Trapping of single magnetic beads in a micro flow

Superparamagnetic microbeads have been widely applied in immunoassay and separation of target analytes from biological samples in microfluidics, since their large surface-area-to-volume ratio is able to increase the detection sensitivity or separation efficiency and they can be easily manipulated using an external magnetic field. This paper reports both experimental and numerical studies of the single bead trapping on a permalloy microarray in the presence of a micro flow and an external magnetic field. The permalloy microarray consists of microstructures measuring 4 µm (width) ×20 µm (length) x 0.3 µm (thickness). The width of the magnetic trap formed between two adjacent microstructures is 3 µm. After microbeads (5-5.9 µm in diameter) were injected onto the permalloy microarray in a microchannel, single beads were captured in the traps. The retention of beads at a given magnetic field and various fluid flow rates was experimentally studied. A single-bead model is developed to numerically study the magnetic force and fluid-induced drag on the bead. The magnetic force is studied to determine the position of the beads with maximal magnetic force. Finally, a mechanical equilibrium model is introduced to explain the single bead trapping mechanism. This paper provides a guide for the design of microstructures trapping magnetic microbeads in microfluidic channels with fluid flow.