Magnetoresistive Detection of Magnetic Beads Flowing at High Speed in Microfluidic Channels

An integrated device including microfluidic channels and incorporated magnetoresistive spin valve sensors has been developed and used to detect single magnetic bead motion at cm/s velocities. The sensitivity of the system is high enough to determine the magnetic orientation, the flowing height and the speed of each particle for velocities between 10-23 mm/s and the counting capability of the device is fully working for this range of velocities and below this limit. Sensor signals (between 3-100 muVp-p) correspond to bead moments at different directions indicating a physical rotation of the beads, and a slow response (seconds) of the bead moment to magnetizing field changes. Subsequent magnetic characterization of the micron-sized beads indicates that they are composed of strongly interacting magnetic nanoparticles.