Magnetic forces are now being utilised in an amazing variety of microfluidic applications. Magnetohydrodynamic flow has been applied to the pumping of fluids through microchannels. Magnetic materials such as ferrofluids or magnetically doped PDMS have been used as valves. Magnetic microparticles have been employed for mixing of fluid streams. Magnetic particles have also been used as solid supports for bioreactions in microchannels. Trapping and transport of single cells are being investigated and recently, advances have been made towards the detection of magnetic material on-chip. The aim of this review is to introduce and discuss the various developments within the field of magnetism and microfluidics.

Biochemical sample mixtures are commonly separated in batch processes, such as filtration, centrifugation, chromatography or electrophoresis. In recent years, however, many research groups have demonstrated continuous flow separation methods in microfluidic devices. Such separation methods are characterised by continuous injection, real-time monitoring, as well as continuous collection, which makes them ideal for combination with upstream and downstream applications. Importantly, in continuous flow separation the sample components are deflected from the main direction of flow, either by means of a force field (electric, magnetic, acoustic, optical etc.), or by intelligent positioning of obstacles in combination with laminar flow profiles. Sample components susceptible to deflection can be spatially separated. A large variety of methods has been reported, some of these are miniaturised versions of larger scale methods, others are only possible in microfluidic regimes. Researchers now have a diverse toolbox to choose from and it is likely that continuous flow methods will play an important role in future point-of-care or in-the-field analysis devices.

The ability to separate living cells is an essential aspect of cell research. Magnetic cell separation methods are among some of the most efficient methods for bulk cell separation. With the development of microfluidic platforms within the biotechnology sector, the design of miniaturised magnetic cell sorters is desirable. Here, we report the continuous sorting of cells loaded with magnetic nanoparticles in a microfluidic magnetic separation device. Cells were passed through a microfluidic chamber and were deflected from the direction of flow by means of a magnetic field. Two types of cells were studied, mouse macrophages and human ovarian cancer cells (HeLa cells). The deflection was dependent on the magnetic moment and size of the cells as well as on the applied flow rate. The experimentally observed deflection matched well with calculations. Furthermore, the separation of magnetic and non-magnetic cells was demonstrated using the same microfluidic device.

The separation of magnetic microparticles was achieved by on-chip free-flow magnetophoresis. In continuous flow, magnetic particles were deflected from the direction of laminar flow by a perpendicular magnetic field depending on their magnetic susceptibility and size and on the flow rate. Magnetic particles could thus be separated from each other and from nonmagnetic materials. Magnetic and nonmagnetic particles were introduced into a microfluidic separation chamber, and their deflection was studied under the microscope. The magnetic particles were 2.0 and 4.5 microm in diameter with magnetic susceptibilities of 1.12 x 10(-4) and 1.6 x 10(-4) m(3) kg(-1), respectively. The 4.5-microm particles with the larger susceptibility were deflected further from the direction of laminar flow than the 2.0-microm magnetic particles. Nonmagnetic 6-microm polystyrene beads, however, were not deflected at all. Furthermore, agglomerates of magnetic particles were found to be deflected to a larger extent than single magnetic particles. The applied flow rate and the strength and gradient of the applied magnetic field were the key parameters in controlling the deflection. This separation method has a wide applicability since magnetic particles are commonly used in bioanalysis as a solid support material for antigens, antibodies, DNA, and even cells. Free-flow magnetophoretic separations could be hyphenated with other microfluidic devices for reaction and analysis steps to form a micro total analysis system.

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