# A Flexible Surgical Instrument for Robot-Assisted Minimally Invasive Surgery
## 1 Introduction  
With the rapid development of robot-assisted minimally invasive surgery (RMIS), the accuracy and dexterity of surgical operations have improved [1; 2; 3; 4; 5]. At present, the primary surgical instruments for RMIS are rigid instruments with wrist motions in which the ability to adjust the posture of rigid instruments in a narrow surgical environment is not sufficient compared with flexible surgical instruments [6]. Flexible surgical instruments have the advantages of multiple degrees of freedoms (DOFs) and good environmental adaptability [7], rendering them highly suitable for performing surgical tasks in narrow workspaces. To meet the requirements for reaching a target position in a small surgical space, flexible surgical instruments are often required to have dexterity equivalent to that of a human hand [8]. In addition, since the surgical instrument needs to be able to probe into the body cavity to complete a surgical task, considering the limitations of the working space, the surgical instruments are required to have smaller size parameters [9]. These requirements increase the difficulty of designing and manufacturing flexible instruments. Moreover, redundant high degrees of freedom (DOFs) also increase the difficulty of accurately establishing kinematic models [10; 11; 12]. In recent years, several researchers have conducted studies on flexible surgical instruments. [PERSON] et al. designed a single-port surgical robotic system [13; 14]. The system included two 6-DOF flexible continuum manipulator arms, and the flexible continuum joint near the poke end was passively supported by a six-bar mechanism. Due to the singularity point of the six-bar mechanism and the configuration of two flexible joints connected in a series, the system had low dexterity, slow operation, and a small load capacity; thus, it could not complete complex surgical tasks. [PERSON] et al. developed a single-hole robot [15; 16]. The robot manipulator was composed of two flexible continuum joints. Each flexible continuum joint was driven by three superelastic nickel-titanium memory alloywires. The continuum was completely connected to form a whole, and the rear-end driving device generated forward and backward movement by pulling the nickel-titanium memory alloy wire, thereby driving the joints of the continuum to perform overall deflection. The system has the advantage of being compact but has a small end-load capacity. [PERSON] et al. designed a flexible surgical instrument using a nickel-titanium alloy tube as a supporting framework [17] and achieved bending by cutting grooves on the tube wall of the iron-nickel alloy tube. Then, they optimized the design of the grooves. It could better resist the axial compression deformation caused by the tension force of the wire. Compared with an articulated joint configuration, this structure had higher flexibility, but its precise motion control was more complicated. [PERSON] et al. developed interleaved continuum-rigid manipulation with a soft, flexible robotic manipulator [18]. They used flexible shafts to remotely drive cable motion in the manipulator. While the 1-DOF flexible continuum portion of the manipulator was small (6 mm in diameter), the additional 2-DOF located in the proximal section were larger and contained complex miniature gear transmissions. [PERSON] et al. proposed a soft fiber-reinforced bending finger with three chambers actuated by ECF (electro-conjugate fluid) pumps [19]; a flexible bending finger with three chambers can easily realize 2-DOF motion. This research has guiding significance for soft robotic arms, especially for minimally invasive surgery with variable stiffness. [PERSON] et al. presented a design strategy that takes a kinematic trajectory as its input and uses analytical modeling based on nonlinear elasticity and optimization to identify the optimal design parameters for an actuator that will follow this trajectory upon pressurization [20]. Finally, they verified the modeling method through experiments.