Jan 23, 2017 | By Benedict

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute of Biologically Inspired Engineering at Harvard University have used multimaterial 3D printing in developing a general framework to design reconfigurable metamaterials.

Metamaterials, materials engineered to exhibit unusual properties not found in nature, can be used to bend light and sound, to improve antenna performance, and to dampen seismic waves from earthquakes. Scientists have even speculated that metamaterials could someday be used to create an invisibility cloak. Each of these applications is, however, dependent on the unique mechanical structure of the metamaterial in question, and these specific structures are often limiting in terms of what each material can actually be used for. Faced with this limitation, a group of Harvard scientists sought to create a framework for making metamaterials that can autonomously switch between several geometries and consequently several functions.

The research group, containing scientists from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute of Biologically Inspired Engineering at Harvard University, developed this general framework for designing reconfigurable metamaterials over the course of several years, and has now published its findings in the journal Nature. Importantly, because the framework is not dependent on scale, it can be applied to anything from the meter scale to the nano scale, from shock-absorbing building materials to photonic crystals, allowing metamaterial structures to take on new and multiple functions.

“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,” said Katia Bertoldi, John L. Loeb Associate Professor of the Natural Sciences at SEAS and senior author on the paper. “Through a collaboration with designers and mathematicians, we found a way to generalize these rules and quickly generate a lot of interesting designs.”

The exciting metamaterials project dates back to 2014, when Chuck Hoberman, of the Harvard Graduate School of Design (GSD) and associate faculty at the Wyss, showed Bertoldi some designs for a set of foldable structures, including a prototype of an extruded cube. “We were amazed by how easily it could fold and change shape,” said Bertoldi. “We realized that these simple geometries could be used as building blocks to form a new class of reconfigurable metamaterials but it took us a long time to identify a robust design strategy to achieve this.”

The research team, which soon contained Johannes Overvelde, first author on the paper and a former graduate student of Bertoldi’s, and James Weaver, a senior research scientist at the Wyss, soon realized that assemblies of polyhedra—solid figures with (usually) more than six faces—could be used as a template for designing extruded, reconfigurable, thin-walled structures. With a combination of design and computational modeling, they were able to create a number of different arrangements, as well as a blueprint for quickly and accurately building similar materials for a range of purposes.

With the computational models in place, the Harvard researchers were able to quantify all the different ways in which the material could bend, and calculate how such movements would affect properties like stiffness. They can now use their digital framework to rapidly cycle through millions of different designs, letting the computer pick the one with the ideal property set for a given purpose, be it shock absorbing, light refracting, or a combination of several functions. Once a given design had been chosen, the scientists were able to use a multimaterial 3D printer, as well as laser-cut cardboard and double-sided tape, to create working prototypes of the material.

At present, these prototypes are just a way to visualize how the complex structures of the materials look at a manageable scale, but the researchers are already hard at work figuring out how they might produce the metamaterials for their actual chosen functions. “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,” said Weaver.

According to the research group, this new framework for developing metamaterials could be useful for structural and aerospace engineers, material scientists, physicists, robotic engineers, biomedical engineers, designers, and architects. “This framework is like a toolkit to build reconfigurable materials,” commented 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.”



Posted in 3D Printing Materials



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