Towards a discretely actuated steerable cannula

Several percutaneous needle-based and intravascular procedures require guidance of the diagnostic or therapeutic tool to the target location by maneuvering the needle or catheter to correct for the error in reaching the target location. Hence, in this paper we present our work towards developing a discretely actuated steerable needle/cannula with multiple degrees-of-freedom to `steer´ the cannula by discrete actuation along the cannula length. We are interested in using the cannula to introduce both diagnostic and therapeutic tools, which may otherwise be difficult to deliver to the appropriate location. We use two antagonistic SMA wires as actuators to generate the required bending forces at each joint. SMA wires were annealed through a customized training process to an arc shape and mounted in the machined grooves on the outer surface of the cannula to generate local bending upon thermal actuation. We propose to use temperature feedback to control the position of the SMA actuators. To use temperature feedback as the feedback signal for enabling individual joint actuation, we had to fully characterize the SMA actuator. Classical uniaxial testing devices and experimental setup used in characterizing straight annealed SMA wires are not applicable in this work since the SMA wire is annealed in an arbitrary shape. Hence, we also present an experimental setup and a procedure for characterizing an SMA actuator that transforms into an arc shape upon thermal actuation.