Presentation on theme: "Biopotential Electrodes. Introduction Electrical Contact point Electrical Contact point Transducer Transducer Biopotential electrodes Biopotential electrodes."— Presentation transcript:
Introduction Electrical Contact point Electrical Contact point Transducer Transducer Biopotential electrodes Biopotential electrodes –Metal (Al, Cu, Fe, Ag,…..) –Non-metal
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 -
Metal Electrolyte Interface I To sense a signal a current I must flow !
The Interface Problem I To sense a signal a current I must flow ! But no electron e - is passing the interface!
Metal Cation No current What’s going on? leaving into the electrolyte
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.
Metal cation: joining the metal What’s going on? No current
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
Half-cell Voltage No current
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
Electrode Double Layer No current ? ??
Electrode Double Layer No current ? ?
Electrode Double Layer No current ?
Electrode Double Layer No current Oxidation or reduction reactions at the electrode- electrolyte interface lead to a double-charge layer
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.
Measuring Half Cell Potential Note: Electrode material is metal + salt or polymer selective membrane
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)
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
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
Ag/AgCl Electrode Fabrication of Ag/AgCl electrodes 1. Electrolytic deposition of AgCl 2. Sintered AgCl: process forming pellet electrodes