models, and in the
consumer goods industry for prototype development for companies like Trek and
Black and Decker. The applications of 3D printing in private and
government defense have been rapidly recognized. For example, applications in
gun prototyping and manufacturing processes for the military have already been
established. Medical applications of 3D printing date back to the early 2000s,
with the production of dental implants and prosthetics. Applications in
the food industry, as well as in fashion, have also emerged.
With regard to research settings, 3D printing has been limited to
biomedical applications and engineering, although it shows tremendous potential
in the chemical sciences. This feature aims to present and compare the basic
printing methods available and discuss some of the current work in chemistry as
well as in other research and teaching efforts that utilize 3D printing
technology.
3D
Printing Methods
Overview
and .STL Format
3D printing is utilized for the rapid prototyping of 3D models
originally generated by a computer aided design (CAD) program, e.g., AutoDesk,
AutoCAD, SolidWorks, or Creo Parametric. The original design is drafted in a
CAD program, where it is then converted to an .STL (Standard Tessellation
Language or STereoLithography) file. The .STL file format, developed by Hull at
3D systems, has been accepted as the gold standard for data transfer between
the CAD software and a 3D printer. The .STL file stores the information for
each surface of the 3D model in the form of triangulated sections, where the
coordinates of the vertices are defined in a text file. By increasing the
number of triangles that define a surface, more data points exist in the text
file to spatially define the part surface. This increase in vertices results in
an increased resolution of the printed device.
The 3D printer interprets the digitally supplied coordinates
derived from the .STL file by converting the file into a G-file via slicer
software present in the 3D printer. The G-file divides the 3D .STL file into a
sequence of two-dimensional (2D) horizontal cross sections (25–100 μm,
depending on the fabrication technique), which allows the 3D object to be printed, starting at
the base, in consecutive layers of the desired material, essentially
constructing the model from a series of 2D layers derived from the original CAD
file. Development of better slicing algorithms to improve the finished
product characteristics is an active area in engineering research.
In the medical field, several other methods are utilized to
generate 3D object renderings, e.g., computerized tomography (CT), laser
scanning, and magnetic resonance imaging (MRI), which generate data that can
all be converted to the .STL format. When merging this digital scanning
technology with 3D printing, physicians are able to model the digital images
obtained through CTs or MRIs by utilizing CAD software to create an .STL file
and subsequently an exact replica of the original scan is printed.
There is an assortment of 3D printing techniques ranging from
well-established methods, which have been employed in industrial settings for
years, to more recent techniques under development in research laboratories
that are used for more specific applications. In the next section, we will
expand on five of the more pertinent systems: stereolithography (3D systems),
inkjet printing (Z Corporation), selective laser sintering (EOS GmbH), fused
deposition modeling (Stratasys), and laminate object manufacturing (Cubic
Technologies).
Stereolithography
(SLA)
Developed by Chuck Hull at 3D Systems,( SLA
was the first commercialized rapid prototyping method. There are several
different approaches to SLA, including direct/laser writing and mask-based writing digital light projection). The various
approaches can be broken up into a free surface or constrained surface
technique depending on the orientation of the laser source. The
direct/laser writing technique contains the common components of a movable
base, a tank of liquid resin, a UV light beam, and a computer interface. The
mask-based writing also contains the movable platform, resin vat, computer, and
UV beam as well as a “mask” in the form of a digital mirror device (DMD) that
allows for the curing of a single layer at once.
In the bath configuration, the UV beam traces a 2D cross section
onto a base submerged in a tank of liquid photoactive resin that polymerizes
upon illumination. The thickness of the cured resin is dependent upon such
factors as duration of exposure, scan speed, and intensity of the power source,
all of which depend on the energy of the UV light. After completion of the 2D
cross section, the base lowers further into the resin by a predefined distance,
and the UV beam begins the addition of the next layer, which is polymerized on
top of the previous layer. In between layers, a blade loaded with resin levels
the surface of the resin to ensure a uniform layer of liquid prior to another
round of UV light exposure. This process is repeated, slice by slice, until the
3D object is completed. The bath configuration is the oldest technique for SLA,
and drawbacks include the size of the vat restricting the height of the desired
object, resin waste, and extensive cleaning procedures, making the layer
configuration an attractive alternative. The layer configuration requires the
same components as the bath configuration. However, the movable platform is
suspended above the resin reservoir, in contrast to the bath configuration
where it is submerged. The light source is located beneath the vat, which has
an optically clear bottom. The change in setup requires lower volumes of resin,
and the height of the printed part is unrestricted. A thin layer of resin fills
a reservoir where it comes in contact with the movable platform. After the
initial layer cures, the platform raises, with uncured resin filling the gaps
left from the cured layer. If the resin has a high viscosity, the filling step
can become quite tedious. The previous steps are repeated until the device is
completed (with minimal cleaning stages in comparison to those of the bath
configuration).
In both configurations, a post fabrication step, using a UV light
to guarantee all reactive groups of the resin are polymerized, is required to
strengthen the bonding in the final 3D object. The direct laser writing method,
while able to generate detailed 3D objects, is time-consuming. The mask-based
approach uses the same fundamentals as the aforementioned direct/laser writing,
but in a high throughput application where a DMD employs millions of mirrors
that can be simultaneously controlled. The specific control of the mirrors
allows an entire layer to be cured at once, greatly reducing layer production
time. It is important to optimize the layer thickness to increase the
curing efficiency, and this information can also be utilized to guide resin
choice based on critical energies. The vertical resolution, which is dependent
on the cured layer thickness, has been reported at submicrometer to
single-digit micrometer resolution, and the lateral resolution depends directly
upon the diameter of the UV beam (80–250 μm).
The choice of UV light source varies depending on the resin, but
common sources include the HeCd laser (325 nm) and the xenon lamp. Two photon
polymerization has also been used in SLA fabrication to reach higher resolutions
of final printed objects. Resins are one of the main limitations of SLA, not
only due to the cost associated with the resin but also due to the fact that
only one resin can be used in the printing process at a time, thus limiting
overall device design. Resins are limited to either epoxy or acrylic bases, and
the majority of these materials are brittle and can shrink upon polymerization.
SLA 3D printers are traditionally expensive, but high resolution (25 μm
layers) and efficient (1.5 cm/h building speed) desktop models are becoming
more accessible to laboratories and even personal homes.)
Bethany
C. Gross, Jayda
L. Erkal, Sarah
Y. Lockwood, Chengpeng
Chen, and Dana
M. Spence*
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
