Technical Brief

Design, Fabrication, and Testing of a Needle-Sized Wrist for Surgical Instruments

[+] Author and Article Information
Philip J. Swaney

Department of Mechanical Engineering,
Vanderbilt University,
Nashville, TN 37212
e-mail: philip.j.swaney@vanderbilt.edu

Peter A. York

Department of Mechanical Engineering,
Harvard University,
Cambridge, MA 02138

Hunter B. Gilbert

Department of Mechanical Engineering,
Vanderbilt University,
Nashville, TN 37212

Jessica Burgner-Kahrs

Associate Professor
Center of Mechatronics,
Leibniz Universität Hannover,
Hannover 30167, Germany

Robert J. Webster, III

Associate Professor
Department of Mechanical Engineering,
Vanderbilt University,
Nashville, TN 37212
e-mail: robert.webster@vanderbilt.edu

Manuscript received March 8, 2016; final manuscript received August 26, 2016; published online December 21, 2016. Assoc. Editor: Carl Nelson.

J. Med. Devices 11(1), 014501 (Dec 21, 2016) (9 pages) Paper No: MED-16-1191; doi: 10.1115/1.4034575 History: Received March 08, 2016; Revised August 26, 2016

This paper presents a miniature wrist that can be integrated into needle-sized surgical instruments. The wrist consists of a nitinol tube with asymmetric cutouts that is actuated by a single tendon to provide high distal curvature. We derive and experimentally validate kinematic and static models for the wrist and describe several prototype wrists, illustrating the straightforward fabrication and scalability of the design. We experimentally investigate fatigue life, the concept of tip-first bending, and practical use of the wrist with a concentric tube robot in an endonasal surgical scenario.

Copyright © 2017 by ASME
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Grahic Jump Location
Fig. 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.

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Fig. 2

The difference between a symmetric cutout design (left) and an asymmetric design (right) is shown. Note the significantly longer moment arm and reduced tendon force required to actuate the asymmetric design.

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Fig. 3

The arc parameters and kinematic values for a single cutout section of the wrist are shown. Cut height is denoted by h, while the chord, t, followed by the tendon is equal to h − Δl, where Δl is the tendon displacement. The angle γ defines the angle the tendon must navigate at each corner and is used to calculate the friction on the tendon in Sec. 4.

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Fig. 4

The geometric parameters that can be selected for the wrist design include the uncut section height c, the cutout height h, the cut depth g, and the outer and inner tube radii ro and ri. Section view A–A illustrates the areas Ai and Ao used to calculate the neutral bending plane location.

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Fig. 5

Wrist schematic showing wrist going from unactuated to fully actuated with kinematic frames defined. The maximum bending angle and minimum radius of curvature are both labeled.

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Fig. 6

A wrist with a curette at the tip is shown bending from 0 to 90 deg. The sequence order is in the top left of each image. Note that the curette is also being rotated while the wrist is actuated.

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Fig. 7

The fixturing setup used for the CNC manufacturing process. The slotted aluminum fixture used to make the wrists, with a nitinol tube glued into the fixture, is held in place by the CNC vise.

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Fig. 8

(a) A detailed view of the wrist manufactured using the low-cost CNC milling method. Note the square corners of the cutout made by the square end mill. (b) This wrist was manufactured using wire EDM, and the rounded corners produced by the wire can be seen clearly in the inset.

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Fig. 9

The use of an end cap to attach the tendon to the tip of the wrist is shown. This is the tendon attachment method used in this work.

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Fig. 10

The experimental setup used for the kinematics and statics experiment is shown. The wrist was placed directly below a microscope and was held in place using a collet. The tendon was then affixed to a force sensor that was mounted to a linear slide. This allowed for simultaneous collection of force and displacement data.

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Fig. 11

The kinematic model predicted and experimental wrist tip spatial trajectory is shown. The wrist starts at top of the figure and rotates counterclockwise from 0 to 140 deg. These results show that the constant curvature assumption is a reasonable approximation for this geometry, since the wrist tip closely follows the path predicted by the model. It also validates the predicted maximum angle of rotation (138 deg) of the wrist.

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Fig. 12

The statics model predicted and experimental results shown here correspond to the tendon force required to actuate the wrist. Note that the model captures the superelastic material behavior, as seen by the change in slope at about θ = 30 deg.

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Fig. 13

A 0.46 mm OD wrist was made using wire EDM manufacturing and is shown being deflected to approximately 90 deg. It is overlaid here on a penny to provide a sense of scale. For this photograph, the tendon was tied at the end of the wrist since the end caps made for the 1.16 mm wrist prototype were much larger than the 0.46 mm tube.

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Fig. 14

The kinematic model is validated for the submillimetric wrist (0.46 mm OD). These results again indicate that the constant curvature assumption is a reasonable approximation. Close inspection of the wrist at high articulation reveals that the small cutouts are not closing fully, preventing the wrist from reaching the full articulation predicted by the model.

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Fig. 15

The statics model is also validated for the submillimetric wrist (0.46 mm OD)

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Fig. 16

This sequence shows the wrist bending from the tip first. In order to accomplish this behavior, the wrist is manufactured such that the most distal cutout has the deepest cut depth, and each sequential cutout is shallower than the cutout before it. This lowers the required actuation force for the most distal cutout and results in the behavior seen here.

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Fig. 17

(a) Photograph of our wrist integrated directly into the inner-most tube of a concentric tube robot, with an inset showing a closer view of the wrist and surgical curette attached to the wrist. The diameter of the wrist is 1.16 mm. (b) Here, an endoscope view shows the concentric tube robot with integrated wrist being used to remove a gelatin tumor from a skull base phantom model.




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