MIT researchers used kirigami to develop ultrastrong, lightweight materials that have tunable mechanical properties, like stiffness and flexibility. Image courtesy of Neil Gershenfeld, Alfonso Parra Rubio, Klara Mundilova, et al.
MIT researchers used kirigami to develop ultrastrong, lightweight materials that have tunable mechanical properties, like stiffness and flexibility. Image courtesy of Neil Gershenfeld, Alfonso Parra Rubio, Klara Mundilova, et al.

Cellular solids are materials composed of many cells that have been packed together, such as a honeycomb. The shape of those cells largely determines the material’s mechanical properties, including its stiffness or strength. Bones, for instance, are filled with a natural material that allows them to be lightweight while also stiff and strong.

Inspired by bones and other cellular solids found in nature, humans have used the same concept to develop architected materials. By changing the geometry of the unit cells that make up these materials, researchers can customize the material’s mechanical, thermal or acoustic properties. Architected materials are used in many applications, from shock-absorbing packing foam to heat-regulating radiators.

Using kirigami, the ancient Japanese art of folding and cutting paper, researchers at Massachusetts Institute of Technology (MIT) have now manufactured a type of high-performance architected material known as a plate lattice, on a much larger scale than scientists have previously been able to achieve by additive fabrication. The technique allows them to create these structures from metal or other materials with custom shapes and specifically tailored mechanical properties.

“This material is like steel cork. It is lighter than cork, but with high strength and high stiffness,” says Neil Gershenfeld, who leads the Center for Bits and Atoms (CBA) at MIT and is senior author of a new paper on this approach.

The researchers developed a modular construction process in which many smaller components are formed, folded and assembled into 3D shapes. Using this method, they fabricated ultralight and ultrastrong structures, as well as robots that, under a specified load, can morph and hold their shape.

Because these structures are lightweight but also strong, stiff and relatively easy to mass-produce at larger scales, they could be especially useful as architectural, airplane, automotive or aerospace components.

Architected materials, like lattices, are often used as cores for a type of composite material known as a sandwich structure. To envision a sandwich structure, think of an airplane wing, where a series of intersecting, diagonal beams form a lattice core that is sandwiched between a top and bottom panel. This truss lattice has high stiffness and strength yet is very lightweight.

Plate lattices are cellular structures made from three-dimensional intersections of plates, rather than beams. These high-performance structures are even stronger and stiffer than truss lattices, but their complex shape makes them challenging to fabricate using common techniques like 3D printing, especially for large-scale engineering applications.

The MIT researchers overcame these manufacturing challenges by using kirigami, a technique for making 3D shapes by folding and cutting paper that traces its history to Japanese artists in the seventh century.

Kirigami has already been used to produce plate lattices from partially folded zigzag creases. But to make a sandwich structure, flat plates must be attached to the top and bottom of this corrugated core, which means onto the narrow points formed by the zigzag creases. This often requires strong adhesives or welding techniques that can make assembly slow, costly and challenging to scale.

By modifying a common origami crease pattern, known as a Miura-ori pattern, the MIT researchers were able to transform the sharp points of the corrugated structure into facets. These facets, like those on a diamond, provide flat surfaces to which the plates can be attached more easily, with bolts or rivets.

“Plate lattices outperform beam lattices in strength and stiffness while maintaining the same weight and internal structure,” says Alfonso Parra Rubio, a research assistant in the CBA and co-lead author of the paper. “Reaching the H-S upper bound for theoretical stiffness and strength has been demonstrated through nanoscale production using two-photon lithography. Plate lattices construction has been so difficult that there has been little research on the macro scale. We think folding is a path to easier utilization of this type of plate structure made from metals.”

Moreover, the way the researchers design, fold and cut the pattern allows them to tune certain mechanical properties, such as stiffness, strength and flexural modulus (the tendency of a material to resist bending). They encode this information, as well as the 3D shape, into a creasing map that is used to create these kirigami corrugations.

For instance, based on the way the folds are designed, some cells can be shaped so they hold their shape when compressed while others can be modified so they bend. In this way, the researchers can precisely control how different areas of the structure will deform when compressed. Because the flexibility of the structure can be controlled, these corrugations could be used in robots or other dynamic applications with parts that move, twist and bend.

To craft larger structures like robots, the researchers introduced a modular assembly process. They mass produce smaller crease patterns and assemble them into ultralight and ultrastrong 3D structures. Smaller structures have fewer creases, which simplifies the manufacturing process.

Using the adapted Miura-ori pattern, the researchers can create a crease pattern that will yield their desired shape and structural properties. Then they utilize a unique machine – a Zund cutting table – to score a flat, metal panel with this pattern, so it can be folded into the 3D shape.

“To make things like cars and airplanes, a huge investment goes into tooling,” Gershenfeld says. “This manufacturing process is without tooling, like 3D printing. But unlike 3D printing, our process can set the limit for record material properties.”

Using their method, the researchers produced aluminum structures with a compression strength of more than 62 kilonewtons, but with a weight of only 90kg per m2 (cork weighs about 100kg per m2). Their structures were so strong they could withstand three times as much force as a typical aluminum corrugation.

The versatile technique could be used for many materials, such as steel and composites, making it well-suited for the production lightweight, shock-absorbing components for airplanes, automobiles or spacecraft.

However, the researchers found that their method can be difficult to model. So, in the future, they plan to develop user-friendly CAD design tools for these kirigami plate lattice structures. In addition, they want to explore methods to reduce the computational costs of simulating a design that yields desired properties.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.