Characterization of Precurved Needles for Use in Distal Tip Manipulation Mechanisms

Precurved needles are used in a variety of medical applications for both passive and active control of instrument position. In one application, the deployment of a precurved stylet from a concentric outer cannula can be used to achieve lateral positioning of the distal tip of the stylet. This paper outlines how the material and geometry of the stylet can be chosen to ensure that it will not yield and thus repeatedly return to its precurved shape when deployed from a cannula. Using this methodology, we calculate the maximum strain for a range of stylet geometries and show that nitinol is required for the stylet material so that it will not plastically deform. Then, 16 stylets of varying diameters (0.508 mm, 0.635 mm, 0.838 mm, and 0.990 mm) and radii of curvature (10 mm, 20 mm, 30 mm, and 40 mm) were manufactured. Experiments were performed with four different diameter cannulas (20, 18, 16, and 14 gauges) to measure the forces required to deploy the stylets from and retract them back inside the cannulas. Retraction forces were measured between 0.3 N and 13.9 N, and deployment forces were measured between 0.2 N and 7.0 N. For a given cannula, it was found that force increases as the stylet diameter increases and as the bend radius decreases.