The Next Layer

Like milling machines, 3D printers also evoke a feeling of wonder during the layer-by-layer printing process as solid objects miraculously appear out of a vat of liquid or on a metal tray. From the very beginning in the dental industry, there were affordable 3D printing options available, but material options were lacking. Just as the milling machine and material options were improved, 3D printers and their materials have now been improved to match performance requirements. The technology has been refined to the point where it can now be used to print polymer resin long-term temporary restorations or restoration patterns with suitable accuracy and performance, but there is still more work to be done.

In an effort to improve mechanical properties, some companies now offer polymer resin printer materials with ceramic fillers. However, the ratio of ceramic material to resin is very low. While the filler may enhance or improve some of the resin's properties, the physical properties of the final printed product remain closer to those of resin than ceramic.

Could this 3D printing technology be applied to zirconia or other ceramic materials which may have more desirable properties for dental applications? The answer is a resounding yes.

From a technical standpoint, the method of printing ceramic materials is not entirely dissimilar to the 3D printing technologies already in use in the dental industry. The same principals involved in SLA, DLP, jetted-resin, SLS/SLM, and extrusion technologies can be employed in the printing of ceramics once a few challenges are overcome.

Slurry-based material printing technologies such as SLA, DLP, and resin jetting are the most common ones in the dental industry today. They all rely on a similar method that uses a light source to cure a liquid polymer resin into a solid.

In a slurry-based ceramic material printer, the raw print material is predominately a ceramic material slurry—but mixed with a photosensitive resin. During the printing process, a UV light source in the printer cures the photosensitive resin, just as it does in the resin printers. The actual printing process or sequence may be slightly modified from traditional resin-only processes to accommodate the different properties of a ceramic material slurry. This may include differences in light source wavelength or intensity. It may also include the environmental conditions surrounding the slurry reservoir or delivery apparatus. Ceramic particle size, particle density, light scatter, and material viscosity are all factors¹ which have to be balanced in material development in order to achieve desired final material density, strength, and dimensional accuracy. 

Not surprisingly, the post-processing of 3D printed ceramic materials differs from the process followed by resin printer materials. Anyone familiar with zirconia processing will appreciate the steps involved.

In ceramic 3D printing, once the printed objects are removed from the printer, they are considered to be in the green or green body state. They must go through a drying process that evaporates water or solvents in the slurry, which are used to help with material viscosity requirements. Remember, the green state parts are still held together by the photopolymer network that was cured during the 3D printing process. Once the parts are dried, that resin network is removed through a process called debinding. Parts are placed into an oven and heated to a temperature that burns off the polymer network, leaving a pure ceramic in a white body state. This is effectively the same process that zirconia disk manufacturers follow to remove binders in their material fabrication process. It is often referred to as pre-sintering the zirconia. After debinding is complete, printed parts are then run through a sintering cycle, very much like the cycle used for zirconia milled from disks or blocks. Then the resulting restorations can be stained and glazed just like any other dental zirconia.

Much like the pre-sintering process for zirconia disks and blanks, green-state printed zirconia parts go through a slight shrinkage during debinding. The sintering process for printed zirconia parts also results in additional shrinkage as the zirconia phase transformation takes place.

Currently, there are no FDA clearances for 3D printed zirconia restorative materials, so this prevents their use for laboratory-printed restorations in the US at this time. However, 3D printing of ceramics—and zirconia in particular—is occurring in geographical markets where FDA clearance isn't required. In these markets, 3D printing technology is being used for dental implants and surgical scaffolds to facilitate bone development in dental arches.

Lithoz, an Austrian company, has created the CeraFab 7500 Dental 3D printer using lithography-based ceramic manufacturing (LCM) technology. LCM is a modified form of DLP-style printing for ceramic materials. This printer has lateral resolution and layer thickness specifications that are accurate enough for crown-and-bridge work. Materials available for this printer include zirconia for restorations, hydroxyapatite for implants, and tricalcium phosphate for bioresorbable bone augmentation. Printers with larger form factors and build platforms for printing high volumes of dental implants are available both from this vendor and others in markets where they have received the appropriate regulatory clearances.

DWS Additive Manufacturing is also working on a new zirconia material called Irix Z for their DFAB 3D printers. They intend to offer the material in several shades for use in both the desktop and chairside versions of their SLA technology-based printer.

For dental laboratories, the printing of fixed and removable restorative prosthetics is probably the most interesting and viable option for 3D printed ceramics. Research is showing that 3D printed zirconia can achieve flexural strength and material density very close to what is seen with the disks and blanks used in milling workflows. In small to mid-sized production volumes, the milling workflow will likely continue to prevail due to simpler post-processing steps.

However, as production volume increases, 3D printing often gains an advantage in process speed. At some point, large build trays and optimized print settings will act as an equalizer.

The shrinkage factor normally associated with zirconia disks in CAM milling is also a factor with 3D printing, so this does not provide an advantage for one method or the other. An STL file for a 3D printed zirconia crown also has to be up-sized to compensate for shrinkage during sintering. In a 3D printing workflow, that scaling has to be done by the print project software before it is virtually sliced and sent to the printer.

Dipping or painting for custom shading of pre-sintered zirconia works the same way for both printed and milled zirconia. As with different brands of milled zirconia materials, slight modifications of techniques may be necessary.

Zirconia disks or blanks are currently available in multi-shaded or multi-translucency layers. That is not easy to duplicate in today's SLA- or DLP-style 3D printing technology. DWS claims to have a process to customize resin shade using their Photoshade technology, but it is unclear how significant of a color change is possible. This is one area that jetted-material printing technology may have an advantage, as multiple resins can be mixed to achieve significant shade and translucency transitions. Ultimately, 3D printing is a layer-by-layer manufacturing process. It will be interesting to see how much control or color change is possible with a 3D printed zirconia material and if it can be accomplished in a cost-effective, production-efficient, and consistent manner.

For now, at least in the US, we await regulatory clearance on 3D printable ceramic materials so that the next generation of 3D printers can come to our shores and our dental laboratories. Fortunately for the US market, research is already showing promise in the materials. Dental laboratories continue to embrace and learn new technology and will no doubt find a way to make the next generation of equipment and materials work in their favor, adding yet another layer to the already complex landscape of digital dentistry.

Watch a video interview with Chris Brown, BSEE, about the potential for 3D printing ceramics: insidedentaltech.com/idt1198