Hydrogel artificial cilia enable precise microrobotic actuation

Cilia are micrometer-sized structures found throughout the body. Their characteristic three-dimensional beating supports key functions: neuronal maturation in the brain, clearance of the respiratory tract in the lungs, and transport processes in the reproductive system. Conversely, impaired or damaged cilia can contribute to neurodevelopmental disorders, respiratory dysfunction, infertility, or embryo malformations.

Each microactuator measures about 18 micrometers in length and around 2 micrometers in diameter, close to the scale of real cilia. The team placed hundreds of cilia on a flexible, foil-like substrate with integrated electrodes. Four small electrodes around each cilium generate an electric field that drives ion movement inside the hydrogel. This controlled ion migration produces motion.

Depending on how the electrodes are powered, the hydrogel cilia bend or rotate. Activating electrodes on one side shifts ions in that direction, causing bending.

“At small scales, using electrical signals to drive ion movement for actuation has proven to be a highly effective and efficient method. For example, the human body relies on electrical muscle signals to control the distribution of ions in muscle tissue, which then generates motion,” says Zemin Liu, first author of the study. “Inspired by this principle, we developed micrometer-scale ion-driven hydrogels. Just like human muscle, these hydrogels move when electrical signals stimulate the ions inside them. In our work, we use only 1.5 volts, which is below the electrolysis threshold in aqueous environments and is completely safe, for instance inside the human body.”

To fabricate the arrays, the scientists used Two-Photon Polymerization, printing nanometer-thin layers to tune the hydrogel network and actuation performance.

“The fluid inside our hydrogel moves fast because we created tiny, nanometer-scale pores throughout the material. These pores act like miniature highways that let the fluid flow more quickly and in greater volume, which produces stronger and more effective motions,” says Wenqi Hu, Assistant Professor at The Hong Kong University of Science and Technology. “With our fabrication technique, even a very low voltage is enough to create a strong electric field, which pushes the ions to move rapidly. Thanks to both the pore structure and the strong electric field, our artificial cilia can react extremely fast.”

The team tested the microrobotic cilia more than 330,000 times with minimal wear and demonstrated operation in multiple fluids, including human serum and mouse plasma.

“In the past, researchers could only observe how natural cilia behave. Now we finally have a robotic platform that lets us study cilia in action: how they move, how they work together as a collective group, and what kinds of fluids they can transport or mix,” says Metin Sitti, President of Koç University in Istanbul.

COMPAMED-tradefair.com; Source: Max Planck Institute for Intelligent Systems