Design and Precision Control of an Mr-compatible Flexible Fluidic Actuator

Magnetic resonance imaging (MRI) offers many benefits to image-guided interventions, including excellent soft tissue distinction, little to no repositioning of the patient, and zero radiation exposure. The closed, narrow bore of a high field MRI scanner limits clinician access to the patient, such that an MR-compatible robot is essentially required for many potential interventions. A robotic system of this kind could additionally provide the clinician increased accuracy and more degrees of freedom within the minimally invasive context. Fluid power is an excellent type of actuation to use inside the MRI scanner, as such actuators can be designed free of magnetic and electrical components. However, there are no fluid power actuators readily available that are suitable for use in the operating room. This paper reports a compact, intrinsically safe, sterilizable fluid power actuator. Using additive manufacturing processes, the actuator was printed in a single build. Thus, it is composed of several integrated parts in a compact design. Employing an inchworm-like behavior, the linear actuator can advance or retract a needle or mechanism rod in discrete steps; thus the device is intrinsically safe. The actuator is fluid agnostic, but a pneumatic prototype is presented here with initial testing results. For the pneumatic case, sub-step positioning control has been tested using a nonlinear, modelbased controller, and the mean steady-state error was 0.025 mm. Thus this type of actuator appears to be promising solution for use in MRI-guided interventions. INTRODUCTION Medical robotic systems for image-guided interventions require safe, sterilizable (or disposable), precision actuators. Imaging modalities like computed tomography (CT) and magnetic resonance imaging (MRI) impose further limitations on the design of such actuators. It is desirable for the robot and its actuators to be transparent to the imager and to not produce artifacts, noise or distortion in the images. Thus, actuator design is restricted to materials and principles of operation that are compatible with the imaging environment. Together with the requirements for safety, sterilizability and precision control, these restrictions present a challenging design problem. Over the past 20 years, numerous custom MR-compatible actuators have been reported in the literature. Due to the high strength magnetic field of the MR imager, these actuators cannot contain ferromagnetic materials. Pneumatic actuation has been commonly employed because the working principle