Microfluidic Tango: Sorting Without Diffusion

Separations of complex biological mixtures such as the contents of a cell require biomolecules to be sorted by their size or density. To accomplish this, molecular biologists usually employ methods that rely on diffusion, the often gradual migrations of particles from one zone to another. However, diffusion-based sorting requires patience, since the particles must randomly wander over a large number of possible paths.

Now, a multidisciplinary Princeton team (Robert Austin, Austin@princeton.edu) has produced a potentially faster, non-diffusion-based sorting method. The researchers tap into the power of microfluidics, the control of liquids using microscopic structures. Their microfluidic method allows them to sort objects in a nonrandom (deterministic) fashion.

In their technique, a smooth fluid carries the biomolecules of interest in a downward stream. Encountering arrays of obstacles staggered in a certain way, smaller molecules zig-zag back and forth through the obstacles but must proceed on the average straight down. However, if a biomolecule is big enough, it moves steadily at an angle to the zig-zag motion, taking tango-like dance steps as it veers to the left or right, thereby separating itself from the smaller molecules.

In their initial demonstrations, the researchers have sorted fragments of artificial bacteria chromosomes to within 12% of their molecular weight in 10 minutes, already an order of magnitude faster than conventional methods. In tests with sub-micron polymer bead particles, the device can rapidly and continuously sort them into an array of output channels with a resolution of 1% of the particles' radius or less. Thus the device may find applications in the area of sorting inorganic nanoparticles as well. (Huang et al., Science, 14 May 2004.)