A propulsion system small enough to let microscopic robots navigate inside the human body has been designed by a group of researchers in North Carolina.

The absence of a workable means of propulsion had been a major stumbling block to developing microbots for this purpose, because of the limitations in miniaturizing motors that require onboard fuel or batteries for their power. However, the answer could lie in a simple electronic diode, according to Orlin Velev at North Carolina State University in Raleigh.

Working in conjunction with scientists from the University of Hull in the U.K., Velev's team recently conducted an experiment in which a diode floating in a tank of salt water was exposed to an alternating electric field. By inducing a current within the diode, an electric field was created between the diode's electrical contacts to produce locomotion.

The propulsion occurs as the current in the diode accelerates ions in the surrounding fluid. Those ions push in one direction, allowing the diode to move in the opposite direction. In fact, the "particle-localized electro-osmotic flow" even provides a steering mechanism of sorts. The flow can be direct toward either the cathode or the anode, depending on the surface charge. Velev reported in an interview with New Scientist that the microdevices were able to achieve speeds of several millimeters per second.

The theory has excited the research community, but a great deal of work remains to be done to recreate the feat on a true nano scale. Velev's devices can be measured in millimeters, making  them an order of magnitude too large for working inside the human body.

Velev's work, "Remotely powered self-propelling particles and micropumps based on miniature diodes," was recently published in the journal Nature Materials. Co-authors included Suk Tai Chang, Vesselin Paunov, and Dimiter  Petsev.