Scientists at the University of California, San Diego, have created what they claim is the first self-propelling, hydrogen-bubble-powered "microrocket" requiring no external source of fuel. In the most acidic solutions, these micromotors can reach speeds of 100 body lengths per second. It's claimed that the breakthrough could pave the way (or rather line the esophagus) towards stomach-going nanomotors which could provide imaging or precisely targeted drug treatment. In addition to self-propulsion, the gut-rockets can be steered, and made to collect and release a payload.
The speed of the microrockets is determined by the acidity of the solution into which they are inserted. The "rockets" are in fact polyaniline tubes ten micrometers long and two to five micrometers in diameter. The inner surface of the tube is lined with zinc. In acid, the zinc loses electrons, promoting the production of hydrogen bubbles - an action which propels the gut-rocket forward. Though there's only a narrow pH band in which the microrocket functions, it's a band that encompasses the acidity of the human stomach.
The top speed clocked by the scientists thus far is 1,050 micrometers per second. So, they're talking about the microrocket's own body length, not a human's, when they say it achieves 100 body lengths per second. That may only be a (fairly chelonian) speed of 0.0023 mph (0.0038 km/h), but scaled, that's roughly the equivalent of a human swimming at 380 mph (612 km/h) - which is even harder than it sounds in pH 1 acidity.
The lifespan of the microrockets spans ten to 120 seconds depending on the rate at which the zinc dissolves. More zinc, and a higher (i.e. less acidic) pH extend the lifespan.
The more elaborate functionality comes courtesy of magnetism. The outside of the tubes can be coated with a magnetic layer, allowing the microrocket to be guided to a desired location. This way, the scientists were able to collect a polystyrene payload, travel, and then release it with a sudden change in the magnetic field - something akin to a poorly secured bicycle falling off a roof rack by taking a corner too quickly.
"With further improvements and optimization, we hope to improve and expand the working environments to milder conditions and extend the lifetime of such microrockets to longer periods," researcher Joseph Wang told PhysOrg. "We are also exploring new materials to broaden the scope of our microengines towards new environments."
The study, undertaken by researchers Wei Gao, Aysegul Uygun and Joseph Wang, was published recently in the Journal of the American Chemical Society. See below for a video of one of the microrockets in action.