Researchers from Hanyang University have developed an innovative micropillar array capable of collective and rapid magnetic oscillations, demonstrating strong potential for advanced applications in robotics, fluid transport, and dynamic surface control.

In nature, many organisms exhibit collective movements to accomplish tasks that would be challenging for individuals alone. A prominent example is the coordinated motion of marine cilia, which collectively regulate fluid flow, facilitate locomotion, or enhance adhesion to surrounding surfaces. Although artificial micropillar structures have been explored to manipulate surface functionality, achieving dynamic actuation with both rapid response and sufficiently large deformation remains a significant challenge.

Led by Jeong Jae (JJ) Wie, an Associate Professor in the Department of Organic and Nano Engineering at Hanyang University, and Jun Oh Kim, a collaborator from the Korea Research Institute of Standards and Science (KRISS), the team developed arrays of micrometer-scale structures that respond instantly to changes in a rotating magnetic field, producing rapid, synchronized oscillations with high deformation amplitudes.

These findings were recently in the journal ACS Nano.

Conventional soft actuators suffer from reduced deformation magnitude at high oscillation frequency due to their inherent viscoelastic delays, limiting their ability to rapidly reach equilibrium configurations that minimize the magnetic moment. This leads to diminished performance with increasing oscillation frequency.

To overcome these limitations, the researchers embedded hard magnetic microparticles into a silicone-based elastomer and programmed their magnetization profile. This design enabled the micropillar arrays to achieve various controlled deformation modes, including simple bending, twisting, and torsional oscillations. Researchers changed the magnetization profile to generate bending and twisting deformations, while the magnetic field gradient control led to torsional line- or point-symmetric oscillations.

Furthermore, hard magnetic microparticles enable micropillar arrays to actuate under a moderate magnitude of magnetic fields, which operate under a commercial magnetic stirrer. In contrast, micropillar arrays with conventional soft magnetic microparticles, such as iron (Fe) microparticles, require a strong magnitude of magnetic flux density.

Remarkably, these magnetically programmed micropillar arrays maintained their large deformation magnitudes up to 15 Hz without delay in output frequency. With their height of just 400 渭m, the micropillars achieved a remarkable peak velocity of 81.8 mm/s鈥攎ore than 200 times their body length per second鈥攄emonstrating an exceptional speed-to-size ratio in soft material actuation.

The researchers also showcased how these collective, oscillatory micropillar arrays could be applied in soft robotics and microfluidics鈥攖ransporting cargo or mixing liquids via magnetically driven motion.

The micropillar array directed fluid to circulate in a clockwise or counterclockwise direction through torsional line- or point-symmetric oscillations. Additionally, micropillar multiarray carpets served as microfluidic paddles, generating controlled liquid flow in a petri dish-sized canal, effectively mixing fluids without the need for external pumps or tubing.

In another setup, the micropillar array is also inverted so that micropillar tips act as the legs of a soft robot, thereby enabling walking locomotion. Rather than relying on traditional wheels or mechanical limbs, the robot advances through the collective torsional motion of the micropillars, driven entirely by a magnetic stirrer placed beneath the surface.

"This breakthrough of collective magnetic oscillations can be an emerging template for many applications, beyond soft actuators by incorporating other functional materials for dynamic photonics and energy transfer," said Jisoo Jeon, the co-first author of this work.

"This work represents a significant step forward in the development of untethered, high-performance microactuators for next-generation soft robotics and microfluidic technologies," added another co-first author, Hanyang University researcher Hojun Moon.