 Hello everybody, welcome to a joint bioengineering seminar series. Today, we are hosting Dr. Ben Schultz. He has expertise in the design, processing, testing, and characterization of high-performance and light-weight hybrid and composite materials. These include metal alloys, composite materials, forms, nanocomposites, self-lubricating, and other tribo-bio materials. Biomaterials and materials for additive manufacturing. He has published over 40 peer-reviewed articles on the topic of advanced materials and manufacturing, and he has presented his work to a broad audience including conference attendees, industry professionals, students ranging in preparation from elementary to graduate school. His current research is on bio-resorbable magnesium alloy form biomaterials produced using additive manufacturing and casting processes. His today topic is design and synthesis of metal forms for structural and biomedical applications. Please join me in welcoming Dr. Schultz. Thank you for that introduction. Well, I'm going to try to breeze through my presentation a little faster so that I give people time to get back to where they need to go, and thank you for coming this afternoon. My alarm is going to go off at a point where it's supposed to be five minutes of questions, so that's on purpose. So I'm going to talk about some of my past research on metal foams and syntactic foams, and it's going to be, the presentation is a little bit heavier on the structural side than the biomedical side, because the biomedical side for me is pretty new. I'm getting into this area. And the reason I'm starting with the structural side as well is to give you some idea of how these materials could be designed to mimic some of the properties of biomaterials such as like bone. So to give you that background, I'm just going to get into this. Has anybody taken a course or are familiar with composite materials in this room? Well, composite materials are combinations of at least two materials. One is a matrix that surrounds the material, and then there's another phase that we call reinforcement, and they could be in the form of fibers. There's fiber-reinforced composites everywhere now for reducing weight and a lot of applications. They can also be particulate. So this is a picture of a metal matrix composite that is a commercial alloy used for things like brake rotors. And these little dark specks are silicon carbide particles, and this is an aluminum silicon alloy. And there are a variety of ways of manufacturing these composites. And from a material science perspective, we know that depending on how you process something, you're going to be able to affect the structure and affect its properties as well. So there's a relationship between the process, the structure of the material, the microstructure, and its properties. There's also a cost element of it as well. This is the various ways that we can make a material like this. We can use diffusion processes that require kind of expensive equipment and time. We have powder metallurgy processes where we combine powdered metals and these silicon carbide particles, mix them up, compress them, and center them. And then we have liquid metal processes where we take this metal, we melt it, we add the silicon carbide some way, and then we cast it into a shape. And it turns out that these are going to be the most expensive and these are going to be the least expensive. And that's why kind of traditionally we focused on these liquid metal processes because they're lower cost. So just a really quick overview of some applications of these things. I've worked a lot with metal matures composites like the picture you saw previously. These are some examples, brake drums, brake rotors, drive shafts. And they all offer opportunities to reduce weight. You could potentially replace the steel component with something that's about half the weight and save a lot on energy. My dissertation was on metal matrix nanocomposites and the promise with these materials rather than using micro-sized reinforcements like I showed in the previous slide. I'm using nano-sized reinforcements that can reduce the grain size and also strengthen the material through processes that we describe as Orowan strengthening. And these could, if you were able to get a composite where you have about 17 volume percent of a 10 nanometer size particle fully dispersed into the composite, we could get a 1 gigapascal strength in aluminum which is stronger than any aluminum and rivaling steel. We also have unique properties that are not just strength but also lubrication properties. This is an aluminum composite with graphite particles. So we can add graphite to the aluminum and make itself lubricating, reduce this friction coefficient. There are a lot of tribological problems in the body in joints. When you have a joint replacement, if you're using a metal component that is rubbing against the metal component that will lead to some wear debris and that wear debris can be toxic and cause the implant to fail. So looking at unique ways, creative ways of making those metal components not wear as quickly or not leave toxic debris inside the body, metal matrix composites might be a way of attacking that problem. Thermal applications heat sinks done a lot of work on things like aluminum, cubic boron nitride that have very high thermal conductivity that can be used for things like heat sinks and electronic components. And the topic of today's talk is about metallic foams and metal matrix syntactic foams. These are kind of an interesting class of material and I've got some handouts I'll pass around here. These are very porous metals. The porosity can be greater than 90% in some cases. Metal foams are just, the porosity is just a hole. Sometimes they can be closed so that each hole is separate from each other. And in this case this is an open cell porosity so you can almost, you can see through this if you hold it up to the light. These are extremely light materials and I'm going to pass this around and be careful when holding onto this because it's, don't grab it hard because it has some little sharp edges on it if you really squeeze on it. So that type of material has been used for things like energy absorbers and my current research I'm exploring these types of materials for bone scaffolds that I'll talk about a little later. The syntactic foams are a type of composite foam so instead of having just pores we have hollow particles creating the porosity. And this is aluminum, this is A206 which is aluminum copper alloy, high strength alloy. This has aluminum oxide hollow spheres in it and if you look very closely at this when I pass this around you can see these little pinholes, those are the spheres that have been cut in half. So on that surface you're seeing the holes that are inside those spheres. So I'll pass this around too. So these have all kinds of applications for energy absorption and also reducing the weight. We've made some lead syntactic foams for example that are half the density of lead. So you could save weight in lead acid batteries. And they have some really unique behavior when you look at the mechanical deformation. Metal foams, what they typically do, if you were to crush that metal foam that I'm passing around if you were to plot that on a, are you familiar with stress strain curves? You've taken the engineering courses, introductory engineering courses. Metal foams, when you crush them they do something like this. So they will start to deform elastically and then once they start to yield rather than continuing to go up like most metals do, does anybody know what this is called? This portion of it. It's the reason why it goes up when it's plastic deformation and it goes up because it's strain hardening. So it's getting harder and harder as you're crushing it. Metal foams don't do that, they plateau. And that's because all those little pores are crushing. And eventually it starts to go up to site the metal because it's fully densified. Sintactic foams behave similarly except they have a little dip here. And then they have some things that sometimes go on here where they start to shear and then crush and then shear and then crush and shear and crush until they finally densify. So the analysis that you'll see is based on that. Now this is a Ashby chart. This is showing the relative properties of various foam materials. And this is a specific energy absorption which is the area under this curve divided by the density. And then a specific peak strength which is this value like the yield strength divided by the density. And this is a way of comparing things by weight. This is a metal foam, one I passed out earlier, and these are the syntactic foams. They are capable of absorbing more energy than the metal foams are at reduced weights or more energy in higher peak strengths. Some of our composites are up here. These stars are some of our foams that we've created in the lab. So lots of processes for making these. We focus on these liquid metal processes and I'm just going to skip these slides because we'll get into the more data stuff. So how can we design better composites? One of the ways is of course going to be selecting high quality materials to go into those composites. Another way is to look at things like the structure of these spheres and control the thickness and the diameter of the sphere. So the thickness of the wall and the diameter of the sphere. If you plot the peak strength versus the t by d ratio which is the thickness by diameter ratio we get some plots that look like this whereas you increase the t by d ratio your peak strength goes up. Also as you increase the strength of the alloy that you're working with your peak strength bumps up. Same thing for magnesium alloys that has that relation increasing t by d, increasing peak strength and increasing alloy strength. You have increasing peak strength. We did some work to evaluate this. We looked at the t by d ratio of these spheres. These are some of the foams that we created with various sizes of the spheres. Size ranges. These are the compression curves and we can see that there's an initial peak and then it drops off and then you have some densification events happening here that cause it to eventually crush and densify the composite. Let's see if this, actually I do have a little video. I forgot about that one. This is showing how that crushes. This material is compressed. As you see this, see how it shears on these angles. This is very common shear behavior in metals. What's a little bit uncommon with these foams is that they not only shear in this manner but they shear and fracture these spheres to densify the material. This is some obstacle images and SCM images, scanning electron microscope images of the spheres that have sheared and started to fracture within the metal. You can see, this is a cross section, regions in these bands that are oriented at those 45 degree angles, as you saw in the little video. These images are showing, within these bands the spheres are crushed and they're sheared in relation to each other. So they're no longer, they're not crushed in place but they're actually physically sheared. Other properties that are important, like the toughness, that's looking at the energy absorption. We see effects of the T by D ratio as T by D ratio increases. We have increasing properties across the board. This is toughness. Size ranges of the spheres give you better specific energy absorption with increasing sphere content. We've also looked at high strain rate properties of these materials. His strain rate is, most of these tests that we've been doing are quasi-static tests. It's low and that's what most of your engineering data is based on, what you're looking up in the handbook. High strain rate data is done using specialized equipment to look at how something would behave under a kind of a drastic load or a blast or even something shot at it. So this is a Split Hopkinson bar test. We have a transmitter bar and an incident bar with strain gauges attached. The specimen goes between these two. And then a striker bar hits this incident bar, smashes the specimen, and we can get a relatively high strain rate measurement of the deformation of that material. And our material under these conditions, it was only around 1,000 per second strain rate compared to 10 to the minus 3. You see a lot of scatter there, but it's more or less constant. At even higher strain rates, something that you'd see in a blast event, thousands per second, 3,000, 4,000, 5,000. You might start seeing some strain rate dependence, but we don't see it. We've also done the same work with magnesium. It can be done with magnesium and had similar results with that material. I already mentioned the effect of matrix properties. So this is three different compositions and three different treatments, heat treatments. ASCAS, T4 and T7 are different solutionizing heat treatments and aging heat treatments. And we see increased mechanical properties with the heat treated composites compared to the ASCAS composites. I'm just going to skip through this. Other reinforcement materials, silicon carbide as opposed to aluminum oxide, you can get tailored properties based on not only the properties of the matrix, the size of the reinforcement, the thickness of the walls. You can get different properties by changing the reinforcement itself. So these are some data that we found or we measured for A206, which is an aluminum alloy, AZ91, which is a magnesium alloy with two different reinforcements. Highest strain rate properties also reveal the same kind of shearing. As you see, this is a big crack within this and some shearing of the spheres. And similar properties compared to the quasi-static results. Taking this data, we looked at what analytical models were out there for evaluating the properties based on things that we can change, like the matrix properties, the reinforcement properties, the size and thickness of the walls and tried to apply them to our data and the plots that we're getting from the various models that are out there were very far from the experimental results. So along this red line, this would be a one-to-one match between the empirical models and the models and the empirical data. And what we're getting is far off from that. So we took several parameters and these are the parameters that we evaluated and developed some models that fit rather well with the experimental results. Looking at some properties, how we define them is less important than just showing that we've got some relationships based on these parameters that are measurable from doing microscopy and from doing some initial testing. We can get a very close relationship between experimental and the empirical calculation. This is toughness predicted versus experimental. Density is a little harder to evaluate because of defects that are present within these spheres and within the composites, but that is also measurable. Now looking at some applications of this, when I did this work, I was looking at it for things like blast resistance and crushing absorption. There was kind of a theme there. So we did some evaluation based on how it would resist a blast, how it would behave in a blast, and a metal foam versus a syntactic foam. If you were to create a barrier, a metal foam could withstand this 100 kilograms of TNT at 2 meters if it's 87 inches thick and weighed over 1,000 pounds. The same amount of energy can be absorbed by for the same amount of TNT in all the same situation. About 5 sixteenths of an inch of syntactic foam are on a little less than half the weight. So this is a calculation, not an actual measurement, but it gives some idea of some of the promise of that and why we look at things like composites. These could be used for things like energy absorbers to protect against under vehicle blasts. So far I've talked about, and I'm going to rush through, the work that I've done on syntactic foams that are looking at design for the mechanical properties. But there are a lot of interesting feature prospects related to implants. One researcher that I've got here produced titanium ceramic microsphere composites by powder metallurgy processes to try to match the stiffness and flexural strength of cancelled bone. If you insert and you all know this is more better than I do, if you insert a implant that has a difference in mechanical properties from the bone that it's going into, it can lead to some damage because this is going to deform differently than the surrounding material. So you're putting pressure or tension or compression depending on the axis on this femur that can lead to some damage to that femur over time. Also, this titanium material is there permanently and over time can lead to some damages and need to be replaced. So there's some interest in creating some kind of temporary fix that helps that bone to grow or helps as long as it needs to be. Now, this isn't related to bones. This is a bioresorbable stent material. This is a magnesium scaffold that's dissolvable about 95% over a year. So within a year is about 95% resorbed into the body. And this is a commercial product that's just recently introduced. So this concept of putting a material into the body that is resorbable in the case of magnesium is not entirely new. So biomaterials that we're looking at are within this region. And the metals that I've been talking about are kind of up here, right? They do not match perfectly with this. We've got stainless steel is there, titanium is represented here, right? But if we can match something that's closer to your cancerous bone properties, we're in better shape. So magnesium alloys that I've been working with have very attractive properties. They're very lightweight. But they are very difficult to cast. And there are, at this time, no 3D printing methods that are used to create magnesium components. So there's kind of some difficulties there in their processing that prevent that. But magnesium foams could be ideal bone implant materials because they can be custom made to match not only the structures, but also the properties of cancerous bone. So the metal foams could be within this region shown at the dash marks. So the technology, and I apologize for this kind of amateurish picture, but the technology on developing would allow us to take some CAD model based on even a scan of somebody's bone, take that to create a pattern, which we can 3D print, and then finally cast liquid magnesium into it, remove that pattern, and we have a magnesium bone scaffold. Some people have done some similar things, various methods. One person has taken magnesium ribbons that you can purchase from next Sigma Aldrich and cinched them together in stacks to create a structure like this. This obviously has some drawbacks and is not very scalable. Here we have a researcher that took titanium wire and kind of bundled it up and then cast magnesium surrounding it and then dissolved away the titanium with hydrofluoric acid. And I'm just going to use that for bone scaffold. There's obvious problems with that because hydrofluoric acid is extremely toxic. There is my thing. So the third is someone used sodium chloride as a template and then tried to dissolve it away and it left some corrosion and some issues with matching that template that we want to create. So our objectives are to create kind of a scalable casting process using a strudelow material that can recreate structures found in bone. We did an I-Core program. I'm going to skip through this real quick. But I'll share the slides and you can take a look at them on your own time. Some interesting things here about how if you're designing something for commercial purposes, you want to do some more than just a literature review. You want to talk to people and find out what they need and what are the problems. You have some ideas in your head of what the problems might be. But until you talk to someone and find out do they actually need this product, you are kind of left in the dark. So that was the whole purpose of these slides. And I guess these are some of the design parameters that we're trying to hit. We're trying to match porosity. The types of porosity, multi-scale porosity that might be in bone. We're trying to match pore sizes and structure. Something that's highly interconnected. We're trying to mimic the properties of bone. We're also trying to control the rate at which these magnesium alloys degrade inside the body. So we're doing some alloy development with this as well. We also want this to be compatible. We can't use anything that's toxic. And we also want this to be sterilizable so that the sterilizing processes that are used don't destroy the material. So I'm going to stop here and let you ask some questions out of time. So are there any questions? I really rushed through. Any questions? I have one, sir. Currently, titanium is used for those implants. Are we able to then... Is it possible to make those magnesium scaffolds surrounding those titanium implants because this is bioresorbable? It's going to be the first interaction point for the tissue. Then it's going to help integration better. But at the back bone, we have already titanium in there. So we are both using titanium at a point that it is already approved in terms of its strength and durability, but also using the magnesium because it's bioresorbable and because it's porous. And how about the adhesion in between? Okay. Okay, there's several strategies that you could potentially do that, I think. That would say something interesting. I haven't seen anybody doing that, but I think it would be possible. One way that is done, this is actually a very common thing to do in the foundry industry for things, not for bones, but for things like engines where they will cast in a liner into the piston cylinder liner engine using kind of mechanical interlocking between the two materials. The liner has some roughness that the metal surrounding it can grab onto. So if you were to develop this titanium backbone with some structure on the surface that you later cast the magnesium surrounding it, that can help that bond between the titanium and the magnesium. I'm not sure what the reactions would be between magnesium and titanium with the kind of temperatures that we'd be talking about, but it also is possible that there might be some chemical bonding that would allow that to be done. But yeah, I could see that being a method of developing an implant. I'm curious to know more about the degradation of magnesium, the whole degradation, a certain degradation of what kind of degradation products are formed That's an important problem with magnesium because when magnesium dissolves, it also evolves hydrogen. And that can be kind of captured or kept within the body surrounding, the tissue surrounding that implant. So it's very important, I mentioned very, very briefly that we're doing some alloy development. We're trying to control that corrosion rate. Depending on what alloying elements that you have there, most of the alloys that have been used are fairly pure, so they're not an alloy. And that is going to be kind of a surface degradation that can be kind of controlled by surface coatings. There's also some concern that magnesium part is dissolving in the body. It can lead to some sharp edges as it's dissolving away that could cause tissue damage. So these are things that we're looking at and how to control that corrosion rate and how it actually occurs. One of the ways that people try to prevent that is through kind of developing a passivating film on the surface that helps prevent that corrosion from occurring so quickly. Kind of like if you painted your car. Thank you very much.