Magnetic particle handling microsystems for miniaturized analytical applications

Microfluidic systems can be advantageously used for analytical applications due to (i) the possibility of using minute quantities of sample and reagents, (ii) relatively fast reaction times, as short molecular diffusion lengths are of the order of the microchannel dimension, and (iii) a large surface-to-volume ratio offering an intrinsic compatibility between the use of a microfluidic system and surface-based assays. The use of functionalized nano- and micro-particles (´beads´) inside microfluidic systems presents additional advantage, as they offer a large specific surface for chemical binding. Such small particles can be advantageously used as a ´mobile substrate´ for bio-assays or even for in vivo applications; they can be easily recovered from a dispersion, reversibly re-dispersed, etc. Magnetic nano- and microparticles still are more interesting: having embedded magnetic entities, they can be magnetically manipulated using permanent magnets or electromagnets, independent of normal microfluidic or biological processes. This extra degree of freedom is at the basis of an improved exposure of the functionalized bead surface to the surrounding liquid and of higher sample pre-concentration efficiencies, due to the increased relative motion of the bead with respect to the fluid [1]. We present an overview of recent developments in the use of magnetic beads in microfluidic systems. Nearly all important functions in a bio-assay can be realized using magnetic beads: sample purification, providing a solid substrate to the sample, mixing, labeling, separation and transport. Droplet-based systems in which aqueous magnetic particle containing droplets are transported in an oil medium are particularly attractive for their simplicity, reconfigurability, self-containment, and the reduction of sample evaporation effects.