Continuous flow processes on single magnetic and diamagnetic particles in microfluidic devices (2011)

Magnetic microparticles have seen increasing interest in (bio)chemical processes in recent years due to their various surface functionalities, high surface-to-volume ratio, small sizes, and ease of manipulation via magnetic fields. However, conventional reactions and assays that use magnetic particles as solid supports are typically performed in multi-step procedures that require consecutive reaction and washing steps. While offering high capture efficiencies, these are batch processes that, due to the consecutive steps required, are typically time-consuming and laborious. Their incorporation into microfluidic devices has brought about benefits including finer control over the movement of particles and reagent/sample solutions, as well as the ability to place a magnet closer to the area of interest. However, most instances of on-chip magnetic particle based procedures rely on trap-and-release methodology, essentially requiring the same stepwise routine as with conventional systems. A method of reducing these inefficiencies is to perform the reaction or separation in continuous flow, thereby allowing continuous sample introduction and analysis of the process in rapid times, and with minimal reagent consumption and waste production.Two methods of performing continuous flow procedures on single particles in microfluidic devices via the application of magnetic forces were investigated: 1) the use of magnetic microparticles as mobile solid supports for performing rapid separations, reactions, and immunoassays via magnetic attraction, and 2) the use of diamagnetic repulsion forces for performing similar procedures on non-magnetic particles, with a view to the label-free processing of diamagnetic species such as polymer particles and biological cells, based on their intrinsic properties.For the magnetic attraction experiments, a study into the effect of temperature on magnetic particle deflection behaviour and separations was performed, whereupon it was found that an increased temperature of the system yielded increased deflection distances and separation resolution due to the reduced viscous drag. This was followed by several investigations into the deflection of particles through laminar flow streams containing alternating reagents and washing buffers for performing multistep reactions and assays. The setup was used to demonstrate amide bond formation and polyelectrolyte deposition in continuous flow, before being used to detect clinically relevant levels (5 and 10µg mL-1) of the inflammatory biomarker, C-reactive protein. Thus, these findings show great potential for rapid, continuous processing of particles for a number of chemical and biological applications, as well as in clinical diagnostics.For the diamagnetic repulsion studies, diamagnetic polystyrene particles were suspended in paramagnetic media and deflected away from a magnetic field in continuous flow. The effect of particle size and the magnetic susceptibility of the paramagnetic media on particle deflection were investigated using high magnetic fields, where it was found that larger particles in a medium with higher susceptibility yielded the greatest deflection. This work was extended via a proof-of-principle setup in which polystyrene particles were repelled out of a reagent stream and into a buffer stream using permanent magnets, with a view to performing continuous flow reactions through laminar flow reagent and washing buffer streams, akin to those achieved via magnetic attraction. Finally, flow focussing of polystyrene particles and label-free cells was achieved via diamagnetic repulsion forces applied by permanent magnets, demonstrating the ability to manipulate cells in continuous flow by magnetic forces based on their intrinsic properties. This work could be applied to the label-free processing of particles and cells for separations, reactions, and assays.