# A Flexible Surgical Instrument for Robot-Assisted Minimally Invasive Surgery
## 2 Structural Design of the Flexible Surgical Instrument  
At present, soft robots are generally driven by cable active polymers, pneumatic artificial muscles, deformable memory alloys, embedded pneumatic networks, and other elastic materials to achieve flexible motion and ensure dexterity [26; 27; 28; 29]. In this study, a wire-driven mode was adopted for flexible surgical instrument design, because it retains the advantages of force and motion transmission despite the limitations of structural dimensions [30]. Surgical instruments for MIS mostly have a diameter of 5, 8, or 10 mm in clinical practice. Considering the implementation of the overall strength and flexible function of the instrument and the convenience of use with the sleeve needle, the diameter used in this study was 8 mm. The surgical instrument was composed of an end effector, a flexible part, a rigid rod, and a drive box, as shown in Figure 1.  
As shown in Figure 1a, the flexible part was composed of seven cylindrical segments connected in sequence by pin rolls, and the number of segments could be increased or decreased with the requirements of the operating space. The segments are marked R\({}_{i}\) in turn (\(i\) represents the number of segments, and the segment connected to the rigid rod is marked as R\({}_{1}\)), and the pin rolls connecting the two segments are marked A\({}_{k}\) in turn (\(k\) represents the number of pin rolls, and the pin roll connecting the rigid rod is marked as A\({}_{0}\)). The front segment was connected to the next segment by a pin, the stiffness of the  
Figure 1: Structural analysis of the flexible surgical instrument: (**a**) surgical instrument; (**b**) connection of two segments; (**c**) traction cables of the flexible part; (**d**) structure of the clamps. flexible section could be increased by connecting the segments in a series with the pin rolls, and each segment could be bent approximately 15\({}^{\circ}\) around the center axis of the pin shaft, such that the maximum bending angle of the instrument's flexible portion was 105\({}^{\circ}\). The connection between two segments is shown in Figure 1b, where holes H\({}_{i}\)/ (_j_ represents the number of holes) were used to pass through the cables on each segment. The end surface of the segment is shown in Figure 1c, where the four small circular holes, H\({}_{i}\)\({}^{1}\)-H\({}_{i}\)\({}^{4}\), were used to pass the four cables, C\({}_{1}\)-C\({}_{4}\), and the central hole, H\({}_{i}\)\({}^{5}\), was used to pass two cables, C\({}_{5}\) and C\({}_{6}\). The curved DOF r\({}_{1}\) of the flexible components was controlled by cables C\({}_{1}\) and C\({}_{3}\); when cable C\({}_{1}\) was pulled, cable C\({}_{3}\) was released, and the instrument bent to cable C\({}_{1}\)'s side, and vice-versa. The length of each segment was 20 mm, and the flexible portion of the instrument was 140 mm long, allowing for a large range of motion. The surgical clamps were the end effectors of the surgical instrument, and the designed surgical clamps had an opening and closing DOF and a rotational DOF. For operational and control convenience, as shown in Figure 1d, the surgical clamps were designed so that one was movable and one was fixated. The fixed surgical clamp was designed with a rotating shaft and was connected through the sleeve and the end joint. The circlip effect was to limit the operation of the surgical clamp's axial movement, the bearing was used to reduce the friction between the rotating components and to maintain the roller alignment when the load was loaded. The rotational DOF, r\({}_{2}\), of the end effector was controlled by the cables C\({}_{2}\) and C\({}_{4}\), and in order to ensure that the traction direction of C\({}_{2}\) and C\({}_{4}\) rotated along the rotating shaft, cables C\({}_{2}\) and C\({}_{4}\) were guided through a 90 degree turn to deliver force perpendicular to the rotating shaft. Two grooves are designed in the rotating shaft, and two small-stepped holes were made on the two grooves.