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A linearly-actuated capsule endoscope and an artificial intestine in which to test it

Paul Glass1, Kartik Thumbavanam Padmanabhan2

1 Department of Biomedical Engineering, pglass@andrew.cmu.edu2 Robotics Institute, kthumbav@andrew.cmu.edu

Carnegie Mellon University, Pittsburgh PA 15213, USA

Abstract – This paper presents the conceptual designfor a linearly-actuated stopping mechanism to improve the functionality of currently existing capsule endoscopes under development at Carnegie Mellon University and elsewhere. Mathematical and physical modeling of one optimized solution is presented. An idea for a device to simulate the peristaltic contractions of the human intestine is also presented. This would allow future capsule designs to be tested in-vitro in a physiologically realistic environment to ascertain their viability for eventual clinical use.

Index terms: micro robots, biomedical robots, capsule endoscopy

I. INTRODUCTION

Capsule endoscopes are pills containing a light source, a camera and a transmitter which are swallowed by a patient to image their intestinal tract to aid in disease diagnosis. Current capsule endoscopes on the market [1] are passive and the clinician has no control over the capsule’s orientation and position, resulting in a potential for missed diagnoses. Other research groups have attempted to add mechanisms that would allow a capsule to be controlled by a clinician while it passes through the gastrointestinal tract, allowing the operator to stop and/or move the capsule along the length of the tract at will [2-6]. No stopping or locomotion capsules have yet been tested successfully in animal or human trials. The current gold standard at the NanoRobotics Laboratory at Carnegie Mellon University is a three-legged stopping mechanism actuated by shape memory alloy [7]. While the legged mechanism for attachment and adhesion has shown promise in preliminary testing, there are some disadvantages to working with SMA wire: its behavior is nonlinear, it has a low frequency of actuation, it is

extremely inefficient and it tends to heat to high temperatures which may damage the body’s tissues. There is also a lack of a suitable environment for testing the capsule to adequatelymimic the dynamic environment of the human intestinal tract.

Because of these limitations to capsule endoscopy development at that NanoRobotics laboratory, a linearly-actuated capsule mechanism that avoids using SMA wire is presented in Section II of this paper. Section III discusses a design for a simulated intestine environment for testing capsule endoscope.Section IV examines the promise of these two proposals and looks at directions for future research.

II. LINEARLY ACTUATED CAPSULE

By having a linearly actuated stopping mechanism, it will be possible to reduce the number of actuators from one per leg (as with the SMA-actuated capsule) to one for the entire device. The motion of actuation is analogous to that of an umbrella, where you actuate in one degree of freedom (along the central axis) and the umbrella opens outward. In this case, a linear actuator would deploy a similar three-legged stopping mechanism to that developed previously [7].

One possible solution for such a mechanism would be to actuate a single plate which is in contact with three legs. These legs would be fixed to a pin joint and shaped in such a way that a small linear displacement of the plate would result in a large angular displacement of the leg.

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A visualization of this concept is shown in Fig. 1. This figure shows the initial configuration of the leg when the capsule would be in an undeployed (passive) state and the actuated configuration when the legs would be deployed.

Fig. 1: Model geometry in initial and final configurations

With the application of an applied force Fapp on the plate, the legs bend from the initial angle of θ degrees from the horizontal to an angle (θ + φ). L is the distance between the undeployed leg and the intestine wall, x is the leg length, y is length of the angled portion of the leg, h1 is the initial radial position of the contact point of the leg with the plate and h2 is the final position of this contact point. Tapplied is the torque induced on the hinge by Fapp and d is the distance that the moving plate travels between the initial and final configurations. The geometry of the system can be defined by the equations:

a = y cosθ [1]h1 = y sinθ [2]

a - d = y cos (θ+φ) [3]h2 = y sin (θ+φ) [4]L = x sin (φ) [5]

The torque Tapplied applied on the mechanical hinge in the leg mechanism can be calculated as the cross product of the force applied on the intestine and the perpendicular distance from the centre of the legs. For each leg, this torque is divided by three since the applied force of actuation will be evenly divided between the three legs of the stopping mechanism.

Tapplied = Fapp . y sin (θ) / 3 [6]

The net resultant force on the tip of the leg is the product of the torque and the leg length x:

Fresult = Tapplied . x [7]

The preload force on the walls of the intestine is the portion of this force that is perpendicular to the intestine wall. It is obtained from:

Fwall-preload = Fresult * cos (φ) [8]

We are interested in optimizing the value of Fwall-preload for a given L and Fapp by determining values for the following geometrical parameters:

Design Parameters: x, y, θ, h1, h2, a

Further constraints are needed to limit the capsule to a size that can be comfortably swallowed and pass through the digestive system. These constraints are comparable to dimensions for existing capsules being developed in the NanoRobotics Laboratory.

Design Constraints: y = 1-8 mm, θ = 5-75o, Total leg Length = (a+x) = 20 mm, h2 = <7.5 mm.

Matlab was used to run numerical simulations to optimize the geometrical parameters to

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maximize the preload force applied on the intestinal wall. A Monte Carlo Analysis was conducted torandomly generate values for uncertain variables continuously to simulate a model. The simulation calculated multiple scenarios of the model by repeatedly sampling values from the probability distributions for the uncertain variables and using those values for the cell. Random values are assigned for each of the design constraints within their specified range and the corresponding force calculations are analyzed by an iterative procedure until the best possible solution is found. The optimum geometric values obtained through this method were: x = 12.8 mm, y = 7.8 mm, a = 7.2 mm, h1 = 3 mm, h2 = 6.59 mm.

The predicted normalized preload force Fwall-preload / Fapp for this geometry is plotted against the distance from the intestinal wall in Fig. 2.

Fig. 2: Preload force induced on the intestinal wall for varying L.

Using the parameters generated from this simulation, a CAD model was created using SolidWorks to assess the viability of this solution. Fig. 3 shows a side view of this model with a single leg in orange in the undeployed state. When the moving plate (in blue) moves upwards, the tip of the legs will move outward towards the intestinal wall into the deployed state, shown in Fig. 4.

Fig. 3: CAD model of device in undeployed state (side view)

Fig. 4: CAD model of device in deployed state (top view)

III. SIMULATED INTESTINE ENVIRONMENT

In order to determine the viability of anycapsule prototype, testing should be conducted to explore how it will react in a dynamic environment that closely mimics the conditions in the small intestine. While a proposed stopping or locomotion mechanism may show promise in a static, rigid tube, the environment in the intestine is quite different. As many of the physical parameters of the intestine as possible should be duplicated. Such parameters include the peristaltic forces exerted by the intestine wall on the capsule, the mechanical properties of the wall itself, the friction properties at the interface

Normalized Preload Force vs L

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of the capsule and the intestine wall, the body pressure both within the intestine lumen and outside the lumen in the abdominal cavity and the chemical properties of the contents of the lumen.

The mechanical properties of the intestine and the frictional properties between the capsule and the wall can be recreated by using freshly obtained porcine intestine samples and incorporating them into the design on the simulator. If this tissue can be actuated to recreate intestinal peristalsis, a capsule placed within it will be in a physiologically realistic environment.

There are two dominant muscle layers in the intestinal wall which contribute to the peristalticpropulsion of the lumen contents. Circular muscle fibers are oriented perpendicularly to the axis of the intestine to form muscle rings which constrict the intestine when contracted. Longitudinal muscles are oriented along the axis of the intestine. Contraction of these fibers shortens the intestine. The synchronized contraction and relaxation of these muscle fibers results in the peristaltic waves that force chyme (food and digestive secretions) through the intestinal tract. To simplify this system, the resultant force on a capsule endoscope can be broken down into the contributions from the circular muscle fibers which act in the radial direction Fr and the contributions from the longitudinal muscle fibers which act in the axial direction Fl, as shown in Fig 5.

Fig. 5: Forces acting on a capsule in the small intestine.

Literature values for these forces obtained through numerical simulations of the electrical activity in the small intestine are Fr = 26.9 g/cm, Fl = 17.2 g/cm with a wave propagation speed of 0.08 cm/s [8].

A two degree-of-freedom system that can recreatethis behavior is presented in Fig. 6 and 7. In this device, a length of fresh pig intestine would be firmly fixed to hose connections attached to the walls of a

tank. A capsule could be placed within the intestine and a carriage traveling along a linear track could be pulled by the weights attached to a pulley system to duplicate the longitudinal peristaltic forces Fl and wave propagation speed as shown in Fig. 6.

Fig. 6: Longitudinal force application in intestine simulator

A close-up of the carriage in Fig. 7 shows how the radial forces would be applied. Springs mounted to brackets push rollers into the intestine, resulting in force values proportional to the spring constant and bracket displacement. The weight attached to the pulley and spring constant of the carriage springs can be chosen to generate physiologically realistic force values.

Fig. 7: Radial force application in intestine simulator

The tank and intestine can also later be pressurized to match the apparatus conditions with the conditions in the abdominal cavity. A “proof of concept” device using this design was

Fr

Fl

Fl

Fl

Fr Fr

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machined out of acrylic and aluminum and is shown in Fig. 8 and 9. The carriage travels along two aluminum rails, actuated by the force of a falling mass (not shown). The two rollers in Fig 9 simplify the radial forces that act around the intestine circumference into two unidirectional forces.

Fig. 8: Proof of concept intestine simulator

The simulator is still under construction at this time and several issues need to be resolved before proceeding. Binding in the longitudinal direction is preventing smooth motion along this axis. Some imprecision induced by milling the acrylic panels of the carriage have generated friction in the radial direction. This will be improved by cutting rectangular slots in new panels with a laser cutter. Also, the springs currently in use were incorrectly selected for duplicating the radial intestinal forces. With two rollers and approximate radial displacements of 1 cm per spring, springs with 80 N/m spring constants should be chosen while the springs currently in use are rated at 1270 N/m. These last two factors currently result in radial forces much greater than the physiological values presented above, preventing any useful testing from being possible at this time.

IV. CONCLUSIONS

The linearly-actuated capsule presented in Section II does not seem to be a viable actuation method. Because of constraints on the size of the capsule, the legs cannot be deployed far from the shell wall, potentially making it hard to adhere to a surface that the legs are not initially in contact with. Although not optimized solutions, potential models with shorter values for y should be explored to see if an actuator could be located to provide sufficient forces for this design to be clinically useful in diagnosing and treating diseases of the small intestine.

The intestine simulator under construction does show promise as a tool for testing stopping and locomotion capsule endoscope prototypes. Future efforts should be made to improve the assembly of the device to eliminate the binding and friction issues currently exhibited. Also, a method to more accurately simulate the radial forces around the entire intestine diameter and not in a single uniaxial direction should be made. This could possibly be accomplished by replacing the spring-mounted rollers with an elastic ring circling the intestine tissue with the material properties of the ring chosen to emulate the intestine’s radial forces.

Fig 9: Proof of concept radial force application

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V. REFERENCES

[1] http://www.givenimaging.com

[2] B. Kim, S. Lee, J.H. Park, and J.O. Park. “Design and fabrication of a locomotive mechanism for capsule-type endoscopes using shape memory alloys (SMAs)”, IEEE/ASME transactions on mechatronics, vol. 10, no. 1, 2000.

[3] A. Menciassi, A. Moglia, S. Gorini, G. Pernorio, C. Stefanini, and P. Dario. “Shape memory alloy clamping devices of a capsule for monitoring tasks in the gastrointestinal tract”, J. Micromech. Microeng., vol. 15, pp. 2045-2055, 2005.

[4] D. Reynaerts, J. Peirs and H. Van Brussel. “Shape memory micro-actuation for gastro-intestinal intervention system”, Sensors and Actuators, vol. 77, pp. 157-166, 1999.

[5] Y. Guozheng and Z. Jianyong. “A self-propelling endoscope system by squirmy robot”, Proc. of 2003 international symposium on micromechatronics and human science. pp. 159-163, Nagoya, October 2003.

[6] P. Breedveld. “Development of a rolling stent endoscope”, IEEE / RAS-EMBS international conference on biomedical robotics and biomechatronics. Pisa, February 2006, presented.

[7] M.E. Karagozler, E. Cheung, J. Kwon, and M. Sitti. “Miniature endoscopic capsule robot using biomimetic micro-patterned adhesives”, IEEE / RAS-EMBS international conference on biomedical robotics and biomechatronics. Pisa, February 2006, presented.

[8] Miftahof, RN., “The Wave Phenomena in Smooth Muscle Syncytia”, In Silico Biology, vol 5(5), 479-498, 2005.