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

1 the interface I To sense a signal a current I must flow !

2 the interface problem I To sense a signal a current I must flow ! But no electron e - is passing the interface!

3 metal cation No current What’s going on? leaving into the electrolyte

4 metal cation 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. leaving into the electrolyte

5 metal cation What’s going on? No current joining the metal

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

7 half-cell voltage No current

8 half-cell voltage No current metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au V h / Volt -3,0 negativ 0 0,223 positiv 1,68

9 electrode double layer No current ? ??

10 electrode double layer No current ? ?

11 electrode double layer No current ?

12 electrode double layer No current

13 current influence n withcurrent n with current flowing the half-cell voltage changes n n

14 current influence n withcurrent n with current flowing the half-cell voltage changes overpotentialpolarization: n this voltage change is called overpotential or polarization: n

15 current influence n withcurrent n with current flowing the half-cell voltage changes overpotentialpolarization: n this voltage change is called overpotential or polarization: n V p = V r + V c + V a activation, depends on direction of reaction concentration (change in double layer) ohmic (voltage drop)

16 polarizable electrode displacement capacitor n “perfectly” polarizable electrode: - only displacement current, electrode behave like a capacitor n

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

18 nonpolarizable electrode overpotential n “perfectly” nonpolarizable electrode: -current passes freely across interface, -no overpotential n

19 nonpolarizable electrode overpotential n “perfectly” nonpolarizable electrode: -current passes freely across interface, -no overpotential n examples: -silver/silver chloride (Ag/AgCl), -mercury/mercurous chloride (Hg/Hg 2 Cl 2 ) (calomel)

20 Question! n How is the current doing this to “pass freely”? Electrons can’t live in liquids! Can they?

21 chemical reactions silver / silver chloride

22 electrical behaviour equivalent circuit

23 electrical behaviour equivalent circuit ??

24 electrical behaviour equivalent circuit ?

25 electrical behaviour equivalent circuit

26 equivalent circuit electrode-electrolyte

27 body-surface electrode anatomy

28 again el. behaviour additional the skin: an additional interface! ??

29 again el. behaviour additional the skin: an additional interface! ?

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

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

32 expanded equiv. circuit dominating parts highlighted

33 total equivalent circuit simplified

34 results n high impedance Z E n high impedance Z E :... n changing half-cell voltage V h* n changing half-cell voltage V h* :...

35 high impedance n interference with main power-lines (!!) n n c

36 high impedance n interference with main power-lines (!!) n potential devider with R input n c

37 high impedance n interference with main power-lines (!!) n potential devider with R input n frequency dependant. c

38 changing half-cell voltage n influenced by local concentration n

39 changing half-cell voltage n influenced by local concentration n saturation of amplifier n

40 changing half-cell voltage n influenced by local concentration n saturation of amplifier n motion artefacts by changing the the gel-skin potential (V ep ). simulation:

41 body-surface electrode half-cell voltage drift simulated

42 body-surface electrode impedance locus diagram