Researchers Teach Old Artificial Hairs New Tricks

Magnetic cilia are small artificial hair-like structures whose movement is controlled by tiny magnetic particles embedded inside of them. They are a popular material, frequently being used in soft robotics, droplet and particle control systems, sensors and microfluidics applications.

Despite their popularity, the cilia suffer from one notable drawback – most are made in such a way that their movement is completely fixed. Now, researchers at North Carolina State University (NC State) have developed a new technique for making magnetic cilia that are reprogrammable, allowing them to be repurposed for new functions or uses. Their research is published in Advanced Materials Technologies.

“What’s novel about this work, is that we have demonstrated a technique that allows us to not only program magnetic cilia, but also controllably reprogram them,” said study author Joseph Tracy, a professor of materials science and engineering at NC State. “We can change the direction of the material’s magnetization at room temperature, which in turn allows us to completely change how the cilia flex. It’s like getting a swimmer to change their stroke.”

Making a reprogrammable cilia “carpet”

Most magnetic cilia rely on the action of so-called “soft” magnets. These materials are easily magnetized and de-magnetised and, in the majority of cases, only become magnetic while in the presence of an external magnetic field. Very few previous magnetic cilia have made use of “hard” magnets – also known as permanent magnets – which are capable of retaining their own magnetic field.

Hard magnets are interesting to researchers looking at magnetic cilia because they can be “programmed” to have a specific magnetic polarization. Controlling this polarization allows for very precise control over how the cilia will flex and move when exposed to a magnetic field.

The NC State researchers have developed a new way to make magnetic cilia that are not just programmable, but also reprogrammable.

Their cilia consist of a polymer embedded with particles of powerful neodymium magnet. To make them, the researchers introduced neodymium microparticles into a polymer dissolved in liquid. The resulting slurry is then exposed to an electromagnetic field that is powerful enough to polarize all of the magnetic microparticles in a uniform direction. By applying a second, less powerful, magnetic field as the polymer slurry dried, the researchers were able to form cilia with regularly spaced, embedded magnetic microparticles.

“This regularly ordered cilia carpet is initially programmed to behave in a uniform way when exposed to an external magnetic field,” Tracy explained. “But what’s really interesting here, is that we can reprogram that behavior, so that the cilia can be repurposed to have a completely different actuation.”

Teaching “hard” magnets new tricks

The first step in reprogramming the cilia is to freeze them in ice, rendering them completely immobile. With the cilia frozen in place, the researchers could then expose them to a damped, alternating magnetic field, which introduces some disorder back into the polarization of the microparticles. This effectively erases the very ordered polarization that was induced when the cilia were first synthesized.

“The reprogramming step is fairly straightforward,” Tracy said. “We apply an oscillating field to reset the magnetization, then apply a strong magnetic field to the cilia which allows us to magnetize the microparticles in a new direction.”

“By mostly erasing the initial magnetization, we’re better able to reprogram the magnetization of the microparticles,” said first author Matthew Clary, a PhD student in the Nanomagnetism Group at NC State. “We show in this work that if you leave out that erasing step you have less control over the orientation of the microparticles’ magnetization when reprogramming.”

“We also found that when the magnetization of the microparticles is perpendicular to the long axis of the cilia, we can cause the cilia to ‘snap’ in a rotating field, meaning they abruptly change their orientation,” added Tracy.

To further improve the reprogramming of the cilia, the researchers have also developed a computational model that can predict the bending behavior of the hard magnet-based cilia, depending on their polarization orientation.

“This model could be used in the future to guide the design of hard-magnetic cilia and related soft actuators,” said coauthor Benjamin Evans, a professor of physics at Elon University.

“Ultimately, we think this work is valuable to the field because it allows repurposing of magnetic cilia for new functions or applications, especially in remote environments,” Tracy added. “Methods developed in this work may also be applied to the broader field of magnetic soft actuators.”

Reference: Clary MR, Cantu SN, Liu JA ‐C., Evans BA, Tracy JB. Magnetic reprogramming of self‐assembled hard‐magnetic cilia. Adv Mater Technol. 2024:2302243. doi: 10.1002/admt.202302243

This article is a rework of a press release issued by North Carolina State University. Material has been edited for length and content.