Dielectrophoretic trapping of molecules is typically carried out using metal electrodes to provide high field gradients. In this paper we demonstrate dielectrophoretic trapping using insulating constrictions at far lower frequencies than are feasible with metallic trapping structures because of water electrolysis. We demonstrate that electrodeless dielectrophoresis (EDEP) can be used for concentration and patterning of both single-strand and double-strand DNA. A possible mechanism for DNA polarization in ionic solution is discussed based on the frequency, viscosity, and field dependence of the observed trapping force.

The effect of confinement on the dynamics of polymers was studied by observing the transient extension and relaxation of single DNA molecules as they interacted with obstacles in a specially designed thin slit. Viscous drag was found to increase with the degree of confinement, which we interpret in terms of hydrodynamic screening by the planar surfaces of the slit. Since the DNA was driven by an electrophoretic force, the experimental data support the notion that an electric field acts on a tethered polyelectrolyte equivalently to a hydrodynamic flow. [S0031-9007(98)

A separation technique employing a microfabricated sieve has been demonstrated by observing the motion of DNA molecules of different size. The sieve consists of a two-dimensional lattice of obstacles whose asymmetric disposition rectifies the Brownian motion of molecules driven through the device, causing them to follow paths that depend on their diffusion coefficient. A nominal 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 method is that samples can be loaded and sorted continuously, in contrast to the batch mode commonly used in gel electrophoresis.DNA fractionation ͉ Brownian motion ͉ microfabricated array T he separation of DNA fragments from restriction enzyme digests plays a central role in molecular biology (1), and there is a growing realization that new and faster methods are needed to accelerate and simplify genomic analysis. One route to this goal is the exploitation of technologies in the microfabrication industry to create miniature silicon-based devices in which sample preparation, separation, and analysis are integrated in a single microenvironment (2-5). The task is complicated by the fact that, when forced to move by a flow or by the application of an electric field, DNA molecules of different sizes all migrate at the same speed. Traditionally, a sieving medium such as a gel or a solution of polymers has been used to alter the mobility. Recently, however, Duke and Austin (6) and Ertas (7) proposed an alternative approach that takes advantage of the fact that, as the molecules migrate, they also diffuse-and this they do at a size-dependent rate. On theoretical grounds, they showed how a two-dimensional obstacle course could be constructed to sort the swift diffusers from the slow. The basic concept is to use a regular lattice of asymmetric obstacles to rectify the lateral Brownian motion of the molecules so that species of different sizes follow different trajectories through the device. A mixture of molecules injected in a fine stream would be sorted continuously.We have implemented this idea by using a microfabricated silicon array. That such a device works in principle has been demonstrated by making microscopic observations of the dynamics of fluorescently labeled latex beads and DNA molecules † † and also by the recent work of van Oudenaarden and Boxer (8) with lipids. The experiments are technically demanding, however, and the resolution of different species has not yet been achieved. Progression from a demonstration of principle to practical continuous stream sorting of molecules requires perfection of the technology. In the work reported here, our aim has been to conduct a systematic, quantitative study of the separation process, using molecules of a well-defined size and experimental conditions that are close to optimal. The close correspondence of the results with theoretical predictions permits us to predict, with some confidence, how to construct a device that will work well in practice. Ma...