A new way of directing the flow of water could revolutionise microfluidics, say researchers from Linköping University in Sweden.

Their method involves controlling the way water spreads out down the channels in a microfluidic chip, a property known as wettability. Nathaniel Robinson, who led the research, describes the effect as similar to waxing a car. 'Water is attracted to the paint and dirt on the car and spreads out over its surface,' he says, 'but after applying a thin layer of wax the water is repelled and forms drops which often run right off the car.'

If water is attracted to the surfaces of a capillary channel it needs very little pressure to move forward, but if it is repelled by the channel then a much higher pressure is needed to move it forward, according to Robinson. Thus by changing the wettability of different parts of the chip, the researchers were able to control the path the water took through a series of channels.

The floor of the chip was made of a polymer, poly(3-hexylthiophene), which changes its wettability when an electrical potential is applied to it. By applying potential to particular channels, the researchers were able to direct water down those channels. Robinson sees this as the starting point for programmable chips where an electronic circuit can decide where to send chemicals within a complex chip.

The polymer layers used 'can be produced with inexpensive printing processes similar to those used to print magazines and milk cartons', said Robinson. The same or similar materials could also be used to print electronic circuits to control the flow of chemicals within the chip.

The work is 'an example of research at its best', said Chiara Neto from the Australian National University, Canberra. 'A key feature of the new approach is that the polymer films retain memory of the electrochemical treatment, so they can be modified and then stored for several minutes prior to their use.'

Clare Boothby