Fascination Technology Magnetic microalgae are turned into robots

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In our "Fascination Technology" section, we present impressive research and development projects to engineers every week. Today: how a microalga can swim despite a magnetic coating.

In nature, single-celled microalgae are fantastic swimmers. Two antenna-like feelers at the front perform a swimming motion that propels the cell forward. Scientists at the Max Planck Institute for Intelligent Systems have coated a single-celled green microalga with magnetic material. This miniature robot was put to the test: Would the microalga with its magnetic coating be able to swim through tight spaces—and as if that wasn't challenge enough—through a viscous liquid, as found in the human body? Would the tiny robot be able to find its way through these difficult conditions?

Biohybrid microswimmers reach 115 micrometers per second

The scientists found that their biohybrid microswimmers were hardly affected by the coating. Thanks to the breaststroke-like movement of the small feelers at the front, the coated algae darted forward at an almost unchanged swimming speed: they reached 115 micrometers per second—about 12 body lengths per second. For comparison, Olympic champion Michael Phelps reached a speed of 1.4 body lengths per second at his best. However, the alga is just a single cell without legs and feet.

Birgül Akolpoglu and Saadet Fatma Baltaci, who jointly led the research work, investigated several years ago how bacteria-based microswimmers can be magnetically controlled in liquids to eventually use them as a means of transporting drugs. Now they have turned their attention to microalgae. The goal of the two researchers was to functionalize the surface of the single-celled organisms with a magnetic material so that the cells can be directed in any desired direction—and thus turn the microalga into a microrobot.

Algae coated with magnetic nanoparticles

The coating of the cells took only a few minutes and almost always worked: nine out of ten algae could be successfully coated with magnetic nanoparticles by the women. They initially tested their biohybrid robot in a liquid as thin as water. With the help of external magnetic fields, they could steer the robots in any desired direction. Then the researchers guided their robots along tiny 3D-printed tubes—a highly confined environment only up to three times the width of the microalgae. To see if control worked there as well, the team set up two different systems: one with magnetic coils and one with permanent magnets around the microscope. They generated a uniform magnetic field and repeatedly changed its direction.

Magnetic control helped the microswimmers to align with the direction of the field. They showed real potential for navigating tight spaces—as if they were equipped with a sort of tiny GPS!

Birgül Akolpoglu

"We found that the algae navigated the 3D-printed microchannels in three ways: reverse movement, crossing, and magnetic crossing. Without magnetic control, the algae often got stuck and moved back to the start. However, with magnetic control, they moved more smoothly and circumvented obstacles," says the co-first author of the publication, Birgül Akolpoglu. "The magnetic control helped the microswimmers align with the direction of the field. They showed real potential for navigation in tight spaces—as if they were equipped with a kind of tiny GPS!"

Viscosity affects the swimming ability of the microalgae

In the next step, the team increased the viscosity of the liquid and sent their microrobots through the narrow channels again. "We wanted to test how our swimmers behave in an environment that resembles mucus. We found that viscosity affects the swimming ability of the microalgae. Higher viscosity slows them down and changes the way they move. When we applied the magnetic field, the swimmers swung back and forth—they moved forward in a zigzag pattern. This shows how fine-tuning viscosity and magnetic alignment can optimize the navigation of microrobots in complex environments," adds Baltaci.

"Our vision is to deploy the microrobots in complex and small environments that are highly confined, such as those found in our tissue. Our results open doors for applications like targeted drug delivery and offer a biocompatible solution for medical treatments with exciting potential for future innovations in biomedicine and beyond," the team concludes.

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