Brinley Macnamara (host) (00:00):

Welcome to the Tech Futures Podcast. I’m your host Brinley Macnamara. I’m also a Networks Engineer here at MITRE and in today’s episode, I’m going to tell you about a recent MITRE investigation into an emerging technology called Mechanical Metamaterials. In this episode, I hope to inform you about what Mechanical Metamaterials are, how they are manufactured and how they can be applied to solve a broad range of problems from evading underwater detection to constructing more resilient body armor to perhaps one day making a ride to Mars a little less bumpy. But before we begin, I wanted to say a huge thank you to Dr. Kris Rosfjord, the Tech Futures Innovation Area Leader. This episode would not have happened without her support. Now, without further ado, I bring you the Tech Futures Podcast episode number four.

Dr. Dave Hurley (01:03):

Will Metamaterials themselves get us to Mars? Maybe, maybe not. But, I do think they will help get us there sooner. One of the most promising applications is vibration control or vibration mitigation, and launch is a pretty violent experience getting into orbit or beyond. So if you can develop a metamaterial solution that basically reduces the weight of your payload while also reducing the vibration profile of the process of getting into orbit. So it’s basically, you can create a smoother ride for your payload while making it lighter. It’s going to reduce the cost of getting things into space and that will play its role in more experimentation and learning more and eventually setting us on that path to getting to Mars.

Brinley Macnamara (host) (02:27):

That was Dr. Dave Hurley talking. He’s a Mechanical Engineer in MITRE Labs and was the Principal Investigator on a project that focused on assessing the state of the art of Mechanical Metamaterials. At this point, I think it’s time for me to take a step back and actually define Mechanical Metamaterials. It’s a complicated topic, so to help me with this, I turn to Casey Corrado, another Mechanical Engineer at MITRE, and one of Dave’s Co-Principal Investigators on the Mechanical Metamaterials project.

Casey Corrado (02:56):

Metamaterials are engineered materials where their properties don’t just depend on the material itself, but its geometry on a small scale. You change the geometry on a small scale, so a large piece of that material has different bulk properties than just the material it’s made of. So if you make a metamaterial out of steel, it might not have the same material properties as traditional steel because it has that engineered small geometry.

Brinley Macnamara (host) (03:28):

This engineered small geometry is often referred to as a unit cell and recent advancements in 3D printing technology have allowed these unit cells to be manufactured at super small scales. We’re talking really small, like on the order of microns, about the size of the width of a human hair. And when you use as the building blocks for material, these tiny yet geometrically sophisticated unit cells give the resulting material unique bulk properties that are not found in conventional materials. One such bulk property is the ability to hide or cloak objects from various spectra of radiation. In practice, this means that we could one day fabricate cloaks that could make an underwater vehicle undetectable by sonar or cloaks that could increase an object’s resistance to heat, or even invisibility cloaks, often referred to as optical cloaks by Materials Scientists.

Dr. Kris Rosfjord (04:24):

Cloaking of materials with metamaterials, it’s just very exciting, and it was first experimentally demonstrated about 15 years ago.

Brinley Macnamara (host) (04:36):

That’s Dr. Kris Rosfjord talking. She’s MITRE’s Tech Futures Innovation Area Leader. Her work as a PhD student at the Lawrence Berkeley National Lab, postdoc at MIT, and Electrical Engineering professor largely focused on the intersection of electromagnetics and materials science. So I was excited to hear her thoughts on the state of the art of metamaterials.

Dr. Kris Rosfjord (04:57):

And it got a lot of interest. It received a lot of interest in the popular media. Perhaps we’re all familiar with the cloaking from Harry Potter, so it was very relatable. That, I believe was a slightly longer wavelength, perhaps microwaves, but there has been the demonstration of a simple optical cloaks more recent than that. Let’s say about 10 years ago. These things in order to steer the light with the metamaterials around the object that one is trying to cloak, you have to have those units cells that have features that are on the order or smaller than the wavelength of light.

Dr. Kris Rosfjord (05:39):

When the earliest cloak that was at microwaves, it’s a longer wavelength than that for the optical regime. As you’re shrinking these things in wavelength, you then are needing to shrink the small characteristic feature of that unit cell. So you’re just increasing the fabrication and then the manufacturing challenge as you go to shorter and shorter wavelengths.

Brinley Macnamara (host) (06:04):

Dave, Casey, and their other Co-Principal Investigator, Dr. DJ Shim, were not naive about the challenges that Nano scientists faced early on with the fabrication and metamaterials at the nanoscale. So they decided to narrow their focus to three subcategories of Mechanical Metamaterials that are a bit easier to fabricate and show more promise for large scale manufacturing and deployment in the coming decades. These include thermal metamaterials, acoustic metamaterials, and metamaterials for shock and impact absorption. I wanted to know more about each of these types of metamaterials, so I started by asking Dave about what makes thermal metamaterial so special.

Dr. Dave Hurley (06:44):

Typically when things heat up or as temperature increases things expand. That’s described by the coefficient of thermal expansion, which is basically an intrinsic material property, but through some pretty clever architectures and the use of multiple materials, geometries have been developed so that as their temperature increases, the local thermal expansion of these unit cells actually results in a bulk reduction in size. So it’s actually taking this behavior that’s in conventional materials and flipping it upside down. Instead of expanding as the temperature increases, there’s metamaterial concepts that actually contract or reduce their size. And that has a lot of interesting applications, particularly when you look at any assembly that has to operate over a very wide temperature range. If you’re assembling a satellite on Earth and then sending it on a mission around the Sun, it’s going to experience a huge range in temperatures and so being able to manage the thermal expansion becomes a very important piece of the success of that.

Brinley Macnamara (host) (08:20):

Metamaterials aren’t just useful in space. In fact, according to Casey, we might see the first application of metamaterials under water with the deployment of acoustic metamaterials.

Casey Corrado (08:31):

Acoustic metamaterials are metamaterials that are designed to specifically manipulate mechanical waves within a structure or in a media (you can do this under water, you can do this in air) that have plenty of different applications. One that I like to talk about for acoustic metamaterials, that’s definitely the most grabbing, fun, exciting is acoustic cloaking.

Casey Corrado (08:57):

So this idea that you could design either a structure that is invisible to acoustic sensing, so sound can kind of just pass right through and it looks like nothing’s there, or you could create a structure that goes around something else to essentially shield that from a sound waves. That has lots of different applications, especially from an underwater lens because you could use it for sonar applications. You could essentially create something like an underwater unmanned vehicle or UUV, or even on a bigger scale, a submarine, that has this acoustic shield over it and it’s hidden from sonar, which would be incredibly fantastic for a lot of our sponsors.

Brinley Macnamara (host) (09:41):

The third and final subcategory of Mechanical Metamaterials that the MITRE team focused on was metamaterials for shock and impact absorption.

Dr. DJ Shim (09:51):

So in blunt shock and impact, there’s been very promising work where metamaterials have been shown to absorb more energy and therefore mitigate that blunt shock and impact better than conventional materials.

Brinley Macnamara (host) (10:14):

That’s Dr. DJ Shim talking. He’s a Chief Engineer in MITRE Labs and was another Co-Principal Investigator on the Mechanical Metamaterials project. I asked DJ if metamaterials for shock and impact absorption could ever be leveraged to make better protective equipment for our war fighters.

Dr. DJ Shim (10:31):

Yes, absolutely. So that is definitely an area that people have been looking at, and something we’re interested in too, as well in looking further into. So metamaterials for body armor and more generally kind of war fighter protective equipment, so those would include other types of armor for vehicles and things like that as well.

Brinley Macnamara (host) (11:00):

Given the growing body of evidence that metamaterials could be a game changer for so many industries, I was curious to know more about why it’s been so challenging to scale the technology.

Brinley Macnamara (host) (11:10):

You describe in the paper the development of metamaterials being mainly academic, and this is in part due to manufacturing limitations. So, I’m wondering why is it so hard to manufacture metamaterials?

Dr. Dave Hurley (11:22):

I think it goes back to a few points that we’ve mentioned earlier, and the first thing is the multiple scales required. So you need intricate fine detail to create let’s say the unit cells, but then you also need large capacity so that you can create a metamaterial large enough for your specific application. These are three dimensional geometries. So you need some sort of additive, or it lends itself toward additive manufacturing capabilities, or often requires some sort of additive it requires additive manufacturing capabilities to produce a metamaterial. Currently, we’ll say the resolution or the detail and the build volume so the size of the part that you can produce with an additive manufacturing method scale together. So you can make something really detailed and small, or you can make something that is pretty coarse and large, but it’s hard to do both. And so metamaterials really require both, they require fine detail, but then also it has to be large enough so that it’s usable for your specific application.

Brinley Macnamara (host) (12:54):

What is the state of the art for fabrication of metamaterials these days? Is it some sort of like micro-sized 3D printer, or how does that work?

Dr. Kris Rosfjord (13:05):

That’s a really great question, but it’s a complicated question because it really depends on the size scale that you’re looking for. And if we think about the subset of metamaterials that is mechanical metamaterials, the size scale that you need is a little larger. You might be talking about tens to hundreds of microns, rather than tens to hundreds of nanometers. And because of that to hinge in size scale, you’re now able to use commercially available, advanced manufacturing or additive manufacturing techniques. Now we all get excited about additive manufacturing and the promises it might hold, and this is one field where these capability on the hundreds of microns scale can be a true game changer. We’re having to use much smaller E-beam lithography-type techniques for photonic metamaterials was very challenging for the community.

Brinley Macnamara (host) (14:04):

From her earliest days on the campus of UC Berkeley, Dr. Rosfjord has had an unshakable passion for studying the world at the nanoscale. I suspect that the alert of the nanoscale to her and her fellow nanoscientists lies in how much we can learn about the larger scale phenomena that govern our world from studying its tiniest components.

Brinley Macnamara (host) (14:25):

Within the field of mechanical metamaterials lies the same truth. By manipulating the tiny structures of unit cells at the micron scale, we can then assemble these cells to create materials on a much larger scale, whose properties transcend those found in nature. While the fabrication of materials at the nanoscale for applications like optical cloaking is still very much confined to a small number of nanofabrication facilities, Dr. Rosfjord noted that there is much more opportunity when you look three orders of magnitude up the size scale to resolutions that are supported by commercial 3D printers, which also happens to be the sweet spot for mechanical metamaterials. This means that metamaterials that will enhance our militaries underwater detection evasion capabilities and outfit our war fighters with better protective equipment might not be as far off. We must act swiftly to deploy mechanical metamaterials to our underwater and armored vehicles as this will ensure that we’ll see the day when mechanical metamaterials will take our space vehicles to Mars.

Brinley Macnamara (host) (15:40):

The music in this podcast is brought to you by Trevor Kowalski, August Wilhelmsson, and Benjamin Cling. That’s it for today’s episode. Thank you so much for listening.