Researchers at the Max Planck Institute for Intelligent Systems (MPI-IS) in Stuttgart created a biohybrid micro swimmer coated in magnetic material, but the coating has little effect on the swimmer's ability to swim. The study was published in the journal Matter.
Animation of how the microswimmer is coated with magnetic nanoparticles and how it swims in water and viscous liquids. Image Credit: Max Planck Institute for Intelligent Systems
With the help of their two whip-like flagella at the front, the 10 µm tiny, single-cell microalgae are excellent swimmers in the wild. It was unclear, nevertheless, what would happen if researchers applied a thin layer of magnetic nanoparticles combined with the natural polymer chitosan (for good adhesion) to the algae.
If navigating confined spaces was not difficult enough, could the tiny swimmer still make its way through a viscous liquid with a density comparable to mucus?
The researchers discovered that the additional load had very little effect on their green algae-based microswimmers. The algae launched themselves forward like a bullet, using their flagella to make a breast-stroke motion.
After magnetization, they continued to swim at a speed of 115 µm per second, or roughly 12 body lengths per second, despite the coating. In contrast, Michael Phelps, an Olympic swimmer, can swim 1.4 body lengths per second. Algae are merely cells without legs or feet.
The study's Co-Leaders, Birgül Akolpoglu and Saadet Fatma Baltaci are scientists from MPI-IS's Physical Intelligence Department. They looked into the possibility of magnetically controlling bacteria-based micro swimmers in fluidic spaces for drug delivery applications a few years ago.
Microalgae are now the focus of their attention. Their goal was to turn the microalgae into microrobots by functionalizing their surface with a magnetic substance that would allow them to be guided in any direction.
It only took a few minutes to coat the cells, and nine out of ten algae were eventually successfully coated with the magnetic nanoparticles. The biohybrid robot was first tested by the team swimming in a liquid as thin as water. They were able to regulate the microalgae's swimming direction by applying external magnetic fields.
Then, to create a highly confined environment where the largest dimension was only three times the size of the tiny microalgae, the researchers guided their robot along tiny 3D-printed cylinders.
The team installed two distinct systems one with permanent magnets around their microscope and another with magnetic coils to test the steering's effectiveness. They produced a consistent magnetic field and shifted its direction several times.
We found that microalgal biohybrids navigate 3D-printed microchannels in three ways: backtracking, crossing, and magnetic crossing. Without magnetic guidance, the algae often got stuck and backtracked to the start. But with magnetic control, they moved more smoothly, avoiding boundaries. Magnetic guidance helped the biohybrids align with the direction of the field, showing real potential for navigating in confined spaces – kind of like giving them a tiny GPS!
Birgül Akolpoglu, Study Co-First Author, Max Planck Institute for Intelligent Systems
The team then sent their microrobots through the narrow channels once more after increasing the fluid's viscosity.
We wanted to test how our swimmers would perform in something that is similar to mucus. We found that viscosity affects how the microalgal biohybrids swim. Higher viscosity slows them down and changes the way they swim forward. When we applied the magnetic field, the swimmers oscillated, moving forward in a zigzag pattern. This highlights how fine-tuning viscosity and magnetic alignment can optimize the navigation of microrobots in complex environments.
Birgül Akolpoglu, Study Co-First Author, Max Planck Institute for Intelligent Systems
“Our vision is to use the microrobots in complex and small environments that are highly confined, such as those found in our tissues. Our findings open doors to applications such as targeted drug delivery, providing a biocompatible solution for medical treatments with exciting potential for future innovations in biomedicine and beyond,” the team concluded.
Magnetic Microalgae on a mission to become robots
Video Credit: Max Planck Institute for Intelligent Systems
Journal Reference:
Akolpoglu, M. B., et al. (2025) Navigating microalgal biohybrids through confinements with magnetic guidance. Matter. doi.org/10.1016/j.matt.2025.102052.