In James Cameron’s sci-fi classic, Terminator 2: Judgment Day, Arnold Schwarzenegger’s iconic time-traveling robot assassin, the T-800, faces off against a next-generation model, played by Robert Patrick. This upgraded Terminator proves rather difficult to disable or escape, mainly because of its liquid metal body, rendered by special effects that were groundbreaking at the time of the film’s release in 1991.
Thirty years later, we may not have mastered time travel or created killer assassin robots that will overthrow humanity (although Boston Dynamics is working hard on the required technology) but a new study from researchers at Carnegie Mellon University has taken us a step closer to the T-1000’s phase-shifting exploits. In a new publication in the journal Matterthe team shows off a tiny lego-like robot that can switch between matter states using the power of magnets.
Soft or hard robots?
Modern robotics efforts have produced both rigid and soft robots. The former have hard metal or plastic bodies, while the latter use more flexible substrates. Soft robots may have improved functionality due to their more mobile structures, but they are also often weaker and harder to control.
This is a video of a person-shaped robot liquefying 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/ Sun Yat-sen University
In a press release, study lead Dr. Chengfeng Pan, an engineer at The Chinese University of Hong Kong said, “Giving robots the ability to switch between liquid and solid states endows them with more functionality.”
Forget The Terminator, enter the sea cucumber
The team’s inspiration to create the phase-shifting robot did not come from Hollywood films. Instead, they give credit to a less glamorous source – the lumpy but highly dynamic sea cucumber. This leathery echinoderm lives on the sea floor and can alter the stiffness of its body to bolster its weight-bearing ability and withstand external damage.
Pan and colleagues wanted to make a robot that could similarly tune its stiffness. Previous efforts to create such machines have used phase-changeable polymers embedded with magnetic microparticles. These polymers are melted by external heat sources, such as lasers, while the microparticles steer shape change when exposed to a magnetic field.
But these robots, write the authors, “are solid and quasi-solid machines with limited morphological adaptability or are liquid or paste-like with low mechanical strength, poor mechanical integrity, poor controllability and low locomotion speed.” A fast-moving, robust magnetic melting machine has proved elusive thus far.
Pan’s team aimed to overcome these challenges by using a new phase-altering material they called a “magnetoactive solid-liquid phase transitional machine”. This is a composite of magnetic particles and a metal with a low melting point, such as gallium, which becomes liquid at just 29.8 °C/86 °F. That point could be brought even lower by using alloys, such as the gallium–indium–tin mixture dubbed “Galinstan”, which melts at -19 °C/-2.2 °F.
This combination allowed the robot to switch phases through inductive heating when exposed to an alternating magnetic field. The robot also had an unusually flexible liquid phase compared to more jelly-like predecessors that could flow at up to 15 cm/s. The robot’s solid phase, on the other hand, was strong – capable of bearing the load of an object 30 times its own weight.
The team quickly put the bot through a series of tests. Its flexible form helped it climb walls, split into two halves that worked together to move objects, jump moats and, as seen in the video above, escape from an enclosure that a solid robot would have been trapped in, before being remolded into its original solid shape on the other side.
How to use a shape-shifting robot
The team quickly moved past these exploratory physical tests to show off the robots’ potential applications. The robots could prove useful as tools for engineering: one demonstration showed how the liquid form of the robot could become a “universal screw” for hard-to-access spaces by flowing into a screw socket and then solidifying.
Other use cases were biomedical. In a model stomach, the team showed how the micromachine could switch to a liquid phase to wrap itself around other foreign objects in the stomach, before becoming solid again to enable both the robot and its cargo to be extracted. A reverse of this test saw the robot drop off cargo at set points in the model stomach, which the authors say is a demonstration of its potential as a drug delivery system.
Thankfully, the team is not planning to explore how the robots could be used to crush the remains of humanity’s resistance in a distant post-apocalyptic future. “Future work should further explore how these robots could be used within a biomedical context,” said senior author and mechanical engineer Professor Carmel Majidi of Carnegie Mellon University. “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. 2022. doi:10.1016/ j.matt.2022.12.003.