Design and Manufacturing of Embedded Pneumatic Actuators for an MRI-Compatible Prostate Cancer Binary Manipulator

Magnetic Resonance Imaging (MRI) compatible robots can assist physicians in precisely inserting biopsy needles or therapeutic instruments directly into millimeter-size tumors using MR imaging feedback. MRI systems although present a challenging environment, including high magnetic fields and limited space, making the development of MRI-compatible robots complex. This paper presents an MRI-compatible pneumatic actuation technology consisting of molded polymer structures with embedded air-muscle, operated in a binary fashion. While having good positioning accuracy, the technology presents advantages of compactness, perfect MRI-compatibility, simplicity, and low cost. Here we specifically report the design and validation of a transperineal prostate cancer manipulator prototype having 20 embedded air-muscles distributed in four star-like polymer structures. Structures are made of silicone elastomer, using lost-core injection molding. The molded compliant joints of the muscles eliminate sliding surfaces, for low motion hysteresis and good repeatability. A simple and effective two-level design method for polymer air-muscles is proposed, using a manipulator model and three muscle models: geometrical, finite elements and uniaxial analytic. Binary control of each air-muscle assures stability and accuracy with minimized costs and complexity. The manipulator is tested MRI-compatible with no effects on the signal-to-noise ratio and, with appropriate image feedback, reaches targets with repeatability and accuracy under 0.5 mm. The embedded approach reveals to be a key feature since it reduces hysteresis errors by a factor of 6.6 compared to a previous non-embedded version of the manipulator. The successful validation of this binary manipulator opens the door to a new design paradigm for low cost and highly capable pneumatic robots, specifically for the intra-MRI manipulation.