3D Printing: Present and Future
Building on current technology for future innovation
Josh Throndson, CDT
Since Charles Hull first patented stereolithography back in 1986, much has changed, but the principles remain the same. CAD technology creates a 3-dimensional geometry that is producible through combining layers, one on top of the next, to construct a 3D form. 3D printing has gained widespread acceptance in dentistry as an efficient, dynamic technology with uses in a multitude of dental applications. Subtractive manufacturing has long been a standby in the dental laboratory, but 3D printing's additive process offers some advantages: it is more energy efficient, creates less waste, involves less production time, and is able to produce and reproduce complex geometries that the milling process simply cannot handle. As a result, 3D printing has become an expansive, multi-use tool in the field of dental technology.
An Overview of Dental 3D Printers
The most common types of dental 3D printers utilize the following technologies: fused deposition modeling (FDM), stereolithography (SLA), digital light processing/projecting (DLP), selective laser sintering (SLS), and selective laser melting (SLM).
SLA is a process by which a particular wavelength of light is selectively applied to a vat of liquid containing photopolymerizable resin. The light source polymerizes layer by layer to create the 3D object. The SLA 3D printer is widely regarded as the first low-cost and commoditized 3D printer in dentistry.
DLP is similar to SLA in that a light source of a particular wavelength is projected into a vat of resin to create the 3D geometry layer by layer. This process originally utilized an LCD screen to flash images layer by layer into a vat, which dramatically sped up the creation of 3D objects. In contrast, SLA only focused a single concentrated light source into the vat. DLP allows an entire layer to be polymerized in an instant instead of a single light beam selectively maneuvering through the layer to complete polymerization. DLP technology was further innovated in 2001 when digital micromirror devices (DMDs) replaced LCD screens and allowed a higher-intensity light source to be projected onto the micromirrors, resulting in greater contrast and resolution. The result was faster polymerization in conjunction with improved accuracy.
Post-Processing with SLA and DLP
With the resin-based methods of fabrication used in SLA and DLP, a considerable amount of time, energy, equipment, and cleaning agent are required to finalize the end product. Once the object is done printing, it must be rigorously washed, generally in a vat of isopropyl alcohol of 94% or higher—sometimes requiring two baths to clean all of the uncured resin off of the print. It is then necessary to place the object into a light-curing unit that provides a specific wavelength of light at a specific temperature (depending on the resin) to finish curing the product. This process is often messy and time consuming. Expect future printers to come market with the manufacturing capabilities to eliminate these post-processing steps.
FDM technology maintains the principle of creating a 3D object layer by layer, but the method is much different than DLP or SLA. Thermoplastic polymers are fed as filaments through a heated nozzle that moves across a build plate to erect the 3D printed form, much like a hot glue gun. Once the material is extruded, it cools and solidifies, the build plate lowers, and the next layer is extruded. This relatively simple technology makes it easy to use, and this type of printer has a low cost of acquisition, making it advantageous for those just getting started with 3D printing. It is possible to employ multiple materials in a single print by utilizing several nozzles or by changing filaments within a single nozzle. Furthermore, the end product does not require nearly as much post-processing as it would with SLA or DLP. The post-processing consists of simply removing the sprues or support structures and polishing the object to the desired finish. However, the limited number of filaments applicable to dental technology is a disadvantage, as well as lack of speed and accuracy. Future innovations could mitigate some of the current disadvantages. Recently, one manufacturer announced that their printer will be able to print PMMA teeth with a flexible thermoplastic material in a single build, reducing the steps needed to finish the end product. Other materials that can be printed with FDM technology include PEEK and PEKK for use in substructures.
SLS is a process in which a high-powered laser beam is aimed into a powder bed to sinter a layer of the desired object. Once that layer is complete, the object is covered with a new layer of powder, and another layer is sintered. Because the materials used for SLS are generally rigid, the need for support structures is eliminated, thereby cutting down on the time and labor required to achieve the final product. Materials most closely associated with SLS are plastics and polymers such as ABS, PVC, and PEEK. PEEK frameworks are gaining traction in the dental community as an alternative to metal frameworks, which require laborious milling or casting processes.
SLM is a similar process to SLS; the difference lies in the laser. SLM uses a much higher-energy, higher-density laser to melt the powder into a liquid phase, thus creating an intensely dense, homogenous part with a higher tensile strength than a cast part. The end product requires little post-processing since there is no need for washing, curing, or sintering. This technology is commonly used for copings, bridge frameworks, and partial metal frameworks. These types of printers are rather costly, generally have a large footprint, and require a substantial amount of energy, as well as an extensive knowledge of materials and lasers. This creates a high barrier to entry for most dental laboratories, and outsourcing this type of printing is a common practice.
Presently Available Applications
A vast number of 3D printers and associated resins have currently been cleared or approved by the FDA for use in creating dental appliances in the United States. To date, approved indications include:
• surgical guides
• temporary crowns
• dental models for crown and bridge
• models for clear aligners
• permanent and try-in dentures
• denture bases and teeth
• metal partial frameworks
• castable partial framework patterns
• custom impression trays
• indirect bonding trays
• copings for crowns and bridges
• PEEK and PEKK frameworks
It is expected that product offerings will continue to grow with further advancements in materials and technologies.
Two material aspects that will likely come to market in the future are 3D printed clear aligners and 3D printed ceramics. The clear aligner market has seen rapid disruption in recent years due to expiring patents, the lowering costs of 3D printers, and new orthodontic planning software coming to market. Even with these recent advancements, the production of clear aligners remains a laborintensive process. The process begins with creating the orthodontic plan and then proceeds to 3D printing a succession of orthodontic models; thermoforming the plastic aligner trays to the models; and finally trimming and polishing the final trays. Several companies are hard at work to produce a resin capable of achieving the physical properties these thermoformed trays possess. This material would require both the thickness and rigidity necessary to move teeth into ideal esthetic positions. Even when these resins receive FDA approval, as it currently stands there will still be the need for post-processing, which includes sprue removal, polishing, washing, and the final curing of the trays. The only way to improve the process for making 3D printed aligners is to eliminate the need for postprocessing. Numerous 3D printer manufacturers are working to develop this technology.
Perhaps the most disrupting and exciting upcoming application of 3D printing in dental technology is in 3D printed ceramics. Ceramics make up the vast majority of dental restorations and are considered ideal due their biocompatibility and esthetic properties. They do not suffer from corrosion or release of ions like metal. Compared to PFM crowns, ceramic crowns are not only better able to mimic the colors and shades of natural teeth, but also have a much higher translucency than metal. Zirconia has been a widely adopted ceramic because of its inherent strength and ever-increasing translucency. Traditionally, ceramics in dentistry have been limited to production through milling or subtractive manufacturing, but in the future, production of ceramics will include the additive manufacturing process. 3D printed ceramics are actively being developed and tested, and BEGO recently received its FDA 510(k) Premarket Notification (see sidebar) for the VarseoSmile Crown plus, which it touts as “the world’s first tooth-colored, ceramic-filled hybrid material for 3D printing of permanent single crowns, inlays, onlays, and veneers.” The final 3D printed restoration achieves a flexural strength of 116 MPa. These advancements—along with more we have yet to imagine—will ensure that 3D printing remains relevant in the dental laboratory for years to come.
Two Giant Leaps
Two major developments took place in the dental 3D printing world on June 29.
The FDA posted a 510(k) Premarket Notification for BEGO VarseoSmile Crown plus, a ceramic-filled hybrid material for the 3D printing of permanent restorations. IDT Editorial Advisory Board member Chris Brown, BSEE, covered this material in-depth in a recent article that can be seen at insidedentaltech.com/idt1204.
On the same day, manufacturer XJet announced a global partnership with Straumann Group to boost the 1400 AM System from concept to production. Straumann VP Stephan Oehler says XJet’s NanoParticle Jetting (NPJ) technology has been used for developing product iterations and proof of concept to this point, but that this will be “the first product development project to reach the next level of bringing this technology to production of ceramic end-use parts.” IDT Executive Editor Daniel Alter, MSc, MDT, CDT, explored XJet’s NPJ technology in a 2017 article that can be seen at insidedentaltech.com/idt1205.
About the Author
Josh Throndson, CDT, is the Director of Operations and co-owner of Innovative Dental Technologies in Memphis, Tennessee.