Elevation and Azimuth Rotational Actuation of an Untethered Millirobot by MRI Gradient Coils

Magnetic resonance imaging (MRI) devices provide magnetic field gradients, which can be used to remotely actuate magnetic robots that may one day carry out medical tasks, such as diagnosis, drug delivery, or therapeutic-laser-based procedures, in addition to the high-resolution tissue images for diagnosis. However, in comparison with magnetic systems that are custom designed solely for magnetic actuation, the magnetic environment of an MRI device is constrained by the requirements for imaging, which reduces the number of active degrees of freedom available for magnetic actuation. Moreover, the current MRI-powered untethered robots are limited to translational magnetic pulling in three dimensions only. In this article, we propose a design for an untethered magnetic robot that can rotate in a three-dimensional liquid volume in magnetic environments, like those in MRI devices. We demonstrate rotational actuation of our robot inside a commercially available MRI gradient coil. Two kinds of near-neutrally buoyant robot designs are proposed, where each design has particular advantages. Design methodologies, analysis of rotational performance, closed-loop orientation control of up to 2.51 rad of orientation change with maximum net displacement of 18.4 mm, and angular velocity control (0.2-0.63 rad/s) of these robot designs are presented.