A Lego minifig made out of a new material can “melt” through a cage’s metal bars before reassembling into its solid form on the other side. (credit: Q. Wang et al., 2023)

One of the many iconic moments in Terminator 2: Judgment Day was seeing the T-1000 briefly morph into a liquid to pass through the metal bars separating him from its target: a teenage John Connor. A team of engineers mimicked that famous scene with a soft robot in the shape of a Lego minifig. The robot “melts” into liquid form in response to a magnetic field, oozing between the bars of its cage before re-solidifying on the other side. The team described its work in a recent paper published in the journal Matter.

As we’ve previously reported, we traditionally think of robots as being manufactured out of hard, rigid materials, but the subfield of soft robotics takes a different approach. It seeks to build robotic devices out of more flexible materials that mimic the properties of those found in living animals. There are huge advantages to be gained by making the entire body of a robot out of soft materials, such as being flexible enough to squeeze through tight spaces to hunt for survivors after a disaster. Soft robots also hold strong potential as prosthetics or biomedical devices. Even rigid robots rely on some soft components, such as foot pads that serve as shock absorbers or flexible springs to store and release energy.

For instance, Harvard researchers built an octopus-inspired soft robot in 2016 that was constructed entirely out of flexible materials. Soft robots are more difficult to control precisely because they are so flexible. So, for the “octobot,” they replaced the rigid electronic circuits with micro-fluidic circuits. Such circuits regulate the flow of water (hydraulics) or air (pneumatics), rather than electricity, through the circuit’s microchannels, enabling the robot to bend and move. In 2021, engineers at the University of Maryland built a three-fingered soft robotic hand that is sufficiently agile to be able to manipulate the buttons and directional pad on a Nintendo controller—even managing to beat the first level of Super Mario Bros. as proof of concept.