Deployable, Variable Stiffness, Cable Driven Robot for Minimally Invasive Surgery

Abstract

Minimally Invasive Surgery (MIS) imposes a trade-off between non-invasive access and surgical capability. Treatment of early gastric cancers over 20 mm in diameter can be achieved by performing Endoscopic Submucosal Dissection (ESD) with a flexible endoscope; however, this procedure is technically challenging, suffers from extended operation times and requires extensive training. To facilitate the ESD procedure, we have created a deployable cable driven robot that increases the surgical capabilities of the flexible endoscope while attempting to minimize the impact on the access that they offer. Using a low-profile inflatable support structure in the shape of a hollow hexagonal prism, our robot can fold around the flexible endoscope and, when the target site has been reached, achieve a 73.16% increase in volume and increase its radial stiffness. A sheath around the variable stiffness structure delivers a series of force transmission cables that connect to two independent tubular end-effectors through which standard flexible endoscopic instruments can pass and be anchored. Using a simple control scheme based on the length of each cable, the pose of the two instruments can be controlled by haptic controllers in each hand of the user. The forces exerted by a single instrument were measured, and a maximum magnitude of 8.29 N observed along a single axis. The working channels and tip control of the flexible endoscope remain in use in conjunction with our robot and were used during a procedure imitating the demands of ESD was successfully carried out by a novice user. Not only does this robot facilitate difficult surgical techniques, but it can be easily customized and rapidly produced at low cost due to a programmatic design approach.

Keywords: deployable; minimally invasive surgery; rapid manufacture; soft robotics; variable stiffness.

Figure 1

Deployable cable-driven parallel robot (A) folded beside 12 mm diameter flexible endoscope (black) (B) undeployed robot placed around flexible endoscope (C) inflatable structure deployed and force-transmission cables pretensioned to allow the two surgical instruments (anchored inside gold and red shafts) to be controlled by changing each cable length.

Figure 2

The three weld designs generated using programmatic approach (A) the weld pattern to produce a circumferential beam layer, with key dimensions highlighted (B) weld pattern to produce stiffening beams at the prism vertices, stiffening beam width highlighted (C) weld pattern to join layers produced in A and B where their weld patterns intersect (D) process of welding layers together, followed by (E) cutting and (F) manually welding along the red lines in (D) results in a cylindrical shape (G) Lastly, the completed inflatable structure can be pressurized, taking its prismatic shape.

Figure 3

Workflow and schematic of laser welding system showing the progression from the welding parameters input by the user to gcode commands interpreted by the microcontroller (MCU), which controls not only the motion of the optics but also the triggering of the laser diode. The Cartesian robot constitutes three linear actuators whose stages are driven by belts and timing pulleys coupled to stepper motors.

Figure 4

Laser welding for planar designs (A) Cartesian robot with red pilot laser beam tracing a weld pattern at the interface between thermoplastic sheets (B) Cartesian robot within safety cabinet (C) Six DOF robot arm setup in the same cabinet for planar welds on the vacuum table.

Figure 5

Cable sheath and CYCLOPS schematic diagram (A) cable sheath design in blue showing Bowden cables and force-transmission cables, where the vertical line welds on the right and left are later manually welded to produce a cylinder (B) front view of CYCLOPS where the inflatable structure and sheath are in place, the cables are attached to the brass overtubes and also showing the origin of the coordinate system in red (C) cross sectional view showing entry points of cables to the structure interior, the attachment points on the overtubes and the robot's coordinate system in red. Not to scale.

Figure 6

Radial stiffness measurement setup (A) the initial position where the Aluminum block is in contact with the inflatable structure (enclosed by green dashed line) (B) the inflatable structure deflecting and exerting force on the force transducer during testing.

Figure 7

Force exertion test setup showing the right overtube rigidly clamped to the force transducer and the CYCLOPS structure held by a clamp.

Figure 8

Plots of radial stiffness against chamber pressure (A) stiffness of identical structures with 12 and 15 mm stiffening chambers with equal pressure in each layer (B) stiffness of each structure with a constant circumferential chamber pressure of 1,500 mbar and varying stiffening chamber pressure (C) radial stiffness measured at six different orientations (D) diagram showing which face was in contact with the force sensor for each orientation in (C).

Figure 9

Force measurements taken during typical motions of the master controller (A) force exertion by the instrument in the X, Y, and Z axes (B) force magnitude and cable tension forces during the same test period.

Figure 10

Demonstration of flexibility, insertion and deployment of the CYCLOPS (A) the endoscope is still free to bend (B) the CYCLOPS robot around the endoscope fits comfortably through a trans-anal port (C) the endoscope advances around curved, narrow path (D) further advancement (E) the assembly before deployment (F) deployed CYCLOPS robot ready to carry out surgical tasks (G) overview of robot after navigating the curved path and deploying.

Figure 11

ESD procedure setup and results. (A) Setup of CYCLOPS system with haptic controllers, control computer, motor unit, syringe pump, imaging hub, diathermy unit, and grasper actuator. (B) View of inner structure from retracted flexible endoscope camera. (C) CYCLOPS robot in its calibration position. (D) Both instruments at minimum X position. (E) Both instruments at maximum X position. (F) Chicken breast after removal of skin. (G) Measurement of removed skin.