Robotics

Mesh-like structures morph into predetermined shapes

MIT mechanical engineers have designed 3-D-printed mesh-like structures that morph from flat layers into predetermined shapes, which could lead to developments such as adaptive robotic fins.
By Jennifer Chu October 4, 2019
Courtesy: Massachusetts Institute of Technology (MIT)

Researchers at MIT have designed 3-D printed mesh-like structures that morph from flat layers into predetermined shapes, in response to changes in ambient temperature. The new structures can transform into configurations that are more complex than what other shape-shifting materials and structures can achieve.

As a demonstration, the researchers printed a flat mesh that, when exposed to a certain temperature difference, deforms into the shape of a human face. They also designed a mesh embedded with conductive liquid metal, that curves into a dome to form an active antenna, the resonance frequency of which changes as it deforms.

The team’s new design method can be used to determine the specific pattern of flat mesh structures to print, given the material’s properties, in order to make the structure transform into a desired shape.

The researchers say that down the road, their technique may be used to design deployable structures, such as tents or coverings that automatically unfurl and inflate in response to changes in temperature or other ambient conditions.

Such complex, shape-shifting structures could also be of use as stents or scaffolds for artificial tissue, or as deformable lenses in telescopes. Wim van Rees, assistant professor of mechanical engineering at MIT, also sees applications in soft robotics.

“I’d like to see this incorporated in, for example, a robotic jellyfish that changes shape to swim as we put it in water,” van Rees said. “If you could use this as an actuator, like an artificial muscle, the actuator could be any arbitrary shape that transforms into another arbitrary shape. Then you’re entering an entirely new design space in soft robotics.”

Van Rees and his colleagues are currently investigating ways to apply the design of complex shape-shifting to stiffer materials, for sturdier applications, such as temperature-responsive tents and self-propelling fins and wings.

Gift wrap’s limit

Two years ago, van Rees came up with a theoretical design for how to transform a thin flat sheet into a complex shape such as a human face. Until then, researchers in the field of 4-D materials — materials designed to deform over time — had developed ways for certain materials to change, or morph, but only into relatively simple structures.

For the domes: A set of lattice structures that has transformed into spherical caps, or dome-like shapes, after application of a temperature difference. The individual samples range from 3x3 cells to 20x20 cells, with further variations cell sizes and rib dimensions. Courtesy: Massachusetts Institute of Technology (MIT)

For the domes: A set of lattice structures that has transformed into spherical caps, or dome-like shapes, after application of a temperature difference. The individual samples range from 3×3 cells to 20×20 cells, with further variations cell sizes and rib dimensions. Courtesy: Massachusetts Institute of Technology (MIT)

“My goal was to start with a complex 3-D shape that we want to achieve, like a human face, and then ask, ‘How do we program a material so it gets there?’” van Rees said. “That’s a problem of inverse design.”

He came up with a formula to compute the expansion and contraction that regions of a bilayer material sheet would have to achieve in order to reach a desired shape, and developed a code to simulate this in a theoretical material. He then put the formula to work, and visualized how the method could transform a flat, continuous disc into a complex human face.

However, van Rees and his collaborators found the method wouldn’t apply to most physical materials if they were trying to work with continuous sheets. While van Rees used a continuous sheet for his simulations, it was of an idealized material, with no physical constraints on the amount of expansion and contraction it could achieve. Most materials, in contrast, have very limited growth capabilities. This limitation has profound consequences on a property known as double curvature, meaning a surface that can curve simultaneously in two perpendicular directions — an effect that is described in an almost 200-year-old theorem by Carl Friedrich Gauss called the Theorema Egregium, Latin for “Remarkable Theorem.”

To impart double curvature to a shape-shifting sheet, the researchers switched the basis of the structure from a continuous sheet to a lattice, or mesh. The idea was twofold: first, a temperature-induced bending of the lattice’s ribs would result in much larger expansions and contractions of the mesh nodes, than could be achieved in a continuous sheet. Second, the voids in the lattice can easily accommodate large changes in surface area when the ribs are designed to grow at different rates across the sheet.

The researchers also designed each individual rib of the lattice to bend by a predetermined degree in order to create the shape of, say, a nose rather than an eye-socket.

For each rib, they incorporated four skinnier ribs, arranging two to line up atop the other two. All four miniribs were made from carefully selected variations of the same base material, to calibrate the required different responses to temperature.

When the four miniribs were bonded together in the printing process to form one larger rib, the rib as a whole could curve due to the difference in temperature response between the materials of the smaller ribs: If one material is more responsive to temperature, it may prefer to elongate. But because it is bonded to a less responsive rib, which resists the elongation, the whole rib will curve instead.

The researchers can play with the arrangement of the four ribs to “preprogram” whether the rib as a whole curves up to form part of a nose, or dips down as part of an eye socket.

Massachusetts Institute of Technology (MIT)

www.mit.edu

– Edited by Chris Vavra, production editor, Control Engineering, CFE Media & Technology, cvavra@cfemedia.com. See more Control Engineering robotics stories.


Jennifer Chu
Author Bio: MIT News Office