Biopotential Biopotential ElectrodesElectrodes

IntroductionIntroduction

Electrical Contact pointElectrical Contact point Transducer Transducer Biopotential electrodesBiopotential electrodes

– Metal (Al, Cu, Fe, Ag,…..)Metal (Al, Cu, Fe, Ag,…..)– Non-metalNon-metal

metal electrolyte

M+

A-

e -I

Metal Electrolyte Metal Electrolyte InterfaceInterface

To sense a signala current II must flow !

metal electrolyte

M+

A-

e -I

The Interface ProblemThe Interface Problem

To sense a signala current II must flow !

But no electron e- ispassing the interface!?

Metal CationMetal Cation

No current

What’s going on?

leaving into the electrolyte

Metal Cation: Metal Cation: leaving into the leaving into the electrolyteelectrolyte

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: Metal cation: joining the joining the metalmetal

What’s going on?

No current

Metal Cation: Metal Cation: joining joining the metalthe 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 VoltageHalf-cell Voltage

No current

Half-cell VoltageHalf-cell VoltageNo current

metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au

Vh / Volt -3.0 negativ 0 0.223 positiv 1.68

Electrode Double Electrode Double LayerLayer

No current

? ? ?

Electrode Double Electrode Double LayerLayer

No current

? ?

Electrode Double Electrode Double LayerLayer

No current

?

Electrode Double Electrode Double LayerLayer

No current

Oxidation or reduction reactions at the electrode-electrolyte interface lead to a double-charge layer

Contact (Half Cell) Contact (Half Cell) PotentialPotential

•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 Measuring Half Cell PotentialPotential

Note: Electrode material is metal + salt or polymer selective membrane

Half Cell Potential (VHalf Cell Potential (Vhh))

Iron Iron -440 mV -440 mV LeadLead -126 mV-126 mV CopperCopper +337 mV+337 mV Platinum Platinum +1190 mV+1190 mV Compare to electrophysiological Compare to electrophysiological

Signals ???Signals ??? Two Similar electrodes ??? (Ag/Agcl Two Similar electrodes ??? (Ag/Agcl

5 mV and steel 5 mV and steel 100mV)100mV)

Ag/AgCl ElectrodeAg/AgCl Electrode

Fabrication of Ag/AgCl electrodes

1. Electrolytic deposition of AgCl

2. Sintered AgCl: process forming pellet electrodes

Electrolysis ProcessElectrolysis Process

eAgAg

AgClClAg