Silicone retractor with embedded force-sensing function for
attachment to surgical suction pipes
Toshio.Koyama,Takanobu.Iwai,Takeshi.Yoneyama,Hiroyuki.Kagawa,Yutaka.Hayashi,Mitsutoshi,Nak
ada,Tetsuyou.Watanabe,Member, IEEE

Abstract—A silicone retractor that can be attached to suction
pipes was developed in order to enhance the usability [1]. The
measurement of the retracting force is desired in order to avoid
damage to brain tissue due to an unexpected large force. This
paper presents a force-sensing embedded silicone retractor thatcan be attached to suction pipes. The developed silicone
retractor can provide three functions at the same time: suction,
retracting, and retracting force measurement. The force-sensing
system is based on a visualization mechanism that displays theforce as a colored pole motion. The surgeon can then roughly
estimate the retracting force. With a fiberscope, the retracting
force can be measured with a resolution of 0.05–0.3 N. The
retractor is made of silicone and has the advantages ofdisposability, low cost, and easy sterilization/disinfection. The
system was validated through finite element method analysis
and experiments.

neurosurgery, tissues are very soft and fragile, and damage tonormal tissues has to be avoided. Therefore, a force-sensing
system should be provided for the hybrid retraction/suction
device. The present paper presents a force-sensing function
embedded in a silicone retractor that can be attached to suction
devices. The main features are as follows.

I. INTRODUCTION

Multiple functions: Similar to [1], functions for suction,
retraction, and retracting force-sensing are provided at thesame time.

Recently, many medical devices for surgical operations
have become popular. For example, medical devices forneurosurgery include the electrical scalpel, forceps, retractor,
bipolar forceps, endoscope, and suction. These devices arevery convenient for the surgery itself, but surgeons are
required to change medical devices many times. Thisdecreases the usability for complex surgical procedures andincreases the surgery time, which places a burden on thepatient. Devices with more functions are desired. A key
resolution is for a single medical device to provide more than
two functions.

Force visualization: A mechanism to visualize the force
is utilized so that surgeons can understand the magnitude ofthe load on tissues just by looking. If an endoscope or camera
is used, the force can be quantified with high resolution.
Easy setup: The sensor system itself is constructed fromonly silicone parts and does not need any electric components.
It is easy to attach to the suction pipe and has the advantages ofdisposability, low cost, and easy sterilization/disinfection.

The remaining parts of this paper are organized as follows.
Related works are described below. The developed
force-sensing systems are described in Section II, and theexperimental results for validation are presented in Section III.
Section IV summarizes the results.

In neurosurgery, some of the most frequently used devices
are suctions and retractors. Retractors pull back tissues to
produce a visible field and space for surgery. Suctions suction
blood and soft tissues. Very frequent changes are usuallyrequired, and surgeons sometimes use suctions as a retractor.
Based on this situation, the neurosurgeons among the authors
previously developed a silicone retractor that can be attachedto the tip of suction devices [1] (see Fig. 1). By providing an
area for retracting to the suction tip, surgeons can not only
suction but also retract tissues at the same time. However, theforce applied by this device to the tissue is unknown. In
Figure 1. Suction pipe/device and silicone retractor for suction pipe
*Resrach partly supported by JST AS262Z.
T. Koyama, T. Iwai are with the Department of mechanical Engineering,
College of science and engineering, Kanazawa University, Ishikawa, Japan(corresponding author to provide e-mail: toshio83@stu.kanazawa-u.ac.jp;
iwai.t@stu.kanazawa-u.ac.jp ).
H. Kagawa, T. Yoneyama, and T. Watanabe are with School of
Mechanical Engineering, College of Science and Engineering, Kanazawa
University, Ishikawa, Japan (e-mail: kagawa@t.kanazawa-u.ac.jp;
yoneyama@t.kanazawa-u.ac.jp; te-watanabe@ieee.org )Y. Hayashi and M. Nakada are with Faculty of Medicine, Institute ofMedical, Pharmaceutical and Health Sciences, Kanazawa University,
Ishikawa, Japan

A. Related works
Many studies have focused on developing force-sensing
systems. One approach has been developing force and tactile
sensors for medical surgeries [2–7]. The objective of thisapproach is to provide haptic feedback to surgeons so that theycan easily manipulate the devices and find the tumor. One of
the most commonly used force-sensing systems is based on
strain gauges [8–10]. Electrical components have a significant
role in transferring the force information. However, medical

devices need sterilization or disinfection, which is difficult to
do with electrical components. In addition, extra parts such as
an amplifier are required, and the overall system is usuallylarge and expensive. Noise in the wiring for signal transfer isanother problem. Thus, several research groups havedeveloped force-sensing systems that are free of electricalcomponents. Takaki et al. [11] developed a forceps that
includes a force-sensing function based on force visualization
using moiré fringe patterns. Tadano and Kawashima [12]developed force sensation feedback system with a pneumatic
servo system. Kawahara et al. [13] developed an organ
stiffness measurement system that utilizes an air jet andendoscope. Peirs et al. [14] developed a forceps with anembedded force sensor based on a mechanism for detecting
the deformation of flexible optical fibers. Tada et al. [15]developed a force sensor based on detecting the change inillumination of a light source attached to an elastic frame.
Watanabe et al. [16, 17] developed a force-sensing system thatcan be attached to a fiberscope using a highly elastic fabric.
However there has been no system aimed at providing thesuction, retracting, and force-sensing functions at the sametime.
With regard to force visualization-based sensors in otherfields, Ohka et al. [18] developed a three-axis force sensor
where the force is estimated from the poses of conical feelers.
Kamiyama et al. [19] developed a force distribution sensor
where forces are detected from the poses of two-layer
spherical markers. However, the target ranges for the force
and size differ from those of the force sensors forneurosurgery. Miniaturization is an important issue, as notedin [6, 7].

Figure 3. Target situation II: retracting force is captured with fiberscope

II. FORCE-SENSING SYSTEM
A. Target situation
This study focused on the suction and retraction inneurosurgery. As shown in Fig. 2, the proposed silicone
retractor with the embedded force-sensing function works byattaching the silicone retractor to the tip area of the suction
pipe. The suction pipe suctions not only tumors but also bloodand soft tissues to clear the visible area. The silicone retractor
extends the visible area by retracting brain tissue that disturbs
the viewing area of the surgeons. Embedding a force-sensing
and visualization mechanism into the silicone retractor allows
surgeons to control the retracting force based on the received
information and avoid unexpected fracture or breaking of
normal brain tissues because of unexpected large forces.
In the situation shown in Fig.2, whether surgeon can get
the visualized force information depends on the viewing
angle of the surgeon. Then a fiberscope or endoscope is used
if quantitative force information is needed. This was anothersituation considered in this study, as shown in Fig. 3. The
visualized force information is captured by the camera in thefiberscope or endoscope, and the force values are calculated.
The obtained force value provides feedback to the surgeons.
In this case, the surgeons can understand the force
information with higher resolution and accuracy.
B. Design requirements
The design requirements for the proposed retractor are as
follows.
1.

The force can be visualized.

2.

Retraction and retracting force measurement can be
performed at the same time.

3.

The device can be attached to suction pipes.

4.

There should be no electric components.

5.

The dimensions should have a width of less than 30mm, length of less than 20 mm, and thickness of lessthan 15 mm.

6.

The force range is 0–0.3 N with a resolution of 0.05
N.

Figure 2. Target situation I: surgeon evaluates force visually

In order to consider the above two situations, a force
visualization-based mechanism is required. To enhanceusability, the device should be able to provide the suction,

retracting, and retraction force measurement functions at thesame time. If the silicone retractor can be attached to thesuction pipe, suction can be provided by the suction system.
The silicone retractor itself is then required to provide theretraction and retracting force measurement functions. In
order to facilitate easy setup and usage, a disposable andlow-cost silicone retractor is preferred. A disposable retractor
means easy sterilization and disinfection. For this purpose,the silicone retractor should not have any electricalcomponents. The nominal dimensions of the original silicone
retractor are a width of 23 mm, length of 18 mm, andthickness of 2 mm. Realizing the same dimensions is difficult.
As a preliminary step, the target size was set to be a width ofless than 30 mm, length of less than 20 mm, and thickness ofless than 15 mm. The required force range and resolution
were determined based on the experimental results of
conventional studies [8]–[10], where the typical force
magnitude was around 0.1 [N]. The force information
obtained by the silicone retractor is not for force control but
for display to surgeons. The resolution of 0.05 N was thus
considered to provide sufficient accuracy.

operators. The pole position is on the (second) stair so that thepole can enlarge and show the deformation whiletilting/moving without contacting any other areas/walls. The
maximum moving point is the tip of the pole: 𝑥 [mm]. The
moving distance depends on the magnitude of the retracting
force. Thus, when calibrating the relationship between theretracting force F [N] and moving distance of the pole tip x[mm], the magnitude of the retracting force can be derived
from x [mm]. Therefore, the moving distance of the tip of thepole can be used to visualize the force information.
D. Structure of silicone retractor with embedded
force-sensing function

C. Principle of force sensing

Figure 5. Schematic top and side views for structure of silicone retractor
with an embedded force-sensing function

Figure 4. Principle of force sensing

The developed force-sensing system is based on a force
visualization mechanism. Fig. 4 shows a schematic view ofthe principle of the force-sensing system. The X- and Y-axes
are defined in Fig. 4. F [N] denotes the force in the Y
direction and corresponds to the magnitude of the retracting
force. 𝑥 [mm] denotes the moving distance of the tip of thepoles (markers) in the X direction from the initial position.
The silicone retractor mainly comprises a base, deformation
part, pole, and contact part. The base is the reference for
deformation and is made of a relatively hard material. Thereis a hole at the center of the base so that the silicone retractor
can be attached to the tip of the suction pipes. The contact partadds the function of retracting a wide area and is made of a
relatively hard material. The force-sensing is available atroughed or curved surfaces because force is detected at thecenter area of the contact part. Note that we don’t consider thecase when retracting tissue with a part of contact part because
retractor is for extending viewing area and the entire part ofthe contact area is needed for the extending. The stair-likestructure for the deformation part enlarges and shows thedeformation resulting from the retracting force. When
retracting tissues, the stair-like structure deforms. The joint
part for each step is thin, and stress concentrations occur.
Because of the stress concentrations, large deformations canbe expected. The pole is attached to the stair as a marker that
displays the enlarged deformation as force information to theFigure 6. Overview of manufacture and assembly processes for silicone
retractor with embedded force-sensing function

TABLE I.

MATERIAL PROPERTIES IN FEM ANALYSIS

Base part
Deformation part
Contact part

Figure 7. Manufactured silicone retractor with embedded force-sensing
functionFig. 5 shows a schematic view for the structure of thesilicone retractor with an embedded force-sensing function.
The silicone retractor consists of a base, deformation part,contact part, and poles. All parts are made of silicone so thatthe retractor is disposable, low-cost, and easy to sterilize or
disinfect. The base and contact parts are made from hardsilicone, and the other parts are made from soft silicone. All
parts were made from base materials and hardeners produced
by Shin-etsu Silicone Company. The base part is transparent
so that the movement of the pole can be viewed from the top.
The pole was colored with K-COLOR-BL-70 produced by
Shin-Etsu Chemical Co., Ltd. Colored lines are drawn on thebase so that the operators can visualize the force limits. Thelines were drawn by colored pen harmless to human tissue. If
the tip of the pole goes over the line of the force limit, thismeans that the retracting force is over the allowablemaximum value (in this case, 0.3 N). By checking whether
the tip of the pole is over the line or not, the operator canmaintain the retracting force under the allowable maximum
value in order to avoid damage to brain tissues. The top viewis intended for observation by surgeons, while the side viewshould be captured with a camera. The dimensions of thesilicone retractor are a width of 28 mm, length of 20 mm, andthickness of 15 mm. The poles have a width of 2 mm, lengthof 3 mm, and thickness of 3 mm.
Fig. 6 shows an overview of the manufacturing andassembly processes. First, the poles are manufactured with amold. After the poles are placed into the holes in the mold forthe deformation part, liquid silicone is poured into the molds,
and the deformation part is manufactured. The base ismanufactured by utilizing the mold with a bar. The contact
part is also manufactured with a mold. For the manufacture of
the base, the base and deformation parts can be joined.
Similarly, the contact can be joined with other parts during its
manufacture. For easy manufacture of the prototype, bonds
were used for jointing. The manufactured retractor is shownin Fig. 7.
E. FEM analysis on relationship between retracting force
and moving distance of pole tip

Young’s modulus
[MPa]
0.5
0.2
0.5

Poisson’s ratio
0.3
0.3
0.3

Figure 8. Results of FEM analysis with applied load of 0.1 N

Finite element method (FEM) analysis was performed inorder to determine the relationship between the retracting
force F [N] and moving distance of the pole tip in the X
direction x [mm]. SolidWorks Simulation (SolidWorks) wasused for the analysis. The undersurface of the contact part was
fixed, and a load was applied at the hole in the base part in thenegative Y direction (see the coordinates in Fig. 4). The load
was changed from 0 N to 0.3 N in increments of 0.05 N. Table
I lists the material properties of each part. Note that thematerial properties were determined based on the stress–
strain diagram derived from experimental results. Fig. 8
shows the result when the applied load was 0.1 N. The largest
moving distance was obtained at the top left and right pointsof the right and left poles. Therefore, by checking the moving
distances of the pole tips, the force information can beobtained with high resolution and accuracy. In order toestimate the force value, the relationship between the load F[N] and moving distance of the pole tip x [mm] had to becalibrated. The moving distance of the pole tip x [mm] forevery F [N] was calculated, and the results are plotted in Fig.
13. The derivation of the relationship is described in the next
section with the experimental results.
III. EXPERIMENTAL EVALUATION
Experiments were conducted in order to validate thedeveloped silicone retractor with an embedded force-sensing
function and determine the relationship between theretracting force F [N] and moving distance of the pole tip (inthe X direction) x [mm]. Fig. 9 shows a schematic overview ofthe experimental setup, and Fig. 10 shows a photo of theactual setup. A suction pipe equipped with the developed
silicone retractor was attached to the automatic positioning
stage (IMADA MX2-500N), which precisely controlled themagnitude of the load by moving the suction pipe in thevertical direction. The silicone retractor was placed on thestage of the electronic weighing instrument (SHIMADZU
TW223N) to measure the load applied to the silicone retractor.
A digital camera (CANON PSS120BK) was used to capture
the movement of the poles in the silicone retractor. Table 2
presents the related specifications of the experimental
devices.
TABLE II.

RELATED SPECIFICATIONS OF EXPERIMENTAL DEVICESCameraElectronic weighing instrument
Automatic positioning stage

Resolution: 2816 2112 pixels
Resolution: 0.001 [g]Resolution: 0.01 [mm]Speed: 10 [mm/min](b) Load of 0.20 N
Figure 11. Photos of silicone retractor

B. Procedure to derive moving distance of pole tip in

Figure 9. Schematic view of experimental setup

silicone retractor
The image processing toolbox of MATLAB (MathWorks)
was used to derive the moving distance of the pole tips.
Imtool of MATLAB was used to obtain the pole positions andthe information for unit conversion. As shown in Fig. 12,
𝒑(𝐹) was the position of the top left point of the right pole,
where F was the applied load. Imtool was used to derive 𝒑(𝐹)for every load F. For the unit conversion, a ruler was also
captured, as shown in Fig. 12. The pixel value corresponding
to 10 mm was derived, and the unit conversion from pixels tomillimeters was conducted. The moving distance of the pole
tip in the X direction was derived with,
where 𝑝𝑋 (𝐹) denotes the X coordinate of 𝒑(𝐹). Note that the
movement of the X coordinate was significant, as shown in
Fig. 4.

Ruler

Figure 10. Photo of experimental setup
Pixel information

Figure 12. Photo to derive moving distance of pole tip with Imtool

C. Experimental relationship between retracting force andmoving distance of pole tip
0.35

0.3

Applied force [N]A. Procedure
First, the automatic positioning stage was moved so thatthe silicone retractor was in contact with the stage of theelectronic weighing instrument while the latter read 0.000 g(0.00 N). Then, the automatic positioning stage wascontrolled to increase the magnitude of the load by 0.05 N.
After the control, a photo was taken with the camera in order
to measure the moving distance of the pole tip x [mm]. This
procedure was repeated for loads of 0.00 N to 0.30 N at
increments of 0.05 N. The entire procedure was repeated fivetimes. In total, 35 photos were obtained. Fig. 11 shows thephotos when the load was 0.00 and 0.02 N as representative
results.

0.25
0.2
0.15
Measured Value

0.1

FEM results

0.05
0

0

0.5

1

1.5

2

2.5

Moving distance of tip of pole from the initial state [mm]Figure 13. Relationship between retracting force and moving distance of tip
of pole(a) Load of 0.00 N

Fig. 13 shows the results of the experiment (bluediamonds) and FEM analysis (red squares). Mean values witherror bar expressing standard deviation are given for theexperimental results. The regression curve for theexperimental results is given by𝐹 = −0.029 x 2 + 0.21 𝑥
Note that we don’t consider a force over 0.3[N] because it isover-load.
Table 3 presents the obtained coefficient of determination
and RMSE. The calculation was performed with the curve
fitting toolbox of MATLAB.
TABLE III.

PROPERTIES OF REGRESSION CURVE

Dimension ofpolynomial function

Coefficient ofdetermination: R2

RMSE (Root Mean
Squared Error)

2

0.99

0.013

D. Discussion
The results of the experiments and FEM analysis showeda small difference. This may be because it was difficult tomake the silicone retractors from the exact same materials as
those used for the FEM analysis. The shapes of the
manufactured silicone retractors were also not exactly thesame as those used in the FEM analysis. However, theobtained values were close to each other. Thus, the results ofthe experiments and FEM analysis were validated. The
moving distance of the pole tips monotonically increased
with the applied load. The regression results (Table 3)
showed that a quadratic function can express the relation very
well. Therefore, the retracting force can be estimated from themoving distance of the pole tips with a high degree accuracy.
The resolution of the force-sensing system was less than 0.05
N, and the sensor has a measurement range of 0–0.3 N. Fig.
11 (right) shows that the relation between the blue poleposition and line expressing the force limit can provide arough estimate of the force. Even if there is no camera or
fiberscope, a surgeon can estimate the retracting force using
the developed silicone retractor with an embedded
force-sensing function.
IV. CONCLUSION
In this paper, a silicone retractor with an embedded
force-sensing function that can be attached to a suction pipe
was presented. Suctions and retractors are the most frequently
used medical devices in neurosurgery. Recently, a device
combining the two functions was developed [1]. However, aforce-sensing function was not included. This paper presented
devices that can provide the three functions of suction,
retraction, and force measurement at the same time. The
force-sensing system is embedded into the silicone retractor,
which can be attached to the tip of a suction pipe. The force issensed through a force visualization mechanism. The
embedded color pole moves/tilts if a retracting force isapplied. The moving distance of the pole is associated withthe magnitude of the retracting force. The moving distance ofthe pole can be used to estimate the retracting force. The force
can be visualized, and the surgeon can then roughly estimate
the force. If a camera or fiberscope is used, the force can bemeasured with a resolution of 0.05 N up to a maximum valueof 0.3 N. Because the developed retractor is made of silicone,
it is disposable and low-cost, and sterilization anddisinfection are easy. The developed silicone retractor is
expected to not only reduce the number of times tools need tobe switched but also provide safer retraction. However, forpractical application, further miniaturization and evaluationin real situations are needed. These issues will be consideredin future works.
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