MIT engineers create a system for building shape-changing smart devices

A new set of modular components allows users to create reconfigurable smart devices with electrical connections that keep working no matter which shape the structure forms.This electrical modularity can enable engineers to design interactive devices that can sense which shape they have taken, without the need for external wires. For instance, the modular components, which the researchers call “bifur-circuits,” could be used to rapidly design and prototype adaptable smart devices, like assistive furniture that helps individuals change body positions while recovering from injuries or reconfigurable robotic grippers that remain electrically connected when they change shapes for different applications. Developed by MIT researchers, these 3D-printed building blocks, which are a type of structure known as a mechanical metamaterial, can be combined to form many more possible configurations than traditional metamaterial structures. In a study presenting the new system, the researchers demonstrated several interactive objects, including a chair that converts to a table with storage and can also flatten for stowing. The structure senses its configuration and sends corresponding messages to an electronic display. These new metamaterials could also be used to design antennas for communications and sensing that form new shapes to adjust their frequencies in changing environmental conditions, without bulky mechanical parts. “Metamaterials can make complex mechanical assemblies easy to manufacture just by using repeating units. Our work expands on this design space. If we think of mechanical metamaterials as building blocks, then our work is one way to take advantage of their geometry to embed intrinsic intelligence into hardware, which could open many possibilities,” says Marwa AlAlawi, a mechanical engineering graduate student and lead author of a paper on the devices.AlAlawi is joined on the paper by co-senior authors Ticha Sethapakdi, an electrical engineering and computer science (EECS) graduate student at MIT; and Stefanie Mueller, an associate professor in MIT’s departments of EECS and Mechanical Engineering and leader of the Human-Computer Interaction Group at the Computer Science and Artificial Intelligence Lab (CSAIL). Their co-authors include others at MIT, the University of Tokyo, and the University of Michigan. The research will be presented at the ACM Symposium on User Interface Software and Technology.Shape-changing interactive structuresMechanical metamaterials are programmable, three-dimensional structures of repeating units that can form complex shapes due to their geometries. When squeezed, pushed, or pulled, metamaterials can bend or twist in precise ways. For instance, “auxetic” metamaterials get wider when stretched, instead of narrowing.In prior work, the MIT researchers used auxetic metamaterials to build reconfigurable antennas that formed three shapes depending on how the structure was stretched. This allowed the antenna to dynamically adjust its frequency range without complex, moving parts.Next, the team wanted to expand the number of antenna configurations but were limited because the auxetic metamaterials could only form three fixed states.In this work they created “bifur-circuits,” which are auxetic metamaterials that can form many more shapes based on how the modular units are connected and rotated. The units are also designed to be electrically modular. Due to the way conductive material is integrated into the bifur-circuits, electrical connections throughout the structure are maintained no matter how the object is rotated, pressed, or twisted to form new shapes. To create interactive objects with many possible configurations, bifur-circuits leverage a property known as mechanical bifurcation. Mechanical bifurcation is a sudden change in how a mechanism behaves when a force exerted on it passes a tipping point. For instance, when you gently bend the ends of a plastic ruler, once that force reaches a critical threshold, the ruler buckles.In bifur-circuits, this bifurcation occurs when connected blocks are rotated in certain ways around a pivot point. The property allows connected blocks to form more stable configurations than one block could on its own.Adding more bifur-circuits to a structure exponentially increases the number of potential configurations.“Bifurcation allow us to significantly expand on this reconfigurability space. Just adding one extra unit gives us so many more combinations out of the same structure,” says AlAlawi.Connecting and rotating components activates a unique circuit between adjacent units. This interactivity allows the units to communicate with one another, enabling the structure to sense its configuration.   One of the biggest challenges the researchers faced was incorporating a conductive material that was flexible enough to bend, but still offered enough efficiency in the flow of electricity.“The conductive material was a constraint we had to work around in the design process, and it dictated how the sensing between blocks would happen,” AlAlawi says.Once they perfected the design, the researchers tested the durability of reconfigurable structures by compressing them more than 10,000 times. The structures showed no degradation in electrical connectivity.The researchers also developed a user-friendly construction and simulation tool to simplify the bifur-circuit design process. The software generates instructions for a multimaterial 3D printer, which can fabricate the reconfigurable objects in one pass.They demonstrated the versatility of bifur-circuits by fabricating a chair that can sense its geometry when its shape is changed to a tea table, as well as a shape-shifting controller that will launch one of several video games based on its configuration.Bifur-circuits could someday be used in applications like interactive rehabilitation tools, shape-changing grippers for modular soft robots, or reconfigurable shelters that could respond to changing environmental conditions after a natural disaster.In the future, the researchers want to explore more applications for bifur-circuits. They also want to add more interactivity into the structures and investigate additional metamaterial shapes.“Bifur-circuits are one step toward developing mechanical building blocks with integrated intelligence. It would be interesting to build on this work and come up with building blocks that allow us to create a structure with any form or shape we want, and which are structurally stable and can be actuated,” AlAlawi says. This work was funded, in part, by Japan’s Science and Technology Agency and the Bahrain Crown Prince International Scholarship Program.