Mechanical metamaterials are engineered to behave strangely. Unlike a rubber band, which becomes narrower in width when stretched, a metamaterial might bulge, for example. These materials owe their counter-intuitive properties to their unusual geometrical structures: a honeycomb-like lattice of interconnected polygons that deform in unison to give rise to counterintuitive behavior, for instance. One popular lattice shape is a bow-tie, which widens in the vertical direction when stretched horizontally.

In particular, such materials are valuable for form-fitting robotic prosthetics and other soft robots, flexible machines that offer more adaptability and safety than their hard-edged counterparts. Researchers tune a metamaterial’s properties by varying its underlying geometrical pattern structure or by alternating between multiple materials to stitch a single sample. To create a material with just the right flexibility and shape morphing behavior for a particular application, researchers have begun to develop computer modeling techniques to aid the design process.

Now, researchers at Delft University of Technology (TU Delft) (The Netherlands) and Politecnico di Milano (Italy) have demonstrated a new modeling method for mechanical metamaterial design. The method allows them to tune two mechanical properties of the metamaterial independently: its Poisson’s ratio and its elastic modulus. The researchers applied this design technique to produce metamaterials composed of two polymers with different hardness characteristics with an underlying polygonal structure using a three-dimensional (3D) printer. Their computer model lets them efficiently vary the geometry and layout of the polymer polygons to fit their design criteria.

“This is the start of a new way to design metamaterials,” says Amir Zadpoor of TU Delft, who led the work. Previously, he says, researchers only played around with the internal geometries of the metamaterial structure. This new method gives researchers an efficient way to explore metamaterials composed of multiple materials. They can specify certain parts of the honeycomb to be stiff or soft, giving them an additional parameter to tune to achieve the desired mechanical properties.

To demonstrate the design process, the researchers created and tested 15 different metamaterials. First, they printed and experimentally characterized cellular structures with three different shapes, or unit cells, which included a rectangle, a hexagon, and a bow-tie shape, using the two different polymers. They chose a softer polymer, Agilus30 Black, with a Shore hardness of 30-35, and a harder polymer, VeroCyan, with a Shore hardness of 83-86, which also had an elastic modulus about 1000 times higher. Then, the researchers fed the shapes’ mechanical properties into a computer model to simulate the Poisson’s ratio and elastic modulus of a metamaterial consisting of variations of these unit cells. They chose 15 designs to 3D-print and test using a machine that could stretch the material with controlled force. They found that the materials’ Poisson’s ratio and elastic modulus spanned a wide range of values, which means that materials resulting from this design approach should befit various material design objectives.

“They’ve really shown a good way to make these materials from a manufacturing point of view,” says Fabrizio Scarpa of the University of Bristol, who was not involved with the work. He says that this methodology could benefit the design of many different real-world applications, such as shape-shifting aircraft wings that aeronautical researchers are pursuing, or antennae whose mechanical flexibility improves their transmission or reception. “With their strategy, you can really tailor the way you make these types of structures,” he says.

Read the abstract in Applied Physics Letters.