New nano-materials bring opportunities for constructions of all sizes
Julia Greer and her group of researchers are experimenting with new materials at the nano scale. This is bigger than the atomic scale, having a varying thickness of roughly one thousand atoms. The new material is assembled in a lattice. In this form, and at the nano scale, a material’s properties change. All metals become stronger. And materials come with other new properties. Some exhibit bounce back in shape following compression. And they can be printed at smaller scales. Current 3D printing has a lower limit of about 50 micrometres. Printing with new nano-materials can be achieved at almost 200 times smaller. As Julia Greer explains, today’s building materials can come with flaws such as low fatigue resistance. This will be a thing of the past with new nano-based building materials.
Robyn Williams: The Science Show on RN. And so back to nano.
Did you really make a TARDIS out of your materials?
Julia Greer: We definitely did. I had a student who was a big fan of Doctor Who and he's actually the student who almost is the pioneer, he's the one who started this entire nano architected materials field, at least in our group, and he was having fun one day and so he actually wrote all the characters, there's the phone booth, there's the TARDIS, he wrote all the characters of Doctor Who just to show that we are not limited to periodicity. So I guess Doctor Who was our big claim to fame at that time.
Robyn Williams: Like writing 'IBM' in atoms, nano-style, at Caltech they do Doctor Who and the TARDIS in atoms because that's where they make the new materials from molecules upwards. Meet the amazing Professor Julia Greer, who is like a film star on skates.
You have the most wonderful office. In fact you've got a most wonderful door which is covered with drawings and letters from your daughters, I presume. Is it your daughters?
Julia Greer: They are, yes. I have two daughters, their age is seven and 10 it now, but when they first started producing art they were about six months, and so you can see that there are layers of different kinds of art on the door, and so as you start peeling them you can go back in time.
Robyn Williams: Excellent, and all over your office as well, plus some models. And I must say that, Julia Greer, when you arrived you were on skates. How come?
Julia Greer: So I am perpetually late for every meeting in my life, so I usually rollerblade to try to get to multiple places at the same time. I still haven't mastered the art of being in two places at the same time, so I still try to just hurry, hurry.
Robyn Williams: You don't do entanglement.
Julia Greer: Not yet!
Robyn Williams: Well, one of the wonderful things about your table, which has got a model of a bear on it for some reason, plus all sorts of models of…is it materials?
Julia Greer: Yes, so what we make in our group is nano architected materials, they are lattices in three dimensions. And in a normal material when you start zooming in with a microscope, for example, at some point you get to atoms, so you zoom in to microns, which are one-millionth of a metre, and then eventually you get to a nanometre. And right around that length scale, which is a one-billionth of a metre, you start detecting interactions of individual atoms.
In our materials we built in an extra level of organisation much before you get to the atomic scale, so you start seeing lattices with dimensions of several microns. And that organisation, which we are calling an architecture, can be based on little beams. So it's effectively like nano Legos. So as you construct your lattice out of little nano Legos, each individual component may have different properties because at the nanoscale material properties change. And to capitalise on these properties you have to build it up into a larger structure which we are calling an architecture where the dimensions of the solid, anywhere you poke at the structure, are still just that little wall thickness of the beam. And so that single material embodies every length scale, from some nanometres, to microns, to tens of microns, to eventually millimetres and centimetres, and so you can build up this entire office where we are sitting right now full of these nano lattices and it will weigh as little as an eyelash.
Robyn Williams: As an eyelash? How extraordinary. But how do you get them into place because you can't do it individually bit by bit, how do you train them to go where they are supposed to?
Julia Greer: We do actually tell them exactly where to go. So we use a computer model to design specifically the architecture that we are interested in making, and we have reasons for making different kinds of architectures. It could just be…imagine a square cubic lattice, just in three dimensions. It could be very sophisticated geometrical unit cells which we call maybe call tetrakaidecahedrons, some kind of a geometrical feature, and then you can tessellate it in space in the three dimensions, and we are in fact able to control it so well. Like I said, we can write a little Doctor Who…we can write a little nano TARDIS and characters from Doctor Who, and I can write a nano version of you, of any kind. So to us, complexity comes for free effectively. So writing a simple square cubic lattice versus writing a nano version of you takes about the same amount of time. So as long as we can design it in the computer, we are generally able to make it. Now, what kind of material you make it out of is a whole different story, and so that of course the writing process, which is similar to 3-D printing, is done in a polymer resin. We write these scaffolds out of polymer. What we often do after is we use some kind of a post process to convert it into a different material.
Robyn Williams: And what do you use them for? What's it all about?
Julia Greer: You know, there are so many uses. Sometimes you make a thing and you immediately see the one use. We are on the opposite end of the spectrum. They are so applicable to such a wide range of opportunities. For example, they are tremendously good at energy absorption and that is if you compress them by more than half of their height, maybe even 80%, they will spring right back. So this is the kind of material you would want to have in your helmet, or this is the kind of material that you'd want to have in your vest, say you were a soldier in the field or something like that and you need to have a bullet-proof vest, usually those things are tremendously heavy so you have to load yourself up and run with that.
So what our materials will eventually enable us to do once we scale them up is to effectively print these sheets and then you can stack them up like a textile to develop something like the thickness of your shirt, but it will have the bullet-proof properties that you may want. Or imagine people who have real winter…here in California we don't really have a winter, but if you live somewhere north or in a place where it's cold, these puffy very heavy winter jackets, the reason why they are so large and so voluminous is because of the thermal insulating properties. So imagine now we are able to write a thin sheet of paper that you can roll up and put it in your purse or something like that that is just as warm.
Robyn Williams: Could you make a battery out of it, could it store energy?
Julia Greer: Absolutely. In fact I have quite a few students working on batteries. So these micro lattices can actually revolutionise the entire battery technology because they offer a platform to make something that is all solid state. So in a typical battery you usually have a lot of liquid called electrolyte, and it is effectively wasting space. So first of all the electrolyte is unsafe and unstable. Most of the fires that happen in batteries come from the liquid being flammable. So what our three-dimensional platforms enable us to do is to coat layers of active materials, the anode, the cathode and the electrolyte, all in solid-state form, and they are very, very porous, so all of your products have enough space to go into them. And so these batteries are exceptionally lightweight, they have the energy density and the power density that can exceed the conventional lithium ion batteries and they are fully rechargeable, and so they will maintain their capacity over many, many cycles. That is what we are working towards. This is actually a very exciting pursuit in our group right now.
Robyn Williams: Imagine the possibilities. Professor Julia Greer at Caltech, creating a materials revolution, even with metals.
Julia Greer: One thing that we just figured out, and in fact a press release is just coming out today, we just published a paper in Nature Communications, we figured out how to additively print metals. So up to this point there have been processes for additive manufacturing of metals, but they choke at about 15 microns. So that means that you can make a relatively sophisticated part for the airline industry, for anything that requires a very small sophisticated screw, for example, or for a space application, something that is very exotic, you can certainly print a few of them but you can't get too small. So if you are making a device or if you are making a tiny stent to put into your eye or into your ear, so say you don't want to use a hearing aid and you want to just write a very small Cochlear-type bone-like material and directly insert it into the ear, we are now able to do that because we can write these sophisticated shapes out of the material that we want.
And so up to this point it hasn't been possible to additively manufacture metals with dimensions below about most reliably 50 microns. But we are able to get them down to the beam dimensions of 300 nanometre, so we are pushing it way down into the nanometre range. So we are now going to be able to print all these devices in one shot, so you don't have to have these exhaustive multistep processes, each of which of course has the probability of accumulating defects or shorting the device or killing the device somehow, but we are now able to just print it. And so 3-D logic circuits and many, many other devices.
Robyn Williams: That's extraordinary. But you talked about defects. Isn't it possible that at that nano level where the behaviour of materials, of atoms, is different, they muck around in ways that are almost boggling (I mentioned entanglement), those sorts of strange things, can't they go off and do their own thing in a funny, strange, unpredictable way?
Julia Greer: Well, we are not quite that small. The unpredictable way happens when you put an external stimulus at the atomic level and then you don't really know. Maybe there will be atomic diffusion, maybe the electron hopping will be different. And so then there is some unpredictability. Luckily we are still at the nanometre scale so we still have hundreds of atoms. And when you have that many atoms playing collectively together, the probability of losing one goes way down. Or rather, if you lose one it's okay.
So one thing about these nano architected materials is…you mentioned defects, so they are all beam based, so imagine you take out one of these beams or one of these Legos. Is that a defect? Probably not, it will still stand. What if you take out two? What if you take out three? What if you take out ten? So at what point does it become a defect? And so that's one thing that's a very interesting research pursuit in our group, which is effectively how many beam members or nodes (we call them nodes because it's a junction) do we need to take out before we start losing the beneficial properties? And so what we are finding out is that it's actually quite a few. So it is exceptionally resilient against defects, which is a nice thing.
Robyn Williams: Were you inspired slightly by human, possibly biological examples such as bone?
Julia Greer: We were hugely inspired by…in fact many of our materials are bio inspired. So a lot of the hard biological materials like seashells or turtle shells or anything that requires protection. There are these exceptional sea creatures called diatoms and they seem to be relatively useless, but they live in every ocean and everywhere and they range in diameter from 20 to 500 microns, and we have no idea why nature designed them to be so intricate. They have these layer-by-layer architected features like holes and walls. And if you look at nacre, which is the pearly stuff on the inside of a shell, it has this brick and mortar-type construction.
And bone, like you mentioned, human bone, is a particularly sophisticated example of hierarchical organisation. It goes from 2-nanmetre fibrils that are effectively interwoven fibres of collagen that are then organised into fibrils which can be in the order of 100nm to 200nm which are then self-organised into osteons which can then be on the order of microns, and then further and further. So the layering, even in something as mundane as human bone, is unbelievably sophisticated. And what we have discovered is that each level of hierarchy carries a particular meaning. And so there is a purpose to each of this.
The properties of materials absolutely must be assessed at the relevant length scale. It doesn't make any sense to take something as complicated as bone and to only do mechanical tests at the level of the entire bone, as in break the entire bone. Of course that's what matters if you are a human being. But if you are really trying to understand how to make either better prosthetics or how to make better scaffolds so the human bone will grow onto it, you really need to break it down to the individual levels of hierarchy at each individual length scale.
I'll give you an example of this. We discovered about a decade ago that metals, when you reduce their dimensions to the nanoscale, they become tremendously stronger. So take something as malleable as copper or gold or something like that and reduce their dimensions to about 200nm, which is one-ten-thousandth of a hair diameter, and it becomes as strong as steel, and there's nothing else that you have to do. So you just reduce its dimensions. And then you look at that and you compare it to other metals, and it turns out it's a ubiquitous size effect. So at the nanoscale, all metals, as long as it's just a clean, pristine chunk of metal, they all become stronger. And the smaller/stronger phenomenon, turns out that it's prevalent in many other materials, including bone. So what we discovered is that strength almost always benefits from being tinier.
Toughness, which is the resistance of material to fracture, to failure, doesn't work this way at all. The smaller you make the material, it doesn't change at all. It's because it's the collective combination of all of these mechanisms together that gives rise to toughening mechanisms. You probably know that antlers and teeth and bone and all those kinds of hard biomaterials, they are actually very hard to break, they are meant to be hard to break. But it's not because each individual part in this small scale is tougher, it's because when you combine all of those individual little nano parts, combine them together into this hierarchical incredibly sophisticated fascinating construct, that's when the toughening occurs. And so strength and toughness are very much orthogonal to one another.
Robyn Williams: It sounds like a revolution in materials. When are we going to see something that's ready, that's made?
Julia Greer: Okay, so our biggest challenge right now is how to scale them up. So we are able to make tiny, tiny samples, and you can see sitting on my desk here, the tiny ones are probably…the largest one that we are so proud of would be maybe half a centimetre by half a centimetre. So you are not going to make a protective vest out of this, right? And for batteries you certainly need much larger electrodes, possibly for implantable biomedical devices, those are okay, but it takes us 12 to 20 hours to write each one of those.
So the biggest obstacle to the insertion of these materials into real technological applications is scalability. What we are trying to do now is to scale up their production, and in fact we just started a company that is aiming to do exactly that, to print sheets of paper effectively, something that looks like a sheet of paper, but it's untearable, unbreakable, it absorbs energy, unwettable, and it has many, many other beneficial properties. We need to figure out a way to print sheets of these nano lattices, and then you can think of them as textiles. You can cut out whatever shape you want, you can sew something that you want, you can form factor them into anything that you want and then start using them for whatever the desired application is. So it's a very 'materials by design'-type of an approach.
Robyn Williams: And if it's happening at Caltech, it's likely to be a winner. Julia Greer is Professor of Materials Science there. Astonishing woman. On skates.
- Julia Greer
- Professor of Material Science, Mechanics and Medical Engineering
Division of Engineering and Applied Science
Pasadena CA USA
- Robyn Williams
- David Fisher