A compliant actuator for steering the needle within tissue with feedback of FBG-based shape sensing

Needle steering within tissue is a challenge of minimally invasive medical robotics but is critical for targeting lesions that are difficult to reach by straight paths. This work introduces the design of a steerable needle robot (SNR-I) that is only 1.4 mm in diameter. Its unique three-layer cannula structure enables independent actuation and sensing while retaining the traditional function of the needle. The inhomogeneous stiffness design based on the loop-shaped compliant mechanism is compatible with the smoothness of the needle shape and strong distal steerability. The kinematics of the robot in free space based on a dual-stage constant curvature model was discussed and the numerical solutions was provided in this work. Experiments show that the mean errors in the axial and lateral directions of the estimation of the tip position based on this model in free space are 0.67 mm and 1.72 mm, respectively. An incremental tissue-parameter-independent controller is proposed for planar trajectory tracking within tissue. Three test trajectories show that the actuator and controller proposed in this work can achieve trajectory tracking within the tissue with an error range of +/- 1 mm.