The use of magnetic markers and magnetoresistive sensor elements is a new research area that is very promising for the development of small and powerful biosensors. The magnetic approach has three major advantages over the conventional optical biosensors. The magnetoresistive sensors can be read out very easily, their overall production costs are very cheap because they are standard micro-chip techniques, and the magnetic markers, and with it the attached biomolecules, can be directly manipulated with an outer magnetic field.
This thesis presents an on-chip manipulation and positioning system for single magnetic markers. The setup is simple and easily customisable. Conducting lines are patterned with optical lithography on a Si-wafer chip. An applied current through the conducting lines creates a magnetic gradient field that interacts with the magnetic markers, so the magnetic markers follow the gradient of the field to a local maximum.
It is shown that this manipulation and positioning technique works in principle and, moreover, several applications are introduced. One application is a special design that allows the transportation of several markers and the positioning in predefined places. Additionally, the trapping of markers inside a ring structure is studied and the effects of an applied electric field on the magnetic markers are investigated. Single magnetic markers were positioned directly on top of small TMR sensors using a specifically designed structure. The magnetic stray field of the magnetoresistive sensors helped to position the magnetic markers on top of the sensors, although this is not necessary. The accuracy of the positioning system only depends on the accuracy of the used lithography. So for e-beam lithography the accuracy is below 100nm. The manipulation system is eminently suited for a small handheld biosensor device because it fits together with the sensors directly on the Si-wafer chip.
As an application of the manipulation technique, bond-force measurements
between two biomolecules were done. One biomolecule is attached to the
sample surface and the other is connected to a magnetic marker. After
the biomolecules bound, a magnetic field is turned on using the
conducting lines on the chip. The magnetic gradient field is slowly
increased until the bonds between the biomolecules break. This event is
monitored with a CCD-camera and evaluated to calculate the corresponding
bond-force. Compared to other bond-force measurements, this method has
an extremely low loading-rate (
1fN/sec) and, therefore, the
measured bond-forces are extremely low as well. The two well known bonds
streptavidin-biotin and avidin-biotin were investigated, and very low
bond-forces were measured (full bond of streptavidin-biotin:
245fN, avidin-biotin: 58fN). The measured
bond-forces are about 1000 times lower than produced by earlier
measurements, which fits well to the loading-rate, which is also about
1000 times lower. This method also provides evidence for the
cooperativity of these ligand-receptor bonds.
Several enhancements for the techniques are planned to improve future experiments. First of all, it is planned to use a CVD coating technique to improve the protection layer of the TMR elements, so they survive the aqueous conditions during the positioning. Afterwards, different layer stacks for the sensor can be tested, as well as smaller magnetic particles. With a single bead that binds to the surface of a magnetoresistive sensor and an applied out of plane magnetic field, force-distant curves can be measured using the sensor signal. To accelerate the development time, finite element simulations are proposed as a first step in the evolution of new designs.
Because of the exceptionally low loading rates, the method for the measurement of bond-forces is very interesting for other molecules too. In order to enhance the maximum possible force, ASTRIT SHOSHI started experiments in our group to add magnetic materials to the conducting lines. For example, he added a GMR trilayer below the conducting line to enhance the magnetic field strength, without having to apply an external magnetic field.
Additionally, the combination of this manipulation technique with microfluidic systems [83,125] will be very interesting for future experiments. The fluidic system allows a fast transportation of magnetic markers over large distances, and the magnetic manipulation system facilitates the exact positioning.
At a later stage, it would also be very interesting to apply this on-chip manipulation technique to a complete cell that has to be grown directly on top of the structured design. With a magnetic marker inside the cell and a sensor array below, measurements of the viscosity in different areas of the cell could be done. It would also be very interesting for in-vivo measurements of intermolecular forces, and to determine where in the cell a specific biomolecule can bind.