Versatile building blocks create structures with surprising mechanical properties

Researchers at MIT’s Center for Bits and Atoms have created tiny building blocks that exhibit a variety of unique mechanical properties, such as the ability to produce twisting motion when squeezed. These sub-units could potentially be assembled by tiny robots into an almost limitless variety of objects with built-in functionality, including vehicles, large industrial parts, or specialized robots that could be repeatedly reassembled into different forms.

The researchers created four different types of these subunits, called voxels (a 3D variation on pixels in a 2D image). Each type of voxel exhibits special properties not found in typical natural materials, and in combination they can be used to fabricate devices that respond to environmental stimuli in predictable ways. Examples may include airplane wings or turbine blades which react to changes in atmospheric pressure or wind speed by altering their overall shape.

The results, which detail the creation of a family of discrete “mechanical metamaterials”, are described in a paper published today in the journal Scientists progressauthored by recent MIT student Benjamin Jenett PhD ’20, Professor Neil Gershenfeld and four others.

Metamaterials get their name because their large-scale properties are different from the micro-level properties of their component materials. They are used in electromagnetism and as “structured” materials, designed at the level of their microstructure. “But little has been done to create macroscopic mechanical properties as a metamaterial,” says Gershenfeld.

With this approach, engineers should be able to build structures incorporating a wide range of material properties — and produce them all using the same shared production and assembly processes, says Gershenfeld.

Voxels are assembled from flat-framed pieces of injection molded polymers and then combined into three-dimensional shapes that can be joined into larger functional structures. They are mostly open spaces and thus offer an extremely light but rigid frame when assembled. Besides the basic rigid unit, which offers an exceptional combination of strength and lightness, there are three other variants of these voxels, each with a different unusual property.

“Auxetic” voxels have a strange property in which a cube of the material, when compressed, instead of inflating outwards actually inflates inwards. This is the first demonstration of such a material produced by conventional and inexpensive manufacturing methods.

There are also “conforming” voxels, with zero Poisson’s ratio, which is somewhat similar to the auxetic property, but in this case, when the material is compressed, the sides don’t change shape at all. Few known materials exhibit this property, which can now be produced using this new approach.

Finally, “chiral” voxels respond to axial compression or stretching with a twisting motion. Again, this is an uncommon property; the research that produced such a material through complex manufacturing techniques was hailed as a significant discovery last year. This work makes this property easily accessible at macroscopic scales.

“Each type of material property that we show has once been its own domain,” says Gershenfeld. “People would write about one property. It’s the first thing that shows them all in one system.”

To demonstrate the true potential of large, LEGO-like objects built from these mass-produced voxels, the team, in conjunction with Toyota engineers, produced a super-mileage functional race car, which they demonstrated on the streets at an international robotics conference earlier this year.

They were able to assemble the lightweight, high-performance structure in just one month, says Jenett, whereas building a comparable structure using conventional fiberglass construction methods previously took a year.

During the demonstration, the streets were slippery from the rain and the race car ended up crashing into a barrier. To the surprise of everyone involved, the car’s internal lattice-like structure buckled and then rebounded, absorbing the shock with little damage. A conventionally built car, says Jenett, would likely have been badly dented if it was metal, or shattered if it was composite.

The car provided a vivid demonstration that these tiny parts can indeed be used to make functional devices on a human scale. And, Gershenfeld points out, in the structure of the car, “these are not parts connected to anything else. Everything is made of nothing but these parts”, except for the motors and the power supply.

Since voxels are uniform in size and composition, they can be combined in any way necessary to provide different functions to the resulting device. “We can cover a wide range of material properties that were previously considered very specialized,” says Gershenfeld. “The point is that you don’t have to choose a property. You can create, for example, robots that bend in one direction and are stiff in another direction and only move in certain ways. And so the big change from our previous work is this ability to cover several mechanical properties of materials, which until now were considered in isolation.”

Jenett, who did much of this work as the basis for his doctoral thesis, says, “These parts are inexpensive, easy to produce, and very quick to assemble, and you get that range of properties in one system. They’re all compatible with each other, so there’s all these different types of exotic properties, but they all work well with each other in the same scalable, low-cost system.”

“Think of all the rigid parts and moving parts of cars, robots, boats, and planes,” says Gershenfeld. “And we can cover all of that with this one system.”

A key factor is that a structure composed of one type of these voxels will behave in exactly the same way as the subunit itself, Jenett says. “We were able to demonstrate that the joints do indeed disappear when you put the parts together. It behaves like a continuous, monolithic material.”

While robotics research has tended to be split between hard and soft robots, “it’s really neither,” says Gershenfeld, due to its potential to mix and match these properties to the within a single device.

According to Jenett, one of the first possible applications of this technology could be the construction of wind turbine blades. As these structures get bigger and bigger, transporting the blades to their operating site becomes a serious logistical problem, whereas if they are assembled from thousands of tiny sub-units, this job can be carried out on site, thus eliminating the transport problem. Also, the disposal of used turbine blades is already becoming a serious problem due to their large size and lack of recyclability. But the blades made up of tiny voxels could be disassembled on site, and the voxels were then reused to make something else.

And in addition, the blades themselves could be more efficient, because they could have a mix of mechanical properties designed into the structure that would allow them to react dynamically, passively, to changes in wind strength, he says.

Overall, Jenett says, “We now have this low-cost, scalable system, so we can design whatever we want. We can do quadrupeds, we can do swimming robots, we can do flying robots. This flexibility is one of the key advantages of the system.”

The research team included Filippos Tourlomousis, Alfonso Parra Rubio and Megan Ochalek from MIT, and Christopher Cameron from the US Army Research Laboratory. The work was supported by NASA, the US Army Research Laboratory and the Center for Bits and Atoms Consortia.