In the swim

A transatlantic team brings the idea of micro-robots a step nearer by supplying devices with sufficient energy to propel themselves through liquid

The concept of tiny robots capable of use in microsurgery or continuously monitoring water supplies for biological toxins is one step closer to being a reality thanks to a recent development by researchers from two US universities and the University of Hull.

The transatlantic team devised a method to supply microscopic devices with sufficient energy to propel themselves through liquid, an impressive feat in the world of microfluidics. The micro-devices are powered and controlled with external energy from electric fields but there is also the potential to power them by radio waves.

The researchers took various types of millimetre-sized diodes, or electronic devices containing two electrodes, and put them in a dish of liquid with two external electrodes placed on the outer edges of the dish. They then applied alternating electric currents to the electrodes at the outer edges of the dish, which provided energy for the diodes to move on their own. The diodes absorb the external field energy and convert it into motion in a process called electro-osmosis.

The ability for micro-devices to move around, in liquid especially, is difficult because the smaller a device is scaled down, the more viscosity plays a role. For the devices, it is a bit like swimming in molasses.

In a recent paper the researchers, including chemist Vesselin Paunov of Hull, wrote that ‘micro-sensors and micro-machines that are capable of self-propulsion through fluids could revolutionise many aspects of technology. Few principles to propel such devices and supply them with energy are known.’

Nature has solved the problem of micro-scale chemical-mechanical ways of self-propulsion, with flagellar and ciliar bacterial motors. There has also been research done to synthesise artificial molecular machines including molecular motors, shuttles and nanocars.

But while molecular machines can in principle propel micro-devices or act as micro-pumps when attached to walls, it is hard to adapt their complex natural or artificial molecular structures to engineered devices.

The other problem with these self-propulsion devices is that many can only move in certain kinds of liquid, whereas the researchers’ new method is expected to work in a wide variety of liquids.

While it might be enough of an accomplishment to move through liquid, the researchers say the self-propelling devices will also be able to perform other tasks as well. The voltage induced within the devices’ electrodes can be used to emit light or spin around.

They will also be able to respond to light. The researchers were able to control the speed at which some diodes moved by directing a laser beam at the devices, which proves that their ability to sense their environment and react. This sensing function could prove important if the devices are used to look for a particular protein and analyse it.

Also, if the devices prove to be able to move through biological fluids, they could be used for applications such as drug delivery or microsurgery.

Now they have proven the concept, Paunov and the rest of the team are working to create better microfluidic devices, where the flow of microscopic volumes of liquid can be steered and controlled by electronic diode pumps, valves and mixers.

With those improvements, the microfluidic devices will be able to achieve better analysis of biological samples, assist in development of drugs or perform other biotechnology operations.