Self-Growing 3D Objects

Ed Brown

Tawfick and his colleagues fabricated a pinecone, raspberry, and squash. (Image: The researchers)

Tech Briefs: What got you started with this? I was curious whether you were intrigued by the technology first or were you trying to solve a specific problem.

Sameh Tawfick: This was a technology-first situation. We had discovered the process of self-curing polymers called frontal polymerization. Our collaborators had been working on the chemistry, on different methods of manufacturing with this interesting process that transforms the liquid into a solid following a moving front.

We were intrigued with the visual aspects of the process and the fact that it doesn't need external energy, it can produce very high-quality materials, and it's beautiful to watch when the front moves. It looks similar to what happens If you put water in the freezer in a pressurized container and take it out and it's still liquid and then hit it and, all of a sudden, the super-cold liquid freezes following the front. It's fun to watch; this process looks exactly like that. Liquid transforms into a solid following a moving front. So, all of these ideas, including the good behavior of the material and the low energy, made us think outside the box about how you could manufacture things, given these properties.

Tech Briefs: What is the type of liquid you’re working with?

Tawfick: The liquid is a monomer, which is basically the building block of a polymer — polymer means many monomers. A monomer and a polymer are two words that describe molecules. Monomer is the building block of a long molecule. Polymer is when all the building blocks come together. Any plastic of any sort is basically a polymer, which means it has these long chains. They are strong and lightweight, all the things we like about polymers. Even our biological tissue is a biopolymer, still same idea, long chains of molecules that are held together sort of like a pot of noodles. The long chains are kind of intertwined, and this is what gives them structural integrity.

So, the curing that happens in our process takes the building blocks, the monomers, and makes them into a polymer, but in a very intriguing way, because it follows a moving front. We are not aware of any other biological process, for example, that takes the building blocks and makes them into a chain, which becomes the material, with such a fast and sharp moving front.

Tech Briefs: Approximately how fast?

Tawfick: About 1mm per second.

Tech Briefs: So, you start with a liquid monomer and then you put it in an ice bath?

Tawfick: Yes, we put it in a glass and then put the glass in an ice bath.

Tech Briefs: Then I read you heat the center of the liquid while it is in the ice bath. How do you get the heat in there?

Tawfick: We use what we call a heated tip — think, for example, of a soldering iron, where just the tip gets hot. And it is connected to a moving stage so that we can move it in a controlled way. We move the heated tip to about a millimeter below the surface. As soon as we do that, we wait a few seconds, and the front starts moving in all directions — we call that the dwell time. We wait until we get approximately the size and shape we want to make, and then we start moving the heated tip up outside the bath. We start to move the solidifying part out of the liquid bath while the front is still growing. So, the part is growing and being pulled out of the bath at the same time. The balance between how fast we're taking the part out and how fast the front is moving gives us the shape of the part.

Tech Briefs: Does the motorized stage withdraw the heater in a linear path?

Tawfick: It moves in a single direction to get symmetric shapes. If you want to create complex shapes — we did a Kiwi bird and a helix, for example — then we move in all three directions, X, Y, and Z.

Tech Briefs: How do you figure out how to move it to get your desired shape?

Tawfick: Our team includes people who do numerical simulations of the chemistry and of the desired shape and how the chemistry can lead to the desired shape. And then they can calculate a path — we call it the motion program. We then enter that program into our controller and the machine follows the calculated path. So, there is a simulation process that happens first.

That makes our process very different than typical 3D printing, which deposits material directly where you want it to be. In our case, we have a self-growing front — we just manipulate how the growth occurs.

We need the simulation first, because you don't know what the chemistry will do if you let it run by itself. So, we do what we call an inverse design process. We manipulate the simulation until we get to the desired shape, which happens very quickly in the computer. Then we back down in the simulation to learn the path that will most accurately give us that shape.

Tech Briefs: Is what happens at the front a chemical reaction?

Tawfick: Yes, it's a specific type of polymerization reaction, which is any reaction that takes a monomer into a polymer. This is a specific type, however; it's called a ring-opening metathesis.

Tech Briefs: Does your computer simulation include the timing of how you withdraw the tip as well as the path?

Tawfick: Yes, all these details are captured in our computer simulation. It took us about two and a half years to develop the computer simulation.

The reason we're excited about this is not just because it's a completely new manufacturing paradigm, but the material we're making has very good properties. It can be recycled, it's very stable against chemicals, it's very strong, very tough, it can be used in composite materials — it's a really great polymer.

Also, the process is super-fast compared to other 3D printers. Comparing the same shape side-by-side made by our process vs. a commercial desktop 3D printer, we are about 100 times faster. And it's extremely energy-efficient: You just apply the heat tip for the first second, then the rest is all the exothermic reaction.

Tech Briefs: What kinds of applications do you see?

Tawfick: We’re looking into many different objects. The thing with this process is you need to develop it for the specific geometry you want to make. So, as an example, let's say you want to make baseball bats. You need to use a simulation to develop the motion process that will create the baseball bat, that geometry. Or, if you want to make the blade of a fan or a turbine, you can make those kinds of geometries much more quickly and easily than with the current commercial processes, because you don’t need a mold, you don’t need to remove the part from a mold, you don’t need to clean it up, no specialized equipment, it's just a bath. You pull your object directly from the liquid and you have the geometry — it's kind of magical.

We're also currently developing new materials that can be a little bit rubbery, which might be used for shoe soles. You wouldn’t have to mold them and cut them into shape. You create it in the open and it takes the shape you want.