A Method for Obtaining Contact Force between Catheter Tip and Vascular Wall in Master-slave Robotic System

Xiaoliang Jin1, Shuxiang Guo1,2,3*, Jian Guo3*, Peng Shi1 and Dapeng Song1 1Department of Intelligent Mechanical Systems Engineering, Faculty of Engineering and Design, Kagawa University,

2217-20 Hayashi-cho, Takamatsu, Kagawa, Japan. 2Key Laboratory of Convergence Medical Engineering System and Healthcare Technology, the Ministry of Industry and

Information Technology, School of Life Science, Beijing Institute of Technology, Beijing, China. 3Tianjin Key Laboratory for Control Theory and Application in Complicated Systems and Intelligent Robot Laboratory,

Tianjin University of Technology, Tianjin, China E-mail: s19d505@stu.kagawa-u.ac.jp, *corresponding author: guo.shuxiang@kagawa-u.ac.jp

Abstract - In the research of vascular interventional surgery robotic system, the force information acquisition of surgical tools (guidewires and catheters) has a significant affect on the safety of surgery, especially the contact force between the catheter tip and vascular wall. Therefore. In order to obtain this contact force, we noticed the pressure sensitive rubber because of its piezoresistive effect and easy processing into any shape. a force sensor based on the pressure sensitive rubber was developed in this paper. Once a force is applied on the force sensor attached to the catheter tip, the contact force can be approximated by the piezoresistive effect of the pressure sensitive rubber. The feasibility of the developed force sensor was verified by the experiments in “Vitro”. And the results showed that the force sensor was developed in this paper can accurately detect the contact force between the catheter tip and vascular wall at different positions in the vascular model, the extraction of contact force information of catheter tip is of great significance to the safety of surgery, and the application potential of pressure sensitive rubber. Index Terms - Guidewires and catheters. Contact force. Force sensor. Pressure sensitive rubber and System safety.

I. INTRODUCTION

Nowadays, vascular diseases have brought a great mental and economic burden to the patient, there are two ways to treat vascular disease, one is traditional open surgery, and the other is VIS (Vascular Interventional Surgery). Compared with the traditional surgery, the VIS has the characteristics of minimal trauma, less blood loss and quick recovery, it greatly reduces the patient's pain. The basic process of vascular interventional surgery is that a surgeon operates a guidewire and a catheter in the operating room to reach the location of the lesion for treat ment, which needs a surgeon with rich experience. In addition, Because VIS takes a long time, it is also a great challenge for surgeons. Once a mistake occurs, it will bring serious dangers for patients. In order to overcome the above difficulties, more and more researchers have focused their attention on robotics. Replacing the surgeon with a robot to complete the operation not only relieves the surgeon’s pressure but also protects the surgeon from X-ray radiation. In addition, a robotic system with haptic force feedback also plays a vital role in training a novice surgeon. So, the vascular interventional surgery robotic system has become a hot topic because of its high application prospect and research value.

In recent years. The master-slave robotic systems for VIS has been known for its high precision, high stability and high comfort, as well as its ability to reduce the radiation exposure to patients and doctors. There are some related researches, the research group led by Prof. Guo has achieved good results in the research of haptic force feedback, and the development of master-slave robotic system. Among them, the research group of Kagawa University developed a master manipulator based on magnetorheological fluids. The manipulator can generate a tactile force acting on the doctor's hand according to the force information of the slave side, which improves the operation transparency and safety during the surgery [1]-[4] the research group of Beijing Institute of Technology developed a surgical robot with the collaborative operation function of a guidewire and catheter, the robot completed the evaluation experiment in the human body in 2019 and proved to have good application potential [5]-[11], and the research group of Tianjin University of Technology developed a monitoring system to improve the performance of the robotic system through the combination of force feedback and vision feedback, and bring convenience to the operator [12]. In addition, The Hamlyn Centre for Robotic Surgery, Imperial College London, a versatile robotic platform for endovascular interventions was developed by M. E. M. K. Abdelaziz et al [13], the robot can easily assemble the surgical instrument and quickly switch between the manual and robotic operations. The master manipulator used in this robotic system was developed by G. Dagnino et al [14], It realized the haptic force feedback through a voice coil motor. Chinese Academy of Sciences, a framework based on the HMM (Hidden Markov Model) for the analysis of surgeon’s natural behaviors and the recognition of surgical instrument motion patterns during PCI (Percutaneous Coronary Intervention) is proposed by X. Zhou et al [15]-[17]. This research has demonstrated the potential of relevant behaviors for encoding surgical tool motions. Institute of Biomedical Engineering, Polytechnique Montréal, a method named fringe field navigation was proposed by A. Azizi et al [18][19], which solved the problems of low rigidity and poor operability of instruments in narrow vascular. Department of Intelligent Robot Engineering, Hanyang University, a robotic system with the haptic function was developed by J. Woo et al [20], the robotic system can manipulate a steering catheter to accurately select the vascular branches, reducing the radiation exposure time for patients and surgeons. At present, there are

some mature robotic systems have been used in the clinic. The list of some typical robotic systems is shown in TABLE I [21], the research of vascular interventional surgery robotic system has always been significant and challenging. Therefore, more and more researchers devote themselves to their research. The main contribution of this paper is to develop a force sensor for obtaining the contact force between the catheter tip and vascular wall in the operation. The design principle of the force sensor is described in detail. The force sensor is proved to be effective and feasible, and it can extract the contact force and determine the position of the contact point by the sudden change of the force value. The remained section is organized as follows. Robotic system overview is introduced in Section II. Development of the force sensor is described in Section III. Experiments and results are described Section IV. and Section V is conclusion and future work.

II. MASTER-SLAVE ROBOTIC SYSTEM

The concept of a master-slave robotic system is shown in Fig.1. The whole system includes the master side and the slave side. The surgeon operates a master manipulator has the haptic force feedback function to control the slave robot complete the surgery. The force between the vascular and the surgical tools (Guidewire and Catheter) will be detected by the force sensors on the slave robot and fed back to the master side, the master manipulator will generate the haptic force according the force of slave side and act on surgeons’ hand. In addition, a camera is used to feed back the scenes of slave side in real time. With the combination of the force and vision feedback, the surgeon will perform the surgery safely and efficiently. The force from the slave side can be divided into three categories, the contact force, the friction force and the viscous force. However. The contact force of the catheter tip is the most important factor affecting the safety of the surgery. Therefore, the focus of this paper is to extract the contact force during surgery.

TABLE I ROBOTIC SYSTEMS FOR CLINICAL APPLICATIONS

Fig. 1 The concept of master-slave robotic system

Fig. 2 The master manipulator based on MR (Magnetorheological) fluids A. Master Manipulator The master manipulator of master-slave robotic system is shown in Fig.2. This device was developed based on previous research of our group [1]-[4], includes a guidewire control unit and a catheter control unit. These two controls both have two degrees of freedom, linear and rotation motion. The linear and rotation motion of the input guidewire and catheter is detected by the encoder (MTL, MES020-2000P, Japan). So, there are 4 encoders located on the master manipulator. The outstanding contribution of this master manipulator is to implement haptic force feedback to surgeons based on MR (magnetorheological) fluids. The design and performance evaluation experiment of the master manipulator based on MR fluids are introduced in the previous research work of our group. Therefore, it will not be described in detail here. B. Slave Robot The developed slave robot of master-slave robotic system is shown in Fig.3, includes a guidewire manipulation unit and a catheter manipulation unit. this slave robot can achieve three modes of motion, linear and rotation motion of the guidewire, linear and rotation motion of the catheter, simultaneous linear and rotation motion of the guidewire and catheter. These three modes can basically meet the needs of collaborative operating of a guidewire and catheter. During the surgery, the guidewire supports and guides the catheter, the collaborative operating of a guidewire and catheter enables them to quickly pass through the narrow space and accurately locate the target position. And whether linear or rotation motion, the main driving component is a stepping motor with a resolution of 0.36 degrees (ASM46 AA, ORIENTAL MOTOR, Japan) and synchronous belt. The advantage of using synchronous belt drive is that there is no sliding, high transmission efficiency and accurate transmission ratio. In addition, the force detection mechanism of this robot can detect the force information of the guidewire and catheter in vascular in real time through a load cell (TU-UJ5N, TEAC, Japan), the detection range of the load cell is -5N to 5N. There is a grasper device inside the force detection mechanism, it is driven by two small hollow stepping motors (LIKO MOTOR, 20BYGH30-0604A-ZK3M5), the designed grasper device like a doctor’s hand, which can clamp or release the guidewire and catheter without damaging them. The detailed design method of grasper device and the simulation results has been described in Ref. [30].

Fig. 3 The slave robot for operating guidewire and catheter

Fig. 4 The design drawing and physical drawing of the force sensor

III. DEVELOPMENT OF A FORCE SENSOR

The type of forces on the catheter in the vascular basically include 3 categories, (1) the contact force between the catheter tip and vascular wall (2) the friction force between the catheter and vascular wall and (3) the viscous force, which comes from the blood. Among them, the contact force between the catheter tip and vascular wall is the most important factor, and its will directly affect the safety of surgery. So, in this section, a force sensor based on the pressure sensitive rubber is developed for the robotic system described in section II to extract the contact force between the catheter tip and vascular wall. A. The Design of Force Sensor The pressure sensitive rubber is a sensitive material with a resistance strain effect. So, it is often used to make a series of components, such as pressure sensors, tactile sensors, contact switching elements and so on. In addition, because it also has excellent processing properties and can be easily cut into any shape. Therefore, this paper chooses pressure sensitive rubber as the core material of force sensor. The design of force sensor is shown in Fig.4 (a) and Fig.4 (b), two copper electrodes are used as signal output terminals, and the pressure sensitive rubber is filled between them. The ring force sensor has an outer diameter of 4 mm and a length of 6 mm, and Fig.4 (c) is the physical drawing and installation drawing of force sensor. This force sensor is mounted on the catheter tip. The advantage of this is that it does not affect the collaborative operation of the guidewire and catheter and can also successfully extract the contact force between the catheter tip and vascular wall during the surgery. The catheter adopted in this paper is 4Fr.

B. The Working Principle of Force Sensor The force sensor is developed based on pressure sensitive rubber. The measuring circuit of the force sensor is shown in Fig.5, it is a classic measuring circuit. The input voltage of the circuit is 5V and the resistances R2, R3 and R4 are 330kΩ, the resistance R1 can be considered as a pressure sensitive rubber sensor. When the bridge is balanced, that is, R1 = R2 = R3 = R4, the output voltage U0 = 0. When the bridge is unbalanced, the output voltage U0 can be calculated by the formula (1).

3 20

3 4 1 2

( )iR RU U

R R R R= −

+ + (1)

In the following part, we will get the relationship between

the output voltage and contact force through the calibration experiments, and calculate the magnitude of the contact force between the catheter tip and vascular wall in according to the change of output voltage U0.

C. The Calibration of Force Sensor The calibration of force sensor is shown in Fig.6. There is a load cell with standard output (-5N ~ 5N) that is attached to the plate. The output shaft of the load cell was pushed through the developed force sensor. The data of the two sensors were collected at the same time, and the sampling time was 200ms. Experimental results are shown in Fig.7. We used Matlab software (Version: 2014) to fit the results obtained from the calibration experiment and the related mathematical formula obtained is shown in formula (2).

3 20 0 00.2184 0.4449 0.8171 2.388contF U U U= − + − + (2)

Where, Fcont represents the contact force between the catheter tip and vascular wall, and U0 represents the output voltage of measuring circuit.

Fig. 5 The measuring circuit of force sensor

Fig. 6 Experimental setup for calibration of force sensor

Fig. 7 Experimental results for calibration of force sensor

IV. EXPERIMENTS AND RESULTS

It is necessary to verify the developed force sensor. So, in this section, the experiment in vascular model was completed, the results were obtained and analyzed, the performance of the force sensor was discussed. A. Experimental Setup The catheter tip is easy to contact with the vascular in the curved and thin vascular environment. In order to pass through the narrow vascular space quickly and safely the collaborative operating of a guidewire and catheter is necessary. The guide wire guides and supports the catheter, the catheter follows the path of the guidewire, and their mutual coordination makes it easier for them to correctly select vascular branches and pass through the narrow vascular space. The vascular model has an inner diameter of 5 mm and an outer diameter of 7 mm was selected in this experiment is shown in Fig. 8, the catheter and guidewire was pushed from point A, go through point B and point C, and arrive the target point D by the slave robot. The force sensor was installed at the catheter tip and monitors its force information in real time. The data of force sensor were collected, and the sampling time was 200ms.

Fig. 8 Experimental Setup. The starting point is A and target point is D

B. Experimental Results As shown in Fig.9. The contact force between the catheter tip and vascular wall, it corresponds to Fig. 8. From point A to point B, the catheter tip has been in contact with the vascular wall. This moment, there was no obvious collision, therefore, the contact force is in a constant state. At point B, the catheter tip collides with the vascular wall for the first time, therefore, the contact force suddenly increases. Then, when the catheter tip passed point B to point C, because point C was difficult to pass, we performed push-pull and rotate the catheter. At this time, there were two obvious collisions, therefore, between the point C and point D, there are two peaks appeared. We defined the position of the peak as the point of collision and mark it as 1st, 2nd and 3rd. It can be seen from the experiment results that the developed force sensor has initially achieved the extraction of the contact force between the catheter tip and the vascular wall and has achieved the purpose of the research, indicating the feasibility of the design of the force sensor. C. Discussion In vascular interventional surgery, the force on the guide wire and catheter are more complex. Among them, the contact force between the catheter tip and vascular wall is particularly important, which directly affect the safety of surgery. In order to extract the contact force, a force sensor is developed based on the pressure sensitive rubber. The developed force sensor was proved to be effective in detecting the force at the contact point (as shown in Fig.8 and Fig.9), By analyzing the contact force, we can judge the insertion status of the catheter and the safety of the current operation. It also implies that the method of obtaining the contact force between the catheter tip and the vascular wall proposed in this paper is feasible. It is worthy to emphasize that the developed force sensor is only the preliminarily realization of the extraction of contact force. But there are some limitations need to be improved, for instance, the size of the force sensor needs to be reduced in the future research. In addition, the accuracy of the force sensor maybe affected due to the characteristics of the material, and the measured contact force does not consider the hysteresis of the force sensor, the accuracy of the force sensor also needs to be improved in the future research.

Fig. 9 The contact force detected by the developed force sensor

A B C D

1st 2nd 3rd

Collisions for 3 times

V. CONCLUSION AND FUTURE WORK

In this paper, A force sensor was developed based on the pressure sensitive rubber to extract the contact force between the catheter tip and vascular wall. The developed force sensor was manufactured in a ring shape and mounted on the catheter tip to monitor the contact force between the catheter tip and vascular wall in real time. We have completed the calibration experiment and the collision detection experiment of the force sensor. Experimental results showed that the design method of the force sensor is feasible and can effectively detect the force information of the contact point. The contact force information between the catheter tip and vascular wall is important. In this paper, the purpose of extracting the contact force is achieved preliminarily by using the developed force sensor. It provides a basis for the safety of vascular interventional surgery. In the future, the hysteresis of developed force sensor will be studied and the accuracy of force sensor will be improved.

ACKNOWLEDGMENT

This work was supported by National High-tech Research and Development Program (863 Program) of China (No.2015 AA043202), SPS KAKENHI (15K2120), National Natural Sci ence Foundation of China (61703305), Key Research Program of the Natural Science Foundation of Tianjin (18JCZDJC385 00), and Innovative Cooperation Project of Tianjin Scientific and Technological Support (18PTZWHZ00090).

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