I’ve been very surprised by the impact of our article in the orthodontic community. We published in a materials science journal, Nature Materials, but most of the people contacting us were asking how we make the tooth? what polymer did we use? what about collaborating? … for dental applications.
Indeed, it is true that the last figure and the final highlight of our paper were about an artificial tooth. This synthetic tooth has the shape of a natural one: a wisdom tooth. My top right wisdom tooth to be precise. It also has similar internal structure and mechanical properties as a real tooth. This means a harder outer part, which is the enamel, and a “softer” part the dentin. We did not reproduce the pulp and the root but this could be easily done given all the synthetic plastics and gels available around.
Pictures of the natural wisdom tooth, entire (left) and broken part (right).
I would love to see our artificial tooth used as a replacement when it is needed. But it should be noted that we did not look at its biocompatibility, if it would increase the pH in the mouth, if was giving a weird feeling, etc. We did not also investigate if it would be economically advantageous to produce them on the market.
The point we wanted to make with this artificial tooth was to demonstrate how the manufacturing platform we developed could be used for producing objects with complex shape, and where we control the mechanical response by controlling the design of the internal structure and chemical composition.
Like a 3D printing technique.
But our method is faster, with better finishing, more controllable, and can make harder composite materials containing a lot of hard ceramic and a little bit of soft plastic.
In the era where additive manufacturing is expected to revolutionize the production of goods, our method can provide new capabilities to complement current 3D printing technologies. For example, 3D printers use inks made of molten plastics that can then harden into a pure plastic, or beds of powders that become metals or ceramics after local melting. Because of the use of temperature, it is very hard to make composite materials via 3D printing: the addition of hard particles into molten polymer usually turns the inks into very thick pastes or the heating of beds containing ceramics and polymer will burn away the polymer.
This is a huge limitation of current 3D printing because composite materials are used in one of the biggest industry in the world: transportation. Wings of planes, sits of buses, body of cars… are now being made of composite materials instead of the traditional metal, because it is way lighter, thus demands less energy consumption, and with much more performance. In addition, many other industries are using composites of various kinds and the demand keeps increasing.
We named our new method MASC for “Magnetically-Assisted Slip-Casting”. I like to call it “The” Masc in reference to Jim Carrey’s crazy character I loved as a child.
For a new idea, open an old book. The technique thus combines an old ceramic processing that has been developed in the XX century to make pots and figurines, slip-casting, with a more recent one based on magnetic fields. As my supervisor said: “But you know, people have been doing slip-casting for many years, it is very well-established. Try something new”. Well, we turned the well-established boring technique into a super cool additive manufacturing platform!
Let’s deal first with the old technique. Slip-casting is very simple, you can do it at home. It consists in making a mold replicating the shape of the object you are interested in, then pouring a suspension of particles, called the slip, into the mold. In our case, the suspension of particles is made of water, some soap, anisotropic tiny ceramic platelets (imagine little tiles) and an additive that will link the tiles together when the water gets out. The soap helps to disperse the platelets so that it is very liquid and that each particle is separated from its neighbor. Slip-casting was mostly used for clay potteries, in that case, the slip is only clay with water.
Schematics of the “slip” composition we used before and after drying.
The mold is simply made of plaster as it is porous and can take any shape. It starts as a powder that you manually mix with water in a ratio powder: water 2:1. It gets quite viscous as you mix (like flour and water). Just need to pour it on your object, and let it stiffen, then dry it and the object can be removed. It can be sometimes a bit tricky to remove the object but by covering it with oil or plastic it is easier.
Pictures of a mold of a molar.
When you pour the suspension, the slip, into the mold, the water gets sucked into the porous plaster. When it is fully dry, the packed particles are maintained together by the additive and therefore the replicate can be taken out of the mold. The replicate is thus a concentrate of platelets hold together by the additive. It is therefore weak and porous. We measured it contains a little bit less than 40 % particles for 60 % air.
Pictures of the replicate of the molar just after slip-casting and drying. It is brown because we used brown particles in the slip.
Now, along with slip-casting, we used magnetic fields. We placed a rotating magnet close by to our mold. Since our suspension contains these tiny tiles, we can orient and align them using the rotating magnet (see the post “You know you twist so fine” for explanation of why we needed to rotate the magnet). Of course, we modified the platelets before to make them respond to the magnetic field. Those platelets are brown because of the iron content. The purpose of platelet orientation is to imitate the natural structure of the tooth.
Indeed, the natural enamel also contains mineral crystals, hard anisotropic material, that is oriented perpendicularly to the surface. The dentin, on the contrary, has minerals oriented perpendicularly to those in the enamel. Using magnetic fields, we can thus align the tiles as the crystals in the natural tooth.
Schematic and picture of the internal structure of a natural molar.
In addition, the enamel of our teeth is hard so that it can break the food. To increase the hardness, we modified our slip. The composition we choose contains platelets, made of aluminium oxide, or alumina (the component that makes porcelain white) and silica nanoparticles which are hundred times smaller than the longest dimension of the tiles. Silica or silicon oxide is the component of quartz, where it is pure, but also of common glass, where it is mixed with other components. After doing our MASC technique, we will sinter the replicate at high temperature, 1600°C. At this temperature, the aluminium oxide does not change much but the silica will melt and bridge each little tiles like a cement. This will make the fake enamel harder.
For the dentin part, we do not add silica, it will be just tiles without cement.
So to sum up the casting of our replicate: we make a slip with enamel-like composition, pour it in the porous mold, align using the magnet in one direction. Then we wait for it to dry and make the slip with the dentin-like composition, pour it on the enamel-like part that sits in the mold, and align in the perpendicular direction. After drying, we take it out of the mold and cook it at super high temperature.
Images of the original molar and of its replicate.
The final step was to infiltrate our synthetic tooth with a polymer. We used a dental cement, which is a plastic-like material that is commonly used in dentistry and has the advantage to further increase the hardness while filling the remaining gaps.
But is our tooth as we designed it to be? We used several methods to look at the orientation of the tiles, the local composition, and also the mechanical properties: hardness and crack propagation.
Using a diamond-shaped diamond, we measured the hardness by a method called indentation. We made indents of the diamond look at the imprint. The larger the imprint the softer the material. The trend looks quite similar to that of a natural tooth, harder outside, softer inside!
Imprints of a diamond on the artificial tooth.
In a wall made of tiles, cracks propagate along the edges of the tiles. This is what we observed. Since the orientation of the tiles is changing at the interface between enamel and dentin, the crack is deviated there and will not reach the pulp. We also observed similar case in our artificial tooth.
Image of a crack in the artificial tooth.
We successfully achieved our goal of controlling particle orientation, composition and mechanical properties in a complex shaped sample. From there, the range of possible objects is infinite!
Any suggestion? 😉
H. Le Ferrand, F. Bouville, T.P. Niebel, A.R. Studart, Magnetically assisted slip-casting of bio-inspired heterogeneous composites, Nature Materials 14 (2015), 1172.