It’s not just engineers or technology sites that have got excited by the new programmable mechanical metamaterial developed by researchers at Harvard University.
Mainstream media were no doubt seduced by the futuristic opening line in a new report from Harvard’s John A. Paulson School of Engineering and Applied Sciences: “Imagine a house that could fit in a backpack or a wall that could become a window with the flick of a switch.”
This foldable material can change shape, size and volume and was inspired by a technique in the art of origami called snapology. We interviewed Johannes T. B. Overvelde, lead author on the paper for Nature Communications and graduate student in the lab of Katia Bertoldi, the John L. Loeb Associate Professor of the Natural Sciences at the School of Engineering.
Could you tell us a little about your research background?
Johannes T. B. Overvelde: I did both my BSc and Msc in Mechanical Engineering at Delft University, with a focus on computational engineering and optimization in fields related to solid and fluid mechanics. After this, I started my PhD at Harvard University in the field of Applied Math (in Katia Bertoldi’s group), where I explored the use of compliance in the the design of materials and devices.
What is a metamaterial?
Johannes T. B. Overvelde: A metamaterial is a material with (typically) unusual properties that are derived from their internal geometry, not their chemical composition. Changing the material properties therefore comes down to the design of their microstructure.
(Courtesy Johannes T. B. Overvelde)
What inspired this particular project and how did the idea of a ‘folding’ material emerge?
Johannes T. B. Overvelde: Typically the properties (such as stiffness) of materials are fixed - they can’t be changed after producing them. We are interested in creating materials with tunable properties, so as to embed more functionality, and explore different geometries to achieve this. Many people have in fact explored similar directions in the ancient art of origami; in specific we were inspired by a technique called snapology, in which paper strips are ‘snapped’ together to create complex extruded polyhedra.
How does it work?
Johannes T. B. Overvelde: The metamaterial can be made by assembling unit cells; extruded cubes with 24 faces and 36 edges. Like origami, the cube can be folded along its edges to change shape. In the article we demonstrate, both theoretically and experimentally with a centimetre-scale prototype, that the cube can be deformed into many different shapes by folding certain edges, which act like hinges. We furthermore embedded pneumatic actuators (air pockets) into the structure, which can be programmed to deform specific hinges, changing the cube’s shapes and size, and removing the need for external force.
By strategically placing and pressurising the embedded actuators, we were able to control and deform the structure along these three degrees of freedom. The large assembly of extruded cubes (4x4x4) contained a total of 96 actuators, 32 for each degree of freedom, which we pressurized manually using three syringes.
The structure responded directly, such that the deformed states could be achieved within a few seconds. Moreover, when releasing the pressure, the energy stored in the elastic hinges immediately pushed the structure back to its initial fully expanded state.
In what ways do you think it could be deployed in building construction or architecture?
Johannes T. B. Overvelde: The design can be used to make foldable and reprogrammable objects of arbitrary architecture, whose shape, volume and stiffness can be dramatically altered and continuously tuned and controlled. Importantly, the structure is completely scalable; it works from the nanoscale to the metre-scale and could be used to make anything from surgical stents to portable pop-up domes for disaster relief.
In fact, by using various type of actuation, this material can be programmed to interact with its environment in different ways as it changes its shape. If one would make a wall from this material, for example, it could be folded in different ways to manipulate sound or light. Or a roof that on a hot day autonomously opens up its microstructure to allow fresh air to come in, or closes when it starts to rain.
Are you working on any other new materials projects?
Johannes T. B. Overvelde: We are currently exploring more designs based on similar principles. This metamaterial is just one expression of the vast design space. Moreover, I’m really interested in materials that autonomously adapt to their environment, for example through embedded sensing and computation.
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