Download BME 301: Introduction to Biomedical Sensors

Introduction to Biomedical
Sensors
BME 301 Biomedical Sensors
Lecture Note 1
BME 301 Biomedical Sensors - Ali Işın 2014
What is a Biomedical Sensor?
• Any instrumentation system can be described
as having three fundamental components:
• a sensor,
• a signal processor,
• and a display and/or storage device.
BME 301 Biomedical Sensors - Ali Işın 2014
• Although all these components of the
instrumentation system are important, the sensor
serves a special function in that it interfaces the
instrument with the system being measured.
• In the case of biomedical instrumentation, a
biomedical sensor is the interface between the
electronic instrument and the biological system.
BME 301 Biomedical Sensors - Ali Işın 2014
Important Concerns
• There are some general concerns that are important
for any sensor in an instrumentation system interface
function especially for biomedical sensors:
1. The sensor can affect the system, for that sensors must
be designed to minimize their interaction with the
biological host. It is important that the presence of the
sensor does not affect the variable being measured in
the vicinity of the sensor via interaction between the
sensor and the biologic system. This may change the
quantity being sensed in the vicinity.
BME 301 Biomedical Sensors - Ali Işın 2014
2. The biological system can affect the
performance of the sensor. The foreign body
reaction might cause the host’s system to
attempt to break down the materials of the
sensor in order to remove it. This may, in fact,
degrade the sensor package so that it can no
longer perform in an adequate manner. So the
material of package must be proper.
BME 301 Biomedical Sensors - Ali Işın 2014
3. Sensors that are implanted in the body are
not accessible for calibration. Thus, they must
have extremely stable characteristics so that
frequent calibrations are not necessary.
BME 301 Biomedical Sensors - Ali Işın 2014
Classification of Biomedical Sensors
• Biomedical sensors can be classified according to
how they are used with respect to the biological
system:
1. Noninvasive biomedical sensors do not even
contact the biological system being measured.
Sensors of radiant heat or sound energy coming
from an organism are examples of noncontacting
sensors. Noninvasive sensors can also be placed
on the body surface like Skin surface
thermometers, biopotential electrodes, and
strain gauges placed on the skin.
BME 301 Biomedical Sensors - Ali Işın 2014
2. Indwelling sensors (minimally invasive
sensors) :are those that can be placed into a
natural body cavity that communicates with
the outside. Examples: oral–rectal
thermometers, intrauterine pressure
transducers, and stomach pH sensors.
BME 301 Biomedical Sensors - Ali Işın 2014
3. Invasive sensors: are those that need to be
surgically placed and that require some tissue
damage associated with their installation.
BME 301 Biomedical Sensors - Ali Işın 2014
• We can also classify sensors in terms of the
quantities that they measure:
• 1. Physical sensors: are used in measuring
physical quantities such as displacement,
pressure, and flow.
BME 301 Biomedical Sensors - Ali Işın 2014
2. Chemical sensors: are used to determine the
concentration of chemical substances within the
host. A sub-group of the chemical sensors that
are concerned with sensing the presence and the
concentration of biochemical materials in the
host.
3. Bio-analytical sensors or biosensors: used to
measure some internal quantities like enzymes.
BME 301 Biomedical Sensors - Ali Işın 2014
1. Bio-analytical Sensors:
• A special class of sensors of biological
molecules has evolved in recent years. These
bioanalytical sensors take advantage of one of
the following biochemical reactions:
(1) enzyme–substrate.
(2) antigen–antibody.
(3) ligand–receptor.
BME 301 Biomedical Sensors - Ali Işın 2014
• The advantage of using these reactions in a
sensor is that they are highly specific to a
particular biological molecule, and sensors
with high sensitivity and selectivity can be
developed based upon these reactions.
BME 301 Biomedical Sensors - Ali Işın 2014
• The basic structure of a bio-analytical sensor;
BME 301 Biomedical Sensors - Ali Işın 2014
• There are two principal regions of the sensor. The first
contains one component of the biological sensing reaction
such as the enzyme or the antibody, and the second region
involves a means of detecting weather the biological
reaction has taken place.
• This second portion of a bioanalytical sensor is made up of
either a physical or chemical sensor that serves as the
detector of the biological reaction. This detector can
consist of an electrical sensor such as used in
electrochemical sensors, a thermal sensor, a sensor of
changes in capacitance, a sensor of changes in mass, or a
sensor of optical properties.
BME 301 Biomedical Sensors - Ali Işın 2014
Example bio-analytical sensor:
• One example of a bioanalytical sensor is a glucose
sensor. The first portion of the sensor contains the
enzyme glucose oxidase. This enzyme promotes the
oxidation of glucose to glucuronic acid and hydrogen
peroxide while consuming oxygen in the process.
• Thus, by placing a hydrogen peroxide or an oxygen
sensor along with the glucose oxidase in the
bioanalytical sensor, one can determine the amount of
glucose oxidized by measuring the amount of hydrogen
peroxide produced or oxygen consumed.
BME 301 Biomedical Sensors - Ali Işın 2014
• Stability is important for bioanalytical sensors,
especially those that are used for long-term
measurements.
• The stability issues are also related to preservation of
the biological molecules used in the first portion of the
sensor. These molecules can often be degraded or
destroyed by heat or exposure to light.. Thus, an
important issue in dealing with bioanalytical sensors is
the preservation of the biochemical components of the
sensor.
BME 301 Biomedical Sensors - Ali Işın 2014
2.Chemical Sensors:
• There are many biomedical situations where it is
necessary to know the concentration or chemical
activity of a particular substance in a biological
sample. Chemical sensors provide the interface
between an instrument and the specimen to
allow one to determine these quantities.
• These sensors can be used on a biological
specimen taken from the host and tested in a
laboratory, noninvasive or invasive sensors.
BME 301 Biomedical Sensors - Ali Işın 2014
• The below table indicates the most famous biomedical
sensors, the most common three types of the
biomedical sensors are electrochemical, optical and
thermal biomedical sensors:-
Electrochemical
Optical
Thermal
a. Amperometric
a. Colorimetric
a. Calorimetric
b. Potentiometric
b. Emmision and
absorption
spectroscopy
b. Thermo
conductivity
c. Coulometric
c. Fluorescence
BME 301 Biomedical Sensors - Ali Işın 2014
• A Clark amperometric electrode for sensing
oxygen;
BME 301 Biomedical Sensors - Ali Işın 2014
• It consists of an electrochemical cell
separated from the specimen being measured
by an oxygen-permeable membrane. The cell
is biased at a fixed potential of 600 mV, and
under these conditions the following reaction
occurs at the noble metal cathode:
O2 + 2H2O → 4e- + 4OH-
BME 301 Biomedical Sensors - Ali Işın 2014
• This reaction involves the reduction of molecular
oxygen that diffuses into the cell through the
oxygen permeable Membrane. Since the other
components of the reaction are in abundance,
the rate of the reaction is limited by the amount
of oxygen available. Thus, the rate of electrons
used at the cathode is directly related to the
available oxygen. In other words, the cathode
current is proportional to the partial pressure of
oxygen in the specimen being measured.
BME 301 Biomedical Sensors - Ali Işın 2014
• The electrochemical cell is completed by the silver
anode. The reaction at the anode involves forming the
low-solubility salt, silverchloride, from the anode
material itself and the chloride ion contained in the
electrolytic solution. The cell is designed with these
materials in abundance so that their activity does not
affect the sensor performance.
• This type of sensor is an example of an amperometric
electrochemical sensor that is the current in the cell is
proportional to the concentration of the analyte; in this
case, oxygen.
BME 301 Biomedical Sensors - Ali Işın 2014
3. Physical Sensors
• Physical variables measured include
temperature, strain, force, pressure,
displacement, position, velocity, acceleration,
optical radiation, sound, flow rate, viscosity,
and electromagnetic fields.
BME 301 Biomedical Sensors - Ali Işın 2014
• Temperature Sensors;
• Temperature is an important parameter in many
control systems, most familiarly in environmental
control systems. Several distinctly different
transduction mechanisms have been employed to
measure temperature.
• The mercury thermometer is a temperature sensor
which produces a non electronic output signal. The
most commonly used electrical signal generating
temperature sensors are thermocouples, thermestors,
and resistance thermometers.
BME 301 Biomedical Sensors - Ali Işın 2014
• Thermocouples;
• Thermocouples employ the Seebeck effect, which
occurs at the junction of two dissimilar conductors. A
voltage difference is generated between the hot and
cold ends of the two conductors due to the differences
in the energy distribution of electrons at the two
different temperatures. The voltage magnitude
generated depends on the properties of the conductor,
e.g., conductivity and work function, such that a
difference voltage will be measured between the cold
ends of two different conductors. The voltage changes
fairly linearly with temperature over a given range,
depending on the choice of conductors.
BME 301 Biomedical Sensors - Ali Işın 2014
• To minimize measurement error, the cool end
of the couple must be kept at a constant
temperature and the voltmeter must have
high input impedance. A commonly used
thermocouple is made of copper and
constantan wires.
BME 301 Biomedical Sensors - Ali Işın 2014
• A thermocouple is an ‘‘auto-generator,’’ i.e., it
produces a usable output signal, in this case
electronic, directly in response to the
measurand without the need for auxiliary
power.
BME 301 Biomedical Sensors - Ali Işın 2014
•
•
Resistance thermometer;
The resistance thermometer relies on the increase in resistance of a metal wire
with increasing temperature.
•
As the electrons in the metal gain thermal energy, they move about more rapidly
and undergo more frequent collisions with each other and the atomic nuclei.
These scattering events reduce the mobility of the electrons, and since resistance
is inversely proportional to mobility, the resistance increases.
Resistance thermometers typically consist of a coil of thin metal wire. Platinum
wire gives the largest linear range of operation. The resistance thermometer is a
‘‘modulator’’ or passive transducer. In order to determine the resistance change, a
constant current is supplied and the corresponding voltage is measured (or vice
versa).
•
•
A measure of the sensitivity of a resistance thermometer is its temperature
coefficient of resistance (TCR).
BME 301 Biomedical Sensors - Ali Işın 2014
• Thermistors;
• Thermistors are resistive elements made of semiconductor
materials and have a negative temperature coefficient of resistance.
The mechanism governing the resistance change of a thermistor is
the increase in the number of conducting electrons with increasing
temperature, due to thermal generation, i.e., the electrons that are
the least tightly bound to the nucleus (valence electrons) gain
sufficient thermal energy to break away (enter the conduction
band) and become influenced by external fields.
• Thermistors are measured in the same manner as resistance
thermometers, but thermistors have up to 100 times higher TCR
values.
BME 301 Biomedical Sensors - Ali Işın 2014
• Displacement and Force Sensors;
• Many types of forces can be sensed by the
displacements they create.
• For example, the force due to acceleration of a
mass at the end of a spring will cause the spring
to stretch and the mass to move. Its displacement
from the zero acceleration position is governed
by the force generated by the acceleration (F =m ·
a) and by the restoring force of the spring.
BME 301 Biomedical Sensors - Ali Işın 2014
• Another example is the displacement of the center of a
deformable membrane due to a difference in pressure
across it.
• Both of these examples require multiple transduction
mechanisms to produce an electronic output: a
• primary mechanism which converts force to
displacement (mechanical to mechanical) and then an
intermediate mechanism to convert displacement to
an electrical signal (mechanical to electrical).
BME 301 Biomedical Sensors - Ali Işın 2014
Applications Of Biomedical Sensors:
• Biomedical research:
• One of the application fields of the biomedical
sensors is using them in continuous
biomedical research. So, these sensors can be
used to improve the quality of the biomedical
product).
BME 301 Biomedical Sensors - Ali Işın 2014
• Patient care applications:
Sensors are used as a part of instruments that
carry out patient monitoring by making
measurements such as blood pressure, oxygen
saturation, body temperature and ECG.
BME 301 Biomedical Sensors - Ali Işın 2014
• Specimen analysis:
• This can include analyses that can be carried out
by the patients themselves in their homes such as
it is done with home blood glucose analyzers.
• Sensors also are a part of large, multicomponent, automatic blood analyzers used in
the central clinical laboratories of medical
centers.
BME 301 Biomedical Sensors - Ali Işın 2014