Shape-locked Geometry Reduces Snapping Effects in Concentric Tube Robots

Concentric tube robots (CTRs) increase the dexterity of surgical instruments while maintaining small instrument diameters. The tip is bent by rotating and sliding the individual, nested tubes without the need for additional tendons. However, this design may become unstable under certain conditions due to the torsional wind- up created by rotating the tubes, which is at some point released by a sudden snapping movement. Several groups have demonstrated techniques to reduce snapping 1, 2, 3, 4. Mostly, this was achieved through anisotropic cut-outs along the tubes. Still, some regions of the workspace remain difficult to access. Swaney et al. propose an alternative approach in which two tubes with an elliptical cross-section can be used at a locked relative tube rotation that would otherwise be unstable and not reachable 5. However, the rotation angle was not intended to vary during use. This paper resumes the concept of shape-locked CTRs in order to provide discrete stable tube configurations throughout the entire workspace, including otherwise unstable regions. The tube cross-sections are designed as matching regular polygons with n edges to prevent unintended rotation. Unlike existing approaches, the tubes can still be rotated if the applied torque is high ◦ The aim of this work is to experimentally assess whether polygonal CTRs offer more uniform coverage of the workspace than annular CTRs.