Catching cells in droplets has enabled scientists to make a highly efficient microchip for cell sorting.

Andrew Griffiths, at the University of Strasbourg, France, and colleagues have improved on an established technique for sorting cells. Fluorescence-activated cell sorting (FACS) separates mixtures of cells into different containers based on the tagging of cell-surface proteins. Griffiths' microfluidic system uses a similar approach but in the new fluorescence-activated droplet sorting (FADS) method, each cell is trapped in an individual aqueous droplet, instead of being in a continuous stream of liquid, as in FACS. The droplets then flow over the microfluidic chip, which sorts each one according to the enzyme activity of the cell inside. 

Microfluidic sorting of surfactant cells at an asymmetric Y-shaped junction

To show the technique's potential, the researchers used a mixture of Escherichia coli cells expressing an active galactosidase enzyme and cells expressing an inactive enzyme. The cells were captured in droplets containing a substrate that fluoresces when acted on by the active enzyme. This meant that only the active cells became fluorescent and their corresponding droplets could be identified. A high-speed computer is programmed to recognise the fluorescent droplets, feeding back to the chip, which collects them - and so the cells containing the active enzyme - separately. The technique is simple, inexpensive and can be used for high-throughput applications.

This is the first time the droplet and microfluidic approaches have been combined for this use. Charles Baroud, an expert in multiphase flows in microfluidic devices at the Polytechnic School in Palaiseau, France, commends this integration. 'While the individual basic operations have already been demonstrated, their integration has added tremendous complexity. Studies such as this are instrumental in the development of truly useful droplet-based lab-on-a-chip devices,' he says. 

Griffiths says that in future work the team will try to put different microfluidic operations on the same chip to select cells with multiple optimised properties. 'This would enable cell selections that are currently impossible to do,' he comments. He says he hopes that this will allow the chips to be useful for industrial and biomedical applications. 

Roxane Owen

Enjoy this story? Spread the word using the 'tools' menu on the left or add a comment to the Chemistry World blog.