Figure 1. A significant advantage of this material and design philosophy is its scalability. We've produced these materials with unit cell sizes as large as 12 inches, which are ideal for applications like building and bridge construction. We’ve also created very small materials with unit cells smaller than the thickness of a human hair (20 microns). (Image: Pablo Zavattieri)
Tech Briefs: What got you started on this project?

Professor Pablo Zavattieri: In 2011, we had the idea of connecting some bistable and metastable mechanisms together. As an exploratory project, we built a material using those mechanisms as unit cells and found that the behavior was very unusual and unexpected. After some further analysis we discovered that the materials could be used to absorb energy without accumulating damage, therefore they could be reused. Once we published the results of that project, a company contacted us and suggested that they could use the materials to fabricate structures that would absorb impact. With that company we applied for grants, and we received a couple. One was for developing non-pneumatic tires — tires that don't have air. The other project was to make runway mats for airports.

The materials are designed for fully recoverable, energy-dissipating structures, akin to what is referred to as architected shape-memory materials, or phase-transforming cellular materials, known as PXCM.

The idea is that we are using geometry and local properties to control their emerging behavior. The behavior of these materials doesn't come from their chemistry or processing — it comes from designing them to bend and twist in a way that is well-controlled. A key aspect of these materials is that they can change from one stable state to another and back again.

So, we decided to use our idea to make a mat that would be super light and could be put on the ground, where it would absorb energy due to impacts. For example, we were thinking about airplanes hitting the ground very hard. Instead of causing damage, the energy would be absorbed by the panel in an elastic way — it would just cause an elastic deformation and then go back to its original shape — that's why we call it intelligent.

Tech Briefs: There are some terms you use that I don’t understand, like architected material and phase-transforming material.

Zavattieri: Architected materials are designed with geometry. We can design them as building blocks, like rooms in a big building. For example, if you have a sponge in your hand and you compress it, you will see that all the little bubbles in the foam bottom out at the same time. So, we’ve designed a material using that approach. We call it architected because we use architectural concepts. The big difference is that, although our materials could be thought of as rooms in a building, each of our rooms is very tiny — they might be micron scale or millimeter scale. We are able to do that with 3D printing.

We borrowed the idea of phase transformation from metallurgy. For example, you can have a very specific phase of steel with some type of crystal structure. When you apply stress to it, it can transform, causing an energy-absorbing plastic deformation. That happens with steel because of its crystalline structure. We don’t rely on a crystal lattice, but because of our architecture, there are little mechanisms that take place, not at the atomic scale, but at the millimeter scale.

The most remarkable feature of these intelligent architected materials (or phase-transforming cellular materials) is their ability to dissipate energy and facilitate functions like actuation, morphing, and configuration changes, while the base material only deforms in the elastic regime. This crucially means that these materials experience minimal internal deformation, preserving their structural integrity without any damage or inelastic behavior. In essence, we design these materials primarily by leveraging geometric and mechanical principles based on their inherent material properties. As a result, controlled snap-through instabilities occur without causing harm or accumulating wear and tear in the material. What's truly fascinating is that we can employ virtually any material — be it metal, polymers, rubber, concrete, or others — as long as they are designed to remain in the elastic regime. While it's true that more brittle materials present greater design challenges, one of my PhD students even successfully crafted a single-unit cell using concrete, a material known for its brittleness in tension. It's all about effective design, making material choices remarkably versatile.

Tech Briefs: You say that these deformations happen in a very controlled way. How do you control them?

Zavattieri: We use computational tools to predict forces and deformations. When you apply a force to the surface of a material, on the inside, forces are transmitted through columns and beams similar to the way they would in a building. Imagine that you have a huge building, and maybe there is snow on the roof, or even a big helicopter. The forces are transmitted through the columns of the building. If instead of putting a helicopter on the roof, you put there a large extremely heavy structure, the forces will be transmitted through the columns and damage the building, even the rooms on the bottom floor.

The way we design it is to use the same concepts that are used for analyzing mechanical structures, for example, visual mechanics and finite element analysis. Once we design them with our computer models, we have a good idea of how they will behave, so we can go ahead and complete the application.

Tech Briefs: I read that the energy is dissipated by means of the snap-through mechanisms. Could you explain that.

Zavattieri: Sure. The transformation of elastic energy as you're applying load is a dynamic event, which produces a noise. That noise is caused by a sudden release of elastic energy which is converted into kinetic energy. A similar thing happens with a spring that has a mass attached to it and is given an initial velocity. The mass will vibrate, and that vibration will release heat and noise. The energy of the vibrations in the system is eventually going to dissipate in the material.

That's the trick, the transformation from elastic to kinetic energy.

Tech Briefs: What about size? What sizes are you working on? Can you scale them up to make big panels?

Zavattieri: The beauty of these materials is that the mechanism is scalable. You can design the little beams to be 3 millimeters, three microns, or three meters.

The trick is that elastic materials behave the same, whether at the kilometer scale, meter scale, millimeter scale, micrometer, even nanometer. Right now, we are making unit cells that are centimeters, but we are designing structures that can be scaled to micrometers. You just need to scale the numbers. We know the rules, we know the calculations that go into that size. The beauty of all this is that everything is the same except that the numbers will scale.

Tech Briefs: But isn’t there a limit to the size of the panel you could make?

Zavattieri: Absolutely. Basically, there are two things that limit the size. One is you are limited by the manufacturing process. If you want to build something on the scale of meters, then you have to use a lot of material, so 3D printing would not be the best way to go. If you want to do something on the micron scale, you could print using lithography and other techniques that are typically used for electronics.

The other limitation is the plastic deformation for a given material. For example, if you use polymer, it will deform plastically before it fails. But you can calculate those limits, using equations from material science.

A significant advantage of this material and design philosophy is its scalability. We've produced these materials with unit cell sizes as large as 12 inches, which are ideal for applications like building and bridge construction, where they can absorb and harness energy. Conversely, we’ve also utilized Nanoscribe (which uses two photon lithography) to create very small materials with unit cells smaller than the thickness of a human hair (20 microns). This scalability opens up a world of possibilities, from macro to micro applications.

Tech Briefs: Would it be possible to manufacture separate panels, and then connect them together?

Zavattieri: In fact, that's one of the concepts that we are proposing for the airport runway application. The mat is about 6 feet by 12 feet, which is the right size for a human to carry. You can put them all in place like a Lego type structure.

Tech Briefs: Besides the airport mat, do you have any other thoughts about what the first kinds of applications might be?

Zavattieri: Many things that are lightweight and require impact resistance. I see applications in walls and in structures for rapid deployment — to deploy structures for humans in places where they are in danger. You can use this because it's lightweight, and it would offer protection.

We are currently also evaluating our material for earthquake-resistant structures. In tall buildings, the capability of the material to absorb energy might be just enough to prevent failure and damage.

There could also be applications for cars and airplanes because they are lightweight and can absorb energy. Also seats that can accommodate the human body and absorb energy, and also protective helmets.

People are evaluating many different applications for these materials. I would say everything that is lightweight for civil engineering applications, transportation, vehicles.

Tech Briefs: Do you have any sense of the time frame for when it might be practically usable?

Zavattieri: That depends on the industry. If an industry is interested, we can start developing structures for particular applications — we have all the tools. We just need an application and then maybe a couple of years. But for the mats, we are ready to go. So, our goal is to start working with industry.

Tech Briefs: Is there anything you want to add?

Zavattieri: I think the last question was the crucial one. This technology is ready to go, so we can start developing applications for any specific industry or research laboratory.



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This article first appeared in the November, 2023 issue of Tech Briefs Magazine.

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