In the 1960's book and film Fantastic Voyage a team of people are briefly shrunk to the point they can sail a miniature submarine through a scientist's bloodstream to remove a potentially fatal blood clot. Soon, perhaps, tiny robots powered by soundwaves and driven by magnetic fields could do the same thing, hopefully without all the detours.
The idea of using robots smaller than the width of a human hair to fix problems within the body has been a dream of scientists for some time. Unsurprisingly, the obstacles are intimidating, including the slowness of the robots involved, and the fact prototypes are hard to steer close to surfaces such as the walls of blood vessels. That's a problem because it is near the walls that blood flow is smoothest.
Dr Amirreza Aghakhani of the Max Plank Institute for Intelligent Systems has built an early model of a potential solution by trapping an air bubble inside the body of the robot. Sound waves produced outside the object through which the robot is moving drive it to speeds much faster than other machines with similar goals have been able to achieve.
In Proceedings of the National Academy of Sciences, Aghakhani describes 3D-microprinting what looks a little like R2D2's top half 25 μm across. A gap in the bottom allows an air bubble to form when inserted in liquid. A fin at the front provides stability and causes the bot to move in the direction desired, instead of in spirals.
Propulsive force for Akhakhani's device is provided by sound waves oscillating the air bubble, while he steered it by coating the outside with a film of magnetic nickel and applying external magnetic fields.
Although the bot started off orientated at random, the application of the sound waves causes it to flip so that its flat bottom is orientated towards a glass substrate, here substituting for the blood vessel wall.
Although the unnamed bots look clumsy they slide along the surface at 1 millimeter per second (0.04 inches/second), which doesn't sound like much until you realize its 90 body lengths a second. Moreover, they could travel uphill and make sharp turns without difficulty.
Most bacteria are even smaller than the microrobots Aghakhani and others have built, and they move under their own power. So far, however, we are a long way from building objects that size with onboard energy supplies powerful enough to make them operate at the speeds we need. Consequently, machines small enough to be injected into human bodies usually leave their energy sources behind, with power transmitted through electric or magnetic fields.
Aghakhani is not the first to try using acoustic waves, but previous attempts have run into problems when they get close to the walls of the vessel through which they are traveling.