Engineers at MIT, collaborating with researchers from EPFL and the University of Cincinnati, have developed a groundbreaking method to fabricate microscopic soft magnetic robots known as magno-bots. Published in the journal Matter, the study details a new hydrogel material that enables the creation of complex, three-dimensional structures smaller than a millimeter that can be controlled remotely by magnets. These structures feature micron-scale precision and can deform or move in intricate ways, opening new possibilities for applications in medicine, such as targeted drug delivery and minimally invasive biopsies. While moving microscopic objects with magnets has been explored before, existing designs often rely on mixing magnetic particles directly into printable resins. This approach frequently leads to structural weaknesses because metal particles scatter light during the 3D printing process or clump together, preventing the creation of detailed shapes. The new MIT technique overcomes these limitations through a unique double-dip process. First, researchers use standard two-photon lithography to 3D print a microstructure using a non-magnetic polymer gel. Next, the printed structure is dipped into a solution containing iron ions, which the gel absorbs. A second dip in a hydroxide solution causes these ions to react, forming iron-oxide nanoparticles within the gel. This method infuses the structure with magnetic properties after printing, avoiding the issues of light scattering and sedimentation associated with pre-mixing particles. A key innovation of this process is the ability to spatially tune the magnetism of different parts of a single structure. By adjusting the laser power during the initial printing phase, engineers can control how tightly the gel is cross-linked. Tighter gel formations absorb fewer magnetic particles, while looser areas absorb more. This allows researchers to create multifunctional devices where some components are highly magnetic and others are less so, all within the same tiny object. To demonstrate the technology, the team created ball-and-stick structures resembling tiny lollipops. The spherical heads were smaller than a grain of sand and infused with varying levels of magnetism. When an external magnet was passed over the liquid dish, these structures snapped together like the jaws of a Venus flytrap, forming an active robotic gripper. The researchers also fabricated a bistable switch featuring a central rectangular body flanked by tiny oar-like structures. Applying a magnet to either side caused the oars to flip, locking the switch in a new position. This mechanism could serve as a microfluidic valve to control fluid flow inside the body. Carlos Portela, an associate professor of mechanical engineering at MIT and a lead author of the study, noted that the material offers instantaneous response to magnetic stimuli without the delays associated with chemical reactions. Andrew Chen, another co-lead author, emphasized that the ability to manipulate these materials from a distance without physical contact is crucial. Rachel Sun, also a co-author, highlighted that the new method provides unprecedented design freedom for printing deformable, multifunctional microstructures. The team believes this technology paves the way for soft, miniature robots capable of performing complex tasks inside the human body, guided by external magnetic fields.