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Biopotential Electrodes. Introduction Electrical Contact point Electrical Contact point Transducer Transducer Biopotential electrodes Biopotential electrodes.

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Presentation on theme: "Biopotential Electrodes. Introduction Electrical Contact point Electrical Contact point Transducer Transducer Biopotential electrodes Biopotential electrodes."— Presentation transcript:

1 Biopotential Electrodes

2 Introduction Electrical Contact point Electrical Contact point Transducer Transducer Biopotential electrodes Biopotential electrodes –Metal (Al, Cu, Fe, Ag,…..) –Non-metal

3 Electrode – Electrolyte Interface Electrode Electrolyte (neutral charge) M+ and A- in solutionM M M A-A- A-A- M+M+ M+M+ e-e- e-e- Current flow M+ : CationA- : Anione- : electron M+M+ e-e- M  M + + e - A -  A + e -

4 Metal Electrolyte Interface I To sense a signal a current I must flow !

5 The Interface Problem I To sense a signal a current I must flow ! But no electron e - is passing the interface!

6 Metal Cation No current What’s going on? leaving into the electrolyte

7 Metal Cation: leaving into the electrolyte No current One atom M out of the metal is oxidized to form one cation M + and giving off one free electron e - to the metal.

8 Metal cation: joining the metal What’s going on? No current

9 Metal Cation: joining the metal One cation M + out of the electrolyte becomes one neutral atom M taking off one free electron from the metal. No current

10 Half-cell Voltage No current

11 Half-cell Voltage No current metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au V h / Volt -3.0 negativ positiv 1.68

12 Electrode Double Layer No current ? ??

13 Electrode Double Layer No current ? ?

14 Electrode Double Layer No current ?

15 Electrode Double Layer No current Oxidation or reduction reactions at the electrode- electrolyte interface lead to a double-charge layer

16 Contact (Half Cell) Potential Depends on: The metal, Concentration of ions in solution and Temperature. Half cell potential cannot be measured without a second electrode. The half cell potential of the standard hydrogen electrode has been arbitrarily set to zero.

17 Measuring Half Cell Potential Note: Electrode material is metal + salt or polymer selective membrane

18 Half Cell Potential (V h ) Iron -440 mV Iron -440 mV Lead-126 mV Lead-126 mV Copper+337 mV Copper+337 mV Platinum mV Platinum mV Compare to electrophysiological Signals ??? Compare to electrophysiological Signals ??? Two Similar electrodes ??? (Ag/Agcl  5 mV and steel  100mV) Two Similar electrodes ??? (Ag/Agcl  5 mV and steel  100mV)

19 Polarization If there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization. Overvoltage Difference bet. observed and zero-current half cell potentials Resistance Current changes resistance of electrolyte and thus, a voltage drop results. Concentration Changes in distribution of ions at the electrode- electrolyte interface Activation The activation energy barrier depends on the direction of current and determines kinetics

20 Polarizable and Non-Polarizable Electrodes Perfectly Polarizable Electrodes No actual charge crosses the electrode-electrolyte interface when a current is applied. (e.g Platinum electrode) Perfectly Non-Polarizable Electrode Current passes freely across the electrode-electrolyte interface. These electrodes see no overpotentials. (e.g. Ag/AgCl Electrode) Example: Ag-AgCl is used in recording while Pt is use in stimulation

21 Ag/AgCl Electrode Fabrication of Ag/AgCl electrodes 1. Electrolytic deposition of AgCl 2. Sintered AgCl: process forming pellet electrodes

22 Electrolysis Process


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