Incompressible liquid based Force sensible silicone retractor
attachable to surgical suction instruments
Toshio Koyama, Takeshi Yoneyama, Mitsutoshi Nakada, and Tetsuyou Watanabe, Member, IEEE

Abstract— This paper presents a silicone retractor, which isa continuation and extension of a previously developed systemthat had the same three functions as the old version: 1)
retracting, 2) suction, and 3) force sensing. These features makethe retractor a safe choice for use in neurosurgery. Suction isachieved by attaching the retractor to a suction pipe. The
retractor has a deformation area filled with an incompressible
liquid that is displaced in proportion to the extent of
deformation; fiberscopes or human eyes detecting thedisplacement get a visual representation of the force. The new
design improves on the old one in three ways—miniaturization,
made possible by the incompressible-liquid-based mechanism,and measurement of force distribution by distribution of theareas deformed by force. The system was validated byconducting experiments.

applicable cases.
This paper presents an extended version of the silicone
retractor that solved this issue. The key features of the new
silicone retractor include the following.
Suction pipe
Suction catheter

Silicone retractor
Tank

Pump

I. INTRODUCTION
Recently, several instruments are used in the medical field,e.g., electric scalpel, bipolar, endoscope, and suction device.
The number of different tools available leads to an increase in
the number of times an instrument is changed. Instrumentchanges can disturb a surgeon’s concentration and increase
the operation time for surgery, which presents a burden to thepatients. Hence, a reduction in the number of times aninstrument is changed is required.
A possible approach could involve embedding multiple
functions in an instrument. Based on this concept, wepreviously developed a silicone retractor that could be usedby attaching it to the tip of the suction pipe [1] as shown in Fig.
1. To prevent surgeons from applying an overload on a tissue,
the retractor had a force sensing function in addition to
retraction and suction. This function is important to avoid
damaging a normal (brain) tissue. The deformation part wasembedded in the silicone retractor. This part deformed whenload was applied to the retractor. Cameras or fiberscopes
visually captured the deformation. The system was based on aforce visualization mechanism [2]–[4]. However, the systemdrawbacks included the large thickness of the retractor, which
decreased the retracting function and limited the number of*Research supported by JST Astep AS262Z01299K and JSPS KAKENHI
Grant Number 16H04298.
T. Koyama is with the Department of mechanical Engineering, College ofscience and engineering, Kanazawa University, Ishikawa, Japan(corresponding author to provide e-mail: toshio83@stu.kanazawa-u.ac.jp).
T. Yoneyama, and T. Watanabe are with School of MechanicalEngineering, College of Science and Engineering, Kanazawa University,
Ishikawa,
Japan(e-mail:yoneyama@t.kanazawa-u.ac.jp;
te-watanabe@ieee.org )M. Nakada is with Faculty of Medicine, Institute of Medical,
Pharmaceutical and Health Sciences, Kanazawa University, Ishikawa, Japan.

Figure 1. Silicone retractor with the multifunctions of retraction, suction,and force sensing as developed in a previous study.

Force visualization mechanism utilizing incompressible
liquid: Incompressible liquid was installed into the retractor.
The tube was connected to the retractor to observe liquid level,
which is displaced accordance to the load. The load could be
visually captured by the liquid displacement. The
liquid-based system facilitated miniaturization of the force
sensor parts. This was a key element in reducing the
thickness.
Distributed force measurement: The miniaturization of
the force sensor parts enabled the embedding of five force
sensor parts in different areas of the silicone retractor.
Consequently, the distributed forces were measurable.
High usability: In a manner similar to extant literature [1],the silicone retractor could be easily set up by installing asuction pipe into the retractor’s hole. The retractor wascomposed entirely of silicone and connected to PTFE tubes.
Thus, the system benefits included the absence of electricalcomponents, disposability, easy sterilization/disinfection,
MRI compatibility, and low cost.
Multiple functions: In keeping with the retractor
developed in a previous study [1], the current retractor also
simultaneously provided the functions of retraction, suction,and force sensing via attachment to the suction pipe.
The rest of this paper is organized as follows. The
subsequent related works are presented followed by adescription of the proposed system. The experimental
calibration and validation are detailed. Finally, the results aresummarized.
A. Related works
This paper involves novel medical instruments with themultifunctions of retraction, suction, and force sensing. This
paper focused on the key mechanism involved, namely, theforce sensing system. Several force sensing systems related to
medical operations were developed by extant literature. A
detailed analysis can be obtained from several review papers[5]–[11]. The commonly used force sensing systems are
based on strain gauges [12]–[14]. Further, tactile/force
sensing systems based on magnet and magnetic sensors weredeveloped [15], [16]. Electrical components are notpreferable in medical operations due to the requirement ofsterilization or disinfection. Moreover, magnetic components
are not preferable in medical operations because of the MR
incompatibility. Additionally, large amplifiers and otherrelated components are required for the measurement. This
increases the size of the system as a whole and subsequently
increases the cost. Several approaches were proposed toresolve these issues.
Takaki et al. utilized moiré fringe patterns to visuallydisplay force information on forceps [17]. A pneumatic force
could be transferred via an air tube. Hence, a forceps with aforce sensing system and a tip area without electricalcomponents was developed [18], [19]. With respect tostiffness sensors, Kawahara et al. utilized air pressure and acamera/endoscope for estimating organ stiffness [20].
Acoustic information could also be transferred without any
electrical components. Fukuda et al. utilized acoustic
reflection for estimating tissue softness [21]. Additionally,
optical information could be transferred from the tip to theroots of the forceps without any electrical components. Peirs
et al. utilized optical fibers to transfer the visualized
deformation of a flexible structure [22]. Tada et al. detected
the peak illumination changes of a light source embedded to a
deformable structure [23]. Ohka et al. tracked the position andorientation of columnar and conical feelers made of silicone
through a camera [24]. Kamiyama et al. estimated the motionof two layered markers with cameras to obtain the magnitude,
distribution, and direction of load [25]. We utilized highelastic fabric for visualizing a small force magnitude [2], [3].
We also proposed a stiffness sensing system based on theforce visualization mechanism [4].

safer and easier operations. Fig. 2 shows an example in which
two functions, suction and retraction, are combined. The
silicone retractor is attached to the suction pipe, and surgeons
can handle the two functions without changing the instrumentby operating the suction pipe.
Fig. 3 shows the target situation where a silicone retractor
attached on the tip of a suction pipe pulls fluids (for instance,
blood in Fig. 3), thereby retracting periphery tissue. As thesilicone retractor has width, it can retract the tissue.
Furthermore, an incompressible liquid was embedded into thesilicone retractor for force sensing. The sensor parts wereconnected to tubes. If a (retracting) force was applied to theretractor, the sensor parts deformed and the liquid levelchanged. The change in liquid level inside the tubes could be
visually captured. However, the liquid level can be different
depending on the instrument’s angle. In addition, surgeons
may not be able to detect small changes in the liquid level.
Therefore, the quantitative force value could be obtained if acamera or fiberscope that is fixed on the suction pipe wasused. Surgeons could also approximately understand themagnitude of the retracting force by observation and avoid
the fracture in normal tissues due to an overload.
Suction pipe
Retract

Suck

Silicone retractor
Figure 2. How to use silicone retractor attached to the suction pipe

Eye
Looking

II. FORCE SENSING SYSTEM
A. Target situation
This study proposes a medical instrument with the aim ofsupporting suction and retraction in neurosurgery. Suction
devices are used to suck blood, saline, and soft tumor. A
retractor is used to retract tissues to extend the visible field forsurgeons. Operations with both devices are very popular andfrequent in neurosurgery; thus, frequent instrument changes
are commonly required. The effort of the surgeons is reducedby combining the two functions in an instrument. Moreover,embedding force sensing and display are useful in realizing

SuctionSilicone
retractor

Retracting

Blood

Figure 3. Target situation

B. Design requirements
1. Suction, retraction, and retracting force measurement
are available simultaneously.
2. There is no electrical component in the silicone
retractor.
3. The silicone retractor can be attached to the suction

pipe.
The force can be visually captured at the area awayfrom the tissue (retraction and suction areas).
5. The dimensions should include a width of less than 25mm, length of less than 15 mm, and thickness of lessthan 4 mm.
6. The force range should be 0.00 N–1.00 N, and theresolution should be 0.05 N.
7. Distributed forces could be measured.
The above points are the design requirements for thesilicone retractor that included the force sensing function. We
realized the three functions by attaching the force sensing
silicone retractor to the suction pipes. The absence ofelectrical components in the silicone retractor led to several
advantages including easy setup, disposability, easysterilization/disinfection, MRI compatibility, and low cost.
The force visualization mechanism was used so thatsurgeons could directly visualize the magnitude of theretracting force and avoid an overload on the tissues. In thepreviously developed system [1], visualized force
information was displayed on the silicon retractor. However,there were some cases where the silicon retractor could not beseen due to spreading blood or the occlusion of the peripherytissue that covered the retractor. To resolve this issue, thecaptured area for the visualized force was set away from thetissues.
This paper contributed in reducing the dimensions. The
previously developed retractor [1] had dimensions of a widthof 28 mm, length of 20 mm, and thickness of 15 mm. In thispaper, the requirement for thickness was dramaticallyreduced to 4 mm. Thus, the proposed system was available ina deep and narrow space where the previous system could not
be applied. The required range and resolution were the sameas the previous system [1] and were based on the studies of
Budday et al. [26]. The main objective included avoiding
overload, and the resolution of 0.05 [N] was considered to be
sufficiently accurate. In the previous system [1], only the total
force at the center area of the retractor (around the suctionpipe) could be measured. There could be the cases where thetarget tissue was nonuniform, and hence force distribution
was not uniform. To avoid fractures, measurement of
distributed forces is preferable.
4.

Back side

Colored Water
Tube
Sensor part (made of silicone)

F [N]The end of tube

x [mm]

Figure 4. Principle of force sensing.

Form of letter “V”

Figure 5. Shape of deformation for the developed force sensible silicone
retractor.

C. Principle of force sensing
Fig. 4 shows a schematic of the principle of force sensing.
A silicone bag filled with incompressible liquid (water) wasused to construct the sensor part. The internal portion of thesensor was connected to the tube. The tube end was open sothat water could freely move inside the tube. If force wasapplied at the sensor part, it deformed and water moved
toward the tube end due to the incompressibility of water.
Moreover, thanks to surface tension, the water level does notchange because of the motion of tube. Therefore, the moving
distance of water could be used to estimate the magnitude of
the applied force. The diameter of the tubes was so small thatthe moving distance of water could be captured. Note that thismethod was valid when the pressure of the magnitude (thestiffness of the sensor part) was equal to or lower than thecontact pressure (the stiffness of the target object/tissue), as
indicated by previous experimental results [27]. Therefore,
the system could have a dead zone where the water does not
move even if load was applied to the system. The dead zone
appeared when the load was small, specifically 0–0.2 [N].
Therefore, it was not a problem for avoiding the overload.
Furthermore, the system range and resolution depended on
the elasticity of the sensor part’s material.
Let F [N] be the magnitude of the retracting force, and x[mm] be the moving distance of the edge of the colored waterfrom the initial position. We calibrated the relationshipbetween F [N] and x [mm] in advance. Then, load (F [N])could be estimated by the displacement of water (x [mm]). To
simplify the detection of the displacement (x [mm]), coloredwater was used. By tracking the edge of the colored water, thedisplacement of water (x [mm]) could be determined. The
volume of water did not influence the relationship between F[N] and x [mm] on account of the incompressibility of thewater.
To measure the distributed forces, the five sensor parts
were installed as shown in Fig. 4. The retractor deformed in
the form of letter “V,” as can be seen in Fig. 5, when thetissues were retracted with the silicone retractor. This isbecause the tissues and the silicone retractor are soft; however
the suction pipe is hard. Therefore, the force was applied only
to a portion of the suction pipe. The five sensor parts werearranged such that the distribution of deformation could be
captured.

Non-through hole

16 mm

4mm

Hole for suction pipe
Hole for water (φ 1)Retracting part
Sensor part
1

2

3
4
Sensor Number

5 mm

22 mm

5

Tube
Inside diameter: 0.5 mm
Outside diameter: 1.0 mm

Figure 6. Top and side views for the structure of the force sensing silicone
retractor.

1mm

SiliconeBar

water was colored by an edible blue colorant (food coloring)produced by Kyoritsu Foods Co. Inc. The used tube was a
PTFE tube (TUF-100 series) produced by Chukoh Chemical
Industries Ltd. These parts did not include any electricalcomponents and were low cost, disposable, and easy tosterilize/disinfect. The five sensor parts are denoted by No.1,
No.2, …., No. 5 in increasing order from left to right in Fig. 6.
The dimensions of the developed silicone retractor had awidth of 22 mm, length of 16 mm, and thickness of 4 mm.
Fig. 7 shows the overview of the manufacturing andassembly processes of the proposed force sensing silicone
retractor. Fig. 8 shows the manufactured force sensing
silicone retractor. The retractor and sensor parts were madewith a monolithic molding. Initially, liquid silicone waspoured into the mold to manufacture the retractor and sensor
parts. The bars were inserted into the mold before the liquidsilicone solidified. After the silicone solidified, the bars were
removed, and the holes for a suction pipe and the sensor parts
were obtained. The colored water was poured into the holes ofthe sensor parts and tubes. Finally, bonding joined thesilicone part and the tube. This was a prototype, and a bond(CA-522) produced by CEMEDINE Co., Ltd. was used toglue the tube into the sensors. A biocompatible bond could
also be used.

Silicone retractor

Tube
Colored Water

Endoscope

Force sensible
silicone retractor

Down

Tubes

Bump

Figure 7. Overiew of the manufacturing and assembly processes for the
proposed force sensing silicone retractor.

Force gauge
Automatic positioning stage

Stage

Figure 9. Schematic view of the experimental setup

III. EXPERIMENTAL CALIBRATION

Figure 8. Manufactured force sensing silicone retractor.

D. Structure of force sensible silicone retractor
Fig. 6 shows a schematic of the structure of the silicone
retractor developed in this paper. As shown in Fig. 6, theretracting part, five sensor parts, colored water, and tube wereused to construct the developed silicone retractor. The
retracting and sensor parts were made of silicone (Liquidsilicone: KE-1308, Hardener: CAT1300, Liquid silicone [g]:Hardener [g] = 1:0.06) produced by Shin-Etsu Co., Ltd. The

We conducted experiments to derive the relationshipbetween the applied loads on the sensor parts (F [N]) and themoving distance of the edge of the colored water (x [mm]).
Fig. 9 shows a schematic of the experimental setup. Fig. 10shows a photograph of the setup. The developed silicone
retractor was attached to a suction pipe. The suction pipe was
attached to the automatic positioning stage (IMADA
MX2-500N). The force gauge (IMADA MX2-500N) was
fixed to measure force in the vertical direction. The
handmade stage was placed on the force gauge. The stage had
a bump to calibrate the performance of each sensor part, asshown in Fig. 9. The width of bump was the same as that of
the sensor part for calibration. The endoscope (Vktech) was
attached to the automatic positioning stage so that thephotograph of the colored water in the tubes could be
captured at the same position (the relative pose and positionof the tubes from the endoscope could be the same). Table 1

presents the related specifications of the experimental
instruments.

Initial state(a) Load of 0.00 [N]Figure 10. Photo of the experimental set up.
TABLE I.

SPECIFICATIONS OF THE EXPERIMENTAL INSTRUMENTS

Instruments
Endoscope
Force gauge
Automatic positioning stage

Specification
Resolution: 640 × 480
Resolution: 0.01 N
Resolution: 0.01 mm
Speed: 10 mm/min

A. Procedure
The contact between the sensor part of the silicone retractor
with the bump of the stage was enabled by the automatic
positioning stage. The position of the stage was controlled sothat the value of the force gauge was 0.00 N. Then, theautomatic positioning stage was controlled to increase theload by an increment of 0.05 N until it reached 1.00 N or thecontact (between the bump and sensor part) was lost. As
shown in Fig. 5, the silicone deformation increased in theform of a letter “V.” Then, a large deformation could result ina deviation of the contact area and lose the contact. The
contact was lost when the load was 0.55 N for sensor part Nos.
2 and 4, and 0.2 N for sensor part Nos. 1 and 5. At each step,
the photograph of the colored water in the tube was taken to
estimate the moving distance of the edge of the colored water.
The experiment was repeated five times for every sensor part.
B. Deviation of the moving distance of the colored water

Load of 0.00 [N]Moving distance of theedge of water x [mm]Load of F [N]Figure 11. Schematic of the procedure to obtain the moving distance of thecolored water in the tube.

(b) Load of 1.00 [N]Figure 12. Image of movement of colored water

Fig. 11 illustrates the procedure to derive the moving
distance of the colored water. The displacement of the edge
corresponding to the moving distance of the colored water
was obtained by focusing on the edge of the colored water. As
indicated in Fig. 11, the position when the load value was
0.00 N was set as the reference, and the displacement from thereference position was derived. The image analysis was
conducted by Adobe Photoshop. Fig. 12 shows images of theload at 0.00 and 1.00 N as representative results.
C. Calibration based on the experimental results
Fig. 13 shows the experimental results for the relationshipbetween the applied load (F [N]) and the moving distance ofthe edge of the colored water (x [mm]). The mean values withthe error bar expressing the standard deviation are shown.
The following model was used for the regression:
𝐹 = 𝑎3 𝑥 3 + 𝑎2 𝑥 2 + 𝑎1 𝑥 + 𝑎0
(1)
where 𝑎0 , 𝑎1 , 𝑎2 , and 𝑎3 are the parameters. The fitting wasdone by the curve fitting toolbox of Matlab. The parametervalues for each sensor part are shown in Table II, while Table
III shows the results for the fitting accuracy
From Table III, it could be observed that sufficient fitting
accuracy was obtained for the design requirements of theresolution of 0.05 N. As shown in Fig. 13, there were somecases where the increasing rate of the load with respect to themoving distance decreased to become almost flat (constant).
This was due to the buckling of the sensor part. To take theeffect of the buckling into account, a three-dimensional
polynomial function was used as a regression model. The
curve for sensor part No.2 was different from that for sensor
part No.4. This was due to the manufacturing inaccuracy ofhole position, which affected the directions of the applied
forces and the corresponding deformation.

A. Results
Let 𝐹i be the load 𝐹 for the sensor part No. 𝑖 and 𝐹i at 𝑥 isdenoted by 𝐹i (𝑥) . The distributed forces ( 𝐹i (𝑥) ) were
calculated by using (1). The total force (𝐹t (𝑥)) was calculatedby the following expression:𝐹t (𝑥) = 𝐹1 (𝑥) + 𝐹2 (𝑥) + 𝐹3 (𝑥) + 𝐹4 (𝑥) + 𝐹5 (𝑥)
(2)

1.2

Magnitude of the load F [N]

1

1

2
3
4
5
Sensor Number

0.8

0.6
NO.1

NO.2

NO.3

NO.4

0.4

0.2
NO.5

0
0

10

20

30

40

50

We compared the derived total force (𝐹t (𝑥)) with the actual
value of the load. The results were shown in Fig. 15. Fig .16
shows the corresponding distributed forces.

Moving distance of the Water edge x [mm]Numberof thesensor
part
No. 1
No. 2
No. 3
No. 4
No. 5

PARAMETER VALUES FOR THE REGRESSION MODEL

𝑎3

𝑎2
−3

1.78× 10
7.32× 10−4
3.21× 10−5
2.88× 10−5
7.74× 10−3
TABLE III.

Number of the sensor
part
No. 1
No. 2
No. 3
No. 4
No. 5

𝑎1
−2

-1.86× 10-1.33× 10−2
-2.44× 10−3
-1.61× 10−3
-4.51× 10−2𝑎0−2

8.76× 10
1.15× 10−1
6.32× 10−2
3.80× 10−2
1.04× 10−1

−2

2.72× 10
4.17× 10−2
5.17× 10−2
9.08× 10−2
2.93× 10−2

FITTING ACCURACY OF REGRESSION
Coefficient ofdetermination (R2 )
0.99
0.99
0.99
0.99
0.99

RMSE (Root Mean
Squared Error)
0.005
0.025
0.027
0.018
0.006

IV. EXPERIMENTAL EVALUATION
Fig. 14 shows the photograph of the experiment ofretracting gelatin. To confirm the validity of the developed
force sensing system, we conducted experiments to check
whether the applied load could be estimated when retracting
gelatin. The experimental setup was the same as the one
shown in Fig. 9, except for the stage. The flat stage was usedinstead of the stage with bump. The gelatin (Maruha Nichiro
Corporation, blending ratio: gelatin powder 2 [g] and water
25 [ml]) was located on the stage. Initially, the silicone
retractor was in contact with the gelatin with a load of 0.00 N.
The load was increased in increments of 0.05 N by controllingthe automatic positioning stage. At each step, a photograph
was taken to derive the moving distance of the water for eachsensor part. The experiment was conducted five times.

1

0.8

Actual valueFirst trial

0.6

Second trial
0.4

Third trial
Fourth trial

0.2

Fifth trial

0
0

0.2

0.4
0.6
0.8
Actual load value [N]1

1.2

Figure 15. Comparison of the derived total force from the proposed sensing
system with the actual value of the load.
Derived force Fi at each sensor [N]TABLE II.

Derived total force [N]1.2

Figure 13. Relationship between the applied load (F [N]) and the moving
distance of the edge of the colored water (x [mm]).

0.4
0.35

0.3

NO.1

0.25

NO.2

0.2

NO.3

0.15

NO.4

0.1

NO.5

0.05
0
0

0.2

0.4
0.6
0.8
Actual load value [N]1

1.2

Figure 16. Comparison of each force derived from the proposed sensing
system with the actual load value.

B. Discussion
As indicated in Fig. 15, there was a small difference
between the derived total force and actual load. A resolutionof 0.05 N was obtained, suggesting the validity of thedeveloped force sensing system. The reason for the difference
could be the low resolutions of the endoscopes. Note that theforce sensing system worked between 0.2 and 1.0 N. As
observed in Fig. 13, each sensor could not work in the range
0.0–0.05 N. As illustrated in Fig. 15, the total forces (under
0.2 N) were distributed, and the load for each sensor part
became small (under 0.05 N). This could be the reason for thedead zone of 0–0.2 N.
Fig. 16 shows that the distributed forces could be observed.
The difference between sensor part Nos. 2 and 4 was due tothe manufacturing inaccuracy of the hole position (asmentioned above).
V. CONCLUSION

Figure 14. Photo of the experiment of retracting gelatin.

This paper presented a novel instrument for neurosurgery.

The proposed instrument had three functions: retraction,
suction, and force sensing. Suction instruments and retractors
are frequently used in neurosurgery. To reduce the number oftimes an instrument is changed and enhance operation safety,
we developed a silicon retractor with force sensing [1]. This
worked by attaching the instrument to the tip of a suction pipe.
The system had the three key functions, but its large thicknesspresented a serious problem by limiting its usability. To
overcome this drawback, this paper presented a new force
sensible silicon retractor, which also worked by attaching it to
the suction pipe. The usage of an incompressible liquid was
the key solution for reducing the thickness. The sensor partwas constructed by a silicone bag with the liquid, namely,
colored water. Given the incompressibility, the deformationof the silicon bag resulted in the movement of water. The
applied force was estimated by visually detecting thedisplacement of the colored water. The obtained sensor
resolution was less than 0.05 N and the range was 0.05–1.0 N.
Other features of the sensor system included its low cost anddisposability. Five additional sensor systems were installed
for measuring the distributed forces. The experimental results
showed the validity of the system. When the total load wasderived, the range 0–0.2 N was a dead zone. The dead zone ata low range is not a serious issue in avoiding the overload andensuring the safety of operations. However, it is preferable forforce sensing to be available at a low range. Future work
could include the improvement of sensing at a low range.

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