You have likely heard of horsepower, but how about algae power?

Like a sled drawn by a team of dogs, or a plough pulled by oxen, researchers in Japan have created microscopic vehicles which can be moved by tiny, lively, green algae. The single-celled swimming organisms are caught in baskets attached to the micromachines, and in the future could assist with environmental monitoring, engineering and research.

To date, two types of algae transport have been created using a specialised 3D-printing technology that uses light to create microstructures from plastic. Both were carefully designed to allow the algae enough room to swim.

The team worked at a scale of 1 micrometer, equal to 0.001 millimetre. According to the researchers, the most challenging part was optimising the design of the basket-shaped trap, so that it could effectively capture and hold the algae when they swam into it.

“We were inspired to try and harness Chlamydomonas reinhardtii, a very common algae found all over the world, after being impressed by its swift and unrestricted swimming capabilities,” said Naoto Shimizu, a graduate student at the University of Tokyo, who initiated the project. “We’ve now shown that these algae can be trapped without impairing their mobility, offering a new option for propelling micro-machines which could be used for engineering or research purposes.”

The traps were attached to two different micro-machines. The first, called the scooter, has two traps which hold an alga in each and looks a bit like a podracer from Star Wars. It was intended to move in a forward direction.

The second - the rotator - spins like a Ferris wheel and has four traps holding a total of four algae. The size and shape of the baskets allowed the alga’s two flagella - small, whiplike appendages - to continue moving, propelling the machines along.

“As we had hoped, the rotator displayed a smooth rotational movement. However, we were surprised by the scooter," says Shimizu.

"We thought it would move in one direction, as the algae face the same way. Instead, we observed a range of erratic rolling and flipping motions,” explained project research associate Haruka Oda. “This has prompted us to further investigate how the collective movement of multiple algae influences the motion of the micro-machine.”

The team is planning to try different and more complex designs for their next vehicles, enhancing the rotator to make it spin faster. They say the main advantage of these micro-machines over those driven by different organisms is that neither the machine nor the algae require any chemical modification.

The algae also do not need external structures to guide them into the trap, which grants greater freedom of movement to the micro-machine, as well as simplifying the process.

The researchers do not yet know how long these micro-chariots and their tiny steeds can survive and continue to function. Individual Chlamydomonas reinhardtii can live for about two days, multiplying to produce four new algae. The experiments were carried out over several hours, during which the micro-machines maintained their form.

“The methods developed here are not only useful for visualising the individual movements of algae, but also for developing a tool that can analyse their coordinated movements under constrained conditions,” said Professor Shoji Takeuchi from the University of Tokyo. “These methods have the potential to evolve in the future into a technology that can be used for environmental monitoring in aquatic environments, and for substance transport using microorganisms, such as moving pollutants or nutrients in water.”