Light provides control for 3D printing with multiple materials

Photo: Woman with lab coat stands next to equipment.

By Stephanie Blaszczyk
Science Communicator & Graduate Student (Tang Group)

Additive manufacturing, more commonly known as 3D printing, has revolutionized the fields of healthcare, biomedical engineering, manufacturing and art design.

Surprisingly, breakthroughs in these areas have succeeded, despite the general limitation that most 3D printing techniques can only produce parts made of one material at a time. An industrial renaissance could be just on the other side of one long-standing challenge, to juxtapose different materials and create multimaterial parts.

New research uses different wavelengths of light to achieve this juxtaposition. Scientists at the University of Wisconsin-Madison developed a novel 3D printer that uses visible or ultraviolet light to dictate which of two monomers are polymerized to form a solid material. Patterned combinations of light provide spatial control to yield multimaterial parts. The work was published Feb. 15 in the journal Nature Communications (DOI:10.1038/s41467-019-08639-7).

Graphic: digital designs produced by 3D printers.

The top images show the digital design and its printed form. Purple corresponds to ultraviolet cured stiff epoxide regions, whereas the gray regions are visible light cured acrylate regions that are soft and compliant. At the bottom, the logo for the 3D printing group, MASC, is turned into a printed object composed of both stiff, opaque regions and soft, transparent regions. IMAGE COURTESY A.J. BOYDSTON AND JOHANNA SCHWARTZ

“As amazing as 3D printing is, in many cases it only offers one color with which to paint,” says Professor A.J. Boydston, who led the work with his graduate student Johanna Schwartz. “The field needs a full color palette.”

Boydston and Schwartz knew that improved printing materials required a chemical approach to complement the advances provided by existing engineering perspectives.

“This is a shift in how we think about 3D printing with multiple types of materials in one object,” Boydston says. “This is more of a bottom-up chemist’s approach, from molecules to networks.”

3D printing is the process of making solid three-dimensional objects from a digital file by successively adding thin layers of material on top of previous layers. Most multimaterial 3D printing methods use separate reservoirs of materials and then deposit them or switch out reservoirs to get different materials in designated positions.

Taking a cue from his chemistry background, Boydston realized that a one vat, multiple component approach – similar to a chemist’s one-pot approach when synthesizing molecules – would be more practical than multiple reservoirs with different materials. This approach is based on the ability of different wavelengths of light to control which starting materials polymerize into different sections of the solid product. Those starting materials start as simple chemicals, known as photoresins, that polymerize together into a longer string of chemicals, like how plastic is made.

“If you can design an item in PowerPoint with different colors, then we can print it with different compositions based on those colors,” Schwartz says.

Researchers create multiple digital images that, when stacked, produce a three-dimensional design. The images control whether ultraviolet or visible light is used to polymerize the starting materials, which controls the final material and its properties, like stiffness. The researchers simultaneously direct light from two projectors toward a vat of liquid starting materials, where layers are built one-by-one on a platform. After one layer is built, the build platform moves up, and light helps build the next layer.

The major hurdle Boydston and Schwartz faced was optimizing the chemistry of the new materials. They had to consider how physical properties, like miscibility, in conjunction with chemical phenomena, such as the rates of acrylate and epoxide curing, would impact the final product.

Schwartz developed combinations of acrylate and epoxide monomers to make multimaterial items that contained both stiff epoxide networks and soft hydrogels. Acrylate and epoxide curing can be photochemically initiated, but the wavelength of light and chemical reactions responsible for the initiation of each monomer differ – making these components chemically orthogonal. Visible light preferentially cures acrylates, while ultraviolet light more readily cures epoxides. These components combine to form 3D printed products with spatially-defined chemical heterogeneity.

With the right chemistry in place, Boydston and Schwartz could now dictate exactly where each material cured within the printed object by using ultraviolet or visible light.

“At this stage, we’ve only accomplished putting hard materials next to soft materials in one step,” Boydston admits. “There are many imperfections, but these are exciting new challenges.”

In the immediate future, Boydston wants to address these imperfections and answer open questions, such as what other monomer combinations can be used and whether different wavelengths of light can be used to cure these new materials.

Boydston also hopes to assemble an interdisciplinary team that can increase the impact of wavelength-controlled, multimaterial 3D printing.

Although this project benefitted from many interactions with colleagues across chemistry and engineering disciplines, the printer was primarily built by Schwartz, lead author and a senior graduate student in the Boydston group.

“Johanna was really the intellectual driver and hands-on muscle behind the project,” Boydston says. “She built a custom 3D printer from scratch, and keep in mind she is a chemistry PhD student who previously focused on chemical synthesis.”

This project is a manifestation of the interdisciplinary training received by members of the Boydston group. Johanna’s previous expertise in organic synthesis, coupled with her newly developed skills in optical alignment and mechanical optimization, has led to her recruitment by high-caliber research programs across the country.

Using chemical methods to eliminate an engineering bottleneck is exactly what the 3D printing industry needs to go beyond revolutionary and move toward a new manufacturing renaissance.

“It is this interface of chemistry and engineering that will propel the field to new heights,” Schwartz believes.

“We think there is great potential to expand the chemistry involved in our technology, but it is going to require tremendous help and collaboration from experts across chemistry and engineering disciplines,” according to Boydston.

Multimaterial actinic spatial control (MASC) printing setup in action. Two projectors are aligned to project images of different wavelengths simultaneously into the vat of resin, which cures the multi-material object one layer at a time. (Photo courtesy of A.J. Boydston and Johanna Schwartz)
Multimaterial actinic spatial control (MASC) printing setup in action. Two projectors are aligned to project images of different wavelengths simultaneously into the vat of resin, which cures the multi-material object one layer at a time. (Photo courtesy of A.J. Boydston and Johanna Schwartz)

The researchers’ novel approach to multimaterial 3D printing could enable designers, artists, engineers, and scientists to conceive and realize significantly more complex systems from 3D printing.

“The most sophisticated designs we can think of are usually biological in nature,” Boydston says. “My vision is to rival these biological systems.”

Achieving increased complexity of design could improve the manufacture of adaptive, personalized medical devices, such as orthotics or prostheses. Another avenue for benefit could be in the development or creation of improved training materials, such as simulated organs and tissues. Medical students could use these synthetic biological mimics for training instead of, or before working with, live patients.

What is the most exciting aspect of the new technology? According to Schwartz, “Just as we have the full spectrum of light to play with, correlating this to a full spectrum of materials and chemistries would be really exciting.”

Boydston adds, “We are poised to look in places that 3D printing has never looked before.”

This work was funded by the Army Research Office (Grant No. W911NF-17-1-0595) and the National Science Foundation Graduate Research Fellowship Program (J.J.S. – Grant No. DGE-1256082).