Tech Briefs: Let's begin with how you got started on the project.
Professor Michael Dickey: Well, that's a good question. The truth is that I had a really good student who was curious and kind of got us going down this path. But there's a bit of history that preceded that. Our group has been working about 15 years or so on liquid metals. Usually when I tell people liquid metals, they think of mercury. But these are actually metals made of alloys of gallium.
If you look at the periodic table, gallium is right below aluminum. You probably remember from freshman chemistry that if things are in the same column in the periodic table, they're like brothers or sisters. So, gallium and aluminum are very similar, except that gallium has a very low melting point. We've been studying this material, mostly trying to come up with ways to pattern it, because it allows us to make things like electric circuits.
At the surface of liquids, including water, surface tension typically causes them to form spherical shapes. But the liquid gallium has a kind of funky shape. The reason it's in that shape is because the metal reacts with the air and forms an oxide. If you take a little bit of acid, the vapors are enough to dissolve away the oxide. And without it, the metal beads up like a sphere.
So, this has been the last 10 years of my life.
We've been using that phenomenon to our advantage. Basically, a thin shell quickly forms on the metal and is strong enough so that you can actually 3D print it. This is interesting because you can print metals at room temperature and make shapes. That would not be possible if you tried to do this with water or coffee or pretty much any other liquid that you come across in your day-to-day life.
Tech Briefs: Is this a gel?
Dickey: You asked about the background — this is the space we were working in. When we dispense the liquid from a nozzle, it comes out as a droplet, because the bulk way that this stuff flows is like water, but then it forms a shell on the surface. For a long time, I would have preferred that this stuff dispense more like toothpaste or like a gel.
The challenge is how do you get it to come out and extrude like a cylinder? You need it to do a couple of things. One is to be able to dispense it from a nozzle. And then once it's out of the nozzle, you want it to hold its shape — those are the bare minimum requirements. But beyond that you want it to be conductive and hopefully have decent mechanical properties. We wanted it to be a liquid so we could use it to make stretchable wires, with the liquid metal inside of a rubber tube. The wires are really stretchy because they’re liquids inside of the rubber. It's stretchy like the rubber but still conductive like a metal.
But I've always wondered whether you could print a metal at room temperature and then have it solidify. And the answer is yes. That's what we did with this metallic gel. For that work, we were inspired by sandcastles, which are built out of grains of sand. It won’t work if they're too dry or too wet, but if they get a little bit of water like wet sand, you can build a castle. The reason you can build it is that there's a little bridge called a pendular bridge, or liquid bridge, that kind of pulls the sand particles together. In our case the liquid-metal-filled spheres would be like the sand particles. So, we were wondering whether we could do this, but instead of big sand particles, we have big copper particles — would the bridges be liquid metal? Again, the answer is yes.
We mix copper particles with the liquid metal and then some water. The pH is 1 because we don't want oxide there so that we can form metal-metal bonds. We just mix these things together, and if you get the composition right, then you form a gel — sort of like toothpaste.
You can imagine if you're an electron, there's now a path to go through, whereas without the liquid metal, the copper particles aren't in good contact.
Tech Briefs: The copper particles forming the bridge — is that what’s required to make the gel?
Dickey: The key thing with the gel is that you need to have a network. Imagine a network where all the particles, or most of them, are connected together. There's a path that goes through the entire material. It’s almost like a skeleton inside of the liquid — a framework that holds the material together.
But you need the right proportion. If there’s too much copper, then it's kind of like what’s called a granular solid, it feels gritty. And if you don't have enough, then you've got basically a liquid because there's not enough to hold the whole thing together. So, there's a sweet spot, not all compositions work. It's got to be the right amount of solid particles and liquid particles. Whereas with the sand castle, the liquid bridges are surrounded by air; here, the liquid bridges are surrounded by water.
When it’s just right, we get a formulation that works, which we call a metallic gel — it really is a gel. It's got gel-like properties in terms of the way it feels, and although it is composed mostly of metals, we are able to print it.
We were able to push it out of a nozzle and print our North Carolina State logo — which is kind of cool. Typically, when people print metal particles, for a number of reasons, it doesn't work. For example, If I had a water bottle and put sand particles in it, the sand is just going to settle to the bottom. If I tried to push it out of a nozzle, the liquid will go through, but not the solid, or the solid is going to jam. But even if you could print it, the particles will typically not be in very good contact, so they're not going to be good electrical conductors. Our metallic gel is naturally conductve with no additional processing.
Tech Briefs: Is gallium naturally conductive on its own?
Dickey: Yes, the stretchy wires that I described were just gallium.
Tech Briefs: Does the copper increase the conductivity? Why copper?
Dickey: Well, that's a good question, and the answer may be that it doesn't need to be copper. However, there are several reasons that I think copper is a good choice. For one, it's commercially available, so it’s easy to get our hands on. Second, the liquid metal and the copper wet each other. They form metal-metal bonds, so the mixture sticks together really nicely. And copper is a good conductor of both heat and electricity, so it's good in that way. It's not the cheapest metal, but it's not the most expensive either, so that's also nice. Another thing, which we didn't anticipate, is that when we're printing this, we think we have basically solid copper and liquid gallium, but over time these things diffuse into each other and form a solid.
I had bad teeth as a kid, so I had to get a lot of fillings, and some of them are metal. The way that the fillings are made is the dentist drills out the cavity, which leaves a hole in your tooth. Somehow, they've got to get the metal into that hole. So, believe it or not, they mix a droplet of mercury, which is toxic, with silver, which is solid. It's kind of like the copper particles we're using. They mix them together and it forms a paste and then they jam it into your tooth. And for a short period of time, it's a paste; it's like a gel. But then it becomes solid. It forms a new phase — an alloy of mercury and silver and a few other things.
So, the point is that our materials are just like a gel for a decent amount of time. But after we print them, they solidify. We didn't initially seek to do that, but it was kind of neat that it ended up forming a solid.
The other thing we did not anticipate, which is pretty interesting, is that when you squeeze this stuff through a nozzle, like dispensing toothpaste, the liquid particles will elongate. When they elongate, that results in an interesting property when the stuff dries. Remember, there's water also, so once we've printed it, the water can start to evaporate. And since the particles are aligned, it shrinks more in the radial direction than it does in the printed direction. If you imagine you have a bunch of rods, the space between them needs to go away because of evaporation. That creates what people call anisotropy, which means it's not the same in every direction.
That can create stress in the material that we can use to make shapes that change. The student, who deserves all the credit here, printed this structure, and based on the way that the beams are laid out, you can get curvature or twisting. We're not the first people to do this — people before us have called this 4D printing. But I think we're the first people to do this with conductive metal gel.
I have mixed feelings about this whole 4D printing thing. The basic idea is, you 3D print an object and then the fourth dimension is time. So, it's changing with respect to time, and in some ways that’s good and in some ways it's bad. You might think well, I already printed what I want, I don't want it to change. It turns out that if you don't want it to change, you can just leave it at room temperature, and it will stay in shape. But if you heat it, that drives the water out faster and you get the stresses that cause it to change. My student decided to make a little spider that would self-assemble and 4D printed it so it would change shape over time.
We started with liquid metal — there are some real nice benefits to liquid metal, but there are also some drawbacks. It’s interesting because we can print at room temperature, it's conductive, and it also mainly ends up being a solid — to me, that's the unique thing. There are lots of ways to print plastics and rubber and stuff like that, and there are ways to print metal. But those use very expensive equipment at high temperatures, while this is super simple — it's like dispensing toothpaste.
Tech Briefs: What makes the objects solidify after they've been printed? Is it just the water evaporating over time?
Dickey: Yes, the water evaporates, which happens pretty quickly. Although we haven't really studied this aspect of it, I'm pretty sure I know what's happening. I think the gallium and the copper diffuse into each other, forming an alloy — a copper-gallium tube that's a solid. It's very much like what happens with dental fillings. With fillings you wait for a short period of time, it's like a paste, and then it forms this new phase. So, that's something we're starting to study more now. But I think that's exactly what's happening.
I had initially thought the mechanical properties were not going to be so good because, if it's toothpaste coming out, it's probably not going to make a very sturdy spider. But over time it solidified to the point where the student had to cut it with a saw to make it into the shape to make the measurement. So, it's not as good as steel or anything like that, but it's better than other plastics.
Tech Briefs: When you heat it to change the shape, does that affect the mechanical properties?
Dickey: We haven't studied that, so I shouldn’t speculate too much, but my guess is no. The reason it’s changing shape is something that I don't fully understand myself, but I think what's happening is that, number one, you're evaporating the water, but there's also an ingredient that I didn't mention. There's a little bit of polymer that we add because it makes it easier to dispense from the nozzle. So, just to be clear, we were just trying to print metals, we weren't trying to do 4D printing. It was only when we got impatient and started to heat the water that we saw the 4D printing. But I think that the polymer is shrinking. Imagine a wet sponge — if you let a wet sponge dry it, it shrinks. And that shrinkage is what's creating the stress. I think the polymer is causing the 4D printing aspect. Since there’s such a small amount of polymer, it's not really contributing to the overall strength. So, I don't think the heating could cause a change in the mechanical properties, although it may cause it to solidify faster.
Tech Briefs: You’ve said that you could predictably control the shape. How do you predict what the shape will be?
Dickey: To be honest, we stood on the shoulders of giants. There were people before us who, although they were not printing metallic materials, they were printing polymers that had little rods inside of them. When they pushed the material through the nozzle, those little rods all lined up, so they were able to get the same kind of anisotropic drying that we see. And they used some really hideous-looking math to figure out the way that the stresses would cause things to shrink. I don't know a simple way to describe it, but these are lattice structures. If you imagine two rods aligned in different directions, when one shrinks, they will get closer together. If it does that anisotropically, you can imagine instead of them all just coming together, it could create a curvature in one direction based on the stress of it shrinking, how it pulls all of them together.
The short answer is people had previously done this with other materials, so we just said, well, it works, let's copy their design. I like to mention that because, for one, it's a beautiful way that science works — you build on other people's work.
But the other thing that's nice is that sometimes I worry that when people see our videos, they think OK, it changes shape. But my student said: ‘I want to make a spider, and I want its legs to have this shape.’ Of course, there's a limit to what we can do, you can't make just any old shape, but the point is he intentionally made the legs bend in one direction based on the way he printed it.
Tech Briefs: Have you been thinking about applications?
Dickey: There are a couple ways to go with this question. For the longest time my motivation was what I call multi-material printing. Typically, the name multi-material means you're printing more than one material at a time. Particularly with commercial applications, when people say multi-material, they usually just mean, ‘I'm going to print polymer A and polymer B,’ but it's still polymers. As far as I know, there are no good examples, at least in the consumer space, where you could print polymers with metals.
What I've always wanted to be able to do is print plastics, metals, and other types of materials together, ideally without sacrificing any material properties. That's been a problem, because although we’ve been able to print metals, it requires such enormous temperatures that you just can't print them with plastics.
Why would we want to do that? Well, most things in our day-to-day lives are made from multiple materials. For example, I’m sitting at a table that's got a metal stand and a wooden surface. Most things that have any sort of function in our day-to-day lives have multiple materials.
That was one of the big-picture motivations. The other was that although the conductivity and the mechanical properties are not as good as pure metal yet, I was thinking initially that if it stayed as a metallic paste or a metallic gel, you could use it to print conductors that are stretchable and deformable.
We kind of stumbled into it, but now that we know we can print solids, I think that's also interesting because you could print more structural materials, like a little Eiffel Tower or something.
I was motivated by wanting to do something that no one's ever done before, not ‘There’s a problem let's go solve it.’ There wasn't a particular application I had in mind. That's kind of how I like to do research. I like to think, ‘What is something that nobody has ever done before and how can we do it?’ And then I know if we can do it, something good is going to come out of it.
One of the special things about our material is that it has both thermal and electrical conductivity. So, for the thermal conductivity, you could use it for removing heat from, say, a computer chip or something. In terms of the electrical conductivity, right now the smallest dimensions we can print are nowhere close to what's being used for making things like transistors and computer chips. But you could print conductors, although we're still a little bigger than the conductors that you would find on printed circuit boards. But they're in the ballpark. So, if you're a hobbyist, you could use it to add, let's say, antennas or sensors, or even structural elements to make your 3D printed part stronger.
I think the applications are really just limited by creativity.
Tech Briefs: Do you want to add anything?
Dickey: I want to say that my student really gets the credit here. I'm just so lucky to have a job where I get to work with such amazing people. I consider myself the middleman, the people who deserve the credit are the students.