In 1959, famed theoretical physicist Richard Feynman imagined a future in which microrobots swam through our bloodstreams, repairing our intestines or delivering medicine along the way.
Sixty-five years later, scientists are getting closer to that reality.
Engineers at the University of Tokyo have found a way to motorize tiny microscopic structures without the need for an external energy source.
The solution? A team of freely moving, single-celled organisms, tethered to a “chariot” like tiny horses.
The research wasn’t just a quest for cuteness, although it looks just as cute as it sounds. One of the problems with the “microbots” designed so far is that fluids like blood, because they’re so small, can take on the viscosity of syrup.
This makes it harder for the robot to move, which is why scientists have been trying for years to create small motors powerful enough to move such structures more easily.
Exploiting the fast swimming capabilities of green algae Chlamydomonas reinhardtiiJapanese engineers have come up with a unique solution.
Every cell of C. reinhardtii is only 10 micrometers wide, which is a third of the size of Benchy, the tugboat – the smallest ship in the world, 3D printed in 2020.
Together, however, they can pull machines five times larger than their own individual size – “opening up a whole new realm of possibilities for the development of complex micromachines,” say the machines’ designers.
The algae, which are considered safe for human consumption, are controlled by two flagella. These flagella propel each unit forward, in a manner similar to the breaststroke.
The cell is attached to a specially designed basket that resembles a bridle. The cell’s flagella extend forward, allowing the cell to drag the rest of the vehicle behind it as it paddles.
Unlike other micromotors that scientists have designed – which often rely on external energy sources such as magnetic or electric fields – living motors like C. reinhardtii can move autonomously.
Lead author Haruka Oda and his colleagues designed two different 3D-printed plastic vehicles for the algae to drive, each measuring 50 to 60 microns wide. To put that in perspective, the average human hair is about 100 microns thick.
One of the micro machines is called the “Scooter”. It has two baskets to catch two cells of algae, both facing the same direction and connected to a ‘cart’ at the back.
Without reason, C. reinhardtii take up their positions in each cockpit.
The researchers were surprised to find that the scooter did not drive straight, even when every basket was occupied. Instead, it twisted and turned in complicated ways. It even performed 15 backflips and 10 rolls.
The other form of vehicle, called the “Rotator”, moved more smoothly. It was designed with four baskets, all pointing in the same direction and connected by spokes in a wheel-like formation.
Because each of the four baskets contains one algae cell, the structure ‘rotates’ at an average speed of 20 to 40 micrometers per second, comparable to a ride on a microscopic fairground.
C. reinhardtii can reach speeds of 100 micrometers per second if left unhindered. That’s why researchers are now trying to see if they can make these micromachines move faster and more precisely.
The Rotator, which was just 56 micrometers in size, is five times larger than another previously designed microvehicle created in 2017 to be powered by self-propelled bacteria. Unlike algae, however, these bacteria had to have their speed controlled by a special light modulator.
“The methods developed here are useful not only for visualizing the individual movements of algae, but also for developing a tool to analyze their coordinated movements under confined conditions,” said Shoji Takeuchi, who supervised the project.
“These methods have the potential to develop in the future into a technology that can be used for environmental monitoring in aquatic environments and for transport of substances using microorganisms, such as moving pollutants or nutrients in water.”
One day, these lines of research could even realize Feynman’s dream: a microbot that can deliver “small cargo,” such as drugs, into a liquid environment, such as blood, and that is controlled by life itself.
The research was published in Small.