A COMBINED DIELECTROPHORETIC AND FIELD-FLOW FRACTIONATION MICROSYSTEM FOR BIOMEDICAL SEPARATION AND ANALYSIS Jody Vykoukal, Jun Yang, Frederick F. Becker and Peter R. C. Gascoyne The University of Texas M. D. Anderson Cancer Center Peter Krulevitch, Harold Ackler and Julie Hamilton Lawrence Livermore National Laboratory Abstract The ability to separate and identify cells and other particulate matter is a fundamental requirement of microsystems designed for biomedical and other applications. Here we describe a method that combines dielectrophoresis and field-flow fractionation to separate and identify particles in a microfluidic environment. This method is applicable not only to the analysis of cellular and other particulate analytes but also to the detection of toxins using sensitized test particles. We show proof of principle by achieving differential separation of human peripheral blood mononuclear cell subtypes in a microsystem based on the method. Keywords: cell separation, dielectrophoresis, field-flow fractionation 1. Introduction Dielectrophoresis (DEP) is particularly appealing as a means of manipulating analytes in microsystems because it acts directly upon the particles of interest, moving them with respect to the suspending medium. DEP can be used to manipulate particles so that contact with microsystem surfaces is eliminated, reducing contamination. DEP forces are dependent only upon the dielectric properties of the particles and of their suspending medium. Additionally, the required inhomogeneous electrical fields can be produced by applying relatively small electrical signals (on the order of one volt or less) to readily fabricated microelectrodes and, unlike electrophoresis, DEP utilizes high frequency AC electrical fields, so electrolytic products are not a concern. Furthermore, by selecting appropriate frequencies for the applied field, frequency-dependent differences in the dielectric properties of particles can be exploited to achieve separation of analytes in mixed samples [1, 2]. We describe a microsystem that combines dielectrophoresis with hyperlayer field-flow fractionation (FFF) wherein particles are separated by being maintained at differential positions within a pressure-driven parabolic hydrodynamic flow profile by an applied force field. In our system, this is accomplished by balancing DEP and gravitational forces (Fig. 1). Different particles emerge from the device at times indicative of their positions in the flow profile in accordance with their dielectric properties. Fig. 1. Forces on particles during DEP-FFF separations. Particles having different dielectric properties are levitated by the DEP electrode (chamber bottom) to different equilibrium heights within the hydrodynamic flow profile (represented by vectors on shaded area) and are carried through the chamber at different rates (V1 and V2). V 2 F DEPz F grav V 1 F DEPz F grav flow