Magnetophoresis-based microfluidic devices offer simple and reliable manipulation of micro-scale objects and provide a large panel of applications, from selective trapping to high-throughput sorting. However, the fabrication and integration of micro-scale magnets in microsystems involve complex and expensive processes. Here we report on an inexpensive and easy-to-handle fabrication process of micrometer-scale permanent magnets, based on the self-organization of NdFeB particles in a polymer matrix (polydimethylsiloxane, PDMS). A study of the inner structure by X-ray tomography revealed a chain-like organization of the particles leading to an array of hard magnetic microstructures with a mean diameter of 4 µm. The magnetic performance of the self-assembled micro-magnets was first estimated by COMSOL simulations. The micro-magnets were then integrated into a microfluidic device where they act as micro-traps. The magnetic forces exerted by the micro-magnets on superparamagnetic beads were measured by colloidal probe atomic force microscopy (AFM) and in operando in the microfluidic system. Forces as high as several nanonewtons were reached. Adding an external millimeter-sized magnet allowed target magnetization and the interaction range to be increased. Then, the integrated micro-magnets were used to study the magnetophoretic trapping efficiency of magnetic beads, providing efficiencies of 100% at 0.5 mL/h and 75% at 1 mL/h. Finally, the micro-magnets were implemented for cell sorting by performing white blood cell depletion.

Here we report a novel technology that requires simple fabrication process, to obtain highly efficient magnetic micro-traps. Developed micro-traps consist in chains of iron particles diluted in PolyDiMethylSiloxane (PDMS). X-ray tomography was used to analyze the microstructure of particle ordering in the PDMS membrane and revealed the predominance of chain-like agglomerates. Largest formed traps, with diameter ranging from 4 to 11 µm are found to be the most efficient. The self-organized trap arrays are characterized by a density of 1300 magnetic microtraps/mm 2 , with an average nearest neighbour distance, center-to-center, of 21 µm. Implemented in a microfluidic channel operating at a flow rate of 3 mL/h-a fluid flow of 8,3 mm/s-we measured trapping throughputs up to 7100 beads/min with an average distribution of 750 beads/mm 2. At fluid velocity up to 9,7 mm/s a trapping efficiency of 99,99% were measured. This novel technology allows to trap thousands of beads with throughputs that permit to compete with hydrodynamic trapping functions, while requiring simple fabrication process, and handling. Manuscript Click here to download Manuscript Arrays of high aspect ratio magnetic microstructures .pdf