Wheel of Life: Bacteria provide horsepower for tiny motor

For millennia, people have hitched beasts to plows to exploit the animals’ strength and energy. In a modern variant of that practice, scientists have chemically harnessed bacteria to a micromotor so that they can make the device’s rotor slowly turn.

PULLING MY LEGS. When bacteria crawl clockwise in the circular groove underlying this motor, they brush past the tabs that support the motor’s star-shaped rotor. Molecular bonds between the microbes and a coating on the rotor tug the device around. Hiratsuka et al./PNAS

The new work might lead to improved lab-on-a-chip devices and engines to propel microrobots, says Yuichi Hiratsuka, now of the University of Tokyo, who codeveloped the bacteria-powered micromotor. He and his colleagues describe the research in an upcoming Proceedings of the National Academy of Sciences.

The novel micromachine “is an important step in integrating biological components into microengineered systems,” comments bioengineer William O. Hancock of Pennsylvania State University in University Park.

To make the motors, Hiratsuka’s team, led by Taro Q.P. Uyeda of the National Institute for Advanced Industrial Science and Technology in Tsukuba, Japan, borrowed fabrication techniques from the microelectronics industry.

The machinery of each motor consists of two parts: a ring-shaped groove etched into a silicon surface, and a star-shaped, six-armed rotor fabricated from silicon dioxide that’s placed on top of the circular groove. Tabs beneath the rotor arms fit loosely into the groove.

To prepare the bacterial-propulsion units, the team used a strain of the fast-crawling bacterium Mycoplasma mobile that was genetically engineered to crawl only on a carpet of certain proteins, including one called fetuin. The researchers laid down fetuin within the circular groove and coated the rotor with a protein called streptavidin.

The scientists then coated the micrometer-long, pear-shaped bacteria with a solution containing biotin, a vitamin that readily binds to streptavidin.

The team released the treated bacteria into the grooves in a way that sent them mostly in one direction around the circle. As the microbes passed each of a rotor’s supporting ridges, their biotin-treated cell membranes clung to the streptavidin coating, causing tugs on the tabs and thereby turning the rotor.

Slow and weak, the rotors circle at about twice the speed of the second hand on a watch and generate only a ten-thousandth as much torque as typical electrically powered micromachines do. By using more bacteria, the scientists could boost the torque 100-fold, Hiratsuka predicts.

In earlier work, many specialists in biologically inspired micromotors—including Uyeda’s group—used components of cells, such as filaments called microtubules (SN: 10/27/01, p. 268: Available to subscribers at Natural micromachines get the points), to devise microscale systems that transport objects.

Other teams have also used complete, living microbial cells to drag tiny loads (SN: 8/20/05, p. 117: Bitty Beasts of Burden: Algae can carry cargo) or to move fluids.

By using whole microbes as machine components, the Japanese team “adds a new direction to our field,” comments biomolecular-motor specialist Henry Hess of the University of Florida in Gainesville.

“The micromotor system points the way to self-sustaining and self-repairing machines, since the active units … can multiply and replace each other,” he adds. “Living machines rock!”

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