Effects of contaminants on oxygen sensors
INTRODUCTIONWe are familiar with?a wide range of sensors in the field of?electronics. They are used widely in the various experiments and research activities too. This microelectronic pill is such a sensor with a number of channels and is called as a multichannel sensor. As the name implies this sensor is a pill. That is it is meant to go inside the body and to study the internal conditions.Earlier it was when transistor was invented, that radiometry capsules were first put into use. These capsules made use of simple circuits for?studying the gastrointestinal tract. Some of the reasons that prevented their use was their size and their limitation of?not to transmit through more than a single channel. They had poor reliability and sensitivity. The lifespan of the sensors were also too short. This paved the way for the implementation of single channel telemetry capsules and they were later developed to overcome the demerits of the large size of laboratory type sensors. The semiconductor technologies also helped in the formation and thus finally the presently seen microelectronic pill was developed. These pills are now used for taking remote biomedical measurements in researches and diagnosis. The sensors make use of the micro technology to serve the purpose. The main intention of using the pill is to perform an internal study and recognize or detect the abnormalities and the diseases in the gastrointestinal tract. In this GI (Gastro Intestinal) tract we cannot use the old endoscope as the access is restricted. A number of parameters can be possibly measured by these pills and they include conductivity, pH temperature and the amount of dissolved oxygen in the gastrointestinal tract.Figure 1: Microelectronic PillBLOCK DIAGRAMThe design of the microelectronic pill?is in the form of a capsule. The encasing it has is biocompatible. Inside this are multi- channel (four channels) sensors and a control chip. It also comprises of?a radio transmitter and two?silver oxide cells. The four sensors are mounted on the two silicon chips. In addition to it, there are a control chip, one access channel and a?radio transmitter. The four sensors commonly used are a temperature sensor, pH ISFET sensor, a dual electrode conductivity sensor and a three electrode electrochemical oxygen sensor. Among these the temperature sensor, the pH ISFET sensor and the dual electrode conductivity sensor are fabricated on the first chip. The three electrode electrochemical cell oxygen sensor will be on chip 2. The second chip also consists of a NiCr resistance thermometer which is optional.Figure 2: Block diagramMicroelectronic pill consists of 4 sensors (2) which are mounted on two silicon chips (Chip 1 & 2), a control chip (5), a radio transmitter (STD- type1-7, type2-crystal type-10), silver oxide batteries (8), 1-access channel, 3-capsule, 4- rubber ring, 6-PCB chip carrier.The microelectronic pill consists of a machined biocompatible (non-cytotoxic), chemically resistant polyether-terketone (PEEK) capsule and a PCB chip carrier acting as a common platform for attachment of sensors, ASIC, transmitter & batteries. The fabricated sensors were each attached by wire bonding to a custom made chip carrier made from a 10pin, 0.5pitch polymide ribbon connector. The connector in turn was connected to an industrial STD, flat cable plug (FCP) socket attached to the PCB carrier chip of the microelectronic pill, to facilitate the rapid replacement off the sensors when required. The PCB chip carrier was made from 2 STD. 1.6 mm-thick fiber glass boards attached back to back epoxy resin which maximized the distance between the 2 sensor chips. The sensor chips are connected to both sides of the PCB by separate FCP sockets, with sensor chip 1 facing the top face, with the sensor chip 2 facing down. Thus, the oxygen sensor on chip 2 had to be connected to the top face by three 200nm copper leads soldered onto the board. The transmitter was integrated in the PCB which also incorporated the power supply rails, the connection points to the sensors, as well as the transmitter & the ASIC & the supporting slots for the capsule in which the carrier is located.The ASIC was attached with double-sided Cu conducting tape prior to wire bonding to the power supply rails, the sensor inputs & the transmitter (a process which entailed the connection of 64 bonding pads). The unit was powered by 2 STD. 1.55V SR44 Silver oxide (Ag2O) cells with a capacity of 175mAh. The batteries were connected & attached to a custom made 3-pin, 1.27mm pitch plug by electrical epoxy. The connection on the matching socket on the PCB carrier provided a three point power supply to the circuit comprising a negative supply rail (1.55V). The capsule was machined as two separate screw-fitting compartments. The PCB chip carrier was attached to the front section of the capsule (fig 2). The sensor chips were exposed to the ambient environment through access ports & were sealed by 2 stainless steel clamps incorporating a 0.8 ?m thick sheet of Viton fluoroelastometer seal. A 3mm diameter access channel in center of each of the steel clamps (incl. the seal), exposed in sensing regions of the chips. The rear section of the capsule is attached to the front section by a 13mm screw connection incorporating a Viton rubber O-ring. The seals rendered the capsule water proof, as well as making it easy to maintain (e.g. during sensor & battery replacement). The complete prototype was 16*55mm & weighs 13.5g including the batteries.BASIC COMPONENTSSensorsFigure 3: SensorsThere are basically 4 sensors mounted on two chips- Chip 1 & chip 2. On chip 1 (shown in fig 2 a), c), e)), temperature sensor silicon diode (4), pH ISFET sensor (1) and dual electrode conductivity sensor (3) are fabricated. Chip 2 comprises of three electrode electrochemical cell oxygen sensor (2) and optional Ni Cr resistance thermometer (5).Sensor chip 1An array consisting of both temperature sensor & pH sensor platforms were cut from the wafer and attached onto 100-?m- thick glass cover slip cured on a?hot plate. The plate acts as a temporary carrier to assist handling of the device during level 1 of?lithography when the electric connection tracks, electrode bonding pads are defined. Bonding pads provide electrical contact to the external electronic circuit.Lithography was the first fundamentally new printing technology since the invention of relief printing in the fifteenth century. It is a mechanical Plano graphic process in which the printing and non-printing areas of the plate are all at the same level, as opposed to intaglio and relief processes in which the design is cut into the printing block. Lithography is based on the chemical repellence of oil and water. Designs are drawn or painted with greasy ink or crayons on specially prepared limestone. The stone is moistened with water, which the stone accepts in areas not covered by the crayon. Oily ink, applied with a roller, adheres only to the drawing and is repelled by the wet parts of the stone. Pressing paper against the inked drawing then makes the print.Lithography was invented by Alois Senefelder in Germany in 1798 and, within twenty years, appeared in England and the?United States. Almost immediately, attempts were made to print pictures in color. Multiple stones were used; one for each color, and the print went through the press as many times as there were stones. The problem for the printers was keeping the image in register, making sure that the print would be lined up exactly each time it went through the press so that each color would be in the correct position and the overlaying colors would merge correctly.Early colored lithographs used one or two colors to tint the entire plate and create a water color-like tone to the image. This atmospheric effect was primarily used for landscape or topographical illustrations. For more detailed coloration, artists continued to rely on hand coloring over the lithograph. Once tinted lithographs were well established, it was only a small step to extend the range of color by the use of multiple tint blocks printed in succession. Generally, these early chromolithographs were simple prints with flat areas of color, printed side-by-side.Increasingly ornate designs and dozens of bright, often gaudy, colors characterized chromolithography in the second half of the nineteenth century. Overprinting and the use of silver and gold inks widened the range of color and design. Still a relatively expensive process, chromolithography was used for large-scale folio works and illuminated gift books that often attempted to?reproduce the handwork of manuscripts of?the Middle Ages. The steam-driven printing press and the wider availability of?inexpensive paper stock lowered production costs and made chromolithography more affordable. By the 1880s, the process was widely used for magazines and advertising. At the same time, however, photographic processes were being developed that would replace lithography by the beginning of the?twentieth century.Chip 1 is divided into two- LHS unit having the temperature sensor silicon diode, while RHS unit comprises the pH ISFET sensor.DT-670-SD Silicon Diode FeaturesFigure 4: DT-670-SDIt measures the body core temperature.Also compensates with the temperature induced signal changes in other sensors.It also identifies local changes associated with tissue inflammation & ulcers. ISFETFigure 5: ISFETIon Selective Field Effect Transistor ISFET; this type of electrode contains a transistor coated with a chemically sensitive material to measure pH in solution and moist surfaces. As the potential at the chemically active surface changes with the pH, the current induced through the transistor varies. A temperature diode simultaneously monitors the temperature at the sensing surface. The pH meter to a temperature compensated pH reading correlates the change in current and temperature.This device has an affinity for hydrogen ions, which is the basis for the determination of the pH. The surface of the sensitive area of the sensor contains hydroxyl groups that are bound to an oxide layer. At low pH values hydrogen ions in the sample will bind to these hydroxyl groups resulting in a positively charged surface. In alkaline environments hydrogen ions are abstracted from the hydroxyl groups, leading to a negatively charged surface.Thus, each pH change has a certain influence on the surface charge. On its turn, this attracts or repulses the electrons flowing between two electrodes in the semiconductor device. The electronics compensates the voltage in order to keep the current between the two electrodes at its set point. In this way this potential change is related to the pH. Attachment of a polymer membrane on the ISFET introduces the possibility to go beyond the measurement of pH toward other ions. In this plastic layer certain chemicals (ionophores), which can recognize and bind the desired ion, are put in. Now, complex formations of the ionophore and the ion introduce a charge. The potential change is a measure for the ion concentration. Typically, these sensors can be used in a concentration range between app. 10.5 up to 1 mol/l.Sensor chip 2The Level 1 pattern (electric tracks, bonding pads, and electrodes) was defined in 0.9?m UV3 resist by electron beam lithography. A layer of 200nm gold (including an adhesion layer of 15nm titanium and 15nm palladium) was deposited by thermal evaporation. The fabrication process was repeated.Oxygen sensor detection principle Most portable or survey instruments used for workplace evaluation of oxygen concentrations make use of "fuel cell" type oxygen sensors. "Fuel cell" oxygen sensors consist of a diffusion barrier, a sensing electrode (cathode) made of a noble metal such as gold or platinum, and a working electrode made of a base metal such as lead or zinc immersed in a basic electrolyte (such as a solution of potassium hydroxide). Oxygen diffusing into the sensor is reduced to hydroxyl ions at the cathode: O2 + 2H2O + 4e- OH – Hydroxyl ions in turn oxidize the lead (or zinc) anode: 2Pb + 4OH 2PbO + 2H2O + 4e – This yields an overall cell reaction of: 2Pb + O2 2PbO Fuel cell oxygen sensors are current generators. The amount of current generated is proportional to the amount of oxygen consumed (Faraday's Law). Oxygen reading instruments simply monitor the current output of the sensor. An important consideration is that fuel cell oxygen sensors are used up over time. In the cell reaction above, when all available surface area of the lead (Pb) anode has been converted to lead oxide (PbO), electrochemical activity ceases, current output falls to zero, and the sensor must be rebuilt or replaced. Fuel cell sensors are designed to last no more than one to two years. Even when installed in an instrument which is never turned on, oxygen sensors which are exposed to atmosphere which contains oxygen are generating current, and being used up. Oxygen sensors are also influenced by the temperature of the atmosphere they are being used to measure. The warmer the atmosphere, the faster the electrochemical reaction. For this reason oxygen sensors usually include a temperature compensating load resistor to hold current output steady in the case of fluctuating temperature. (Microprocessor based instrument designs usually provide additional signal correction in software to further improve accuracy.) Another limiting factor is cold. The freezing temperature of electrolyte mixtures commonly used in oxygen sensors tends to be about 5oF (-20oC). Once the electrolyte has frozen solid, electrical output falls to zero, and readings may no longer be obtained. There are two basic variations on the fuel cell oxygen sensor design. These variations have to do with the mechanism by which oxygen is allowed to diffuse into the sensor. Dalton's Law states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the various gases. The partial pressure for oxygen is the fraction of the total pressure due to oxygen. Partial atmospheric pressure oxygen sensors rely on the partial pressure (or pO 2) of oxygen to drive molecules through the diffusion barrier into the sensor. As long as the pO 2 remains constant, current output may be used to indicate oxygen concentration. On the other hand, shifts in barometric pressure, altitude, or other conditions which have an effect on atmospheric pressure will have a strong effect on pO 2 sensor readings. To illustrate the effects of pressure on pO 2 sensors, consider a sensor located at sea level where atmospheric pressure equals 14.7 PSI (pounds per square inch). Now consider that same sensor at an elevation of 10,000 feet. Although at both elevations the air contains 20.9 percent oxygen, at 10,000 feet the atmospheric pressure is only 10.2 PSI! Since there is less force driving oxygen molecules through the diffusion barrier into the sensor, the current output is significantly lower. "Capillary pore" oxygen sensor designs include a narrow diameter tube through which oxygen diffuses into the sensor. Oxygen is drawn into the sensor by capillary action in much the same way that water or fluid is drawn up into the fibers of a paper towel. While capillary pore sensors are not influenced by changes in pressure, care must be taken that the sensor design includes a moisture barrier in order to prevent the pore from being plugged with water or other fluids. Figure 6: Capillary pore type oxygen sensorEffects of contaminants on oxygen sensorsOxygen sensors may be affected by prolonged exposure to "acid" gases such as carbon dioxide. Most oxygen sensors are not recommended for continuous use in atmospheres which contain more than 25% CO 2. Substance-specific electrochemical sensorsOne of the most useful detection techniques for toxic contaminants is the use of substance-specific electrochemical sensors installed in compact, field portable survey instruments. Substance-specific electrochemical sensors consist of a diffusion barrier which is porous to gas but nonporous to liquid, reservoir of acid electrolyte (usually sulphuric or phosphoric acid), sensing electrode, counter electrode, and (in three electrode designs) a third reference electrode. Gas diffusing into the sensor reacts at the surface of the sensing electrode. The sensing electrode is made to catalyze a specific reaction. Dependent on the sensor and the gas being measured, gas diffusing into the sensor is either oxidized or reduced at the surface of the sensing electrode. This reaction causes the potential of the sensing electrode to rise or fall with respect to the counter electrode. The current generated is proportional to the amount of reactant gas present. This two electrode detection principle presupposes that the potential of the counter electrode remains constant. In reality, the surface reactions at each electrode causes them to polarize, and significantly limits the concentrations of reactant gas they can be used to measure. In three electrode designs it is the difference between the sensing and reference electrode which is what is actually measured. Since the reference electrode is shielded from any reaction, it maintains a constant potential which provides a true point of comparison. With this arrangement the change in potential of the sensing electrode is due solely to the concentration of the reactant gas. Figure 7: Three electrode electrochemical sensorThe oxidation of carbon monoxide in an electrochemical sensor provides a good example of the detection mechanism.Carbon monoxide is oxidized at the sensing electrode: CO + H2O CO2 + 2H+ + 2e – The counter electrode acts to balance out the reaction at the sensing electrode by reducing oxygen present in the air to water: 1/2 O2 + 2H+ + 2e- H 2O Similar reactions allow for the electrochemical detection of a variety of reactant gases including hydrogen sulphide, sulphur dioxide, chlorine, hydrogen cyanide, nitrogen dioxide, hydrogen, ethylene oxide, phosphine and ozone. A bias voltage is sometimes applied to the counter electrode to help drive the detection reaction for a specific contaminant. Biased sensor designs allow for the detection of a number of less electrochemically active gases such as hydrogen chloride and nitric oxide. Several other contaminants (such as ammonia) are detectable by means of other less straight forward detection reactions. Electrochemical sensors are stable, long lasting, require very little power and are capable of resolution (depending on the sensor and contaminant being measured) in many cases to 0.1ppm. The chief limitation of electrochemical sensors is the effects of interfering contaminants on toxic gas readings. Most substance-specific electrochemical sensors have been carefully designed to minimize the effects of common interfering gases. Substance-specific sensors are designed to respond only to the gases they are supposed to measure. The higher the specificity of the sensor the less likely the sensor will be affected by exposure to other gases which may be incidentally present. For instance, a substance-specific carbon monoxide sensor is deliberately designed not to respond to other gases which may be present at the same time, such as hydrogen sulphide or methane. Even though care has been taken to reduce cross-sensitivity, some interfering gases may still have an effect on toxic sensor readings. In some cases the interfering effect may be "positive" and result in readings which are higher than actual. In some cases the interference may be negative and produce readings which are lower than actual. Electrochemical sensor designs may include a selective external filter designed to remove interfering gases which would otherwise have an effect on the sensing electrode. The size and composition of the filter are determined by the type and expected concentration of the interfering contaminants being removed.Control ChipASIC (Application-Specific Integrated Circuit) is the control chip that connects together the external components of the micro system.Figure 8: Interfacing of ASIC with external components of the systemApplication-Specific Integrated Circuit (ASIC) An integrated circuit designed to perform a particular function by defining the interconnection of a set of basic circuit building blocks drawn from a library provided by the circuit manufacturer.ASIC is a novel mixed signal design that contains an analog signal conditioning module operating the sensors, 10-bit ADC & DAC converters & a digital data processing module. An RC relaxation oscillator (OSC) provides the clock signal.The analog module is based on the AMS (Automated Manifest System), which offer a lot of power saving scheme (sleep mode) & a compact IC design. The temperature circuitry biased the diode at constant current, so that a change in temperature would result in corresponding change in diode voltage. The pH ISFET sensor was biased as a simple source & drain follower at constant current with D-S voltage changing with threshold voltage & pH. Conductivity circuit operated at direct current measuring the resistance across the electrode pair as an inverse function of solution conductivity. An incorporated potentiostat operated the amperometric oxygen sensor with a 10-bit DAC controlling the working electrode potential with respect to reference. The analog signals were sequenced through a MUX prior to being digitized by the ADC. The bandwidth for each channel was limited by the sampling interval of 0.2ms. The digital data processing module conditioned the digitized signals through the use of a serial bit stream data compression algorithm, which decided when transmission was required by comparing the most recent sample with the previous one. This minimizes the transmission length & particularly effective when the measuring environment is at quiescent, a condition encountered in many applications. The entire design is based on low power consumption & immunity from noise interference. The digital module is clocked at 32 kHz & employed in sleep mode to conserve power from analog module.Radio TransmitterIt’s assembled prior to integration in the capsule using discrete surface mount components on a single-sided PCB. The footprint of the standard transmitter measured 8*5*3mm including the integrated coil (magnetic) antenna. It’s designed to operate at a transmission freq. of 40.01MHz at 20?C generating a signal of 10kHz band width. A second crystal stabilized transmitter was also used. This unit is similar to the free running STD transmitter, having a transmission frequency limited to 20.08MHz at 20?C, due to crystal used. Pills incorporating the STD transmitter are Type 1, where as the pills having crystal stabilized unit is Type 2. The transmission range was measured as being 1m & the modulation scheme FSK (Frequency Shift Keying), with a data rate of 1kb/s. PERFORMANCEFigure 9: a) Temperature Channel Performance, b) pH channel performanceTemperature Channel Performance The linear sensitivity was measured over a temperature range from 0?C to 70?C & found to be 15.4 mV/?C. This amplified signal response was from the analog circuit, which was later implemented in the ASIC. The sensor (fig a), once integrated in the pill, gave a linear regression of 11.9 bits/?C , with a resolution limited by the noise band of 0.4?C (Fig b). The diode was forward biased with a constant current (15 ?A) with the n-channel clamped to the ground, while p-channel was floating. Since the bias current supply circuit was clamped to the negative V rail, any change in the supply voltage potential would cause the temporary channel to drift. Thus, it was seen that o/p signal changed by 1.45mV change in supply expressed in mV, corresponding to a drift of – 41.7mV/h in the pill from a supply voltage change of –14.5mV/h.pH Channel PerformanceThe linear performance from pH 1 to 13 corresponded to sensitivity of –41.7mV/pH unit at 23?C. The pH ISFET sensor operated in a constant current mode (15 ?A), with drain voltage clamped to positive supply rail & the source voltage floating with the gate potential. The Ag/AgCl reference electrode, representing the potential in which the floating gate was referred to, was connected to ground. The sensor performance, once integrated in the pill (fig b), corresponded to 14.85 bits/pH which give a resolution of 0.07pH/data point. The sensor exhibits a larger responsivity in alkaline solutions. The sensor life time of 20h was limited by Ag/AgCl reference electrode made from electroplated silver. The ph sensor exhibited a signal drift of –6mV/h (0.14pH), of which –2.5mV/h was estimated to be due to the dissolution of AgCl from the reference electrode. The temperature sensitivity of the pH sensor was measured as 16.8mV/?c. The changing of the pH of the solution at 40?c from pH 6.8 to 2.3 and 11.6 demonstrated that the two channels were completely independent of each other and there was no signal interference from the temperature channel (fig b).ADVANTAGESIt is being beneficially used for disease detection & abnormalities in?human body. Therefore it is also called as MAGIC PILL FOR HEALTH CARE.Adaptable for use in corrosive &?quiescent environment.It can be used in industries in evaluation of water quality, Pollution Detection, fermentation process control & inspection of pipelines.Micro Electronic Pill utilizes?a PROGRAMMABLE STANDBY MODE, So power consumption is very less.It has very small size, hence it is very easy for practical usage.High sensitivity, Good reliability & Life times.Very long life of?the cells (40 hours), Less Power, Current &?Voltage requirement (12.1mW, 3.9mA, 3.1 V).Less transmission length & hence has zero noise interference.OTHER APPLICATIONSThe generic nature of microelectronic pill makes it adaptable for use in corrosive environments related to environmental & industrial applications, such as the evaluation of water quality, pollution detection, fermentation process control & inspection of the pipelines. The integration of radiation sensors & the application of indirect imaging technologies such as ultrasound & impedance tomography, will improve the detection of tissue abnormalities & radiation treatment associated with cancer & chronic inflammation. LIMITATIONSIt cannot perform ultrasound & impedance tomography.Cannot detect radiation abnormalities.Cannot perform radiation treatment associated with cancer & chronic inflammation.Micro Electronic Pills are expensive & are not available in many countries.Still its size is not digestible to small babies.Further research is being carried out to remove its draw backs.CONCLUSIONWe have therefore described about the multichannel sensor, which has been implemented in remote biomedical using micro technology, the microelectronic pills, which is designed to perform real time measurements in the GI tract providing the best in vitro wireless transmitter, multi channel recordings of analytical parameters. FUTURE DEVELOPMENTSFurther developments focus on the photo pattern able gel electrolyte and oxygen and cation selective membranes. Also in the future, these measurements will be used to perform physiological analysis of the GI tract. For e.g., Temperature sensors can be used to measure the body core temperature, also locate any changes corresponding to ulcers or tissue inflammation; pH sensors may be used for determination of presence of pathological conditions associated with abnormal ph levels etc.FUTURE CHALLENGESIn the future, one objective would be to produce a device, analogous to a micro total analysis system (?TAS) or lab on a chip sensor which is not only capable of collecting & processing data, but which can transmit it from a remote location. The overall concept would be to produce an array of sensor devices distributed throughout the body or environment, capable of transmitting high-quality information in real time.REFERENCES NoIntroduction1Block Diagram3Basic Components6Performance17Advantages19Other Applications20Limitations21Conclusion22Future Developments23Future Challenges24References25ABSTRACTThe invention of transistor enabled the first use of radiometry capsules, which used simple circuits for the internal study of the gastro-intestinal (GI) tract. They couldn't be used as they could transmit only from a single channel and also due to the size of the components. They also suffered from poor reliability, low sensitivity and short lifetimes of the devices. This led to the application of single-channel telemetry capsules for the detection of disease and abnormalities in the GI tract where restricted area prevented the use of traditional endoscopy.They were later modified as they had the disadvantage of using laboratory type sensors such as the glass pH electrodes, resistance thermometers, etc. They were also of very large size. The later modification is similar to the above instrument but is smaller in size due to the application of existing semiconductor fabrication technologies. These technologies led to the formation of "MICROELECTRONIC PILL".Microelectronic pill is basically a multichannel sensor used for remote biomedical measurements using micro technology. This is used for the real-time measurement parameters such as temperature, pH, conductivity and dissolved oxygen. The sensors are fabricated using electron beam and photolithographic pattern integration and were controlled by an application specific integrated circuit (ASIC). ................
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