Researchers at Massachusetts Institute of Technology (MIT) have developed a fabrication method for soft magnetic hydrogels that can be 3D printed into structures smaller than 1mm and activated remotely by an ordinary magnet.
According to the research paper published in Matter, the study also saw contributions from École Polytechnique Fédérale de Lausanne (EPFL) and the University of Cincinnati.
The core technical problem the team solved is one that has blocked this line of research for years. Standard microstructure fabrication relies on two-photon polymerization (2PP). When researchers have tried to load that resin with magnetic nanoparticles beforehand, the metal particles scatter or absorb the laser light, weakening the final structure or preventing it from forming altogether.
The MIT team’s answer was to separate the printing step from the magnetization step entirely. They first print a structure using ordinary polymer gel, with no magnetic material added. The printed structure is then soaked in a solution of iron ions, which the gel absorbs, and dipped a second time in a hydroxide ion solution. The two sets of ions react inside the gel to form iron-oxide nanoparticles, which are inherently magnetic. Because magnetism is introduced after printing, the laser operates without interference, and the structural resolution of the technique is preserved.
The method also gives researchers control over how strongly magnetic any individual feature of a structure will be. By adjusting the laser’s power while printing a given section, they can change how tightly cross-linked the gel is in that area. A tighter gel absorbs fewer ions and ends up less magnetic. A looser gel absorbs more. This means a single printed structure can have components with meaningfully different magnetic responses, all within the same sub-millimeter architecture.
Moving Individual Features Within Microstructures
As a proof of concept, the team printed a cluster of lollipop-shaped structures, each <1mm tall, with spherical tips smaller than a grain of sand. Each sphere was infused with a different concentration of magnetic particles. When a refrigerator magnet was passed over the dish, the structures bent and pulled toward it in a graded sequence that resembled a closing hand.
A second demonstration used the same gel to build a bistable switch: a 1mm-long rectangle flanked by oar-like magnetic arms ~8µm thick, roughly the width of a red blood cell. A magnet applied to one end flipped the oars and locked the rectangle in one position; moving the magnet to the other end reversed it. The researchers suggest such a device could function as a magnetically controlled valve in a microfluidic circuit.
The practical applications being discussed are primarily medical. Structures of this kind, guided through the body by an external magnet, could in principle be used to deliver drugs to specific locations or collect tissue samples without surgical access. Similar magnetic microdevices have been proposed before, but earlier designs mostly function by pulling an entire structure in the direction of a magnet. What the MIT team has built moves individual features within a single structure, which is what makes more complex tasks, like gripping, plausible at this scale.
That kind of selective, localized movement within a microscopic object is what distinguishes this from earlier magnetic microrobotics work. Whether the fabrication process can be adapted for clinical-grade manufacturing, or scaled in any meaningful way, remains an open question the paper does not address. The research was supported in part by the National Science Foundation and a MathWorks seed grant.
The approach of embedding magnetic particles directly into the resin before printing is not new. In 2014, researchers at ETH Zurich used 2PP to fabricate helix-shaped microactuators from a biocompatible epoxy resin loaded with magnetic nanoparticles, producing structures about 60µm long that could be guided through liquids by an external magnetic field.
In that approach, magnetic functionality had to be established during fabrication, alongside the printing process itself. The MIT double-dip process removes this dependency by separating the two steps: structures are first printed without magnetic material, then magnetism is introduced afterward, avoiding interference with the fabrication method while enabling spatial variation in magnetic response within a single microstructure.
Titled, “Magnetically responsive microprintable soft nanocomposites with tunable nanoparticle loading,” the study was conducted by Rachel M. Sun, Andrew Y. Chen, Yiming Ji, Eric M. Stewart, Daryl W. Yee, and Carlos M. Portela.
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Featured image shows the team fabricated ball-and-stick structures resembling tiny lollipops. The structures were less than a millimeter in height, with balls that were smaller than a grain of sand. The balls were infused with magnetic particles. Image via Rachel M. Sun et al., Matter.
