Metamaterials — materials whose function is determined by structure, not composition — have been designed to bend light and sound, transform from soft to stiff, and even dampen seismic waves from earthquakes. But each of these functions requires a unique mechanical structure, making these materials great for specific tasks, but difficult to implement broadly.
But what if a material could contain within its structure, multiple functions, and easily and autonomously switch between them?
Researchers from Harvard’s Wyss Institute of Biologically Inspired Engineering and the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a general framework to design reconfigurable metamaterials. The design strategy is scale independent, meaning it can be applied to everything from meter-scale architectures to reconfigurable nano-scale systems such as photonic crystals, waveguides, and metamaterials to guide heat.
The research is published in Nature.
“In terms of reconfigurable metamaterials, the design space is incredibly large and so the challenge is to come up with smart strategies to explore it,” says Katia Bertoldi, John L. Loeb Associate Professor of the Natural Sciences at SEAS and senior author of the paper. “Through a collaboration with designers and mathematicians, we found a way to generalize these rules and quickly generate a lot of designs.”
Bertoldi and former Graduate Student Johannes Overvelde collaborated with Chuck Hoberman, an Associate Faculty member of the Wyss Institute and the Pierce Anderson Lecturer in Design and Engineering at the Harvard Graduate School of Design (GSD), and James Weaver, a Senior Research Scientist at the Wyss Institute, to design the metamaterial.
“It all started back in 2014 when Chuck stopped by my office with a foldable prototype. It was an extruded cube and we were amazed by how easily it could fold and change shape. We also instantly realized that this simple geometry could be used as a building block to form a new class of space-filling reconfigurable materials, but it took us a long time to identify a robust design strategy to achieve this,” says Bertoldi. In a culmination of the following years of research, the team realized that space-filling assemblies of polyhedra can be used as a template for the design of reconfigurable thin-walled structures that consist of rigid plates connected by flexible hinges, dramatically simplifying the design process.
The team used computational models to quantify all the different ways the material could bend and how that affected functionality like stiffness. This way they could quickly scan close to a million different designs, and select those with the preferred response.
Once a specific design was selected, the team constructed working prototypes of each 3D metamaterial both using laser-cut cardboard and double-sided tape, and multimaterial 3D printing approaches. Like origami, the resulting structure can be folded along their edges to change shape.
“By combining design and computational modeling, we were able to identify a wide range of different deformations and rearrangements and create a blueprint or DNA for building these materials in the future,” says Overvelde, now Scientific Group Leader of the Soft Robotic Matter group at FOM Institute AMOLF in the Netherlands.
“This framework is like a toolkit to build reconfigurable materials,” says Hoberman. “These building blocks and design space are incredibly rich and we’ve only begun to explore all the things you can build with them.”
This formalized design framework could be useful for structural and aerospace engineers, material scientists, physicists, robotic engineers, biomedical engineers, designers, and architects.
“Now that we’ve solved the problem of formalizing the design, we can start to think about new ways to fabricate and reconfigure these metamaterials at smaller scales, for example through the development of 3D-printed self-actuating environmentally responsive prototypes.” says Weaver.