Nearly all materials can be manipulated with electric fields. If a material is charged, it can easily be moved in an electric gradient field (the direction depends on the charge). If a material is not charged, the electric field induces a dipole moment within in the material and the induced dipole moment is then affected by the electric field (e.g. this is used in dielectrophoresis experiments, see below). In the experiments presented in this chapter the magnetic beads are also charged, and so they can easily be manipulated.
Figure 3.9 shows a setup that uses magnetic and electric fields to manipulate magnetic markers. Currents through the two lines in the middle (wide and thin line) create magnetic fields, and the top and bottom lines are electrodes of a capacitor creating an electric field between them.
![\includegraphics[width=\textwidth]{Bilder/E-M-Feld}](img166.png)
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In the experiments, the magnetic markers are collected with a small current on the lines in the middle (see the CD for the complete video). Then, the current is turned off and an electric field is applied on the outer electrodes. The movement of the beads is viewed and recorded through the optical microscope, as before.
When the electric field is turned on, many markers are drawn quickly towards one of the electrodes. Only very few don't move at all. Although this observation shows that the markers can be manipulated with electric fields, it also reveals the major problem of this method. The experiment shows that the markers are charged and, therefore, attracted or rejected in the electric field. This is quite similar to the oil-drop experiment of MILLIKAN3.1 in 1907 [94]. But here it is unclear how much the markers are charged and if they are positively or negatively charged. The video shows all three possibilities (positively charged, negatively charged and uncharged) in the same sample. Therefore, this method is not usable for a controlled manipulation of magnetic markers. However, the electric force exerted on many markers seems to be much stronger than the force that can be applied with the magnetic field. Therefore, we will briefly refer to this method in chapter 4.1.
Another approach to manipulate markers with electric fields, that is not used in this thesis, is to use ac fields that exert a dielectrophoretic force on the markers. Dielectrophoresis describes the movement of particles caused by the interaction of the induced dipole moment and an external electric field [34]. The dielectrophoretic force depends on the frequency of the external ac electric field and on the magnitude of the complex dielectric constant of the particle in relation to that of the used medium. This method is especially interesting, because just by changing the frequency, you can change from an attractive force to a repulsive force. But using electric fields also has the disadvantage that everything on the sample is manipulated nonselectively. All kinds of particle traps can be built with this technique, and so there is a lot of research done about dielectrophoresis. Manipulation [81] and separation [96] of bio-particles or nanoparticles [74] are done with dielectrophoresis as well as the use of dielectrophoresis in diagnostic instruments [48]. Furthermore, microspheres were specially engineered for their dielectric properties [128]. The combination of magnetic on-chip manipulation techniques with on-chip dielectrophoresis is a promising field for future research.