Application of SMA Wire for an Active Steerable Cannula

In this article we present the feasibility of using the shape memory alloy (SMA) wires, namely Nitinol, as an actuator for a steerable surgical cannula. A 3D finite element (FE) model of the actuated steerable cannula was then developed in ANSYS to show deflection of the surgical cannula under the actuation force. The behavior of SMAs was simulated by defining the isothermal stress-strain curves using the multi-elasticity capability of ANSYS. The transformation temperatures of the Nitinol wire at different levels of stress were gathered to form the transformation diagram. Using the one-dimensional Brinson model, the isothermal stress-strain response of the wire was obtained. The thermomechanical characteristics of SMAs were also studied completely by a series of experiments performed on the wires. Birth and death method was used in the solution procedure to have the prestrain condition on Nitinol wire prior to the actuation step. A prototype of the actuated steerable cannula was also developed to validate the numerical simulation. Finally a study was done on design parameters affecting the deflection such as Young’s modulus of cannula, SMA diameter and its offset from the neutral axis of the cannula which can be useful in design optimization.