Small intestinal model for electrically propelled capsule endoscopy
© Woo et al; licensee BioMed Central Ltd. 2011
Received: 30 July 2011
Accepted: 16 December 2011
Published: 16 December 2011
The aim of this research is to propose a small intestine model for electrically propelled capsule endoscopy. The electrical stimulus can cause contraction of the small intestine and propel the capsule along the lumen. The proposed model considered the drag and friction from the small intestine using a thin walled model and Stokes' drag equation. Further, contraction force from the small intestine was modeled by using regression analysis. From the proposed model, the acceleration and velocity of various exterior shapes of capsule were calculated, and two exterior shapes of capsules were proposed based on the internal volume of the capsules. The proposed capsules were fabricated and animal experiments were conducted. One of the proposed capsules showed an average (SD) velocity in forward direction of 2.91 ± 0.99 mm/s and 2.23 ± 0.78 mm/s in the backward direction, which was 5.2 times faster than that obtained in previous research. The proposed model can predict locomotion of the capsule based on various exterior shapes of the capsule.
With changes in social life patterns and stress levels, the incidences of digestive diseases are increasing every year. One of the conventional diagnosis methods for digestive diseases is endoscopy, which causes pain and discomfort to patients. In addition, an endoscopy cannot easily monitor the small intestine because it is difficult to insert the endoscope through the pylorus. In order to solve these problems, many biomedical devices have been developed and the capsule endoscopy is one of the successful devices that can automatically capture internal images of the gastrointestinal tract [1–5]. One disadvantage of the capsule endoscope is its lack of locomotive ability; the capsule naturally goes in an aboral direction by peristalsis and there is no way to go backward and get detailed images when the capsule passes a suspicious position.
In order to solve this problem, many studies have been carried out to implement self-propelled robotic capsule endoscopes using various mechanisms such as motors [6–9], shape memory alloys (SMA) [10, 11], magnets [12–14], and electrical stimuli [15–17]. The motor and the SMA were found to provide enough force to propel the capsule and can work together with various types of conventional gears and wheels. One disadvantage of those techniques was the large power consumption and the inability to operate from conventional batteries in the capsule endoscope. Therefore, this method was tested with power lines and the site of operation was limited to the large intestine.
Another mechanism used magnetic power to propel the capsule. M. Sendoh et al. implemented an exterior capsule with a screw shape and applied the external magnet to spin the capsule . During the spinning, the screw shape generated forward or backward force to move the capsule. One disadvantage of this mechanism was the meandering path of the small intestine. In order to propel the capsule, the external magnetic force had to be perpendicularly applied to the direction of the movement and it was difficult to apply the magnetic force as the path of the small intestine twisted in various directions.
Another mechanism used an electrical stimulus to propel the capsule [15–17]. Park et al. implemented a capsule with a practical size that can propel itself in an aboral direction; Moon et al. reported that the capsule can propelled in both aboral and oral directions using four electrodes. This mechanism did not require complex circuits and consumed less power that make possible to operate by small batteries. Previous researches showed the feasibility of using electrical stimulus for locomotion, but did not indicated the optimal exterior shape and position of the electrodes because there was no proper model for locomotion.
Many studies have been conducted to determine the physiological properties of the small intestinal tract using a sodium channel , wave equation [19–21], nonlinear equation , and neural network . These methods were focused on determining the electrophysiology that is the control signal of the peristalses. Therefore, those models did not provide basic information about the friction, contraction forces, and viscoelastic properties of the small intestine. Another method was assuming the small intestine as a viscous fluid and using fluid engineering [24, 25]. This model did not consider the elastic properties of the small intestine and the values of the friction were lower than in the actual intestinal environment. Still another method used biomechanical modeling of the small intestine for robotic endoscopy . The model was focused on finding an ischemia problem occurring in the small intestine when it was excessively extended by the robotic endoscope. Therefore, this model also did not report information on friction and contraction forces induced by electrical stimulation.
In this paper, a small intestinal model for an electrically propelled capsule was proposed and verified from in vitro experiments. The model took into account friction and the contraction force properties of the small intestine, and then conducted a simulation to choose the proper exterior shape of the capsule. Through the simulation, two shapes of capsules had chosen based on the velocity and internal volume of the capsule. After implementation of the chosen capsules, there were inserted into fresh small intestines that were temporally reactivated, and then the velocity of the moving capsule was repeatedly measured while applying electrical stimuli. From the experiments, the velocities of the capsules showed a similar tendency to the simulation results.
2.1 Mechanism of the electrically propelled capsule
where is the total moving force, is the moving force is friction, and is the drag force.
where ε c is strain, t is time, f (x) is the exterior shape of the capsule, and d 0 is the initial diameter of the small intestine. Since the capsule extends the small intestine, the strain depends on the exterior shape of the capsule. The exterior shape of the capsule will be discussed later.
Figure 4 (a) depicts the electrical stimulus parameters, Figure 4 (b) shows the maximum contraction pressure depending on various electrical stimulus parameters, and Figure 4 (c) shows transient response of contraction. In order to reduce the number of experiments, the duration was fixed at 5 ms that is twice of chronaxie of the smooth muscle. The experimental results shows that the maximum contraction force increased nonlinearly when the voltage was increased.
where P m is the maximum contraction pressure, f s is the stimulus frequency, and A s is the stimulus amplitude. Standard coefficient error is lower than 0.2 and the P-value showed less than the significance level (< 0.01).
where P s (t) is the contract pressure, τ s is the rising time constant, P m is the maximum contraction force.
The value of the rising time (τ s), which is defined as a time to reach 63.2% of its maximum contraction pressure, was measured from for three different stimulus parameters (10, 20, and 40 Hz @ 6 V and 5 ms) with four different samples from two different swains (N = 20). The average (SD) rising time constant value was measured as 17.3 ± 8.3 seconds.
where x is the x-axis, x 0 is the center placement of the electrode, and ω is the variance. The variance value was empirically set as 0.22.
Where μ is the frictional coefficient and it is set at 0.1 based on Baek's experiments . The friction increases with increasing contraction force and grooves at the capsule surface.
3. Simulation and experimental results
Six Landrace porcine small intestines (6~7 months old) were collected from a local abattoir. The small intestines were rinsed several times and transported to the laboratory in ice cold oxygenated Krebs-Ringer bicarbonate solution. This experiment was performed according to the guidelines of the Committee on Animal Experimentation of Kyungpook National University. Six different small intestines were taken and inserted into the Krebs' solution to await activation . After the small intestine is activated, it naturally performs peristalsis, secretes digestive juice, and can be contracted by electrical stimuli.
4. Conclusion and discussion
In this paper, a simulation model for an electrical stimulus capsule was proposed and compared with ex-perimental results. The model represented friction and drag related to the shape of the capsule and the contraction force from electrical stimuli. From the model, the acceleration and velocity of the capsule were calculated. The simulation model was compared with the experimental results and the two were found to be well matched.
After the electrical stimulus, there were no visual signs of bleeding or obstruction and the capsule moved freely to the oral and aboral direction in the small intestine.
One interesting finding was that after the electrical stimulus was completed, sometimes-large contractions occurred and moved the capsule. In this experiment, movement after stopping the capsule was not included, but was sometimes observed in certain positions of the small intestine. This phenomenon was not observed at every points of small intestine, but it was reproducible at certain points where this phenomenon was observed. This could interfere with stopping the capsule at certain positions and additional breaking mechanisms for the capsule should be researched [33, 34].
Additional files 2: A movie file of locomotion of various types of electrical stimulus capsules. A movie file that shows the electrical stimulus capsule can be used to be locomotion method. (WMV 3 MB)
In this study, the size of electrodes were fixed at 5 × 6 mm based on previous research data. The size of the electrodes could influence the bulk impedance and contraction area. In order to determine the optimal size and shape of the electrodes, an electrical physiological model and Gauss equation mixed model is required.
The meandering path of the small intestine will cause additional friction from the walls of the small intestine. The present simulation assumes that the small intestine is a long straight valve and so does the in vitro experiments, and the results of both were similar. In order to straighten the small intestine, the mesentery was cut and straightened; this caused some parts of small intestine to wrinkle and sometimes the capsule was blocked in certain positions. When the capsule was passing a meandering curve, the capsule will asymmetrically stretched a part of the small intestine. Therefore, additional research is required for asymmetrical modeling of the portions of the small intestine with a curved structure.
The current model does not represent difference in the aboral and oral moving speeds. Since the friction does not depend on the direction of movement, and the contraction force from the electrical stimulus was high enough to ignore peristalsis, there was no speed difference from the simulations in either direction. It could be assumed that the small intestine would naturally resist the capsule, going in an oral direction, and it could be explained by "law of the intestine," which indicates that the small intestine pushes its contents toward to an aboral direction. Therefore, additional research is required into the velocity difference between the movement in the aboral and oral directions.
In spite of the above problems, this paper presents a practical simulation model for an electrical stimulus capsule that will help us to design the proper shape of the capsule and positions of the electrodes. Further, this model could be used to study and to reduce the number of capsule endoscopes that do not naturally go in an oral direction.
This study was supported by a grant of the Korea Healthcare technology R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea. (Number:A092106). And this work was supported by the Grant of the Korean Ministry of Education, Science and Technology" (The Regional Core Research Program/Anti-aging and Well-being Research Center). Also This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology(2010-0025322)
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