Now a team lead by Orlin Velev at North Carolina State University in Raleigh, US, has found that a simple electronic diode could overcome both these problems. Velev and Vesselin Paunov from the University of Hull, UK, floated a diode in a tank of salt water and zapping the set-up with an alternating electric field. They reached speeds of several millimetres per second using electro-osmosis.
But there are still significant challenges ahead. Velev's diodes are millimetre-sized but any robot designed to work within the human body would have to be an order of magnitude smaller. In the past, attempts to shrink propulsive mechanisms have run up against a fundamental barrier in fluid dynamics: fluids become progressively more viscous on smaller scales. "It's like moving through honey" says Velev.
But extrapolations of the team's measurements indicate the propulsive force will work just as well at smaller scales. "The propulsive force scales in exactly the same way as the drag. That's quite significant," says McKinley.
Another challenge is that electro-osmosis occurs only at higher pH levels, when the ionic content of the water is high. Changing the pH from acidic to alkaline reverses the direction of thrust and there is zero thrust when the pH is about 6. Blood is only weakly alkaline so Velev will have to make adjustments to generate significant propulsive forces inside the body. He thinks the problem might be overcome by covering the diode with a polymer that shifts the pH at which zero thrust occurs.
Other types of micropropulsion have all run up against significant barriers. One idea exploits the phenomenon in which an electric current in a magnetic field experiences a force. The idea is to bathe a robot in a magnetic field and then switch on a current to generate a force. "The trouble is you need to power the current which requires an onboard battery. How do you do that?" asks McKinley.
Ultrasound can create pressure gradients within liquids that can move particles around. The problem here is that ultrasound can be hard to focus and can also cause bubbles to form and collapse, a process called cavitation that can damage cells.
Yet another option is to carry an onboard supply of hydrogen peroxide which dissociates into steam and oxygen. Expelling these gases generates a force - the attitude thrusters on the space shuttle work in the same way. But the fuel supply uses up the space available for sensors.
Richard Jones and his co-authors announced at Softmachines a platinum catalyzed version of the hydrogen peroxide propulsion of microbots They are using a directed random walk where the propulsion interacts with Brownian motion. the abstract for their paper is here