Jul 28, 2016 | By Alec

As Minecraft proved to the world, blocks – regardless of their size – are at the core of every possible structure. But what if those blocks themselves could perform certain functions, without the need for complex electronics and motors? That, in a nutshell, is what a team of Dutch researchers have developed in collaboration with Israeli colleagues. Their 3D printed metamaterial blocks hide programmable geometric patterns that perform a certain function when exposed to pressure. Perfect for customizable prostheses that can bend in a certain direction, or for custom shoes.

This remarkable material breakthrough was realized by researchers Corentin Coulais, Eial Teomy, Koen de Reus, Yair Shokef and AMOLF group leader professor Martin van Hecke. It has just been unveiled in a paper entitled Combinatorial Design of Textured Mechanical Metamaterials, published in the influential Nature journal.

While a complex achievement, it is best illustrated by the remarkable rubber block visible above. Actually consisting of numerous building blocks, these have been programmed to deform in a certain pattern when exposed to pressure. In this particular case, several blocks bulge out to form a smiley face – but the same principles can be applied to very large 3D printed cubes with predictable and functional deformations as well. “In science the smiley seems to be the gold standard when you claim you are able to make any shape you want, but we could have made anything else,” says Coulais.

But programming those cubes can be really tricky. Combining several blocks with different orientations quickly creates a complex 3D jigsaw puzzle that can seize up if not properly designed. “The orientation of the blocks in the metamaterial is important. Under pressure, all of the hollow and bulging sides must fit exactly together. Most of the stacks are 'frustrated': somewhere within two hollows or bulges meet. However a large number of fitting solutions for this three-dimensional puzzle were found,” professor Van Hecke explained.

The trick is to ensure that a bulging block is always surrounded by neighbors who can accommodate that bulge. “When you only have two, it’s fairly easy to do. What becomes hard is when you have a full stack,” says Coulais. “Whenever we add a new cube, we have to make sure it fits with the previously arranged ones.” But if done right, very complex functions can be realized.

This is so remarkable because it goes against deformation principles to some extent. Generally speaking, atoms and molecules determine the properties of the materials. But metamaterials (which are man-made) seem to break this rule. “In the case of metamaterials, the spatial structure determines the material's behavior,” explains professor van Hecke. “For example, a pattern of holes in a sheet of material gives rise to a mechanical response that is completely different than in the same material without holes. We also wanted to investigate this phenomenon for a three-dimensional pattern of holes.”

Originally, the Dutch scientists used software to bring the metamaterial blocks together one by one. But soon they realized that the process can be much simpler, for adjacent slices of a cube have to be mirror images of each other to fit. This means that the configuration of all the blocks that make up a metamaterial cube can be determined by rearranging the surface blocks.

What’s more, a lot of different options are possible. Van Hecke’s colleagues from the Tel Aviv University in Israel calculated the number of possibilities to create non-blocking stacks of different cubes. “For one cube of 14x14x14 building blocks that is a number with no less than 65 figures,” says Van Hecke. “For each possible stack the deformation within the cube results in a specific pattern on the sides of the cube. By smartly combining the building blocks we can program the material such that every desired pattern appears on the sides of a compressed cube. Surprisingly such a cube can also be used to analyze patterns. If we press it against a pattern of hollows and bulges then we measure a force that is dependent on the patterns.”

Theoretically at least, this creates a lot of possible applications. Coulais believes that this ability to program materials with certain motions can be very useful for prosthetic limbs, for instance. For those limbs, it’s obviously the goal to make joints bend in a certain direction, and that is exactly what these 3D printed materials can do. “Being able to handle and play is really part of the research, because some of the things we see often come up as a surprise,” Coulais added.

But Coulais is also thinking about another interesting property of the material. For the material becomes squishier if you push against it with the pattern encoded inside it. “If you push on this object with a smiley texture, it’s going to be softer than if you push with another pattern,” says Coulais. This could make it perfect for wearable objects that need to be comfortable and supporting, such as custom shoe insoles that feature an outline of your foot.

Other more complex applications are also on the horizon, such as 3D printed wearables that need a close fit to the body. “If we can make the building blocks more complex or produce these from other materials then the possibilities really are endless,” the professor adds. One thing seems clear: metamaterials are the future.



Posted in 3D Printing Application



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