Engineers have developed a new soft magnetic hydrogel that can be 3D-printed into microscopic structures. 

Compared with previous magnetic materials that move as a single unit, this new gel allows individual parts of a tiny robot to deform and move independently in response to an external magnet.

The development comes from the Massachusetts Institute of Technology (MIT), the Swiss Federal Institute of Technology of Lausanne (EPFL), and the University of Cincinnati. 

These magnetically controlled soft robots, or magno-bots, could be used in healthcare to collect tiny medical samples or deliver medicine into the body.

“We can now make a soft, intricate 3D architecture with components that can move and deform in complex ways within the same microscopic structure. For soft microscopic robotics, or stimuli-responsive matter, that could be a game-changing capability,” said Carlos Portela, study author from MIT. 

New breed of micro-bot

MIT researchers are prioritizing magnetic stimuli over other triggers — like light or chemicals — because of their unique speed and convenience. 

Magnetic fields can help achieve instantaneous, wireless control from a distance, bypassing the need for slow chemical reactions or physical contact. This “programmable” approach enables immediate manipulation of a material’s properties for high-precision, remote-controlled micro-robotics.

The new work has created tiny 3D-printed “lollipops” made of a special magnetic gel, each smaller than a grain of sand. Interestingly, it can instantly transform into robotic grippers when a magnet is waved near it. 

To create magnetically responsive structures smaller than a millimeter, researchers typically rely on two-photon lithography, a high-resolution 3D printing technique that uses lasers to solidify resin. 

However, standard 3D printing of magnetic materials is difficult because magnetic nanoparticles — essentially tiny bits of metal — scatter the laser light and clump together. This interference reduces the laser’s power and compromises the structural integrity of the print, often rendering it impossible to produce intricate, functional microdesigns.

“Directly 3D printing deformable micron-scale structures with a high fraction of magnetic particles is extremely difficult, often involving a tradeoff between magnetic functionality and structural integrity,” said Rachel Sun, co-lead author. 

To overcome these printing obstacles, a “double-dip” fabrication process was used in this work. It adds magnetic properties after the 3D printing is complete. 

The team first prints a clean polymer microstructure and then submerges it in successive chemical baths to grow iron-oxide nanoparticles directly within the gel. 

Furthermore, the gel’s density can be controlled by adjusting the laser power during the initial print. A tighter gel absorbs fewer ions, enabling precise tuning of the magnetism of individual components within a single microscopic robot.

Lollipop gripper robot

To demonstrate the material’s precision, the 3D-printed “lollipop” structures were magnetized to varying levels. When exposed to a simple refrigerator magnet, these individual components reacted with different strengths, allowing the collection of lollipops to move in coordination. 

This synchronized movement mimicked the motion of gripping fingers, demonstrating that these microscopic structures can function as complex, remotely controlled robotic tools.

“You could imagine a magnetic architecture like this could act as a small robot that you could guide through the body with an external magnet, and it could latch onto something, for instance, to take a biopsy,” Portela said. “That is a vision that others can take from this work.”

The researchers also engineered a “bistable” switch using a millimeter-long gel rectangle equipped with magnetic “oars” the size of a red blood cell. 

Applying an external magnet flips these oars to pull and lock the device in either an on or off position, functioning like a remote-controlled toggle. This mechanism could serve as a microscopic valve to regulate fluid flow in medical devices.