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Integration of Low-power ASIC and MEMSSensors for Monitoring Gastrointestinal Tract

using a Wireless Capsule SystemMd Shamsul Arefin, Student Member, IEEE, Jean-Michel Redoute, Senior Member, IEEE,

and Mehmet Rasit Yuce, Senior Member, IEEE

Abstract—This paper presents a wireless capsule microsys-tem to detect and monitor pH, pressure, and temperatureof the gastrointestinal (GI) tract in real-time. This researchcontributes to the integration of sensors (microfabricatedcapacitive pH, capacitive pressure, and resistive temperaturesensors), frequency modulation and pulse-width modulationbased interface IC circuits, microcontroller, and transceiverwith meandered conformal antenna for the development ofa capsule system. The challenges associated with the systemminiaturization, higher sensitivity and resolution of sensors,and lower power consumption of interface circuits are ad-dressed. The layout, PCB design, and packaging of a miniatur-ized wireless capsule, having diameter of 13 mm and length of28 mm, have successfully been implemented. A data receiverand recorder system is also designed to receive physiologicaldata from the wireless capsule and to send it to a computer forreal-time display and recording. Experiments are performedin vitro using a stomach model and minced pork as tissuesimulating material. The real-time measurements also validatethe suitability of sensors, interface circuits, and meanderedantenna for wireless capsule applications.

Index Terms—Wireless Capsule, Microsystem, Applicationspecific integrated circuit (ASIC), Frequency modulation basedinterface circuit, Pulse-width modulation based interface cir-cuit, Capacitive pH sensor, meandered conformal antenna.

I. INTRODUCTION

A wireless capsule system is used for real-time andcontinuous measurement of physiological parameters in thegastrointestinal (GI) tract to replace the traditional endo-scope system [1]–[12]. The traditional endoscopy method,which involves inserting an endoscope to the GI tract, isuncomfortable and time-consuming for patients [13], [14].This method can not measure physiological parameters ofthe small intestine [10], [13]. A wireless capsule systemovercomes these limitations and provides more in-depthanalyses of physiological parameters and motility of the GItract [1], [3], [4], [10], [13]–[17]. A wireless capsule containsseveral sensors with interface integrated circuits (IC) todetect physiological parameters, a digital microcontroller tomanage and process all data from sensors, and a transceiversystem with antenna for wireless communication. The pH,

The authors are with the Department of Electrical and Computer SystemsEngineering, Monash University, Melbourne, VIC-3800, Australia.E-mail: [email protected]; [email protected];[email protected]

pressure, and temperature are the essential physiologicalparameters to detect dysfunctions of the GI tract [13].

Early attempts were to monitor motility of the GI tractas pressure change and to measure body core temperature[18]–[22]. An oscillator circuit is used as an interfacecircuit for pressure or temperature sensor to obtain frequencyvariations for changes in physiological parameters as wellas to transmit the data using low frequencies. Those sys-tems were bulky due to large electronic components andlarge sensor systems. A prototype named IDEAS, whichis 55 mm in length and 16 mm in diameter, is designedto measure pH, temperature, conductivity, and dissolvedoxygen concentration in the GI tract and to transmit data at38.242 MHz frequency [1], [2]. A wireless capsule systemnamed as lab-in-a-pill (LIAP) has a dimension of 36 mmin length and 12 mm in diameter [9]. It consists of pHand temperature sensors, interface circuits, controller circuit,and transmitter to measure and transmit pH and temperaturesignals of the GI tract. The data is transmitted using aloop antenna. For both IDEAS and LIAP systems, voltageamplifier based interface circuits are used to achieve lowerpower consumption. These circuits are affected by flickerand thermal noises and minimize the sensor readout rangeand resolution of these circuits. Commercial products fromGiven Imaging, Bravo pH system, and Smart Pill are alsoavailable [8], [23]. Recently, a wireless capsule system,having a dimension of 26 mm in length and 11 mm indiameter, can measure pH, pressure, and temperature of theGI tract [10]. Wheatstone bridge based interface circuits areutilized to convert sensing signals into measurable quantitiesof voltages. Though these circuits consumes lower power,they are affected by noises and provide lower sensitivityand resolution. The data is transmitted using a cylindricalhelix antenna which occupies large space inside the capsule.A flexible gastric battery or a wireless power link is alsoadopted to replace traditional bulky battery to reduce the sizeof the wireless capsule [24], [25]. Overall, capsule minia-turization, power consumption, sensitivity and resolution ofsensors, wireless communication, and miniaturized antennaare key challenges for the development of a wireless capsulesystem.

In this paper, a wireless capsule system with a datareceiver and recorder system is developed to measure pH,

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pressure, and temperature. The wireless capsule contains amicrofabricated pH, pressure, and temperature sensors todetect and convert physiological signals in the GI tract toelectrical quantities with interface IC circuits. The microfab-ricated capacitive pH sensor is employed to measure highacidic pH levels in the stomach. A frequency modulation(FM) and a pulse-width modulation (PWM) based interfaceIC circuits are designed and implemented to translate sensorchanges in the GI tract into electrical quantities for thepH sensor and pressure sensor, respectively. A miniaturizedmeandered conformal antenna with omnidirectional radiationpattern and center frequency of 433 MHz is also integratedfor the transmission of sensor data to an external receiversystem. This antenna is wrapped around the inner cylindricalsurface of the capsule to increase radiation efficiency and toprovide extra space for sensors and circuits inside the cap-sule. A flexible printed circuit board (PCB) is fabricated ona biocompatible polyimide material [26], [27] which allowsrolling the antenna as a cylindrical shape and to fold andfit remaining parts inside the capsule. The transmitted datafrom the wireless capsule is received for remote monitoringand recording with a data receiver system.

Most of the research works performed for the developmentof wireless capsule endoscopy are focused on system leveldesign, with less emphasis on each system components toaddress challenges. It is important to focus on the design ofsensors, interface circuits, and antenna, which are essentialcomponents for the implementation of wireless capsule, inorder to tackle the challenges. This work contributes tothe integration of the capacitive pH sensor, FM-based andPWM-based interface circuits, and the compact meanderedconformal antenna to design and implement a wirelesscapsule system platform that forms a foundation for theembodiment of a multisensor microsystem. Along with thesesystem components, system level design using those systemcomponents are also verified experimentally.

The components of the wireless capsule system is de-scribed in Section II. The layout of system components,printed circuit board (PCB) design, and packaging of thewireless capsule are described in Section III. The design of areceiver and data recorder system is presented in Section IV.The device is also experimented and evaluated in vitro tovalidate the design and implementation of the system. Theexperimental results are provided in Section V. Finally,Section VI presents a summary of this paper.

II. SYSTEM DESCRIPTION

The block diagram of the wireless capsule system isshown in Fig. 1. The system consists of two main sections.The first section is the wireless capsule, as presented inFig. 1(a). The wireless capsule contains pH, pressure, andtemperature sensors to detect and convert physiologicalsignals in the GI tract to electrical quantities with interfaceIC circuits. The capacitive pH sensor [28], [29] with thefrequency modulation (FM) based interface IC circuit [30]is used to translate capacitive changes due to pH changes into

Fig. 1. Block diagram of the wireless capsule system. (a) Wireless capsulehaving sensors, interface IC circuits, a microcontroller, and a wirelesstransceiver with meandered antenna. (b) An external data receiver andrecorder system consist of a microcontroller, a wireless transceiver withinverted-L antenna (ILA), a UART to USB bridge, and a computer.

voltage changes. A capacitive pressure sensor with the pulse-width modulation (PWM) based interface circuit [31] is usedto obtain pulse-width variation due to pressure changes. Aresistive temperature sensor with a voltage divider circuit isused to convert resistance shifts due to temperature shiftsinto voltage shifts. The wireless capsule is powered bytwo batteries with commercial voltage regulator circuits forsupply voltage of 1.8 V. The microcontroller processes allthe sensor data and controls interface circuits, regulators, anda wireless transceiver circuit. The microcontroller controls aregulator connected with interface circuit to reduce powerconsumption. Sensor data is processed with a microcon-troller and then transmitted using the wireless transceiverwith a meandered conformal antenna [32].

The external data receiver and recorder systems is shownin Fig. 1(b). This receiver system consists of a micro-controller, a wireless transceiver with inverted-L antenna(ILA), a universal asynchronous receiver/transmitter (UART)to universal serial bus (USB) bridge, and a computer. Thetransmitted data from the wireless capsule is received usingthe transceiver and transmitted using a UART to a computerfor remote monitoring and recording of data. A graphicaluser interface (GUI) developed using MATLAB software isused for visualization of physiological data in real-time.

A commercial microcontroller with transceiver circuitfrom Silicon Labs [33] is selected for the proposed wirelesscapsule system. The major blocks, which are utilized for thewireless capsule, are analogue-to-digital converter (ADC),programmable counter array (PCA), and wireless transceiver.The UART block of the microcontroller is used for thedata receiver and recorder system to send the received datafrom wireless capsule to a computer. The baud rate of theUART is 115.2 kbps. The microcontroller also includes a

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programmable precision internal oscillator as a system clockoperating at 24.5 MHz frequency. The ADC uses an internalprecision voltage reference of 1.68 V. A repeat count of 8,which increases the effective ADC resolution, is used toincrease accuracy for pH and temperature measurements.The PCA, which is an enhanced 16-bit counter or timertriggered by both positive and negative edges of a PWMsignal, is configured to capture values of the counter andtype of transitions of a signal. From captured counter valuesfor both transitions of a signal, the pulse-width of thePWM signal is calculated for pressure measurements. Thetransceiver, which is integrated with the microcontroller forwireless sensor data transmission, is configured for Gaussianfrequency shift keying (GFSK) modulation and operating at433.9 MHz of center frequency having 128 kbps data rateand 250 kHz channel spacing. This is sufficient for sensordata transmission in real-time. It is also possible to trans-mit multiple wireless capsule data using other transmissionchannels.

The power management system includes batteries, reg-ulators, and algorithm to supply and control power to allcircuits. The wireless capsule is powered by two silver oxidebatteries (RENATA 370), each having 9.5 mm in diameterand 2.05 mm in height, nominal voltage of 1.55 V, andcapacity of 40 mAh. Two regulators (Texas Instruments,TPS73118) are used to supply 1.8 V separately to theinterface circuit and the microcontroller. The microcontrolleris always powered by a regulator, while the regulator forthe interface circuit is controlled by the microcontroller toreduce the power consumption. The power consumption ofthe microcontroller is also optimized using algorithm tocontrol different blocks, such as the transceiver, ADC, PCA,and system clock.

A. Sensors

The pH, pressure, and temperature sensors are used todetect physiological signals inside the GI tract. The photo-graph of the sensors are depicted in Fig. 2. The dimensionof the pH sensor is 10 mm×10 mm. The dimension of thepackaged pressure sensor is 3 mm×3 mm. The diameter ofthe temperature sensor is 2.413 mm.

The digestive fluid in the stomach, also known as gastricacid, usually has a pH level between 1 and 2 [34]. Thecapacitive pH sensor, as shown in Fig. 2(a), is employedto measure strong acidic ranges of gastric acid [28], [29].The pH sensor consists of interdigitated electrodes fabricatedon a quartz substrate and passivated by silicon nitridelayer using conventional microfabrication steps. Instead ofmeasuring solid-solution interfacial potential, the changes ofbulk solution capacitance, corresponding to pH levels, aremeasured using the sensor.

The pressure in the GI tract is necessary for clinical orphysiological examinations. The normal pressure values inesophagus and stomach are slightly lower than atmosphericpressure of 101 kPa while the pressure in colon is slightlyhigher than atmospheric pressure [14], [35]. A capacitive

Fig. 2. (a) Photograph of the capacitive pH sensor. (b) Photograph ofthe capacitive pressure sensor. (c) Photograph of the resistive temperaturesensor.

Fig. 3. Block diagram of interface circuits of the proposed wireless capsule.

pressure sensor from Microfab [36], as shown in Fig. 2(b),is utilized for the wireless capsule.

The temperature of the GI tract is measured using adiscrete thermistor (192 series) from Honeywell [37], asshown in Fig. 2(c). The resistance of this thermistor is100 KΩ at 25 oC with negative thermal coefficient.

B. Interface Circuits

The block diagram of interface circuits for pH, pressure,and temperature sensors are shown in Fig. 3. There are threeseparate interface circuits for each sensor. For pH sensor, theFM-based interface IC circuit [30] is used to convert capac-itance changes from pH sensor into voltage changes. TheFM interface circuit provides higher resolution and sensingfor pH measurements [38]. The changes in capacitance dueto pH levels are converted into frequency variations usinga voltage-controlled oscillator (VCO) circuit. The controlvoltage (Vctrl) and inductance (L0) controls the nominalfrequency of the VCO. A sine-to-square converter circuitconverts frequency shifts from the VCO into time-periodvariations. The time-period changes are then converted tovoltage changes using a frequency-to-voltage (FVC) circuit.

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The FVC circuit consists of a logic controller circuit anda charge pump circuit. The logic controller circuit, thatproduces three control pulses (S1, S1, and S2) to controlthe charge pump circuit, translates the time-period variationinto pulse-width variations. An external delay control voltage(VBD), a feedback voltage (VFV C) from charge pump cir-cuit, and charging current (Ich) of the charge pump circuitprovide controls to obtain voltage changes (∆VpH ) at theoutput of the charge pump circuit.

The capacitive pressure sensor is connected with thePWM-based interface IC circuit to translate capacitancechanges from pressure variations into pulse-width or dutycycle changes variations of a square wave. The square waveclock from a ring oscillator drives both reference and sensingRC controlled pulse generators each containing a high-passfilter, a reference resistor (R), and a capacitor (Cref orCpressure). The pressure sensor (Cpressure) is connectedto the sensing RC circuit while a fixed capacitor (Cref ) isconnected to the reference RC circuit. The voltages acrossthe pressure sensor and the reference capacitor are comparedwith a set threshold voltage (α) using a self-tuning invertercomparator circuit. The PWM signal having pulse-widthdependent on the capacitance of the pressure sensor isobtained using an XOR gate. The circuit provides highersensitivity and linearity with lower power consumption.

The interface circuit for resistive temperature sensor isthe voltage divider circuit where the sensor is in series witha reference resistor to produce a varying output voltagewith respect to sensor resistance changes due to temperaturevariations.

The interface circuits are fabricated in the UMC 0.18 µmprocess and packaged in a standard 48-pin QFN-packagehaving dimensions of 7×7 mm2. As shown in Fig. 4, thereare two FM-based interface circuits and one PWM-basedinterface circuit. For the wireless capsule, one of the FM-based interface circuit is used for pH measurement and theother one is retained for future extension. The PWM-basedinterface circuit is used for pressure measurement. The activearea of FM-based and PWM-based interface circuits are0.18 mm2 and 0.17 mm2, respectively.

C. Antenna and Propagation

The meandered conformal antenna [32], as shown inFig. 5, is used for the wireless capsule to communicate withthe external receiver system. The antenna that is fabricatedon a flexible polyimide material having a length and widthof 21 mm and 36.2 mm, is rolled up or wrapped insidethe wireless capsule as a cylindrical shape having radius of5.76 mm.

For measurements, the capsule containing the antennais inserted in a plastic container filled with pork mince.The antenna operates at center frequency of 436 MHz with124.4 MHz bandwidth (18.5% fractional bandwidth) withoutoverlapping of segments, as shown in Fig. 6. For the firstcase of 25% overlapping (half of first and fourth segmentsare overlapped) while rolled up, as illustrated in Fig. 6(a),

Fig. 4. Die photograph for wireless capsule comprising both FM-basedand PWM-based interface IC circuit.

Fig. 5. (a) Schematic of the rolled meandered antenna having 4 identicalsegments. (b) Photograph of the fabricated antenna on Polyimide material.

the radius of rolled up antenna becomes 4.45 mm. For theother case, the radius of the rolled up antenna, as illustratedin Fig. 6(b), becomes 3.12 mm due to 50% overlapping.The measured results of return loss, as shown in Fig. 6(c),does not exhibit significant changes in resonance frequencydespite its lower gain. Therefore, the antenna is suitable forcapsules having smaller radius.

A low frequency transmission through skin layer is pre-ferred due to higher efficiency [39], [40]. A band-pass filtertype impedance matching circuit from Johanson Technologyis used which operates in the range of 424 to 444 MHZfrequencies [41]. For the external receiver system, the ILAantenna operates at 434 MHz center frequency with 5.1 MHzbandwidth (from 431.7 to 436.8 MHz) for voltage standingwave ratio (VSWR) less than 2. The distance betweenthe external receiver system and the meandered antennainside the wireless capsule traveling through the GI tract,as depicted in Fig. 7(a), varies significantly depending onbody sizes. A significant portion of transmitted power isabsorbed by the surrounding tissue. From Fig. 7(b), the in-body path loss, which is measured considering the stomachis full of lossy tissue medium, increases rapidly with thepropagation distance. The experimental results shows path

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Fig. 6. Schematic of overlapping of antenna while rolled up. (a) 25% over-lapping. (b) 50% overlapping. (c) Measurement results for the overlappingeffect on antenna return loss (S11) of the antenna.

Fig. 7. (a) Schematic of the GI tract having varying distance from theexternal receiver system and the meandered antenna inside the wirelesscapsule. (b) Measured in-body path loss variation with propagation distanceat 433 MHz frequency.

loss of 17.24 dB for in-body propagation of 140 mm. Thetransmit power of 13 dBm is used for the wireless capsulesystem. The maximum communication distance between thewireless capsule and the transmitter is 15 cm. The requiredreceiver sensitivity is -103 dBm. The antenna gain is roughlyuniform in the azimuth plane and is slightly nonuniformin the elevation plane. The maximum measured far fieldantenna gain at 433 MHz is -36.86 dBi.

III. WIRELESS CAPSULE DESIGN

The schematic of the wireless capsule is illustrated inFig. 8. The cylindrical shaped capsule is 28 mm in outerlength and 13 mm in outer diameter. Since, the thickness ofthe capsule body is 0.5 mm, the available inner length anddiameter of the capsule for sensors, circuits and antenna are

Fig. 8. Schematic of the proposed wireless capsule.

27 mm and 12 mm, respectively. The top side of the capsuleis partially opened for sensors to interact with the stomachfluids and other end of the capsule is closed. The PCB isfabricated on a flexible polyimide material to allow foldingand fitting components inside the capsule. The batteries areplaced in between sensors and interface circuit to utilize theempty space created due to the shape of the pH sensor. Thespace between sensors and batteries are sealed using DowCorning sealant to protect the circuits inside the capsule fromleaking of stomach fluids [42].

The flexible PCB contains sensors, circuits, and antenna,as presented in Fig. 9(a)-(b). The PCB is designed in twolayers, where the top layer (Fig. 9(a)) contains most ofthe circuits, sensors, and an antenna and the bottom layer(Fig. 9(b)) contains battery connectors, a reed switch, andregulator circuits with input and output capacitors for noisereduction. The structure of the PCB allows to roll the antennaas a cylindrical shape and to fold and fit the remaining partsinside the capsule. Three components of wireless capsulebody, as shown in Fig. 9(c), are fabricated using a 3D printer.The inner and outer diameter of the capsule is 12 mm and13 mm, respectively and the thickness is 0.5 mm. The firstcylindrical component holds the circuits and battery. Thesecond component is used to separate circuits and sensors.A part of the PCB containing sensors are routed through asmall opening in the middle of the component to positionthe sensors on the top side of capsule. The last componentis positioned and sealed on top of the capsule to protectsensors as well as to allow contact with stomach fluids. Theside view and top view of the packaged capsule is shown inFig. 9(d)-(e). The outer diameter and length of the capsuleis 13 mm and 28 mm, respectively. The partially opened topside of the capsule allows the stomach fluid to interact withsensors. The soldering parts of sensors are also sealed usingDow Corning sealant to avoid short-circuit due to stomachfluids [42].

The microcontroller is programmed to meet controlling,processing, and transmission requirements. The system flowchart for the wireless capsule is shown in Fig. 10. Allmodules used in the microcontroller are initialized whenpower is on. Then the sleep mode is enabled for a spe-cific time by activating a low frequency clock instead ofthe system clock to reduce the power consumption of the

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Fig. 9. Photograph of a wireless capsule system. (a) Top side of the flexiblePCB. (b) Bottom side of the flexible PCB. (c) Components of the wirelesscapsule body. (d) Side view of a packaged wireless capsule. (e) Top viewof a packaged wireless capsule.

microcontroller. The physiological data, which is a slowvarying signal, is collected in every 18 seconds. To measureand collect data, the interface circuit is enabled along withpH, pressure, and temperature settings. Using two separateinterrupt service routines for the ADC and PCA, voltagevalues for temperature and pH levels as well as pulse-width of pulses for pressure levels are obtained. Whendata from all sensors are available, interface circuits areturned off to limit the power consumption of circuits andthe transceiver is turned on for data transmission. After thedata is transmitted, the transceiver operates as a receivermode to acquire acknowledgment data from the external datareceiver system. If the acknowledgment data is not receivedwithin a predefined time, the data is transmitted again.If the acknowledgment signal is received, the transceiveris disabled to decrease the power consumption. Then themicrocontroller enters into the sleep mode for 18 seconds.

IV. RECEIVER DESIGN

The photograph of the receiver system is shown in Fig. 11.The system contains a microcontroller [33] and UART toUSB bridge [43] from Silicon Labs, impedance matchingcircuit from Johanson Technology [41], and an ILA antenna.

Fig. 10. System algorithm for the wireless capsule.

The ILA antenna operates at 434 MHZ with 5.1 MHzbandwidth.

The microcontroller for data receiver system is pro-grammed to communicate with the wireless capsule anda computer. The system flow chart for the receiver sys-tem is shown in Fig. 12. The required blocks, such astransceiver and UART are initialized at the beginning. Thenthe transceiver is turned on as a receiver mode for acquiringdata from the wireless capsule. If data is received, thevalidity for the data packet configuration is checked toensure that data is from the wireless capsule. If valid datais obtained, the transceiver operates as a transmitter tosend acknowledgment data to the wireless capsule. Thenthe UART is enabled to transmit the data to a computerusing a UART-to-USB bridge. After the transmission ofacknowledgment data, the transceiver changes to receivermode for acquiring next data from the wireless capsule. AMATLAB based GUI is developed to communicate with thereceiver system and to display and record the data fromwireless capsule in real-time.

V. EXPERIMENTAL RESULTS

The packaged wireless capsule prototype is experimen-tally evaluated through in vitro measurements of pH, pres-

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Fig. 11. Photograph of the data receiver system.

Fig. 12. System flow chart for the data receiver system.

sure, and temperature of pH buffer solutions. Sensor re-sponses are experimentally obtained using the wireless cap-sule and then real-time measurements are performed tovalidate the feasibility of the system for wireless capsuleapplications.

A. Experimental Setup and Procedures

The performance of the wireless capsule is experimentedand evaluated in vitro using a setup, as shown in Fig. 13(a),mimicking the stomach environment1. Figure 13(b) shows astomach model which is fabricated using a 3D printer andplaced inside a cylindrical shaped container. Minced pork isused as a tissue simulating material in between the stomachmodel and the container, as the electrical property of mincedpork and stomach tissue at 433 MHz frequency is similar[44]–[46]. During the experiments, the stomach model iscompletely immersed in the minced pork. The wirelesscapsule is inserted in the stomach model for measurements.

1Due to ethics difficulty, we experimented in vitro.

Fig. 13. (a) Photograph of the experimental setup. During experiments,the container is filled with minced pork to immerse the stomach model. (b)Photograph of the stomach model.

The pH solutions, made from standard buffer solutions bymixing deionized water, is used for pH measurements, andthe temperature is applied using a hot plate. The pressuremeasurement is controlled and verified using a syringepressure system with a precision positioner and a pressuregauge [31]. The pH and temperature levels of the solutionis verified by a commercial pH glass-electrode (HQ40d,HACH) sensor. The temperature of the solution is alsomeasured using a mercury based thermometer. The receivercircuit, containing the ILA antenna, is placed outside thecontainer.

For the experiments, a set of circuit parameters for in-terface circuits shown in Fig. 3 are selected to convertsensor signals to measurable electrical quantities. For thepH sensor interface circuit, the LC-tank of the VCO circuitis set by L0 of 15 nH and Vctrl of 1.0 V to convert thecapacitance changes to frequency shifts. The Ich of 41.7 Aand VBD of 1.3 V is used to convert the frequency shiftsto voltage change. The Vctrl of 1.0 V and VBD of 1.3 Vis generated using a voltage divider circuit from a 1.8 Vsupply. To produce Ich of 41.7 A, Rch of 10 kΩ is used.For temperature sensor interface circuit, a voltage dividercircuit having 50 kΩ of reference resistor is used. For thepressure sensor interface circuit, ring oscillator of the PWM-based interface circuit is operated at 12.5 kHz frequency, theRC circuit is set by R of 4 MΩ and Cref of 4.7 pF, and thecomparator threshold is set by α of 0.9.

B. Sensor Responses

The capacitance changes due to pH levels are converted tofrequency changes and then into voltage changes with FM-based interface IC circuit. The ADC of the microcontrollerconverts the voltage levels into digital codes. The ADCoutput codes for pH levels ranging from 1.0 to 4.03 isshown in Fig. 14. The changes of ADC output codes are

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Fig. 14. ADC output codes for pH values ranging from 1.0 to 4.03.

1221 for pH 1.0 to 2.0, 701 for pH 2.0 to 3.0, and 186 forpH 3.0 to 4.0. There is a slight difference in ADC outputcodes between room temperature and body temperature. Thedifference is much smaller in high acidic levels than theoutput code variations due to pH change. Since the output ofthe ADC includes a repeat count of 8, the average resolutionsof the pH sensor are 0.0066 for pH 1.0 to 2.0, 0.0114 forpH 2.0 to 3.0, and 0.0430 for pH 3.0 to 4.0.

For pressure sensor, the capacitance changes due to ap-plied pressure is converted into pulse-width variations usingthe PWM-based interface circuit. The pulse-width of thePWM signal is calculated using PCA of the microcontrollerby capturing counter values for both positive and negativeedges of the PWM signal. The PCA output codes forpressure values ranging from 50 kPa to 200 kPa is shownin Fig. 15. The changes of PCA output codes are 143 for101 kPa to 200 kPa, 18 for 80 kPa to 101 kPa and 12 for50 kPa to 80 kPa. The average resolutions of the pressuresensor are 0.69 kPa for 101 kPa to 200 kPa, 1.17 kPa for80 kPa to 101 kPa and 2.5 kPa for 50 kPa to 80 kPa.

The resistance changes due to temperature levels areconverted into voltage changes using a voltage divider circuitwhich is translated into ADC output codes using the ADC ofthe microcontroller. The ADC output codes for temperaturevalues ranging from 25 oC to 50 oC are shown in Fig. 16.The response from the temperature sensor exhibits linearcharacteristics for temperature values ranging from 25 oCto 50 oC. The difference of ADC output code for 25 oCand 50 oC is 2302, which provides the average resolution of0.087 oC for the temperature sensor.

C. Real-time Measurements

For real-time measurements, the wireless capsule mea-sures pH, pressure, and temperature values and transmits tothe receiver system which is connected to a computer. AMATLAB based GUI is developed to communicate with thereceiver system and to record and display the data from thewireless capsule in real-time. The acquired data is ADC andPCA output codes which are converted into pH, pressure,and temperature values using a third order fitted regressioncurve obtained from responses of sensors (Fig. 14, 15, and16). A snapshot of the GUI for real-time display and record

Fig. 15. PCA output codes for pressure values ranging from 50 kPa to200 kPa.

Fig. 16. ADC output codes for temperature values ranging from 25 oC to50 oC.

of received data from the wireless capsule is shown inFig. 17.

The ADC and PCA output codes of measurements forpH, temperature, and pressure sensors, which are acquiredin 18 seconds intervals for 3000 seconds, are shown inFig. 18. In the first part, pH and temperature are varied andrecorded for 1500 seconds. At the same time, the pH andtemperature of the buffer solution are also verified usingcommercial pH and temperature meters. The pH values of0.98, 1.12, 2.49, 3.23, and 3.78 are used by mixing deionized(DI) water with standard buffer solutions. The ADC outputcode due to pH changes is shown in Fig. 18(a). Sinceit takes some time to completely stabilize the pH of amedium, the ADC output response contains transient spikesat the same time of mixing DI water. The temperature ischanged linearly in between 36 oC to 42 oC, as shown inFig. 18(b). In the second part, the syringe pressure systemwith a pressure gauge is placed to verify the pressure and theopening of the stomach model is sealed to induce slightlyhigher and lower than atmospheric pressure of 101 kPainside the stomach model. The PCA output code due topressure change is shown in Fig. 18(c). Though the pressureis not varied from 0 to 1500 seconds, transient spikesare observed due to induced force from mixing DI waterduring pH measurements. The pressure values of 105, 110,120, 95, 90, and 80 kPa are induced in this experimentfor 1500 seconds to 3000 seconds. Sensor responses andreal-time measurement results validate the potential of the

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Fig. 17. A snapshot of the graphical user interface (GUI), developed using MATLAB, for real-time display and record of received data from the wirelesscapsule.

wireless capsule system for real-time monitoring and displayof physiological parameters of GI tract.

The FM-based and PWM-based interface circuits consume11.772 mW and 98 µW, respectively, from a 1.8 V powersupply. The power consumption of the microcontroller is1.08 µW in sleep mode and 7.38 mW in active mode froma 1.8V supply. The power consumption of the transceiver is32.4 mW in transmitter mode and 18 mW in receiver mode.The microcontroller stays in active mode for 1.3 millisec-onds and in sleep mode for 18 seconds. The measurementtime for the FM-based and PWM-based interface circuitsare 40 and 80 microseconds, respectively. Since the powerconsumption of the transmitter is much higher, all sensordata is transmitted at the same time. The average powerconsumption is 207 µW including the power dissipationfrom the voltage divider circuit for temperature sensor andtwo regulators. The wireless capsule can function for 200hours with two silver oxide button batteries.

Table I shows a comparison with the performance pa-rameters of the wireless capsule system with previous cap-sule systems in the literature. The implemented capsulesystem integrates sensors with low power, low noise, andhigher resolution interface IC circuits. Although the powerconsumption of FM-based interface circuit is higher thanamplifier-based and Wheatstone bridge-based circuit, thelower measurement time effectively reduces the overallpower consumption. A microfabricated capacitive pH sensoris utilized to measure high acidic pH levels which provideshigher sensitivity, low noise, fast response, and consistentresults. A meandered conformal antenna, which uses thewhole cylindrical capsule surface, is integrated to achievebetter radiation pattern and improved performance as wellas to provide extra space for sensor and circuits. Thedimensions of the system, which is comparable with [10],is smaller than other systems in the literature.

Fig. 18. Real-time measurement results of pH, temperature, and pressuresensors in 18 seconds intervals for 3000 seconds. (a) pH measurements. (b)Temperature measurements. (c) Pressure measurements.

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TABLE IPERFORMANCE COMPARISON OF WIRELESS CAPSULES USED FOR PHYSIOLOGICAL SIGNAL MONITORING.

Capsule CapsuleSensors

Range Interface Wireless Antenna Power Ref.Name Dimensions (Resolution) Circuit Transmitter Type Source (Year)

(H×D) Technique (Modulation)

Gutnic 28×9 mmpH -

- -Loop Gold-Iron [18]

Pressure - antenna electrode (1957)Temperature - battery

ITMS 22.6×10.7mm

Temperature (Crystal) 32-44 oC (-) Oscillator 262 kHz(FM)

- Nickel-Cadmiumbattery

[21](1988)

RigelResearchLtd.

8.8×6mm

Pressure (Inductive) 0-40 kPa (-) Oscillator 250-570 kHz(FM)

Ferriteaerial

Mercurybattery

[20](1997)

55×16 mm

pH (ISFET) 4-10 (0.64)

-

SilverIDEAS Temperature (Si Diode) 0-70 oC (0.4 oC) Voltage 38.342 MHz oxide [1], [2](Prototype) Conductivity (Dual Elec.) 0.05-10 mS/cm (0.02

mS/cm)amplifier (FSK) battery (2002)

Dissolved O2 (Three Cell) 0.0-8.2 mg/L (0.08mg/L)

- 27×19mm

Pressure (Resistive) 0-60 kPa (-) Wheatstonebridge

433.92 MHz(ASK)

Cylindricalhelixantenna

Lithiumbattery

[16](2004)

- - pH (ISFET) 1-7 (-) Voltageamplifier

433 MHz (-) - Silveroxidebattery

[11](2005)

26×13 mmpH -

- - -Lithium [8]

Smart PillTM Pressure - battery (2006)(Commercial) Temperature -

LIAP 36×12 mm pH (ISFET) 1-10 (0.1) Voltage 433.92 MHz Loop Silver [9]Temperature (Si Diode) 10-50 oC (0.25 oC) amplifier (OOK) antenna oxide

battery(2006)

- 26×11 mmpH (ISFET) 0-14 (0.01) Wheat- 433 MHz Cylindrical Silver [10]Pressure (Resistive) 95-120 kPa (0.01 kPa) stone (FSK) helix oxide (2014)Temperature (Thermistor) 35-42 oC (0.01 oC) bridge antenna Battery

- 22×11 mmpH (ISFET) 0-10 (0.5) Wheat- 433.92 MHz

-Silver [12]

Pressure (Resistive) 70-150 kPa (0.5 kPa) stone (FSK) oxide (2015)Temperature (Thermistor) 34-42 oC (0.2 oC) bridge Battery

This Work 28×13 mmpH (Capacitive) 1-4 (0.0066) FM-FVC 433 MHz Meandered Silver

(2016)Pressure (Capacitive) 50-200 kPa (0.69 kPa) PWM (GFSK) conformal oxideTemperature (Thermistor) 25-50 oC (0.087 oC) Volt. di-

viderantenna battery

VI. CONCLUSIONS

A complete wireless capsule system is designed and im-plemented to detect and monitor pH, pressure, and tempera-ture of the GI tract in real-time. A microfabricated pH sensoris employed to measure high acidic pH levels ranging frompH 1.0 to 4.0. The pressure values from 50 kPa to 200 kPaare measured using the pressure sensor. The temperaturevalues ranging from 25 oC to 50 oC are also measured.The FM-based and PWM-based sensor interface IC circuitsare adopted to provide a low noise, low power, and highresolution alternative for interface circuit. The processed data

from the microcontroller is transmitted using the wirelesstransceiver with a meandered conformal antenna. The in-tegration of sensors, interface IC circuits, microcontroller,and transceiver circuit with meandered conformal antennaalso form a basis for future multisensor microsystems.Components of the wireless capsule are packaged and sealedinside a cylindrical shaped capsule having length of 28 mmand diameter of 13 mm. The packaged wireless capsule isexperimented in vitro and validated for real-time monitoringand display of physiological parameters of the GI tract.Future works will be emphasized on further miniaturization

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of both sensors and circuits, sensor diversity, including otherfunctionalities, adopting energy harvesting techniques, andvalidating the system through human experimentations andclinical trials.

ACKNOWLEDGMENT

Authors acknowledge support received from Monash Uni-versity Postgraduate Publication Award.

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Md Shamsul Arefin (S’06) received the B.Sc.degree in Electrical and Electronics Engineeringfrom Bangladesh University of Engineering andTechnology (BUET), Dhaka, Bangladesh in 2007,and the M.S. degree in Electrical Engineeringfrom Northern Arizona University (NAU), Ari-zona, USA in 2010. He has recently receivedhis Ph.D. degree in Electronic Engineering fromMonash University, Melbourne, Australia. His re-search interests include low power MEMS sensorinterface circuits, mixed signal integrated circuit,

microtransducers, bio-sensors, energy harvesting, embedded systems, andmicrofluidics for biomedical applications.

Jean-Michel Redoute (M’09-SM’12) receivedthe degree of M.S. in electronics at the UniversityCollege of Antwerp, in 1998, and the degree ofM.Eng. in electrical engineering at the Universityof Brussels (VUB), in 2001. In August 2001,he started working at Alcatel Bell in Antwerp,Belgium, where he was involved in the designof analog microelectronic circuits for telecom-munications systems. Between January 2005 andAugust 2009, he worked at the ESAT-MICASlaboratories of the University of Leuven as a

Ph. D. research assistant. In September 2009, he joined the BerkeleyWireless Research Center at the University of California, at Berkeley asa postdoctoral scholar. In September 2010, he started working at MonashUniversity as a Senior Lecturer. His research interests include robust mixed-signal integrated circuit design with a high immunity to electromagneticinterference, electromagnetic compatibility and exposure, biomedical circuitdesign and radio frequency integrated circuit design.

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Mehmet Rasit Yuce (S’01-M’05-SM’10) is anassociate professor in the Department of Electri-cal and Computer Systems Engineering, MonashUniversity, Australia. He received the M.S. de-gree in Electrical and Computer Engineering fromthe University of Florida, Gainesville, Florida in2001, and the Ph.D. degree in Electrical andComputer Engineering from North Carolina StateUniversity (NCSU), Raleigh, NC in December2004. He was a post-doctoral researcher in theElectrical Engineering Department at the Univer-

sity of California at Santa Cruz in 2005. He was an academic member inthe School of Electrical Engineering and Computer Science, University ofNewcastle, New South Wales, Australia until Jul 2011. In July 2011, hejoined Monash University.

His research interests include wearable devices, Internet-of-Things (IoT)for healthcare, wireless implantable telemetry, wireless body area network(WBAN), bio-sensors, integrated circuit technology dealing with digital,analog and radio frequency circuit designs for wireless, biomedical, andRF applications. Dr. Yuce has published more than 150 technical articles inthe above areas and received a NASA group achievement award in 2007 fordeveloping an SOI transceiver. He received a best journal paper award in2014 from the IEEE Microwave Theory and Techniques Society (MTTS).He received a research excellence award in the Faculty of Engineeringand Built Environment, University of Newcastle in 2010. He is an authorof the books: Wireless Body Area Networks published in 2011 andUltra-Wideband and 60 GHz Communications for Biomedical Applicationspublished in 2013. He is a senior member of IEEE. He is a topical editorfor IEEE Sensors Journal and a guest editor for IEEE Journal of Biomedicaland Health Informatics in 2015.

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