1. BIO POTENTIAL ELECTRODES

2. ELECTRODES What is an electrode? Device that converts ‘ionic potentials’ into ‘electronic potentials’ They are employed to pick up signals from our body. They play the role of ‘transducers’

3. Electrode-Skin Interface Model Figure:A body-surface electrode is placed against skin, showing the total electrical equivalent circuit obtained in this situation. Each circuit element on the right is at approximately the same level at which the physical process that it represents would be in the left-hand diagram. Sweat glands and ducts Electrode Epidermis Dermis and subcutaneous layer RuRu ReRe E se E he RsRs RdRd CdCd EPEP RPRP CPCP CeCe Gel

4. Types of electrodes MICROELECTRODES: Used to measure near or within the cell SKIN SURFACE ELECTRODES: Used to measure from the surface of the skin NEEDLE ELECTRODES: Used to penetrate the skin

5. MICRO ELECTRODE They are normally used to measure the potential within a single cell. The microelectrodes are very small in diameter and so it will not damage the human cell. Microelectrodes are classified into Metallic & Non-Metallic

6. MICROELECTRODE

7. The metal microelectrodes are formed by electrolytically etching the tip of fine tungsten filament into a minute structure. ‘ELECTROPOINTING’. Bioelectric potentials requires two electrodes, the voltage measured is difference between potential of microelectrode & reference electrode

8. N = Nucleus C = Cytoplasm B A R mb R ma E mb E ma E mp RiRi ReRe C mb C ma C di C d2 RsRs CwCw C d1 Metal-electrolyteinterface Referenceelectrodemodel Lead wire capacitance Shaftcapacitance Electroderesistance Tissue fluid resistance Shankcapacitance Cellmembrane Cytoplasmresistance To amplifier

9. SIMPLIFIED EQUIVALENT CKT E mp Membrane and action potential C ma R ma C d + C w E ma - E mb E 0 A B

10. Area of reference electrode is many times greater than metal’s electrode tip(cross section is small),its impedance is negligible Impedance of microelectrode ∞ 1 area of tip Electrode output is coupled with amplifier Input impedance is not high ‘HIGH PASS FILTER’

11. MICROPIPET

12. It consists of glass micropipet whose diameter= 1 micrometer Pipet is filled with electrolyte 3M of KCl (usually). A thin flexible metal wire is inserted to stem of micropipet ClAg,Stainless steel or tungsten

13. SIMPLIFIED EQUIVALENT CIRCUIT

14. When micropipet is coupled with amplifier then membrane potential is coupled with High series resistance & moderate shunt capacitance Impedance of electrode places a limit on response time of circuit & it behaves like “LOW PASS FILTER”

15. SURFACE ELECTRODES The surface electrodes are used to measure the potential available from the surface of the skin and used to sense the potential from heart, brain and nerves. The smaller area surface electrodes are used to measure EEG,EMG potentials and the larger area surface electrodes are used to measure ECGpotentials. Depends on construction, the surface electrodes are classified into 1.Metal plate electrode 2.Suction cup electrode 3.Adhesive tape electrode 4.Multipoint electrode 5. Floating electrode

16. METAL PLATE ELECTRODE It is made up of Ag-AgCl (Silver-Silver Chloride). It is used to pick up ECG from the limb lead positions. It is fixed to the skin surface by means of conductive gel &rubber belt. Resistance value range 2 to 10 K ohms

17. METAL DISC ELECTRODE: It is made up of Ag-AgCl. It is used to pick up EEG from the scalp. It is fixed to the scalp by means of adhesive tape.

18. Disposable foam-pad electrode: It is made up of Ag-AgCl. It is used to pick up ECG or EEG for those patients with contagious skin diseases. It is fixed to the skin surface by means of adhesive tapes attached to the electrode

19. Suction Electrodes Well suited for attachment to flat surface body & to regions underlying tissue is soft. A paste is introduced into the cup, & only rim is in contact with the skin.

20. FLOATING ELECTRODE Principle of this electrode is to eliminate movement artifact by avoiding direct contact of metal with skin Conductive path is electrolyte paste, which forms electrolyte bridge Generally attached to skin by two adhesive rings, which adhere to both the plastic surface of electrode & skin

21. MULTIPOINT ELECTRODE Practical electrode used for ECG easurement. It contains ‘active points’ Low resistance is established

22. NEEDLE ELECTRODE Used to record peripheral nerve action potential (electroneurography) Needle resembles medicine dropper Consists of fine insulated wires with bare tips, which is in contact with muscle or tissue Reminder of wire is insulated Wire electrodes of copper or platinum are often used for EMG pick up

23. Metal-electrolyte interface takes place between the un-insulated tip of wire & electrolytes of the body. ‘unipolar electrode’ measures the potential at the point of contact with respect to reference. ‘bipolar electrode’ one wire is active electrode & other acts as reference

25. Simplified equivalent circuit of a Needle type EMG electrode pair and equivalent circuit of the input stage of an amplifier

26. DEPTH ELECTRODE Used to study the electrical activity of neurons in superficial layers of brain. It consists of teflon insulated platinum(90%)- iridum (10%)alloy wires, bounded to central supporting stainless steel wire which acts as indifferent electrode Electrode rests on subcortial nerve cells Area=0.5mm2 Used to inject medicine into brain Used to measure oxygen tension