We developed a novel microfluidic device in which magnetic particles are maintained dynamically in a microchamber and undergo hydrodynamic recirculation. From a physical point of view, this dense dynamical system is reminiscent of fluidized beds and its behaviour has been interpreted in this frame. Gravitational fluidized beds were widely investigated in the macroscopic worlds but their integration in microfluidics is hardly possible as they provide a very limited dynamic range. Magnetic forces offer the opportunity to extend the concept of fluidized bed to miniaturized systems and to our knowledge, this is the first implementation of a magnetically stabilized fluidized bed in microfluidics. Previous systems consisted in either low density static or dynamic systems in which columns are trapped in a strong and local field potential. These systems have a higher potential capacity, but limited capture efficiency. The new system developed here bridges the gap between these two previous strategies and opens new regimes of operation combining a high density of magnetic particles, for high capacity, an active hydrodynamic stirring for increased kinetics and a high flow rate allowing concentration of very dilute analytes from a rather large sample volume. This system was validated in an immunoassay using a stop-and-go strategy taking full advantage of the dynamic nature of the magnetic microcolumn. We believe, however, that it also has strong potential of preconcentration of analytes from ml to µl or nl sized sample downstream microfluidic analysis modules. This laboratory is involved in the EU Nadine, aimed at developing early diagnosis of Alzheimer's disease in blood.
