How A Liquid Metal Could Transform Soft Electronics with Michael Dickey, professor at North Carolina State University

(Intro Narrator at 00:00:03) Hello, my friends. Today, we're talking to Michael, professor at North Carolina State University, and we discuss the properties of the metal element known as gallium, where gallium is being used in the marketplace, and how it could revolutionize efforts to reduce carbon dioxide in our atmosphere. All of this right here, right now on the Modern CTO podcast.

(Joel Beasley at 00:00:33) Here we go. This is the Modern CTO podcast. So what is that? There's metal or something? What is this stuff?

(Michael at 00:00:46) Yeah, so the heart of it is gallium. Gallium is just an element that's on the periodic table, and what makes gallium really special is that it's got a low melting point. So if you were to hold it in your hand, it would melt. But I have a little bit of it here.

(Joel Beasley at 00:01:05) Is it toxic?

(Michael at 00:01:07) No, so that's kind of the other funny thing. Usually when I tell people I work with liquid metals, they think of one of two things. They either think of mercury, which is toxic, or they think of the Terminator, which is, you know, scary bad guy. And so, no. I always have to be a little bit careful because I don't think that gallium has been super carefully studied, but one way to think about it, or what I tell people, is that the melting point of a material doesn't correlate with its toxicity. So if you think about water, water melts at zero degrees Celsius, but that's obviously not toxic. The only reason people think metals would be toxic is because of mercury, which is toxic. Gallium is not found in our diet, so it's not something that you would have naturally in your body. So I usually tell people to be a little bit careful. But if it does get inside your body, your body sort of treats it like it's iron. So if you take—my wife sometimes takes an iron supplement to keep from getting anemic, and that ultimately makes its way through your body, and gallium does the same thing. So there's that.

(Michael at 00:02:12) And then the other kind of weird thing about it is it has no vapor pressure. So what that means is, you know, if you take a shower and you dry off with a towel, you hang up the towel, the water will evaporate. You come back the next day and the towel is dry. But this does not evaporate, at least not at room temperature. And so that's important when you ask about the toxicity because it means you're not going to worry about breathing it. So, you see, I mean, I've got this right here in the office. I would open it, but—

(Joel Beasley at 00:02:38) And if you ate it, it would just be like iron?

(Michael at 00:02:41) Well, I mean, I wouldn't—I probably wouldn't eat it. I've gotten it on my skin before, not on purpose. But yes, it is considered to have low toxicity. So we had a collaborator who came to me a few years ago. He was a really brilliant guy, but he had this idea of using liquid metal for drug delivery. And, yeah, I looked at him the same way you just looked at me—interesting, but kind of a weird idea. And it turned out to work. And as part of that study, we—well, when I say we, it was, you know, a collaborative effort—but we made little particles and put them into mice. And the mice were able to—we're able to kind of figure out where it goes. And, also, just kind of a funny story. I had a doctor contact me, gosh, it was probably about a decade ago, and they were interested in using liquid metal for wires for pacemakers.

(Michael at 00:03:29) So it would literally be inside the body. Now it would be encapsulated in rubber or something like that. But the idea is that if you have a pacemaker, there's a battery source, and then there's the pacemaker that's sending the signal to tell your heart to beat. And that wire looks kind of like a spring. And 99.9% of the time, it works great. But if it breaks, that's bad news because that means your heart's not getting the pacemaker signal. So he had the idea of using liquid metal as a wire to sort of be indestructible so it could stretch, and that's one of the things that's really cool about it. But anyway, one of the concerns was whether it was going to be toxic. And so he said they were going to do some experiments, and I told him, "Oh, that's great. I'd really love to see the results of those." And he said, "Okay." And then I didn't make much of it. Well, about a month later, I got a package in the mail, like a FedEx envelope, and I took it out, and it looked like shrink-wrapped chicken, kind of like flesh or something. And I was like, "What is this?" And so I contacted the doctor and said, "I got this package. I can't make any sense of it. What is that?" And he said, "Oh, that's the rabbit." And I was like, "When I said I want to see the results, I was thinking of a plot or a graph or just a write-up, but I didn't actually need to see them."

(Michael at 00:04:41) So anyway, they—this is a long answer to a short question—but they had injected it into rabbits as part of a controlled study. And the rabbit was fine, other than the fact they had to dissect the rabbit to figure out, to make sure everything was okay. So anyway, I'm still not at the point where I would swallow it on purpose, but I also don't panic if I get it on my skin. Or even if I got a little bit in my mouth, I'd probably—I wouldn't be happy about it, but I wouldn't panic.

(Joel Beasley at 00:05:07) That must be an interesting colleague you have. I didn't know if you should call him your friend, but to ship you the rabbit. Yeah.

(Michael at 00:05:13) It was not the whole rabbit. It was like what you'd see if you bought chicken at the grocery store or something.

(Joel Beasley at 00:05:19) Well, he did the work for you, right? He cleaned it for you?

(Michael at 00:05:21) Yeah.

(Joel Beasley at 00:05:23) Did you ultimately get the data on it?

(Michael at 00:05:25) You know, it wasn't like a full-fledged study as far as I know, but they injected it in the rabbit. Of course, when you inject anything into your skin, you're going to get a little bit of irritation. So I think there was some irritation, but it didn't cause the rabbit to die. It didn't cause any—I'm not a doctor—but it didn't cause anything to cause concern. And actually, gallium has been FDA approved, so that's kind of the ultimate thing. But it's been FDA approved for different pharmaceutical applications, and it was FDA approved as an MRI contrast agent. So when you go get an MRI done, but it turns out there's ones that work better, so we don't use it. For a while, it was explored as a replacement for mercury for—actually, I just went to the dentist yesterday and got a filling. Now they don't use metal anymore, but when we were kids, you know, they use metal amalgams, and those use mercury. So they looked at gallium as a replacement.

(Michael at 00:06:13) So there's kind of been a bunch of here-and-there studies where people have looked at it. But all of these things that I'm mentioning to you are—it's kind of a subtlety—but, you know, what I'm showing you here is gallium metal. But if you take a vitamin that has iron or magnesium or any of these micronutrients that you hear about, those are metal salts. Just like if you put table salt on your french fries, you know, sodium is a metal, but when you're eating it on your french fries, you're eating a sodium salt. And so those metals don't have really a way to dissolve into your body. It's the salts that are ultimately being used for a lot of these applications. But for that one study I mentioned, we did put actual blobs of little blobs of liquid metal in the body, and that was also fine. Again, for your listeners, I wouldn't recommend eating it.

(Joel Beasley at 00:07:05) And is it being used in any commercial application today?

(Michael at 00:07:09) Yeah. So there are, and it's kind of funny the timing of this talk. Earlier today, I was just hosting a group of, I don't know, eight or so people that flew down that work at a company that makes some of these materials, and they use them for a number of different applications, and they're here just visiting. But probably the most high-profile one that just came out recently was in the Sony PlayStation. They are using liquid metals to help dissipate heat from the computer chip that goes inside of the PlayStation. So there's videos you can find online. They call them delidding videos where they take apart the PlayStation. And sure enough, they have liquid metals in there. And the idea is that as transistors get smaller and smaller and you pack more and more of them onto a chip, it generates a lot of heat, and metals are a really good way to remove that heat. That's not the most exciting example, but the point is that there are examples where people probably have this in their house, and they probably don't even realize it because it's inside the packaging.

(Michael at 00:08:06) Gallium itself is used for some other things. It's used as a precursor for some semiconductor materials called gallium nitride, gallium arsenide. These are high-performing semiconductors, so there's that. People use them at research universities for a very sophisticated tool that's called a focused ion beam. Have you ever heard of that?

(Joel Beasley at 00:08:26) No.

(Michael at 00:08:26) No. So they call it FIB. It's a beam of—it's inside of a vacuum chamber—it's a beam of ions that they literally focus down, kind of like a laser beam but with ions, and they can use it to cut materials. So it's pretty wild. So those are things that are in existence now that use this particular material. But what we're trying to do, and other people, we're trying to use it for more futuristic things.

(Joel Beasley at 00:08:53) Like what?

(Michael at 00:08:54) Yeah, like what? So one of them is trying to make electronics that are soft or stretchable, soft and/or stretchable. So, you know, most electronic devices that you encounter in your day-to-day life, like the computer that we're talking on now, your cell phone, all these things are made out of rigid materials. But our bodies do all these amazing things, and they're essentially soft. If you take away the bones, the stuff that does most of the work, like your brain, your fingers, your muscles, all these are made out of soft materials. So part of this is sort of a little science fiction-y, you know? Can we use that as inspiration? But I think there's also some practical things. People are looking at wearable devices or making electronics into clothing that could actually deform, be worn comfortably on the body.

(Michael at 00:09:41) There's also this entire little subfield that's called soft robotics, and I'm kind of on the periphery of it. But the idea is to try to make—you know, when at least when I think of a robot, I think of something that you might see in a car factory with sparks shooting and these robotic arms moving around. Not a very safe place for humans. But there's this idea of trying to make robots soft, more like an octopus, because you have sort of unlimited degrees of freedom of motion. You know, an octopus can fit through a beer bottle. It can contort itself. It can wrap around and grab things. It can do all these complex things, but it's completely soft. And so, you know, if you can start imagining putting sensors or other electronics inside of a material that's like that, that's also kind of of interest. Actually, part of the group of people that are here today, they're interested in 3D printing. So, you know, there's these 3D printers that can print plastics, print polymers, but they're not compatible with printing metals. So one of the videos I can show you later, we take 3D-printed parts and then we inject liquid metal. So it allows you to pattern the metals in ways that just aren't possible with copper or aluminum because it's a liquid at room temperature. And I haven't even said this yet, but probably the most important thing for us and the reason a lot of this works is that the metal reacts with air on its surface, and it forms a skin on its surface. Actually, let me—you'll appreciate this. Let me—it's just right over here—grab it. Yeah. So this is just gallium, and my student molded it into the shape of North Carolina. And I'm holding it like this because if I held it, you know, like this for too long, it would melt and turn into a puddle.

(Michael at 00:11:20) And the viscosity of this metal, when it melts, it's like water. So think of it like a metallic water. Another interesting thing about this material, it's got the largest surface tension. So remember, it doesn't evaporate. That's very, very weird. It's got the largest surface tension of any liquid. So to demonstrate that to you, you know, if you take salad dressing, like oil and water, and you shake it, it forms a bunch of little droplets. Well, this one, if I shake it as violently as I can—well, it did form a couple of droplets, but it very quickly goes back together. So the surface tension of water is—I'm not going to tell you the units because it's not important—but it's about 70. This is like 700.

(Joel Beasley at 00:12:02) Wow.

(Michael at 00:12:02) So it'd be like walking around on Earth and seeing a bunch of people that are five feet tall, six feet tall, five feet tall, six feet tall, and all of a sudden, someone that's 50, right? It's the same—

(Joel Beasley at 00:12:13) Dwayne Johnson.

(Michael at 00:12:14) Yeah, like 50 is like king of the rock. So that's very unusual. It's not like shades of gray. Most liquids you encounter day to day are down here, and this one's way up. But anyway, so this other one, which is exactly the same material, but because it's reacted with air, it forms a shell on its surface. So, you know, think about what happens to a waterbed. If you remove the shell of the waterbed, the liquid just flows out. But this one's got the shell that holds it in that shape.

(Joel Beasley at 00:12:38) How tough is that shell? Is it like normal metal?

(Michael at 00:12:41) That shell is an oxide, so it's actually a glass. So it's not very strong, and it's only three nanometers thick.

(Joel Beasley at 00:12:48) How did you get introduced to all of this?

(Michael at 00:12:51) I was very, very lucky. So actually, my PhD, believe it or not, was on a different topic. I was studying polymers, which has come in handy, but I've kind of gone off the reservation a little bit here. So I didn't even know what liquid metals were when I was a PhD student. But one of my friends, a good friend during my postdoc—postdoc is like an apprenticeship that you do between once you get your PhD and then before you become a professor—so I met somebody then, and still a good friend to this day. And he had made the observation that if you took these liquids and you touched a drop to a surface and then you pulled, it acted more like bubblegum. In other words, it formed like a cone shape. They call that necking, where the liquid kind of—tension. Whereas if you do that with a droplet of water and you touch a droplet of water, I mean, heck, I can do it right now. I've got my water here. You know, if I touch a droplet to my hand and then pull away, I just end up with two puddles, one puddle on the tip of my finger and one on my skin. And so he made this observation that it formed more like what bubblegum would do. And so that raised the question, "Why is it behaving this way?" And, well, the answer ends up being pretty simple. It's reacting with air, and it's forming a solid shell on its surface.

(Michael at 00:14:08) Oh, by the way, I didn't mention this, but that shell forms so fast that it's basically immediate. So one time on accident, we accidentally squirted some of the metal out of a syringe, out of a nozzle, and it came out and it formed a fiber. It formed a wire. So, you know, when you turn on your garden hose, it forms a cylinder of liquid, but that cylinder eventually breaks up into little droplets because of surface tension. This is like shooting a liquid out of a hose and having more hose form around the liquid. It's like forming its own shell as it goes.

(Joel Beasley at 00:14:42) Oh, because the syringe or the nozzle was metal?

(Michael at 00:14:45) Well, the syringe was metal, but you just—when you shoot it out, it starts reacting with air. And the air reacts and forms this shell as you go. So you can sort of form its own container in a way.

(Joel Beasley at 00:14:58) Have you looked at it performing that action under a high-speed camera?

(Michael at 00:15:02) We have done some—we do have a high-speed camera in our lab, and those are super fun to play with because you can see—I think a normal video plays at like 30 frames per second, like probably what we're seeing on the computer. But these can do tens to hundreds of thousands. I think the state-of-the-art cameras can do a million frames per second. It's just incredible. And we have looked at that, but just not that particular process.

(Michael at 00:15:24) What we have done is we've looked at what happens when you bring two of these droplets together. You know, the question is, like, do they sit on each other kind of like building a snowman where you just stack liquids, or do they merge together and form a bigger droplet? And the answer is it sort of does a little bit of both. It merges together, but then it stops.

(Michael at 00:15:43) So you end up with—it kind of looks like a figure eight where they are. They merge together, but not into a single drop. So we've used that actually to our benefit to make self-healing circuits.

(Joel Beasley at 00:15:53) Wait. So they penetrate each other's shells, but they don't fully merge?

(Michael at 00:15:57) Yeah. So I believe—what it's really hard to study because you can't see inside. You can't see what's happened when you touch the material together because it's a metal. It's shiny. So you can't see it optically. A lot of like the what we call spectroscopic techniques don't really work.

(Michael at 00:16:13) So we kind of infer it based on the way it flows and also what happens when you pull them back apart. So if you touch two droplets very gently, you can pull them back apart. But anything other than that, they actually will merge back together. And I think what's happening is that very thin shell is breaking. Again, it's not a perfect analogy, but it's sort of like touching two water beds and then the water connecting between the beds.

(Michael at 00:16:36) Now the question is what happens to that shell of the water bed. And what I think is happening is it's just breaking, and you're able to make metal-to-metal contact. So we've actually used this—and there's another cool video we can see—but where you can take a wire of this stuff and cut it. And, you know, when you cut your skin, sometimes blood comes out, but usually it forms a scab.

(Joel Beasley at 00:16:57) Hopefully. Hopefully.

(Michael at 00:16:59) I don't know what else could come out. But yeah, when the blood does start coming out, it forms a scab. And actually, part of that is—there's a cascade mechanism whereby that happens, but it's triggered by oxygen. And this is sort of the same thing. When you cut it, the metal gets exposed to more oxygen and forms a new skin, and so it doesn't leak out.

(Michael at 00:17:19) So like this thing that I showed you, the little sophisticated circuit, if you were to cut this with scissors, the liquid doesn't come out. It stays flush with where you cut it. So then this is just—this is actually sort of boring. It's just silicone, kind of like bathroom caulk. It's real common.

(Joel Beasley at 00:17:35) I can see why you're so interested in this stuff and you're spending time. I get to talk to scientists, you know, here and there or researchers and professors. I can almost always see like the full extent of it, you know, in the conversation. I'm like, oh, okay. Cool. I kind of wrap my mind around this. This seems like there's still stuff to discover about this material.

(Michael at 00:17:57) Yeah. Yeah. So sometimes I describe this material as like solutions looking for problems because it's got so many unique properties. And sometimes people, they have the same reaction you did. Oh, liquid metal must be toxic. Oh, liquid metal, it must be mercury or something. So I feel kind of funny saying this because, like I said, I didn't even know what it was when I was a grad student, but I'm kind of going around telling people how cool this material is. And we haven't even—I mean, we barely talked about the electrochemistry stuff that's really interesting. It also can do some really interesting things like, electrochemically—I'm sorry. It's called catalytically, so it can help drive certain reactions, including reducing carbon dioxide. So there's some really nice work done in Australia, some of my friends in Australia, where they've shown that you can convert CO2 back into carbon. That sounds like a miracle material. And, of course, you know, there's more to the story. You'd have some energy source to do this. But the point is it does things that just you wouldn't necessarily think of.

(Joel Beasley at 00:18:58) So one of the breakthroughs here as far as science goes is that this is a novel non-contact method to manipulate and shape fluids. Are there other fluids that can be shaped without contact that can also act as like a metal?

(Michael at 00:19:15) No. It's very difficult. So usually when we want to manipulate fluids, you know, we put them into pipes. You know, like if we want to flow water to our house or something, we put it in a pipe and flow it underground. There's tricks to manipulate droplets, like to suspend droplets and kind of get them to levitate and stuff like that. But as far as I know, there's nothing that could—first of all, take a stream and make it into a cylinder and then also just get it to move at will. So I don't know. There could be some kind of cool stuff that maybe could be done here, particularly with like patterning the metal, using it somehow for circuits.

(Joel Beasley at 00:19:50) Dude, this is awesome. This is—you're a really good teacher, man. You're really good at communicating and explaining things, and you're pleasant to talk to. So your students are very lucky because I remember going through school, and it was rare that I would get a teacher that was great like you.

(Michael at 00:20:05) Oh, thank you. There's not a lot of things I do well, but I do like sharing our work and teaching it. But I probably flip that on its head a little bit and say that I'm lucky to work with the students. We have really good students here, and everything I've shown you was really done by students. So yeah, it kind of cuts both ways, but thank you for saying that.

(Joel Beasley at 00:20:23) Yeah. If we've got parents that are—we got a lot of parents that listen, and they might want to direct their kids at some of this liquid metal type stuff. Tell me a little bit about what you would suggest to them as far as this being something in the future that they might want to start early learning about.

(Michael at 00:20:40) Yeah. So they're at the right place if they're listening to this podcast. I do have a YouTube channel where if it's a young kid, I think a lot of times I wonder about the impact of our work. You know, we're scientists. We publish papers. Sometimes I like to joke, you know, I'm happy if 20 people read one of our research papers. And, of course, it's more than that, but it's on that scale, you know, tens to hundreds of people. But these YouTube videos, it really helps make a lot of this accessible. So a lot of the work that we do is essentially funded by taxpayer dollars in many cases. And so I liked putting it out there and kind of giving back and hopefully getting people excited about it. So that'd probably be the first place I'd start. You know, I've got a TEDx talk that I think does an okay job at sort of explaining why the material is interesting. But then, you know, beyond that, believe it or not, you can buy gallium on Amazon. Again, you don't want to eat it. If you eat a little bit, it's not going to hurt you. But yeah, this is not super exotic material. So actually, gallium is right below aluminum in the periodic table. I didn't mention this earlier. And you might remember from like high school chemistry that if they're in the same column, that means they're like brothers or sisters. They've got very similar properties.

(Michael at 00:21:51) But the big difference is gallium melts at a very low temperature. And, oh, by the way, so gallium melts at 30 Celsius, and it boils at over 2,000. It's like 2,400 or something like that.

(Joel Beasley at 00:22:01) I don't know a lot about this stuff. Could it turn into a gas?

(Michael at 00:22:04) So if you heat it up enough, you would start evaporating it. But I think people—in the literature, the lowest temperature where I've seen somebody even be able to measure, we call vapor pressure, is 500 degrees Celsius. So that's way hotter than your like your oven can go at home. Yeah. So yeah, in practice, if you're just playing with this stuff, it'd be difficult. Now, you can react it with other things, and when you react it—when if you cook on a charcoal grill, like you turn a piece of charcoal into carbon dioxide and it becomes a gas. So you could do something like that. People call those like metal-organic precursors and stuff like that. So there's—you can make gases that contain gallium, but if you just bought the metal, it would not be. But anyway, that's another place.

(Michael at 00:22:45) There are, I think it used to be, at least like if you go to Walgreens or, you know, drugstore kind of place, some old-fashioned thermometers. I don't know why anybody would buy old-fashioned thermometers when you can just buy kind of a digital one. But anyway, I have one of these here, and it's got gallium. I think this is actually gallium-indium and tin. And again, those are like adding salt to lower the melting point. So there are—that's another commercial product. It's not super high-tech or anything, and we're trying to think a little bit beyond that. But so those are places where you could if you really wanted to get your hands on some of this stuff. I will mention another—it's kind of good and well, it's an interesting thing, but it and it's kind of a bad thing about this. There were two things.

(Michael at 00:23:27) One is that gallium is expensive. So a gram of this stuff is about, I don't know, a dollar or less than a dollar, which depending on what you compare it to is could be cheaper, expensive. But I can't imagine a world where we're going to be making like power lines with this material. But the little thingy that I showed you here, this is actually an antenna. This is like a few pennies worth of material. So in that sense, it's not cost prohibitive. So but anyway, that's kind of one downside of this material. And then the other—I don't know. I call it a feature. It's going to be good or bad, but it will form metal bonds with other metals and in some cases can do so very aggressively. So in the case of aluminum, it will diffuse into what's called the grain boundaries.

(Michael at 00:24:12) So the boundaries between aluminum crystals. If you have a piece of aluminum like aluminum foil, there's a bunch of crystals that are all connected together, and it will go into the boundaries between those, and it'll cause it to fall apart. So that's called embrittlement. And that's bad news if you have something valuable made out of aluminum because you can literally take a piece of aluminum foil and just—it'll just crumble apart when aluminum touches or gallium does.

(Joel Beasley at 00:24:36) And that's what we should make all the Android robots out of.

(Michael at 00:24:40) Exactly. That's how we'll take them down.

(Joel Beasley at 00:24:42) Yeah. Gallium guns. We're doing it. Got super, super cool gallium.

(Michael at 00:24:46) Yeah. I love it. Because the thing I was telling you, really, you could squirt it out, and it would form a—look like a laser beam.

(Joel Beasley at 00:24:52) Yeah. That's what we could do.

(Michael at 00:24:53) No. It doesn't do it with all metals. Like, we found stainless steel and copper. It doesn't do those things. But anyway, it's one of the concerns people have. It's like, well, if I put gallium in my fancy computer, like, is it going to destroy the computer chip and that kind of thing?

(Joel Beasley at 00:25:10) Is there anything that does that to plastic?

(Michael at 00:25:12) Yes. And from a different mechanism, but not nearly as kind of cool. So there's what people call plasticizers, which are actually these days kind of a no-no because plasticizers are sometimes used in plastics, and they can end up in drinking water and that kind of thing. But plasticizers are just like molecules that can go in between the polymer chains and help them kind of expand and become softer. So there's a great example of this is, you know, PVC pipe that you probably have in your house. Right? It's like really hard, rigid material. That's PVC, polyvinyl chloride. PVC can also be used for shower curtains, which are nice and soft and pliable.

(Michael at 00:25:58) And the difference is the shower curtains have plasticizers in them. Actually, the new car smell—I was told, I don't—I never actually looked this up, but I was told that those are also plasticizers that are in the plastics in the car. So it smells nice, but you're not really supposed to be breathing them.

(Joel Beasley at 00:26:11) That sounds about right for humans. Right?

(Michael at 00:26:13) Yeah. But anyway, it's not as dramatic. The aluminum, it like just crumbles apart. You know?

(Joel Beasley at 00:26:19) So we should write Elon Musk on Twitter and say, hey, when you're making your army of humanoid robots, make them out of aluminum, please. Thank you.

(Michael at 00:26:27) Great idea.

(Joel Beasley at 00:26:29) Well, dude, Michael, man, we made a podcast. How do you feel?

(Michael at 00:26:31) Oh, it's good. It made me feel real welcome. So thank you.

(Joel Beasley at 00:26:35) Thank you so much for listening. And if you found this episode useful, please share it with a friend or colleague who you think would get value from it. And if you have topics that you'd like to hear discussed on the podcast, either add me on LinkedIn, or send me an email [email protected]. Every time I get an email or LinkedIn message, it absolutely makes my day and inspires me to keep going.