Some soft milli-robots have already been developed for various biomedical applications due to their small size and the ability to be externally powered, often by magnetic fields. Their unique structure allows them, for example, to pass through and roll through the rough tissue of the gastrointestinal tract. One day, we might even be able to coat it with a drug solution to deliver it exactly where it’s needed in the body. However, most milli-robots are made of non-degradable materials such as silicon, which must be surgically removed for clinical use. Additionally, these materials are not very flexible and the robot’s properties cannot be fine-tuned, limiting its adaptability. So Wanfeng Shang, Yajing Shen and others wanted to build a milli-robot out of soft, biodegradable materials that could grab, roll, and climb, and would easily melt when the job was done.

As a proof of concept, the researchers used a gelatin solution mixed with iron oxide microparticles to create a milli-robot. When the material was placed over a permanent magnet, the microparticles in the solution pushed the gel outward, forming insect-like “legs” along the lines of the magnetic field. The hydrogel was then placed in the cold to make it more rigid. The final step was to soak the material in ammonium sulfate to create crosslinks in the hydrogel and make it even stronger. This allowed the researchers to adjust the characteristics. For example, if you move the hydrogel away from the magnet, it will have fewer legs, but it will be longer.

Because the iron oxide microparticles form magnetic chains within the gel, moving the magnet near the hydrogel flexed the legs and produced a claw-like grasping motion. In the experiment, the material grabbed a 3D-printed cylinder and a rubber band and carried each to a new location. Additionally, the researchers tested the milli-robot’s drug-delivery capabilities by coating it with a dye solution and rolling it through a model of the stomach. Upon reaching its destination, the robot deployed and strategically used magnets to release dye. Made using water-soluble gelatin, the milli-robots easily decompose in water in two days, leaving only tiny magnetic particles. He said it could open up new possibilities for 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 American Chemical Society. Note: Content may be edited for style and length.