Multimaterial Printing: On the Horizon?
Hurdles persist, but research and development are underway
Chuck Stapleton
3D printing has been well established in the dental industry for several years, with the leading technology so far being Digital Light Processing (DLP). However, most current technologies focus on single-material printing. This can be limiting for the type of work dental laboratories perform because it does not allow for multiple shades or multiple physical properties within a single print. 3D printed dentures offer a perfect example of this limitation: For a full 3D printed denture, the process involves printing the base and the teeth separately and then curing them together. In order to achieve any sort of shading, the teeth must be stained by hand. Ideally, this could be accomplished in a single print.
Some systems can accomplish multi-material printing with MultiJet Printing (MJP) technology. They are extremely accurate, and there are applications within dentistry that make sense. However, compared to the DLP printers that have taken over the dental market, these MJP systems are more expensive to purchase, run, and maintain. Wouldn't it be great to print a monolithic denture using lower-cost DLP technology?
I was able to spend some time with Johanna Schwartz, PhD, a researcher at Lawrence Livermore National Laboratory, and Al Siblani, founder of EnvisionTEC, while writing this article. While previously working with Andrew J. Boydston, PhD, Schwartz has done research in the area of Multimaterial Actinic Spatial Control (MASC), and EnvisionTEC is the inventor of DLP printing and the first company to commercialize it.
There are two factors that most people in dentistry think of when talking about printing multiple materials: property differences and color differences. Color and shade differences could be achieved by using multiple materials, but Siblani explains that "exposure energy and the concentration of photosensitive additives in the material can control the color that is produced, even when using a single material. The more energy the resin receives, the darker the final photopolymer." No one seems to have really utilized this aspect of 3D printing, even though it is theoretically possible. Photosensitive additives would provide variations in a single color—darker or lighter—but to achieve dramatically different colors, multiple materials are required. Furthermore, achieving various material properties within one object is a bit more complicated and does require multiple materials at the same time.
The driving reason behind this DLP printing limitation is the vat of liquid in the 3D printer holding only one liquid at a time. Theoretically, someone could change materials according to layer, but it would be extremely time consuming. Even then, this would only allow a material change layer by layer, rather than anywhere within an object (Figure 1).
Some companies coat a DLP printed part with other materials to change its properties, but that is not the same as taking a readily usable part straight from the machine. Other materials undergo a thermal curing process after being printed, which helps strengthen them. Doing this changes the properties of the entire object, and the process cannot be controlled for specific parts of the object.
This is where segmented wavelength or MASC printing becomes interesting. It is all about the photo-initiators used in the material. Photo-initiators absorb energy from a light source and start the reaction of photopolymerization for the liquid resin, acting as a catalyst. Photo-initiators have different spectral distribution and absorption on different wavelengths (Figure 2).
"Most manufacturers will use visible light projectors that work at 405 nm, but they can cause issues with improper curing. Visible light projectors are used because they are lower-cost than ones that work in the UV spectrum. If you use a projector designed to work in the UV spectrum, you get a more complete cure of methacrylate resins used in dentistry. The reason for this is that the photo-initiators used in dental methacrylates operate below the 400-nm range," Siblani says.
But what if you had two projectors in a single machine that covered different wavelengths? You could then use two different photo-initiators in a single vat. That is exactly what Schwartz did.
"I had to build a specialized projector that would only produce light at the 365-nm wavelength," she says, "and then used a second projector that could produce visible light. By combining the two together, we can control both photo-initiators in the same solution."
Schwartz combined 70% epoxy and 30% acrylate in her research. When using 365-nm wavelength, the epoxy and acrylate are cured together. When using the 405-nm wavelength, only the acrylate is cured (Figure 3). The result was a single printed object that had material properties at two extremes. One material was flexible (visible-light-cured acrylate) and the other was hard and brittle (UV-cured epoxy and acrylate) (Figure 4). The printed objects are not limited to flexible or hard; they can have physical properties anywhere in between (Figure 5).
Schwartz continues, "There are options beyond just using two light sources. Infrared or more finely controlled visible light can open up more than just printing with two materials at once."
There are so many possibilities once we can start printing with multiple materials. The aforementioned possibility of monolithic printed dentures does not seem so distant. Producing hard-soft night guards directly from the printer is one advancement that I am anticipating eagerly.
Schwartz also explains that thermoplastic-like properties could potentially be achieved if the curing ratio is controlled. If you had a rigid material mixed with a softer material, then printing something with flexibility that maintains shape is possible. Printable aligners, as well as flexible removable dentures, have been chased for a long time by the dental industry, and this may open up that avenue.
There is still quite a way to go and a few challenges that must be overcome. We do not expect to see much development in the very near future. For example, epoxy is not used often in the oral environment and new materials would require regulatory approval. The CAD software companies would need to modify their systems to allow designers to select which materials go where and then transfer that data to the 3D printer software. Design times would increase substantially; just designing a single restoration takes time, so imagine adding additional steps around how flexible the material should be or shading. Designers would need to improve their skills, the software would need to become smarter, or both. The best option, as with any emerging technology, likely will be the turnkey closed systems because open systems may struggle with the new material mixtures.
3D printing has advanced rapidly over the past decade, and there are still areas that can be explored. Several 3D printing companies would not discuss multimaterial technology because it either it was under development or the company was not familiar with it. The fact that some are actively pursuing it is encouraging, and we should see this coming to market at some point.
When multimaterial printing on DLP machines becomes common in dentistry, we will probably be talking about Schwartz's current research: volumetric printing. As she explains, "When you take a CT scan of an object, it captures everything in 3D. Now, just imagine reversing that process, using the CT waves to project a movie into a vat of resin, and thereby creating a 3D product all at once." If you can wrap your head around that explanation, then you are doing better than I.
References
1. Schwartz JJ, Boydston AJ. Multimaterial actinic spatial control 3D and 4D printing. Nat Commun. 2019;10(1):791. https://doi.org/10.1038/s41467-019-08639-7. Published Feb 15, 2019. Accessed May 21, 2020.
2. Chirumbolo A. Understanding Acuvue contacts and ultraviolet light. https://covalentcareers.com/resources/understanding-acuvue-contacts-and-ultraviolet-light. Published January 9, 2015. Accessed May 21, 2020.
More from the Expert
Watch a video interview with Chuck Stapleton about multimaterial 3D printing: insidedentaltech.com/idt1197
![FIG 1. Diagram of projector light source with a single vat of material. (Figure reproduced from Nature Communication.[1])](/media/thumbnail/85770/150x150 "Figure 1")
![FIG 2. A light spectrum image showing UV and visible light. (Figure courtesy of Antonio Chirumbolo, OD.[2])](/media/thumbnail/85771/150x150 "Figure 2")
![FIG 3. A visual of the acrylate and epoxy solution. (Figure reproduced from Nature Communication.[1])](/media/thumbnail/85772/150x150 "Figure 3")
![FIG 4. At left, a CAD model of tetragonal lattice. Purple corresponds to regions printed with UV light, and white corresponds to regions printed with visible light. At right, the printed tetragonal lattice. (Figure reproduced from Nature Communication.[1])](/media/thumbnail/85774/150x150 "Figure 4")
![FIG 5. Hand printed with a more rigid bone structure inside. (Figure reproduced from Nature Communication.[1])](/media/thumbnail/85785/150x150 "Figure 5")
