INTRODUCTION The term 'smart sensor' has been used by various researchers in a number of different contexts, ranging from sensors incorporating a few active devices to pro- vide a more reliable interface to the sensor to enhance the quality of the sensed signal, to integrated sensors incorporating a sophisticated electronic circuit block including both digital and analog circuitry that helps transform the sensor from a passive component into a 'smart' periphery of a control/instrumentation system. However, over the past few years, a rather broad consensus has been achieved whereby a smart sensor is defined as one that is capable of (i) providing a digital output; (ii) communicating through a bidirectional digital bus; (iii) being accessed through a specific address; and (iv)executing commands and logical functions . In the context of this paper, the smart sensor is defined as one that should possess these features. In addition, it is also desirable for the smart sensor to perform such functions as compensation of secondary parameters (e.g. temperature), failure prevention and detection, self-testing, auto- calibration and various computationally intensive operations. The development of such sensors will greatly enhance the capabilities of many control and instrumentation systems that are nowadays lacking in their interface with the external world. Figure 1 shows the elements of a typical electronic control system. Sensors provide analog information on the system being monitored through signal conditioning circuits to a microprocessor-based controller. The processor interprets the information, makes appropriate decisions (perhaps in conjunction with higher level control), and implements those decisions via the actuators. Sensors currently represent the weakest link in the development of most next-generation instrumentation and control systems. Where sensors exist at all, they are frequently unreliable, rarely attain an accuracy of 8 bits and may cost more than the processor. This situation is especially true in biomedicine, where the problems are particularly difficult. Only in the past few years has this situation begun to change. Sensors have progressed through a number of identifiable generations, as shown in figure 2. First- generation devices had little, if any, electronics associated with them, while second-generation sensors were part of purely analog systems with virtually all of the electronics remote from the sensor. By the third generation, where the majority of the systems currently reside, at least the first stage of amplification occurred in the sensor module or on the sensor chip itself. Thus, the output from these systems is a high- level analog signal, encoded either as a voltage amplitude or as a variable pulse rate. This signal is digitized remotely and then processed by a microcomputer. Many automotive sensing systems fit into this category. We are now evolving into the fourth- generation sensors, where more analog and digital electronics are on-chip, making the sensor addressable action between the sensor and the host microcomputer. Some large-area visible imaging devices and pressure sensors represent fourth-generation components. On the horizon are fifth-generation sensors, in which data conversion is accomplished on the sensor (or, at least, in the sensor module) so that the bidirectional communication link with the microcomputer is digital. These devices will likely be digitally compensated using field-programmable read-only memories (PROMS) to accuracies are not available in sensors today. From a system viewpoint, the standardization of communication protocols and formats is badly needed for this fifth generation of devices. Figure 3 shows the block diagram of a generic fifth-generation VLSI sensor. The device is addressable, self-testing and communicates over a bi- directional digital bus. It can measure the outputs of a variety of sensors, use some of these outputs for secondary parameter compensation, perform digital signal processing tasks and retain only the useful and required signals and then communicate the acquired signals with the higher level computer. For example, a gas sensor might sense gas pressure, temperature, flow and include an array of gas sensors to improve selectivity and sensitivity. The required device counts for such a sensor, estimated in the range of ten thousand, are still low enough that the circuitry would occupy no more than a few square millimeters and would not be a major cost factor as most of the cost would be in testing and packaging. Based on the evolution of both sensor technologies and electronic systems, it is evident that solid-state sensors will continue to incorporate more sophisticated electronic circuits. The integration of high-performance solid-state sensors and sophisticated signal processing and control circuitry into a smart sensor module serves a dual purpose: it allows the sensor to become a much more versatile component in terms of its sensing function and accuracy; and will enable the smart sensor to become an active and compatible component of an overall ‘sensing system’. It is believed that unless these systems are developed and demonstrated to be capable of surpassing the performance of many of the existing sensors, solid-state sensors will not be fully utilized in terms of their capabilities and features. The development of such systems will naturally require the development of individual components and circuit blocks that make up the overall system. 3. RETINAL AND CORTICAL IMPLANTS Proposed retina implants fall into two general categories: • Epiretinal, which are placed on the surface of the retina. • Sub retinal, which are placed under the surface of the retina. Both approaches have advantages and disadvantages. The main advantages of the sub-retinal implant are that the implant is easily fixed in place, and the simplified processing that is involved, since the signals that are generated replace only the rods and cones with other layers of the retina processing the data from the implant. The main advantage of the epiretinal implant is the greater ability to dissipate heat because it is not embedded under tissue. This is a significant consideration in the retina. The normal temperature inside the eye is less than the normal body temperature of 98.6o Fahrenheit. Besides the possibility that heat buildup from the sensor electronics could jeopardize the chronic implantation of the sensor, there is also the concern that the elevated temperature produced by the sensor could lead to infection, especially since the implanted device could become a haven for bacteria. There are also two options for a cortical implant. One option is to place the sensors on the surface of the visual cortex. At this time, it is unknown whether the signals produced by this type of sensor can produce stimuli that are sufficiently localized to generate the desired visual perception. The other option is to use electrodes that extend into the visual cortex. This allows more localized control of the stimulation, but also presents the possibility of long-term damage to the brain cells during chronic use. It should be noted, however, that although heat dissipation remains concern with a cortical implant, the natural heat dissipation within the skull is greater than within the eye. Given the current state of the research, it is unclear which of these disadvantages will be most difficult to overcome for a chronically implanted device. Therefore, different research groups are investigating different solutions. Here we describe our proposed solution. An implantable version of the current ex-vivo micro sensor array, along with its location within the eye, is shown in Figure 1. The micro bumps rest on the surface of the retina rather than embedding themselves into the retina. Unlike some other systems that have been proposed, these smart sensors are placed upon the retina and are small enough and light enough to be held in place with relatively little force. These sensors produce electrical signals that are converted by the underlying tissue into a chemical response, mimicking the normal operating behavior of the retina from light stimulation. The chemical response is digital (binary), essentially producing chemical serial communication. A similar design is being used for a cortical implant, although the spacing between the micro bumps is larger to match the increased spacing between ganglia in the visual cortex. As shown in Figure 1, the front side of the retina is in contact with the micro sensor array. This is an example of an epiretinal implant. Transmission into the eye works as follows. The surface of the retina is stimulated electrically, via an artificial retina prosthesis, by the sensors on the smart sensor chip. These electrical signals are converted into chemical signals by the ganglia and other underlying tissue structures and the response is carried via the optic nerve to the brain. Signal transmission from the smart sensors implanted in the eye works in a similar manner, only in the reverse direction. The resulting neurological signals from the ganglia are picked up by the micro sensors and the signal and relative intensity can be transmitted out of the smart sensor. Eventually, the sensor array will be used for both reception and transmission in a feedback system and chronically implanted within the eye. Although the micro sensor array and associated electronics have been developed, they have not yet been tested as a chronic implant. Another challenge at this point is the wireless networking of these micro sensors with an external processing unit in order to process the complex signals to be transmitted to the array. 4. SMART SENSOR CHIP DESIGN Figure 2 shows a close-up of the smart sensor shown in figure 1. Each micro bump array consists of a cluster of extrusions that will rest on the surface of the retina. The small size of the micro bumps allows them to rest on the surface of the retina without perforating the retina. In addition, the slight spacing among the extrusions in each micro-bump array provides some additional heat dissipation capability. Note that the distance between adjacent sets of micro bumps is approximately 70 microns. These sensors are bonded to an integrated circuit. The integrated circuit is a multiplexing chip, operating at 40KHz,with on-chip switches and pads to support a grid of connections. Figure 1 shows a 4 × 4 grid for illustrative purposes, although the next generation of sensor chip has a 16 × 16array. The circuit can the ability to transmit and receive, although not simultaneously. Each connection has an aluminum probe surface where the micro machined sensor is bonded. This is accomplished by using a technique called backside bonding, which places an adhesive on the chip and allows the sensors to be bonded to the chip, with each sensor located on a probe surface. Before the bonding is done, the entire IC, except the probe areas, is coated with a biologically inert substance. The neural probe array is a user-configured 1:100 DE multiplexer/ 100:1 multiplexer, where an external switch controls the configuration. The neural array is a matrix of 100 microelectrodes constructed as bi-directional switched-probe units that will stimulate or monitor the response state of an aggregate of neurons, more specifically, bipolar cells, which are two-poled nerve cells. When the array is configured as a DE multiplexer, the switched-probe units serve to stimulate the corresponding aggregate of neurons; thus, the array functions as a neurostimulator. When the array is configured as a multiplexer, the units serve to monitor the evoked response of the aggregate of neurons in the visual cortex; thus, the array functions as a neural response monitor. The array has an additional bi-directional port called the signal carrier, where the direction of the signal flow to and from this port depends on the configuration of the array. As a neuro-response monitor, the neural signals from each aggregate will be relayed through the signal carrier port on a single line. As a neurostimulator, the external signal, whose magnitude will depend on the intensity of the signal required to revive the degenerate neurons, will be injected into the circuit through the signal carrier (bypassing the amplifier) to be distributed to each aggregate through the corresponding unit. Each switched-probe unit consists of a neural probe and two n-channel MOSFETs, whose W/L ratio is 6/2. The W/L ratio defines the behavior of the transistor, where W is the width of the active area of the transistor and L is the length of the polysilicon used for the gate channel. For each unit, the probe is a passive element that is used to interface each aggregate of neurons to the electronic system and the transistors are the active elements that are used to activate the units. The second-generation prototype adds the decoder with its outputs connected to the row and to the column ports of the array. The addition of the decoder reduces the number of required contact pads from 22 to 5 (Set, master clock, VDD,VSS, and the signal carrier port) and enhances the reliability of the Scanner with the configuration of 2 inputs, rather than the external connections of 20 inputs. The configuration of the Set and clock cycles will enable the decoder to sequentially activate each unit by sending +5V pulses to the corresponding row and column ports. To establish the required Set and clock cycles, further neural analysis on the periodic stimulation of the bipolar cells must be conducted. The Set signal initiates the scanning of the probe from left to right and from top to bottom. 5 Applications 5.1 Industrial Smart sensors currently have established their strongest presence in the industrial market. Reasons, which can readily be identified, are: the higher cost associated with the more stringent specifications and medium to low volumes prevalent in the industrial market place the relative unimportance of package size the high level of importance attached to data reliability associated with very large plants and coupled with difficulty of access to sensors for diagnostics. In addition, the development of a number of industrial digital data bus standards [1, 8]. It is expected that the trend to introduce smart sensors in the industrial area will continue as data bus standards are ratio11nalized and the benefits of remote diagnostics are better understood. Design layouts of few smart sensors are shown in Figs 4-8 for pressure, flow and temperature etc. 5.2 Aerospace The aerospace industry is looking to adopt smart sensors for different sensors. The issues of data integrity are equally important or more so, but another major concern is that of the weight of cable associated with large numbers of sensors which are frequently multiplicities for redundancy purposes. Safety is A number of smart sensors for biomedical applications [1, 3, 4, 12, and 13] have also been developed (Figs 9 and 10) by using chip technology. Calibration aspects of such sensors for use in standards have been studied in detail11−13. 5.7.1 Biochips A special class of biosensors, typically known as biochips [14-18], has multiple transducer elements and is based on integrated circuit microchips. The term “biochip” is like a “computer chip”, which is a silicon-based substrate used in the fabrication of miniaturized electronic circuits. Therefore, in the generic sense, there is a certain element of integrated circuit involved in the term ‘biochip’. However, over the years, the term biochip has taken on a variety of meanings. A biochip is now generally defined as a material or a device that has an array of probes used for biochemical assays. In general, any device or component incorporating a two-dimensional array of reaction sites and having biological materials on a solid substrate has been referred to as a biochip. Biochips often involve both miniaturization, usually in microarray formats, and the possibility of low-cost mass production, are of two types, • Array plate biochips, which consist of plate-based or gel-based substrates; and Integrated biochips, which also include the detector-array microchips. Attached to the bottom sealing plate allowing the microfluidic device to be mounted onto the IC biochip [15]. 5.7.2 DNA Biochip using a phototransistor integrated circuit Substrates having microarrays of bio receptors are often referred to as biochips, although most of these systems do not have integrated micro sensor detection systems. These array plate biochips usually have separate detection systems that are relatively large and are only suitable for laboratory-based research applications. They can have large numbers of probes (tens of thousands) that can potentially be used to identify multiple bio targets with very high speed and high throughput by matching with different types of probes via hybridization. Therefore, array plates are very useful for gene discovery and drug discovery applications, which often require tens of thousands of assays on a single-plate. On the other hand, integrated biochips are devices that also include an integrated circuit microsensor chip, which makes these devices very portable, and inexpensive. These devices generally have medium-density probe arrays (10-100 probes) and are most appropriate for medical diagnostics at the physician’s office. 5.7.3 Microfluidic biochips An integrated sensor based on phototransistor integrated circuits has been developed [15, 16] for use in medical detection, DNA diagnostics, and the mapping. The evaluation involves various system components developed for an integrated biosensor microchip. Methods of development include microarray of DNA tubes on nitrocellulose substrate. The chip device has sensors, amplifiers, discriminators, with logic circuitry on board. Integration of light-emitting diodes into the device is also possible. To achieve improved sensitivity, an IC system having the phototransistor sensing element composed of 220 phototransistor cells connected in parallel may be used. 5.7.4 Cytosensor microphysiometer: biological applications of silicon technology The attachment of enzymes to glass microfluidic channels has been achieved [14, 15] using a highly reactive poly (maleic anhydride-alt-α-olefin) (PMA)-based coating that is supplied to the micro channel in a toluene solution. The PMA reacts with 3-aminopropyltriethoxysilane groups linked to the glass surface to form a matrix that enables additional maleic anhydride groups to react with free amino groups on enzymes to give a mixed covalent- monovalent immobilization support. In a typical microfluidics-based IC biochip system, the sample chamber and all access ports are machined into a 2.5 cm × 2.5 cm × 0.3 cm piece of Plexiglas. Glass or quartz plates (1 mm thick) affixed to the top and bottom of the Plexiglas seal the sampling chamber and serve as excitation and detection windows, respectively. While the bottom plate is permanently fixed in place, the top plate is removable for the routine replacement of sampling platforms within the chamber. A pair of rubber-lined Plexiglas rails is attached to the bottom sealing plate allowing the microfluidic device to be mounted onto the IC biochip [15]. 5.7.4 Cytosensor microphysiometer: biological applications of Silicon technology Asilicon-baseddevice, calleda 16-19microphysiometer, can be used to detect and monitor the response of cells to a variety of chemical substances, especially ligands for specific plasma membrane receptors. The microphysiometer measures the rate of proton excretion from 104 to 106 cells. This article gives an overview of experiments currently being carried out with this instrument with emphasis on receptors with seven transmembrane helices and tyrosine kinase receptors. As a scientific instrument, the microphysiometer can be thought of as serving two distinct functions. In terms of detecting specific molecules, selected biological cells in this instrument serve as detectors and amplifiers. The microphysiometer can also investigate cell function and biochemistry. A major application of this instrument may prove to be screening for new receptor ligands. In this respect, the microphysiometer appears to offer significant advantages over other techniques. 5.7.5 Microscale integrated sperm sorter: biochip-lab Biomedical engineers have recently developed18-19 a prototype lab-on-a-chip for harvesting healthy sperm cells to increase male fertility. The microscale integrated sperm sorter (MISS) separates strong swimmers⎯sperm cells that are most likely to fertilize an egg⎯from aimlessly drifting spermatozoa that are essentially incapable of fertilization. The system allows separation and detection of motile sperm from small samples that are difficult to handle using conventional sperm sorting techniques. The device, slightly larger than a penny, is an inexpensive, easy-to-use sorter that men could ultimately use at home to measure fertility or test the outcome of a vasectomy or vasectomy reversal. Since sperm motility is a sensitive indicator of toxicity, the MISS may also be useful as a toxicology test. The rectangular device has been built using conventional techniques similar to those used in making computer chips. It has two input chambers at one end and two collection chambers at the other. A channel extends from each input chamber, merging into a single pipe for some distance, then separating before entering the two collection chambers. The MISS requires no power source. Fluids are set into motion by the forces of surface tension and gravity, which combine to produce a steady flow. The device also takes advantage of laminar flow, in which two liquid streams can be made to run side-by-side without mixing. This is what happens in the middle of the MISS when the two fluid channels merge. To operate the device, a semen sample is placed in one input chamber. A second chamber is filled with salt water. The two fluids move toward the opposite end of the device, combining in the middle into a single laminar flow. While the two streams are side- by-side, swimming sperm will cross the laminar boundary and enter the salt water. Nonswimmers go with the flow and stay in the original stream. When the two streams separate into the collection chambers, one chamber will have swimmers, the other will not. The purity of sorted sperm samples that arrive in the collection chamber is nearly 100 percent. This approach to sperm sorting has additional advantages compared to conventional sorting techniques. It avoids processes, such as centrifugation, that can damage sperm. It can be combined with a color- coded readout for a self-contained, easy-to-use home test. The MISS, which sorts by motility, coincidentally selects sperm cells that lack physical abnormalities, such as a misshapen head or crimped tail. In theory, a single sperm cell is all that is needed to fertilize an egg. But it can be impractical to isolate and harvest the most viable sperm cells using conventionallaboratorytechniques.Doctors frequently resort to hand sorting through dead sperm and debris to find a 'good' sperm, a procedure that can take hours in some cases. 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