September 30, 2015
Cross section of an artificial tooth under an electron microscope (pseudocolor). Ceramic platelets are oriented vertically in the enamel and horizontally in the dentin. (credit: Hortense Le Ferrand/ETH Zürich)
A new procedure that can mimic the complex fine structure of biological composite materials, such as teeth or seashells, has been developed by ETH Zurich researchers. It could allow for creating synthetic materials that are as hard and tough as their natural counterparts.
The secret of these hard natural biomaterials is in their unique fine structure: they are composed of different layers in which numerous micro-platelets are joined together, aligned in identical orientation in each layer.
Although methods exist that allow material scientists to imitate nacre (mother of pearl), it was a challenge to create a material that imitates the entire seashell, with comparable properties and structural complexity, according to the researchers, led by André Studart, Professor of Complex Materials.
The new procedure mimics the natural model almost perfectly. It recreates the multiple layers of micro-platelets with identical orientation in each layer in a single complex piece by using a “magnetically assisted slip casting” (MASC) procedure.
How to create a tooth
Here’s how the procedure works:
Create a plaster cast to serve as a mold.
Pour in a suspension containing magnetized ceramic platelets, such as aluminum oxide platelets. The pores of the plaster mold slowly absorb the liquid from the suspension, which causes the material to solidify and to harden from the outside in.
Create an ordered layer-like structure by applying a magnetic field during the casting process, changing its orientation at regular intervals. As long as the material remains liquid, the ceramic platelets align to the magnetic field. In the solidified material, the platelets retain their orientation.
This continuous process can produce multiple layers with differing material properties in a single object and are almost perfect imitations of their natural models, such as nacre or tooth enamel, says Florian Bouville, a post-doc and co-lead author of the study, which is published in the journal Nature Materials. “Our technique is similar to 3-D printing, but 10 times faster and much more cost-effective.”
The left structure is showing the natural tooth in its gypsum mold. The middle structure is the artificial tooth (sintered but not yet polymer infiltrated). The model on the right has been sintered and polymer infiltrated. It is embedded in a “puck” to enable polishing and coated with platinum to prevent charging in the electron microscope. (credit: Tobias Niebel/ETH Zurich)
To demonstrate the process, Studart’s research group produced an artificial tooth with a microstructure that mimics that of a real tooth. The surface of the artificial tooth is as hard and structurally complex as real tooth enamel, while the layer beneath is as tough as the dentine of the natural model.
They began by creating a plaster cast of a human wisdom tooth. They then filled this mold with a suspension containing aluminum oxide platelets and glass nanoparticles as mortar. Using a magnet, they aligned the platelets perpendicular to the surface of the object. Once the first layer was dry, the scientists poured a second suspension without glass particles into the same mold. The aluminium oxide platelets in the second layer were aligned horizontally to the surface of the tooth using the magnet.
This double-layered structure was then sintered (“fired” in a kiln) at 1,600 degrees C to densify and harden the material. Finally, the researchers filled the pores that remained after the sintering with a synthetic monomer used in dentistry, which subsequently polymerized (formed into a complex material).
Artificial teeth that behave just like the real thing
“The profile of hardness and toughness obtained from the artificial tooth corresponds exactly with that of a natural tooth,” says Studart.
The current study is an initial proof-of-concept, which shows that the natural fine structure of a tooth can be reproduced in the laboratory, he says. “The appearance of the material has to be significantly improved before it can be used for dental prostheses.
He noted that the base substances and the orientation of the platelets can be combined as required, “which rapidly and easily makes a wide range of different material types with varying properties feasible.” For example, copper platelets could be used in place of aluminum oxide platelets, allowing for use in electronics.
One part of the MASC process, the magnetization and orientation of the ceramic platelets, has been patented.
Abstract of Magnetically assisted slip casting of bioinspired heterogeneous composites
Natural composites are often heterogeneous to fulfil functional demands. Manufacturing analogous materials remains difficult, however, owing to the lack of adequate and easily accessible processing tools. Here, we report an additive manufacturing platform able to fabricate complex-shaped parts exhibiting bioinspired heterogeneous microstructures with locally tunable texture, composition and properties, as well as unprecedentedly high volume fractions of inorganic phase (up to 100%). The technology combines an aqueous-based slip-casting process with magnetically directed particle assembly to create programmed microstructural designs using anisotropic stiff platelets in a ceramic, metal or polymer functional matrix. Using quantitative tools to control the casting kinetics and the temporal pattern of the applied magnetic fields, we demonstrate that this approach is robust and can be exploited to design and fabricate heterogeneous composites with thus far inaccessible microstructures. Proof-of-concept examples include bulk composites with periodic patterns of microreinforcement orientation, and tooth-like bilayer parts with intricate shapes exhibiting site-specific composition and texture.
Hortense Le Ferrand, Florian Bouville, Tobias P. Niebel, André R. Studart. Magnetically assisted slip casting of bioinspired heterogeneous composites. Nature Materials, 2015; DOI: 10.1038/nmat4419
Biomimetic dental prosthesis