GMI detection of magnetic-particle concentration in continuous flow

We describe a microfluidic device for the determination of the concentration of magnetic beads under continuous flow of the carrier fluid by means of the Giant Magneto-Impedance effect (GMI). The microfluidic chip is composed of a 10 μl chamber situated on top of the GMI sensing material, which is inserted in a microstrip transmission line for the impedance measurements. Two different GMI materials have been used for the experiments: an amorphous ribbon and a permalloy based multilayer, sputtered onto the same polymeric material of the microfluidic chamber. Detection tests in continuous flow have been performed using solutions containing two types of beads: one made of ferromagnetic microparticles and the other made of superparamagnetic nanoparticles. A basic calculation of the magnitude of the fringe field created by the beads assures a detectable signal on the sensors, but the experimental difficulties severely condition the measurements. The results show a promising sensitivity for both types of particles but also reveal an important number of detection problems that must be overcame before the prototype become functional.