A new technique to print soft materials that undergo complex and rapid shape changes when a magnetic field is applied to them can be used to create tiny, untethered, robots capable of useful movement, such as rolling, jumping and grasping objects. Such objects might be used in a host of biomedical applications, like minimally invasive surgery or targeted drug delivery.

“Existing robots are often heavily tethered because they need to be actuated pneumatically, which makes them unsuitable for biomedical applications,” explains study lead author Yoonho Kim of the Massachusetts Institute of Technology. “Soft active materials that change shape in response to external stimuli, such as heat, light, solvents, electric and magnetic fuels, are better alternatives in this context because they can be controlled remotely. Magnetic fields are a particularly good stimulus option because they are a safe, fast and effective way to actuate magnetically responsive soft materials. The problem with most of the materials made so far, however, is that their shape change has been limited to simple bending or elongation.

“Our new way to print ferromagnetic domains in soft materials has allowed us to make far more complex shape-morphing structures that transform between different 3D shapes within fractions of a second.”

Toothpaste-like ink

Current methods to create magnetic materials rely on using “already cured” elastomers containing non-magnetized particles. These elastomers need to be temporarily deformed into the desired shapes and the embedded particles magnetized by applying a strong magnetic field in a certain direction. “One of key differences between these techniques and our approach is that we can directly inscribe magnetic polarity in complex 3D structures from the start to form complicated patterns of magnetic domains,” Kim tells Physics World.

Unlike previous methods, which were limited to simple geometries and simple deformation, the new technique is based on a 3D direct ink writing. “Toothpaste is the best way to describe the inks employed in this technique,” says Kim. “When we squeeze a tube of toothpaste, we in fact apply a shear-yield stress to it, which allows the paste to come out of the nozzle. The paste then maintains its cylindrical shape if we apply no further stress.

“Direct-writable ink materials also possess this rheological property and the ‘paste’ we used in our study is an ‘uncured’ elastomeric composite containing already-magnetized microparticles.”

 Specific transformations in a magnetic field

The researchers, led by Xuanhe Zhao, of the Mechanical Engineering Department at MIT, made their ink by mixing microparticles of ferromagnetic neodymium-iron-boron (NdFeB) with silicon resin. To introduce the shear-yielding behaviour, they added fumed silica nanoparticles as a rheological modifier. “These particles form a network based on van der Waals interactions that helps the whole elastomer matrix maintain its shape,” says Kim. “It also helps the embedded magnetic particles to disperse throughout the matrix rather than agglomerating to form large clusters.

“We then apply a magnetic field to magnetize the embedded particles. Each microparticle (which is around 5 microns in size) becomes a strong permanent magnet.”

Before printing (that is, before squeezing the toothpaste-like composite out of a nozzle), the magnetic particles are randomly oriented. Thanks to the applied magnetic field, the particles reorient along the applied field direction during printing. “In this way, we can control the magnetic polarities of the magnetic fibres and thereby programme different regions of the printed material to undergo specific transformations in a magnetic field. For example, they can switch between different static shapes or morph dynamically in response to changing magnetic fields.

“A 3D construct built by arranging and stacking these fibres maintains its shape during the printing process. Once printed, we then cure the structure to make a rubber-like elastic solid, which is encoded with intricate patterns of magnetic domains.”

A hexapedal spider-like grabber

As a proof-of-concept, the researchers printed several structures with programmed magnetic domains capable of performing multiple tasks. One example is a hexapedal spider-like grabber. “By applying magnetic fields in different directions and of different strengths to different parts of its structure, this robot can be made to crawl, roll over, carry a drug cargo and even catch and release a fast-moving ball,” says Zhao.

“These demonstrations prove that our shape-morphing structures are strong and agile enough to interact with fast-moving objects. We hope our technology will allow us to develop untethered magnetically-remote-controlled magnetic soft robots that can operate in confined and enclosed spaces, like the human body.”

Indeed, the team, reporting its work in Nature 10.1038/s41586-018-0185-0, says that it is now focusing on developing specific biomedical applications for its technology. “This line of work will also require us to improve our materials and fabrication platform and advance magnetic field control for actuating such soft robots,” adds Zhao.