Design of a dexterous robotic surgical instrument with a novel bending mechanism

Abstract

Background
The robot-assisted minimally invasive surgery (RMIS) has developed rapidly in recent years, requiring highly articulated instruments to enable surgeons to perform complicated and precise procedures.

Methods
A novel wrist-type surgical instrument was proposed for RMIS. The wrist consists of superelastic-wire-driven snake-like joints and universal joints, which could perform two deflections and one distal rotation. The bending mechanism and the kinematics of universal joints were analyzed. The forward and inverse kinematics of the wrist were derived.

Results
The performances of the instrument were evaluated using a prototype by experiments. The average motion deviation of the wrist’s deflection was 0.15 ± 0.08 mm, and the maximum deviation was 0.52 mm. The maximum payload capability was 10 N. The suture task and ex vivo procedure verified the effectiveness of the instrument.

Conclusions
The proposed instrument has high dexterity and payload capability, which contributes to improving the quality of the RMIS procedures.
This article is protected by copyright. All rights reserved.

Full text

Received: 15 April 2021
-
Revised: 12 September 2021
-
Accepted: 16 September 2021
DOI: 10.1002/rcs.2334
ORIGINAL ARTICLE
Design of a dexterous robotic surgical instrument with a
novel bending mechanism
Yingkan Yang
|Jianmin Li |Kang Kong |Shuxin Wang
Key Laboratory of Mechanism Theory and
Equipment Design of Ministry of Education,
Tianjin University, Tianjin, China
Correspondence
Kang Kong, Key Laboratory of Mechanism
Theory and Equipment Design of Ministry of
Education, Tianjin University, Tianjin 300354,
China.
Email: kongkang103@tju.edu.cn
Funding information
National Key R&D Program of China, Grant/
Award Numbers: 2019YFC0118003,
2019YFB1311502; National Natural Science
Foundation of China, Grant/Award Numbers:
51805362, 51875390
Abstract
Background: The robot‐assisted minimally invasive surgery (RMIS) has developed
rapidly in recent years, requiring highly articulated instruments to enable surgeons
to perform complicated and precise procedures.
Methods: A novel wrist‐type surgical instrument was proposed for RMIS. The wrist
consists of superelastic‐wire‐driven snake‐like joints and universal joints, which
could perform two deflections and one distal rotation. The bending mechanism and
the kinematics of universal joints were analysed. The forward and inverse kine-
matics of the wrist were derived.
Results: The performances of the instrument were evaluated using a prototype by
experiments. The average motion deviation of the wrist's deflection was 0.15 0.08
mm, and the maximum deviation was 0.52 mm. The maximum payload capability
was 10 N. The suture task and ex vivo procedure verified the effectiveness of the
instrument.
Conclusions: The proposed instrument has high dexterity and payload capability,
which contributes to improving the quality of the RMIS procedures.
KEYWORDS
bending mechanism, RMIS, robotic surgical instrument, universal joint
1
|
INTRODUCTION
Minimally invasive surgery (MIS) is usually performed by inserting
several rigid, slim instruments into a patient's body through three or
four small incisions with image guidance by an endoscopy. Compared
with open surgery, MIS has numerous benefits for patients, such as
less blood loss, less post‐operative pain and shorter hospital stays.
1
However, there exist some drawbacks due to the small incisions,
including limited degrees of freedom (DOFs) of the instrument, hand‐
eye incoordination due to fulcrum effect, a long learning curve and
easy fatigue. To solve these problems, surgical robotic systems have
been developed to improve the quality of the MIS.
2
Dexterous sur-
gical instrument is the core component of the surgical robot, and its
performance determines the quality of the surgery.
To improve the dexterity of the instrument, a wrist mechanism
with two or more DOFs is usually added to its distal end.
3
Various
kinds of dexterous instruments have been designed for surgical ro-
botic system, as shown in Figure 1. To achieve compact structure and
remote transmission, the cable‐driven mechanism is widely used in
the design of robotic surgical instruments. The multi‐DOF instrument
of the da Vinci system is a typical cable‐pulley mechanism, as shown
in Figure 1A.
4
It has a dexterous two‐DOF (pitch‐yaw, PY) wrist and a
rotational shaft. A cable‐pulley drive instrument with a yaw‐roll (YR)
wrist was described by Wang et al., and the rotation at the distal end
of the instrument is demonstrated to assist the performance of su-
turing and knotting, as shown in Figure 1B.
5
Cable guide channels,
which eliminate the need for pulleys and reduce the overall length to
facilitate miniaturization, were utilized to develop a dexterous and
compact surgical instrument, as shown in Figure 1C.
6
However, the
slip, friction and stretching of cables reduce the motion accuracy and
stability of the instrument.
16
To gain precise transmission and
enhanced stiffness, Wang et al. designed a surgical manipulator with
Int J Med Robot. 2021;e2334. wileyonlinelibrary.com/journal/rcs © 2021 John Wiley & Sons Ltd.
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https://doi.org/10.1002/rcs.2334

a multi‐gear array mechanism, as shown in Figure 1D,
7
and a motor‐
driven laparoscopic needle holder with the bevel gear mechanism
was designed by Dexterite Surgical, as shown in Figure 1E.
8
However,
the size of such instruments is limited by the size of gears, and their
transmission structure is complicated and bulky. Adding the link drive
mechanism is also an effective way to design surgical instruments
due to its characteristics such as high accuracy and high load capa-
bility. Hong et al. designed an instrument with a three‐DOF parallel
link mechanism, as shown in Figure 1F.
9
It can perform deflection in
two directions and axial rotational motion of the forceps. A robotic
visible forceps with a serial‐linkage mechanism was developed by
Zhang et al., as shown in Figure 1G.
10
The forceps can perform one
bending motion and 3 N lifting force at the tip. Though such in-
struments have high payload capability, their structures are complex
and bulky. Moreover, shape memory alloys (SMAs), pneumatic actu-
ators, hydraulic actuators and magnetic actuators have been used to
design dexterous surgical tools.
17
Although these methods allow the
tools to achieve high strength and dexterity, they are difficult to
implement precise control due to the nonlinearity.
In recent years, flexible joints are increasingly used in the design
of surgical instruments to enhance dexterity and compliance. A
diameter of 5 mm instrument with a snake‐like wrist was developed
for the surgery in confined space, as shown in Figure 1H.
11
The wrist
can perform bending motions in two orthogonal planes. The instru-
ment of the da Vinci SP system has a double‐jointed distal end with
elbow and snake‐like wrist joints, as shown in Figure 1I.
12
The wrist
and elbow joints have two‐DOF, respectively. A highly flexible and
compliant instrument was designed for the SPORT single‐port ro-
botic surgical system, as shown in Figure 1J.
13
The instrument con-
sists of three‐section continuum joints, and each section has two
bending movements. Francis et al. designed two miniaturized flexible
instruments for operations in small and confined space, such as
neurosurgery, head and neck surgery, and paediatrics, as shown in
Figure 1K.
14
The instrument has a diameter of 2 mm with a three‐
DOF wrist. It is highly dexterous and compliant, but less rigid. The
instrument with flexible joints has high dexterity and compliance, but
its rigidity is generally low, which hinders its precise operation and
payload capability. To address this problem, researchers have
explored many methods. A flexible instrument with enhanced stiff-
ness has been developed, and its wrist is composed of a precision
spring and four superelastic wires, as shown in Figure 1L.
15
Due to
the rigidity of the spring and wires, the stiffness of the instrument is
improved. However, the machined spring is easy to fatigue. Wang
et al. designed a controllable stiffness continuum with phase change
materials to enhance the stiffness of the instrument, but the
switching time of the stiffness is long.
18
Methods to increase cable
tension
19
or optimize cable path
20
have been proposed to increase
the payload capability. Nevertheless, these designs are complex and
occupy a substantial space, making it difficult to create a compact and
lightweight instrument.
To develop a dexterous and compact surgical instrument with
reliable payload capability, a four‐DOF wrist‐type robotic surgical
instrument is proposed in this paper by using a novel cable‐driven
bending mechanism with serial universal joints. First, the overview
FIGURE 1 Various instruments. (A) Cable‐driven Endowrist
4
; (B) Microhand instrument
5
; (C) instrument with cable guide channels
6
;
(D) gear‐driven instrument
7
; (E) needle holder with bevel gear mechanism
8
; (F) parallel‐linkage‐actuated instrument
9
; (G) serial‐linkage‐
actuated instrument
10
; (H) snake‐like wrist Endowrist
11
; (I) da Vinci SP instrument
12
; (J) SPORT instrument
13
; (K) miniaturized flexible
instruments
14
; (L) flexible instrument with enhanced stiffness
15
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and detail design of the proposed instrument are given. Then, the
forward and inverse kinematics of the wrist are discussed. Further-
more, the performances of the prototype are evaluated by experi-
ments. Finally, discussions and conclusions are provided.
2
|
MATERIALS AND METHODS
2.1
|
Mechanical design
A four‐DOF robotic surgical instrument with a snake‐like wrist was
developed, as shown in Figure 2. The instrument is composed of four
parts, including the end‐effector, wrist, shaft and transmission
interface. The end‐effector performs opening and closing motions to
grip and dissect tissues. The wrist can bend in two directions and
rotate around the distal end of the instrument. The bending and
distal rotation movements can adjust the position and orientation of
the instrument tip, which contributes to performing dexterous
operation. The direction of jaws of the end‐effector can be adjusted
by the distal rotation with ease. A compact cable‐driven mechanism
is adopted to carry out remote drive from the transmission interface
to the wrist and the end‐effector. The instrument can be attached to
the minimally invasive surgical robot system by the transmission
interface to implement operations.
The wrist with a compact structure is designed by using snake‐
like joints integrated with serial universal joints, as shown in
Figure 2. The snake‐like joints consist of four hinge joints with five
links and are driven by cables to realize bi‐directional bending. The
axes of the two adjacent hinge joints are perpendicular to each other.
The detailed structure of the wrist is shown in Figure 3. Two yokes
and one spider make up one universal joint, which is equipped in the
centre of the hinge joints, as shown in Figure 3A. The rotation axis of
the universal joint is designed to coincide with the rotation axis of the
hinge joint all the time, which ensures that the motion of the uni-
versal joint is in sync with the motion of the hinge joint, as shown in
Figure 3B. Two pairs of antagonistic drive cables slide in holes evenly
distributed on the links, which control the wrist to bend in two di-
rections, as shown in Figure 3C. At the same time, the universal joint
can rotate around its axis to realize the distal rotation of the wrist, as
shown in Figure 3D. The distal end of the universal joint and the base
of the end‐effector are fixedly connected by the set pin, as shown in
Figure 2. Thus, the rotation of universal joints can realize the rotation
of the end‐effector.
The end‐effector is used to perform clamping, cutting, dissec-
tion and coagulation in surgery. Most of the end‐effectors have two
jaws doing pivotal motions relative to each other. The pivotal mo-
tion is usually generated by linear displacement. Concerning the
proposed end‐effector, the required linear movement is produced
by the rotary actuator coupled with a cable that has a reset spring,
as shown in Figure 2. The pin slides along the slot to change the
angle of the two jaws around the pivot pin. The closing of the
end‐effector is implemented by the tension of the cable, and
the opening is performed by the restoring force of the spring. The
relationship between the angular position of the jaws and the lon-
gitudinal variation of the cable can be derived by geometry, which
can be expressed as follows:
FIGURE 2 Structure of the robotic surgical instrument
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Δl¼sgsin φg1
sinφgþθg1=2−1
sinφgþθg=2;ð1Þ
where Δlimplies the longitudinal variation of the cable, θ
g
and θ
g1
imply two different arbitrary angular position of the jaws
(θ
g
≤θ
g1
), and s
g
and φ
g
are the design parameters of the sliding
slot.
The compact cable‐driven structure is used in the instrument to
perform long distance transmission, as shown in Figure 4. The
bending joint is driven by two pairs of antagonistic cables. The ends
of cables are attached to the distal end of the wrist and the other
ends are attached to drive shafts to form a closed‐loop cable‐driven
structure. The distal rotation joint is connected to a rigid rod, which is
also driven by a closed‐loop cable‐driven structure. An open‐loop
cable‐driven structure with a reset spring is used to drive the end‐
effector. In this structure, the cable is connected to the drive shaft
by passing through the central axis of the serial universal joints and
the rigid rod.
2.2
|
Cable‐driven mechanism
The hinge joints are fundamental elements of the wrist, and the total
bending angle of the wrist depends on the number of hinge joints and
the bending angle of each hinge joint. Figure 5A illustrates the
bending of one hinge joint. The black lines represent links, the blue
and red lines represent cables, and the circle is the rotation axis.
When the joint bends θleftwards, the length of the left cable de-
creases to l
l
(θ), and the length of the right cable increases to l
r
(θ). The
lengths of cables after bending can be expressed as follows
8
>
>
<
>
>
:
llðθÞ ¼ 2hgcos θ
2−dsin θ
2
lrðθÞ ¼ 2hgcos θ
2þdsin θ
2;ð2Þ
where h
g
is the distance from the rotation axis to the link and dis the
distance from the centre of the cable hole to the centre of the link.
Based on the Equation (2), the length variations of the two cables
can be expressed as follows
8
>
>
<
>
>
:
Δll¼llðθÞ−llð0Þ ¼ 2hgcos θ
2−1−dsin θ
2
Δlr¼lrðθÞ−lrð0Þ ¼ 2hgcos θ
2−1þdsin θ
2:ð3Þ
According to the Equation (3), the sum of two length variations
(Δl
l
+Δl
r
) is less than zero. Consequently, the cable slack tends to
occur in the closed‐loop cable‐driven structure, which reduces the
motion accuracy and stiffness of the wrist. In order to address this
problem, superelastic wires can be used as drive cables and back-
bones of the wrist. With the superelastic wires, the discrete hinge
joints can form a continuous joint, whose centreline length is
invariant.
19
Figure 5B shows the schematic of the hinge joint with
superelastic wires. Based on the geometrical analysis, the length
variations of the two superelastic wires can be expressed as follows
ΔLl¼ ðR−dÞΘ−RΘ¼−dΘ
ΔLr¼ ðRþdÞΘ−RΘ¼dΘ;ð4Þ
where Rrepresents the bending radius of the wrist and Θis the
bending angle of the distal end of the wrist. The sum of two length
variations (ΔL
l
+ΔL
r
) is zero, which contributes to the elimination of
cable slack and variable cable tensions.
21
2.3
|
Universal joint kinematics analysis
According to the analysis above, the wrist consists of four hinge
joints with the same number of universal joints. The universal joint is
not a constant‐velocity joint, and it needs to satisfy certain conditions
to realize a constant‐velocity output.
22
The kinematic schematic of
universal joints is shown in Figure 6. The relationship between the
input and output of the double universal joint can be deduced by
geometry, which can be expressed as follows
FIGURE 3 Structure of the wrist. (A) Front view; (B) side view;
(C) top view; (D) axonometric drawing
FIGURE 4 Transmission of the instrument
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ET AL.

tan β3¼tanarctantan β1
cos θ1−γ1þψ1cos θ2;ð5Þ
ω3
ω1¼cos θ1
1−sin2θ1cos2β1
⋅cos θ2
1−sin2θ2sin2arctantan β1
cos θ1−γ1þψ1;
ð6Þ
where β
1
is the input angle, β
3
is the output angle, ω
1
is the input
angular velocity, ω
3
is the output angular velocity, γ
1
is the angle
between the plane containing the input yoke and intermediate
yoke (S
1
O
1
S
2
) and the plane containing intermediate yoke and output
yoke (S
2
O
2
S
3
), ψ
1
is the phase angle of the two ends of the inter-
mediate yoke, θ
1
is the angle between the input yoke and the in-
termediate yoke, and θ
2
is the angle between the output yoke and
the intermediate yoke.
Because the rotation axis of the universal joint coincides with
that of the hinge joint, as shown in Figure 3, the angle between the
yokes of the universal joint is equal to the bending angle of the hinge
joint. Due to the perpendicular configuration of the two adjacent
hinge joints, as shown in Figure 2,γ
1
is 90°. In order to gain a stable
output, ψ
1
is set to 90°. Hence, Equations (5) and (6) can be rewritten
as follows
tan β3
tan β1¼cos θ2
cos θ1
;ð7Þ
ω3
ω1¼cos θ1cos θ2
cos 2θ1cos 2β1þcos 2θ2sin 2β1
:ð8Þ
Further, the relationship of input angle and output angle of three
universal joints can be calculated as
tanðβ4−90∘Þ ¼
tanarctantan β1cos θ2
cos θ1−γ2þψ2
cos θ3
:ð9Þ
Based on the above analysis, the superelastic wires can make the
discrete hinge joints form a continuous joint. Hence, the hinge joints
bending in the same direction have the same angular movement.
According to the structure of the wrist, as shown in Figure 2, the
angular movements of the first joint (θ
1
) and the third joint (θ
3
) are
the same and the angular movements of the second joint (θ
2
) and the
fourth joint (θ
4
) are the same. Due to the structure of the wrist, γ
2
is
90°. Hence, ψ
2
is set to 0° to gain a stable output. Equation (9) can be
rewritten as
tan β4¼tan β1cos θ2:ð10Þ
The relationship of input angle and output angle of four universal
joints can be deduced as
tan β5¼tanðarctanðtan β1cos θ2Þ−γ3þψ3Þ
cos θ4
:ð11Þ
According to the structure of the wrist, γ
3
is 90° and θ
2
is equal
to θ
4
. Hence, ψ
3
is set to 90° to gain a stable output. Equation (11)
can be rewritten as
tan β5¼tan β1:ð12Þ
Hence, the input angle (β
1
) is equal to the output angle (β
5
), and
the four universal joints could achieve a constant‐velocity output
(ω
1
=ω
5
) with the configuration analysed above.
2.4
|
Wrist kinematics
The snake‐like wrist consisting of multiple links has two bending
DOFs. The angles of the wrist are determined by the length varia-
tions of cables. The position and orientation of the distal end of the
wrist in the space can be confirmed with given angles of the wrist.
Consequently, the kinematics analysis can be divided into two parts:
the mapping between actuator space and configuration space and the
mapping between configuration space and task space.
FIGURE 5 Kinematic diagram of the hinge.
(A) Hinge with cables; (B) hinge with
superelastic wires
FIGURE 6 Kinematic schematic of universal joints
YANG ET AL.
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2.4.1
|
Mapping between actuator space and
configuration space
As shown in Figure 7, there are two pairs of antagonistic drive cables
which control the wrist to bend in arbitrary direction in space. The
bending angle is Θand the bending direction angle is Φ, as shown in
Figure 8. The length variations of the four cables after bending can be
calculated as follows
8
>
>
>
>
>
>
>
>
>
>
>
>
<
>
>
>
>
>
>
>
>
>
>
>
>
:
ΔL1¼dcosπ
4−ΦΘ
ΔL2¼dcos3π
4−ΦΘ
ΔL3¼dcos5π
4−ΦΘ
ΔL4¼dcos7π
4−ΦΘ
:ð13Þ
According to the cable length variations, the bending direction
angle and the bending angle of the wrist can also be determined
8
>
>
>
>
<
>
>
>
>
:
Φ¼π
4þarctan 2ΔL2−ΔL4
ΔL1−ΔL3
Θ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðΔL1−ΔL3Þ2þ ðΔL2−ΔL4Þ2
q2d
:ð14Þ
The inverse mapping between the actuator space and the
configuration space is defined by Equation (13). The forward mapping
is given by Equation (14).
2.4.2
|
Mapping between configuration space and
task space
In consideration of the wrist with superelastic wires, the centreline
length of the wrist is invariant and the hinge joints bending in the
same direction have the same bending angle. Hence, the curvature of
the wrist can be approximately regarded as a constant. The piece‐
wise constant curvature model
23
can be used to calculate the kine-
matics of the wrist. The coordinate of the wrist is shown in Figure 8.
The position and orientation of the end‐effector in the space can be
represented by the following matrix
T0
n¼2
6
6
4
nxsxoxdx
nysyoydy
nzszozdz
0 0 0 1
3
7
7
5;ð15Þ
where [n
x
n
y
n
z
]
T
, [s
x
s
y
s
z
]
T
and [o
x
o
y
o
z
]
T
represent orientation vec-
tors in the x
n
,y
n
and z
n
directions in the coordinate o
0
x
0
y
0
z
0
.
[d
x
d
y
d
z
]
T
represents the vector from o
0
to o
n
in the coordinate
o
0
x
0
y
0
z
0
.
The forward kinematics of the wrist can be derived from the
homogeneous matrix transformation, and the position and orienta-
tion in the space can be calculated as follows
8
>
>
>
>
>
>
>
<
>
>
>
>
>
>
>
:
dx¼Ltcos Φsin Θ−Lccos Φðcos Θ−1Þ
Θ
dy¼Ltsin Φsin Θ−Lcsin Φðcos Θ−1Þ
Θ
dz¼Ltcos ΘþLcsin Θ
Θ
;ð16Þ
8
<
:
nx¼cos θrcos Θ−1cos2Φþ1þcos Φsin Φsin θrðcos Θ−1Þ
ny¼sin θrcos Θ−cos2Φðcos Θ−1Þþcos Φsin Φcos θrðcos Θ−1Þ
nz¼−cosðΦ−θrÞsin Θ
;
ð17Þ
8
<
:
sx¼cos Φsin Φcos θrðcos Θ−1Þ−sin θrcos Θ−1cos2Φþ1
sy¼cos θrcos Θ−cos2Φðcos Θ−1Þ−cos Φsin Φsin θrðcos Θ−1Þ
sz¼−sinðΦ−θrÞsin Θ
;
ð18Þ
8
<
:
ox¼cos Φsin Θ
oy¼sin Φsin Θ
oz¼cos Θ
;ð19Þ
FIGURE 7 Two pairs of antagonistic drive cables on the wrist
FIGURE 8 Coordinate of the wrist
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ET AL.

where Lcis the length of the wrist, Ltis the distance from the wrist to
the distal end of the end‐effector and θ
r
is the angle of the distal
rotation joint. The inverse kinematics can be derived from the posi-
tion and orientation, which can be found as follows
8
>
>
>
>
>
>
>
>
>
<
>
>
>
>
>
>
>
>
>
:
Φ¼arctandy−Ltoy
dx−Ltox
Θ¼2 arctan0
@ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðdx−LtoxÞ2þ ðdy−LtoyÞ2
qdz−Ltoz1
A
θr¼arcsin szox−nzoy
o2
xþo2
y!
:ð20Þ
3
|
EXPERIMENTS AND RESULTS
3.1
|
Prototype design
Based on the above analysis, a prototype of the robotic surgical in-
strument was developed, as shown in Figure 9. The instrument is
compact and lightweight with a total length of 550 mm, a diameter of
9.5 mm and a weight of 180 g. The ranges of two bending motions are
both 50°, and the angle of the distal rotation joint can reach 360°.
The maximum opening angle of the end‐effector is 60°. The param-
eters of the jaw's sliding slot s
g
and φ
g
are 4.4 mm and 33°.
Superelastic NiTi wires of 0.55 mm diameter were used to drive
bending motions and a 7 19 SUS304 cable with a diameter of
0.5 mm was used as the drive cable of the end‐effector. Various in-
struments, including needle holders, forceps, monopolar instruments
and bipolar instruments, as shown in Figure 9C, have been developed
for different surgical procedures. The instrument can be connected to
the “MicroHand” surgical robot
5
to perform operations. “MicroHand”
is a master‐slave robot consisting of a surgeon console and a slave
cart, as shown in Figure 9D.
3.2
|
Motion accuracy
To measure the accuracy of the designed instrument, a motion ac-
curacy test was performed, as shown in Figure 10. The instrument
was actuated by a control device. The programmable multi‐axis
controller (PMAC) processed the input commands from the panel
and output commands to motors to drive the corresponding joints of
the instrument. The instrument was driven repeatedly three times
within 50° in one direction and the position of the distal end was
measured with an electromagnetic tracking system (NDI Aurora,
Waterloo, Canada). The tracking sensor was attached to the end of
the instrument, as shown in Figure 10.
The deviation between the actual trajectory and desired tra-
jectory and the backlash hysteresis of the instrument were
measured. The motion trajectory is shown in Figure 11. The average
deviation was 0.15 0.08 mm, and the maximum deviation was 0.52
mm. The backlash hysteresis is shown in Figure 12. The average and
maximum hysteresis of the prototype were 4.46 2.34° and 7.78°
respectively.
3.3
|
Payload capability
To evaluate the payload capability of the instrument, the payloads at
various angles in one direction were tested by applying vertical
forces at the distal end of the instrument and measuring the
FIGURE 9 Prototype of the robotic surgical instrument. (A) Prototype of the instrument; (B) close‐up view of the wrist; (C) various
instruments; (D) proposed instrument mounted on “MicroHand” surgical robot
YANG ET AL.
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displacements. The distal end of the instrument was connected to a
dynamometer through a stainless cable and the instrument was fixed
with a rigid structure, as shown in Figure 13. According to related
research, the mean of average forces for retraction with grasping is
around 5 N and the force to lift one‐third of an average human liver is
more than 4 N.
24,25
Hence, the maximum load in the test was set to 5
N which was sufficient for surgical operations. The results of the
payload capability test are shown in Figure 14. The largest defor-
mation of the wrist occurred at the horizontal position with a stiff-
ness of about 1.19 N/mm and the smallest deformation occurred at
the angular position of 50° with a stiffness of about 2.88 N/mm. The
extreme payload capability of the wrist at the horizontal position was
about 10 N, as shown in Figure 13.
3.4
|
Suture test
A suture task was carried out to evaluate the dexterity of the in-
strument for performing complicated operations. The “MicroHand”
surgical robot was used as the operating system for the proposed
FIGURE 11 Motion trajectory
FIGURE 12 Backlash hysteresis
FIGURE 10 Motion accuracy test
FIGURE 13 Payload capability test
FIGURE 14 Results of the payload capability test
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instrument in this study. The task is to perform a running suture in
horizontal and vertical directions respectively. Four pairs of dots are
marked symmetrically on both sides of the line, and the diameter of
the dots is approximately 2 mm and the distance between the two
dots is 10 mm, as shown in Figure 15. In the test, the performance of
the proposed robotic instrument was compared with that of the
conventional manual laparoscopic instrument. The task performance
metrics are completion time and operation quality. The operation
quality is scored by the numbers of successful sutures within 5 min. A
successful suture is scored as one point, and non‐successful suture is
not scored. It is a successful suture that starts inside the entry dot
and ends in the exit dot, without digression of the needle from the
dot. Ten participants from Tianjin University were invited to partic-
ipate in the test. None of these participants was a left‐hand user and
had experience in MIS operation. The participants were trained to
acquire basic laparoscopic skills and got familiar with the proposed
instrument and the conventional instrument before the test. The
experiment setup and operation condition were completely the same
for all participants. The data results were compared using Wilcoxon
signed‐rank‐test, a value of p<0.05 was considered significant.
Statistical analysis was run with SPSS version 19 software.
The average time of the horizontal suturing with the robotic
instrument was 221.0 68.29 s, and the average time for the lapa-
roscopic instrument was 282.3 32.89 s, which represents a sig-
nificant difference (p=0.007). The completion time of the vertical
suturing of the robotic instrument was significantly shorter than that
of the laparoscopic instrument (268.6 60.19 s vs. 371.4 77.74,
p=0.012). Compared with the laparoscopic instrument, the robotic
instrument performed a higher quality of the horizontal suturing
(2.3 1.25 vs. 1.7 0.67, p=0.196), and the vertical suturing
(2.0 0.67 vs. 1.6 0.84, p=0.102).
3.5
|
Ex vivo test
Furthermore, an ex vivo single‐layer continuous anastomosis test
with a porcine intestine was performed using the designed instru-
ment with the “MicroHand” system. The test aimed to evaluate the
usability of the instrument in the clinical procedures. A surgeon with
the experience of laparoscopic surgery was invited to perform the
test. Before the test, the surgeon had 30 min to get familiar with the
instrument and robotic system. During the test, as shown in
Figure 16, the ends of the two pieces of intestines were pulled
together using a needle holder on the right and a grasper on the left.
Then, a running suturing was performed to close the wound and the
thread was pulled tight, and a surgical knot was placed to secure the
closure. In the procedure, the suturing and knot‐tying were
performed fluently. The needle could be held stably, and the tissue
could be clamped gently. The anastomosis procedure lasted for
approximately 40 min. The related research shows that the time of
the robot‐assisted laparoscopic intestinal anastomosis is about 25–
55 min and the time of the standard laparoscopic suturing is
approximately 90 min.
26
Hence, the anastomosis time of the ex vivo
test is acceptable.
4
|
DISCUSSION
The purpose of this study is to provide a dexterous and compact
surgical instrument with high payload capability for robot‐assisted
minimally invasive surgery (RMIS). The main contributions and fea-
tures of this study include: (1) a novel three‐DOF wrist structure with
hinge joints and serial universal joints is proposed; (2) the distal
rotation motion with a large range enhances the ability to perform
complicated operations, especially sutures; (3) the wrist has a high
payload capability which improves the safety of the surgery; (4)
various instruments have been developed for different surgeries.
The comparison between this paper and previous studies has
been shown in Table 1. For many instruments,
4–8,10‐13,15
the wrist
generally has two DOFs. The proposed wrist has three DOFs, and its
movement is more dexterous. The rotation range (360°) is larger
than that of other instruments. The dexterity of the instrument has
been demonstrated by the suture and ex vivo test. Compared with
the cable‐driven
4‐6
and gear‐driven mechanisms,
7,8
the proposed
wrist has advantages in terms of the rotation range and payload
capability; Compared with the linkage‐driven mechanism,
9,10
the
proposed wrist has a large distal rotation range; Compared with the
flexible‐jointed mechanism,
11‐15
the proposed wrist has a larger
rotation range and a higher payload capability. Related studies have
shown that the snake‐like Endowrist and SP instruments challenge
the surgeons due to the reduced strength.
27,28
Hence, their payload
capability is not high. Compared with the superelastic‐wire‐driven
mechanism,
13,15
the proposed wrist mechanism has a higher rigidity
due to the composite structure with hinge joints and universal joints.
Overall, the proposed instrument has advantages over previous
studies in terms of rotation range and payload capability.
The structure of the wrist adopts a modular design method,
which makes it have good scalability. According to Figure 3, the
combination of a hinge joint and a universal joint can be considered
as a submodule, and several submodules make up a module. When
the axes of two adjacent hinge joints are perpendicular to each other,
the module can perform three DOFs (pitch‐yaw‐roll, PYR)
FIGURE 15 Scene of the suture task
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simultaneously; when the axes of two adjacent hinge joints are par-
allel to each other, the module performs two DOFs (yaw‐roll, YP)
simultaneously. Furthermore, two or more modules can be connected
as a multi‐DOF continuum arm, which is well suited to confined
spaces, especially for working in MIS, single port access surgery and
natural orifice transluminal endoscopic surgery.
The result of the motion accuracy test shows that the average
motion deviation of the instrument is 0.15 0.08 mm. In the delicate
suture of laparoscopic surgery, the diameter of the vessel is generally
greater than 1 mm, therefore, the motion accuracy of the instrument
can meet the needs of delicate surgical operations. Hence, the motion
of the instrument is precise. The deviation and hysteresis of the in-
strument are mainly caused by the following reasons. Frist, the
diameters of the holes and cables are 0.6 and 0.55 mm, respectively.
There are clearances between the holes and cables, which affects the
length variation that the cables should have when the wrist moves.
Second, the friction between the joints decreases the motion accu-
racy. Hence, reducing the clearances between the holes and cables
and decreasing the friction between the joints will improve the mo-
tion accuracy of the instrument. The control method is also an
important way affecting the motion accuracy. Hence, optimized
kinematic algorithm, dynamical model, feedback control, motion and
friction compensation will be considered to improve motion accuracy
in the future work. Furthermore, backlash hysteresis causes the
nonlinear motion, which makes precise control of the instrument
difficult. Various mathematical models for hysteresis such as Pre-
isach, Krasnosel'skii–Pokrovskii, Prandtl–Ishlinskii, Maxwell‐Slip,
Bouc–Wen and Duhem, have been reviewed and analysed by the
related reference.
29
Several backlash hysteresis models of cable‐
conduit actuators or cable‐pulley mechanisms have been proposed
and used to compensate for the hysteresis behaviour.
30‐32
Hence, to
avoid nonlinearities, the backlash hysteresis model and compensation
control of the instrument need to be analysed and implemented in
future work. Based on the analysis in Section 3.3, a 5 N payload
capability on the tip of instrument is required during a typical surgical
procedure. According to the payload test, the instrument has a high
stiffness and can take on a load of 10 N, which meets the clinical
requirements and ensures the stability of the surgical operation. The
results of the suture test show that the proposed robotic instrument
can perform the suturing in an awkward position which is difficult for
the conventional instrument. The distal rotation joint of the proposed
instrument makes it easy to adjust the posture of the needle, which
contributes to improving the performance of the suturing. The users
can complete the suturing with a high quality in a short time, which
FIGURE 16 Scene of the intestinal
anastomosis
TABLE 1Comparison between this paper and previous studies
Driving mechanism Diameter (mm) Wrist‐DOF Bending range Rotation range Payload capability
Endowrist
4
Cable‐pulley 8 PY 90° 270° High
Wang et al.
7
Gear‐driven 9.5 PY 80° – Medium (2.5 N)
Hong et al.
9
Linkage‐driven 8 PYR 64° 60° High (10 N)
Snake‐like Endowrist
11
Cable‐driven 5 PY 90° 270° Medium
27
SP instrument
12
Cable‐driven 6 PY 90° 270° Medium
28
SPORT instrument
13
Superelastic‐wire‐driven 8 PY 90° – Medium (3.25 N)
Francis et al.
14
Concentric tube 2 RPR 70° 250° Low (0.26 N)
Haraguchi et al.
15
Superelastic‐wire‐driven 10 PY 90° – High (5 N)
This study Superelastic‐wire‐driven 9.5 PYR 50° 360° High (10 N)
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indicates that the proposed instrument can reduce the difficulty of
suturing and the learning time of complicated operations. The
feasibility of the instrument has been demonstrated by the ex vivo
anastomosis procedures. The instrument is dexterous enough to
perform the suturing and knotting. The force of the instrument is
sufficient to lift the tissue and tighten the suture stably. In consid-
eration of the requirements of clinical sterilization, the materials of
the instrument, including stainless steel, NiTi wires, carbon fibre and
polyetherimide plastic, are all biocompatible and autoclavable.
Hence, the proposed instrument can be sterilized by autoclave (134°
C, 201.7∼229.3 kPa).
However, some limitations of the proposed instrument need to
be further optimized. The diameter of the instrument is 9.5 mm
which satisfies the demands of the MIS, but it is still larger than that
of 5 and 8 mm instrument. Due to the constraint of the universal
joint, the range of the bending motion is 50°, which is not very
large compared with some robotic instruments.
4,5
Though the
payload test has investigated the stiffness of the instrument at
various angles in one deflecting direction, the payload capability with
different postures needs to be tested. In future works, the operating
angle of the universal joint will be increased to expand the motion
range of the bending joint. The instrument will be miniaturized,
tested by more comprehensive experiments and evaluated by the
animal experiment.
5
|
CONCLUSION
A novel wrist‐type four‐DOF robotic surgical instrument for MIS is
presented. The wrist can perform three DOFs and provides high
payload capability with a compact structure. The results of the per-
formance evaluation experiments demonstrate that the dexterity,
payload capability, accuracy and feasibility of the designed instru-
ment satisfy the requirements of the MIS. The instrument can pro-
vide effective dexterity and operation force for surgical procedures,
and reduces the difficulty of suturing, which conduces to improving
the efficiency and safety of the MIS. The limitations of the instru-
ment, especially miniaturization and bending range, will be modified
and more comprehensive analysis and evaluation will be imple-
mented in the future.
ACKNOWLEDGEMENTS
This work was supported by the National Key R&D Program of China
(grant no. 2019YFC0118003 and grant no. 2019YFB1311502), the
National Natural Science Foundation of China (grant no. 51805362
and grant no. 51875390).
CONFLICT OF INTEREST
The authors have no conflicts of interest to declare.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
ORCID
Yingkan Yang
https://orcid.org/0000-0003-2525-3269
Kang Kong https://orcid.org/0000-0001-5646-8807
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How to cite this article: Yang Y, Li J, Kong K, Wang S. Design
of a dexterous robotic surgical instrument with a novel
bending mechanism. Int J Med Robot. 2021;e2334. doi:10.
1002/rcs.2334
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