This document provides an overview of transducers for biomedical applications. It defines transducers as devices that convert one form of energy into another for measurement purposes. It classifies transducers as active or passive, analog or digital, and primary or secondary. It also discusses various transducer principles including capacitive, inductive, resistive, and piezoelectric. The document then focuses on specific biomedical applications, describing transducers used to measure electrical activity, blood pressure, blood flow, temperature, respiration, and pulse. Common transducer types for these applications include electrodes, strain gauges, inductive sensors, capacitive sensors, thermistors, and fiber optic sensors.

1. TRANSDUCERS FOR
BIOMEDICAL APPLICATIONS
Presented By:
Jaspreet Singh Mehrok
SLIET, EIE
Department
India

2. Transducers
 Transducer
 a device that converts primary form of energy into other
different energy form only for measurement purposes.
 Primary Energy Forms: mechanical, thermal, electromagnetic,
optical, chemical, etc.
 Sensor
 It is a wide term which covers almost everything from human
eye to trigger of a pistol.
 Senses the change in parameter(specific).

3. CLASSIFICATION OFCLASSIFICATION OF
TRANSDUCERSTRANSDUCERS
 Active & Passive Transducers
 Absolute & Relative Transducers
 Direct & Complex Transducers
 Analog & Digital Transducers
 Primary & secondary Transducers
 On the basis of principle used

4. Active vs Passive Transducers:
 Active Transducers:
 Add energy to the measurement environment as part of
the measurement process.
 Requires external power supply.
 Strain gauge, potentiometer & etc.
 Passive Transducers :
 Do not add energy as part of the measurement process but may
remove energy in their operation.
 Does not require external power supply
 Thermocouple, photo-voltaic cell & etc.

7. ANALOG & DIGITAL TRANSDUCERSANALOG & DIGITAL TRANSDUCERS
 ANALOG TRANSDUCER - The
transducers which convert the input
quantity into an analog output which is a
continuous function of time.
 DIGITAL TRANSDUCERS - The
transducers which convert the input
quantity into digital form means in the
form of pulses.

8. PRIMARY vs SECONDARYPRIMARY vs SECONDARY
TRANSDUCERSTRANSDUCERS
 PRIMARY TRANSDUCERS - Some transducers
contain the mechanical as well as electrical device.
The mechanical device converts the physical quantity
to be measured into a mechanical signal. Such
mechanical device are called as the primary
transducers.
 SECONDARY TRANSDUCERS - The electrical
device then convert this mechanical signal into a
corresponding electrical signal. Such electrical device
are known as secondary transducers

9. CLASSIFICATION ON THE BASIS OFCLASSIFICATION ON THE BASIS OF
PRINCIPLE USEDPRINCIPLE USED
 Capacitive
 Inductive
 Resistive
 Electromagnetic
 Piezoelectric
 Photoconductive
 Photovoltaic

10. Selecting a Transducer
 What is the physical quantity to be measured?
 Which transducer principle can best be used to measure this
quantity?
 What accuracy is required for this measurement?
 Fundamental transducer parameters
 Physical conditions
 Environmental conditions
 Compatibility of the associated equipment
 Reducing the total measurement error :
 Using in-place system calibration with corrections performed
in the data reduction
 Artificially controlling the environment to minimize possible
errors

11. Transducers forPhysiological Variable
Measurements
• A variable is any quantity whose value
changes with time. A variable associated
with the physiological processes of the body
is known as a physiological variable.
• Physiological variables occur in many
forms: as ionic potential, mechanical
movements, hydraulic pressure ,flows and
body temperature etc.
• Different transducers are used for different
physiological variables.

12. Electrical Activity Measurement
 Electrodes:
 Electrodes convert ionic potential into electrical signals.
 Used for EEG, ECG, EMG, ERG and EOG etc.
 Different types of Electrodes are:
1) Surface Electrodes(no. Of muscles)
These electrodes are used to obtain bioelectric potentials from the surface of
the body.
2) Needle electrodes(specific to a muscle)
These electrodes are inserted into body to obtain localized measurement of
potentials from a specific muscle.
3) Microelectrodes(cellular level record)
Electrodes have tips sufficiently small to penetrate a single cell in order to
obtain readings from within cell.

13. Electrical Activity
Measurement(cont.)
 Working of Electrodes:
 When metal electrodes come in contact with electrolyte then ion-electron
exchange takes place as a result of electro-chemical reaction.
One cation M+
out of the electrolyte
becomes one neutral
atom M
taking off one free
electron
from the metal
One atom M out
of the metal
is oxidized to form
one cation M+
and
giving off one free
electron e-
to the
metal.

14. Half-cell potential
Oxidation and reduction processes take place when metal comes in contact
with Electrolyte .
Net current flow is zero but there exists a potential difference depends upon
the position of equilibrium and concentration of ions. That p.d. is known as
half-cell potential.
Over-potential
If there is a current between the electrode and electrolyte then half-cell
potential altered due to polarization is known as over-potential.
Electrical Activity
Measurement(cont.)

15. Electrical Activity
Measurement(cont.)
 Types of Electrodes:
 Perfectly Polarizable Electrodes
- only displacementdisplacement current, electrode behave like a capacitorcapacitor
example: noble metals like platinum Pt
 Perfectly Non-Polarizable electrode
- current passes freely across interface,
- no overpotentialoverpotential
examples:
- silver/silver chloride (Ag/AgCl),
- mercury/mercurous chloride

16. Blood Pressure
Blood pressure is an important signal in determining the functional integrity of
the cardiovascular system. Scientists and physicians have been interested in
blood pressure measurement for a long time.

17. Blood Pressure
Measurement
 Blood pressure measurement techniques are generally put into two
broad classes:
1) DIRECT TECHNIQUES
Direct techniques of blood pressure measurement, which are also
known as invasive techniques, involve a catheter to be inserted into
the vascular system.
2) INDIRECT TECHNIQUES
The indirect techniques are non-invasive, with improved patient
comfort and safety, but at the expense of accuracy.

18. Transducers for Blood Pressure
Measurement
Strain Gauges
Resistance is related to length and area of cross-section of the
resistor and resistivity of the material as
By taking logarithms and differentiating both sides, the equation
becomes
Dimension
al
piezoresistanc
e
Strain gage component can be related by poisson’s ratio as

19. Transducers for Blood Pressure
Measurement(cont.)
Gage Factor of a strain gage
G is a measure of
sensitivity
Think of this as a
Transfer Function!
⇒Input is strain
⇒ Output is dR
⇒Put mercury strain gauge around an arm or chest to measure force of muscle
contraction or respiration, respectively
⇒ Used in prosthesis or neonatal apnea detection, respectively
Strain Gauges

20. Transducers for Blood Pressure
Measurement(cont.)
Strain Gauges

21. Transducers for Blood Pressure
Measurement(cont.)
An inductor is basically a coil of
wire over a “core” (usually ferrous)
It responds to electric or magnetic
fields
A transformer is made of at least
two coils wound over the core: one
is primary and another is secondary
Primary Secondary Displacement Sensor
Inductors and tranformers work only for ac signals
Inductive Pressure Sensors ( LVDT)

22. Transducers for Blood Pressure
Measurement(cont.)
Capacitive Pressure Sensors
When there is difference in P1 & P2
then diaphragm moves toward low
pressure side and accordingly
capacitance varies. So, capacitance
becomes function of pressure and
that pressure can be measured by
using bridge ckt.
It can be used for blood pressure
measurent.

23. Transducers for Blood Pressure
Measurement(cont.)
Capacitive Pressure Sensors
Pressure
An example of a capacitive sensor is a pressure sensor.
In parts a, the thin sensor diaphragm remains parallel to the
fixed electrode and in part b, the diaphragm deflects under
applied pressure resulting in capacitance change

24. Transducers for Blood Pressure
Measurement(cont.)
The other pressure sensing approach, characterized by a
diaphragm in front of the fibre optic link, is based on the
light intensity modulation of the reflected light caused by
the pressure-induced position of the diaphragm.
Fibre-optic pressure sensor

25. Blood Flow
 A measure of the velocity of blood in a major vessel. In a
vessel of known diameter , this can be calibrated as flow
and is most successful accomplished in arterial vessels.
Used to estimate heart output and circulation. Requires
exposure of the vessel. Flow transducer surrounds
vessel. Methods of measurement include
 Electro-magnetic
 Ultrasonic principles
 Fibre-optic laserDopplerflowmetry

26. Blood Flow Measurement
 Based on Faraday’s law of induction that a conductor that moves
through a uniform magnetic field, or a stationary conductor placed
in a varying magnetic field generates e m f on the conductor:
 When blood flows in the vessel with velocity u and passes through the
magnetic field B, the induced emf e measured at the electrodes is.
∫ ⋅×=
L
de
0
LBu
For uniform B and uniform velocity profile
u, the induced emf is e=BLu. Flow can be
obtained by multiplying the blood velocity u
with the vessel cross section A.
Electromagnetic Flow meters

27. Blood Flow Measurement(cont.)
Electromagnetic Flow meter Probes
• Comes in 1 mm increments for
1 ~ 24 mm diameter blood vessels
• Individual probes cost $500 each
•Only used with arteries, not veins,
as collapsed veins during diastole
lose contact with the electrodes
• Needless to say, this is an
INVASIVE measurement!!!
• A major advantage is that it can
measure instantaneous blood
flow, not just average flow.

28. Blood Flow Measurement(cont.)
Ultrasonic Flow meters
 Based on the principle of measuring the time it takes for an
acoustic wave launched from a transducer to bounce off red
blood cells and reflect back to the receiver.
 All UT transducers, whether used for flowmeter or other
applications, invariably consists of a piezoelectric material,
which generates an acoustic (mechanical) wave when
excited by an electrical force (the converse is also true)
 UT transducers are typically used with a gel that fills the air
gaps between the transducer and the object examined

29. Blood Flow Measurement(cont.)
Ultrasonic Flow meters
The Doppler blood-flow measurement
Doppler blood flow detectors
operate by means of continuous
sinusoidal excitation. The
frequency difference calibrated
for flow velocity can be displayed
or transformed by a loudspeaker
into an audio output.

30. Blood Flow Measurement(cont.)
Fibre-optic laser Doppler flow metry
The basic scheme of fibre-optic laser
Doppler flow metry is illustrated in figure
. The light of a He–Ne laser is guided by
an optical fibre probe to the tissue or
vascular network being studied. The
light is diffusely scattered and partially
absorbed within the illuminated volume.
Light hitting moving blood cells
undergoes a slight Doppler shift. The
blood flow rate is derived by the
spectrum-analysis of the back-scattered
signal, which presents aflow-dependent
Doppler-shifted frequency.

31. Temperature
 Systematic Temperature:
A measure of the basic temperature of the complete organism.
Measured by thermometer, oral thermistor probe.
 Skin Temperature:
Measurement of the skin temperature at a specific part of body
surface. Measured by thermistors placed at surface of the
skin , infrared thermometer or thermograph.

32. Temperature Measurement
 Thermistors are made from
semiconductor material.
Generally, they have a negative
temperature coefficient (NTC), that is NTC
thermistors are most commonly used.
Ro is the resistance at a reference point
(in the limit, absolute 0).
Thermistor

33. Temperature Measurement(cont.)
Seebeck Effect
When a pair of dissimilar metals are joined at one end, and there is a temperature
difference between the joined ends and the open ends, thermal emf is generated,
which can be measured in the open ends.
This forms the basis of thermocouples.
In a bimetallic strip, each
metal has a different
thermal coefficient…this
results in electromagnetic
force/emf or bending of the
metals.
Thermocouples

34. Temperature Measurement(cont.)
Fiber Optics
Most of the light is trapped in the core, but if
the cladding is temperature sensitive (e.g. due
to expansion), it might allow some light to leak
through.
-> hence the amount of light transmitted would
be proportional to temperature
-> since you are measuring small changes in
light level, this sensor is exquisitely sensitive

35. Temperature Measurement(cont.)
Liquid-in-glass Thermometer
A common form of mercury-in-glass
is a solid-stem glass thermometer
shown in figure. When bulb comes in
contact with temperature then
mercury expands and gives direct
value on main scale.

36. Respiration sensors
The primary function of the respiratory system are to supply oxygen to the
tissues and remove carbon-dioxide from tissues. Several types of transducers
have been developed for measurement of respiration rate.
1)Strain Gauge type chest transducer
The transducer is held by an elastic band which goes around the chest. The
respiratory movements result in resistance change of the strain gauge element
connected in Wheatstone bridge. The bridge output varies with chest expansion
and yields signals corresponding to respiratory activity.
2) Thermistor
Air is warmed during its passage through the lungs and there is a detectable
temperature difference between inspired and expired air. Temperature change
can be measured by thermistor and it gives rate of change of resistance and
hence calibrated in terms of respiration rate.

37. Pulse sensors
Heart rate measurement is one of the very important parameters of the human
cardiovascular system. The heart rate of a healthy adult at rest is around 72 beats
per minute (bpm).
Basically, the device consists of an infrared transmitter LED and an infrared
sensor photo-transistor. The transmitter-sensor pair is clipped on one of the
fingers of the subject. The LED emits infrared light to the finger of the subject. The
photo-transistor detects this light beam and measures the change of blood volume
through the finger artery. This signal, which is in the form of pulses is then
amplified and filtered suitably and is fed to a low-cost microcontroller for analysis
and display

38. Pulse
Sensor(cont.)
The microcontroller counts the number of pulses over a fixed time interval
and thus obtains the heart rate of the subject. Several such readings are
obtained over a known period of time and the results are averaged to give a
more accurate reading of the heart rate. The calculated heart rate is displayed
on an LCD in beats-per-minute in the following format:
Rate = nnn bpm

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