An Automated Intestinal Biomechanics Simulator for Expediting Robotic Capsule Endoscope Development1 (2014)

Robotic capsule endoscopes (RCEs) are pill shaped devices used for noninvasive imaging, chemical sensing, performing simple surgical procedures, and active locomotion through the bowel lumen [1]. Currently, there is no accurate in vitro method for testing RCEs during development, and thus, they are prematurely tested in vivo at a cost of about $1400 per swine test [2]. The purpose of this research was to develop an in vitro method of testing RCEs without the need for repetitive animal experimentation. We have developed a device that simulates the biomechanical behavior and environment of the small intestine to expedite the development of RCEs. This device (Fig. 1Fig. 1Intestine biomechanics simulator with mounted synthetic intestine (not filled)) supports two independently controlled compressive pressure vessels that mimic peristaltic waves and traverse at a speed corresponding to that of human peristaltic waves: 0.08–2 cm/s [3–6]. The simulator was authenticated by calibrating the cavity pressure of the vessels to exert the main components of peristaltic force within human physiological ranges: traction force (4–40 gf) and contact force (80–500 gf). These values ascertain the biomimicry of the simulator and thus allow for experimentation of RCEs. Mounted intestines are kept moist by humidifying the device, thus creating an environmental chamber. The use of such a device will save researchers both time and money in research and development of RCEs, allowing for sooner commercialization of this emerging noninvasive technology.The human intestine propels chyme using peristaltic and segmentation waves. Peristaltic waves advance the chyme while segmentation waves mix it for better nutrient absorption [3]. The main traversing agents of these waves, and thus the focus of the simulator's calibrations, are traction and contact forces. The coalition of these forces allows for wave propagation velocities of 0.08–2 cm/s [3–6]. The intestinal biomechanics simulator was designed to mimic two traversing waves to allow for biomimicry of both peristalsis and segmentation.The simulator (L.6 m × W.3 m × H.3 m) was designed to accommodate a 3.5 cm diameter reusable synthetic intestine (Fig. 2). During experimentation, a synthetic intestine (O-SIN-A-0005, SynDaver, Tampa, FL) is mounted on two cylindrical openings on each end of the tank housing. Though the simulator can operate with a porcine intestine, a synthetic intestine is used for both sanitation purposes and higher resilience to deformity. The synthetic intestine contains preloaded chyme and is validated against mechanical, physic-chemical, thermal, and dielectric properties of the small intestine. The intestine is also compatible with all known surgical devices, including suction devices, which are a component of the RCE testing we are conducting. As with a real intestine, the properties of the model intestine alter if the tissue dries out. To prevent such occurrence, the intestine-holding environmental chamber is held at 100% humidity. A high humidity also decreases external intestine friction and thus minimizes tissue deformation from the pressure vessels' motion, which would not be experienced in vivo.During experimentation, RCEs are inserted automatically via servo motor linkage at one of the openings. This automation allows for minimal user interaction during experimentation to minimize experimental variance. The simulated peristaltic waves traverse via two independently controlled linear actuators consisting of a stepper motor, lead screw, nut, and linear slide. The compression mechanism, referred to as a pressure vessel (Fig. 3), resembles a donut shape solid with an expandable inner wall. The inner layer of the pressure vessel is a polyisoprene tube that expands upon pressure increase. The pressure vessel contains two ports: for air inlet, modulated via 200 kPa dynamic pressure regulator, and pressure transducer (4426-005G, Digi-Key, Thief River Falls, MN) outlet.To ensure biomimicry, the two main force components of peristaltic force are measured: traction (force measured in the axial direction) and contact (force measured in the radial direction). The traction force is measured by holding a capsule stationary in the intestine via string attachment to a load cell while “waves” pass over it. The contact force is measured using two coupled lead cells that, with 3D printed attachments, resemble a capsule with a 1.5 cm diameter. This measurement is taken statically. The two capsule-like load cells measure the squeezing effect of the pressure vessels. By measuring the traction and contact forces, the pressure inside the vessels can be adjusted to ensure that the generated peristaltic wave can be considered similar to a contraction in the human small intestine.Due to an ability to dynamically adjust pressure in the pressure vessels, the simulated wave must be kept within human physiological values. After each polyisoprene tube is replaced, a linear relationship is determined between vessel pressure and traction or contact forces. Knowledge of such relationship allows for a user to readily adjust applied traction and contact forces.The biomechanics simulator is controlled via microcontroller circuits coupled with stepper motors, servo motors, pressure transducers, and solenoid valves. Microcontrollers allow for the system to be automated allowing for single-user experimentation and minimizes ambiguity of results.Past studies have determined human physiological force values both experimentally and analytically. Measurement experiments have resulted in traction forces of 5.9–9.36 gf [4], and contact forces of 479 gf [4,7]. Active biomechanical properties of the GI tract during bolus progression have been calculated to be 32–60.2 gf and 94–660 gf for traction and contact force, respectively [6,8]. From these values, our optimum force ranges were determined to be 6–40 gf and 80–500 gf for traction and contact force, respectively.A cost-benefit analysis of using the simulator has been estimated over a 12 month period. Considering a year-long timeline of RCE development, a research group is assumed to conduct one porcine experiment every two months ($1400). It is assumed that the simulator's synthetic tissue must be replaced each month ($150) and the pressure vessel must be replaced every three months ($150). The simulator cost is estimated to be $1700 including direct materials and manufacturing cost considering a $30/h assembly cost and a 10 h assembly time. The 12 months will consist of an initial payment of $1700, monthly recurring payments of $150 (synthetic tissue) and $150 (pressure vessel) every three months. With a net cost of $3800 per year, the simulator will produce an annual savings of $4600. Not only is this a monetary savings, but this analysis assumes one in vivo test every two months while multiple in vitro simulator tests can be conducted each week.From the similitude of wave propagation speed as well as traction and contact forces experienced by an RCE in vivo as compared the simulator, we can deduce that this novel device successfully mimics the biomechanical properties of the human intestine while saving experimenters time and money in RCE development. Such validation along with maintaining intestinal moisture allows for iterative testing of RCEs at a low cost, which would otherwise take longer in scheduling and setting up costly porcine experiments.This simulator consisting of two pressure vessel waves operates automatically via microcontrollers. Such automation allows for wave pattern planning as well as accurate test repetition. Wave pattern planning allows a single user to perform an experiment while watching the RCE traverse by either peristaltic or segmentation waves. Dynamic pressure feedback from the vessels allows for monitoring the applied traction and contact forces on the RCEs to be within human physiological range at all times to ensure experimental authenticity.The simulator is currently being used with porcine intestines for the development of an RCE that resists intestinal propulsion. This device does not replicate the vast curvature of the human intestine, thus further work must be performed to better represent the human GI tract. Applying such simulators in laboratories will expedite the advent of noninvasive surgical procedures performed by RCEs.