Design and Manufacturing of Embedded Air Muscles for a Magnetic Resonance Imaging Compatible Prostate Cancer Binary Manipulator

Abstract

Magnetic resonance imaging (MRI) compatible robots can assist physicians with the insertion of biopsy needles and needlelike therapeutic instruments directly into millimetersize tumors, using MR images as feedback. However, MRI systems present a challenging environment with high magnetic fields and limited space, making the development of MRIcompatible robots complex. This paper presents an MRIcompatible pneumatic actuation technology consisting of molded polymer structures with embedded airmuscles operated in a binary fashion. Along with its good positioning accuracy, the technology presents advantages of compactness, perfect MRIcompatibility, simplicity and low cost. Here, we specifically report the design and validation of a transperineal prostate cancer manipulator prototype that has 20 embedded airmuscles distributed in four starlike polymer structures. These compliant structures are made of silicone elastomer, using lostcore injection molding. Low motion hysteresis and good precision are achieved by designing molded joints that eliminate sliding surfaces. An effective design method for such embedded polymer airmuscles is proposed, using a manipulator model and four airmuscle design models: geometrical, finite elements, uniaxial analytic, and experimental. Binary control of each airmuscle ensures stability and accuracy with minimized costs and complexity. The prototype is found MRIcompatible with no observable effects on the signaltonoise ratio and, with appropriate image feedback, is found to reach targets with precision and accuracy under 0.5 mm. The embedded approach reveals to be a key feature since it reduces hysteresis errors by a factor of ≈7 compared to a previous nonembedded 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.