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Microdevices for Dielectrophoretic Flow-Through Cell Separation Comparing Experiments with Numerical Simulation for a Novel System that Separates and Fractionates Particles from a Fluid Stream

M any research groups have developed dielectrophoretic (DEP)-based separation systems for a wide range of particle types, including cells, bacteria, viruses and proteins [1]–[10]. Simple twodimensional (2-D) spatial separation of a binary mixture of particles on a microelectrode array is relatively easy to demonstrate; however, the implementation of a practical flowthrough separation system is rather more difficult and a range of continuous or bulk separation strategies have been devised. One type of DEP separator works by attracting particles out of a suspending medium by positive DEP. The medium flows over an electrode array, which is part of a flow-through chamber as shown in Figure 1. The frequency applied to the electrodes is chosen so that the cells of interest are attracted to the electrodes by positive DEP, and all the other cells experience negative DEP and are eluted from the device. Provided the flow velocity is not too high, the required particles remain trapped on the electrodes and can be released for postprocessing by switching off the field. The separation chamber can be formed using a microelectrode array, for example, an interdigitated electrode, that generates a DEP force that is maximum at the surface and rapidly decreases (exponentially) into the chamber center. The simple device shown in Figure 1 has a number of practical problems; for example, the cells enter the chamber randomly distributed in the fluid carrier and therefore move at different velocities and experience the positive DEP force from the electrodes for different lengths of time. This leads to a completely random distribution of cells on the collection electrode even from a homogeneous population. Also, owing to the exponential profile of the DEP force, particles furthest from the electrodes often transit the device without being influenced by the DEP force and may not be captured. We have therefore developed a new type of DEP separator that consists of two microelectrode arrays integrated into a single device, as shown in Figure 2 [11], [12]. The first part of the device uses negative DEP to force a random distribution of particles entering the device into a well-defined sheet positioned midway between the upper and lower channel walls. This ensures that all particles enter the separation device at the same height, thereby eliminating any error in the collection point in the second array due to the exponential nature of the dielectrophoretic force. The sheet of particles then enters the second separation electrode array, which is energized with a frequency such that the required subpopulation of particles is attracted to the electrode surface. Using this method, different subpopulations will collect on the second electrode array in clearly defined bands depending on their properties. This device can therefore also be used as a cell or particle fractionation device. Another major problem with DEP-based separation systems is nonspecific adhesion of unwanted cells onto the collecting electrodes. Prefocusing the particles eliminates contact between the cells and the electrodes or the glass through sedimentation and therefore prevents adhesion. In this article, we report on the numerical simulation of particle trajectories in this device and compare the simulations with experiments on the dielectrophoretic separation of white blood cells. Possible application of this DEP-separation device might be the separation of live from dead cells, the retrieval of small numbers of particles from a fluid containing debris, or the isolation of rare cells from a large population of unwanted particles with minimum contamination.