Abstract
A system for actively changing the stiffness of a long, thin, flexible robotic manipulator has been designed for cardiologists to use in a range of diagnosis and treatment procedures. Low-stiffness manipulators, such as catheters, are ideal for steering through vasculature with low risk of tissue injury. However, such instruments are not well-suited for applying force to tissue. The proposed system solves this problem by using a series of bead-shaped vertebrae containing pull wires to actively change the stiffness of the catheter, similar to gooseneck surgical retractors. Individual wires steer the catheter to a desired location. All wires are then tensioned to create friction between each vertebra and prevent sliding, therefore resisting motion. While this design concept has been implemented manually in various settings for decades, fine robotic control of the friction and stiffness of the system relies on a thorough understanding of the friction properties between vertebral segments. We have developed an analytical model to understand the interactions between vertebrae and determine the relationships between system parameters and the overall stiffness of the catheter. Experiments validated the calculations from the model and the functionality of the system by applying known loads to the tip of the catheter and measuring the catheter displacement. The catheter stiffness was measured to range from 100 N/m to 800 N/m, which is sufficient for performing many surgical tasks on tissue. This system can be useful in minimally invasive procedures involving direct instrument contact with tissue by improving accuracy, safety, and work flow.