Chapter 3
Miniature Wrists for Surgical Robots
difficult to create dexterous surgical instruments. Consequently, existing needle-sized de- vices have a limited number of degrees of freedom, preventing them from turning sharp corners in the anatomy as would be encountered at the skull base [82], in the middle ear [83], and in the ankle [84]. Additionally, the lack of a wrist on needle-sized devices greatly limits the ability of surgeons to perform other dexterous tasks, such as suturing and tissue resection. Lastly, the small size of the tools make it difficult to use complex mechanisms or actuation schemes in the tool design. Therefore, the goal of this work is to create a de- vice that provides additional degrees of freedom and dexterity to needle-sized surgical tools while employing a simple actuation scheme and straightforward integration with existing needle-sized tools.
There have been a number of different wrist designs presented in the literature, includ- ing designs based on traditional mechanical linkages such as ball joints [85], universal joints [86], cables and pulleys [87, 88, 89], lead screws [90, 91], serial chains in parallel [92], and flexures [8, 93]. The outer diameter of these designs range from 2.4 to 15 mm.
While it may be possible to downscale each of these designs to some extent, continuum structures are often more easily miniaturized than mechanical linkages.
Multiple forms of continuum wrists and dexterous structures have also been designed, including a unique cable-ring design [9], a rolling variable neutral-line mechanism [94], interlocking fiber designs [31, 95], a flexible multi-backbone design [40], a compliant rolling-contact design [96], and a flexible tubular nitinol structure [41], among others. For a thorough review of joints used in existing bendable surgical instruments, see [38]. In gen- eral, designs with fewer components have shown better scalability, and thus designs that involve machining the wrist into the shaft of the manipulator itself are appealing.
Manufacturing a region of compliant bending in nitinol tubing has been investigated by several groups. Kutzer et al. created a 6-mm tool for arthroscopy that used rectangular, symmetric cutouts [45]. Wei et al. used triangular cuts that were made in nitinol tube to create a similar manipulator [97]. A 10-mm tool used for endoscopic camera steering
was developed by Fischer et al. [10]. Various finite element analyses were undertaken by several groups to aid the design of compliant bending regions in nitinol tubes [98, 99, 100].
Catheters with bendable nitinol tips were made by both Haga et al. and Bell et al. [46, 47], and a steerable needle tip was created using a machined nitinol tube by Ryu et al. [101].
In this work, we have created a miniature wrist for needle-sized surgical instruments that is made using asymmetric, rectangular cutouts in a nitinol tube, forming a compliant bending region that is actuated with a single tendon. The wrist can be outfitted with vari- ous surgical end effectors for delivering treatment in tight spaces (see Fig. 3.1). Our wrist most closely resembles the catheter and needle designs of [46, 47, 101], but operates by a simpler principle and can bend more tightly. Our wrist is straightforward to manufacture and does not employ the intricate spring cutout design nor hydraulic actuation of the active catheter of [46], it is small enough for use on needle-sized devices and does not require a nitinol restoring spring like the active catheter design of [47], and it is able to achieve a larger deflection over a smaller radius of curvature compared with the active needle design of [101]. Our wrist is actuated with a single tendon and can be prototyped using inexpen- sive manufacturing methods. The design is scalable, and most importantly, the wrist can be easily integrated into needle-sized surgical tools. One example of such a tool is the concentric tube robot.
Concentric tube robots are needle-sized robotic manipulators that are comprised of a se- ries of precurved, superelastic nitinol tubes, often less than 2 mm in diameter, that are often designed for medical applications [102]. The tubes are independently translated and rotated with respect to one another, creating a controllable tentacle-like motion of the manipulator.
Mechanics-based models of concentric tube robots have been derived [103, 104], and these devices have been investigated for use in prostate removal [105], cardiac applications [106], endonasal skull base surgery [107], and prostate brachytherapy [108]. Providing needle- sized instruments such as the concentric tube robot with a wrist may enable new surgical approaches and techniques that are currently impossible with existing rigid needle-sized
(a) (b) (c)
Figure 3.1: Our wrist can be outfitted with various surgical tools. (a) A curette is attached to the end of the wrist and is affixed to a wire that runs the length of the tube, allowing for rotation of the curette. (b) A gripper is shown attached to the wrist. Note that this gripper was modified from a commercial biopsy tool and is unactuated, but is shown here for illustrative purposes. (c) A laser fiber is deployed through the wrist, illustrating the use of the wrist to aim a laser.
tools.
The contributions of this chapter include the derivation and validation of kinematic and statics models for asymmetric cutout wrists, a low-cost manufacturing process, a de- sign method to achieve preferential bending of the wrist, and experimental validation of the fatigue life and scalability of the wrist. A preliminary version of some results in this manuscript can be found in [77]. Additions and enhancements in this archival manuscript include model validation using a scaled micro-wrist, a method for designing the wrist ge- ometry to achieve tip-first bending of the wrist, a new tendon-attachment design, and fa- tigue testing of the wrist.