Magnetophoretic cell clarification

Abstract

(cont.) the feed fluid was achieved after one pass through the counter current system. In the second case, four permanent magnets were arranged in a quadrupole around a central column to create areas of high magnetic field at the column walls and areas of low magnetic field at the centerline, inducing non-magnetic particles to concentrate at the centerline, where they were removed through a coaxial central outlet tube at the top of the column. Depending on the flow rate, up to 99% of polystyrene beads of different sizes could be removed from the feed after one pass through the quadrupole system. The recovery efficiency decreased with increasing flow rate, i.e. with decreasing residence time in the device. E. coli cells were able to be removed with separation efficiencies as high as 95% at much higher flow rates due to the formation of [approximately]12 micron aggregates in the presence of the magnetic nanoparticles; these large aggregates experienced enhanced magnetic forces over individually-dispersed cells and could be recovered more effectively. The governing equations for magnetophoretic clarification were applied to the quadrupole configuration to predict particle trajectories through the column and to predict the separation efficiency under different flow conditions, which showed a good match to the experimental results. It was also shown that axial magnetic field gradients near the entrance region acted effectively as a barrier to entry of particles in the slow moving regions near the walls; this retardation of their axial movement provided a longer residence time for the particles that allowed them to be moved more efficiently to the centerline ...

A new approach for the removal of micron-sized particles from aqueous suspensions was developed and applied to the problem of cell clarification from raw fermentation broth. The concepts of magnetophoretic separation were exploited to take advantage of the force that acts on a non-magnetic particle when it is immersed in a magnetic fluid (ferrofluid) that is subjected to a non-uniform magnetic field. The magnetic "pressure" difference across the non-magnetic particle owing to the magnetization of the surrounding magnetic fluid forces the particles away from areas of high magnetic field strength and into areas of low magnetic field strength. This force is proportional to the volume of the non-magnetic particles, and is therefore stronger for larger particles. In this way, non-magnetic particles can be focused and moved out of the bulk fluid by applying a non-uniform magnetic field to the system, leading to magnetophoretic clarification. The magnetic fluid used in this work was composed of magnetite nanoparticles coated with a poly(acrylic acid)-poly(ethylene oxide)-poly(propylene oxide) graft copolymer layer that stabilized the nanoparticles in water and prevented their aggregation. The magnetic nanoparticles were approximately 32 nm in diameter, with the magnetite core itself being approximately 8 nm in diameter. Magnetophoretic clarification was investigated using two different flow configurations. In the first case, the particle-laden magnetic fluid was pumped through a flow tube while a series of magnets around the tube moved counter to the direction of the feed flow; the non-magnetic particles in the feed were captured and effectively removed from the bulk fluid by the moving magnets. A removal efficiency of 95% of E. coli cells from