Tiny submarines could make a 'fantastic voyage' into our bodies

It sounds a lot like the plot of the old science fiction movie Fantastic Voyage. Researchers in the U.S. have developed microscale rockets powered by acoustic waves and an onboard bubble motor that may one day navigate through landscapes of cells and blood vessels inside the human body.

In the movie, a submarine and its crew are shrunk to microscopic size and enter into the body of an injured scientist to repair damage to his brain. The ultimate aim of this real-life 'fantastic voyage' is to precisely steer micro- or nanovessels through the body to deliver medicine, clear arteries or provide more accurate diagnosis.

• Paper in Science Advances

This experiment is the culmination of 15 years of research by Thomas Mallouk, Vagelos Professor in Energy Research and Professor of Chemistry at the University of Pennsylvania, and his colleagues.

Their first attempts were similar micro- and nanoscale vessels powered by chemical energy, but the chemical propellants they used, like hydrogen peroxide, were toxic and therefore inappropriate for biomedical use. 

Full-speed ahead

Then came a happy accident that solved that dilemma. A process used to lift particles off a microscope slide provided the answer. Because the particles are so tiny and fragile, high-frequency sound waves are used to move them — a technique called acoustic levitation.

Mallouk and his colleagues noticed that not only did it move them, it moved them at very high speed. This inspired them to see if high-frequency sound waves could also be used to power the experimental microscale rockets.

Tiny rockets with bubble motors

The microscale rockets really are tiny, at only 10 microns in length and 5 microns wide — which is smaller than a human cell. They're shaped somewhat like a round bottomed cup. The outer layer is gold and the inner layers are nickel along with a polymer. In the process of 3D printing them, a tiny bubble forms and is trapped inside the rocket. 

The high frequency ultra-sound waves excite the air bubble to oscillate inside the device, acting a little like an an onboard motor. An external magnetic field is used to steer the vessel. The speed of each individual vessel can also be controlled by altering the frequency of the sound wave.

In a magnified video, the black microscale rocket is being powered by ultrasound and controlled by magnetic fields so as to move individual tiny particles through a liquid. (Liqiang Ren)

As the devices will be used in large numbers rather than individually, the team is now working on how to to steer many different vessels, because no two bubbles will be exactly the same size. They will therefore require different acoustic frequencies to control them. It is akin to each cellphone requiring a different frequency for individual use.

The voyage of the future  

The potential for these bubble-powered, sound-fuelled, microscale vessels is enormous. They might deliver medicine to tumours, for example, or be used as mini tugs or plows to clean out clogged arteries. With on-board sensors, they could perform inside-the-body diagnosis in places that have been difficult to observe.