Among the different types of swimming strokes, freestyle, breaststroke or butterfly, freestyle is the most efficient for human swimmers since it allows us to cover the greatest distance in the shortest time.

The new nanorobot was made by Joseph Wang and colleagues of the University of California at San Diego (UCSD) and co-workers at the Harbin Institute of Technology in China and the Israel Institute of Technology. It is a multi-link two-arm structure consisting of a central gold body segment and two nickel arm segments connected by flexible, porous silver hinges. It was fabricated using a template electrodeposition technique. When exposed to a planar oscillating magnetic field (with a magnetic frequency of 17 Hz) in water, the two arms of the structure begin to swing in a freestyle stroke, pushing the middle link forwards.

Moving at low Reynolds numbers

The robot can also, surprisingly, move at low Reynolds numbers, highly viscous fluids, say the researchers. This has not been possible for many other such devices, in which conventional propulsion mechanisms have broken down because of the large viscous forces experienced. The Reynolds number is the ratio of inertial forces to viscous forces in a fluid and the nanoswimmer needs to overcome a viscous force that is a thousand times greater than the inertial force.

"Our swimmer has flexible arms that can display 'non-reciprocal' propulsion – that is, it basically deforms its body to move forwards," explains team member Jinxing Li of UCSD. "Its powerful freestyle gait, which has never been observed in nature, is a result of the synchronized deformation of the nanorobot under the combined action of the magnetic field and viscous forces in its surroundings."

Up to 12 body lengths per second

"The nanobot is strong and can propel itself at speeds of up 59.6 µm/s in water, which is around 12 body lengths per second, at a magnetic frequency of 25 Hz," he tells "It can rapidly reach a desired location and so might be used in many biomedical applications. These include localized diagnostics and treatment – by propagating through the bloodstream to detect cancer cells, for example, or travelling through arteries to remove blood clots."

The team, reporting its work in Nano Letters DOI: 10.1021/acs.nanolett.7b02383, says that it is now busy studying the fundamental mechanisms behind the nanoswimmer's motion.