One of the main challenges in developing microscale robots lies in miniaturising their power and propulsion. Now, researchers in the US may have found a solution to this problem, by exploiting the natural movement of bacteria to propel micro-objects through water.

Many bacteria propel themselves along in a fluid by rotating their corkscrew-like tails, called flagella, at relatively high speeds. These flagella are only around 20 nanometres in diameter and are about 10,000 nm long.

Motors made from bacterial flagella have been used as novel "nano-actuators" before (see Bacteria harnessed as miniature pumps), but Metin Sitti and Bahareh Behkam of Carnegie Mellon University in Pennsylvania, US, have taken another approach. They use the entire microorganism as the motor and control its on/off motion with chemicals.

Sitti and Behkam began by sticking several S. marcescens – the kind of bacteria that cause pink stains on shower curtains – onto polystyrene beads 10 microns in diameter. These tiny "robots" were suspended in a solution containing water and glucose.

Outboard motoring

The bacteria themselves are only about one-fifth of the size of each bead and adhere to them via electrostatic, van der Waals forces and hydrophobic interactions. As the attached bacteria rotate their flagella, feeding on surrounding glucose, they push their bead forward at speeds of around 15 microns per second. Nanometer Nanotechnology

To stop the bacteria's motion, the researchers add copper sulphate to the solution. The copper ions bond to the rotor of the flagella motor and prevent it from moving. To restart the motion, another chemical called ethylenediaminetetraacetic acid (EDTA) is added. The EDTA traps the copper ions attached to the rotor, allowing it to move again. The rotors can be switched off in this way an unlimited number of times.

Using the entire microorganism as a motor has many advantages, the researchers say. Bacteria are robust machines that can easily be integrated with other microscopic components and do not need to be purified or reconstituted, as detached bacterial components must be.

Moreover, the bacteria motors work using simple nutrients such as glucose and are naturally sensitive to their environment. This means they can be precisely controlled.