Have you ever accidentally put milk in a cup of tea and wished you could take it out again? US scientists have developed a process similar to this, which they claim could make microfluidics easier.

Sung Cho of the University of Pittsburgh, US, and colleagues have developed a method to separate droplets of a mixed solution into droplets with high concentrations of different particles.

In biomedical lab-on-a-chip devices, such as biosensors, the particles in droplets of analyte often need to be separated before analysis. This is usually done using microchannel networks, but this makes device fabrication more complicated and expensive.

Cho's team made an aqueous solution of negatively charged latex and neutral polystyrene particles. The difference in charge enabled them to use electrophoresis to separate the particles into different regions of the droplet. They were able to split the droplet in two using a process called 'electrowetting on dielectric', and move the new droplets around the device. Cho now intends to investigate different separating procedures and wants to try the method on real biological cells and molecules.

Mike Hughes, an engineer from the University of Surrey, UK, said that Cho's work represents a major step forward in the potential applications of droplet-based microfluidics. He explained: 'The idea of droplet microfluidics is that we can take separate drops containing cells, chemicals or antibodies and can manipulate them on a microarray in order to react them and observe the result. Cho and co-workers take this to a whole new level by being able to separate the mixtures into two droplets again. It's almost like taking a cup of tea and being able to separate it back into the tea and the milk!' he said.

Rebecca Gillan