Development of In-Pipe Micro Mobile Robot Using Peristalsis Motion Driven by Hydraulic Pressure

Conference paper
Part of the Mechanisms and Machine Science book series (Mechan. Machine Science, volume 2)

Abstract

In this paper, we developed an in-pipe mobile robot which operates in the blood vessels. For this purpose, the diameter of the robot was designed to be equivalent to or smaller than that of blood vessels, assuming the use of the robot in the human aorta. Thus, the diameter of the robot was designed to be less than 2-3 mm. Furthermore, we adopted a mechanism that operates normally even when machining accuracy is not good. Taking into account the scale effect, a design was considered in which the robot is operated in an environment where the effects of friction and surface tension are large and the effects of gravity and inertia are small. As a result, we did not select a mechanism that moves with the use of wheels, but instead adopted a mechanism that utilizes peristalsis to move the robot. In addition, due to the possibility of leakage of electric current, we did not use an actuator operating on an external power supply but one which is operated by hydraulic pressure. With these conditions, we fabricated an in-pipe mobile robot, and the operational characteristics of the robot were studied.

Keywords

In-pipe robot Scale effect Peristalsis motion Hydraulic pressure Blood vessel 

1 Introduction

We developed an in-pipe mobile robot which operates in the blood vessels. For this purpose, the diameter of the robot was designed to be equivalent to or smaller than that of blood vessels, assuming the use of the robot in the human aorta. Thus, the diameter of the robot was designed to be less than 2-3 mm. Furthermore, we adopted a mechanism that operates normally even when machining accuracy is not good. Taking into account the scale effect, a design was considered in which the robot is operated in an environment where the effects of friction and surface tension are large and the effects of gravity and inertia are small. As a result, we did not select a mechanism that moves with the use of wheels, but instead adopted a mechanism that utilizes peristalsis to move the robot. In addition, due to the possibility of leakage of electric current, we did not use an actuator operating on an external power supply but one which is operated by hydraulic pressure. In this study, we propose a mobile device, a mobile microrobot, that has a pistonlike hydraulic pressure generator at its end and achieves peristaltic motion by changing its body length and width by injecting and ejecting a driving fluid into and from the driving section of the device.

2 Travelling Mechanism and Actuator Suitable for Operation in the Blood Vessel

In designing an in-pipe mobile robot, the travelling mechanism and an actuator suitable for operation in blood vessels were developed. In selecting the travelling mechanism, crawlers and wheels were not considered. Wheels are not suitable for driving the robot in blood vessels whose internal structures are complicated. For example, when wheels are used, it is difficult for the robot to climb over a bump with a height greater than the radius of the wheels [1-3]. The travelling mechanism using a crawler, which is considered to be suitable for traversing rough surfaces, may cause some damage on the internal walls of the blood vessels. Furthermore, both mechanisms require highly accurate manufacturing processes for fabrication of wheels and shaft holes, thus high precision processing is required, and this is an obstacle for miniaturization of the robot.

On the basis of the above considerations, we studied the travelling mechanism of nematodes and earthworms, which use peristalsis to move their bodies.

Now, we consider the scale effect. In general, when the diameter of a machine decreases to 2-3 mm, the effects of inertia and gravity decrease and the effects of friction and surface tension increase. In particular, in water whose viscosity is higher than that of air, these phenomena are apparent. When we study the traveling mechanism of microorganisms in water, we find that they use flagella or generate peristaltic motion. For this study, we noted that many organisms of this size, i.e., nematodes, use peristalsis, in which the driving mechanism shrinks and stretches the body in the direction of movement, and we attempted the fabrication of a similar system to mechanically drive the robot.

In selecting an actuator, considering the use of the robot in the human body, we selected a driving mechanism that does not require heat or electricity, as even a very weak voltage may have a significant effect on the body. Considering various factors, we selected a hydraulic pressure driving mechanism that makes use of an isotonic sodium chloride solution.

3 Mechanisms of Prototype Devices

3.1 Principle of Motion

In this study, we propose a mobile device, a mobile microrobot, that has a pistonlike hydraulic pressure generator at its end and achieves peristaltic motion by changing its body length and width by injecting and ejecting a driving fluid into and from the driving section of the device. We develop an in-pipe mobile device with peristaltic motion, which comprises two segments that are arranged in series in the direction of motion. Each segment is made of crude rubber or silicon, which are highly compatible with living bodies; therefore only physiological saline solution, as the driving fluid, and the crude rubber or silicon are inserted into living bodies. The details of the peristaltic motion are as follows. First, the driving fluid is injected into the rear segment to allow it to expand in both forward and circumferential directions; thus, the rear segment becomes thick and long. Next, the driving fluid is also injected into the front segment, causing it also to become thick and long. At this time, the thickened rear segment is in contact with the inner wall of a pipe and remains stationary because of friction, causing the front segment to be pushed forward. Subsequently, the driving fluid is evacuated from the rear segment to cause it to contract; thus, the rear segment becomes thin and short. At this time, the thickened front segment comes into contact with the inner wall of the pipe and is thus stationary because of friction; therefore, the contracted rear segment is detached from the inner wall and drawn forward owing to the decreased friction. Extensional waves propagate from the front to the rear segments during the repetition of these motions, causing the two-segment device to move by peristaltic motion. Figure 1 shows a schematic of the principle of the two-segment mobile device.
Fig. 1

Schematic illustration of principle of motion

3.2 Experimentally Fabricated Robot

Figures 2(a) and (b) show the in-pipe mobile robot fabricated with the assumption that it moves in pipes of 7 mm inner diameter. In order to enable the operation of the robot in pipes of various diameters, the diameter of the robot was set at 8 mm and was designed to pass through a pipe with a minimum 4 mm diameter. The total length of the robot was 18 mm when shrunk. The weight of the robot was 3.4 g without fluid in the body of the robot. By altering the length of the stretchable section, the total length of the robot can be reduced or increased. Acrylic material was used for the nodes of the robot, and silicone rubber (0.2 mm thick) was used for the external wall. A saline solution was introduced through a vinyl pipe via a 1.5-mm-diamater hole located at the back of the robot. The robot was stretched and shrunk by the pressure applied to the saline solution.
Fig. 2

Mobile device driven by two-tube pumping

4 Driving Experiment

4.1 Outline of System

We designed the robot with the aim of operating it in blood vessels, however, for the following reasons we did not carry out the operation in actual blood vessels.
  1. (i)

    Quantitative observation was difficult due to variability of data caused by the difference in the freshness of blood vessels.

     
  2. (ii)

    Since blood vessels are opaque, we were unable to observe the operation of the robot.

     
  3. (iii)

    Acquisition of blood vessels was difficult

     
In the experiment, we used a clean silicon rubber tubes (inner diameter, 6 mm; outer diameter, 8 mm). A robot was placed inside the acrylic pipe, and the operation was studied. The acrylic pipe was arranged both horizontally and vertically with respect to the ground. The travelling speed and the amount of strokes for one reciprocated movement of the syringe piston were measured. The crude rubber coating the bodies expands and contracts when the driving fluid that fills the syringes is pumped and evacuated. Electric actuators that move straight back and forth are connected to the piston sections of the syringes, thus establishing a system that can control the position and velocity of the piston motion. Figures 3 and 4 show the straight reciprocation actuators attached to the syringes and the outline of the system used in our experiments, respectively. Physiological saline solution was used as a driving fluid considering the minimization of the damage that may be caused by the leakage of the driving fluid inside blood vessels when our devices are used in practice. Moreover, silicon rubber tubes and a 5-mPas-viscosity mixture of glycerin and pure water were used in place of blood vessels and blood, respectively. Table 1 summarizes the mechanical characteristics of human blood vessels and blood and those of the silicon rubber tubes and the mixture used in this experiment.
Fig. 3

Straight reciprocation systems attached to syringes

Fig. 4

Outline of experimental system

Table 1

Characteristics of blood and blood vessels, and mechanical properties of the materials used in this experiment

 

Human

Experimental materials

Artery:

Outer diameter

8[mm]

8[mm]

Artery:

Inner diameter

6[mm]

6[mm]

Young’s modulus

of blood vessel

1[MPa]

2[MPa]

Viscosity coefficient of blood

4.7[mPas]

5[mPas]

4.2 Experimental Methods

Silicon rubber tubes are placed almost horizontally in the mixture, as explained in section 4.1, at room temperature. The amounts of saline pumped into the syringes were 0.30. This value, with which the maximum level of expansion is ensured without breakage of the device, are obtained in the preliminary experiment. The measurement was carried out in nine steps by changing the evacuation rate by 0.09 ml/s from 0.90 ml/s. The time required for the mechanism to move 30 mm in a pipe was measured using the front end of the device as the measurement point. The behavior is photographed using a video camera, and each measurement was carried out by analyzing the obtained images.

5 Experimental Results

Figure 5 shows the experimental results for the peristaltic mobile devices driven by the two pumping methods. The robot can move at a speed of 0.38 mm/s in silicon rubber tubes 6 mm inner diameter. The highest speed was obtained at the evacuation rate of 0.90 ml/s.
Fig. 5

Experimental results for mobile devices driven by two- and one-tube pumping methods

6 Discussion and Conclusions

We proposed a microscopic mobile device that can move inside 2-3-mm-diameter blood vessels by peristaltic motion achieved by repeated expansion and contraction using hydraulic pressure, in particular, using a physiological saline solution as the driving fluid. Therefore, the diameter of the robot was designed to be either equal to or smaller than that of the human aorta. Assuming the use of the robot in the aorta, the diameter of the robot was set at 2-3 mm or smaller. Accordingly, the use of wheels to move the robot was avoided, and instead we adopted a mechanism which uses peristaltic movements. Also, considering the possibility of leakage of electric current, we did not use an actuator which requires an external power supply, and selected an actuator which can be operated by hydraulic pressure. We then fabricated a robot which satisfies the above conditions, and experimentally studied the operation of the robot. Although we were unable to perform driving experiments in actual human blood vessels, we confirmed that the robot can move at a speed of 0.38 mm/s in silicon rubber tube of 6 mm inner diameter.

Notes

Acknowledgments

A Part of this study was supported by Grant-in-Aid for Scientific Research C(20500397).

References

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Copyright information

© Springer Science+Business Media B.V. 2012

Authors and Affiliations

  1. 1.Department of Innovative Systems EngineeringNippon Institute of TechnologySaitamaJapan
  2. 2.Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and TechnologyTokyoJapan

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