TECH SPECIAL Evaluation of 3D Printing and Its Potential Impact on Biotechnology and the Chemical Sciences PART 1

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

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