Often, when we’re dealing with 3D printed shape-shifting structures, they move when responding to different temperatures. But the innovative engineers at MIT, who have prior experience with using 3D printing to create shape-shifting materials, have developed a 3D printing technique to create soft, 3D printed structures that can be manipulated with magnets. The research has applications in creating remotely controlled biomedical devices.
“We think in biomedicine this technique will find promising applications. For example, we could put a structure around a blood vessel to control the pumping of blood, or use a magnet to guide a device through the GI tract to take images, extract tissue samples, clear a blockage, or deliver certain drugs to a specific location. You can design, simulate, and then just print to achieve various functions,” explained Xuanhe Zhao, the Noyce Career Development Professor in MIT’s Departments of Mechanical Engineering and Civil and Environmental Engineering.
The structures were created using a new 3D printable ink, which is infused with tiny magnetic particles. The researchers placed an electromagnet around a 3D printer nozzle, which, as the ink goes through it, causes the particles to swing into one orientation. So by simply using an external magnetic field to control the orientation of individual sections, 3D printed devices and structures can shift immediately into intricate, complex formations, and even move around.
Some of the structures the researchers 3D printed include a self-folding sheet, a tube that squeezes itself shut, a smooth ring that wrinkles up, and a grabber that can roll, crawl, jump up, and snap together quick enough to play catch. It can even be magnetically manipulated to carry a small pill across the table, simply by wrapping itself around the object.
The team published their results in a paper, titled “Printing ferromagnetic domains for untethered fast-transforming soft materials,” in the Nature journal. The project was partially funded by the National Science Foundation, the Office of Naval Research, and the MIT Institute for Soldier Nanotechnologies. Co-authors include Yoonho Kim, Hyunwoo Yuk, and Ruike Zhao of MIT’s Soft Active Materials Laboratory, Shawn A. Chester from the New Jersey Institute of Technology, and Zhao.
The abstract reads, “Here we report 3D printing of programmed ferromagnetic domains in soft materials that enable fast transformations between complex 3D shapes via magnetic actuation. Our approach is based on direct ink writing of an elastomer composite containing ferromagnetic microparticles. By applying a magnetic field to the dispensing nozzle while printing, we reorient particles along the applied field to impart patterned magnetic polarity to printed filaments.”
Hydrogel devices swell with temperature or pH changes, dielectric elastomers stretch under electric voltages, pumping air or water into hydraulic devices can actuate them, and shape-memory polymers are able to deform through stimuli like light or heat – all of these, including MIT’s magnetically activated structures, are in the general soft actuated devices category. But issues still abound – dielectric elastomers need high voltages, devices driven by air or water are inefficient for remote applications, and shape-memory polymers and hydrogels can take hours to change shape.
Kim said, “There is no ideal candidate for a soft robot that can perform in an enclosed space like a human body, where you’d want to carry out certain tasks untethered. That’s why we think there’s great promise in this idea of magnetic actuation, because it is fast, forceful, body-benign, and can be remotely controlled.”
While others have successfully created magnetically activated materials, they’ve only been able to achieve simple movements.
Yuk said, “People have only made structures that elongate, shrink, or bend. The challenge is, how do you design a structure or robot that can perform much more complicated tasks?”
The MIT team rose to the challenge, and searched for ways to make magnetic domains. Rather than creating structures with the same magnetic particles, these individual domains, or sections, each have their own magnetic orientation. This way, when they’re exposed to an external magnetic field, like the electromagnet on a 3D printer nozzle, the domains will move depending on the magnetic field direction its particles respond to – allowing them to complete more complex motions.
The researchers used their new 3D printing platform to fabricate these domains. During printing, they changed the direction of the electromagnet around the nozzle in order to tune the orientation of the magnetic particles.
“This work is very novel. One could use a soft robot inside a human body or somewhere that is not easily accessible,” said Jerry Qi, a mechanical engineering professor at Georgia Tech who was not involved with the research but has notable 4D printing experience. “With this technology reported in this paper, one can apply a magnetic field outside the human body, without using any wiring. Because of its fast responsive speed, the soft robot can fulfill many actions in a short time. These are important for practical applications.”
In addition, the team developed a physical model that can predict how a 3D printed structure under a magnetic field will deform or move, by using the pattern of the structure’s domains, the material’s elasticity, and which direction an external magnetic field is applied. Ruike discovered that the model’s predictions matched closely with the team’s experiments.
“We have developed a printing platform and a predictive model for others to use. People can design their own structures and domain patterns, validate them with the model, and print them to actuate various functions. By programming complex information of structure, domain, and magnetic field, one can even print intelligent machines such as robots,” said Zhao.
In addition to the previously mentioned structures, the team also 3D printed auxetic structures that rapidly expand or shrink along two directions, as well as a ring embedded with electrical circuits and red and green LED lights. The ring can deform, depending on its magnetic orientation, to light up either green or red.
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