American researchers have used hydrogels to control the flow of liquid in microfluidic devices.

Dongshin Kim and David Beebe from the University of Wisconsin, Madison, made plugs that change size to control the flow within a network of channels.

Hydrogels are jelly-like polymers that swell when wetted and have been used to provide actuation for valve and pumps in microfluidic systems. 'However, the swelling phenomenon of hydrogels is not fully utilized yet: most microfluidic components using hydrogels are realizing simple and rudimentary functions,' said Beebe.

Changes in pH can alter the shape of hydrogel bars, and integrating them into the assembly of a chip means it can provide sophisticated functions, not available via traditional assembly, according to Beebe. 'One can change the chip function without fabricating a new chip,' he said.

Aaron Wheeler, University of Toronto, Canada, said that Beebe's work is a 'creative, innovative approach to integrate active components in microfluidic devices. There are many useful micro-scale technologies that are awkward to integrate in microfluidic devices because of the closed nature of microfluidic systems. This innovation, which enables spatial control of macro-scale objects in closed microchannels, is exciting because it suggests future microfluidic systems that will be compatible with micro-scale technologies requiring precise spatial control, which may result in new analytical tools.'

Beebe's ultimate goal is 'to create "smart" systems that can make autonomous decisions based on either the local environment or the result of a previous processing step'. He continued: 'For example, a chemical synthesis chip that "knows" when to move to the next step automatically, based on the autonomous sensing of a product.'

Debora Giovanelli