Manipulation and sorting of magnetic particles by a magnetic force microscope on a microfluidic magnetic trap platform

We have integrated a microfluidic magnetic trap platform with an external magnetic force microscope (MFM) cantilever. The MFM cantilever tip serves as a magnetorobotic arm that provides a translatable local magnetic field gradient to capture and move magnetic particles with nanometer precision. The MFM electronics have been programmed to sort an initially random distribution of particles by moving them within an array of magnetic trapping elements. We measured the maximum velocity at which the particles can be translated to be 2.2 mm/s +/- 0.1 mm/s, which can potentially permit a sorting rate of approximately 5500 particles/min. We determined a magnetic force of 35.3 +/- 2.0 pN acting on a 1 mu m diameter particle by measuring the hydrodynamic drag force necessary to free the particle. Release of the particles from the MFM tip is made possible by a nitride membrane that separates the arm and magnetic trap elements from the particle solution. This platform has potential applications for magnetic-based sorting, manipulation, and probing of biological molecules in a constant-displacement or a constant-force mode.