Adrienne R. Minerick 1 * Ronghui Zhou 1 Pavlo Takhistov 2 Hsueh-Chia Chang 1 1 Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN, USA 2 Department of Food Sciences, Rutgers University, New Brunswick, NJ, USA Manipulation and characterization of red blood cells with alternating current fields in microdevices The motion of a suspension of erythrocytes (red blood cells, RBCs) in response to a high-frequency alternating current (AC) field in a microfluidic device is examined with parallel and orthogonal electrode configurations to delineate the various fundamental driving forces. Cell repulsion from the platinum electrodes due to electrode polariza- tion interacting with cell membrane polarizations is observed to be the strongest force acting on the particles in the first few seconds of field application. We exploit this strong repulsion to concentrate the bioparticles between the microelectrodes to amplify multiparticle aggregation phenomenon and dielectrophoretic (DEP) manipula- tion in a small and well-characterized region within the microfluidic device. Secondary motions include RBC pearl chain formation along field lines due to particle polarization followed by classical dielectrophoretic motion of the chains across field lines to regions of weaker field. These are driven by far weaker dipole-dipole and field-dipole inter- actions than the preliminary electrode repulsions. RBC chain length and total aggre- gated cells are presented for a variety of AC frequencies and are significantly amplified by the electrode repulsion. Motion of particles away from the polarized electrode is found to be species- and age-sensitive and can stand by itself as a promising identifi- cation and separation mechanism. In a 0.1 S/m isotonic phosphate buffer saline medi- um, we observe the largest cell mobilities at an optimal frequency of approximately 1 MHz, corresponding to the inverse diffusion time across the double layer of the cell and across the electrode’s polarized layer. This suggests that the dielectric responses of both particles and electrodes in the low MHz frequency range are mostly determined by normal electromigration of ions from the bulk to their interfaces. Sensitivity to RBC age and species suggests that the surface proteins and membrane ion channels can affect the capacitance of the interface to accommodate the ions from the bulk. Such surface ion accumulation and polarization mechanisms are different from the classical dielectric theories. The resonant frequency of electrode polarization at around 1 MHz falls between positive and negative dielectrophoretic resonant frequency peaks – sug- gesting that the double-layer polarization mechanism is a distinct and potentially important bioparticle manipulation tool. Keywords: Chain translation / Dielectrophoresis / Dipole / Erythrocyte / Microfluidic device / Min- iaturization DOI 10.1002/elps.200305644 1 Introduction 1.1 General aspects The possibility of focusing and separating blood cells based on their conductivity and shape characteristics has great potential in studying fundamental cell proper- ties as well as in the medical diagnostics field. Straight- forward, rapid separations and distinctions between abnormal and healthy blood cells could aid in the diagno- sis or eventual treatment of leukemia [1], malaria [2], exposure to toxicants [3], prenatal diagnosis of genetic abnormalities [4], or other blood-related diseases. This article outlines a special electrokinetic phenomenon that has the potential of aiding in cell focusing and separation applications. The application of nonuniform alternating current (AC) fields to suspensions has seen many uses in the last 25 years and is widely being explored to develop fast bioparticle separation and identification mechanisms. Ex- Correspondence: Dr. Hsueh-Chia Chang, Department of Chem- ical Engineering, University of Notre Dame, 182 Fitzpatrick, Notre Dame, IN 46556, USA E-mail: chang.2@nd.edu Fax: +219-631-8366 Abbreviations: AC, alternating current; DEP , dielectrophoresis; RBC, red blood cells, erythrocytes Electrophoresis 2003, 24, 3703–3717 3703 * Current address: Dave C. Swalm School of Chemical Engineer- ing, Mississippi State University Miniaturization  2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim