Design Considerations for Shape Memory Alloy Actuator Based Active Steerable Needle

This study proposes a design and application of shape memory alloy (SMA) wire actuator for making an active steerable surgical needle using both experimental and computational method. The active steerable needles are able to change direction during insertion into a tissue to compensate for possible misplacement and thus reach desired target. This steering maneuver is possible due to the ability of the needle to bend in a desired direction when actuated. There is a huge interest in these type of active needles because of their prospective benefits in percutaneous interventional procedures like brachytherapy and tissue ablation. In this design, a nitinol wire, which is a SMA of nickel and titanium, is used as an actuating component. This actuator wire is available commercially in various diameters and lengths and is chosen based on design requirement. It can produce a recoverable tensile strain up to 6% when stress is applied (pseudo-elasticity). The strain can recover by contracting to its original length when the wire is heated above its actuation temperature (shape memory effect). This strain recovery of nitinol can take place even under stressed condition. These properties in nitinol wire, in addition to its small size and bio-compatibility, make it a suitable actuator for active steerable needle.  In designing the active steerable needle, the needle cannula is considered as a cantilever beam whose base end is fixed but the tip end is connected to a nitinol wire. The other end of nitinol wire is connected to the base of the needle cannula after pre-straining. When the nitinol wire is heated to activate its shape memory effect, the wire contracts and actuates the bending of the needle. When this is followed by cooling the wire, the actuator expands because of the bias force from the bent needle cannula trying to become straight. The design criteria are based on the maximum tip deflection of needle cannula from the needle axis. A computational study is performed using COMSOL Multiphysics software and verified with the results from prototype testing. A built-in Lagoudas SMA material model is used for nitinol wire modeling. The finite element modeling is performed by including needle cannula geometry as well as nitinol wire geometry connected by a hinged joint. The pre-straining procedure by using mechanical loading-unloading condition as well as thermal actuation procedure by heating and cooling the nitinol wire is simulated. The experimental and computational results from the study of this design can be used as a guide for a multi-actuator active steerable needle that can bend in any direction based on actuation of different nitinol wires.