Novel Magnetic Propulsion System for Capsule ...

Proceedings of the ASME 2009 International Mechanical Engineering Congress & Exposition IMECE2009 November 13-19, Lake Buena Vista, Florida, USA

IMECE2009-10432 Novel Magnetic Propulsion System for Capsule Endoscopy Chengzhi Hu1, Mingyuan Gao1, Zhenzhi Chen1, Honghai Zhang1, and Sheng Liu1,2,*, Fellow, ASME 1

2

School of Mechanical Science & Engineering, Huazhong University of Science & Technology, Wuhan, 430074 China Wuhan National Laboratory for Optoelectronics, Huazhong University of Science & Technology, Wuhan, 430074 China

Abstract—For the purpose of realizing the noninvasive exploration of gastrointestinal tract, a novel magnetic propulsion system is proposed, which includes a patient support, a magnet assembly with two groups of permanent magnets positioned oppositely, and a magnet support. The proposed approach exploits permanent magnet and coupling movement of multi-axis components to generate quasi-static magnetic field for controlling the position, orientation, and movement of a self-propelled robotic endoscope in the gastrointestinal tract. By driving the five coupling axes, the proposed magnetic propulsion system is capable of steering the capsule endoscope through the intestinal tract in multi-directions of 2D space. Experiments in simulated intestinal tract are conducted to demonstrate controlled translation, rotation, and rototranslation of capsule endoscope. Finite Element Method is used to anal yze navigation system’s mechanical properties and the distributions of magnetic field. The proposed technique has great potential of enabling the application of controlled magnetic navigation in the field of capsule endoscopy. Index Terms—Biomedical equipment, capsule endoscopy, gastrointestinal tract (GI), magnetic propulsion, self-propelled capsule

I. INTRODUCTION tomography (CT) and magnetic resonance imaging (MRI) is becoming increasingly important [15]. A commercial computer-aided sugery system (Niobe™, Stereotaxis, St. Louis, MO) using techniques of magnetic navigation and MRI is currently developed for improving the treatment of arrhythmias, heart failure, and coronary artery disease by steering magnetic catheters and guidewires through the heart and the coronary vasculature [16]. Meanwhile, recently researches reported the feasibility of using the technology of magnetic navigation in the field of gastrointestinal endoscopy, and preliminary proof-of-concept investigations have been performed [17]. However, due to many concerns such as safety and cost, few automated navigation systems have been proposed for serving the patients in the field of non-invasive gastrointestinal endoscopy. In this paper, we proposed an automated multi-axis magnetic propulsion system for self-controlling the endoscopic capsule’s location and orientation in the gastrointestinal tract with maximum level of safety and costeffectiveness. In order to judge the feasibility of the proposed system, experimental tests were carried out and it demonstrated controlled translations, rotations, and rototranslations of the capsule/clay complex under this novel propulsion system. II. THEORIES

Capsule endoscopy is an examination of the gastrointestinal track using a wireless capsule endoscope which contains digital camera, ASIC transmitter, antenna, illuminating LEDs assisted imaging, and battery. Capsule endoscope with the size of a normal pill can be easily swallowed by patients in various ages. It takes pictures throughout the gastrointestinal tract by sending images to an external recorder and thus provides useful information for clinic diagnosis of gastrointestinal tract diseases [1]. However, the movement of the commercialized capsule depends solely on the peristalsis system. Therefore, it takes 6-8 hours for an examination and doctors cannot perform a pinpoint analysis once an irregular vision has been found [2]-[4]. In addition, camera in capsule takes thousands of photos in every part of the tract to prevent the possibility of missing an important picture. Therefore, hours of time are needed for doctors to organize and analyze a large number of images, still with a 60-70% rate of success in diagnostics. All of these drawbacks render capsule endoscopy extremely high in cost but extremely inefficient in terms of time spent and resources used. With the advent of micro-electro-mechanical systems (MEMS) technology, development of the self-propelled robotic endoscope has been the topic of interests in the field of gastrointestinal endoscopy for several years [5]-[8]. Various locomotion mechanisms and actuation systems have been developed for self-propelled endoscopic capsule such as shape memory alloy (SMA) actuator, spiral type micro machine, motor legged robot, hydraulic manipulator, earthworm micro-robot, and biomimetic spermatozoa microrobot [9]-[14].

In many surgical fields, including craniomaxillofacial surgery, cardiovascular diagnosis, and ureteral stones treatment, computer-aided surgery (CAS) based on computed

In a current free region, where

H  0

(1)

it is possible to define the scalar magnetic potential

Vm from

the relation

H  Vm

(2)

This is analogous to the definition of the electric potential for static electric fields. Using the constitutive relation between the magnetic flux density and magnetic field

1

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B  0 (H  M)

Now, according to Gauss’ theorem

(3)

   T dV   n T dS  0

together with the equation

1

B  0

(4)

we can derive an equation for

Fext   f ext dV 

(5)

1

Along the boundaries far away from the magnet, the magnetic field should be tangential to the boundary as the flow lines should form closed loops around the magnet. The natural boundary condition is

n  (0Vm  0M)  n  B  0

F

r denotes the coordinates of a material point, T is the stress tensor, and f ext is an external

where

1 1  T2   pI   E  D  H  B  I  ED  HB  2 2 

(11)

  1 1  0      pI   E  D  H  B  I  ED  HB    f ext 2 2    (23) Consider again the case of a solid surrounded by air. To compute the total force, the projection of the stress tensor on the outside of the solid surface is needed,

g ext can represent the

reaction force from another body that the solid is attached to The equations for the balance of forces on the solid now become (14)   T1  fext  0

1 1  n1T2   pn1   E  D  H  B n1 2 2     (n1  E)D  (n1  H)B

(15)

   T

1

 f ext dV 

 n (T 1

2

 T1 )  g ext dS  0

(24)

where n1 is the surface normal, a 1-by-3 vector, pointing

For computing the total force F on the solid these equations need to be integrated over the entire solid and the solid/vacuum boundary 1

(22)

The equation for the balance of forces becomes

On the boundary, the following equations apply

n1 (T2  T1 )  g ext  0

I

expression for the stress tensor can be written as

This changes the force balance equation

The external boundary force

is the air pressure,

stress tensor is also known as the Maxwell stress tensor. Using the fact that, for air, D   0E and B  0 H the

It is sometimes convenient to use a volume force instead of the stress tensor. This force is obtained from the (10) fem    TEM

(13)



(21) is the identity 3-by-3 tensor

(or matrix), and E and B are 3-by-1 vectors. In this expression of the stress tensor, air is considered to be nonpolarizable and nonmagnetizable. When air is approximated by vacuum,   0 . This expression of the

In certain cases, the stress tensor T can be divided into one part that depends on the electromagnetic field quantities and one part that is the mechanical stress tensor (9) T  TEM   M

n1T2  n1T2  g ext

(20)

  1 1 T2   pI   0 E  E  B  B  I   0 EE  BB  2 2   0 0  

(8)

(12)

(19)

1 2

For air, the stress tensor

volume force. This is the equation solved in the structural mechanics application modes for the special case of a linear elastic material, neglecting the electromagnetic contributions. In the stationary case there is no acceleration, and the equation representing the force balance is

n1 (T2 T1 )  0

 n T dS

To summarize, the total force, F, is computed as a boundary integral of the stress tensor in vacuum on the outside of the solid. Note that to obtain this result, the contribution from the air pressure gradient has been neglected. This is equivalent of assuming that   T2  0 .

is the density,

0     M  fem  fext

(18)

Fext  F  0

(7)

0    T  fext

dS

to keep the solid stationary. That is

2



ext

1

(6)

d r    T  f ext dt 2

g

1

is needed to balance the term for the boundary integral of the stress tensor in the surrounding vacuum

Thus the magnetic field is made tangential to the boundary by a Neumann condition on the potential. Cauchy’s equation of continuum mechanics reads

where

1

This means that the external force

Vm

   (0Vm  0M)  0



(17)

1 1

1

out from the solid. This expression can be used directly in the boundary integral of the stress tensor for computing the total force F on the solid.

(16)

1

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III. SYSTEM DESIGN AND LOCOMOTION ANALYSIS

orange color indicates the patient support’s linear movement in the longitudinal direction. By combining movements of five axes in different permutation and sequence, we can navigate magnet built-in capsule endoscope’s locomotion through the gastrointestinal tract in any direction in 2D space. Four main movements of capsule endoscope are shown in (d), the relative translation between patient support and magnet assembly will steer capsule moving in the lateral or longitudinal direction in term of patient support’s motion path; the opposite motion of magnets in the coronal plane will drive capsule rotating about its vertical axis, this motion changes capsule’s orientation, allowing to realize capsule’s motion transition from the longitudinal path to the lateral path and vice versa. The capsule endoscope is also capable of rotating about its longitudinal axis by pivoting the magnet assembly in the magnet support, this movement is not only useful for adjusting capsule’s orientation , but also necessary for enlarging viewing angle of the built-in micro camera to improve diagnosis effect of gastrointestinal endoscopy.

Fig.1 illustrates the assembly drawing of magnetic navigation system. The patient support 1, which can be moved longitudinally forwardly and rearwardly and laterally inwardly and outwardly, is mounted on a pedestal, allowing the patient to be moved relative to the magnet assembly. The magnet assembly 2 comprises several Nd-Fe-B permanent magnets, ball screws, linear ball rails, and gripper device. The ball screws convert motor’s rotary motion to ball nut’s linear motion, exerting necessary force to move the permanent magnets along a longitudinal axis. Once the two groups of oppositely-positioned magnets are moved in the opposite direction parallel to the longitudinal axis, the magnet inside the capsule endoscope will be rotated about its lateral or vertical axis or any axis in the transverse plane according to the different positions of magnet support. The magnet support 3 contains a pair of worm/worm gear, a pivot, and aluminum framing. The magnet assembly is mounted onto the magnet support and can rotate about the longitudinal axis. The relative rotation between magnet assembly and patient support permits adjustment of the positions of magnet, allowing magnet built-in capsule endoscope to rotate about its longitudinal axis.

Fig. 2. Locomotion analysis of magnet built-in capsule endoscope Fig. 1. Overall Assemble Drawing of Magnetic Navigation System

IV. MATERIAL AND METHODS

The proposed magnetic navigation system has five coupling axes which can realize capsule endoscope’s multidirection movement in the gastrointestinal tract. Fig.2 demonstrates the principle of capsule endoscope’s locomotion mechanism. The different movements of five axes are shown with numbers and corresponding arrowhead in (a), (b), and (c). Number 1 with red color indicates magnet assembly’s rotation about its longitudinal axis, number 2 with green color shows two magnets’ linear motion in opposite direction in the coronal plane, number 3 with blue color represents two magnets’ linear motion in opposite direction in the sagittal plane, number 4 with purple color shows the patient support’s linear movement in the lateral direction, and number 5 with

A. Fabrication of the magnets/clay capsule complex One cylindrical permanent Nd-Fe-B magnet is used as the built-in magnet of capsule endoscope. The magnet has a diameter of 6 mm, a height of 6mm, a weight of 2.16g, and generate a field of 0.42T at the center of its surface. We use oil clay as the coating materials to simulate capsule’s geometrical shape. The resulting magnet/clay complex has a diameter of 10mm, a length of 15mm, and a weight of 3.55g as shown in Fig.3.



the following. V. RESULTS AND DISCUSSIONS Fig. 4 shows the relation between the magnetic flux density and the distance. The black line plots the simulation results, whereas the circle plots the experiment measurement values. The experimental data match well with the finite element analysis, demonstrating that our electromagnetic model is appropriate for analyzing the mechanical properties of the proposed propulsion system. As we can see, with the increase of the distance between magnetic sources and built-in magnets, the strength of the magnetic flux density drops rapidly. Thus, electromagnet is a better magnetic resource compared with permanent magnet because it can generate a stronger magnetic field. Fig. 5 is a visualization of the solved magnetic field. The streamlines show the lines of magnetic force, and the background color shows the distribution of the magnetic field. The magnetic field is stronger when the magnetic force lines become denser. A maximum intensity of 0.34 T at the surface is obtained.

(a) (b) (c) Fig. 3. (a)NdFeB magnet; (b)oil clay; (c)Resulting clay/magnet capsule

B. External Magnetic Field Sources Electromagnets or permanent magnets can be mounted on the magnet assembly of proposed magnetic navigation system. In current approach, two groups of permanent Nd-Fe-B magnets are used to provide sufficient magnetic field for driving endoscopic capsule. Each group includes a rectangle Nd-Fe-B magnet and a cylindrical Nd-Fe-B magnet; the rectangle one has a length of 80mm, a width of 60mm, a height of 15mm, the cylindrical one has a diameter of 90mm, a height of 30mm, and generated a field of 0.32T, 0.29T at the center of its surface, respectively. C. Simulated Gastrointestinal Tract Translucent Silicone rubber is used as the simulated gastrointestinal tract for experiments. The length and the diameter of the silicone rubber pipeline is 1.6m and 25mm, respectively. The friction coefficient of the silicone rubber is 0.25-0.7. The friction coefficient of the large intestinal of a pig varies from 0.22-0.35. D. Finite Element Analysis Finite element methods are used to simulate magnetic field distributions and mechanical properties of the proposed magnetic navigation system. Gauss meter is used to measure the actual values of generated magnetic field. E. Magnetic Navigation of the built-in magnets

Fig. 4. Magnetic Flux Density as a function of distance

One type of experimental setups is considered. The magnet/clay capsule is moved on the surface of a 1.6 meter-long silicone rubber pipeline. This tube is shaped as a curved pipe with different radius of curvature for better simulating the configuration of human’s intestinal tract. By taking into account the thickness and the width of human’s abdomen, the distance between two groups of magnets is about 550mm-570mm, corresponding to the order of magnitude of an average abdomen size, and thus can be used in the field of gastrointestinal endoscopy. In this case, the field is generated by two groups of permanent magnets, diametrically opposed with respect to the tube. Furthermore, electromagnets can be used in order to further increase the magnetic field applied inside the structure. The distribution of the resulting magnetic field between the magnetic sources and around the capsule is analyzed by Finite element methods. With this experimental setup, the multiaxis magnetic navigation system is driven to induce different kinds of motions of the capsule/clay complex, as reported in

Fig. 5. Visualization of the solved magnetic field



Fig. 6. Comparison between the Magnetic Force and the Friction Force

Fig. 6 illustrates comparison between the magnetic force and the friction force. Square symbol plots the relationships of distance between external and internal magnets versus the value of magnetic force exerted on the built-in Nd-Fe-B magnets of the magnet/clay capsule complex. Triangle symbol shows the relationships of distance versus frictional resistance force. In order to overcome the frictional resistance of the intestine, the magnitude of distance between external and internal magnets must be adjusted to an appropriate value for generating enough driving force, which can be obtained in terms of square plot line. As we can see, permanent magnets sources with a surface magnetic flux density of 0.34T-0.5T is appropriate for driving the magnet/clay capsule complex; however, taking the visco-elastic feature, peristalsis movement, and complicated surface topography of the intestine into consideration, electromagnet is much more suitable due to its capability for generating larger intensity of magnetic field. Experiments of motion control are presented by the video frames reported in Figs. 7. Fig. 7 shows a rototranslation of the magnet/clay capsule inside a simulated gastrointestinal tract using silicone rubber tube. Due to the visible external manipulation of magnetic navigation system, capsule can move linearly in both longitudinal and lateral direction, rotate about its longitudinal and lateral axes, and pivot about its vertical axis. (a1) – (g1) shows the location and movement direction of the capsule at different time in the simulated gastrointestinal tract. The position and orientation of the magnet assembly and the patient support inducing these movements are reported in the related frames (a2) – (g2).

Fig. 7. Rototranslation of the magnet/clay complex on a simulated intestinal tract. The capsule’s position and orientation is shown by the frames a1–g1 correspond to the arrangement of the magnetic navigation reported in a2–g2.

VI. CONCLUSIONS A novel magnetic propulsion system was proposed and manufactured for controlling endoscopic capsule’s 5 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 10/13/2014 Terms of Use: http://asme.org/terms


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movements in the intestinal tract. Experiments were conducted, demonstrating this proposed multi-axis navigation system was capable of steering capsule’s location and orientation in the simulated 2D intestinal tract. Finite element methods were used to analyze the magnetic field distributions and mechanical properties of proposed navigation system. The proposed technique suggests a feasible approach in the field of capsule endoscopy. Further research is currently in progress for realizing endoscopic capsule’s movements in 3D space and visualizing the locomotion control by ways of ultrasonic image. ACKNOWLEDGMENT The authors greatly appreciate the help rendered by Dr. Dan Xie and engineer Yuli Xu of the School of Mechanical Engineering, Huazhong University of Science & Technology for their help in manufacturing this navigation system. This work was supported by the National High Technology Research and Development Program of China under Ministry of Science and Technology (863 Program) under Grant No. 2008AA04Z313. The work of Mingyuan Gao and Chengzhi Hu was supported by the Graduate Innovation Foundation Program from the Huazhong University of Science & Technology under Grant No. HF0601108100. REFERENCES [1]

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