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This Metal Robot “Shapeshifts” Between Solid and Liquid To Escape Tight Spaces

A close-up photo of a white robot hand.
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Scientists have developed a strange new material that can quickly flip between a liquid and a solid state – all thanks to magnets.

In a scene that wouldn’t look out of place in James Cameron’s Terminator 2: Judgement Day, the researchers demonstrated the potential of their new creation by trapping a miniature robot made of the material in a cage, then having it escape. Using the power of magnets, the Lego-like robot can turn itself into a puddle and ooze past the cage bars, before being remoulded.

The researchers say that robots made from similar materials could one day be used in biomedicine, as a novel way to deliver drugs or remove foreign objects in hard-to-reach places such as the stomach.

This is a video of a person-shaped robot liquifying to escape from a cage after which it is extracted and remolded back into its original shape. Credit: Wang and Pan et al. / Carnegie Mellon University and Sun Yat-sen University.

Why the need for a shapeshifting robot?

Magnetically-driven miniature robots have garnered increasing attention over the past several years. Micro- and nanorobots capable of navigating the human body would open the door to a myriad of potential advances in personalized and precision medicine.

It’s hard to scale down traditional controls and actuation systems to fit the frames of these diminutive robots. This is where magnetic actuation comes in. Magnetic fields are safe for biological tissue and minimally invasive, while also offering high levels of controllability even at these tiny scales.

But this is where researchers have run into another problem. Currently, magnetically-actuated robots tend to be made from solid magnetic elastomers, which can be very rigid and offer little in terms of morphological adaptability. Alternative liquid material systems overcome this obstacle, but at the expense of having very low mechanical strength.

Inspired by the sea cucumber – a deep sea creature that can alter the stiffness of its body to withstand external damage and improve its weight-bearing ability – researchers set out to create a robot that could similarly fine-tune its stiffness, to see if it could be used for these applications.

“Giving robots the ability to switch between liquid and solid states endows them with more functionality,” said study lead Dr. Chengfeng Pan, who was then an engineer at the Chinese University of Hong Kong and is now an assistant professor of mechanical engineering at Zhejiang University, in a press release.

The new phase-changing material was presented in an article published in the journal Matter.

“Now, we’re pushing this material system in more practical ways to solve some very specific medical and engineering problems,” said Pan.

Magnetic particles create puddles out of robots

The key to the team’s robot creation is a new phase-shifting material, dubbed “magnetoactive phase transitional matter” (MPTM). The material is composed of magnetic neodymium-iron-boron microparticles embedded in gallium – a metal with a melting point of just 29.8 °C.

“The magnetic particles here have two roles,” said Prof. Carmel Majidi, a professor of mechanical engineering at Carnegie Mellon University and the paper’s senior author. “One is that they make the material responsive to an alternating magnetic field, so you can, through induction, heat up the material and cause the phase change. But the magnetic particles also give the robots mobility and the ability to move in response to the magnetic field.”

Phase-changing materials themselves also aren’t new; but current options have a rather viscous liquid phase and tend to rely on electrical currents, heat guns or other external heat sources to trigger that phase change. With MPTM, the liquid phase flows much more easily and everything can be controlled by the magnetic field.

Physical tests showed that the MPTM material has a high mechanical strength (21.2 MPa) and stiffness (1.98 GPa) in its solid phase, meaning that it would require high forces to irreversibly deform or fracture the material. It could also bear loads up to 30 times its own weight. In its liquid phase, the MPTM robot could flow at speeds of up to 15 centimeters per second.

Could a shapeshifting robot save your life one day?

As seen in the video above, the robot can quickly change between its solid and liquid phases to escape tight spaces. Additional mobility and strength tests done by the research team also showed the robot shifting into its liquid form to climb walls and split into two halves that could work together to move objects before coalescing back together.

Beyond this obstacle course of testing, the researchers also began to examine some other practical use cases for their robot creation. One of these was an engineering test, where the robot functioned as a sort of “universal screw” – melting into its liquid form to fill up a screw socket, before solidifying to become a hard screw. They also propose that the robots could be used as smart soldering robots for wireless circuit assembly, by oozing into hard-to-reach circuits and acting as both metallic solder and an electrical conductor.

Other biomedical-style tests put the robot to use inside of a model stomach, demonstrating how the robot can be moved through it using a magnetic field. Inside the model stomach, the robot was able to melt into its liquid form to trap foreign debris and carry it out of the organ. The robot could also do the opposite – delivering drugs by melting to drop off its payload before re-hardening to exit the stomach.

“Future work should further explore how these robots could be used within a biomedical context,” said Majidi. “What we're showing are just one-off demonstrations, proofs of concept, but much more study will be required to delve into how this could actually be used for drug delivery or for removing foreign objects.”

Reference: Wang Q, Pan C, Zhang Y, et al. Magnetoactive liquid-solid phase transitional matter. Matter. 2023;6(3):855-872. doi: 10.1016/j.matt.2022.12.003