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Here we report on the fabrication of micro-magnet arrays by powder agglomeration in a polymer matrix. The NdFeB@polydimethylsiloxane (PDMS) inner microstructure and the generated magnetic forces were studied, when prepared under two different magnetic field configurations. The initial process uses the classical crosslinking of PDMS mixed with NdFeB powder under a low magnetic field gradient provided by a permanent magnet (LG set-up for low gradient). In contrast, the optimized process uses an intermediate layer, composed of iron microstructures in a PDMS matrix that amplifies and focuses the magnetic field gradient given by the permanent magnet (HG set-up for high gradient). Both processes result in a heterogeneous material that can be described as an array of permanent micro-magnets diluted in a non-magnetic host matrix. The NdFeB@PDMS microstructure was characterized by X-ray tomography and optical microscopy. The magnetic properties were also measured by magnetometry and colloidal probe AFM. Results showed that the HG set-up leads to an array of micro-magnets localized at the surface, with higher compactness and density, resulting in stronger magnetic performances compared to the LG set-up. This technology only implies easy-to-handle and cheap fabrication processes, paving the way for the development of low-cost lab-on-chip devices integrating magnetophoretic trapping functionality.

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.