Influence of magnetic guidance on the torsional behaviour of Concentric Tube Robots

Inhalt:
In order to improve minimally invasive surgical procedures, efforts are being made to miniaturise existing flexible, cableactuated instruments and entire endoscopes. Concentric Tube Robots (CTR) are considered a promising solution in this context, as they enable high precision with minimal construction volume. However, their practical use has so far been limited by considerable technical requirements – in particular with regard to complex path planning to avoid snapping effects and the need for robotic control due to non-independently controllable degrees of freedom. Against this background, the approach presented aims to make CTR more accessible through targeted technical modifications and to significantly reduce the infrastructure costs. This could not only facilitate integration into existing surgical systems, but also promote the clinical dissemination of this technology. This work investigates the torsional behaviour of CTR, in which magnetic guides were implemented along the tubes. The goal was to reduce torsion in the CTR prototypes and enable more stable discrete positions for the CTR tip by using magnetic guides on the inner and outer tubes. For validation, two prototypes with different magnetic configurations were developed: the 6x1 model and the 5x3 model. These setups were tested in comparison to models in which the outer tubes had no magnetic guides, so no magnetic interaction occurred between the inner and outer tubes. First, suitable polymeric magnetic materials were selected and integrated into polyamide tubes. The validation experiments involved a 720Grad rotation in one direction followed by another 720Grad in the opposite direction, during which the torque at the base of each inner tube and the relative twist angle between the bases and tips of each tube pair were measured. Firstly, it became clear that the torsion effects were influenced by the magnetic guides. Nevertheless, torsion still occurred in the magnetic models, indicating that the magnetic stabilization was insufficient to fully suppress the torsion. Furthermore, the results showed that the 6x1 model exhibited a slight reduction in twist angle and therefore in torsion compared to its control model, while the 5x3 model experienced greater twist. Additionally, the rotation recordings of the tips revealed that neither the 6x1 nor the 5x3 model was able to maintain discrete positions. Despite the presence of magnetic guides, the twist in both models remained continuous. Thus, the desired positions were not stabilized under all circumstances. Future development will investigate the use of segmented magnetic structures, miniaturisation of the mechanical setup, the inclusion of working channels, and the implementation of a model-based control algorithm.