Transportation and positioning
of magnetic markers

In 2004 WIRIX-SPEETJENS and DE BOECK presented [136] an interesting method to transport magnetic particles along a defined pathway using an alternating magnetic gradient field. In a quite simple setup with only two sawtooth shaped conducting lines, they guided single particles along a predefined line. They used 2$\mu$m MICROMOD beads, and an alternating current of 50mA at a frequency of 0.1Hz in their experiments.

Figure 3.10: Transportation of single beads, from Wirix-Speetjens and De Boeck [136]

Figure 3.10 shows their results for a dual metallisation device (a,b,c) and for a single metallisation device (x,y,z). In the images, the white arrows indicate the stepwise movement of the single magnetic particles. The movement in one direction only works if the two conductors are very well aligned. Otherwise, the magnetic particle moves back and forth between two local maxima. The velocity of the magnetic particle can be adjusted by changing the current and by changing the proportion between the width and the length of the sawtooth structure.

Figure 3.11: Moving magnetic particles to several defined positions with a star like structure.

Because this thesis combines magnetic manipulation and detection, a different structure is designed that allows the transportation of a bead to several defined positions. Figure 3.11 presents the star like structure and the positioning of a few beads at defined locations. Outside the optical microscope images of figure 3.11(a-f), it is always sketched where the potential is applied. The time and the actual current during these six images of the video are written in orange at the bottom of the images. At the beginning a few magnetic beads are collected in the corner of the right conducting line (a). Then, the current is turned off in the right line and the top-right line is turned on (b). The beads directly move from the right to the top-right corner (see the attached CD for the complete video). In the same fashion the beads are moved counter-clockwise around until they are finally collected in the middle ring (f).

The accuracy of the positioning inside the corner only depends on the accuracy of the lithography. Using optical lithography, we are restricted by the wavelength of light. So in this experiment, the accuracy of the positioning can only be about 1$\mu$m (about the size of the used magnetic beads). Changing to e-beam lithography would allow a much better positioning accuracy of about 50-100nm.

During the experiment, that takes about 5minutes, more and more beads are collected from the vicinity. Therefore, the number of beads in the corners increase steadily. While in the first corner, there were only about 5 beads, in the end there are probably more than 30 beads. Although it isn't tested, this experiment can certainly be done with single beads. As will be shown in chapter 5, this mainly depends on the concentration of the beads in the solvent.

What cannot be seen in the six images, but in the complete video, is a problem with the top-left conducting line. Immediately after this line is turned on, bubbles rise from the surface and turn the microscope image completely black (that's the reason for not providing the image here). For a discussion of this bubble problem, see section 3.2. Although the top-left conducting line is not working, the experiment can go on. So the principle of this transportation and positioning technique for magnetic particles works very well.