Magnetism-based microsystems, as those dedicated to immunoaffinity separations or (bio)chemical reactions, take benefit of the large surface area-to-volume ratio provided by the immobilized magnetic beads, thus increasing the sensitivity of the analysis. As the sensitivity is directly linked to the efficiency of the magnetic bead capture, this paper presents a simple method to enhance the capture in a microchannel. Considering a microchannel surrounded by two rectangular permanent magnets of different length (Lm = 2, 5, 10 mm) placed in attraction, it is shown that the amount of trapped beads is limited by the magnetic forces mainly located at the magnet edges. To overcome this limitation, a polyethylene terephthalate (PET) microchip with an integrated magnetic track array has been prototyped by laser photo-ablation. The magnetic force is therefore distributed all along the magnet length. It results in a multi-plug bead capture, observed by microscope imaging, with a magnetic force value locally enhanced. The relative amount of beads, and so the specific binding surface for further immunoassays, presents a significant increase of 300% for the largest magnets. The influence of the track geometry and relative permeability on the magnetic force was studied by numerical simulations, for the microchip operating with 2-mm-long magnets.
Microfluidics Magnetism Permanent magnet Magnetic bead Ink Magnetic track array Photo-ablation Polymer Numerical simulation
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Prof. Jean-Philippe Ansermet and Dr. Simon Granville from the Laboratory of the Physics of Nanostructured Materials (EPFL, Switzerland) are thanked for the magnetic ink susceptibility measurements. Prof. Heinrich Hofmann and Carlos Morais from the Powder technology laboratory (EPFL, Switzerland) are thanked for their contribution in the magnetic ink density measurements and fruitful discussions. This work was supported by the Fond National Suisse pour la Recherche Scientifique (FN 200020-113413 “Analytical tools for fast phosphoproteome analysis”, FN-PNR 404740-117324 “Supramolecular phases for protein adsorption”, CTI 9215.1 “Microfluidic plateform for fast immunoassays”).