Multi - capsule endoscopy: Demonstrations of inter - capsular control and (tactile) sensing

Abstract

Wireless Capsule Endoscopy (WCE) is an emerging Gastrointestinal (GI) tract imaging and treatment method that is developed to be a better alternative to the traditional endoscopy/colonoscopy devices and procedures. Thanks to pill shaped design and dimensions (22-26 mm in length, 10-13 mm in diameter), patient can easily swallow a single WCE where the capsule travels through the GI tract, and leaves the body. The complete non-invasive monitoring of the small bowel is possible in this way which is not possible in conventional endoscopy/colonoscopy. Even tough the WCE has very important role in the GI diagnostics in the clinics, there are several limitations to its usage. First of all, it is still cannot take any biopsy sample or make therapeutic actions in the clinical applications. Nonetheless, clinical operator can not control locomotion of the capsule through GI tract, which affect the monitoring process negatively. Also, WCE cannot perform sunctioning, flushing etc. which can be possible with conventional endoscopy. Due to its size limitation because of the patient comfort, battery life is limited and this might result with an incomplete examination. There are exhaustive studies to find solution to these shortcomings in the literature which we will be also working with the similar aim. Commercial capsules have a CMOS camera integrated on both tips and takes video stream and/or photographs while travelling. A clinician interprets this visual data to diagnose any issues with GI tract. Besides imaging, numerous literary studies on capsule endoscopy have demonstrated drug delivery, navigation strategies, tactile sensing for tumor diagnosis, and biopsy to increase the functionality of the WCE. While each function can work individually, using these in conjunction is needed to achieve complex treatment methods without any invasive process. Yet, the size limitation due to patient comfort hampers the availability of multiple features within a single capsule. In the first two parts of our study, in an effort to increase the space and functionality, we propose the usage of multiple capsules in conjunction. For our method, capsules together form a capsule-train in GI tract, whose wagons are connected with magnetic push/pull forces without any contact to each other. We focused on contactless/wireless connection between capsuels due to possible tissue damage. By knowing the distance between capsules, several functions can work in junction, e.g. the first capsule uses camera to find a tumor and second capsule takes a biopsy sample accurately thanks to known distance between capsules. For the first section, we have used passive magnets to achieve a wireless force connection between two capsules. After trying several magnet arrangamets on the tips of the capsules, we finalized a design where two capsules has a balanced constant distance in-between by using both pulling and pushing forces. We have used two large ring magnets that pulls each other and two cylindrical magnets that pushes each other. Cylindrical magnets are placed on the tips of the capsules while the ring magnets are placed with more distance to each other. Here, pulling force is dominant at large distances due to ring magnets and pushing force is dominant at smaller distances due to cylindrical magnets. This arrangament achieves constant distance in-between capsules without any energy consumption. Distance value is determined by the magnet sizes and arrangement. However, this method only works in thight tube-like shapes where tips of the capsules can not misalign, which might result with a clinch between a tip capsule with a ring capsule. To test the passive connection, we have used straight plastic tubes where we move capsules together with a constant in-between distance. Here, we pulled one of the capsules with a stepper motor and monitored in between distance with a camera placed above the test setup where an image processing code is ran to monitor the distance. We have achieved the capsule train without any connection breaks for typical bowel movement speed. As the second section, we improved our capsule train model to be more applicable on more challenging real life environments. Since typical human bowel diameter is ~25 mm while WCE diameter is ~12 mm, two capsules will have an angle between their tips, which results with a difference in between force due to angle. Here, we demonstrate an active distance control model with a closed loop control via the placement of a sphere permanent magnet on one capsule and a solenoid on the other capsule. Hall Effect Sensors have employed to determine distance between capsules. A PID controller have been developed to achieve stabilized desired distance between capsules by manipulating solenoid current. Experiments were conducted by pulling the leading capsule at typical human peristalsis speed. An inter-capsule distance of 1.94 mm was achieved on the average for the desired distance as 2 mm on 3D-printed plastic phantoms, while 0.97 ± 0.28 mm of distance was observed for the ex-vivo bovine tissue, for a set distance of 1mm. By achieving successful demonstration of inter-capsule control, this work substantiates realizability of multi capsule endoscopy for future studies. As the third part of our study, we focused on to develop a novel diagnostic tactile sensing method to use in capsule endoscopy, which supports our multi-capsule approach by adding a new method to WCE functional palette. Since palpation is a widespread diagnostics method for clinicians, using this method inside GI tract will be an useful addition since some of the abnormalities such as inflammation of early stage tumors cannot be seen on visual imagery while having higher elasticity modulus than healthy tissue. Planned tactile sensing model measures the tissue elasticity modulus. In our fourth part this study, we will be presenting our tactile sensing mechanism that adopts the atomic force microscopy (AFM) methodology and fits it into a single WCE volume. AFM is used to achieve surface imaging up to nanoscale levels of resolution by scanning a moving cantilever through targeted area. On each discrete scanning position on the sample, cantilever base moves down to make cantilever tip interact with the surface. Tip deflection occurring due to interaction between the cantilever tip and the surface gets recorded. Mechanical surface properties such as topology, elasticity, adhesivity etc. can be deducted from this data by Hertz contact model to be used to inspect the tissue healthiness. Main difference of our model is having a different cantilever tip displacement measurement method. Conventional AFM uses a lazer beam reflecting from the cantilever tip and lands on a 4 part photodiode sensor where the sensor output is directly related to cantilever tip deflection. Since using such system in a single capsule is not possible with current hardware, we focused on using a different sensing method and decided to use a piezoelectric material attached onto cantilever. As cantilever tip bends, piezomaterial placed onto cantilever is also bends and generates charge. Amount of generated electric charge, therefore the current output, is directly related to piezo deformation. By using this mechanism, we can use the same elasticity modulus measurement procedure with AFM approach, which is widely used in practical applications. We planned to use a micro stepper motor in the capsule due to size limitations. We have 3D modeled the cantilever and inner-capsule holding parts around the typical WCE and motor dimensions. Since each manufactured cantilever and piezo material is unique, we needed to calibrate each assembly on a glass surface where no indentation occurs. Also, we needed a charge amplifier circuit to read piezo signal output since the used piezo sheets has ~10 mm3 area with a very small electrical charge generation. The test procedure calculates the stiffness of the cantilever by finding it's resonant frequency. We need this value to find force acting on the tip of the capsule. By knowing the piezo output on glass and the stiffness of the cantilever, we were able to start our test on different materials and tissues. We have prepared two jellies with different densities to experiment on. Also, we used chicken breast, bovine liver, sheep stomach, cow stomach, and human colon tissue as tissue tests. After all measurements, we were able to compare real elasticity values of jellies, chicken breast and bovine liver tissues to measured values by the cantilever. After all test, we have obtained 30% maximum error on chicken breast, 15% on bovine liver, %33 on stiffer jelly and %49 on the second jelly. Since the tissues with inflammation or tumors have 4x to 5x larger elasticity values than healthy tissues, there results would be usable to determine the state of the inspected tissue which is our main motivation for this section. As an additional study, which can be identified as the fourth section of this study is about 3D locomotion of a single WCE (with passive magnets inside) in stomach by an externally controlled electromagnet array used by an clinician. Thanks to this mobility, clinician can see any desired area in the stomach. Here, WCE position and angle is controlled by 5 electromagnet array placed under the patient in lying position. Capsule tip levitation is controlled by a single, larger electromagnet placed above the patient. As my study, I have simulated the angular position of WCE by alternating the electromagnet currents to achieve any desired distance. Later on, I conducted rotation, levitation and displacement experiments on 3D printed PLA bovine stomach model. Also, same experiments done in ex-vivo environment where bovine stomach tissue is paved into 3D printed PLA model. These experiments succesfully valiated the applicability and accuracy of 3D control with 6 magnet array arrangement.