Scientists have long been trying to create a miniature robot based on magnetic microparticles. But earlier developments demonstrated in this field (we discuss them below) lacked the desired characteristics: they were either too hard and insufficiently adaptive, or had very low mechanical strength.
At the end of January 2023, the research journal Matter published an article about a new liquid robot made of a magnetoactive phase-change substance. According to the scientists, inspiration came not from the famous T-1000 in Terminator 2: Judgment Day, but from the sea cucumber — an invertebrate echinoderm able to switch from a semi-liquid to a solid state in a few seconds.
The basis for the miniature robot was gallium — a metal that melts at 29.7646°C. Adding magnetic microparticles of neodymium, iron, and boron made it possible to quickly switch the substance from solid to liquid phase using a tuned magnetic field. By placing the alloy between two alternating magnets, scientists not only melted the metal without external heat sources but also could adjust the liquid robot’s direction of motion. The presented miniature prototype has:
Potentially the robot can be used to place electronic components on boards. At the presentation the robot was shown as a smart solder: with it a small diode was installed and fixed on a microcircuit. Scientists also demonstrated possible medical use, for example removing foreign objects from the stomach or local drug delivery.
For now this only demonstrates the ability of metal alloys for controlled dynamic reconfigurability. But engineers hope that in the future such technologies will find wide use in biomedicine.
Scientists have shown gallium experiments more than once. In 2018 an international team of researchers from China and Australia created a robot consisting of a plastic wheel, a lithium power source, and a drop of liquid gallium.
The structure rotated when the liquid metal moved through a tube inside the wheel. Movements of molten gallium were in turn controlled by changing the voltage supplied by the built-in lithium battery.
Robotics professor Li Xiangpeng, who worked on this robot, stated in a talk that tiny gallium-based nanorobots will soon be able to deliver cancer drugs targeted in the human body, as well as track and localize new malignant cells. Later he added that such miniature machines could also find use in military intelligence.
The magnetoactive phase-change substance created on a gallium basis is not the first material presented by scientists that can switch from solid to liquid. Engineers have long been developing miniature plastic “robots” able to deliver drugs in a targeted way.
In 2022 a team of scientists from Soochow University presented a microscopic robot able to break into droplets and then reassemble. The study was published in Science Advances. According to the article, the robot consists of ferrofluid — magnetic iron oxide nanoparticles combined with hydrocarbon oil. To demonstrate the development’s capabilities, scientists built a maze with obstacles and narrow sections through which they successfully guided the soft robot. Using an external magnetic field, the inventors controlled liquid droplets: stretched them, compressed them, made them rotate, jump, and perform other actions.
Overall, scientists have long worked on creating ferrofluid microrobots for medical purposes. Back in 2011, engineers from a U.S. Department of Energy national research center created magnetic robots less than 1 mm in size able to move in an organized way and assemble into different shapes under an external magnetic field.
Unfortunately, ferrofluids, like elastomers (silicones) used to create soft robots, have a number of limitations. The former are considered a very unstable material, which significantly narrows the application range. The latter have too low plasticity and therefore respond poorly to programmable deformations.
Not only elastomers, ferrofluids, and gallium are suitable for creating soft-bodied robots. In 2022 other scientists from Hong Kong presented a slime robot consisting of a mixture of PVA (polyvinyl alcohol) and sodium tetraborate (borax). Magnetic particles in the polymer made it possible to control movement of the substance with a magnetic field, and the resulting material’s ability to self-heal — to unite separate particles into a whole.
At the presentation researchers showed that the created material can adapt to changing mechanical conditions (compression, bending, stretching), making it possible to manufacture dynamic motion sensors based on it. The robot can also extract swallowed objects from the stomach without surgical instruments. For now the presented slime is toxic to humans, but if placed in a silicon dioxide shell, toxins can be prevented from entering the body during use.
The field of liquid robotics is only beginning to develop, but already shows great promise. Potential medical applications of new developments will make it possible to diagnose complex diseases early, locally treat severe illnesses without killing healthy cells, and save more lives. Liquid technologies may also help people with disabilities and contribute to a technological breakthrough in biomechanics.
