Experimental investigation of intestinal frictional resistancein the starting process of the capsule robot

Cheng Zhang a,b, Hao Liu a,n, Hongyi Li a

a State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Science, Shenyang 110016, Chinab University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

Article history:Received 24 June 2013Received in revised form16 September 2013Accepted 19 September 2013Available online 30 September 2013

Keywords:FrictionCapsule robotIntestineStarting process

a b s t r a c t

The imperfection of the intestinal friction model is one of the biggest obstacle of the capsule robot'sdevelopment. This paper seeks to study the intestinal frictional resistance in the starting process of thecapsule robot. Experiments are conducted to measure actual frictional resistance, which is found to havesomething to do with the velocity, acceleration and original state of the capsule robot. The analyticalexpression of the frictional resistance can fit the experimental result with R-square equaling 0.9605. Theachievement of the article is hoped to smooth the motion and save the energy in the capsule robot'sstarting process.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Broad challenges exist when fundamental research to studyfrictional characteristics between medical device and biologicaltissue is no exception. Capsule robot, which can examine thewhole gastrointestinal (GI) tract non-invasively and actively whenperformed by doctors who have had special training and areexperienced in the endoscopic procedures, is one of the mostrepresentative medical devices. The imperfection of friction modelbetween the capsule robot and the intestine has been one of thebiggest obstacles of the development of the capsule robot [1]. Thecapsule robot needs to start and stop frequently when working inthe intestine. The frictional resistance increases rapidly from asmall value in the robot's starting process [2]. If the driving force ofthe capsule robot in uniform motion is used in accelerated motion,the capsule robot cannot move smoothly and more energy iswasted.

The study aims to figure out the intestinal frictional resistancevariation in the starting process of the capsule robot when thecapsule robot moves inside the intestine. Some experiments areconducted to measure the actual frictional resistance with ahomemade uniaxial experiment platform. The frictional resistance

is found to changes with the capsule's velocity, acceleration andoriginal state. Further, curve-fitting is used to deduce the analy-tical expression of the frictional resistance in the starting process.The validity of the fitting result is verified by experiment result.The achievement of this study is hoped to make certain theintestinal frictional characteristics in the starting process of thecapsule robot and optimize the control method of the capsulerobot with various driving principle.

2. Background

According to the statistics of World Health Organization(WHO), there are nearly 130 million patients with GI tract diseasein China. More people have suffered with the disease in theworldwide. The capsule endoscopy (CE) has been the mosteffective medical device to examine the GI tract painlessly andnon-invasively. CE has become the gold standard for the diagnosisof most diseases in the GI tract [3–6]. However, the passive featureof CE makes it out of control in the intestine. Missed diagnosis andileus will occur possibly during an 8-h inspection [7]. To improvethe situation, many researchers are engaging in developing anactive CE, which can be called capsule robot, using various drivingmechanisms [8–16]. Before a practicable solution can be devised,the frictional characteristics between the capsule robot and theintestine need to be researched.

Now one of the main limitations that the capsule robot movesin the intestine inefficiently is the imperfection of the frictionmodel due to the presence of complex lumen surface features and

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Tribology International

0301-679X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.triboint.2013.09.019

n Corresponding author at: State Key Laboratory of Robotics, Shenyang Instituteof Automation, Chinese Academy of Science, Shenyang 110016, China.Tel. þ86 24 83602925.

E-mail addresses: zhangcheng@sia.cn (C. Zhang), liuhao@sia.cn (H. Liu),hli@sia.cn (H. Li).

Tribology International 70 (2014) 11–17

a highly motile, tortuous intestine path. The frictional resistancecomes from intestinal peristaltic contractions, mucoadhesion, thecollapsed lumen and the interaction between the capsule surfaceand the intestinal inwall [17]. The surrounding organs, weight ofthe intestinal wall and pressure from the fluid within the intestinealso impart forces on the capsule robot. The uncertainty offrictional characteristics has been the main obstacle of the devel-opment of the capsule robot.

Experimental investigation is the main method to research thefrictional resistance between the capsule and the intestine. Somepeople focus on the affection of the capsule's parameters, whileothers seem more concerned with the material property of theintestine. The characteristics of the capsule, such as weight, shape,dimension, material, contour line, surface preparation methodsand kinestate have great influence on the friction [18–24].Researches show that the friction of smooth cylindrical capsuleis smaller than other shapes [18,19], the influence of the capsule'slength on the friction is greater than that of the diameter [21],capsule's weight has no significant effect on the friction [22], resinmaterial is suitable for making capsule's shell [20] and the frictionare in direct proportion to the capsule robot's velocity [23]. Thebiomechanical properties of the intestine also have a greatinfluence on the capsule's movement. Hoeg et al. studied thetissue distention of the intestine when a robotic endoscope movedthrough. An analytical model and an experimental model weredeveloped to predict the tissue behavior in response to loading[25]. Ciarletta et al. used the theory of hyperelasticity to make astratified analysis of the intestinal wall. The hyperelastic model issignificative to research the frictional characteristics of the intes-tine, but it neglects the time effect [26]. Woo et al. considered thefriction of the intestine using a thin walled model and Stokes' dragequation. However, the model cannot fully describe the materialcharacteristics of the intestine [27]. Kim et al. found that thevariation of resistance was correlated with the viscoelasticity ofthe intestine. A five-element model is used to describe the visco-elastic property of the intestinal stress relaxation. Furthermore,the group first developed an analytical model for the frictionprediction, which was verified by finite element analyses [28].

Most of the above experiments are conducted in the state of restor uniform motion. However, the frictional resistance in the startingprocess has been neglected by most of the researchers until now.Therefore, there is hardly any appropriate control method aiming atthe starting process. By indentifying the parameters that can bemanipulated dynamically, a well-designed capsule robot can workwith or against the friction as needed for movement or positioning.Tubular porcine intestine is chosen as the experimental subject,which is pretreated to maintain the biomechanical characteristic. Ahomemade uniaxial experiment platform is used to conduct theexperiment. An analytical expression of the frictional resistance willbe deduced according to the experimental results. This work ishoped to change the status quo that the capsule robot's frictionalresistance is uncertain in the starting process and guide the designof capsule robot's starting mode.

3. Experimental methods

A homemade uniaxial experiment platform was developed totest the frictional resistance variation in the starting process. Theplatform consists of two parts: drive unit and data acquisition unit,as shown in Fig. 1. The main part of the drive unit is LMX1E serieslinear motor system, which is produced by HIWIN. The positioningaccuracy of the system can reach 71 μm. The left side of the driveunit is a load platform. The rotor of the linear motor is combinedwith a support, on which there fixed a one degree of freedommicro force sensor (FUTEK LSB200), whose accuracy is 1 mN. The

probe of the force sensor is connected to a capsule dummy that isplaced inside the intestine with a polymer string. The capsuledummy can be pulled by the drive unit with different velocitiesand accelerations. Flexible polyurethane foam (PUF) is used as thebasement to support the intestine. This way can simulate anin vivo environment of the intestine as much as possible. PCI6229 data acquisition (DAQ) card produced by NI is used to acquirethe voltage signal of encoder and micro force sensor. Then thesignal is transmitted to computer for saving and analysis.

The capsule dummy used in the experiment is made of resinmaterial. The shape of the capsule dummy is an assembly of acylinder in the center and two hemispheroids in both ends. Thedimension is shown in Fig. 2. The weight of the capsule inside theintestine is 3.9 g.

The intestine used in the experiment was taken from astandardized laboratory pig to ensure the repeatability of experi-mental results. In order to reduce the effect of the food debriswithin the intestine, the pig was kept off food, but not water for24 h. Under the supervision of the medical ethics committee, thepig was sacrificed with an anesthetic overdose and anatomized byprofessionals. Segments of jejunum were excised from the pigimmediately following euthanization and stored in a sink that isfilled with 37 1C Tyrode's solution with continual oxygen supply(1000 ml Tyrode's solution consists: NaCl 8.0 g, KCl 0.2 g, MgSO4

�7H2O 0.26 g, NaH2PO4 �2H2O 0.065 g, NaHCO3 1.0 g, CaCL2 0.2 g,glucose 1.0 g.). When exposed to unconditioned air, intestinaltissue drying had been observed and previously remedied byapplying saline to the tissue surface [29]. Because of lack ofconstant temperature and humidity system in the experimentplatform, changing same type test specimens frequently isadopted to ensure the mechanical property of the test specimen.The mesentery and one side of the intestine specimen are fixed onthe basement.

The drive unit is controlled to pull the capsule with differentvelocities and accelerations. The polymer string is adjusted to a

Fig. 1. Experiment platform for measuring frictional resistance variation.

Fig. 2. The dimensions of the capsule.

C. Zhang et al. / Tribology International 70 (2014) 11–1712

proper tension before each experiment for the purpose of ensuringthe synchronism between the force and the velocity. The quanti-tative relations between the motion parameters and the frictionalresistance in the starting process can be obtained in this way. Thenthe capsule is always in the motion states of uniform acceleration,uniform velocity or uniform deceleration. If the tension acquiredby the micro-force sensor is expressed as F, the frictional resis-tance of the capsule is

f ¼ F�ma ð1Þwhere m is the mass of the capsule and a is the acceleration.According to the motion parameters of existing capsule robot, thevelocity ranges from 1 mm/s to 50 mm/s and the accelerationranges from 1 mm/s2 to 1000 mm/s2 in the experiment. The modeof motion in the experiment is shown in Fig. 3. Each experiment isrepeated three times to ensure the repeatability of the experi-mental result.

4. Results

The experimental results describe the frictional resistance ofthe capsule at a certain velocity and acceleration. One of theexperimental results is shown in Fig. 4, where the velocity is20 mm/s and the acceleration is 100 mm/s2. In the figure, x-axis is

time while y-axes are frictional resistance (blue curve) andvelocity (green curve). All the data are acquired from DAQ cardwith the sampling frequency of 1 kHz. The synchronism betweenthe force and velocity signal is acceptable in this way. Because ofthe power frequency interference and alterable electromagneticwave interference (EMI), the force signal contains substantial noisesignal. Third polynomial order Savitzky–Golay smoothing filter isused to filter the noise. The filtering result is represented by reddotted line.

The variation trend of the frictional resistance can be seen as anirregular trapezoid approximately according to the filtering results.The process of frictional resistance increase is similar to a straightline segment. When the capsule moves at a constant velocity, thefrictional resistance fluctuates uncertainly because of the intestinalsamples' viscoelasticity and pleated surface. The frictional resis-tance decreases rapidly after the capsule stops because of stressrelaxation phenomenon of the intestinal material. However, thefrictional resistance does not decrease to zero in a long timebecause part of capsule's kinetic energy converts to intestinaldeformation energy. The process of frictional resistance increase isthe main research object, which is emphasized by an orangedotted box in Fig. 4. The frictional resistance increase can beapproximated to a first order linear change. Analyzing the fric-tional resistance variation quantitatively will make sense to theoptimization of the capsule robot's control method in the startingprocess. The duration, variation trend and original state of thefrictional resistance variation will be discussed in detail accordingto the experimental results.

5. Discussion

The curve of frictional resistance variation in the startingprocess of the capsule robot can be approximate to a straight linesegment according to the experimental results. If we want to getan analytical expression of a straight line segment, we need tomake certain its upper and lower bound, slope and initial value.The above three conditions can be understood as the duration ofthe frictional resistance increase, the final value of frictionalresistance increase and the original state before the frictionalresistance increases. Next we will discuss them respectively.Fig. 3. Capsule's mode of motion in the experiment.

Fig. 4. Experimental results when the capsule moves at a velocity of 20 mm/s and acceleration of 100 mm/s2.

C. Zhang et al. / Tribology International 70 (2014) 11–17 13

5.1. Duration

The duration of frictional resistance increase is different fromthe acceleration time of the capsule robot, as shown in Fig. 4.When the capsule robot reaches the state of uniform motion, theintestine does not extend completely and the frictional resistancecontinues to increase. In other word, the duration is the time thefrictional resistance changes from original state to final valuerather than the acceleration time of the capsule robot. The averagedurations at a certain velocity and acceleration are shown inTable 1. The data in each column indicates that the durationhas nothing to do with the velocity approximately. But theduration decreases with rising acceleration according to the datain each row.

There are fifteen experimental data of the duration at a certainacceleration. These data consist of three repeatable experimentaldata at five different velocities. The mean value of these experi-mental data is calculated and their quantitative relation will bededuced by curve-fitting. The relation between the duration andthe acceleration is shown by blue round dot in Fig. 5. Base-10logarithm of the acceleration is used as the independent variablebecause the accelerations have too much sample interval. Fig. 5also shows the average and standard deviation of each data.According to the results of calculations, all the coefficients ofvariance are smaller than 15%, which indicates that the dispersiondegree of each data is acceptable. Second order Gaussian approx-imation fitting is used to fit the data based on the variation trendof the experimental result. Fitting result is shown by red line inFig. 5. The analytic formula of the fitting is expressed as

DðaÞ ¼ a1expð�ððlgðaÞ�b1Þ=c1Þ2Þþa2expð�ððlgðaÞ�b2Þ=c2Þ2Þ ð2Þ

where D is the duration, a is the acceleration and the values offitting parameters are shown in Table 2. The goodness of fit isshown in Table 5, which indicates the fitting result correspondswell to the experimental result.

5.2. Variation trend

The process of frictional resistance increase is similar to astraight line segment. The slope of the line can explain thevariation trend directly. The duration of increasing friction hasbeen discussed in Section 5.1. Next, the final value of frictionalresistance increase needs to be analyzed to achieve the variationtrend analysis. According to the experimental result, the final valueincreases with the rising velocity but has nothing to do with theacceleration. This conclusion also has been proved by the others'research results [18,21]. The frictional value at one point cannot beused as the final value because the frictional resistance fluctuatesuncertainly when the capsule robot moves at a constant velocity.There are twenty-four experimental data of the final value at acertain velocity. These data consist of three repeatable experi-mental data at eight different accelerations. The mean value of thefrictional resistance at each velocity is seen as the final value of thefriction resistance, as shown in Fig. 4. The coefficients of varianceof each data is smaller than 8%. The relation between the finalvalue and the velocity is obtained by means of curve-fitting. Theblue round dot is actual frictional resistance, while the red line isthe fitting result, as shown in Fig. 6.

Third order polynomial approximation fitting is used to fit thedata. We can find that there is an obvious smooth phase at the

Table 1Average durations of frictional resistance increase.

1 mm/s2 10 mm/s2 40 mm/s2 70 mm/s2 100 mm/s2 400 mm/s2 700 mm/s2 1000 mm/s2

10 mm/s 5.137 1.261 1.235 0.665 0.723 0.681 0.445 0.54020 mm/s 4.934 1.650 1.146 0.692 0.912 0.737 0.421 0.52830 mm/s 4.677 1.437 1.129 0.617 0.947 0.598 0.469 0.50940 mm/s 4.759 1.755 0.997 0.771 0.717 0.658 0.432 0.37850 mm/s 4.821 1.528 1.167 0.748 0.810 0.661 0.349 0.489

Fig. 5. Relation between the duration and the acceleration.

Table 2Fitting parameters of duration.

Parameter Value

a1 7.613�1015

b1 �35.59c1 5.982a2 15.46b2 �12.50c2 8.331

Fig. 6. Relation between the final value and the velocity.

C. Zhang et al. / Tribology International 70 (2014) 11–1714

velocity of 20–30 mm/s in the fitting result. This phenomenon alsoappeared in my previous research [30]. It is proved that therelation between the frictional resistance and the velocity has aninflection point at the velocity of about 20 mm/s. The analyticformula of the fitting is expressed as

f ðvÞ ¼ p1v3þp2v

2þp3vþp4 ð3Þwhere f is the final value of frictional resistance, v is the velocityand the values of fitting parameters are shown in Table 3. Thegoodness of fit is shown in Table 5. The slope of the straight line isthe ratio of f(v) and D(a).

5.3. Original state

There may exist frictional resistance between the capsule andthe intestine when they are in a relatively static state. If theintestine is extended, the frictional resistance is caused by stressrelaxation of the intestine because of the viscoelasticity of theintestinal material. It is common for the capsule to move inter-mittently in normal work condition. In this case, the original stateof the frictional resistance is not zero, but a variational value overthe time after the capsule stops in the intestine. Fig. 7 shows thefrictional resistance decrease after the capsule stops under thecondition of different velocities and accelerations in the intestine.The experimental results indicate that the frictional resistancedecrease has nothing to do with the capsule's velocity andacceleration approximately. The curves in different cases can alsobe fitted by second order Gaussian approximation.

f 0ðtÞ ¼ a1expð�ððt�b1Þ=c1Þ2Þþa2expð�ððt�b2Þ=c2Þ2Þ ð4Þwhere f0 is the original state of frictional resistance, t is the timeand the values of fitting parameters are shown in Table 4. Thegoodness of fit is shown in Table 5. This conclusion agrees with myprevious work [31]. Both of the two conclusions have the samevariation trend, but different numerical value because of theotherness of the samples.

To sum up, the change of frictional resistance over time in thecapsule's starting process can be expressed by analytical expres-sion. The formula has relations with the capsule's velocity andacceleration. If the capsule moves from stationary state and theintestine has no deformation, it can be expressed as

f ¼ f ðvÞDðaÞt; tA ½0;DðaÞ� ð5Þ

If the capsule moves intermittently, the expression also has todo with the time interval between two successive motions. Inother words, the original state of the frictional resistance is notzero but a changing value over time.

f ¼f 0ðtÞ; tA ½0; t0Þf 0ðt0Þþ f ðvÞ

DðaÞðt�t0Þ; tA ½t0; t0þDðaÞ�

(ð6Þ

The situation described by Eq. (5) is an exceptional case inpractice. The capsule robot needs to start and stop frequentlywhenworking in the intestine. Therefore, the analytical expressionof Eq. (6) has more important practical application value. Anadditional experiment is conducted to test and verify the validityof the above analytical expression with the homemade uniaxialexperiment platform stated in Section 3. The capsule is pulled bythe driving unit in the intestine intermittently at a velocity of25 mm/s and acceleration of 500 mm/s2. The time intervalbetween two successive motions is 5 s. Fig. 8 presents a compar-ison of the frictional resistance variation with respect to timebetween the proposed analytical expression and the experimentalresult and the goodness of fit is show in Table 5. The comparisonresult clearly shows that the intestinal frictional resistance varia-tion in the starting process of the capsule robot obtained from theexperiment corresponds quite well to that of the proposedanalytical expression.

This article aims to reveal a law of the intestinal frictionalresistance in the starting process of the capsule robot and establisha model. The issue stated by the model is always neglected bymost researchers. The parameters of the model are different todifferent parts of the intestine because different parts of theintestine have different mechanical property. The parameters ofthe model are mainly obtained from experiments and furtheranalysis. Therefore, more experiments need to be conducted tomake the model be more universal. The achievement of the paperwill perfect the frictional resistance model of the capsule robotmoving in the intestine.

Table 3Fitting parameters of variation trend.

Parameter Value

p1 1.581�10�6

p2 1.289�10�4

p3 4.291�10�3

p4 3.175�10�2

Fig. 7. Frictional resistance variation with respect to time after the capsule stops.

Table 4Fitting parameters of original state.

Parameter Value

a1 6.263�106

b1 �9.564c1 2.222a2 7.020�109

b2 �439.3c2 84.70

Table 5Goodness of fit.

Goodness of fit Fig. 5 Fig. 6 Fig. 7 Fig. 8

SSE 5.117�10�2 1.56�10�5 4.103�10�3 4.469�10�3

R-square 0.9966 0.9964 0.9919 0.9605Adjusted R-square 0.9881 0.9911 0.9919 0.9589RMSE 0.1599 2.793�10�3 7.776�10�4 0.0013

C. Zhang et al. / Tribology International 70 (2014) 11–17 15

The capsule robot needs to start and stop frequently whenworking in the intestine. The driving force in the starting processof the capsule robot is different from that at uniform motionaccording to research result in the article. If the same driving forceis used in the two different phases, the capsule robot cannot movesmoothly. More energy is wasted to the capsule robot withinternal driving. A thorough friction model will make the controlstrategy more reasonable.

The relation between the resistance force and the character-istics of the capsule, such as weight, shape, dimension, material,contour line, surface preparation methods and so on, has been wellresearched when the capsule is static or in uniform motion.However, the influence of these characteristics in the startingprocess has not been studied intensively. In addition, complexintestinal environment has been one of the greatest obstacles ofthe development of the capsule robot. Different parts of theintestine have different diameters, wall thickness, lengths ofintestinal villi and so on. The influence of the biodiversity of theintestine on the frictional resistance can be revealed by a lot ofexperiments. More work needs to be done to research the materialcharacteristics of the intestine and the interaction characteristicsbetween the capsule robot and the intestine. That will be ourresearch direction in the future work.

6. Conclusion

The capsule robot's frictional resistance variation in the startingprocess has been analyzed in the paper. Actual frictional resistanceis measured by means of experiment. The intestine samplesprepared in the experiment is reasonable and the experimentalprocess is repeatable. The frictional resistance variation in thestarting process is similar to a bounded straight line according tothe experimental results. The upper and lower bound of the line isinversely proportional to the acceleration. The slope of the line hasto do with the capsule's velocity and acceleration. If the capsulemoves intermittently, the initial value of the line is relevant to thetime interval between two successive motions. The analyticalexpression is deduced by curve-fitting and tested to be effectiveby experiment. R-square between the analytical expression andthe experimental result is 0.9605. The phenomenon in the experi-ments is possibly caused by the special material propertiesand structure of the intestine and need to be further studied.

The achievement of the paper will perfect the frictional resistancemodel of the capsule robot moving in the intestine. Then thecontrol strategy will be optimized in the starting process of thecapsule robot according to the better frictional resistance model.The motion will be smoothed and more energy will be saved in thestarting process. Further, it is hoped to contribute to the develop-ment of the capsule robot in the future.

Acknowledgment

This work was supported by the National Natural ScienceFoundation of China (No. 61105099) and the National TechnologyR&D Program of China (No. 2012BAI14B03).

References

[1] El-Matary W. Wireless capsule endoscopy: indications, limitations, and futurechallenges. Journal of Pediatric Gastroenterology and Nutrition 2008;46:4–12.

[2] Faigel D, Cave D. Capsule endoscopy. Philadelphia: Saunders; 2007.[3] Munoz-Navas M. Capsule endoscopy. World Journal of Gastroenterology: WJG

2009;15:1584–6.[4] Saruta M, Papadakis KA. Capsule endoscopy in the evaluation and manage-

ment of inflammatory bowel disease: a future perspective. Expert Review ofMolecular Diagnostics 2009;9:31–6.

[5] Mishkin DS, Chuttani R, Croffie J, Disario J, Liu J, Shah R, et al. ASGE technologystatus evaluation report: wireless capsule endoscopy. Gastrointestinal Endo-scopy 2006;63:539–45.

[6] Lin MC, Dung LR, Weng PK. An ultra-low-power image compressor for capsuleendoscope. Biomedical Engineering Online 2006;5:14.

[7] Carey EJ, Leighton JA, Heigh RI, Shiff AD, Sharma VK, Post JK, et al. A single-center experience of 260 consecutive patients undergoing capsule endoscopyfor obscure gastrointestinal bleeding. The American Journal of Gastroenterol-ogy 2007;102:89–95.

[8] Wang K, Wang Z, Zhou Y, Yan G. Squirm robot with full bellow skin forcolonoscopy. In: Proceedings of the 2010 IEEE international conference onrobotics and biomimetics; 2010. p. 53–7.

[9] Quaglia C, Buselli E, Webster RJ, Valdastri P, Menciassi A, Dario P.An endoscopic capsule robot: a meso-scale engineering case study. Journalof Micromechanics and Microengineering 2009;19:11.

[10] Yang WA, Hu C, Meng MQH, Dai HD, Chen DM. A new 6D magnetic localizationtechnique for wireless capsule endoscope based on a rectangle magnet.Chinese Journal of Electronics 2010;19:360–4.

[11] Gao M, Hu C, Chen Z, Zhang H, Liu S. Design and fabrication of a magneticpropulsion system for self-propelled capsule endoscope. IEEE Transactions onBiomedical Engineering 2010;57:2891–902.

[12] Wang X, Meng QH, Chen X. A locomotion mechanism with external magneticguidance for active capsule endoscope. In: Proceedings of the annual inter-national conference of the IEEE Engineering in Medicine and Biology Society(EMBC); 2010. p. 4375–8.

[13] Kim B, Park S, Park JO., IEEE. Microrobots for a capsule endoscope; 2009. p.729–34.

[14] Kim Y-T, Kim D-E. A novel propelling mechanism based on frictional interac-tion for endoscope robot; 2009.

[15] Ito T, Ogushi T, Hayashi T. Impulse-driven capsule by coil-induced magneticfield implementation. Mechanism and Machine Theory 2010;45:1642–50.

[16] Li H.Y., Katsuhisa F., Chernousko F.L. Motion generation of the capsubot usinginternal force and static friction. In: Proceedings of the 45th IEEE conferenceon decision and control; 2006. p. 6575–80.

[17] Terry BS, Lyle AB, Schoen JA, Rentschler ME. Preliminary mechanical char-acterization of the small bowel for in vivo robotic mobility. Journal ofBiomechanical Engineering-T ASME 2011;133:091010 1–7.

[18] Kim JS, Sung IH, Kim YT, Kwon EY, Kim DE, Jang YH. Experimental investiga-tion of frictional and viscoelastic properties of intestine for microendoscopeapplication. Tribology Letters 2006;22:143–9.

[19] Wang KD, Yan GZ. Research on measurement and modeling of the gastrointestine's frictional characteristics. Measurement Science and Technology2009;20:015803 1–6.

[20] Li J, Huang P, Luo HD. Experimental study on friction of micro machinessliding in animal intestines. Lubrication Engineering 2006;175:119–22.

[21] Wang X, Meng MQH. An experimental study of resistant properties of thesmall intestine for an active capsule endoscope. Proceedings of the Institutionof Mechanical Engineers Part H—Journal of Engineering in Medicine2010;224:107–18.

[22] Zhang C, Su G, Tan R, Li H. Experimental investigation of the intestine's frictioncharacteristic based on “internal force-static friction” capsubot. In: Proceed-ings of the IASTED international conference on biomedical engineering.Innsbruck, Austria; 2011. p. 117–23.

[23] Zhang C, Liu H, Tan RJ, Li HY. Modeling of velocity-dependent frictional resistanceof a capsule robot inside an intestine. Tribology Letters 2012;47:295–301.

Fig. 8. Comparison of the frictional resistance variation with respect to timebetween the proposed analytical expression and the experimental result whenthe capsule moves intermittently with a velocity of 25 mm/s and acceleration of500 mm/s2.

C. Zhang et al. / Tribology International 70 (2014) 11–1716

[24] Lyle AB, Luftig JT, Rentschler ME. A tribological investigation of the smallbowel lumen surface. Tribology International 2013;62:171–6.

[25] Hoeg HD, Slatkin AB, Burdick JW, Grundfest WS. Biomechanical modeling ofthe small intestine as required for the design and operation of a roboticendoscope. In: Proceedings of IEEE international conference on robotics andautomation; 2000. p. 1599–606.

[26] Ciarletta P, Dario P, Tendick F, Micera S. Hyperelastic model of anisotropicfiber reinforcements within intestinal walls for applications in medicalrobotics. International Journal of Robotics Research 2009;28:1279–88.

[27] Woo SH, Kim TW, Mohy-Ud-Din Z, Park IY, Cho J-H. Small intestinal model forelectrically propelled capsule endoscopy. BioMedical Engineering Online2011:10.

[28] Kim JS, Sung IH, Kim YT, Kim DE, Jang YH. Analytical model development forthe prediction of the frictional resistance of a capsule endoscope inside anintestine. Proceedings of the Institution of Mechanical Engineers Part H—Journal of Engineering in Medicine 2007;221:837–45.

[29] Lyle AB. Evaluation of small bowel lumen friction forces for applications inin vivo robotic capsule endoscopy [M.S.]. United States – Colorado: Universityof Colorado at Boulder; 2012.

[30] Zhang C, Liu H, Tan R, Li H. Modeling of velocity-dependent frictional resistanceof a capsule robot inside an intestine. Tribology Letters 2012;47:295–301.

[31] Cheng Z, Gang S, Hongyi L. Motion control research of internal force-staticfriction capsubot in intestine. In: Proceedings of the IEEE/ICME Internationalconference on Complex Medical Engineering (CME); 2011. p. 346–51.

C. Zhang et al. / Tribology International 70 (2014) 11–17 17