Low-Frequency Gradiometers for Closed-Loop Control of Magnetic Microrobots

Abstract

Because they can readily generate gradients and time-varying fields, magnetic particle imaging (MPI) apparatuses are increasingly considered for the dual purpose of manipulating and sensing biomedical magnetic microrobots. Feedback about the position and orientation of microrobots is an essential requirement for closed-loop control, and reading out this information magnetically is especially desirable for physiological targets. A typical approach is to toggle between a low frequency (<100 Hz) actuation mode, where field gradients or low frequency rotating fields act to propel microrobots, and a sensing mode based on MPI in the 10s of kHz for detecting their position and posture. Although making use of sequential actuation and sensing leverages established techniques from MPI, an ideal approach would offer uninterrupted actuation and sensing. We have previously demonstrated the possibility of simultaneously actuating and inductively sensing at frequencies in the Hz or 10s of Hz. Here, we extend these concepts by showing how low-frequency gradiometers can be designed to sense position and torque delivery to microrobots. As an illustrative example, we demonstrate a prototype apparatus that controls a helical swimming magnetic robot with inductive feedback. We further demonstrate how gradiometers can be designed as add-ons to commercial magnetic field generators and how they can be adapted for setups based on rotating permanent magnets. These efforts advance toward instrumentation for simultaneous inductive sensing at frequencies <100 Hz that can be incorporated into magnetic actuation apparatuses.