3D Guiding of Magnetic Nanoparticles with Static Fields: A Permanent-Magnet Approach

We present a novel concept for using permanent magnet systems to guide superparamagnetic nanoparticles (SPPs) or macroscopic ferromagnetic objects along arbitrary trajectories over large volumes [1-3]. In principle, the same system can also magnetic resonance imaging (MRI) by exploiting the inherent contrast of the SPPs [3]. The approach combines two magnetic field configurations: a strong, homogeneous dipolar field that magnetizes and orients the particles, and a superimposed, constantly graded quadrupolar field that exerts a uniform directional force on them. This configuration enables controlled, continuous guidance of particles with constant force across the entire volume. Several prototype systems based on the Halbach principle [4] were developed in various scales to demonstrate two-dimensional particle guidance over centimeter distances [3] and under optical microscopes [5]. The latter enabled precise manipulation of individual cells containing SPPs. An advanced design employing two quadrupoles allows dynamic control of magnetic forces, including complete force cancellation, making it possible to stop and steer particles around sharp edges. Commercial systems are available for both 2D (microscope-compatible) and full 3D guidance applications. Finally, the concept has been extended to liquid crystalline elastomers embedded with SPPs, forming "micro-robots" that combine magnetic coarse positioning with light- or temperature-controlled microscopic actuation [6].