Picture: Max Planck Gesellschaft
Additive manufacturing at the micro- and nanoscale opens up new possibilities for functional materials in biomedicine. Researchers from the Max Planck Institute for Intelligent Systems, the Hong Kong University of Science and Technology, and the Koç University have now developed artificial hydrogel cilia whose motion closely resembles that of natural cilia. The results will be published in the journal Nature in mid-January 2026.
Cilia play a central role in the human body, for example in cleaning the airways, transporting fluids in the brain, or reproduction. Their complex three-dimensional beating motions at frequencies between five and 40 hertz have so far been difficult to replicate technically. The research team therefore used 3D printing via two-photon polymerization to fabricate micrometer-scale hydrogel structures with high geometric control. Each printed cilium is around 18 micrometers long and has a diameter of approximately two micrometers.
The motion of the artificial hairs is triggered by electric fields. Electrodes integrated into the flexible substrate generate targeted ion migration within the hydrogel. Depending on the control signals, the cilia bend or rotate freely in space.
“At small scales, using electrical signals to control ion movement has proven to be an extremely effective and efficient method. The human body, for example, uses electrical muscle signals to control the distribution of ions in muscle tissue, thereby generating movement,” says Zemin Liu, first author of the study. “Inspired by this principle, we developed micrometer-sized, ion-driven hydrogels. Just like human muscles, these hydrogels move when electrical signals control the ions inside them. In our research we used only 1.5 volts, which is below the electrolysis threshold in aqueous environments and therefore completely safe, for example for applications in the human body.”
A key contribution of 3D printing lies in the tailored pore structure of the hydrogel. Nanometer-scale channels accelerate fluid and ion transport and enable fast, repeatable movements. In long-term tests with more than 330,000 cycles, the structures showed hardly any wear. They also functioned reliably in various liquids, including human serum.
“The fluid in our hydrogel moves very quickly because we created tiny nanometer-sized pores throughout the material. These pores act like miniature highways along which fluid can flow faster and in larger quantities, resulting in stronger and more effective movements,” says Wenqi Hu, who led the Bioinspired Autonomous Miniature Robot Group at MPI-IS and is now an assistant professor at the Hong Kong University of Science and Technology. “With our fabrication technique, a very low voltage is sufficient to generate a strong electric field that drives the ions to move rapidly. Thanks to the pore structure and the strong electric field, our artificial cilia can respond extremely quickly.”“In the past, researchers could only observe how natural cilia behave. Now we finally have a robotic platform that allows us to study cilia in action: how they move, how they work together as a group, and what kinds of fluids they can transport or mix,” says Metin Sitti, who led the Physical Intelligence Department at MPI-IS and is now president of Koç University in Istanbul. “These hydrogel cilia could one day be used in biomedicine to restore or replace damaged cilia. As an important advance in micro-actuation technology, they also open up new possibilities for the design of miniature robotic systems, such as the flapping micromachine presented in this work.”
Looking ahead, such 3D-printed microactuators could support both fundamental research and future medical and microfluidic applications.