In this work propose a novel design of a soft magnetic manipulator for Endoscopic Endonasal Surgery (EES). EES is performed by inserting a rigid endoscope and accompanying tools through the nostrils. The coupled constraints of narrow, curved anatomy with straight and rigid tools present a significant challenge in EES, limiting visibility and manoeuvrability within the workspace.  Enhancing the flexibility and controllable Degrees of Freedom (DoF) of the tools used in EES could make procedures safer and easier to perform, and significantly improve surgical outcomes.  

Soft magnetic manipulators present some notable advantages when designing for small scale, high deflection and delicate tissue interaction.In this paper we investigate unstable actuation scenarios of such devices in which magnetic torque causes a twist about the main axis of the soft manipulator, instead of desired deflection. This torsional deformation leads to unpredictable control and to decrease in bending capabilities of the manipulator. To address the issue of twisting, we consider a manipulator design with an elastomeric double helix reinforcement structure. This unique geometry allows us to create a manipulator with variable stiffness between its Y-Z and X-Z planes and low torsional stiffness. We present two designs and determine most optimal solution experimentally under application of magnetic fields. Additionally, we observed an independence in motion between the two planes of the chosen manipulator design. This allowed us to create a modular configuration, where two segments are joined together to make a manipulator with variable stiffness along its length for improved manoeuvrability. We hope this work will contribute to design of soft magnetic tools for EES, where high deflection is required to perform complex procedures.