Under a microscope, a bouquet of lollipop-like structures, each smaller than a grain of sand, gently undulates in a Petri dish of liquid. Suddenly they snap together like the jaws of a flycatcher as the scientist waves a miniature magnet over the plate. What was previously a collection of small passive structures immediately transformed into an energetic robotic gripper.
The Lollipop gripper is one demonstration of a modern type of cushioned magnetic hydrogel developed by MIT engineers and their collaborators at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland and the University of Cincinnati. In the study will appear in the magazine today MIT team reports a modern method for printing and producing a gel that can be transformed into convoluted, magnetically activated three-dimensional structures.
The modern gel could form the basis of cushioned, microscopic, magnetically responsive robots and materials. Such magnobots could be used in medicine, for example to release drugs or take biopsies under the influence of an external magnet.
Setting objects in motion using magnets is nothing modern, at least on a macro scale. For example, we can wave a fridge magnet over a stack of paper clips, which will follow the magnet in response. At the microscale, scientists have designed a variety of magnetic “micro-swimmers” – pieces smaller than a millimeter that can be remotely guided by a magnet to squeeze through miniature spaces. For the most part, these designs work by mixing magnetic particles with a printable resin and attracting the entire float toward an external magnet.
And the MIT team’s modern material can be turned into even more convoluted and deformable structures with micron-scale precision. These functions could enable the magnetic millibot to move individual elements and perform more convoluted maneuvers.
“We can now create a soft, complex 3D architecture from components that can move and deform in complex ways within the same microscopic structure,” says study author Carlos Portela, the Robert N. Noyce Professor of Mechanical Engineering at MIT. “For soft, microscopic robotics or stimuli-responsive matter, this could be a game-changing capability.”
Co-authors of the study from MIT include graduate students Rachel Sun and Andrew Chen, as well as Yiming Ji and Daryl Yee of EPFL and Eric Stewart of the University of Cincinnati.
Instantly
At MIT, Portela’s group is developing modern metamaterials — materials with unique, microscopic architectures that result in supernormal material properties. Portela has produced a variety of metamaterials, including extremely strong and versatile structures and structures that can manipulate sound and withstand violent impacts.
He has recently expanded his research to “programmable” materials, which can be designed to change their properties in response to stimuli such as specific chemicals, airy, and electric and magnetic fields.
From the team’s perspective, magnetic stimuli stand out from the rest.
“With a magnetically responsive material, we can control it from a distance, and the response is immediate,” says co-author Andrew Chen. “We don’t have to wait for a slow chemical reaction or physical process, and we can manipulate the material without touching it.”
In the modern study, the team aimed to create a magnetically responsive metamaterial that could produce structures smaller than a millimeter. Scientists typically produce the microstructures using two-photon lithography, a high-resolution 3D printing technique that involves shining a laser into a miniature puddle of resin. With repeated flashes, the laser draws a microscopic pattern in the resin, which solidifies into the same pattern, eventually forming a miniature, three-dimensional structure, layer by layer.
While 3D printing resin can produce convoluted microstructures, using the same process to print magnetic structures is challenging. The researchers tried combining the resin with magnetic nanoparticles before printing the mixture. However, magnetic particles are essentially pieces of metal that inherently scatter airy or agglomerate and deposit unintentionally. Scientists have found that any magnetic particles in the resin can reduce the laser power in a given place and weaken the resulting structure or completely prevent it from being printed.
“Directly 3D printing micron-scale deformable structures with a high content of magnetic particles is extremely difficult and often involves a trade-off between magnetic functionality and structural integrity,” says Sun, co-author of the paper.
Double-sided print
Scientists have created a modern way to produce magnetic microstructures by combining resin 3D printing with a double-immersion process. The researchers first used conventional resin printing to create a microstructure using a typical polymer gel, without the addition of magnetic particles. They then immersed the printed gel in a solution containing iron ions that the gel could absorb. The iron-soaked structure is then immersed again in a second solution of hydroxide ions. The iron ions in the gel bond with the hydroxide ions to form iron oxide nanoparticles, which are magnetic in nature.
With this modern process, the team can print convoluted structures smaller than a millimeter and, after printing, add magnetic properties to them. Moreover, they are able to control the magnetic strength of individual features of the structure. They found that by adjusting the laser power when printing specific features, they could determine the degree of cross-linking, or “tightness,” of the gel after printing. The denser the gel, the fewer magnetic particles it can create. This way, researchers can determine how magnetic each small element might be.
“This provides unprecedented design freedom for printing multifunctional structures and materials at the microscale,” says Sun.
As a demonstration, the team produced structures made of balls and sticks that resembled small lollipops. The structures were less than a millimeter high, and the spheres were smaller than a grain of sand. Scientists printed lollipops from polymer gel and infused each ball with different amounts of magnetic particles, giving them different degrees of magnetism. Under a microscope, they observed that when they moved an ordinary refrigerator magnet over the structures, the lollipops were attracted to the magnet to varying degrees, in a configuration imitating finger gripping.
“You could imagine a magnetic architecture that could act like a small robot that could be guided around the body using an external magnet and could hook onto something, for example to perform a biopsy,” Portela says. “That is a vision that others can draw from this work.”
The team also produced a magnetically responsive “bistable” switch. First, they printed a miniature, millimeter-long rectangle of polymer gel and attached four small, paddle-like magnetic structures to either side. Each oar was about 8 microns chunky – about the size of a red blood cell. When the team applied a magnet to one end of the rectangle, the oars rotated towards the magnet, pulling the rectangle in the same direction and locking it in that position. When the magnet was applied to the other side, the oars flipped again, pulling the rectangle like a switch in the opposite direction.
“We think this is a new type of bistable mechanism that could be used, for example, in a microfluidic device as a magnetic valve to open or close a certain flow,” Portela says. “For now, we have figured out how to fabricate complex magnetic architectures on the microscale, as well as spatially tailor their properties. This opens up many interesting ideas for future soft miniature robots.”
This research was supported in part by the National Science Foundation and the MathWorks seed grant program.
This work was performed in part at MIT.nano’s manufacturing and characterization facilities.