This paper reports the design, modeling, and control of an MR-compatible actuation unit comprising pneumatic stepper mechanisms. One helix-shaped bellows and one toroid-shaped bellows were designed to actuate in pure rotation and pure translation, respectively. The actuation unit is a two degree- of-freedom needle driver that translates and rotates the base of one tube of a steerable needle like a concentric tube robot. For safety, mechanical stops limit needle motion to maximum unplanned step sizes of 0.5 mm and 0.5 degrees. Additively manufactured by selective laser sintering, the flexible fluidic actuating (FFA) mechanism achieves two degree-of-freedom motion as a monolithic, compact, and hermetically-sealed device. A second novel contribution is sub-step control for precise translations and rotations less than full step increments; steady- state errors of 0.013 mm and 0.018 degrees were achieved. The linear FFA produced peak forces of 33 N and −26.5 N for needle insertion and retraction, respectively. The rotary FFA produced bidirectional peak torques of 68 N-mm. With the FFA’s in full motion in a 3T scanner, no loss in signal-to-noise ratio of MR images observed.

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, model-based 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.

PurposeThe purpose of this paper is to determine the most‐practical means of transforming computer‐aided‐design models of custom clubfoot pedorthoses into functional pedorthoses for testing on patients in a clinical trial.Design/methodology/approachThe materials used in conventional orthosis fabrication are not yet available for solid free‐form fabrication; therefore, to fabricate the pedorthoses, several approaches were considered, including direct manufacturing, additive‐based moulding, laser cutting of foam and combinations of several of these approaches.FindingsThe chosen approach of additively manufacturing the custom hard shell, and moulding the polyurethane‐foam insert, resulted in accurate, durable and effective pedorthoses that fit well, and could be adjusted as needed. The pedorthoses that were produced are currently being tested on the respective patients for their improvement in mobility and degree of clubfoot correction, and will continue through early 2010.Practical implicationsAdditive manufacturing provides an ideal approach for generating the custom, end‐use hard‐ and soft‐layer patterns: each pedorthosis is truly unique; and the soft layer has regions of variable thickness. The advantage of this approach is the reduction in labour and the increase in degrees of design freedom available, compared to conventional methods of fabricating orthotic devices. Replacement inserts can be moulded in a matter of hours using this silicone‐moulding approach.Originality/valueSeveral new approaches for fabricating custom orthotic devices were explored, and the related results are discussed. The goal of this paper is to convey the potential of the fabrication procedure used and lessons learned on this project to the rapid prototyping and orthotic communities.

Since the advent of SFF and RP a number of SFF benchmarking geometries and methodologies have been developed and employed with some similarities but limited standardization. Minimal information has been published in regard to a standard method of measuring the resolution limits or capabilities of SFF and SFF-based hybrid processes. In an effort to benchmark resolution limits of SFF and Hybrid Fabrication processes, several benchmarking geometries were developed to capture the resolution capabilities, specifically hole size and rod size range, of multiple hybrid fabrication path steps and a hybrid path as a whole. These useful geometries are shared with the SFF community and procedures for their use are described in this paper.

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