Microscale acoustic “rockets” navigate the human body

Engineers turn bubbles into motors to propel minute vessels through landscapes of cells and particles suspended in fluid

Ever since nanotechnology became a real branch of engineering, its practitioners have been trying to design tiny structures that can work like submarines to navigate through the human body.

One stumbling block towards this goal has been what fuels and motor analogues could be used to propel and steer such nanovessels around and inside blood vessels and organs without causing harm.

Researchers at Pennsylvania State University and the University of San Diego hit a wall with their research, because they were using toxic materials like hydrogen peroxide as fuel. A fortuitous discovery about the behaviour of bubbles has opened up a new avenue for their research, as they describe in Science Advances.

Working with material scientists at the Harbin Institute of technology in Shenzhen and surgeons at University of Michigan, Thomas Mallouk of the Department of Chemistry at Penn State was trying to move nanovessels with acoustic levitation, a technique used to lift particles off microscope slides. Unexpectedly, he found that high-frequency sound waves made the vessels move at very fast speeds. Investigating this phenomenon further, Mallouk and his team designed microscale “rockets” that can use acoustics to zip around and steer in a liquid medium.

The rockets are not rocket-shaped. They resemble a round-bottomed cup 10µm in length and 5µm wide, 3D printed from a polymer and coated with a 10nm-thick layer of nickel and a 40nm-thick layer of gold.

The inside of the cup is then coated with trichlorosilane, which repels water. When submerged in fluid, an air bubble spontaneously forms inside the cup. When bombarded with ultrasound waves, the bubble vibrates, turning it into a motor and propelling it through the fluid. The vessel can be steered with precision by manipulating an external magnetic field. Each rocket has a characteristic resonant frequency, so individual vessels can be driven independently.

Steering of the vessels is so precise that Mallouk’s team made them move up microscopic staircase structures. The addition of fins to the cup structures allows them to be steered freely in three dimensions.

Moreover, the team describes using the vessels to push other particles or cells around, or tow them with precision through a crowded environment. The key to this is the small size of the vessels, Mallouk claims.

“This wasn’t available on a larger scale,” he said. “There’s a lot of control you can do at this length scale. At this particular length scale, we’re right at the crossover point between when the power is enough to affect other particles.”

Changing the acoustic stimulation adjusts the speed of the vessels. “If I want it to go slow, I can turn the power down, and if I want it to go really fast, I can turn the power up,” explains Jeff McNeill, a graduate student who works on nano-and microscale motor projects. “That’s a really useful tool.”

Mallouk is working with engineers and roboticists at Penn to equip the vessels with computer chips and sensors to give them autonomy and intelligence, which would allow them to be used for tasks including imaging and even surgery. “We’d like to have controllable robots that can do tasks inside the body: delivering medicine, diagnostic snooping,” he said.