Untethered magnetic devices (UMDs) driven by rotating magnetic fields have been proposed for applications in minimally invasive medicine. Previous work has explored the propulsion and guidance of a single UMD using a rotating magnet dipole source, and actuation of multiple UMDs has focused on the use of uniform magnetic fields. This work seeks to extend the methods for remote actuation using a single magnet dipole source to generate two rotating magnetic fields with arbitrary independent rotation axes at any two points with a common rotation frequency. The solutions found show that there are multiple positions and rotation axes of the magnet dipole that can generate these rotating fields. The method is demonstrated with two screw-type UMDs propelled independently within an agar gel block using a single permanent-magnet actuator.

Previous work in the open-loop behavior of magnetically driven screws in soft tissue has focused on the impact of magnetic field rotation speed and pitch angle, measured between the field's rotation axis and the screw's principle axis, on the average magnetic torque, both in-plane and out-of-plane. However, prior work did not rigorously consider the role of screw geometry and tissue material properties on the resulting trajectory. This study seeks to develop a plausible empirical model from experimental measurements in an agar gel tissue phantom for use in further research and feasibility studies of magnetic screws for in vivo applications. Non-dimensional parameters representing rotation speed, pitch angle, screw size, magnetic strength, and tissue stiffness are varied orthogonally. Circular trajectories are fitted to the resulting recorded trajectories. Using physical insight and observation of the trends of the non-dimensional turning radius/curvature, simple models for each parameter group are proposed. A final empirical model is then proposed to unify the individual models.

This paper considers the requirements for stable control of simple devices that consist of a permanent magnet rigidly attached to a screw operating in soft tissue. Placing these devices in a rotating magnetic field causes the device's permanent magnet to rotate synchronously with the applied field, making the attached screw generate forward propulsion. Although there has been significant research in the area of magnetic screws, this paper describes magnetic phenomena that have not been addressed in prior work, yet will be critical in the design of future steering and control strategies for medical applications. Using a time-averaged model of magnetic torque, we analyze the magnetic torque present when intentionally steering the screw or due to naturally occurring disturbances in the screw's heading. We predict and experimentally demonstrate the existence of magnetic torques that simultaneously act to stabilize and destabilize magnetic screws operating in tissue. We find that there exists an applied field rotation speed, above which screws may become unstable, and we find that the interaction between the stabilizing and destabilizing magnetic torques may cause potentially undesired but predictable behavior while steering.