1. 1. Compiled By: Prof. G B Rathod ET department-BVM College, Email: ghansyam.rathod@bvmengineering.ac.in Electrodes
2. 2. Outlines Electrode Theory Biopotential electrodes Biochemical transducers Outcomes References Questions 7/25/20152 BVM, ET
3. 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. 4. Electrode Theory 7/25/20154 BVM, ET
5. 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. 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. 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. 8. Biopotential electrodes 7/25/20158 BVM, ET
9. 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. 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. 11. Biopotential electrodes 7/25/201511 BVM, ET
12. 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. 13. Biopotential electrodes 7/25/2015BVM, ET13
14. 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. 15. Biopotential electrodes 7/25/2015BVM, ET15
16. 16. Biopotential electrodes 7/25/2015BVM, ET16
17. 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. 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. 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. 20. Biopotential electrodes 7/25/2015BVM, ET20
21. 21. Biopotential electrodes 7/25/2015BVM, ET21
22. 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. 23. Biopotential electrodes 7/25/2015BVM, ET23
24. 24. Biopotential electrodes 7/25/2015BVM, ET24
25. 25. Biopotential electrodes 7/25/2015BVM, ET25
26. 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. 27. Biopotential electrodes 7/25/2015BVM, ET27
28. 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. 29. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET29 Reference Electrode The pH electrode Blood Gas Electrodes Specific Ion ELectrodes
30. 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. 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. 32. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET32 Fig: Reference Electrode Basic configuration
33. 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. 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.
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36. 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. 37. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET37 Fig: diagram of Po2 electrode With platinum cathode showin Principle of operation
38. 38. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET38 Fig: combination of Pco2 And Po2 electrode
39. 39. BIOCHEMICAL TRANSDUCERS 7/25/2015BVM, ET39 Fig: Diagram showing Construction of flow- Through liquid membrane Specific ion electrode.
40. 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. 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. 42. References 7/25/2015BVM, ET42 Book: Biomedical instrumentation and measurements ,by L. Cromwell, F .Weibell, and E. Pfeiffer. PHI publication 2nd Edition
43. 43. Questions???? 7/25/2015BVM, ET43