1. Biopotential electrodes A complex interface Summer School Timisoara 2002R. Hinz

2. the interface problem To sense a signala currentImust flow ! But no electron e- ispassing the interface!

3. 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.

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

5. metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au Vh / Volt -3,0negativ 00,223positiv1,68 half-cell voltage No current

6. electrode double layer No current

7. concentration (change in double layer) ohmic (voltage drop) current influence • withcurrent flowing the half-cell voltage changes • this voltage change is calledoverpotential orpolarization: Vp = Vr + Vc + Va activation, depends on direction of reaction

8. polarizable electrode • “perfectly” polarizable electrode:- only displacement current, electrode behave like a capacitor • example: noble metals like platinum Pt

9. nonpolarizable electrode • “perfectly” nonpolarizable electrode:- current passes freely across interface,- no overpotential • examples: - silver/silver chloride (Ag/AgCl),- mercury/mercurous chloride (Hg/Hg2Cl2) (calomel)

10. chemical reactions silver / silver chloride

11. electrical behaviour equivalent circuit

12. equivalent circuit electrode-electrolyte

13. again el. behaviour the skin: an additional interface!

14. total equivalent circuit simplified

15. results • high impedance ZE:... • changing half-cell voltage Vh*:...

16. high impedance • interference with main power-lines (!!) • potential devider with Rinput • frequency dependant. c

17. changing half-cell voltage • influenced by local concentration • saturation of amplifier • motion artefacts by changing the the gel-skin potential (Vep). simulation:

18. body-surface electrode half-cell voltage drift simulated