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. About one in 10 couples have fertility problems and 40 percent
of these cases have absent or abnormal sperm. Conventional fertility treatments are effective in
many cases, but are less effective when sperm counts are very low.
6. Conclusion
The evolution and technology development of smart sensors for various applications in different
fields have been discussed. Fabrication aspects of smart sensors have also been discussed. The
latest trends including biochips have also been presented. Measurements and instrumentation
systems will be developed by using smart sensor in future.
REFERENCE
1. L. Schwiebert, S. K. S. Gupta, J. Weinmann, et. al., Research Challenges in Wireless Networks
of Biomedical Sensors, Proc. Seventh Annual International Conference on Mobile Computing and
Networking (Mobicom’01), pp 151-165, 2001.
2. Singh K, Development of MEMS using ASE technique, presented at 24th Nat Symp on
Instrumentation, Goa, India, Feb 2-6, (2000).
3. Singh K, Proc. Asia-Pacific IEEE Conf on Biomedical Engg, Hangzhou (China), (Sept, 2000)
863.
4. Singh V R, Bhatnagar S, Verma S & Singh R, Physics of Semiconductor Devices (ed. K. Lal et
al.), [Narosa Pub,New Delhi] (1997) 195.
5. Huijsing J H, Sensors and Actuators A, 30 (1992) 167. Heintz F & Zabler E, Sensors and
Actuators, 13 (1989) 39.Giachino J M, Sensors and Actuators, 10 (1986) 239.Smith G & Brown
M, Sensors and Actuators A, 46-47 (1995)521.
6. Singh V R, Proc. All Ind Symp on Modernisation of Automobile Industry with its Impact on
Environment, NewDelhi (1996) 10.
7. Singh V R, Main Group Elements and their Compounds (ed.V G Kumar Das), [Narosa Pub,
New Delhi] (1996) 204.
8. Singh V R, Proc. Conf. On Metrology in Relation to Environment, New Delhi (1997).Agarwal
R & Singh V R, Proc Nat Conf On Biomed Engg,Roorkee (2000) 224.
9. Singh V R, Advances in Metrology and its Role in Quality Improvement and Global Trade (ed.
B.S. Mathur et al.),[Narosa Publ, New Delhi] (1996) 242.
10.Gomez R, Bashir R & Bhunia A K, Sensors and Actuators B,86 (2002) 198.Actuators B, 74
(2001) 2.