Liquifying Robot Points to Shape-Shifting Electronics

Some gaps are too small for animals with skeletons to fit, while squishy creatures can make themselves impossibly thin and pass through. But most animals must pick one form or another—they are either hard as lobsters and solid as sharks or shape-shifty as octopi.
 
Up to now, robots, even so-called soft robots, have been made of materials that had only so much give. But a team of engineers at Sun Yat-sen University in Shenzhen and Zhejiang University in Hangzhou, both in China, and Carnegie Mellon University in Pittsburgh, were inspired by an animal that can change from solid to liquid and back to develop a demonstration system that can melt its way out of a jam, as well as assemble and solder electronics via remote control. The researchers hope their development will lead to new classes of biomedical equipment and ways to firmly assemble products with hard-to-reach parts.
 
Sea cucumbers, which are marine animals related to starfish and sand dollars, can control their degree of stiffness, changing from rigid to jelly-like in a matter of seconds.
 
“This is an important requirement for squeezing through confined spaces and escaping predators,” said Carmel Majidi, a professor of mechanical engineering at Carnegie Mellon and one of the lead authors on the paper, “Magnetoactive liquid-solid phase transitional matter,” published in the journal Matter in January 2023. “We wanted to develop a material that had a similar ability to undergo a dramatic stiffness change in order to improve the ability of machines and robotic systems to maneuver through tightly confined spaces.”
 
While sea cucumbers are flesh and fluid, robots are generally made from metal and electronics, requiring electricity to power them. Metals can change from solid to soft, but generally only at high temperatures. One metal with a roughly room-temperature melting point is gallium.
 
“For over a decade, we’ve been working with gallium and gallium-based liquid metal alloys for applications in stretchable electronics and soft robotics,” Majidi said. Gallium has a melting point of 85.6 °F, or 29.8 °C. “Unlike mercury or lead, gallium can be used for liquid-phase or low-melting metals that are non-toxic and potentially safe for contact with human tissue.”
 

Induction Heating

 
Gallium metal by itself wouldn’t be more than a lump. What Majidi and his colleagues, including Chengfeng Pan, a former doctoral student of Majidi’s who is now an assistant professor of mechanical engineering at Zhejiang University, realized is they needed a way to heat the gallium quickly and uniformly as well as move the molten metal without touching it. Heating a gallium robot from the outside, such as placing it in an oven, clearly wasn’t the answer.
 
Instead, the team embedded the gallium with neodymium-iron-boron microparticles, which are magnetic and heat up when exposed to an alternating electric field, much the way an iron skillet heats up quickly when placed on an induction cooktop. The gallium returned to a solid through ordinary cooling. Magnetic fields also can act on the microparticles to move the gallium in either its liquid or solid form, allowing researchers to guide it across a surface.

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In a demonstration of the concept, the researchers created a tiny human-shaped figurine and placed it in a miniature jail cell. To help it escape, the researchers melted the figurine until it was thin enough to pass between the bars. Once outside, however, the blob needed to be reshaped by hand.
 
“Currently, we don’t have a reliable method for the material to regain its original shape autonomously,” Majidi said. “It would be tremendously impactful if we could achieve this added capability. One potential pathway is to guide the displacement and motion of the magnetic particles to deform the melted material back into the original shape. However, this will require more advanced use of electromagnetic field to individually address the different magnetic particles within the substance.”
 
Another demonstration didn’t require returning to the original shape. Instead, the engineers applied magnetic fields to guide a pill-shape gallium mass through a simulated stomach toward a small object. Once there, researchers melted the gallium and manipulated it so that it covered the object. When cool, the gallium could carry the embedded object with it.

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“Because these systems can be wirelessly manipulated, they are particularly well suited for applications in medicine like drug delivery, biopsy, imaging, and removal of foreign substances,” Majidi said. “Moreover, the magnetic fields required are within the range of what can be achieved using a standard MRI machine.”
 
There are also more straightforward applications. For instance, engineers could guide a gallium pellet to an opening deep inside a piece of electronics and then melt the pellet to fill the crevice. When cooled, the gallium could hold components together like a screw.
 
“The great thing about gallium is that it’s quite strong and stiff in its solid state and has similar load bearing capacity to other metals,” Majidi said.
 
Jeffrey Winters is editor in chief of Mechanical Engineering magazine.