Sorting by diffusion: An asymmetric obstacle course for continuous molecular separation

A separation technique employing a microfabricated sieve has been demonstra ted by observing the motion of DNA molecules of different size. The sieve c onsists of a two-dimensional lattice of obstacles whose asymmetric disposit ion rectifies the Brownian motion of molecules driven through the device, c ausing them to follow paths that depend on their diffusion coefficient. A n ominal 6% resolution by length of DNA molecules in the size range 15-30 kbp may be achieved in a 4-inch (10-cm) silicon wafer. The advantage of this m ethod is that samples can be loaded and sorted continuously, in contrast to the batch mode commonly used in gel electrophoresis.