Steering Rotating Magnetic Swimmers in 2.5 Dimensions Using Only 2D Ultrasonography for Position Sensing

Micro/millimeter-scale magnetic robots have many potential biomedical applications. However, there are major challenges in using these devices for
in vivo
tasks due to limitations of imaging technology for real-time detection and navigation inside the human body. Ultrasound is a popular sensor for localizing such robots because it is easy to use and less expensive than other imaging methods. This letter studies methods to track and control a millimeter-scale rotating magnetic swimmer to perform path following. Detection using ultrasound B-mode and Doppler mode to track the robot during navigation are compared. In addition, an algorithm to perform 2.5D closed-loop control (controlling 3 degrees of freedom for a swimmer to follow a planar path) of the swimmer using only 2D ultrasound feedback was studied and tested experimentally.