Small, flexible caterpillar-like robot bends, rolls, grabs and degrades

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When you hear the term “robot,” you might think of complicated machines working in factories or roaming other planets. But “millirobots” could change that. These are robots about as wide as a finger that could one day deliver drugs or perform minimally invasive surgery. Now researchers who report in ACS Applied Polymer Materials have developed a soft, biodegradable and magnetic millirobot inspired by the walking and grasping abilities of insects.

Some flexible millirobots are already being developed for various biomedical applications, thanks to their small size and ability to be powered externally, often by a magnetic field. Their unique structures allow them to squeeze or roll through the bumpy tissues of our gastrointestinal tract, for example. They might even one day be coated in a medicated solution and deliver the medicine exactly where it’s needed in the body. However, most millirobots are made from non-degradable materials, such as silicone, which means they would need to be surgically removed if used in clinical applications. Moreover, these materials are not very flexible and do not allow to refine the properties of the robot, which limits their adaptability. So Wanfeng Shang, Yajing Shen and their colleagues wanted to create a millirobot out of soft, biodegradable materials that can grab, roll and climb, but then easily dissolve when they’re done.

As a proof of concept, the researchers created a millirobot using a gelatin solution mixed with iron oxide microparticles. By placing the material over a permanent magnet, the microparticles in the solution pushed the gel outward, forming insect-like “legs” along the lines of the magnetic field. Then the hydrogel was placed in the cold to make it more solid. The final step was to soak the material in ammonium sulfate to cause cross-linking in the hydrogel, making it even stronger. Changing various factors, such as the composition of the ammonium sulfate solution, the thickness of the gel or the strength of the magnetic field, allowed the researchers to tune the properties. For example, placing the hydrogel farther from the magnet resulted in fewer but longer legs.

Since iron oxide microparticles form magnetic chains in the gel, moving a magnet near the hydrogel caused the legs to flex and produce a claw-like gripping motion. In experiments, the material grabbed a 3D-printed cylinder and a rubber band and transported each to new locations. Additionally, the researchers tested the millirobot’s ability to deliver medication by coating it in a dye solution and then rolling it around in a model stomach. Once at its destination, the robot uncoiled and released the dye through the strategic use of magnets. Since it is made from water-soluble gelatin, the millirobot easily degrades in water within two days, leaving only the tiny magnetic particles behind. The researchers say the new millirobot could open up new possibilities for drug delivery and other biomedical applications.

The authors acknowledge funding from the National Natural Science Foundation of China, the Hong Kong RGC General Research Fund, and the Shenzhen Key Basic Research Project.

Video: https://youtu.be/1va-OQvfJDg

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Materials provided by American chemical society. Note: Content may be edited for style and length.

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