AbstractThe advantages of using microelectrodesBiological cell separation using dielectrophoresis in a microfluidic device R. Díaz, S. Payen University of California, Berkeley Bio and Thermal Engineering Laboratory EECS 245 Abstract Basic IC fabrication techniques are employed in this paper with the purpose of designing a micro-fluidic device with a 3D electrode arrangement to separate live and dead biological cells. This is done us-ing the concept of dielectrophoresis, which describes the transnational motion of particles due to the appli-cation of a non-uniform electrical field. The simulations where carried out using the protoplast model for mammalian spherical cells [4] in a wide range of electric field frequencies. Analytically, we have found that the Clausius-Mossoti Factor is negligible for live mammalian cells over a frequency range of 50 – 70 KHz whereas is maximum for the dead cells. Since the Clausius-Mossoti factor is the important term in the dielectrophoretic force formulae, we can envision our microfluidic design as a feasible tool to separate live and dead mammalian cells. 1. Introduction In the past few years, there has been an extensive re-search in the manipulation and analysis of biological cells at the micro scale. There is an increase interest in applying microelectromechanical systems (MEMS) for selective trapping, manipulation and separation of bioparticles. Al-though there is a huge demand of automated single-cell manipulation and analysis in immunology, developmental biology and tumor biology calling for the development of suitable microsystems, the approaches currently available to meet those needs are limited [7]. The term dielectrophoresis (DEP) was first introduced by Pohl [9] to describe the transnational motion of particles due to the application of non-uniform electrical fields. The dielectrophoretic motion is determined by the magnitude and polarity of the charges induced in a particle by the ap-plied field [8]. Usually, dielectrophoresis is performed un-der an alternating current (AC) field over a wide range of frequencies. The DEP force is dependent on several parameters: the dielectric properties and size of the particle, the frequency of the applied field and the electrical properties (conductiv-ity and permittivity) of the medium. Therefore, if is desired to achieve a good particle manipulation say cell separation, detailed analysis and careful selection need to be done in order to obtain the desired results. In this paper, we are going to propose and analyze a micro-electrode system incorporated in a microfluidic device, de-signed for the separation of live and dead biological cells using the dielectrophoretic force. As an application, it is desired to separate the cells to selectively apply medicine or for gene therapy using electroporation techniques [1,2]. 2. Theory of Dielectrophoresis Electrophoresis and dielectrophoresis describe the movement of particles under the influence of applied elec-tric fields. Whereas electrophoresis is the movement of charged particles in direct current (DC) or low-frequency alternating current fields, dielectrophoresis is the movement of particles in non-uniform electric fields. The dipole mo-ment m induced in the particle can be represented by the generation of equal and opposite charges (+q and –q) at the particle boundary. The magnitude of the induced charge q is small, equivalent to around 0.1 % of the net surface charge normally carried by cells and microorganisms, and can be generated within about a microsecond. The important fact is that this induced charge is not uniformly distributed over the bioparticle surface, but creates a macroscopic dipole. If the applied field is non-uniform, the local electric field E and resulting force (E.δq) on each side of the particle will be different. Thus, depending on the relative polarizability of the particle with respect to the surrounding medium, it will be induced to move either towards the inner electrode and the high-electric-field region (positive DEP) or towards the outer electrode, where the field is weaker (negative DEP). Following established theory, the DEP force FDEP acting on a spherical particle of radius r suspended in a fluid of abso-lute dielectric permittivity εm is given by: , (*) 213)]}({Re[2EwKRFDEP∇=επwhere Re[K(w)] is the Clausius-Mossoti function and de-termines the effective polarizability of the particle and the factor ∇E2 is proportional to the gradient and the strength of the applied electric field. The polarizability parameter Re[K(w)] varies as a function of the frequency of the ap-plied field and, depending on the dielectric properties of the particle and the surrounding medium, can theoretically have a value between +1.0 and –0.5. The value for Re[K(w)] at frequencies below 1kHz is determined largely by polariza-tions associated with particle surface charge. While in-creasing frequency, first the effective conductivity and sec-ond the effective permeability are the dominant contribut-ing factors. A positive value for Re[K(w)] leads to an in-duced dipole moment aligned with the applied field and to a positive DEP force. A negative value for Re[K(w)] results in an induced dipole moment aligned against the field and produces a negative DEP. The fact that the field appears as ∇E2 in the equation of the DEP force indicates that revers-ing the polarity of the applied voltage does not reverse the DEP force. AC voltages can therefore be employed and, for a wide range of applied frequencies (typically 500Hz to 50MHz), the dielectric properties of the particle, as embod-ied in the parameter Re[K(w)], can be fully exploited. The advantages of using microelectrodes The advantages to be gained by reducing the scale of the electrode design can be illustrated using the example of the spherical electrodes for the case of a particle located one-tenth of the distance from the inner to the outer electrode [12]. For a 100-fold reduction of electrode size, a 1000-fold reduction of operating voltage will therefore produce the same DEP force on a particle in the same relative location. In addition to the practical advantage of being able to use lower operating voltages for a given desired DEP force, there is also a significant reduction in electrical heating and electrochemical effects. The energy deposition form the field is proportional to σE2, where σ is the conductivity of the
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