05-14 magnetic cilia

[Image above] Bending of NdFeB-based magnetic cilia magnetized pointing up in horizontal magnetic fields. Credit: Matthew R. Clary, North Carolina State University

On a growing number of campuses across the U.S., university students have become familiar with seeing small, boxy robots rolling through the grounds to deliver food to hungry residents. By delivering food to where it’s needed rather than requiring people to trek to a central dining hall, many students report the robots help keep them from skipping meals, which is an important factor in maintaining good physical and mental health.

Similarly, on a much smaller scale, robots are helping keep patients healthier by enabling targeted drug treatments. By delivering medicine to the precise location where it’s needed rather than throughout the entire body, drug activity is enhanced in the desired tissue and the intensity of undesired effects elsewhere is reduced.

Navigating through the human body is very different from wheeling down campus walkways, however, so these microsized delivery robots require alternative locomotion strategies to propel themselves to the source of the disease. Fortunately, nature offers multiple strategies that can serve as inspiration.

For example, some microorganisms are covered in tiny, hair-like structures called cilia that beat in a coordinated, whip-like manner to propel the cell. Researchers have been inspired by this motion to create magnetic cilia, or synthetic versions of cilia consisting of hair-like polymers embedded with magnetic particles to power the movement.

Most magnetic cilia make use of “soft” magnets, meaning they only become magnetized in the presence of a magnetic field. In contrast, “hard” magnets can produce their own magnetic field and thus offer greater control over the movement of magnetic cilia. But so far, only a few studies have made use of “hard” magnets in magnetic cilia.

In a recent open-access study, researchers at North Carolina State University explored the creation of magnetic cilia using “hard” neodymium magnets, which consist of neodymium, iron, and boron. But the true novelty of their work lies in the fact that after endowing the particles with a specific magnetic polarization, the particles’ polarization could later be reprogrammed, “which in turn allows us to completely change how the cilia flex,” says senior author Joe Tracy, professor of materials science and engineering at NC State, in a press release.

To create the magnetic cilia, the researchers embedded NdFeB microparticles (5 µm) in Irogran, a thermoplastic polyurethane, through solvent casting. They placed the slurry in a vertical magnetic field during the casting process to give all the microparticles the same magnetization. Then, they applied a less powerful magnetic field as the liquid polymer dried to align the magnetization directions of the microparticles and space the magnetic cilia regularly across the substrate.

To reprogram the microparticles’ polarization, the researchers first embedded the magnetic cilia in ice to restrain them from bending during the reprogramming process. They then exposed the magnetic cilia to a damped, alternating magnetic field, which reset the magnetization of the microparticles. Finally, they applied a strong magnetic field to magnetize the microparticles in a new direction.

In the press release, first author Matt Clary, Ph.D. student at NC State, explains the importance of resetting the magnetization before attempting to reprogram the microparticles.

“If you leave out that erasing step, you have less control over the orientation of the microparticles’ magnetization when reprogramming,” he says.

In addition to the experimental process, the researchers developed a computational model that allows users to predict the bending behavior of magnetic cilia based on the orientation of the microparticles’ polarization. This model could help other research groups explore the development of “hard” magnetic cilia and related soft actuators.

“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 says. “Methods developed in this work may also be applied to the broader field of magnetic soft actuators.”

The open-access paper, published in Advanced Materials Technologies, is “Magnetic reprogramming of self-assembled hard-magnetic cilia” (DOI: 10.1002/admt.202302243).