Hydrogel cilia set new standard in microrobotics

Cilia are micrometer-sized biological structures that occur frequently in nature. Their characteristic high-frequency, three-dimensional beating motions (5 - 40 Hz) play indispensable roles inside the body. In the human brain, ciliary motion is crucial for neuronal maturation; in the lungs, it is essential for clearing the respiratory tract; and in the reproductive system, cilia transport gametes. Conversely, impaired or damaged cilia can lead to neurodevelopmental disorders, respiratory dysfunction, infertility, or malformations of the embryo. Scientists from the Physical Intelligence Department at the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart, from Hong Kong University of Science and Technology and Koç University in Istanbul created artificial cilia from hydrogel, which they can move individually or in groups applying an electric field. Their groundbreaking work will be published in Nature on January 14, 2026 bearing the title “3D-printed low-voltage-driven ciliary hydrogel microactuators”. Each microactuator or microrobot is only 18 micrometers in length with a diameter of around 2 micrometers, nearly as small as real cilia. The scientists placed hundreds of their cilia on a flexible foil-like substrate that contains built-in electrodes. Around each cilium, they placed four small electrodes. When the electrodes are switched on, they create an electric field that causes ions inside the hydrogel to move. This controlled ion migration is what sets the cilia into motion. Depending on how the scientists power the electrodes, the hydrogel cilia can bend or spin. Turning on the electrodes on one side pushes the ions in that direction, causing the cilium to bend toward that side. To make the cilium rotate, the four electrodes are turned on in sequence, which makes the ions move in a circular path. The cilium then follows this motion and rotates smoothly in 3D. “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, who is the 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 build the tiny arrays, the scientists use a method called Two-Photon Polymerization, also known as 2PP. The team printed the hydrogel cilia nanometer small layer by layer to optimize the hydrogel network structure 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, who led the Bioinspired Autonomous Miniature Robot Group at MPI-IS and who is now an Assistant Professor at The Hong Kong University of Science and Technology....

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