Small caterpillar-like soft robot folds, rolls, grabs and degrades

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

Some soft millirobots are already being developed for a variety of biomedical applications, thanks to their small size and ability to be driven externally, often by a magnetic field. Their unique structures allow them to crawl or roll through the bumpy tissues of our gastrointestinal tract, for example. They may even one day be coated in a drug solution and deliver the drug right where it is needed in the body. However, most millirobots are made of non-degradable materials, such as silicone, which means they must be surgically removed if used in clinical applications. In addition, these materials are not as flexible and do not allow much fine-tuning of the robot’s properties, limiting their adaptability. So Wanfeng Shang, Yajing Shen and colleagues wanted to create a millirobot from soft, biodegradable materials that could grab, roll, and climb, but then dissolve easily after its job is 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 out, forming insect-like “legs” along the lines of the magnetic field. Then the hydrogel was placed in the cold to make it firmer. The final step was to soak the material in ammonium sulfate to cause cross-linking in the hydrogel, making it even stronger. By changing several factors, such as the composition of the ammonium sulfate solution, the thickness of the gel or the strength of the magnetic field, the researchers were able to fine-tune the properties. For example, by placing the hydrogel further away from the magnet, fewer, but longer legs are created.

Because the microparticles of iron oxide form magnetic chains in the gel, moving a magnet near the hydrogel caused the legs to arch and produce a claw-like grasping motion. In experiments, the material grabbed a 3D-printed cylinder and a rubber band and carried each to new locations. In addition, the researchers tested the millirobot’s ability to deliver a drug by coating it in a dye solution and then rolling it through a stomach model. Once at its destination, the robot unfolded and released the dye with the strategic use of magnets. Because it’s made with water-soluble gelatin, the millirobot easily degraded in water in 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.