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

The findings, which detail the creation of a family of discrete “mechanical metamaterials,” are described in a paper published in the journal Science Advances, authored by recent MIT doctoral graduate Benjamin Jenett PhD ’20, Professor Neil Gershenfeld, and four others.

“This remarkable, fundamental, and beautiful synthesis promises to revolutionize the cost, tailorability, and functional efficiency of ultralight, materials-frugal structures,” said Amory Lovins, an adjunct professor of civil and environmental engineering at Stanford University and founder of Rocky Mountain Institute, who was not associated with this work.

Metamaterials get their name because their large-scale properties are different from the microlevel properties of their component materials. They are used in electromagnetics and as “architected” materials, which are designed at the level of their microstructure. “But there hasn’t been much done on creating macroscopic mechanical properties as a metamaterial,” Gershenfeld said.

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, Gershenfeld says.

The voxels are assembled from flat frame pieces of injection-moulded polymers, then combined into three-dimensional shapes that can be joined into larger functional structures. They are mostly open space and thus provide an extremely lightweight but rigid framework when assembled. Besides the basic rigid unit, which provides an exceptional combination of strength and light weight, there are three other variations of these voxels, each with a different unusual property.

The “auxetic” voxels have a strange property in which a cube of the material, when compressed, instead of bulging out at the sides, actually bulges inward. This is the first demonstration of such a material produced through conventional and inexpensive manufacturing methods.

There are also “compliant” voxels, with a zero Poisson ratio, which is somewhat similar to the auxetic property, but in this case, when the material is compressed, the sides do not change shape at all. Few known materials exhibit this property, which can now be produced through this new approach.

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

“Each type of material property we’re showing has previously been its own field,” Gershenfeld added. “People would write papers on just that one property. This is the first thing that shows all of them in one single system.”