Thursday, April 15, 2021

3D-Printed Material to Replace Ivory

With an elephant-friendly alternative to ivory, developed by TU Wien and Cubicure, old artefacts can be restored with high precision.

For centuries, ivory was often used to make art objects. But to protect elephant populations, the ivory trade was banned internationally in 1989. To restore ivory parts of old art objects, one must therefore resort to substitute materials - such as bones, shells or plastic. However, there has not been a really satisfactory solution so far.

On the right: The new material Digory, ivory on the left

  Credit: TU Wien 

TU Wien (Vienna) and the 3D printing company Cubicure GmbH, created as a spin-off of TU Wien, have now developed a high-tech substitute in cooperation with the Archdiocese of Vienna's Department for the Care of Art and Monuments and Addison Restoration: the novel material "Digory" consists of synthetic resin and calcium phosphate particles. It is processed in a hot, liquid state and hardened in the 3D printer with UV rays, exactly in the desired shape. It can then be polished and colour-matched to create a deceptively authentic-looking ivory substitute.

Beautiful and Mechanically Stable

"The research project began with a valuable 17th-century state casket in the parish church of Mauerbach," says Prof. Jürgen Stampfl from the Institute of Materials Science and Technology at TU Wien. "It is decorated with small ivory ornaments, some of which have been lost over time. The question was whether they could be replaced with 3D printing technology."

The team already had experience with similar materials: the research group also works with ceramic materials for dental technology, for example. Nevertheless, it was a challenging task to develop a suitable substitute for ivory: "We had to fulfil a whole range of requirements at the same time," says Thaddäa Rath, who worked on the project as part of her dissertation. "The material should not only look like ivory, the strength and stiffness must also be right, and the material should be machinable."

Stereolithography in the 3D printer

Through numerous experiments, Thaddäa Rath and other members of the team from TU Wien and Cubicure succeeded in finding the right mixture: Tiny calcium phosphate particles with an average diameter of about 7 µm were embedded in a special resin, together with extremely fine silicon oxide powder. The mixture is then processed at high heat in Cubicure's 3D printers using the hot lithography process: Layer by layer, the material is cured with a UV laser until the complete object is finished.

Chess figures made of the new magerial with added dark lines to mimic ivory as precisely as possible

"You also have to bear in mind that ivory is translucent," explains Thaddäa Rath. "Only if you use the right amount of calcium phosphate will the material have the same translucent properties as ivory." Afterwards, the colour of the object can be touched up - the team achieved good results with black tea. The characteristic dark lines that normally run through ivory can also be applied afterwards with high precision.

No more tusks!

In the field of restoration, this is a big step forward: With the new material "Digory", not only is a better, more beautiful and easier to work with substitute for ivory available than before, the 3D technology also makes it possible to reproduce the finest details automatically. Instead of painstakingly carving them out of ivory substitute material, objects can now be printed in a matter of hours.

"With our specially developed 3D printing systems, we process different material formulations for completely different areas of application, but this project was also something new for us," says Konstanze Seidler from Cubicure. "In any case, it is further proof of how diverse the possible applications of stereolithography are."

The team hopes that the new material "Digory" will become generally accepted in the future - as an aesthetically and mechanically high-quality ivory substitute, for which no elephant has to lose a tusk.

Contacts and sources:
Prof. Jürgen Stampfl
Institut für Werkstoffwissenschaft und Werkstofftechnologie

Dipl.-Ing. Thaddäa Rath
Institut für Werkstoffwissenschaft und Werkstofftechnologie
Vienna University of Technology (TU Wien)

Stronger Armor Using Sound Waves to Make Patterns that Never Repeat

Mathematicians and engineers at the University of Utah have teamed up to show how ultrasound waves can organize carbon particles in water into a sort of pattern that never repeats. The results, they say, could result in materials called “quasicrystals” with custom magnetic or electrical properties.

The research is published in Physical Review Letters.

“Quasicrystals are interesting to study because they have properties that crystals do not have,” says Fernando Guevara Vasquez, associate professor of mathematics. “They have been shown to be stiffer than similar periodic or disordered materials. They can also conduct electricity, or scatter waves in ways that are different from crystals.”
Non-pattern patterns

Picture a checkerboard. You can take a two-by-two square of two black tiles and two white (or red) tiles and copy and paste to obtain the whole checkerboard. Such “periodic” structures, with patterns that do repeat, naturally occur in crystals. Take, for example, a grain of salt. At the atomic level, it is a grid-like lattice of sodium and chloride atoms. You could copy and paste the lattice from one part of the crystal and find a match in any other part.

A quasiperiodic two-dimensional pattern

 Courtesy of Fernando Guevara Vasquez

But a quasiperiodic structure is deceiving. One example is the pattern called Penrose tiling. At first glance, the geometric diamond-shaped tiles appear to be in a regular pattern. But you can’t copy and paste this pattern. It won’t repeat.

The discovery of quasiperiodic structures in some metal alloys by materials scientist Dan Schechtman earned a 2011 Nobel Prize in Chemistry and opened up the study of quasicrystals.

Since 2012, Guevara and Bart Raeymaekers, associate professor of mechanical engineering, have been collaborating on designing materials with custom-designed structures at the microscale. They weren’t initially looking to create quasiperiodic materials—in fact, their first theoretical experiments, led by mathematics doctoral student China Mauck, were focused on periodic materials and what patterns of particles might be possible to achieve by using ultrasound waves. In each dimensional plane, they found that two pairs of parallel ultrasound transducers suffice to arrange particles in a periodic structure.

But what would happen if they had one more pair of transducers? To find out, Raeymaekers and graduate student Milo Prisbrey (now at Los Alamos National Laboratory) provided the experimental instruments, and mathematics professor Elena Cherkaev provided experience with the mathematical theory of quasicrystals. Guevara and Mauck conducted theoretical calculations to predict the patterns that the ultrasound transducers would create.
Creating the quasiperiodic patterns

Cherkaev says that quasiperiodic patterns can be thought of as using, instead of a cut-and-paste approach, a “cut-and-project” technique.

If you use cut-and-project to design quasiperiodic patterns on a line, you start with a square grid on a plane. Then you draw or cut a line so that it passes through only one grid node. This can be done by drawing the line at an irrational angle, using an irrational number like pi, an infinite series of numbers that never repeats. Then you can project the nearest grid nodes on the line and can be sure that the patterns of the distances between the points on the line never repeats. They are quasiperiodic.

The approach is similar in a two-dimensional plane. “We start with a grid or a periodic function in higher-dimensional space,” Cherkaev says. “We cut a plane through this space and follow a similar procedure of restricting the periodic function to an irrational 2-D slice.” When using ultrasound transducers, as in this study, the transducers generate periodic signals in that higher-dimensional space.

The experimental setup with four pairs of ultrasound transducers surrounding a reservoir with carbon nanoparticles suspended in water. 

 Courtesy of Fernando Guevara Vasquez

The researchers set up four pairs of ultrasound transducers in an octagonal stop sign arrangement. “We knew that this would be the simplest setup where we could demonstrate quasiperiodic particle arrangements,” Guevara says. “We also had limited control on what signals to use to drive the ultrasound transducers; we could essentially use only the signal or its negative.”

Into this octagonal setup, the team placed small carbon nanoparticles, suspended in water. Once the transducers turned on, the ultrasound waves guided the carbon particles into place, creating a quasiperiodic pattern similar to a Penrose tiling.

“Once the experiments were performed, we compared the results to the theoretical predictions and we got a very good agreement,” Guevara says.
Custom materials

The next step would be to actually fabricate a material with a quasiperiodic pattern arrangement. This wouldn’t be difficult, Guevara says, if the particles were suspended in a polymer instead of water that could be cured or hardened once the particles were in position.

“Crucially, with this method, we can create quasiperiodic materials that are either 2-D or 3-D and that can have essentially any of the common quasiperiodic symmetries by choosing how we arrange the ultrasound transducers and how we drive them,” Guevara says.

It’s yet to be seen what those materials might be able to do, but one eventual application might be to create materials that can manipulate electromagnetic waves like those that 5G cellular technology uses today. Other already-known applications of quasiperiodic materials include nonstick coatings, due to their low friction coefficient, and coatings insulating against heat transfer, Cherkaev says.

Yet another example is the hardening of stainless steel by embedding small quasicrystalline particles. The press release for the 2011 Nobel Prize in Chemistry mentions that quasicrystals can “reinforce the material like armor.”

So, the researchers say, we can hope for many new exciting applications of these novel quasiperiodic structures created by ultrasound particle assembly.

Find the full study here.

Contacts and sources:
Paul Gabrielsen
University of Utah

Publication: Wave-Driven Assembly of Quasiperiodic Patterns of Particles.
Elena Cherkaev, Fernando Guevara Vasquez, China Mauck, Milo Prisbrey, Bart Raeymaekers. Physical Review Letters, 2021; 126 (14) DOI: 10.1103/PhysRevLett.126.145501