Jellyfish-shaped robots made of magnetic ferrofluid can be controlled by light through an underwater obstacle course. Swarms of these soft robots could be useful for delivering chemicals throughout a liquid mixture or moving fluids through a lab-on-a-chip.
Ferrofluid droplets are made of magnetic nanoparticles suspended in oil, and they can move across flat surfaces or change shape when coaxed in different directions by magnets. By immersing these droplets in water and exposing them to light, Mengmeng Sun at the Max Planck Institute for Intelligent Systems in Germany and his colleagues have now made them defy gravity.
When ferrofluids absorb light – they are particularly good at that because they are dark – they heat up and any tiny bubbles within them expand. This makes the droplets lighter and more buoyant when they are under water, so they can float upwards, says Sun.
He and his colleagues created soft robots with a droplet of ferrofluid encased in a shell of hydrogel shaped like a jellyfish. Then they put them to the test. The researchers devised an obstacle course at the bottom of a tank of water, which included various platforms at different heights. They directed the robots through it, making them move up and over the platforms.
In one experiment, they arranged three of the jellyfish robots in a line at the bottom of a water tank and used a laser to heat them up. The robots moved upwards in succession, one after another. Sunlight focused through a magnifying glass had a similar effect, making the jellies float vertically.
Hamid Marvi at Arizona State University says controlling a whole swarm of droplets simultaneously could be useful in a future where they deliver drugs or perform other functions inside the human body. He says encasing them in hydrogel allows for complex motion, because light can be used to direct the ferrofluid droplet as well as to make the hydrogel itself move.
However, Marvi says many details, including the safety of ingesting ferrofluids, must be worked out before medical uses are possible. Sun and his colleagues hope to answer some of these open questions. For instance, they want to figure out how to use an optical fibre, which could enter the body, instead of lasers or sunlight to direct the robots.
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