A team of physicists led by Philip Ketterer of the Technical University of Munich have built one of the smallest rotary engines in the world. Some 40 nanometers tall, the device is made of three separate components that click together to form crude versions of an axle bearing and a spinning crank lever. Inspired by the flagella (whip-like propellers) used by many bacteria, the motor looks like a half of a helicopter blade. It creates forward thrust through the rotational motion of that blade.

The device is outlined in a paper published in the journal Science Advances. "It's a step toward the long-term goal of artificial nano-robots," says Ketterer. "You could easily imagine a future where similar such motors are used for propelling nano-robots in our bodies, much in the same way that bacteria naturally move about."

Ketterer's molecular motor comes in a research field called multilayer DNA origami. Scientists use straps of physical DNA material to form devices and machines. They're currently developing the nano-sized equivalents of hinges, clamps and bearings. The rotary motor, with an axle bearing case that clicks together, is easily "the most complex device made so far with this technique," Ketterer says.

Ketterer refrains from calling this creation a "motor" because the device is powered passively. It creates rotational motion by harnessing the energy of the ambient, irregular collisions of molecules around it, also known as Brownian motion. This means that the scientists can't turn these motors off and on, and they also can't control which way the rotors turn.

But Ketterer's team is investigating different ways to power these motors and control their spin. One idea involves using laser light to transfer pulses of energy to the rotary motor via waves. However, Ketterer gets the most animated when he talks about the prospect of using a influx of energy-rich molecules, like ATP, to power his motors. This process is used by bacteria and other cells use to power a whipping flagella.