Non-ideal behaviors of magnetically driven screws in soft tissue

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.