Soft metasurface can throw some shapes

Engineers at Duke University have developed a scalable soft surface that can continuously reshape itself to mimic objects in nature. Relying on electromagnetic actuation, mechanical modeling and machine learning to form new configurations, the surface can even learn to adapt to hindrances such as broken elements, unexpected constraints or changing environments. The engineers report the new soft surface in a paper in Nature.

“We’re motivated by the idea of controlling material properties or mechanical behaviors of an engineered object on the fly, which could be useful for applications like soft robotics, augmented reality, biomimetic materials and subject-specific wearables,” said Xiaoyue Ni, assistant professor of mechanical engineering and materials science at Duke. “We are focusing on engineering the shape of matter that hasn’t been predetermined, which is a pretty tall task to achieve, especially for soft materials.”

According to Ni, previous examples of morphing matter have typically been programmed rather than programmable. That is, soft surfaces equipped with designed active elements can shift between a few set shapes, like a piece of origami, in response to light or heat or other stimuli triggers. In contrast, Ni and her laboratory wanted to create something much more controllable that could morph and reconfigure as often as it likes into any physically possible shapes.

To create such a surface, the researchers started by laying out a grid of snake-like beams made of a thin layer of gold encapsulated by a thin polymer layer. The individual beams are just 8µm thick – about the thickness of a cotton fiber – and less than 1mm wide. The lightness of the gold beams means that magnetic forces can easily and rapidly deform them.

To generate these forces, the surface is put into a low-level static magnetic field. Voltage changes are then used to create a complex but easily predictable electrical current along the golden grid, which drives the surface’s out-of-plane displacement.

“This is the first artificial soft surface that is fast enough to accurately mimic a continuous shape-shifting process in nature,” Ni said. “One key advance is the structural design that enables an unusual linear relationship between the electrical inputs and the resulting shape, which makes it easy to figure out how to apply voltages to achieve a wide variety of target shapes.”

The new ‘metasurface’ shows off a wide array of morphing and mimicking skills. It can create bulges that rise and move around the surface like a cat trying to find its way out from under a blanket, oscillating wave patterns, and a convincing replication of a liquid drop dripping and plopping onto a solid surface. And it produces these shapes and behaviors at any speed or acceleration desired, meaning it can reimagine that trapped cat or dripped droplet in slow motion or fast forward.

With cameras monitoring the morphing surface, it can even learn to recreate shapes and patterns on its own. By slowly adjusting the applied voltages, a learning algorithm takes in 3D imaging data and figures out what effects the different inputs have on the metasurface’s shape. The researchers showed that when a human palm spotted with 16 black dots slowly shifted under a camera, the surface could mirror the movements perfectly.

“The control doesn’t have to know anything about the physics of the materials, it just takes small steps and watches to see if it’s getting closer to the target or not,” Ni said. “It currently takes about two minutes to achieve a new shape, but we hope to eventually improve the feedback system and learning algorithm to the point that it’s nearly real-time.”

Because the surface teaches itself to move through trial and error, it can also adapt to damage, unexpected physical constraints or environmental change. In one experiment, it quickly learned to mimic a bulging mound despite one of its beams being cut. In another, it managed to mimic a similar shape despite a weight being attached to one of the grid’s nodes.

There are many ways to extend the scale and configuration of this soft surface. For example, an array of surfaces can scale the size up to that of a touchscreen. Or fabrication techniques with higher precision can scale the size down to 1mm, making it more suitable for biomedical applications.

Moving forward, Ni wants to create robotic metasurfaces with integrated shape-sensing functions to perform real-time shape mimicking of complex, dynamic surfaces in nature, such as water ripples, fish fins or the human face. The researchers may also look into embedding more components into the prototype, such as on-board power sources, sensors, computational resources or wireless communication capabilities.

“Along with the pursuit of creating programmable and robotic materials, we envision future materials will be able to alter themselves to serve functions dynamically and interactively,” said Ni. “Such materials can sense and perceive requirements or information from the users, and transform and adapt according to the real-time needs of their specific performance, just like the microbots in Big Hero 6. The soft surface may find applications as a teleoperated robot, dynamic 3D display, camouflage, exoskeleton or other smart, functional surfaces that can work in harsh, unpredictable environments.”

This story is adapted from material from Duke University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.