Magnetic fields are increasingly being employed in microfluidics. MEMS technology has enabled the fabrication of miniaturized magnetic devices for use at the cellular level. Combined with microfluidic fabrication, the bio-analyst has at his disposal a fast, flexible and portable tool in “lab-on-a-chip” to trap, transport, separate and sort bio-molecules for detection and analysis.
A recent development, the use of functionalized magnetic beads to manipulate cells, proteins, DNA, etc, has attracted noticeable attention due to the high selectivity of the magnetic separation methods. These beads are used to ‘label’ cells to render them magnetic, and with different magnetic loads assigned, facilitate the bio-analyst to select and separate the cells.
Obviously, the performance of such miniaturized devices is vital for the viability and efficiency of the system, to allow the bio-analyst a high level of control over his samples. A consistently strong magnetic field gradient needs to be generated in order to totally magnetize and transport all the magnetic beads in the microchannel.
Researchers at IME, working in collaboration with the Institute of Bioengineering and Nanotechnology, and the Nanyang Technological University, have successfully applied magnetism through magnetic pillars in novel microcoil structures to move magnetic beads. Instead of using thin-film conductors, the researchers used Cu-filled high aspect ratio trenches embedded in Si, and on top of the microcoils, magnetic pillars were placed as magnetic cores. These structures generated large magnetic field gradients that very effectively attracted magnetic beads. By injecting currents in an array of such microcoils placed in a microfluidic chamber, magnetic beads were guided in different movement modes and step sizes in a continuous flow.
Magnetic separation devices developed in the past used either external magnetic sources or on chip planar microelectromagnets. None of these was specifically targeting to maximize magnetic field gradients.
Various structures were fabricated in the IME-led work, carried out as part of the “Biomedical Sensing Devices” project funded by A*STAR. The first device (Fig. 1a) consists of two meander microcoils, each with 20 turns and with every pair of turns of the second microcoil interdigitated within that of the first one. The second device (Fig. 1b) has two independent sets of parallel microcoils, each with square loops with areas of 50 µm2 and 30 µm2, respectively. The third device type (Fig. 1c) is a spiral microcoil array with 30 turns of conductors 3 µm wide and separated by 3 µm.
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Fig. 1. Schematic configurations of three microcoil designs. The arrows represent the movement of magnetic beads when currents are sequentially switched on through the microcoils of each design. |
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The trapping and movement of magnetic beads for all of the above-mentioned types of microcoil arrays was observed after the cell attachment of one or more beads per cell.
These microcoils exhibited more advantages than the planar ones such as the ability to inject high currents, efficient heat dissipation, and sharply enhance magnetic field gradient. These features render the microcoils highly suitable for subsequent integration with other microcomponents, paving the way for more sophisticated devices increasingly being demanded in a growing field of bioanalytical applications. |
