"These microswimmers provide a new way to manipulate single particles with precise control and in three dimensions, without having to do special sample preparation, labeling, surface modification," said Joseph Wang, a professor of nanoengineering at the University of California San Diego.

Wang, with Thomas Mallouk, a professor of chemistry at University of Pennsylvania, and Wei Wang, a professor of materials science and engineering at Harbin Institute of Technology in China, are senior authors of a paper describing the micromotors, published Oct. 25 inScience Advances.

Researchers used the micromotors to push around individual silica particles and HeLa cells in aqueous media without disturbing neighboring particles and cells. In one demonstration, they pushed around particles to spell out letters. Researchers also controlled the micromotors to climb up microsized blocks and stairs, demonstrating their ability to move over three dimensional obstacles.

The micromotors are hollow, half capsule-shaped polymer structures coated with gold. They contain a small piece of magnetic nickel in their bodies, which allows them to be steered with magnets. The inner surface is chemically treated to repel water so that when it is submerged in water, an air bubble spontaneously forms inside the micromotor.

This trapped bubble allows the micromotor to respond to ultrasound. When ultrasound waves hit, the bubble oscillates inside the micromotor, creating forces that propel its initial movement. To keep the micromotor moving, researchers apply an external magnetic field. By changing the direction of the magnetic field, researchers can steer the micromotor in different directions and alter its speed.

"We have a lot of control over the motion, unlike a chemically fueled micromotor that relies on random motion to reach its target," said Fernando Soto, a nanoengineering Ph.D. student at UC San Diego. "Also, ultrasound and magnets are biocompatible, making this micromotor system attractive for use in biological applications."

Future improvements to the micromotors include making them more biocompatible, such as building them from biodegradable polymers and replacing nickel with a less toxic magnetic material such as iron oxide, researchers said.