Gtu topic 4_electrodes_Biomedical
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- 1. Compiled By: Prof. G B Rathod ET department-BVM College,
Email: ghansyam.rathod@bvmengineering.ac.in Electrodes
- 2. Outlines Electrode Theory Biopotential electrodes
Biochemical transducers Outcomes References Questions 7/25/20152
BVM, ET
- 3. Electrode Theory Electrodes: Devices that convert ionic
potentials into electronic potentials are called electrodes. The
interface of metallic ions in solution with their associated metals
results in an electrical potential that is called the electrode
potential. At the equilibrium, the double layer charge produce with
opposite sign. The hydrogen is taken as a reference electrode in
international agreement. The other potentials are taken by taking
hydrogen as a reference electrode. The electrodes potentials for
variety of other electrodes are listed in table. 7/25/20153 BVM,
ET
- 4. Electrode Theory 7/25/20154 BVM, ET
- 5. Electrode Theory When the ionic movement occurs and the new
potential developed at the membrane, the value of that potential
can be found out by Nernst Equation. Where R=gas constant T =
absolute temperature, degrees kelvin n=valence of the ion F=Faraday
constant C1,C2 = two concentrations of the ion on the two sides of
the membrane 1 1 2 2 ln C fRT E nF C f 7/25/20155 BVM, ET
- 6. Electrode Theory f1,f2=respective activity coefficients of
the ion on the two sides of the membrane This above f1 and f2 are
depend on such factors as the charges of all ions in the solution
and the distance between ions. The product of C1f1 of concentration
and its associated activity coefficient is called the activity of
the ion responsible for the electrode potential. The
metal-electrolyte interface developed and the potential generated.
7/25/20156 BVM, ET
- 7. Biopotential electrodes Basically three types. 1.
Microelectrodes: Electrodes used to measure bioelectric potentials
near or within a single cell. Skin surface electrodes: Electrodes
used to measure ECG, EEG, and EMG potentials from the surface of
the skin. Needle electrodes: Electrodes used to penetrate the skin
to record EEG potentials from a local region of the brain or EMG
potentials from a specific group of muscles. The equivalent circuit
of the electrode in upcoming figure. 7/25/20157 BVM, ET
- 8. Biopotential electrodes 7/25/20158 BVM, ET
- 9. Biopotential electrodes Two electrodes are require to do
measurements. If the same type of electrodes are used, the
potential difference is usually small and depends on the actual
difference of ionic potential between the two points of the body.
If the electrodes are different, the dc voltage generated which is
nothing but a electrode offset voltage. Which can cause an error in
the measurement. Some dc also produce in the same type of
electrodes we use. To reduce that error by choice of materials, or
by special treatment, such as coating the electrodes by somecontd
7/25/20159 BVM, ET
- 10. Biopotential electrodes .contd.electrolytic method to
improve stability. E.g : silver silver chloride electrode is very
stable prepared by electrolytically coating a piece of pure silver
with silver chloride. We can see the equivalent diagram of the use
of two electrodes for the biopotential measurements. In that the
impedance is varies according to the polarization which is a result
of direct current passing through the metal electrolyte interface.
Size and type of electrodes also affects the impedance . Higher the
size lower impedance. E.g surface electrodes.have 2 to 10 kohm,
where as small needle electrodes have much larger value.7/25/201510
BVM, ET
- 11. Biopotential electrodes 7/25/201511 BVM, ET
- 12. Biopotential electrodes Microelectrodes: Electrodes with
tips sufficiently small to penetrate a single cell in order to
obtain readings from within the cell. Basically two types: 1. Metal
, 2. Micropipet. Metal type are formed by electrolytically etching
the tip of a fine tungsten or stainless steel wire to the desired
size. Then wire is coated with the an insulating material.
Micropipet as shown in upcoming diagram. The problem with such
electrodes is that high impedance and for that amplifier with very
high impedance required. 7/25/201512 BVM, ET
- 13. Biopotential electrodes 7/25/2015BVM, ET13
- 14. Biopotential electrodes 7/25/2015BVM, ET14 Body Surface
Electrodes: The earliest bioelectric potential measurements used
immersion electrodes, which were buckets of saline solution into
which the subject placed his hands and feet, one bucket for each
extremity. Shown in upcoming image. After that improvements done
and plate electrodes introduced in 1917. These electrodes are
separated from the skin by cotton or felt pads socked in saline
solution. After that jelly introduced.
- 15. Biopotential electrodes 7/25/2015BVM, ET15
- 16. Biopotential electrodes 7/25/2015BVM, ET16
- 17. Biopotential electrodes 7/25/2015BVM, ET17 Another most
popular old type electrodes used today also is a suction cup
electrode shown in figure.
- 18. Biopotential electrodes 7/25/2015BVM, ET18 One difficulty
in using plate electrodes is that possibility of electrode slippage
or movement. This also occurs with the suction cup electrode after
a sufficient length of time. Number of attempts were made to
overcome this problem. All the preceding electrodes suffer from a
common problem. They are sensitive to movement, some to a greater
degree than others. The adhesive tape and nutmeg grater electrodes
reduce this movement artifact by limiting electrode movement and
reducing the interface impedance, but neither is satisfactory
insensitive to movement.
- 19. Biopotential electrodes 7/25/2015BVM, ET19 A new type of
electrode, the floating electrode, was introduced in varying forms
by several manufacturers. This principle of this electrode is to
practically eliminate movement artifact by avoiding any direct
contact of the metal with the skin. The only conductive path
between metal and skin is the electrolyte paste or jelly. Floating
electrodes are generally attached to the skin by means of two sided
adhesive rings. ECG measurement for long time can make some
problem.
- 20. Biopotential electrodes 7/25/2015BVM, ET20
- 21. Biopotential electrodes 7/25/2015BVM, ET21
- 22. Biopotential electrodes 7/25/2015BVM, ET22 Various types of
disposable electrodes have been introduced in recent years to
eliminate the requirement of cleaning and care after each use.
Special types of have been developed for other applications. For
example, a special ear-clip electrode was developed for use as a
reference electrode for EEG measurements. Scalp surface electrodes
for EEG are usually small disks about 7 mm in diameter or small
solder pellets that are placed on the cleaned scalp, using an
electrolyte paste.
- 23. Biopotential electrodes 7/25/2015BVM, ET23
- 24. Biopotential electrodes 7/25/2015BVM, ET24
- 25. Biopotential electrodes 7/25/2015BVM, ET25
- 26. Biopotential electrodes 7/25/2015BVM, ET26 Needle
Electrodes: To reduce interface impedance and, consequently,
movement artifacts, some electroencephalographers use smalls
subdermal needles to penetrate the scalp for EEG measurements. In
animal research longer needles are actually inserted into the brain
to obtain localized measurement of potentials from a specific part
of the brain. Sometimes a special instrument, called stereotaxic
instrument, is used to hold the animals head and guide the
placement of electrodes.
- 27. Biopotential electrodes 7/25/2015BVM, ET27
- 28. Biopotential electrodes 7/25/2015BVM, ET28 Needle
electrodes for EMG consist merely of fine insulated wires, placed
so that their tips are in contact with the nerve muscle. Or other
tissue from which the measurement is made. Wire electrodes of
copper or platinum are often used for EMG pickup from specific
muscles. A single wire inside the needle serves as a unipolar
electrode, If a two wire placed inside the needle, the measurement
is called bipolar and provide a very localized measurement between
the two wire tips.
- 29. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET29 Reference
Electrode The pH electrode Blood Gas Electrodes Specific Ion
ELectrodes
- 30. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET30 Reference
Electrode: Normally Hydrogen is used as a reference electrode.
These electrodes make use of the principle that an inert metal,
such as platinum, readily absorbs hydrogen gas. Unfortunately, the
hydrogen electrode is not sufficiently stable to serve as a good
reference electrode. Measurement of electrochemical concentration
simply requires a change of potential proportional to a change in
concentration. Two types: silver-silver chloride and the
calomel
- 31. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET31 The
silver-silver chloride electrode used as a reference in
electrochemical measurements utilizes the same type of interface
described before. In chemical transducer silver chloride side of
the interface is connected to the solution by an electrolyte bridge
which is filling solution KCl. The reference electrode with 0.01
mole solution, potential is 0.343 V and for 1.0 mole solution,
potential is 0.236 V The another is calomel electrode which is also
called mercurous chloride same as a Silver-silver chloride. 0.01
mole, potential will be 0.388 V and 3.5
- 32. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET32 Fig: Reference
Electrode Basic configuration
- 33. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET33 The pH Electrode
To know chemical balance in the body, pH of the blood and other
fluids are very important. Equation of pH is pH is a measure of the
acid base balance of a fluid. A natural solution has a pH of 7.
Lower pH numbers indicate acidity, whereas higher pH values define
a basic solution. Most human body fluids are slightly basic. The pH
of normal arterial blood ranges between 7.38 and 7.42. The pH of
venous blood is 7.35, because of the extra CO2. 10 10 1 log [ ] log
[ ] pH H H
- 34. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET34 In the
measurement of pH and in any electrochemical measurement, each of
the two electrode required to obtain the measurement is called half
cell and its sometimes called the half cell potential. The glass
electrodes quite adequate for pH measurements in physiological
range(around pH 7). Special hydroscopic glass that readily absorbs
water provides the best pH response.
- 35. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET35
- 36. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET36 Blood Gas
Electrodes: One of the important physiological chemical
measurements is pressure of oxygen and carbon dioxide in the blood.
The effectiveness of both the respiratory and cardiovascular
systems is reflected in these important parameters. The diagram of
Po2 electrode with platinum cathode will be in upcoming slide which
shows a principle of operation.
- 37. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET37 Fig: diagram of
Po2 electrode With platinum cathode showin Principle of
operation
- 38. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET38 Fig: combination
of Pco2 And Po2 electrode
- 39. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET39 Fig: Diagram
showing Construction of flow- Through liquid membrane Specific ion
electrode.
- 40. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET40 Fig: specific
ion electrodes With pH glass electrode (1) Sodium ion (2) Cationic
electrode (3) pH glass (4) ammonia
- 41. Outcomes 7/25/2015BVM, ET41 From this unit, we come to know
about various types of transducers used for the physiological
potential measurements. The real time use and the benefit with some
major and minor artifacts also discussed. Biochemical transducers
and related to pH measurement is also focused and shows it own
stability related advantages in various measurements.
- 42. References 7/25/2015BVM, ET42 Book: Biomedical
instrumentation and measurements ,by L. Cromwell, F .Weibell, and
E. Pfeiffer. PHI publication 2nd Edition
- 43. Questions???? 7/25/2015BVM, ET43