Int J Artif Organs 2013; 36 (12): 900-906
DOI: 10.5301/ijao.5000261
ORIGINAL ARTICLE
Novel artificial anal sphincter system based on transcutaneous energy transmission system tested in vivo Yongbing Wang 1, Hua Liu 2, Qianqian Xu 3, Guozheng Yan 4 Department of General Surgery, Pudong New District People's Hospital, Shanghai - China Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai JiaoTong University, Shanghai - China 3 Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai JiaoTong University, Shanghai - China 4 Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai JiaoTong University, Shanghai - China 1
2
This paper proposes a novel artificial anal sphincter system (AASS) for severe fecal incontinence. The AASS is composed of an artificial anal sphincter (AAS), an external transcutaneous energy transmission system (TETS), and an external control device. The AAS is composed of a cuff, a micropump, a reservoir, and a remote control device. It is designed to be implanted into the body of the patient. The function of the AAS is to open and close the patient’s natural anus. Patients suffering from loss of their natural sphincter lose rectal sensation and are thus unable to perceive imminent fecal incontinence. In order to restore rectal sensation, a pressure sensor in the AAS cuff is designed to detect pressure in the colon. The pressure reflects the present quantity of colon contents, allowing patients to control the AAS to open or close the anus according to the pressure. The TETS is designed to provide electrical energy to the implanted AAS without wire connections. The external control device is designed to receive the pressure information from the AAS and send the patient’s command to the implanted device. This paper provides a thorough discussion of the design of the novel AASS and describes the performance of the AASS when tested in vivo on two Beagle dogs who were chosen to be the subjects for receiving the implant. The experimental results verified that the performance of the AASS met the functional requirements it was designed for; however, the trial also revealed some challenges to be further studied. Keywords: Artificial anal sphincter system (AASS), Artificial anal sphincter (AAS), Transcutaneous energy transmission system (TETS), Wireless communication, Pressure sensor Accepted: August 3, 2013
INTRODUCTION Severe incontinence can arise from a variety of causes. While it is difficult to assess the exact prevalence of severe incontinence, it is estimated that in 1995 the prevalence of incontinence was about 2.2% of the population in the 900
United States. Among the survey population, the prevalence of incontinence among elderly people over 65 was about 30% (1). In cases of severe incontinence, such as a result from colon cancer, the sphincter needs to be excised, which in turn results in permanent fecal incontinence. Aid must be provided to patients in such cases to provide a
© 2013 Wichtig Editore - ISSN 0391-3988
Wang et al
better quality of life. This paper proposes a solution developed specifically for patients whose sphincters have been excised due to colon cancer or other causes. Much research work has been done regarding the treatment and handling of severe fecal incontinence, for which an artificial anal sphincter (AAS) presents one successful solution (2). Currently, there is already an AAS in clinical application. There are a lot of documents about the implantation of an AAS. Numerous research efforts (4-11) have also focused on evaluating the long-term prognosis of patient recipients of AAS implants, where the overall efficacy for cases of severe fecal incontinence has been proved satisfactory. However, one major drawback of the current AAS is that, using an artificial sphincter, a patient generally has no ability to know the imminent occurrence of fecal incontinence. Many other research works have explored alternatives to AASs. For example, Scharg et al (12) proposed a remotecontrolled artificial bowel sphincter. The artificial sphincter included a fluid reservoir, an occlusive cuff, and a micropump based on piezo-technology driven by electricity. The design of the artificial bowel sphincter eliminated the need for manual operation of AASs. Yasuyuki Kakubari et al (13) proposed an artificial sphincter that utilizes shape memory alloy (SMA). In order to drive this artificial sphincter, a transcutaneous energy transmission system (TETS) was designed. This AAS was also driven by electricity and eliminated the need for manual operation of AASs. Chengren Shi et al (14) proposed a new artificial pump designed for an anal sphincter. He used a pressure sensor to get feedback from the colon, which made it possible for the patient to operate the AAS with conscious control. This anal sphincter was powered from the outside with a wire connection, which increased the likelihood of infections, since the skin was perforated to allow a cable to connect the implanted AAS to the outside. This study proposes a novel artificial anal sphincter system (AASS) that allows both transcutaneous energy transmission for the implanted system and conscious control of the AAS by the patient. The paper provides a thorough description of the complete AASS, followed by presentation of the in vivo experiments performed on two Beagle dogs in an effort to evaluate the basic functions of the implemented AASS. The paper then provides a discussion of the experimental results, as well as the revealed challenges concerning the AASS, which must be addressed before it can be employed in clinical applications.
MATERIALS AND METHODS The AASS is composed of three parts: an artificial anal sphincter, a transcutaneous energy transmission system, and an external control device. These three parts will be discussed thoroughly in this section.
The implanted artificial anal sphincter (AAS) The AAS was designed to mimic the function of the natural sphincter and be able to be remotely controlled based on feedback from the pressure sensor. A sphincter is an anatomical structure, a circular muscle that normally maintains constriction of a natural passage or an orifice within the body and which relaxes as required by normal physiological functioning. The two basic functions of the anal sphincter are restoring continence and relaxing continence. The AAS was designed to meet these two basic functions. Since the AAS represents the implanted portion, its size should be as small as possible to achieve minimal invasion. Based on the anatomic structure of a natural anus, the AAS should mimic the shape and be circular to enclose the natural anus. Subcomponents of our proposed, implanted AAS include a cuff, a micropump, a liquid reservoir, and a remote control device. The cuff was designed to enclose the anus circularly. It is a silicone elastomer driven by the inner liquid. Liquid is an optimal choice for inflating or deflating the cuff since liquids such as water or physiological saline are compatible with the environment in the body. When it is driven by the liquid in itself, the cuff inflates centripetally and perfectly seals the anus. The pressure in the cuff is symmetrical around the anus. When the liquid in the cuff is drained out, the cuff is able to rapidly relax and open the anus. A reservoir was needed to accommodate the liquid needed for to operate the cuff. In order to maintain the pressure in the cuff to either close or open the anus, a valve was needed to isolate the liquid between the reservoir and the cuff. Figure 1 depicts the working states of the artificial anal sphincter, where the micropump, a valve, a reservoir, and the cuff work together. In order to minimize the implanted device’s size, the valve and the micropump were integrated into one unit to achieve a smaller size. Then the integrated unit had the abilities to pump liquid and to cut off the connection between the cuff and the reservoir. For pumping, the unit used a bidirectional micropump shown in Figure 2.
© 2013 Wichtig Editore - ISSN 0391-3988
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Novel artificial anal sphincter system
Fig. 1 - Scheme of the artificial anal sphincter. This figure comprises the four working states of AAS. The valve is integrated into the micropump to get smaller size. (a) Deflating status; (b) Deflated maintaining status; (c) Inflating status; (d) Inflated maintaining status.
Fig. 2 - Structure of the micropump. This micropump is driven by a motor. It is a peristaltic pump in principle. 1. motor; 2. planet carrier; 3. roller; 4. bearing; 5. pressure sensor; 6. control circuit. (a) Inner structure of motor and three rolls; (b) micropump in the final solution.
Considering that the AAS needs to be remotely controlled, electrical components are necessary parts of the implant. In addition, a pressure sensor was designed to acquire pressure information in the colon. Circuitry to control the micropump, for the pressure sensing and wireless communication, is presented in the following sections.
Biofeedback pressure sensor Patients with their sphincter excised lose the ability to feel rectal sensations, hence some information must be provided to the patient to alert them of the need to defecate. A pressure sensor was designed to detect the pressure within the colon. Due to its anatomical structure, the colon presents as soft tissue. Hence a pressure sensor cannot 902
Fig. 3 - Pressure transmitting system. When the gastrointestinal contents increase, the colon exerts pressure on the circular cuff. The pressure can be transmitted to the pressure sensor by sealed water in the circular cuff.
measure the pressure in the colon directly. Instead, a circular cuff was designed to measure the pressure in the colon indirectly. Figure 3 depicts the structure of the mechanism used to measure the pressure in the colon. The circular cuff is made of silicone elastomer. It is filled with liquid and sealed, and designed to be able to inflate centripetally or deflate centrifugally while not blocking the passage of the gastrointestinal contents in the colon. In operation, when the amount of gastrointestinal contents increases in the colon, the colon will exert pressure on the elastic cuff, which will be transmitted equally in all directions throughout the liquid. The transmitted pressure is measured by a pressure sensor placed inside the cuff, and gives an indirect indication of the amount of gastrointestinal contents present in the colon. The pressure signal is measured in real time by a microprocessor and an analog to digital converter. For incontinent patients, the average anal pressure for both the resting state and a state of exertion are low, at 59.1 mmHg and 74.6 mmHg respectively (15). To trigger an alert for defecation, a “high” pressure threshold is defined according to the literature (15). When the pressure detected by the microprocessor approaches the pressure threshold, a warning signal is sent to the patient through the wireless communications transceiver. Upon receiving the warning signal, the patient can choose to send a start signal to the AAS (through the wireless communication transceiver as well) to allow opening of the anus. When the microprocessor receives the start signal from the patient, it immediately changes the valve position and starts the micropump to drain liquid from the anal cuff to the reservoir. The cuff will become loose and let the gastrointestinal contents pass out of the body, reduceing the pressure measured by the pressure sensor. When
© 2013 Wichtig Editore - ISSN 0391-3988
Wang et al
Fig. 4 - Controlling components. This scheme shows the controlling components of the artificial anal sphincter. The PIC16F690 is a microprocessor manufactured by Microchip Technology Inc., Chandler, AZ, USA. The Serial Peripheral Interface (SPI) bus is a synchronous serial data link that operates in full duplex mode (manufacturer, city, country). It helps the PIC16F690 communicate with other chips. The AD7683 (Analog Devices, Norwood, MA, USA) is an analogto-digital chip with 16-bit resolution. The LTC2053 (Linear Technology, Milpitas, CA, USA) is an instrument amplifier. The max4684 (Maxim Integrated, San Jose, CA, USA) is an analog switch. The IA4420 (or Si4420; Silicon Labs, Austin, TX, USA) is a wireless communication transceiver.
the pressure signal goes below a preset “low” pressure threshold, the microprocessor turns off the micropump. Since the AAS is equipped with a valve, the pressure in the cuff will be maintained through the time duration needed to allow the passage of gastrointestinal contents. After a fixed time period of loosening the anal cuff, the microprocessor will again start the micropump to pump the liquid from the reservoir back into the cuff. The cuff will inflate centripetally and once again close the colon opening. As soon as the pressure of the liquid in the cuff reaches a fixed preset level, the microprocessor stops the micropump and closes the valve to maintain cuff pressure.
Communications and control electronics This section describes the electronic circuitry enabling the AAS to be remotely controlled under patients’ conscious control. Figure 4 depicts the structure of the controlling components. The microprocessor is the PIC16F690 (Microchip Technology Inc., Chandler, AZ, USA), chosen for its small size and ultralow power consumption. The output of the sensor is preconditioned by an LTC2053 instrument amplifier (Linear Technology, Milpitas, CA, USA) to get the pressure signal within the appropriate voltage for the AD7683 (Analog Devices, Norwood, MA, USA), which samples the pressure signal. An IA4420 integrated circuit (or Si4420; Silicon Labs, Austin, TX, USA) is chosen to provide wireless communication between the AAS and the patient’s external remote control. The Max4684 is an analog switch
(Maxim Integrated, San Jose, CA, USA), chosen to implement the bidirectional drive of the motor of the micropump. Both the ADC and the IA4420 communicates with the PIC microprocessor via a four-wire, serial peripheral interface (SPI) bus. The electronic circuitry was integrated into the micropump in order to attain a small overall size for the device and minimize the invasion of the implantation procedure.
Transcutaneous Energy Transmission System (TETS) The TETS is designed to provide energy for the implanted AAS from outside for a prolonged period. Figure 5 is the scheme of the TETS. Direct current is inverted by a full bridge inverter and it turns into alternating current. An alternating magnetic field generates while the alternating current flows through the primary coil. The secondary coil can couple the alternating magnetic field and generate an alternating voltage in itself. This alternating voltage can be rectified into direct current power source. The rectified direct current power source is fed into all electrical components in the AAS. There are numerous factors affecting the efficiency of the TETS. In this project, the structures and sizes of the primary coil and the secondary coil are very important. Figures 6 and 7 show the secondary coil and the primary coil in the TETS. Energy is transmitted through the skin tissue by magnetic coupling between the primary coil outside the patient’s body and the secondary
© 2013 Wichtig Editore - ISSN 0391-3988
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Novel artificial anal sphincter system
Fig. 5 - Scheme of the TETS. Energy transmitter is situated outside of the patient’s body. Energy receiver is implanted under the patient’s skin.
Fig. 6 - The secondary coil of the transcutaneous energy transmission system (TETS). The secondary coil is implanted under the patient’s skin to create a short distance to the primary coil. The planar spiral shape is helpful for the implantation procedure.
Fig. 7 - The primary coil of the transcutaneous energy transmission system (TETS), which is also planar spiral. Since the primary is placed on the surface of the patient’s skin, it can be larger in size than the secondary coil in order to improve the efficiency of the TETS.
coil implanted under the patient’s skin. In order to achieve minimal invasion, the size of the secondary coil was made as small as possible. A limiting factor on the size of the secondary coil, however, is that the efficiency of the coil is heavily dependent on its size. In particular, the smaller the secondary coil, the lower the efficiency. Since the TETS must have high efficiency, this paper proposes a planar spiral structure for the secondary coil. In order to strengthen the magnetic flux coupled between the primary and the secondary coils, a flat circular ferrite core is designed to be the base of the secondary coil. The thickness of the ferrite was kept to only 1.0 mm, and the diameter was 25 mm. A tiny copper wire was wound spirally 904
Fig. 8 - Structure of external control device. PNP is a type of transistor that consists of a layer of N-doped semiconductors between two layers of P-doped material. The PIC16F690 and IA4420 are described in Fig. 4. S1 and S2 are two press buttons.
on the surface of the ferrite core by hand. The diameter of the copper wire used was 0.15 mm. Glue was used to fix the copper winding in the proper position on the surface of the ferrite core, and when the copper winding covered the full of the surface of the ferrite core, epoxy was applied across the surface of the copper winding to provide electrical insulation between the copper and body tissue once it is implanted into the body. The rectified circuit is integrated into the secondary coil, again to minimize the overall size of the implanted device. The primary coil was also designed to be planar spiral. A planar ferrite core is designed to strengthen the magnetic coupling between the primary coil and the secondary coil. As on the secondary coil, Litz cables were wound spirally on the surface of the primary ferrite core, and an FR-4 board is used to maintain the shape of the spiral Litz cables. The diameter of the primary coil is 48 mm and the height is 5 mm.
External control device The purpose of the external control device is to enable the patient to wirelessly control the implanted AAS. The external control device includes a wireless communication transceiver, a microprocessor, and human interfaces such as push buttons, a buzzer, and a liquid crystal character display (LCD) for displaying messages to the patient. The structure of the external control device is in Figure 8. The character LCD is used to quantitatively show the measured colon pressure value to patients. The buzzer is used to give an audio warning signal to the patient of elevated colon pressure, indicative of the need to allow defecation. In the circuit, the buzzer is driven by a PNP transistor which
© 2013 Wichtig Editore - ISSN 0391-3988
Wang et al
Fig. 9 - Actual AASS. Only the AAS is sterilized since it will be implanted into the body of the dogs. (a) Artificial anal sphincter; (b) TETS (including the primary coil and the secondary coil); (c) External control device.
Fig. 10 - In vivo experiment of the AASS. This photo shows that AAS was already implanted into the body of the dog and that the AASS performed well.
can provide a large current for the buzzer. Push buttons S1 and S2 are designed to allow the patient to issue the start or stop signals controlling the implanted AAS. The microprocessor (PIC16F690) and wireless communication transceiver (IA4420) chips used are the same as those shown in Figure 4. Figure 9 shows the actual AASS before it was used in the in vivo experiment. The implanted part of the AASS is sterilized. In order to meet the requirement of bio-compatibility, the surface of the micropump is treated with a thin layer of silicone. The cables connecting the secondary coil and the control device in the micropump is protected by small diameter silicone tube.
ingested nothing except water. They slept in their cages for most of the time, then some soft food with a large quantity of water was provided to the subjects. Figure 10 shows the surgical procedure implanting the AASS into the second subject. Before the surgeon sewed the incision of the dog closed, the basic functions of the AASS were tested. TETS was able to transmit electrical power to the implanted AAS. The pressure information in the cuff of the implanted AAS was sent out and displayed on the LCD of the external control device. The author was able to send a command to the implanted AAS to open the anus and send a command to the implanted AAS to close the anus. The test results proved that the performance of AASS met the expected functional requirements.
RESULTS
DISCUSSION
After the AASS was designed and assembled, the functions of every module were thoroughly tested by experiments; thereafter, in vivo experiments were conducted on two one-and-half-year old Beagle dogs. The first subject weighed 11.5 kg; the second weighed 11.6 kg. The subjects were fasted one day before the experiments. Only water was freely provided. During the implantation procedure, intravenous injection was provided to the subjects. In the first three days after the implantation, the subjects
Although the in vivo experiment verified the basic functions of the AASS, it revealed new challenges for its clinical application. In particular, the long-term biocompatibility between the AAS and the surrounding tissue needs to be further studied. The long-term reliability of the AASS should also be tested through more experiments. Lastly, while the TETS has been verified to be able to provide enough power to the implanted AAS, the heat resulting from the TETS should be considered.
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CONCLUSIONS This paper proposed a novel AAS system as a solution for severe fecal incontinence. The in vivo experiment verified the basic functions of the AASS. A main feature of the proposed system was the use of a pressure sensor to measure the colon pressure, which can be used to mimic biofeedback, thereby restoring patient’s access to information conveyed by natural rectal sensations. With such feedback, the patient can wirelessly control the AAS.
to thank Yi Gu, a doctoral student in the Radiology Department at Stanford University, for his help in preparing the manuscript. Financial Support: This research work was supported by the Nature Science Foundation of China under grants 30800235 and 31271069. This work was also funded by the Key Discipline Construction Project of Pudong Health Bureau of Shanghai (Grant No. PWZxk2010-01). Conflict of Interest: None.
The authors appreciate all the editors and reviewers for their valuable comments on the manuscript. The authors wish
Address for correspondence: Hua Liu Department of Instrument Science and Engineering School of Electronic Information and Electrical Engineering Shanghai JiaoTong University Shanghai, China 200240 [email protected]
REFERENCES
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ACKNOWLEDGEMENTS
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Zutshi M, Salcedo L, Hammel J, Hull T. Anal physiology testing in fecal incontinence: is it of any value? Int J Colorectal Dis. 2010;25(2):277-282. 2. Wong WD, Jensen LL, Bartolo DC, Rothenberger DA. Artificial anal sphincter. Dis Colon Rectum. 1996;39(12): 1345-1351. 3. Christiansen J, Sparsø B. Treatment of anal incontinence by an implantable prosthetic anal sphincter. Ann Surg. 1992;215(4):383-386. 4. Michot F, Costaglioli B, Leroi AM, Denis P. Artificial anal sphincter in severe fecal incontinence: outcome of prospective experience with 37 patients in one institution. Ann Surg. 2003;237(1):52-56. 5. Christiansen J, Rasmussen OO, Lindorff-Larsen K. Longterm results of artificial anal sphincter implantation for severe anal incontinence. Ann Surg. 1999;230(1):45-48. 6. Altomare DF, Binda GA, Dodi G, et al. Disappointing long-term results of the artificial anal sphincter for faecal incontinence. Br J Surg. 2004;91(10):1352-1353. 7. Lehur P-A, Michot F, Denis P, et al. Results of artificial sphincter in severe anal incontinence. Report of 14 consecutive implantations. Dis Colon Rectum. 1996;39(12): 1352-1355.
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Devesa JM, Rey A, Hervas PL, et al. Artificial anal sphincter: complications and functional results of a large personal series. Dis Colon Rectum. 2002;45(9):1154-1163. Finlay IG, Richardson W, Hajivassiliou CA. Outcome after implantation of a novel prosthetic anal sphincter in humans. Br J Surg. 2004;91(11):1485-1492. Michot F, Lefebure B, Bridoux V, et al. Artificial anal sphincter for severe fecal incontinence implanted by a transvaginal approach: experience with 32 patients treated at one institution. Dis Colon Rectum. 2010;53(8):1155-1160. Mundy L, Merlin TL, Maddern GJ, Hiller JE. Systematic review of safety and effectiveness of an artificial bowel sphincter for faecal incontinence. Br J Surg. 2004;91(6):665-672. Schrag HJ, Ruthmann O, Doll A, Goldschmidtböing F, Woias P, Hopt UT. Development of a novel, remote-controlled artificial bowel sphincter through microsystems technology. Artif Organs. 2006;30(11):855-862. Kakubari Y, Sato F, Matsuki H, et al. Temperature Control of SMA Artificial Anal Sphincter. IEEE Trans Magn. 2003;39(5):3384-3386. Shi CR, Wu YM, Jin LY, et al. New artificial pump development of the anal sphincter and its role observed. Chin J Pediatr Surg. 2001;22(5):301-302. Seong MK, Park UC, Jung SI. Determinant of anal resting pressure gradient in association with continence function. J Neurogastroenterol Motil. 2011;17(3):300-304.
© 2013 Wichtig Editore - ISSN 0391-3988
DOI: 10.5301/ijao.5000261
ORIGINAL ARTICLE
Novel artificial anal sphincter system based on transcutaneous energy transmission system tested in vivo Yongbing Wang 1, Hua Liu 2, Qianqian Xu 3, Guozheng Yan 4 Department of General Surgery, Pudong New District People's Hospital, Shanghai - China Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai JiaoTong University, Shanghai - China 3 Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai JiaoTong University, Shanghai - China 4 Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai JiaoTong University, Shanghai - China 1
2
This paper proposes a novel artificial anal sphincter system (AASS) for severe fecal incontinence. The AASS is composed of an artificial anal sphincter (AAS), an external transcutaneous energy transmission system (TETS), and an external control device. The AAS is composed of a cuff, a micropump, a reservoir, and a remote control device. It is designed to be implanted into the body of the patient. The function of the AAS is to open and close the patient’s natural anus. Patients suffering from loss of their natural sphincter lose rectal sensation and are thus unable to perceive imminent fecal incontinence. In order to restore rectal sensation, a pressure sensor in the AAS cuff is designed to detect pressure in the colon. The pressure reflects the present quantity of colon contents, allowing patients to control the AAS to open or close the anus according to the pressure. The TETS is designed to provide electrical energy to the implanted AAS without wire connections. The external control device is designed to receive the pressure information from the AAS and send the patient’s command to the implanted device. This paper provides a thorough discussion of the design of the novel AASS and describes the performance of the AASS when tested in vivo on two Beagle dogs who were chosen to be the subjects for receiving the implant. The experimental results verified that the performance of the AASS met the functional requirements it was designed for; however, the trial also revealed some challenges to be further studied. Keywords: Artificial anal sphincter system (AASS), Artificial anal sphincter (AAS), Transcutaneous energy transmission system (TETS), Wireless communication, Pressure sensor Accepted: August 3, 2013
INTRODUCTION Severe incontinence can arise from a variety of causes. While it is difficult to assess the exact prevalence of severe incontinence, it is estimated that in 1995 the prevalence of incontinence was about 2.2% of the population in the 900
United States. Among the survey population, the prevalence of incontinence among elderly people over 65 was about 30% (1). In cases of severe incontinence, such as a result from colon cancer, the sphincter needs to be excised, which in turn results in permanent fecal incontinence. Aid must be provided to patients in such cases to provide a
© 2013 Wichtig Editore - ISSN 0391-3988
Wang et al
better quality of life. This paper proposes a solution developed specifically for patients whose sphincters have been excised due to colon cancer or other causes. Much research work has been done regarding the treatment and handling of severe fecal incontinence, for which an artificial anal sphincter (AAS) presents one successful solution (2). Currently, there is already an AAS in clinical application. There are a lot of documents about the implantation of an AAS. Numerous research efforts (4-11) have also focused on evaluating the long-term prognosis of patient recipients of AAS implants, where the overall efficacy for cases of severe fecal incontinence has been proved satisfactory. However, one major drawback of the current AAS is that, using an artificial sphincter, a patient generally has no ability to know the imminent occurrence of fecal incontinence. Many other research works have explored alternatives to AASs. For example, Scharg et al (12) proposed a remotecontrolled artificial bowel sphincter. The artificial sphincter included a fluid reservoir, an occlusive cuff, and a micropump based on piezo-technology driven by electricity. The design of the artificial bowel sphincter eliminated the need for manual operation of AASs. Yasuyuki Kakubari et al (13) proposed an artificial sphincter that utilizes shape memory alloy (SMA). In order to drive this artificial sphincter, a transcutaneous energy transmission system (TETS) was designed. This AAS was also driven by electricity and eliminated the need for manual operation of AASs. Chengren Shi et al (14) proposed a new artificial pump designed for an anal sphincter. He used a pressure sensor to get feedback from the colon, which made it possible for the patient to operate the AAS with conscious control. This anal sphincter was powered from the outside with a wire connection, which increased the likelihood of infections, since the skin was perforated to allow a cable to connect the implanted AAS to the outside. This study proposes a novel artificial anal sphincter system (AASS) that allows both transcutaneous energy transmission for the implanted system and conscious control of the AAS by the patient. The paper provides a thorough description of the complete AASS, followed by presentation of the in vivo experiments performed on two Beagle dogs in an effort to evaluate the basic functions of the implemented AASS. The paper then provides a discussion of the experimental results, as well as the revealed challenges concerning the AASS, which must be addressed before it can be employed in clinical applications.
MATERIALS AND METHODS The AASS is composed of three parts: an artificial anal sphincter, a transcutaneous energy transmission system, and an external control device. These three parts will be discussed thoroughly in this section.
The implanted artificial anal sphincter (AAS) The AAS was designed to mimic the function of the natural sphincter and be able to be remotely controlled based on feedback from the pressure sensor. A sphincter is an anatomical structure, a circular muscle that normally maintains constriction of a natural passage or an orifice within the body and which relaxes as required by normal physiological functioning. The two basic functions of the anal sphincter are restoring continence and relaxing continence. The AAS was designed to meet these two basic functions. Since the AAS represents the implanted portion, its size should be as small as possible to achieve minimal invasion. Based on the anatomic structure of a natural anus, the AAS should mimic the shape and be circular to enclose the natural anus. Subcomponents of our proposed, implanted AAS include a cuff, a micropump, a liquid reservoir, and a remote control device. The cuff was designed to enclose the anus circularly. It is a silicone elastomer driven by the inner liquid. Liquid is an optimal choice for inflating or deflating the cuff since liquids such as water or physiological saline are compatible with the environment in the body. When it is driven by the liquid in itself, the cuff inflates centripetally and perfectly seals the anus. The pressure in the cuff is symmetrical around the anus. When the liquid in the cuff is drained out, the cuff is able to rapidly relax and open the anus. A reservoir was needed to accommodate the liquid needed for to operate the cuff. In order to maintain the pressure in the cuff to either close or open the anus, a valve was needed to isolate the liquid between the reservoir and the cuff. Figure 1 depicts the working states of the artificial anal sphincter, where the micropump, a valve, a reservoir, and the cuff work together. In order to minimize the implanted device’s size, the valve and the micropump were integrated into one unit to achieve a smaller size. Then the integrated unit had the abilities to pump liquid and to cut off the connection between the cuff and the reservoir. For pumping, the unit used a bidirectional micropump shown in Figure 2.
© 2013 Wichtig Editore - ISSN 0391-3988
901
Novel artificial anal sphincter system
Fig. 1 - Scheme of the artificial anal sphincter. This figure comprises the four working states of AAS. The valve is integrated into the micropump to get smaller size. (a) Deflating status; (b) Deflated maintaining status; (c) Inflating status; (d) Inflated maintaining status.
Fig. 2 - Structure of the micropump. This micropump is driven by a motor. It is a peristaltic pump in principle. 1. motor; 2. planet carrier; 3. roller; 4. bearing; 5. pressure sensor; 6. control circuit. (a) Inner structure of motor and three rolls; (b) micropump in the final solution.
Considering that the AAS needs to be remotely controlled, electrical components are necessary parts of the implant. In addition, a pressure sensor was designed to acquire pressure information in the colon. Circuitry to control the micropump, for the pressure sensing and wireless communication, is presented in the following sections.
Biofeedback pressure sensor Patients with their sphincter excised lose the ability to feel rectal sensations, hence some information must be provided to the patient to alert them of the need to defecate. A pressure sensor was designed to detect the pressure within the colon. Due to its anatomical structure, the colon presents as soft tissue. Hence a pressure sensor cannot 902
Fig. 3 - Pressure transmitting system. When the gastrointestinal contents increase, the colon exerts pressure on the circular cuff. The pressure can be transmitted to the pressure sensor by sealed water in the circular cuff.
measure the pressure in the colon directly. Instead, a circular cuff was designed to measure the pressure in the colon indirectly. Figure 3 depicts the structure of the mechanism used to measure the pressure in the colon. The circular cuff is made of silicone elastomer. It is filled with liquid and sealed, and designed to be able to inflate centripetally or deflate centrifugally while not blocking the passage of the gastrointestinal contents in the colon. In operation, when the amount of gastrointestinal contents increases in the colon, the colon will exert pressure on the elastic cuff, which will be transmitted equally in all directions throughout the liquid. The transmitted pressure is measured by a pressure sensor placed inside the cuff, and gives an indirect indication of the amount of gastrointestinal contents present in the colon. The pressure signal is measured in real time by a microprocessor and an analog to digital converter. For incontinent patients, the average anal pressure for both the resting state and a state of exertion are low, at 59.1 mmHg and 74.6 mmHg respectively (15). To trigger an alert for defecation, a “high” pressure threshold is defined according to the literature (15). When the pressure detected by the microprocessor approaches the pressure threshold, a warning signal is sent to the patient through the wireless communications transceiver. Upon receiving the warning signal, the patient can choose to send a start signal to the AAS (through the wireless communication transceiver as well) to allow opening of the anus. When the microprocessor receives the start signal from the patient, it immediately changes the valve position and starts the micropump to drain liquid from the anal cuff to the reservoir. The cuff will become loose and let the gastrointestinal contents pass out of the body, reduceing the pressure measured by the pressure sensor. When
© 2013 Wichtig Editore - ISSN 0391-3988
Wang et al
Fig. 4 - Controlling components. This scheme shows the controlling components of the artificial anal sphincter. The PIC16F690 is a microprocessor manufactured by Microchip Technology Inc., Chandler, AZ, USA. The Serial Peripheral Interface (SPI) bus is a synchronous serial data link that operates in full duplex mode (manufacturer, city, country). It helps the PIC16F690 communicate with other chips. The AD7683 (Analog Devices, Norwood, MA, USA) is an analogto-digital chip with 16-bit resolution. The LTC2053 (Linear Technology, Milpitas, CA, USA) is an instrument amplifier. The max4684 (Maxim Integrated, San Jose, CA, USA) is an analog switch. The IA4420 (or Si4420; Silicon Labs, Austin, TX, USA) is a wireless communication transceiver.
the pressure signal goes below a preset “low” pressure threshold, the microprocessor turns off the micropump. Since the AAS is equipped with a valve, the pressure in the cuff will be maintained through the time duration needed to allow the passage of gastrointestinal contents. After a fixed time period of loosening the anal cuff, the microprocessor will again start the micropump to pump the liquid from the reservoir back into the cuff. The cuff will inflate centripetally and once again close the colon opening. As soon as the pressure of the liquid in the cuff reaches a fixed preset level, the microprocessor stops the micropump and closes the valve to maintain cuff pressure.
Communications and control electronics This section describes the electronic circuitry enabling the AAS to be remotely controlled under patients’ conscious control. Figure 4 depicts the structure of the controlling components. The microprocessor is the PIC16F690 (Microchip Technology Inc., Chandler, AZ, USA), chosen for its small size and ultralow power consumption. The output of the sensor is preconditioned by an LTC2053 instrument amplifier (Linear Technology, Milpitas, CA, USA) to get the pressure signal within the appropriate voltage for the AD7683 (Analog Devices, Norwood, MA, USA), which samples the pressure signal. An IA4420 integrated circuit (or Si4420; Silicon Labs, Austin, TX, USA) is chosen to provide wireless communication between the AAS and the patient’s external remote control. The Max4684 is an analog switch
(Maxim Integrated, San Jose, CA, USA), chosen to implement the bidirectional drive of the motor of the micropump. Both the ADC and the IA4420 communicates with the PIC microprocessor via a four-wire, serial peripheral interface (SPI) bus. The electronic circuitry was integrated into the micropump in order to attain a small overall size for the device and minimize the invasion of the implantation procedure.
Transcutaneous Energy Transmission System (TETS) The TETS is designed to provide energy for the implanted AAS from outside for a prolonged period. Figure 5 is the scheme of the TETS. Direct current is inverted by a full bridge inverter and it turns into alternating current. An alternating magnetic field generates while the alternating current flows through the primary coil. The secondary coil can couple the alternating magnetic field and generate an alternating voltage in itself. This alternating voltage can be rectified into direct current power source. The rectified direct current power source is fed into all electrical components in the AAS. There are numerous factors affecting the efficiency of the TETS. In this project, the structures and sizes of the primary coil and the secondary coil are very important. Figures 6 and 7 show the secondary coil and the primary coil in the TETS. Energy is transmitted through the skin tissue by magnetic coupling between the primary coil outside the patient’s body and the secondary
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Fig. 5 - Scheme of the TETS. Energy transmitter is situated outside of the patient’s body. Energy receiver is implanted under the patient’s skin.
Fig. 6 - The secondary coil of the transcutaneous energy transmission system (TETS). The secondary coil is implanted under the patient’s skin to create a short distance to the primary coil. The planar spiral shape is helpful for the implantation procedure.
Fig. 7 - The primary coil of the transcutaneous energy transmission system (TETS), which is also planar spiral. Since the primary is placed on the surface of the patient’s skin, it can be larger in size than the secondary coil in order to improve the efficiency of the TETS.
coil implanted under the patient’s skin. In order to achieve minimal invasion, the size of the secondary coil was made as small as possible. A limiting factor on the size of the secondary coil, however, is that the efficiency of the coil is heavily dependent on its size. In particular, the smaller the secondary coil, the lower the efficiency. Since the TETS must have high efficiency, this paper proposes a planar spiral structure for the secondary coil. In order to strengthen the magnetic flux coupled between the primary and the secondary coils, a flat circular ferrite core is designed to be the base of the secondary coil. The thickness of the ferrite was kept to only 1.0 mm, and the diameter was 25 mm. A tiny copper wire was wound spirally 904
Fig. 8 - Structure of external control device. PNP is a type of transistor that consists of a layer of N-doped semiconductors between two layers of P-doped material. The PIC16F690 and IA4420 are described in Fig. 4. S1 and S2 are two press buttons.
on the surface of the ferrite core by hand. The diameter of the copper wire used was 0.15 mm. Glue was used to fix the copper winding in the proper position on the surface of the ferrite core, and when the copper winding covered the full of the surface of the ferrite core, epoxy was applied across the surface of the copper winding to provide electrical insulation between the copper and body tissue once it is implanted into the body. The rectified circuit is integrated into the secondary coil, again to minimize the overall size of the implanted device. The primary coil was also designed to be planar spiral. A planar ferrite core is designed to strengthen the magnetic coupling between the primary coil and the secondary coil. As on the secondary coil, Litz cables were wound spirally on the surface of the primary ferrite core, and an FR-4 board is used to maintain the shape of the spiral Litz cables. The diameter of the primary coil is 48 mm and the height is 5 mm.
External control device The purpose of the external control device is to enable the patient to wirelessly control the implanted AAS. The external control device includes a wireless communication transceiver, a microprocessor, and human interfaces such as push buttons, a buzzer, and a liquid crystal character display (LCD) for displaying messages to the patient. The structure of the external control device is in Figure 8. The character LCD is used to quantitatively show the measured colon pressure value to patients. The buzzer is used to give an audio warning signal to the patient of elevated colon pressure, indicative of the need to allow defecation. In the circuit, the buzzer is driven by a PNP transistor which
© 2013 Wichtig Editore - ISSN 0391-3988
Wang et al
Fig. 9 - Actual AASS. Only the AAS is sterilized since it will be implanted into the body of the dogs. (a) Artificial anal sphincter; (b) TETS (including the primary coil and the secondary coil); (c) External control device.
Fig. 10 - In vivo experiment of the AASS. This photo shows that AAS was already implanted into the body of the dog and that the AASS performed well.
can provide a large current for the buzzer. Push buttons S1 and S2 are designed to allow the patient to issue the start or stop signals controlling the implanted AAS. The microprocessor (PIC16F690) and wireless communication transceiver (IA4420) chips used are the same as those shown in Figure 4. Figure 9 shows the actual AASS before it was used in the in vivo experiment. The implanted part of the AASS is sterilized. In order to meet the requirement of bio-compatibility, the surface of the micropump is treated with a thin layer of silicone. The cables connecting the secondary coil and the control device in the micropump is protected by small diameter silicone tube.
ingested nothing except water. They slept in their cages for most of the time, then some soft food with a large quantity of water was provided to the subjects. Figure 10 shows the surgical procedure implanting the AASS into the second subject. Before the surgeon sewed the incision of the dog closed, the basic functions of the AASS were tested. TETS was able to transmit electrical power to the implanted AAS. The pressure information in the cuff of the implanted AAS was sent out and displayed on the LCD of the external control device. The author was able to send a command to the implanted AAS to open the anus and send a command to the implanted AAS to close the anus. The test results proved that the performance of AASS met the expected functional requirements.
RESULTS
DISCUSSION
After the AASS was designed and assembled, the functions of every module were thoroughly tested by experiments; thereafter, in vivo experiments were conducted on two one-and-half-year old Beagle dogs. The first subject weighed 11.5 kg; the second weighed 11.6 kg. The subjects were fasted one day before the experiments. Only water was freely provided. During the implantation procedure, intravenous injection was provided to the subjects. In the first three days after the implantation, the subjects
Although the in vivo experiment verified the basic functions of the AASS, it revealed new challenges for its clinical application. In particular, the long-term biocompatibility between the AAS and the surrounding tissue needs to be further studied. The long-term reliability of the AASS should also be tested through more experiments. Lastly, while the TETS has been verified to be able to provide enough power to the implanted AAS, the heat resulting from the TETS should be considered.
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CONCLUSIONS This paper proposed a novel AAS system as a solution for severe fecal incontinence. The in vivo experiment verified the basic functions of the AASS. A main feature of the proposed system was the use of a pressure sensor to measure the colon pressure, which can be used to mimic biofeedback, thereby restoring patient’s access to information conveyed by natural rectal sensations. With such feedback, the patient can wirelessly control the AAS.
to thank Yi Gu, a doctoral student in the Radiology Department at Stanford University, for his help in preparing the manuscript. Financial Support: This research work was supported by the Nature Science Foundation of China under grants 30800235 and 31271069. This work was also funded by the Key Discipline Construction Project of Pudong Health Bureau of Shanghai (Grant No. PWZxk2010-01). Conflict of Interest: None.
The authors appreciate all the editors and reviewers for their valuable comments on the manuscript. The authors wish
Address for correspondence: Hua Liu Department of Instrument Science and Engineering School of Electronic Information and Electrical Engineering Shanghai JiaoTong University Shanghai, China 200240 [email protected]
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