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Variable stiffness enables the safe and effective operation of the minimally invasive surgical instruments. In this article, we propose a continuously variable stiffness mechanism of the scalable tubular structure. The mechanism consists of multiple coaxial nitinol tubes, and each tube has an anisotropic distribution of flexural stiffness created by nonuniform through-hole patterning. The stiffness of the mechanism is varied by relative rotation and translation among the tubes, resulting in flexural stiffness difference up to 7.2 times in the direction of load. Its flexural stiffnesses along principal axes are independently controlled by the suggested counterrotation algorithm. The stiffness change is validated through analytical modeling, FEM simulation, and the experiments. Thanks to its physically embodied intelligence, the mechanism has a simple scalable structure and the response time is immediate. We applied this mechanism to control the stiffness of the steerable needle. Varying the stiffness grants the additional degree of freedom to control the needle's trajectory, which can expand the workspace of the steerable needle.
Kim et al. (Thu,) studied this question.