Researchers in Sweden have come up with a straightforward method for sorting particles in microfluidics - simply stretching the microchannel.

Sorting particles by size, anything from cells to plastic beads, is very useful in both analytical and preparative chemistry, but it is not always easy to achieve. In particular, a tuneable system, so that particles of different sizes can be sorted by the same device, is a major challenge. Jason Beech and Jonas Tegenfeldt of Lund University have struck upon a solution that makes use of the fact that the material used to make many microfluidic devices is very elastic.

Stretching a microchip changes its separation parameters giving a tuneable sorting system

Several methods have been proposed to sort particles in microfluidic devices, and one of the most promising is 'deterministic lateral displacement'. Originally reported by Huang and co-workers,¹ the technique is able to separate micrometre-sized particles continuously with an uncertainty of only 10 nm. However, the separation depends on distances between obstacles in a narrow channel and so the channel has to be designed to separate only the particle of desired size. Beech and Tegenfeld realised that simply stretching the microchip changes the separation parameters. This gives a tuneable sorting device.

'The concept of macroscopic stretching to change the dimensions of fluidics devices on the nanometre to micrometre scale is something that has not been exploited fully in the microfluidics community and could, with a little imagination, lead to promising applications,' explained Beech 'Having had the idea that stretching the devices would tune them, we began to realise the wealth of interesting applications that this could lead to.'

Beech hopes the idea will spread. 'It will be interesting to see if others put the concept of stretching fluidics devices to use,' he said, 'but the challenges are those of engineering and materials science, namely making more stretchable, more robust and more homogenous devices.'

Edward Morgan