Selective Actuation and Tomographic Imaging of Swarming Magnetite Nanoparticles

Micro- and nanomotors have seen substantial progress in recent years for biomedical applications. However, three grand challenges remain: (i) high velocities to overcome the blood flow, (ii) spatially selective control to enable complex navigation, and (iii) integration of a medical, tomographic real-time imaging method to acquire feedback information. Here, we report the combination of active magnetic matter and a medical imaging technique, namely magnetic particle imaging (MPI), which addresses these needs. We synthesize ∼200 nm magnetic nanoparticles and observe a macroscopic, collective effect in a homogeneous magnetic field with a rotating field vector. The nanoparticles form a millimeter-sized cloud and reach speeds of 8 mm s^-1. This cloud is imaged and selectively steered with an MPI scanner. Our experimental results are supported by a model that highlights the role of the Mason number, the particle’s volume fraction, and the height of the cloud. The successful introduction of a fast swarm of microscopic units and the spatial selectivity of the technique suggest an effective approach to translate the use of micro- and nanobots into a clinical application.