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Wireless powered capsule endoscopy for colon diagnosis and treatment
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Physiol. Meas. 34 1545 (http://iopscience.iop.org/0967-3334/34/11/1545) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
PHYSIOLOGICAL MEASUREMENT
Physiol. Meas. 34 (2013) 1545–1561
doi:10.1088/0967-3334/34/11/1545
Wireless powered capsule endoscopy for colon diagnosis and treatment Wenwen Chen, Guozheng Yan, Shu He, Quan Ke, Zhiwu Wang, Hua Liu and Pingping Jiang Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China E-mail: [email protected]
Received 25 April 2013, accepted for publication 25 September 2013 Published 23 October 2013 Online at stacks.iop.org/PM/34/1545 Abstract This paper presents a wireless power transfer system integrated with an active locomotion and biopsy module in an endoscopic capsule for colon inspection. The capsule, which can move automatically, is designed for non-invasive biopsy and visual inspection of the intestine. To supply enough power for multiple functions and ensure safety for the human body, the efficiency of the current power transmission system needs to be improved. To take full advantage of the volume in the capsule body, a novel structure of receiving coils wound on a multi-core of MnZn ferrite hollow cylinder was used; with this new core, the efficiency increased to more than 7.98%. Up to 1.4 W of dc power can be delivered to the capsule as it travels along the gastrointestinal tract. Three micro motors were integrated for pumping, anchoring, locomotion and biopsy. A user interface and RF communication enables the operator to drive the capsule in an intuitive manner. To gauge the efficacy of the wireless power supply in a simulated real-world application, the biopsy and locomotion capabilities of the device were successfully tested in a slippery, soft tube and gut environment in vitro. Keywords: active capsule endoscopy, wireless power transfer, biopsy, colon (Some figures may appear in colour only in the online journal)
1. Introduction The current enteroscopic technique has been performed in clinical treatment widely. However, with the cable at the end, the diagnosis range would be limited and push action would be painful for the patients. Wireless capsular endoscopic (WCE) devices have gained increasing acceptance as useful tools for early diagnosis of cancer and other diseases affecting the 0967-3334/13/111545+17$33.00
© 2013 Institute of Physics and Engineering in Medicine
Printed in the UK & the USA
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gastrointestinal (GI) tract (Moglia et al 2007). Most commercial devices use two watch batteries that provide an average of 25 mW of power, rendering them operational for only about 6 h (Iddan et al 2000), which is clearly inadequate for incorporating advanced robotic features. A dedicated inductive powering system (with a 1 MHz carrier to limit absorption by the human body) that can support the transfer of over 300 mW has been developed (Carta et al 2011). Moreover, future capsule endoscopes are expected to perform even more complicated and energy-consuming tasks in addition to basic locomotion: biopsy, micro surgical procedures and other related functions. Therefore, significant efforts have been dedicated to developing alternative energy-supply options (Van Gossum and Ibrahim 2010); among these, the wireless power transfer (WPT) (Shiba et al 2010, Arena et al 2005, Lenaerts and Puers 2007), in which power is delivered to the endoscope from a power source outside of the human body, is a promising replacement. Various methods have been developed in an attempt to provide a steady and sufficient supply of energy to an endoscopy capsule. Among these, the multi-dimensional receiving coil proved to be effective for solving randomicity of the capsule attitude (Lee et al 2006, Lenaerts and Puers 2005). A ferrite core was adopted to enhance transcutaneous energy transmission by optimizing coupling with the external field (Miura et al 2006, Lenaerts and Puers 2007). Clearly, power efficiency is essential in WPT-based endoscopes. Limited by the capsule space, the size and corresponding efficiency of wirelessly transmitted power would max out at 5% with the present 200 mm distance between the capsule and the transmitting coil and the 3D orthogonal ferrite receiving core. Locomotion and other functionality would be hindered by this limited power. To address these challenges, this paper presents a WPT system for capsule endoscopes with a hollow circular cylinder multi-core of dimensions 16 mm × 23 mm that can provide a power supply of up to 1.4 W. Most commercial pills rely on peristalsis to propel them through the GI tract. However, capsules for the colon must be equipped with an active locomotion system, designed to cope with the large lumen diameter (typically 2–3 cm) to provide more than a randomized view of the colon (Quirini et al 2007). Modules with legs or paddles have been developed for navigating the intestines (Buselli et al 2010, Hyunjun et al 2007), and an external magnetic field has been used to drive capsules wirelessly (Simi et al 2012). Some devices employ an incremental clampingand-sliding method to move like an earthworm or inchworm (Kundong et al 2008, Guanying and Guozheng 2007, Byungkyu et al 2004). The long, thin body adapts to the intestinal tube environment and can incorporate more therapy tools than basic visual inspection. Two independent motors control the extending mechanism and the pumping balloon for anchoring. The capsule is completely sealed within a soft silicone shell that exerts only gentle force when in contact with the colon. Another important function for WCE is a diagnostic and treatment capability. For example, when a medical professional finds a polyp, WCE may take a sample of the tissue for more precise diagnosis. Two kinds of biopsy mechanism were introduced, and their tissue biopsy sample capabilities under visual control have been tested in vitro. The presented work aims toward the integration of an inductive powers module, a locomotion and a biopsy unit. Based on an inherent RF (radio frequency) communication link and a human–machine interface (HMI), the capsule could be controlled by a doctor with ease. The three functional modules have been successfully combined into a single, miniaturized capsule for endoscopy, which was tested in vitro model setup. 2. Methods An endoscopic capsule should be small enough to be swallowed by a patient, which will require further substantial miniaturization in relation to the presented prototype, and controlled
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Figure 1. Wirelessly powered capsule endoscope system.
wirelessly with minimal negative effects on the patient, especially compared to traditional endoscopy. The capsule’s dimensions should allow for unimpaired mobility through the GI tract. The main elements of the proposed system comprise the inductive power receiver, wirelessly controlled locomotion unit, biopsy mechanism and a vision module. Biopsy and vision modules are designed to be scalable to meet colon examination and surgery requirements. To provide an adequate field of vision and anchor in the colon for surgery, the capsule should be capable of expanding the colon to more than 35 mm. Additionally, the multi-function modules need to be integrated. This results in preliminary dimensions of Ø19 mm × 73 mm with balloon inflated. The design concept of the wirelessly powered capsule endoscopy system is shown in figure 1. With a power transmitting coil outside of human body, the power could be induced by WCE in the colon. By using HMI, the medical professional could control the capsule action wirelessly and observe the colon images in real time. 2.1. Wireless power supply module Based on analyses of loosely coupled transfer systems and magnetic resonant coupling, various power transmission systems have been proposed in our previous studies (Guanying and Guozheng 2007, Kundong et al 2008, Zhiwei et al 2011). The transmitting coil, driven by a square wave current outside the body, generates alternating magnetic fields; the receiving coil induces an ac voltage, which is then rectified and filtered into a dc voltage to provide uninterrupted power. However, before designing a WPT system for real-world applications, the following aspects must be considered. (1) Stability. The posture and location of the receiver changes as the WCE moves in the digestive system via peristalsis, so that both distance and angle misalignments between the transmitter and the receiver always occur. In an attempt to generate greater stability in these situations, a few receivers have been proposed (Lenaerts and Puers 2007).
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Although the fluctuation of output power cannot be completely avoided, the minimum power received by the device needs to be adequate for capsule function. (2) Efficiency. In this loose-coupling system, the transmission distance is approximately 20 cm, yet the transfer efficiency has not been increased to more than 5%. One key parameter for improving efficiency is the distance between the transmitter and the receiver for inductive wireless power transmission; another key parameter is the size of the receiving coil, which is tiny compared to the size of the transmitting coil. However, the distance is constrained by the human body and the coil size is limited by the capsule volume. The structure must be optimized to improve the system. (3) Safety. The electromagnetic field generated by the transmitting coil could damage the human body, and an overheating coil could burn the intestinal wall. To ensure that the electromagnetic field does not exceed safe levels, careful evaluation is necessary. Based on data from the Visible Human Project of the National Library of Medicine, we established a simulation model and adopted a finite integration technique to calculate current density and specific absorption rate (SAR) (Zhiwei et al 2011, Wenhui et al 2009). In addition, to avoid damage to biological tissues, we restricted the temperature to remain below 42.5 ◦ C. Two possible methods can stabilize the receiving power: (a) three or more external magnetic fields from different directions and (b) three or more receiving coils with different orientations. After evaluating these options (Lenaerts and Puers 2005), we choose to use the latter method, which is more efficient and easier to build. Considering human safety, the upper limit of the current density and the SAR value become constraints for the transmitting coil. Thus, to ensure that the capsule has enough power to function, power transmission efficiency must be improved; simultaneously, the resistance of the receiving coil must be reduced to minimize the power dissipation on heat that would result in burning the intestinal wall. To improve transmission efficiency, magnetic flux density should be increased or the resistance of the transmitting coil should be decreased; in doing so, the resistance of the receiving coil should remain low to protect tissues from high temperatures. With a two-layer solenoid transmission coil and three orthogonal receiving coils, the transmitting efficiency was optimized to 2% ∼ 4% (Jia et al 2012). The ferrite cores were designed in square or cylindrical shapes to lengthen the capsule body by a minimum of 11.5 mm. To further improve the efficiency of the power system and reduce capsule volume, we propose a new receiving-coil structure that would fit the capsule structure. According to Faraday’s law of induction, with the equation d di =N dt dt = BS, L
(1)
where L is the coil inductance, i the current, the magnitude flux, and Bw the magnitude induction flux in the area of S (the section area of coil), we get the relationship N 2 Bw Ae . (2) L The magnetic core area (Aw) and cross-sectional area of the magnetic core window (Ae) are related to the energy storage LI2. Ampere turns of IN is a vital measurement of energy storage. The magnetic induction flux Bw, current density of J, and window area Ap have the following relationship: IN =
Aw Ae = AP =
LI 2 , Bw JK0
(3)
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Figure 2. Schematic diagram of a tri-axes micro-power receiver.
where K0 is the space factor of the magnetic core’s window area. To improve the coupling efficiency of the receiving and transmitting coils, a cross-sectional area of the coil ferritecore needs to be maximized while simultaneously occupying a minimal amount of space. Additionally, estimating the coil inductance would help us evaluate the receiving coil’s capacity for inducing power. If the inductance value is high, even if the window area of the core is very large, the induced current still cannot be improved efficiently. To raise induced energy efficiency, we need to increase Ap. When using three orthogonal coils on the outside of a ferrite-core as designed in previous studies, the square ferrite core would be limited to a volume of 6 to 8 mm3. To fit the capsule and gain a bigger Ap area, we designed a frame of a coil wound outside a hollow, cylindrical MnZn ferrite core (Zk7K), as shown in figure 2 (Coil1). However, the power transmission rate would be nearly zero when the receiving coil is mutually perpendicular to the transmitting coils. To ensure consistent power, we introduced a multi-core coil structure as shown in figure 2. Two more coils with the same cylindrical shape were fitted to the slim capsule body and wound vertically to Coil1. Coil2 and Coil3 had the same rectangular shape, which would introduce a large Aw value. By placing these coils vertically from each other, the two coils could induce magnetic flux in two vertical directions. The three coils constitute 3D receiving coils that should theoretically induce consistent, stable power. The multi-core receiving coils were placed concentrically outside the robot body with an 18 mm diameter, 1 mm thickness and 25.8 mm length. Although the coils have a large diameter, this structure saves space in axial direction with the maximum possible Ap area. The efficiency of the power transmitting system should also be improved in theory. 2.2. Locomotion and biopsy module To take full advantage of the capsule volume, we integrated the units as depicted in figure 3(a). The extending mechanism of the locomotion module and biopsy forceps was assembled in the hollow hole of the receiving coils. The vision camera was attached on the front of the capsule body to examine the colon and observe the biopsy process. The control circuit was shielded from electromagnetic fields and packaged with the pump in the tail. The locomotion steps are described in figure 3(b). Although it requires multiple steps, the inchworm-like movement will offer an expanded vision space for examination and biopsy procedures, as well as stable locomotion in the relatively bulky colon. Instead of a legged capsule, that has
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(a) The Modules in the Capsule
Magnetic Link Receive Coil
RF Link
air pipe
Extanding mechanism
cardan Pump
Biopsy Machanism Balloon
valve
bellow
Balloon
(b) Locomotion Actions
air inflation distal clamping and elongation
air exhaust proximal clamping and retraction
Figure 3. (a) The modules in the capsule. (b) Locomotion steps of a robotic capsule.
excellent climbing ability but at the risk of puncturing the intestinal wall, we choose a balloon anchor, which we introduced in a previous prototype (Gao et al 2011). Its soft contact with the colon eliminates this risk of injury to patient, and seals the capsule body well. We have implemented a peristaltic pump with a micro valve in the end. Several kinds of micro-spikes for micro-scale biopsy have already been developed by Byun et al (2004), Kong et al (2005), Kim et al (2007) and Simi et al (2012); in this study we are concerned with designing a precise biopsy jaw with sufficient cutting force, more than 10 N (Greenish et al 2002), and also expect the biopsy process to be safe, repeatable and visible. Two kinds of jaws have been designed for biopsy applications. One biopsy device (nicknamed the ‘big jaw’) consists of a tissue-cutting jaw with a leader pin, as shown in figure 4(a). The jaw stretches out along a guide slot as the nut moves outward. Due to the angle of the slot, the jaw slides open. When the nut moves backward, the big jaw retracts, simultaneously excising the tissue sample. With the bigger teeth in the fore-end, tissues could be cut off. Meanwhile, the smaller teeth behind are used to fix the tissue sample. As shown in figure 6(a), force can be given as F2 = Fp cot θs Ff = Fp / sin θs ,
(4)
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(b)
Figure 4. (a) Biopsy forceps of the big jaw. (b) Biopsy forceps of the micro jaw.
(a)
(b)
Figure 5. (a) Forces of the big jaw. (b) Forces of the micro jaw.
where Fp is the pull force of nut drive by a lead-screw and F1 is the pressure applied on the side of jaws. The tissue-cutting force is F2 = 10 N, and Ff is the possible frictional resistance between the leader pin and the slot. In order to prevent self-locking during slide movement, we set θ s (the friction angle) bigger than 16.7◦ . In this jaw, the biopsy forceps exploited a novel mechanism, making use of the relative displacement between forceps and sleeve as shown in figure 5(b). This tissue-cutting force was determined by the force that results from the sleeve and pressure spring between two teeth. The mechanism has two screws, which are driven by one micro motor of model ZWPD006006. Two nuts, one each embedded in the sleeve and jaw, move in the same direction but with different speeds. The tissue-cutting force is generated by compressing the pressure spring between the upper and lower parts of the jaw while it withdraws into the sleeve; thereafter, the tissue sample would be clamped between the teeth of jaws. As shown in figure 5(b), the relationships of the forces are as follows: Fpull = Ftouch sin ψ Fcut = Ftouch cos ψ − Fspring ,
(5)
where Ftouch is the pressure force to close the forceps of micro jaw; Fpull is the driving force generated from the nuts of forcep; Fspring is the elasticity coming from spring; and is the angle of Ftouch perpendicular to the contact surface. The spring constant K is expected to
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have a value of 0.559 N mm−1. When spring is compressed at the distance x (0 ∼ 5 mm), the pressure force of spring is Fspring = K · x.
(6)
According to the theoretical calculation, the micro jaw could generate a larger bite force and has possibility of more miniaturization. These two payload variations can be used as a modular of capsule platform to support biopsy-related surgical tasks. The biopsy mechanism and extending mechanism would need to be confined in 13.8 mm × 28 mm space. 2.3. Control and vision module To drive the capsule robot wirelessly and ensure that the biopsy is performed safely, we fitted the robot with a RF communication control system to realize the bidirectional data exchange (Wenwen et al 2012), and attached a compact vision module in the front portion of the robot body to detect the colon wall. Two wireless communication chips with model SI4421 by Silicon Labs are used to realize the wireless link. SI4421 is connected with an MCU of PIC16f690 from Microchip Inc. through the SPI bus. SI4421 integrated FIFO and TX data register was designed to be used in the unlicensed frequency band at 433 MHz. The whole control circuit was packaged in a copper cylinder of Ø16 mm × 5 mm with thickness of 0.5 mm to shield it from the magnetic field of a power transmission system, and placed in the tail end of capsule endoscopy as shown in figure 5(a). To receive the communication signals of 433 MHz, a copper wire with the length of 170 mm and diameter of 1 mm was used as antennas, and spiral winded outside the copper shielder. When the number of wireless received data reaches 16, the incoming data are clocked into a 16-bite FIFO buffer, the status of FFIT changes. The chip is a two-way half-duplex circuit. To achieve two-way communication, we need to build a rule to optimize and improve the data transfer process. The design of software flow and a time series communication model makes the communication scheme robust, reliable and optimizes the system for real-time operation (Chen et al 2013). There were three motors in the capsule: one motor controlled the pump and the other two were placed in the extending mechanism and biopsy unit. These motors could be set to move automatically or manually, allowing for a biopsy to be performed by the physician via the human–machine interface. The filling and emptying of balloons were controlled by detecting the air pressure and pumping time. To conduct a biopsy and initiate the extending action automatically, a closedloop control was implemented by integrating a component to detect current parameters. Because the two motors ran in sequence in this prototype, one sensor detected the current on the power line to reflect the force of action in real time. We placed maximum-value limitations on the current value and air pressure value to guarantee patient safety. Unlike the time control (which is only concerned with mobility), the air pressure of the balloon and the force of the clamping mechanism were detected to ensure safety during the clamping, extending and biopsy process. The highest priority for the device is monitoring the current value and air pressure; these parameters must be detected regularly and transmitted wirelessly to the HMI at 0.6 s intervals. The HMI was developed using Visual Studio 2008 (Microsoft), which allowed the operator to input orders for automatic movement, and to change the upper limit of air pressure or the mechanism force, as shown in figure 6. Orders can be sent to the capsule via the telemetry link, and feedback is obtained from the detector. The HMI was designed to include a realtime streaming image display, which displays images from the video of the intestine from a
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Figure 6. HMI interface showing the figure of an inner colon when the capsule was clamping in the vitro colon.
camera attached to the front of the capsule robot. The vision system was packaged in a copper shield that measured 10 mm in diameter and 34 mm in length. The camera could capture 320 × 240 resolution images, compress them into JPEG form, and transmit the compressed image wirelessly. The image transmission speed was close to 30 fps, which was comparable to speeds in the prior capsules. The efficacy of this method of image processing has been verified in previous multiple studies (Pan et al 2010, Gao et al 2012). 3. Experiments and results 3.1. Characterization of the power module To evaluate the wireless power transmission system, we tested the induced power with a custom setup as shown in figure 7. The receiving coils placed on the two-axes, turntable powered with a dc power supply, could turn the coils in two directions of 120◦ each. To identify the attitude of the coils, we defined angle α 1 as the rotating degree of the coil on the XOY plane, and angle α 2 as the rotating degree of the coil on the XOZ plane, as previously defined in figure 2. Because the receiving coils are axisymmetric, we can derive the output power of the coils using an oscilloscope by changing angle α 1 and α 2 between 0◦ and 90◦ , respectively. The transmitting coil was constructed with a double-layer solenoid pair (550 mm in diameter) and 180 strands of AWG 38 enameled copper wire. The coil was wound on an acrylonitrile butadiene styrene hollow cylinder with 25 turns for each layer. The driving device was installed inside the control box, including the serial resonant circuit (SRC) composed of a vacuum capacitor and an adjustable inductor, together with the transmitting coil. A square wave, which could transmit power more efficiently than the sin wave and triangular wave with a changeable frequency, was used to drive an H-bridge circuit to generate an alternate voltage on the SRC. The amplitude of the alternate voltage can be adjusted by turning the knob on the control box. To obtain stable power to drive the robot, we prepared test coils of Coil1 with 28 strands and 80 turns, and Coil2 and Coil3 with 28 strands and 120 turns (according to the parameter L of the three orthogonal coils that we had designed before).
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Figure 7. Experiment setup for testing wireless power transmission.
Figure 8. The output voltage induced by a receiving coil with 27 load resistance.
AC voltage needed to be rectified and filtered into dc voltage to provide power to the capsule. To address this issue, a power conversion board was assembled with coils placed on the side of the extending mechanism. The board was realized on a 0.5 mm thick PCB. Components were selected from the smallest commercially available. DC power was provided at 5.0 V from the receiving coils by using an LDO (LT17635, Linear). When the driving current was 1 A and the frequency was 216 kHz, the output power could exceed 653 mW at any attitude with an efficiency of up to 28%; even in the worst-case scenario, the efficiency was still greater than 7.98% (as shown in figure 8, the induced power was larger than 4.2 V with the load resistant of 27 ). The available power received by the coils was sufficient for
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the capsule to function. Especially when inflating the balloons, the starting power needed to be nearly 700 mW; this would be extremely difficult to achieve with traditional receiving coils, but was shown to be plausible with our novel receiving coils. The attitude stability of power transmission system is defined as Vmax − Vmin , (7) γ = Vmin where Vmax is the largest output voltage and Vmin is the minimum output voltage. We get γ = 90.14% in our coils with ferrite hollow cylinder core compared to the 73.21% of three orthogonal coils in the previous study. That means the novel receiving coils have a good stability in inducing power. The electromagnetic field generated by the transmitting coil was evaluated to ensure that it was within safe parameters for use in patients. In our previous work, the variations in current density and SAR, compared with the restrictions of the International Commission on Non-Ionizing Radiation Protection (ICNIRP), had been verified. At a frequency of 216 kHz and a driving current of about 1 A, the current value was below the ICNIRP restrictive current density, and the SAR was below the upper boundary of the SAR value of 1.98 A (Jia et al 2012). To decrease the heat dissipation on the coils, we needed to decrease the power consumption for the impedance. Increasing strands of the receiving coil was the most straightforward method. In the experiment, we tested coils with 28 strands and 80 turns made of 0.5 mm enameled wire; this reduced the impedance of the coil to nearly one-third that of the original three orthogonal coils. With one layer of coils winding outside the core, heat dissipation occurred faster than would have been possible with the three multi-layer coils. The temperature safety was tested in the vitro colon, which was soaked in 0.9% saline in the sink with constant temperature of 37.5 ◦ C. The receiving coils were sealed in a medical silicon case which is similar to the balloon of capsule endoscope. The final version of the device did not exceed 40 ◦ C, which is safe for human tissue. 3.2. Characterization of the locomotion module To cope with limited power, we improved the pump and proposed locomotion steps that would not require the two motors to operate simultaneously. Approximately 650 mW would be sufficient to sustain all functions in all possible configurations. The pump’s power typically peaks around 800 mW and the power consumption can be totally supported by the inductive link within safe levels. The anchoring force of the custom-made, medical-grade silica balloons was tested in vitro in a simulated colon environment, as described in Lin et al (2012). The anchoring force of the custom made balloons with texture was 1.91 N, which is much bigger than the anchoring force of smooth balloon of 0.24 N. Compared to the legged-shape anchoring mechanism, the balloon anchor would be more comfortable and safer for patients, and the seamless package could prevent the entry of foreign matter and, by extension, decrease the patient’s risk of postoperative infection. The flow rate of air between two balloons was 11.27 mL min−1. The pump can build up backpressure up to 153 kPa, which would be adequate for balloon inflation in the colon. We could adjust the balloon diameter from 20 to 38 mm by controlling the pump motor direction turning it forward or reverse. The custom balloon with a textured top had a maximum volume of 12 ml (only one-third the volume of the traditional ball-shaped balloon). Locomotion tests were performed wirelessly using a closed, flexible silicone tube with slippery mucus on the internal surface as shown in figure 9, and fresh colon in vitro as shown in figure 6. Porcine intestine specimens were obtained in compliance with standard medical
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Figure 9. Wireless powered capsule locomotion inside the slippery tube.
and ethical guidelines from a 50 kg pig, using sections with an average diameter of 35 mm. The in vitro testing in the simulated colon was powered with the optional 5 V dc. From the test results, we concluded the following. (1) The textured surface of the custom balloon enabled it to anchor well on the slippery tube and the simulated colon. Distributing its pressure over a larger surface of the intestine wall than would legged capsules, the balloon made contact without stabbing, which has significant benefits for patient safety. (2) When the front balloon is inflated, the expanded colon is visible to the video camera, enabling diagnosis of the wrinkled and collapsed lumen. The inchworm-like movement centers the capsule in the gut, minimizing potential leg failure and tissue entrapment. (3) The speed of the robot’s movement was influenced by the slope and diameter of the tube or colon. During most of its journey, the robot is engaged in pumping and when the diameter of the tube or colon varied, the pumping time would also change. The speed decreased when the device was climbing ‘uphill’ because of the slippery environment and resistance from its own weight. Conversely, during descent, the speed increased. In a section of the intestine with a diameter larger than 41 mm, the robot would likely slip. (4) When placed in the middle of the capsule bellows, the cardan provided three degrees of freedom, enabling the capsule to navigate corner smaller than 43◦ (with a colon diameter up to 39 mm). When the turning angle of the intestine was larger than 55◦ , the stretching movement was blocked by the intestine wall. The passable angle was smaller for more slender corners, which indicates that in a corner of thin lumen, the capsule is more likely to be blocked. 3.3. Characterization of the biopsy module The cutting force of the two forceps was measured to identify the feasibility of biopsy tasks. The test results of cutting force are shown in figure 10 compared with theoretical value.
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(b)
Figure 10. The idea cut force compared with the test result: (a) big jaw; (b) micro jaw.
Considering the anisotropic characteristics and the uncertainties of the intestine, the desired tissue-cutting force must be over 10 N (Greenish et al 2002). The cutting force of the big jaw ranged from 22 to 10 N at an open angle of 1◦ to 6◦ ; its mechanical efficiency was 79.04%. The cutting force of the micro jaw ranged from 28 to 10 N at an open angle of 10◦ to 31◦ ; its mechanical efficiency was 14.78%. In other words, the force would be adequate to biopsy tissue if the open angle is smaller than 6◦ or 31◦ for the big and micro jaws, respectively. In our present prototype, we integrated the capsular endoscopic device with big jaw. We next will integrate the second generation capsule with the micro jaw and design the mechanism smaller. To demonstrate that the forceps for biopsy can obtain samples from colon tissues, we performed sample excision tests in vitro. The excised pig’s colon was fixed by means of a human hand, and forceps were attached to the small intestine. Several tests were performed to demonstrate the repeatability of the results of the tissue excision procedure; thus, we confirmed that the module could successfully realize its biopsy function. The biopsy process is shown in figure 11 for big jaw and figure 12 for micro jaw.
3.4. System performs with wireless power The whole capsule system was integrated and tested with the wireless power transmission system, as shown in figure 13. The inductive powering system was tested in combination with several capsule modules. The testing of the self-propulsion mechanisms has been discussed in a previous section. The power consumption of the capsule was no more than 800 mW for any action or attitude (the power consumption details are presented in table 1). The power supply was sufficient when the transmission coil frequency was 216 kHz and the driving current was roughly 1 A, which has been shown to be safe for human body. With the wireless control and vision module, the capsule could travel the colon without manual intervention from the human operator. The circuit board was successfully shielded from electromagnetic interference by a copper oval-cylindrical box, and the communication error rate was lower than 1.02% over a distance of 2 m. Error checking code helped in ensuring that the capsule was controlled safely and reliably. Doctors could see the internal colon and control the capsule’s forward, backward, and anchoring functions as well as conduct a biopsy when appropriate.
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(a)
(b)
(c)
(d)
Figure 11. The biopsy procedure of big jaws installed in the prototype capsule. (a) Stretch out the capsule body. (b) Open at maximum degree. (c) Bite tissue and withdraw into capsule. (d) Cut off tissue and store it in the capsule body. Table 1. Power consumption of capsule loads.
Module
Power consumption
Operating time
Image sensor LEDs Autofocus system Control circuit Communication circuit Locomotion actuators Biopsy actuators Biopsy cut action Pump starts to work Locomotion or biopsy
20 mW 4 × 5 mW 12 mW 20 mW 15 mW 220 ∼ 800 mW 200 ∼ 700 mW 700 mW 800 mW 200 ∼ 400 mW
90–100% 90–100% 50–80% 100% 70–90% 60 ∼ 80% 1–2% 0.1% 0.2% 60 ∼ 80%
4. Discussions With an effective wireless power transmitting system, a capsule could travel through and examine a patient’s colon without being limited by an inadequate, portable power supply. In this study, we introduced receiving coils with a novel, hollow, cylindrical-core shape to maximize the use of space in the capsule body, and enhance the safety and efficiency of device. Unlike earlier methods of mobility, including robot legs that could potentially puncture or otherwise injure a patient’s intestines, we implemented a new air-balloon feature to anchor to and expand the colon to enable the physician to see the colon and perform a biopsy, if
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(a)
(b)
(c)
(d)
Figure 12. The biopsy procedure of micro jaw. (a) Stretch out of body and open. (b) Bite tissue. (c) Withdraws into body. (d) Cut off tissue and store it in the body.
Figure 13. Side view of the capsule prototype with the internal modules.
necessary. In addition, to protect the intestine from a puncture as a result of over-inflation of a balloon or the extending mechanism, we added a control system to keep the expansion force within a safe limit. The loosely coupled regime of wireless power transmission system suggests that only a small fraction of the input power is detected by the receiving coil and reaches the load of capsule. To resolve this issue, it would be necessary to find a technical solution that deflects the highest number of field lines into the tiny receiving coil set (Puers et al 2011). Based on our previous studies, even a slight increase in receiving coil volume could increase efficiency
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significantly (Gao et al 2012). The successful use of multiple hollow cylindrical cores led us to hypothesize that enlarging a cross-sectional area of the coil ferrite-core would be an effective way to improve power transmission efficiency. In our design, the core maximized the space for inducing power and thus supplied adequate power for capsule functions. Although our capsule prototype proved the feasibility of the wireless power transmission system during testing, several design features merit additional discussion. The air balloon ensures the safety of the soft, sensitive colon wall. However, locomotion speed presents a problem; each step in the locomotion process would take between 60 to 125 s, which would be too slow for colon examination in clinical situations. This problem could likely be resolved with an optimized pump structure and a more powerful micro motor to decrease inflation time. When the diameter of the colon was smaller than 26 mm, the air could not be exchanged between two balloons. Because the outer diameter of the balloon limits the amount of air it can hold, it leads to capsule getting stuck. This could be resolved by using an airbag to store the excess air, which would require a pipe connected to one balloon and extra space in capsule. The capsule locomotion performance has yet to be tested under in vivo conditions. Although we used the porcine specimen to mimic the GI tract, this may not perfectly imitate the human colon and thus requires further study before it can be used clinically. Peristalsis would assist the capsule’s motion, and may even theoretically be intentionally harnessed during portions of the journey that are not of interest to the desired diagnostic goal. Additionally, the device’s theoretical ability to maximize patient comfort and ensure patient safety during anchoring and extending of the colon should be verified. Thus, future work should implement in vivo studies to optimize design and confirm that safety measures have been effective. 5. Conclusions In this work, we proposed novel receiving coils for capsule-biopsy systems. In the worst-case scenario, the power transmission efficiency was over 7.98%; moreover, receiving power was stable over 650 mW in the context of size and safety constraints. Importantly, this capsule has the capability to be powered wirelessly, allowing for more extensive and complicated surgical and other robotic tasks. Although the anchoring mechanism of the balloon needs improvement, its safety and the property that can be fully sealed show promise for future development. Two types of forceps (for the biopsy task) were developed and tested in vitro; both of these successfully extracted tissue samples from the colon of a pig. The results of this study can serve as a foundation for developing micro-surgical robotic capsules in the future. Acknowledgments This work has been supported by the National Natural Science Foundation of China (NSFC) (no. 31170968); Technical Pre-Research of Manned Spaceflight Research (no. 010203); Shanghai Science and Technology Commission funded project (09 DZ1907400). The authors would like to thank Mr Chenggeng Wu for his help. References Arena A, Boulougoura M, Chowdrey H S, Dario P, Harendt C, Irion K, Kodogiannis V, Lenaerts B, Menciassi A and Puers R 2005 Intracorporeal videoprobe (IVP) Stud. Health Technol. Inform. 114 167 Buselli E et al 2010 Evaluation of friction enhancement through soft polymer micro-patterns in active capsule endoscopy Meas. Sci. Technol. 21 105802 Byun S, Lim J, Paik S, Lee A, Koo K, Park S, Park J, Choi B and Shim E 2004 Novel micro-spiked needles with buried micro-channels for micro-scale biopsy Asia-Pacific Conf. of Transducers and Micro-Nano Technology pp 1095–9
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Byungkyu K, Sunghak L, Jong Hyeon P and Jong-Oh P 2004 Inchworm-like microrobot for capsule endoscope ROBIO’04: IEEE Int. Conf. on Robotics and Biomimetics pp 458–63 Carta R, Sfakiotakis M, Pateromichelakis N, Thon´e J, Tsakiris D and Puers R 2011 A multi-coil inductive powering system for an endoscopic capsule with vibratory actuation Sensors Actuators A 172 253–8 Chen W, Yan G, Liu H, Jiang P and Wang Z 2013 A wireless micro robot for colon endoscope using Archimedes spiral leg Int. J. Med. Robot. Comput. Assist. Surg. 10 1050–5 Gao P, Yan G, Wang Z, Wang K, Jiang P and Zhou Y 2011 A robotic endoscope based on minimally invasive locomotion and wireless techniques for human colon Int. J. Med. Robot. Comput. Assist. Surg. 7 256–67 Gao P, Yan G Z, Wang Z W, Jiang P P and Liu H 2012 Flexible locomotion system for gastrointestinal microrobots Guangxue Jingmi Gongcheng (Opt. Precis. Eng.) 20 541–9 Greenish S, Hayward V, Chial V, Okamura A and Steffen T 2002 Measurement, analysis, and display of haptic signals during surgical cutting Presence: Teleoperators and Virtual Environments 11 626–51 Guanying M and Guozheng Y 2007 Wireless powered microrobot for gastrointestinal detection ICMA’07: Int. Conf. on Mechatronics and Automation (5–8 August, 2007) pp 1085–9 Hyunjun P, Sungjin P, Euisung Y, Byungkyu K, Jongoh P and Sukho P 2007 Paddling based microrobot for capsule endoscopes IEEE Int. Conf. on Robotics and Automation (10–14 April, 2007) pp 3377–82 Iddan G, Meron G, Glukhovsky A and Swain P 2000 Wireless capsule endoscopy Nature 405 417 Jia Z W, Yan G Z, Liu H, Wang Z W, Jiang P P and Shi Y 2012 The optimization of wireless power transmission: design and realization Int. J. Med. Robot. Comput. Assist. Surg. 8 337–47 Kim G, Park S, Koo K, Choi H M, Jung M J, Song S Y and Bang S M 2007 Nickel micro spike for micro scale biopsy using LiGA process AIP Conf. Proc. 879 1499 Kong K, Cha J, Jeon D and Cho D D 2005 A rotational micro biopsy device for the capsule endoscope IROS’05: IEEE/RSJ Int. Conf. on Intelligent Robots and Systems pp 1839–43 Kundong W, Guozheng Y, Pingping J and Dongdong Y 2008 A wireless robotic endoscope for gastrointestine IEEE Trans. Robot. 24 206–10 Lee S-W, Kim J-D, Son J-H, Ryu M-H and Kim J 2006 Design of two-dimensional coils for wireless power transmission to in vivo robotic capsule EMBS’05: IEEE 27th Annu. Int. Conf. Engineering in Medicine and Biology Society, 2005 pp 6631–4 Lenaerts B and Puers R 2005 Inductive powering of a freely moving system Sensors Actuators A 123 522–30 Lenaerts B and Puers R 2007 An inductive power link for a wireless endoscope Biosens. Bioelectron. 22 1390–5 Lin W, Shi Y, Yan G, Wang Y and Li L 2011 A prototype of an anchoring and extending intenstinal micro-robot and an in vitro experiment J. Med. Eng. Tech. 35 410–15 Miura H, Arai S, Kakubari Y, Sato F, Matsuki H and Sato T 2006 Improvement of the transcutaneous energy transmission system utilizing ferrite cored coils for artificial hearts IEEE Trans. Magn. 42 3578–80 Moglia A, Menciassi A, Schurr M and Dario P 2007 Wireless capsule endoscopy: from diagnostic devices to multipurpose robotic systems Biomed. Microdevices 9 235–43 Pan G, Yan G, Qiu X and Song X 2010 A novel JPEG-based wireless capsule endoscope Biomed. Instrum. Technol. 44 519–22 Puers R, Carta R and Thon´e J 2011 Wireless power and data transmission strategies for next-generation capsule endoscopes J. Micromech. Microeng. 21 054008 Quirini M, Scapellato S, Valdastri P, Menciassi A and Dario P 2007 An approach to capsular endoscopy with active motion EMBS’07: IEEE 29th Annu. Int. Conf. Engineering in Medicine and Biology Society pp 2827–30 Shiba K, Morimasa A and Hirano H 2010 Design and development of low-loss transformer for powering small implantable medical devices IEEE Trans. Biomed. Circuits Syst. 4 77–85 Simi M, Gerboni G, Menciassi A and Valdastri P 2012 Magnetic mechanism for wireless capsule biopsy J. Med. Devices 6 017611 Van Gossum A and Ibrahim M 2010 Video capsule endoscopy: what is the future? Gastroenterol. Clin. North Am. 39 807 Wenhui X, Guozheng Y and Wenxin W 2009 Wireless power transmission system for capsule endoscope ICEMI’09: 9th Int. Conf. on Electronic Measurement and Instruments (16–19 August, 2009) pp 1–820 Wenwen C, Guozheng Y, Peng G and Zhiwu W 2012 Control system design of micro diagnosis and treat robot for colonoscopy Opt. Precis. Eng. 20 1296–302 Zhiwei J, Guozheng Y, Zhiwu W and Hua L 2011 Efficiency optimization of wireless power transmission systems for active capsule endoscopes Physiol. Meas. 32 1561
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Wireless powered capsule endoscopy for colon diagnosis and treatment
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Physiol. Meas. 34 1545 (http://iopscience.iop.org/0967-3334/34/11/1545) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
PHYSIOLOGICAL MEASUREMENT
Physiol. Meas. 34 (2013) 1545–1561
doi:10.1088/0967-3334/34/11/1545
Wireless powered capsule endoscopy for colon diagnosis and treatment Wenwen Chen, Guozheng Yan, Shu He, Quan Ke, Zhiwu Wang, Hua Liu and Pingping Jiang Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China E-mail: [email protected]
Received 25 April 2013, accepted for publication 25 September 2013 Published 23 October 2013 Online at stacks.iop.org/PM/34/1545 Abstract This paper presents a wireless power transfer system integrated with an active locomotion and biopsy module in an endoscopic capsule for colon inspection. The capsule, which can move automatically, is designed for non-invasive biopsy and visual inspection of the intestine. To supply enough power for multiple functions and ensure safety for the human body, the efficiency of the current power transmission system needs to be improved. To take full advantage of the volume in the capsule body, a novel structure of receiving coils wound on a multi-core of MnZn ferrite hollow cylinder was used; with this new core, the efficiency increased to more than 7.98%. Up to 1.4 W of dc power can be delivered to the capsule as it travels along the gastrointestinal tract. Three micro motors were integrated for pumping, anchoring, locomotion and biopsy. A user interface and RF communication enables the operator to drive the capsule in an intuitive manner. To gauge the efficacy of the wireless power supply in a simulated real-world application, the biopsy and locomotion capabilities of the device were successfully tested in a slippery, soft tube and gut environment in vitro. Keywords: active capsule endoscopy, wireless power transfer, biopsy, colon (Some figures may appear in colour only in the online journal)
1. Introduction The current enteroscopic technique has been performed in clinical treatment widely. However, with the cable at the end, the diagnosis range would be limited and push action would be painful for the patients. Wireless capsular endoscopic (WCE) devices have gained increasing acceptance as useful tools for early diagnosis of cancer and other diseases affecting the 0967-3334/13/111545+17$33.00
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gastrointestinal (GI) tract (Moglia et al 2007). Most commercial devices use two watch batteries that provide an average of 25 mW of power, rendering them operational for only about 6 h (Iddan et al 2000), which is clearly inadequate for incorporating advanced robotic features. A dedicated inductive powering system (with a 1 MHz carrier to limit absorption by the human body) that can support the transfer of over 300 mW has been developed (Carta et al 2011). Moreover, future capsule endoscopes are expected to perform even more complicated and energy-consuming tasks in addition to basic locomotion: biopsy, micro surgical procedures and other related functions. Therefore, significant efforts have been dedicated to developing alternative energy-supply options (Van Gossum and Ibrahim 2010); among these, the wireless power transfer (WPT) (Shiba et al 2010, Arena et al 2005, Lenaerts and Puers 2007), in which power is delivered to the endoscope from a power source outside of the human body, is a promising replacement. Various methods have been developed in an attempt to provide a steady and sufficient supply of energy to an endoscopy capsule. Among these, the multi-dimensional receiving coil proved to be effective for solving randomicity of the capsule attitude (Lee et al 2006, Lenaerts and Puers 2005). A ferrite core was adopted to enhance transcutaneous energy transmission by optimizing coupling with the external field (Miura et al 2006, Lenaerts and Puers 2007). Clearly, power efficiency is essential in WPT-based endoscopes. Limited by the capsule space, the size and corresponding efficiency of wirelessly transmitted power would max out at 5% with the present 200 mm distance between the capsule and the transmitting coil and the 3D orthogonal ferrite receiving core. Locomotion and other functionality would be hindered by this limited power. To address these challenges, this paper presents a WPT system for capsule endoscopes with a hollow circular cylinder multi-core of dimensions 16 mm × 23 mm that can provide a power supply of up to 1.4 W. Most commercial pills rely on peristalsis to propel them through the GI tract. However, capsules for the colon must be equipped with an active locomotion system, designed to cope with the large lumen diameter (typically 2–3 cm) to provide more than a randomized view of the colon (Quirini et al 2007). Modules with legs or paddles have been developed for navigating the intestines (Buselli et al 2010, Hyunjun et al 2007), and an external magnetic field has been used to drive capsules wirelessly (Simi et al 2012). Some devices employ an incremental clampingand-sliding method to move like an earthworm or inchworm (Kundong et al 2008, Guanying and Guozheng 2007, Byungkyu et al 2004). The long, thin body adapts to the intestinal tube environment and can incorporate more therapy tools than basic visual inspection. Two independent motors control the extending mechanism and the pumping balloon for anchoring. The capsule is completely sealed within a soft silicone shell that exerts only gentle force when in contact with the colon. Another important function for WCE is a diagnostic and treatment capability. For example, when a medical professional finds a polyp, WCE may take a sample of the tissue for more precise diagnosis. Two kinds of biopsy mechanism were introduced, and their tissue biopsy sample capabilities under visual control have been tested in vitro. The presented work aims toward the integration of an inductive powers module, a locomotion and a biopsy unit. Based on an inherent RF (radio frequency) communication link and a human–machine interface (HMI), the capsule could be controlled by a doctor with ease. The three functional modules have been successfully combined into a single, miniaturized capsule for endoscopy, which was tested in vitro model setup. 2. Methods An endoscopic capsule should be small enough to be swallowed by a patient, which will require further substantial miniaturization in relation to the presented prototype, and controlled
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Figure 1. Wirelessly powered capsule endoscope system.
wirelessly with minimal negative effects on the patient, especially compared to traditional endoscopy. The capsule’s dimensions should allow for unimpaired mobility through the GI tract. The main elements of the proposed system comprise the inductive power receiver, wirelessly controlled locomotion unit, biopsy mechanism and a vision module. Biopsy and vision modules are designed to be scalable to meet colon examination and surgery requirements. To provide an adequate field of vision and anchor in the colon for surgery, the capsule should be capable of expanding the colon to more than 35 mm. Additionally, the multi-function modules need to be integrated. This results in preliminary dimensions of Ø19 mm × 73 mm with balloon inflated. The design concept of the wirelessly powered capsule endoscopy system is shown in figure 1. With a power transmitting coil outside of human body, the power could be induced by WCE in the colon. By using HMI, the medical professional could control the capsule action wirelessly and observe the colon images in real time. 2.1. Wireless power supply module Based on analyses of loosely coupled transfer systems and magnetic resonant coupling, various power transmission systems have been proposed in our previous studies (Guanying and Guozheng 2007, Kundong et al 2008, Zhiwei et al 2011). The transmitting coil, driven by a square wave current outside the body, generates alternating magnetic fields; the receiving coil induces an ac voltage, which is then rectified and filtered into a dc voltage to provide uninterrupted power. However, before designing a WPT system for real-world applications, the following aspects must be considered. (1) Stability. The posture and location of the receiver changes as the WCE moves in the digestive system via peristalsis, so that both distance and angle misalignments between the transmitter and the receiver always occur. In an attempt to generate greater stability in these situations, a few receivers have been proposed (Lenaerts and Puers 2007).
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Although the fluctuation of output power cannot be completely avoided, the minimum power received by the device needs to be adequate for capsule function. (2) Efficiency. In this loose-coupling system, the transmission distance is approximately 20 cm, yet the transfer efficiency has not been increased to more than 5%. One key parameter for improving efficiency is the distance between the transmitter and the receiver for inductive wireless power transmission; another key parameter is the size of the receiving coil, which is tiny compared to the size of the transmitting coil. However, the distance is constrained by the human body and the coil size is limited by the capsule volume. The structure must be optimized to improve the system. (3) Safety. The electromagnetic field generated by the transmitting coil could damage the human body, and an overheating coil could burn the intestinal wall. To ensure that the electromagnetic field does not exceed safe levels, careful evaluation is necessary. Based on data from the Visible Human Project of the National Library of Medicine, we established a simulation model and adopted a finite integration technique to calculate current density and specific absorption rate (SAR) (Zhiwei et al 2011, Wenhui et al 2009). In addition, to avoid damage to biological tissues, we restricted the temperature to remain below 42.5 ◦ C. Two possible methods can stabilize the receiving power: (a) three or more external magnetic fields from different directions and (b) three or more receiving coils with different orientations. After evaluating these options (Lenaerts and Puers 2005), we choose to use the latter method, which is more efficient and easier to build. Considering human safety, the upper limit of the current density and the SAR value become constraints for the transmitting coil. Thus, to ensure that the capsule has enough power to function, power transmission efficiency must be improved; simultaneously, the resistance of the receiving coil must be reduced to minimize the power dissipation on heat that would result in burning the intestinal wall. To improve transmission efficiency, magnetic flux density should be increased or the resistance of the transmitting coil should be decreased; in doing so, the resistance of the receiving coil should remain low to protect tissues from high temperatures. With a two-layer solenoid transmission coil and three orthogonal receiving coils, the transmitting efficiency was optimized to 2% ∼ 4% (Jia et al 2012). The ferrite cores were designed in square or cylindrical shapes to lengthen the capsule body by a minimum of 11.5 mm. To further improve the efficiency of the power system and reduce capsule volume, we propose a new receiving-coil structure that would fit the capsule structure. According to Faraday’s law of induction, with the equation d di =N dt dt = BS, L
(1)
where L is the coil inductance, i the current, the magnitude flux, and Bw the magnitude induction flux in the area of S (the section area of coil), we get the relationship N 2 Bw Ae . (2) L The magnetic core area (Aw) and cross-sectional area of the magnetic core window (Ae) are related to the energy storage LI2. Ampere turns of IN is a vital measurement of energy storage. The magnetic induction flux Bw, current density of J, and window area Ap have the following relationship: IN =
Aw Ae = AP =
LI 2 , Bw JK0
(3)
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Figure 2. Schematic diagram of a tri-axes micro-power receiver.
where K0 is the space factor of the magnetic core’s window area. To improve the coupling efficiency of the receiving and transmitting coils, a cross-sectional area of the coil ferritecore needs to be maximized while simultaneously occupying a minimal amount of space. Additionally, estimating the coil inductance would help us evaluate the receiving coil’s capacity for inducing power. If the inductance value is high, even if the window area of the core is very large, the induced current still cannot be improved efficiently. To raise induced energy efficiency, we need to increase Ap. When using three orthogonal coils on the outside of a ferrite-core as designed in previous studies, the square ferrite core would be limited to a volume of 6 to 8 mm3. To fit the capsule and gain a bigger Ap area, we designed a frame of a coil wound outside a hollow, cylindrical MnZn ferrite core (Zk7K), as shown in figure 2 (Coil1). However, the power transmission rate would be nearly zero when the receiving coil is mutually perpendicular to the transmitting coils. To ensure consistent power, we introduced a multi-core coil structure as shown in figure 2. Two more coils with the same cylindrical shape were fitted to the slim capsule body and wound vertically to Coil1. Coil2 and Coil3 had the same rectangular shape, which would introduce a large Aw value. By placing these coils vertically from each other, the two coils could induce magnetic flux in two vertical directions. The three coils constitute 3D receiving coils that should theoretically induce consistent, stable power. The multi-core receiving coils were placed concentrically outside the robot body with an 18 mm diameter, 1 mm thickness and 25.8 mm length. Although the coils have a large diameter, this structure saves space in axial direction with the maximum possible Ap area. The efficiency of the power transmitting system should also be improved in theory. 2.2. Locomotion and biopsy module To take full advantage of the capsule volume, we integrated the units as depicted in figure 3(a). The extending mechanism of the locomotion module and biopsy forceps was assembled in the hollow hole of the receiving coils. The vision camera was attached on the front of the capsule body to examine the colon and observe the biopsy process. The control circuit was shielded from electromagnetic fields and packaged with the pump in the tail. The locomotion steps are described in figure 3(b). Although it requires multiple steps, the inchworm-like movement will offer an expanded vision space for examination and biopsy procedures, as well as stable locomotion in the relatively bulky colon. Instead of a legged capsule, that has
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(a) The Modules in the Capsule
Magnetic Link Receive Coil
RF Link
air pipe
Extanding mechanism
cardan Pump
Biopsy Machanism Balloon
valve
bellow
Balloon
(b) Locomotion Actions
air inflation distal clamping and elongation
air exhaust proximal clamping and retraction
Figure 3. (a) The modules in the capsule. (b) Locomotion steps of a robotic capsule.
excellent climbing ability but at the risk of puncturing the intestinal wall, we choose a balloon anchor, which we introduced in a previous prototype (Gao et al 2011). Its soft contact with the colon eliminates this risk of injury to patient, and seals the capsule body well. We have implemented a peristaltic pump with a micro valve in the end. Several kinds of micro-spikes for micro-scale biopsy have already been developed by Byun et al (2004), Kong et al (2005), Kim et al (2007) and Simi et al (2012); in this study we are concerned with designing a precise biopsy jaw with sufficient cutting force, more than 10 N (Greenish et al 2002), and also expect the biopsy process to be safe, repeatable and visible. Two kinds of jaws have been designed for biopsy applications. One biopsy device (nicknamed the ‘big jaw’) consists of a tissue-cutting jaw with a leader pin, as shown in figure 4(a). The jaw stretches out along a guide slot as the nut moves outward. Due to the angle of the slot, the jaw slides open. When the nut moves backward, the big jaw retracts, simultaneously excising the tissue sample. With the bigger teeth in the fore-end, tissues could be cut off. Meanwhile, the smaller teeth behind are used to fix the tissue sample. As shown in figure 6(a), force can be given as F2 = Fp cot θs Ff = Fp / sin θs ,
(4)
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(b)
Figure 4. (a) Biopsy forceps of the big jaw. (b) Biopsy forceps of the micro jaw.
(a)
(b)
Figure 5. (a) Forces of the big jaw. (b) Forces of the micro jaw.
where Fp is the pull force of nut drive by a lead-screw and F1 is the pressure applied on the side of jaws. The tissue-cutting force is F2 = 10 N, and Ff is the possible frictional resistance between the leader pin and the slot. In order to prevent self-locking during slide movement, we set θ s (the friction angle) bigger than 16.7◦ . In this jaw, the biopsy forceps exploited a novel mechanism, making use of the relative displacement between forceps and sleeve as shown in figure 5(b). This tissue-cutting force was determined by the force that results from the sleeve and pressure spring between two teeth. The mechanism has two screws, which are driven by one micro motor of model ZWPD006006. Two nuts, one each embedded in the sleeve and jaw, move in the same direction but with different speeds. The tissue-cutting force is generated by compressing the pressure spring between the upper and lower parts of the jaw while it withdraws into the sleeve; thereafter, the tissue sample would be clamped between the teeth of jaws. As shown in figure 5(b), the relationships of the forces are as follows: Fpull = Ftouch sin ψ Fcut = Ftouch cos ψ − Fspring ,
(5)
where Ftouch is the pressure force to close the forceps of micro jaw; Fpull is the driving force generated from the nuts of forcep; Fspring is the elasticity coming from spring; and is the angle of Ftouch perpendicular to the contact surface. The spring constant K is expected to
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have a value of 0.559 N mm−1. When spring is compressed at the distance x (0 ∼ 5 mm), the pressure force of spring is Fspring = K · x.
(6)
According to the theoretical calculation, the micro jaw could generate a larger bite force and has possibility of more miniaturization. These two payload variations can be used as a modular of capsule platform to support biopsy-related surgical tasks. The biopsy mechanism and extending mechanism would need to be confined in 13.8 mm × 28 mm space. 2.3. Control and vision module To drive the capsule robot wirelessly and ensure that the biopsy is performed safely, we fitted the robot with a RF communication control system to realize the bidirectional data exchange (Wenwen et al 2012), and attached a compact vision module in the front portion of the robot body to detect the colon wall. Two wireless communication chips with model SI4421 by Silicon Labs are used to realize the wireless link. SI4421 is connected with an MCU of PIC16f690 from Microchip Inc. through the SPI bus. SI4421 integrated FIFO and TX data register was designed to be used in the unlicensed frequency band at 433 MHz. The whole control circuit was packaged in a copper cylinder of Ø16 mm × 5 mm with thickness of 0.5 mm to shield it from the magnetic field of a power transmission system, and placed in the tail end of capsule endoscopy as shown in figure 5(a). To receive the communication signals of 433 MHz, a copper wire with the length of 170 mm and diameter of 1 mm was used as antennas, and spiral winded outside the copper shielder. When the number of wireless received data reaches 16, the incoming data are clocked into a 16-bite FIFO buffer, the status of FFIT changes. The chip is a two-way half-duplex circuit. To achieve two-way communication, we need to build a rule to optimize and improve the data transfer process. The design of software flow and a time series communication model makes the communication scheme robust, reliable and optimizes the system for real-time operation (Chen et al 2013). There were three motors in the capsule: one motor controlled the pump and the other two were placed in the extending mechanism and biopsy unit. These motors could be set to move automatically or manually, allowing for a biopsy to be performed by the physician via the human–machine interface. The filling and emptying of balloons were controlled by detecting the air pressure and pumping time. To conduct a biopsy and initiate the extending action automatically, a closedloop control was implemented by integrating a component to detect current parameters. Because the two motors ran in sequence in this prototype, one sensor detected the current on the power line to reflect the force of action in real time. We placed maximum-value limitations on the current value and air pressure value to guarantee patient safety. Unlike the time control (which is only concerned with mobility), the air pressure of the balloon and the force of the clamping mechanism were detected to ensure safety during the clamping, extending and biopsy process. The highest priority for the device is monitoring the current value and air pressure; these parameters must be detected regularly and transmitted wirelessly to the HMI at 0.6 s intervals. The HMI was developed using Visual Studio 2008 (Microsoft), which allowed the operator to input orders for automatic movement, and to change the upper limit of air pressure or the mechanism force, as shown in figure 6. Orders can be sent to the capsule via the telemetry link, and feedback is obtained from the detector. The HMI was designed to include a realtime streaming image display, which displays images from the video of the intestine from a
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Figure 6. HMI interface showing the figure of an inner colon when the capsule was clamping in the vitro colon.
camera attached to the front of the capsule robot. The vision system was packaged in a copper shield that measured 10 mm in diameter and 34 mm in length. The camera could capture 320 × 240 resolution images, compress them into JPEG form, and transmit the compressed image wirelessly. The image transmission speed was close to 30 fps, which was comparable to speeds in the prior capsules. The efficacy of this method of image processing has been verified in previous multiple studies (Pan et al 2010, Gao et al 2012). 3. Experiments and results 3.1. Characterization of the power module To evaluate the wireless power transmission system, we tested the induced power with a custom setup as shown in figure 7. The receiving coils placed on the two-axes, turntable powered with a dc power supply, could turn the coils in two directions of 120◦ each. To identify the attitude of the coils, we defined angle α 1 as the rotating degree of the coil on the XOY plane, and angle α 2 as the rotating degree of the coil on the XOZ plane, as previously defined in figure 2. Because the receiving coils are axisymmetric, we can derive the output power of the coils using an oscilloscope by changing angle α 1 and α 2 between 0◦ and 90◦ , respectively. The transmitting coil was constructed with a double-layer solenoid pair (550 mm in diameter) and 180 strands of AWG 38 enameled copper wire. The coil was wound on an acrylonitrile butadiene styrene hollow cylinder with 25 turns for each layer. The driving device was installed inside the control box, including the serial resonant circuit (SRC) composed of a vacuum capacitor and an adjustable inductor, together with the transmitting coil. A square wave, which could transmit power more efficiently than the sin wave and triangular wave with a changeable frequency, was used to drive an H-bridge circuit to generate an alternate voltage on the SRC. The amplitude of the alternate voltage can be adjusted by turning the knob on the control box. To obtain stable power to drive the robot, we prepared test coils of Coil1 with 28 strands and 80 turns, and Coil2 and Coil3 with 28 strands and 120 turns (according to the parameter L of the three orthogonal coils that we had designed before).
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Figure 7. Experiment setup for testing wireless power transmission.
Figure 8. The output voltage induced by a receiving coil with 27 load resistance.
AC voltage needed to be rectified and filtered into dc voltage to provide power to the capsule. To address this issue, a power conversion board was assembled with coils placed on the side of the extending mechanism. The board was realized on a 0.5 mm thick PCB. Components were selected from the smallest commercially available. DC power was provided at 5.0 V from the receiving coils by using an LDO (LT17635, Linear). When the driving current was 1 A and the frequency was 216 kHz, the output power could exceed 653 mW at any attitude with an efficiency of up to 28%; even in the worst-case scenario, the efficiency was still greater than 7.98% (as shown in figure 8, the induced power was larger than 4.2 V with the load resistant of 27 ). The available power received by the coils was sufficient for
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the capsule to function. Especially when inflating the balloons, the starting power needed to be nearly 700 mW; this would be extremely difficult to achieve with traditional receiving coils, but was shown to be plausible with our novel receiving coils. The attitude stability of power transmission system is defined as Vmax − Vmin , (7) γ = Vmin where Vmax is the largest output voltage and Vmin is the minimum output voltage. We get γ = 90.14% in our coils with ferrite hollow cylinder core compared to the 73.21% of three orthogonal coils in the previous study. That means the novel receiving coils have a good stability in inducing power. The electromagnetic field generated by the transmitting coil was evaluated to ensure that it was within safe parameters for use in patients. In our previous work, the variations in current density and SAR, compared with the restrictions of the International Commission on Non-Ionizing Radiation Protection (ICNIRP), had been verified. At a frequency of 216 kHz and a driving current of about 1 A, the current value was below the ICNIRP restrictive current density, and the SAR was below the upper boundary of the SAR value of 1.98 A (Jia et al 2012). To decrease the heat dissipation on the coils, we needed to decrease the power consumption for the impedance. Increasing strands of the receiving coil was the most straightforward method. In the experiment, we tested coils with 28 strands and 80 turns made of 0.5 mm enameled wire; this reduced the impedance of the coil to nearly one-third that of the original three orthogonal coils. With one layer of coils winding outside the core, heat dissipation occurred faster than would have been possible with the three multi-layer coils. The temperature safety was tested in the vitro colon, which was soaked in 0.9% saline in the sink with constant temperature of 37.5 ◦ C. The receiving coils were sealed in a medical silicon case which is similar to the balloon of capsule endoscope. The final version of the device did not exceed 40 ◦ C, which is safe for human tissue. 3.2. Characterization of the locomotion module To cope with limited power, we improved the pump and proposed locomotion steps that would not require the two motors to operate simultaneously. Approximately 650 mW would be sufficient to sustain all functions in all possible configurations. The pump’s power typically peaks around 800 mW and the power consumption can be totally supported by the inductive link within safe levels. The anchoring force of the custom-made, medical-grade silica balloons was tested in vitro in a simulated colon environment, as described in Lin et al (2012). The anchoring force of the custom made balloons with texture was 1.91 N, which is much bigger than the anchoring force of smooth balloon of 0.24 N. Compared to the legged-shape anchoring mechanism, the balloon anchor would be more comfortable and safer for patients, and the seamless package could prevent the entry of foreign matter and, by extension, decrease the patient’s risk of postoperative infection. The flow rate of air between two balloons was 11.27 mL min−1. The pump can build up backpressure up to 153 kPa, which would be adequate for balloon inflation in the colon. We could adjust the balloon diameter from 20 to 38 mm by controlling the pump motor direction turning it forward or reverse. The custom balloon with a textured top had a maximum volume of 12 ml (only one-third the volume of the traditional ball-shaped balloon). Locomotion tests were performed wirelessly using a closed, flexible silicone tube with slippery mucus on the internal surface as shown in figure 9, and fresh colon in vitro as shown in figure 6. Porcine intestine specimens were obtained in compliance with standard medical
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Figure 9. Wireless powered capsule locomotion inside the slippery tube.
and ethical guidelines from a 50 kg pig, using sections with an average diameter of 35 mm. The in vitro testing in the simulated colon was powered with the optional 5 V dc. From the test results, we concluded the following. (1) The textured surface of the custom balloon enabled it to anchor well on the slippery tube and the simulated colon. Distributing its pressure over a larger surface of the intestine wall than would legged capsules, the balloon made contact without stabbing, which has significant benefits for patient safety. (2) When the front balloon is inflated, the expanded colon is visible to the video camera, enabling diagnosis of the wrinkled and collapsed lumen. The inchworm-like movement centers the capsule in the gut, minimizing potential leg failure and tissue entrapment. (3) The speed of the robot’s movement was influenced by the slope and diameter of the tube or colon. During most of its journey, the robot is engaged in pumping and when the diameter of the tube or colon varied, the pumping time would also change. The speed decreased when the device was climbing ‘uphill’ because of the slippery environment and resistance from its own weight. Conversely, during descent, the speed increased. In a section of the intestine with a diameter larger than 41 mm, the robot would likely slip. (4) When placed in the middle of the capsule bellows, the cardan provided three degrees of freedom, enabling the capsule to navigate corner smaller than 43◦ (with a colon diameter up to 39 mm). When the turning angle of the intestine was larger than 55◦ , the stretching movement was blocked by the intestine wall. The passable angle was smaller for more slender corners, which indicates that in a corner of thin lumen, the capsule is more likely to be blocked. 3.3. Characterization of the biopsy module The cutting force of the two forceps was measured to identify the feasibility of biopsy tasks. The test results of cutting force are shown in figure 10 compared with theoretical value.
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Figure 10. The idea cut force compared with the test result: (a) big jaw; (b) micro jaw.
Considering the anisotropic characteristics and the uncertainties of the intestine, the desired tissue-cutting force must be over 10 N (Greenish et al 2002). The cutting force of the big jaw ranged from 22 to 10 N at an open angle of 1◦ to 6◦ ; its mechanical efficiency was 79.04%. The cutting force of the micro jaw ranged from 28 to 10 N at an open angle of 10◦ to 31◦ ; its mechanical efficiency was 14.78%. In other words, the force would be adequate to biopsy tissue if the open angle is smaller than 6◦ or 31◦ for the big and micro jaws, respectively. In our present prototype, we integrated the capsular endoscopic device with big jaw. We next will integrate the second generation capsule with the micro jaw and design the mechanism smaller. To demonstrate that the forceps for biopsy can obtain samples from colon tissues, we performed sample excision tests in vitro. The excised pig’s colon was fixed by means of a human hand, and forceps were attached to the small intestine. Several tests were performed to demonstrate the repeatability of the results of the tissue excision procedure; thus, we confirmed that the module could successfully realize its biopsy function. The biopsy process is shown in figure 11 for big jaw and figure 12 for micro jaw.
3.4. System performs with wireless power The whole capsule system was integrated and tested with the wireless power transmission system, as shown in figure 13. The inductive powering system was tested in combination with several capsule modules. The testing of the self-propulsion mechanisms has been discussed in a previous section. The power consumption of the capsule was no more than 800 mW for any action or attitude (the power consumption details are presented in table 1). The power supply was sufficient when the transmission coil frequency was 216 kHz and the driving current was roughly 1 A, which has been shown to be safe for human body. With the wireless control and vision module, the capsule could travel the colon without manual intervention from the human operator. The circuit board was successfully shielded from electromagnetic interference by a copper oval-cylindrical box, and the communication error rate was lower than 1.02% over a distance of 2 m. Error checking code helped in ensuring that the capsule was controlled safely and reliably. Doctors could see the internal colon and control the capsule’s forward, backward, and anchoring functions as well as conduct a biopsy when appropriate.
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Figure 11. The biopsy procedure of big jaws installed in the prototype capsule. (a) Stretch out the capsule body. (b) Open at maximum degree. (c) Bite tissue and withdraw into capsule. (d) Cut off tissue and store it in the capsule body. Table 1. Power consumption of capsule loads.
Module
Power consumption
Operating time
Image sensor LEDs Autofocus system Control circuit Communication circuit Locomotion actuators Biopsy actuators Biopsy cut action Pump starts to work Locomotion or biopsy
20 mW 4 × 5 mW 12 mW 20 mW 15 mW 220 ∼ 800 mW 200 ∼ 700 mW 700 mW 800 mW 200 ∼ 400 mW
90–100% 90–100% 50–80% 100% 70–90% 60 ∼ 80% 1–2% 0.1% 0.2% 60 ∼ 80%
4. Discussions With an effective wireless power transmitting system, a capsule could travel through and examine a patient’s colon without being limited by an inadequate, portable power supply. In this study, we introduced receiving coils with a novel, hollow, cylindrical-core shape to maximize the use of space in the capsule body, and enhance the safety and efficiency of device. Unlike earlier methods of mobility, including robot legs that could potentially puncture or otherwise injure a patient’s intestines, we implemented a new air-balloon feature to anchor to and expand the colon to enable the physician to see the colon and perform a biopsy, if
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Figure 12. The biopsy procedure of micro jaw. (a) Stretch out of body and open. (b) Bite tissue. (c) Withdraws into body. (d) Cut off tissue and store it in the body.
Figure 13. Side view of the capsule prototype with the internal modules.
necessary. In addition, to protect the intestine from a puncture as a result of over-inflation of a balloon or the extending mechanism, we added a control system to keep the expansion force within a safe limit. The loosely coupled regime of wireless power transmission system suggests that only a small fraction of the input power is detected by the receiving coil and reaches the load of capsule. To resolve this issue, it would be necessary to find a technical solution that deflects the highest number of field lines into the tiny receiving coil set (Puers et al 2011). Based on our previous studies, even a slight increase in receiving coil volume could increase efficiency
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significantly (Gao et al 2012). The successful use of multiple hollow cylindrical cores led us to hypothesize that enlarging a cross-sectional area of the coil ferrite-core would be an effective way to improve power transmission efficiency. In our design, the core maximized the space for inducing power and thus supplied adequate power for capsule functions. Although our capsule prototype proved the feasibility of the wireless power transmission system during testing, several design features merit additional discussion. The air balloon ensures the safety of the soft, sensitive colon wall. However, locomotion speed presents a problem; each step in the locomotion process would take between 60 to 125 s, which would be too slow for colon examination in clinical situations. This problem could likely be resolved with an optimized pump structure and a more powerful micro motor to decrease inflation time. When the diameter of the colon was smaller than 26 mm, the air could not be exchanged between two balloons. Because the outer diameter of the balloon limits the amount of air it can hold, it leads to capsule getting stuck. This could be resolved by using an airbag to store the excess air, which would require a pipe connected to one balloon and extra space in capsule. The capsule locomotion performance has yet to be tested under in vivo conditions. Although we used the porcine specimen to mimic the GI tract, this may not perfectly imitate the human colon and thus requires further study before it can be used clinically. Peristalsis would assist the capsule’s motion, and may even theoretically be intentionally harnessed during portions of the journey that are not of interest to the desired diagnostic goal. Additionally, the device’s theoretical ability to maximize patient comfort and ensure patient safety during anchoring and extending of the colon should be verified. Thus, future work should implement in vivo studies to optimize design and confirm that safety measures have been effective. 5. Conclusions In this work, we proposed novel receiving coils for capsule-biopsy systems. In the worst-case scenario, the power transmission efficiency was over 7.98%; moreover, receiving power was stable over 650 mW in the context of size and safety constraints. Importantly, this capsule has the capability to be powered wirelessly, allowing for more extensive and complicated surgical and other robotic tasks. Although the anchoring mechanism of the balloon needs improvement, its safety and the property that can be fully sealed show promise for future development. Two types of forceps (for the biopsy task) were developed and tested in vitro; both of these successfully extracted tissue samples from the colon of a pig. The results of this study can serve as a foundation for developing micro-surgical robotic capsules in the future. Acknowledgments This work has been supported by the National Natural Science Foundation of China (NSFC) (no. 31170968); Technical Pre-Research of Manned Spaceflight Research (no. 010203); Shanghai Science and Technology Commission funded project (09 DZ1907400). The authors would like to thank Mr Chenggeng Wu for his help. References Arena A, Boulougoura M, Chowdrey H S, Dario P, Harendt C, Irion K, Kodogiannis V, Lenaerts B, Menciassi A and Puers R 2005 Intracorporeal videoprobe (IVP) Stud. Health Technol. Inform. 114 167 Buselli E et al 2010 Evaluation of friction enhancement through soft polymer micro-patterns in active capsule endoscopy Meas. Sci. Technol. 21 105802 Byun S, Lim J, Paik S, Lee A, Koo K, Park S, Park J, Choi B and Shim E 2004 Novel micro-spiked needles with buried micro-channels for micro-scale biopsy Asia-Pacific Conf. of Transducers and Micro-Nano Technology pp 1095–9
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