Focused torque delivery and low frequency inductive sensing of micromagnets in rotating magnetic fields

Abstract

Magnetically controlled biomedical microrobots show promise for facilitating site-specific drug delivery and performing other therapeutic functions in the body. Dual use of magnetic particle imaging apparatuses for the control and detection of microrobots is a possibility that is increasingly being explored, avoiding the use of ionizing radiation to provide real-time feedback on their position and trajectory. Nevertheless, the methods that have been demonstrated typically need to switch between distinct actuation and sensing modes, limiting the duty cycle of each and neglecting an opportunity to directly utilize simultaneous inductive feedback during actuation. Here, we show that, under a low-frequency rotating magnetic field (1 to 100 Hz), it is possible to inductively detect magnetic torques applied to a model microrobot. This is made possible by a prototype inductive sensing apparatus that finely adjusts both phase and amplitude to achieve cancellation between a sense and compensation coil, suppressing background signal from the rotating field by 90 dB. Further, by monitoring inductive signals, we show that combining rotating magnetic fields with selection fields enables the selective delivery of torque to multiple microrobots within a working volume. We discuss how the sensitivity of inductive detection using the techniques we are developing compares favorably to alternative methods for measuring the fringing fields produced by the microrobots. These concepts for low-frequency inductive detection of microrobots serve to lay a foundation for future closed-loop control schemes based on simultaneous actuation and sensing.