Inspired by bones and other cellular solids in nature, humans have used the same concept to develop building materials. By changing the geometry of the cells that make up these materials, researchers can tailor the material's mechanical, thermal or acoustic properties. Building materials are used in a wide range of applications, from shock-absorbing packaging foam to heat-regulating radiators.
Using Kirigami, the Japanese art of kirigami and origami, MIT researchers have developed ultra-strong, lightweight materials with tunable mechanical properties such as stiffness and flexibility. These materials can be used in aircraft, cars or spacecraft. Image source: Provided by researchers
MIT researchers modified a common origami crease pattern so that the sharp points of the corrugated structure become facets. These facets, like the facets on a diamond, provide a flat surface to which the plate can be more easily fastened with bolts or rivets. Image source: Provided by researchers
MIT researchers have used Kirigami, the ancient Japanese art of origami and kirigami, to create a high-performance structural material called lattice at a scale far beyond what scientists have previously been able to achieve through additive manufacturing. This technology allows them to create these structures out of metal or other materials with customized shapes and specially tailored mechanical properties.
"This material is like steel cork. It's lighter than cork but has high strength and high stiffness," said Professor Neil Gershenfeld, director of MIT's Center for Bits and Atoms (CBA) and senior author of a new paper on this approach.
The researchers developed a modular manufacturing process in which many smaller parts are formed, folded and assembled into three-dimensional shapes. Using this approach, they create ultra-light, ultra-strong structures and robots that can deform and maintain their shape under specific loads.
The researchers actuated the corrugated structure by tensioning steel wires on compliant surfaces and then connecting them to a system of pulleys and motors, allowing them to bend in either direction. Image source: Provided by researchers
Because these structures are lightweight, strong, stiff, and relatively easy to mass-produce, they are particularly useful in construction, aircraft, automotive, or aerospace components.
Also writing the paper with Gershenfeld are co-first authors, CBA research assistant Alfonso Parra Rubio and MIT electrical engineering and computer science graduate student Klara Mundilova, as well as CBA graduate student David Preiss and MIT computer science professor Erik D. Demaine. The research results were presented at the American Society of Mechanical Engineers Engineering Computers and Information Conference.
Structural materials like lattice are often used as the core of a composite material known as a sandwich structure. To envision a sandwich structure, imagine an airplane wing, where a series of intersecting diagonal beams form a lattice core sandwiched between top and bottom panels. This truss structure has high stiffness and strength yet is very light in weight.
A panel lattice is a honeycomb structure composed of three-dimensional intersections of plates rather than beams. The strength and stiffness of these high-performance structures exceed even those of truss lattices, but due to their complex shapes, fabricating them using common techniques such as 3D printing is challenging, especially in large-scale engineering applications.
MIT researchers overcame these manufacturing challenges using tung paper, a technique of folding and cutting paper to create 3D shapes that dates back to 7th-century Japanese artists.
The researchers used their method to create an aluminum structure with a compressive strength of over 62 kilonewtons, but weighing just 90 kilograms per square meter. Image source: Provided by researchers
Kirigami has been used to create panels utilizing partially folded zigzag creases. But to make a sandwich structure, flat sheets must be attached to the top and bottom of the corrugated core and then to the narrow points created by the herringbone folds. This often requires strong adhesives or welding techniques, making assembly slow, expensive and difficult to scale.
MIT researchers modified a common origami crease pattern so that the sharp points of the corrugated structure become facets. These facets, like the facets on a diamond, provide a flat surface onto which the plates can be more easily fastened with bolts or rivets.
"Plate lattices outperform beam lattices in terms of strength and stiffness while weight and internal structure remain constant," said ParraRubio. "By using two-photon lithography for nanoscale production, theoretical stiffness and strength have reached the H-S upper limit. Plate lattices are very difficult to construct and therefore have been poorly studied at the macroscale. We believe that folding is a route to making it easier to utilize such plate-like structures made of metal."
Additionally, the way the researchers designed, folded and cut the patterns allowed them to tune certain mechanical properties, such as stiffness, strength and flexural modulus (a material's tendency to resist bending). They encoded this information, along with the three-dimensional shapes, into crease maps, which they used to create these jellied paper ripples.
For example, depending on how the pleats are designed, some cells can be shaped so that they retain their shape when compressed, while others can be modified so that they bend. In this way, the researchers can precisely control how different areas of the structure deform under compression.
Because the structure's flexibility can be controlled, these corrugations could be used in robots or other dynamic applications with moving, twisting and bending parts.
To make large structures like robots, researchers use a modular assembly process. They mass-produce smaller crease patterns and assemble them into ultra-light, ultra-strong three-dimensional structures. The smaller structure has fewer creases, simplifying the manufacturing process.
Using a modified Miura-ori pattern, the researchers created a crease pattern that produces the desired shape and structural properties. They then used a unique machine - a Zund cutting table - to cut out flat sheets of metal and fold them into three-dimensional shapes.
"To make products like cars and airplanes, you need to invest heavily in molds. This manufacturing process requires no tools, like 3D printing. But unlike 3D printing, our process can set the limits of recording material properties," Gershenfeld said.
Using their method, they created an aluminum structure with a compressive strength of over 62 kilonewtons but weighing just 90 kilograms per square meter. (Cork weighs about 100 kilograms per square meter) Their structure is very strong and can withstand three times the force of ordinary aluminum corrugations.
This versatile technology can be used in a wide range of materials, including steel and composites, making it ideal for producing lightweight shock-absorbing components for aircraft, cars or spacecraft.
However, the researchers found that their approach could be difficult to model. Therefore, they plan to develop user-friendly CAD design tools for these grid structures in the future. In addition, they hope to explore methods to reduce the computational cost of simulating the performance required by their designs.
Parra-Rubio, Mondilova and other MIT graduate students also used this technique to create three large folded artworks out of aluminum composites, which are on display at the MIT Media Lab. Although each piece is several meters long, the structures only took a few hours to create.
"Ultimately, the artwork is only possible because of the mathematical and engineering contributions we demonstrate in our paper. But we also don't want to lose sight of the aesthetic power of our work," ParraRubio said.