Multiple frequency dielectrophoresis (MFDEP)

Elizabeth Smela and Mario Urdaneta

Department of Mechanical Engineering, University of Maryland
E-mail: smela@umd.edu; Web: www.smela.umd.edu

Multiple frequency dielectrophoresis (MFDEP) manipulates particles using more than one AC frequency, in comparison with standard DEP, which uses only one frequency. In MFDEP these AC signals can come either from a single pair of electrodes, from different electrodes, or even multiple electrodes that each provide multiple signals. At a given point in space, the forces produced by each frequency simply add up to give the total DEP force on the particle. The forces vary in space, depending on the placement of the electrodes and the magnitude of the applied potentials on each of them. (The forces also depend, as they do for DEP, on the particle size and relative polarizability.) Computational tools can be used to determine the net force as a function of position, and the contribution of each signal.

The application of multiple frequencies allows manipulations that are impossible using a single frequency. For example, the quintessential application of DEP is the separation of cells (1). The simplest approach to cell separation is to employ a DEP frequency that attracts the cells of interest and repels all others, or vice versa. However, for some cell mixtures a single frequency that excludes all but the cells of interest may not exist: other cells may have too similar properties and thus also be attracted. For example, Figure 1 shows the Clausius-Mossotti (CM) factors for T- and B- lymphocytes, which are quite similar (gray lines in Fig. 1); thus, only a frequency value in the narrow range a, where the CMs have opposite sign, leads to their separation. A combination of frequencies can, however, tease out small differences in the responses of the cells to target the cells of interest, each frequency affecting the cells somewhat differently to produce a net force difference. Adding a 10 kHz signal on top of the main frequency shifts the CM curves (black lines in Fig. 1) so that they have opposite sign over a range of frequencies b that is wider than a, thus making their separation easier. This application of MFDEP does not require device redesign, just applying the magic combination of frequencies instead of a single frequency on the existing electrode.

Figure 1. Clausius-Mossotti (CM) factors for T- and B- lymphocytes as function of frequency.

Another example of the advantages of MFDEP is illustrated by one of its first implementations, the accurate measurement of the positive DEP response (pDEP, attractive) of canola protoplasts by Kaler et al. (2). With only pDEP, particles move rapidly toward the electrode, making measurements of force difficult. Therefore, this group levitated the particles using negative DEP (nDEP, repulsive), putting them in a stable position, and then added small pDEP perturbations to the main nDEP signal to determine the frequency response of the protoplasts.

MFDEP can provide extraordinary control when multiple electrodes are employed to create complex topographies of attractive and repulsive forces tailored to the device geometry and the application, freeing the design from constraints encountered with single-frequency DEP. We have used MFDEP to attract cells into an insulating micro-scale box. The electric field produced by a single pDEP electrode on the bottom of a box is distorted by the insulating material, resulting in an attraction between the cells and the upper rim that leaves them stuck at the entry. We have seen this experimentally, and it is illustrated in the simulation results depicted in Figure 2, which shows the cross-section of the box and the flow lines (black dots) of cells getting stuck.

Figure 2. Simulations of the electric field distribution in a micro-scale box showing the flow lines (black dots) followed by the cells that get trapped.

The addition of a second nDEP electrode on the rim of the box pushes the cells away and towards the middle, where the pDEP coming from inside the box is able to draw the cells inside (3). Figure 3 shows simulation results for this geometry, and also the top view of a box successfully loaded with cells using this approach.

Figure 3. Simulations of the electric field distribution in a micro-scale box after adding a second nDEP electrode, showing different flow lines (black dots) that the cells follow before getting trapped.

The addition of a second nDEP electrode on the rim of the box pushes the cells away and towards the middle, where the pDEP coming from inside the box is able to draw the cells inside (3). Figure 3 shows simulation results for this geometry, and also the top view of a box successfully loaded with cells using this approach.

In summary, MFDEP is an extension of DEP that enables a wider range of manipulations on mixtures of particles as well as sophisticated manipulations within complex geometries.


References
  1. T. B. Jones, Electromechanics of Particles (Cambridge University Press, New York, 1995).
  2. Kaler, K., Xie, J.P., Jones, T., Paul, R., “Dual-frequency dielectrophoretic levitation of Canola protoplasts”, Biophys. J., 63(1), 58-69, 1992
  3. Urdaneta, M., Smela, E., “Parasitic trap cancellation using multiple frequency dielectrophoresis, demonstrated loading cells into cages”, Lab Chip, 8(4), 550-556, 2008