Biopotential electrode

1. CHAPTER 1
BIOPOTENTIAL ELECTRODES
ENGR. HJH. WAN ROSEMEHAH BINTI WAN OMAR
PSA-JKE

2. Contents
1.0 Introduction
1.1 Principles of Electrode-electrolyte Interface
1.2 Polarization Phenomena
1.3 Polarizable & Non-Polarizable Electrodes
1.4 Electrode behavior and circuit models
1.5 Ag/AgCl Electrode
 Skin-electrode Interface
 Body-surface & Internal Electrodes

3. 1.0 introduction
 Electrode - interface between the body and electronic measuring devices
is required to measure and record bio-potential
 Biopotential Electrode have metalelectrolyte interface.
 Measurement – signal current has to flow through the circuit
 Electrode must have capability of conducting across the interface
between the electronic measuring circuit and the body.
 The electrode actually carries out a transducing
function where current flows in the body by
ions, along with the current flow in the
electrode by electrons
Biopotential generated in the body are ionic potential produced by ionic
current flow

4. Biopotentials Measurement

5. Biopotentials Characteristics

6. Electrode placement

7. Electrode Type

8. ECG measurement

9. 1. Electrons moving in a
direction opposite (The current
crosses it from left to right.
2. Cation , C+ moving the same
direction as the current
3. Anion, A moving opposite
derection of the current
The electrolyte solution
containing cations of the
electrode metal C+ and anions
A–.
1.1 Electrode–electrolyte interface
Figure 1 : electrode-electrolyte interface
Observe the figure 1

10. 1.1 Electrode–electrolyte interface
 Observe the following diagram:
1. no free electrons for charge to cross the interface
- no free electrons in the electrolyte
- No free cations or anions in the electrode
Therefore, chemical reaction occur at the interface
where n is the valence of C and m is valence of A
Figure : electrode-electrolyte interface

11. metal electrolyte
M+
A-
e-
I
Metal Electrolyte Interface
To sense a signal
a current I must flow !

12. metal electrolyte
M+
A-
e-
I
The Interface Problem
To sense a signal
a current I must flow !
But no electron e- is
passing the interface!
?

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

14. 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.

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

16. 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

17. Half-cell Voltage
No current

18. Half-cell Voltage
No current
metal: Al Fe Pb H Ag/AgCl Cu Ag Pt Au
Vh / Volt -3.0 negativ 0 0.223 positiv 1.68

19. Electrode Double Layer
No current
? ? ?

20. Electrode Double Layer
No current
? ?

21. Electrode Double Layer
No current
?

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

23. 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.

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

25. Half Cell Potential (Vh)
 Iron -440 mV
 Lead -126 mV
 Copper +337 mV
 Platinum +1190 mV
 Compare to electrophysiological Signals ???
 Two Similar electrodes ??? (Ag/Agcl  5 mV and steel 100mV)

26. summary
 The net current that crosses the interface, from the
electrode to the electrolyte consists of:
i. Electrons moving in the opposite direction to that of the
the currents in the electrode.
ii. Cations moving in the same direction as the current.
iii. Anions moving in the opposite direction to that of
current in the electrolyte

27. Cont’
 For charge to cross the interface,
i. There are no free electrons in the electrolyte.
ii. There are no free cations or anions in the electrode.
 Something must occur at the interface that transfers
the charge between these carriers.

28. Cont’
 What happens are actually chemical reaction at the
interface:
Where,
n : Valence of C
m : Valence of A



 ne
C
C n



 me
A
Am

29. Cont’
 Reactions involving cations:
i. Electrode is made of some atoms of the same material
as the cations.
ii. Material in the electrode at the interface can oxidize to
form cations and more free electrons.
iii. Cations discharged into the electrolyte.
iv. Electrons remain as charge carrier in the electrode.

30. Cont’
 Reactions involving anions:
i. Anions coming to electrode-electrolyte interface can be
oxidized to neutral atoms.
ii. This gives one or more free electrons to the electrode.
 Both reactions are reversible but not entirely.
 When no current crosses, these reactions still occur.

31. Cont’
 Rate of oxidation equals to rate of reduction reaction.
 So net transfer of charge across the interface is zero.
i. When current flows from electrode to electrolyte,
oxidation reaction dominates.
ii. When current flows from electrolyte to electrode,
reduction reaction dominates

32. Half-Cell Potential
 Half-cell potential determined by:
i. Metal involved.
ii. Concentration of its ions in the solution.
iii. Temperature.
iv. Other second-order factors. (cannot measured without a second
electrode)
 Knowledge of half-cell potential is important to understand
the behavior of biopotential electrode.
 half cell potential of the standard hydrogen electrode has been
arbitrarily set to zero.

34. 1.2 Polarization
 If there is a current between the electrode and electrolyte,
the observed half cell potential is often altered due to
polarization.
 Difference is due to polarization of the electrodes.
 The observed difference between the observed half-cell
potential and the zero-current half-cell potential is known as
overpotential.
 Components of overpotentials:
i. Ohmic overpotential
ii. Concentration overpotential
iii. Activation overpotential

35. Polarization Potential
 These 3 components are additive.
Where,
Vp : Polarization potential
Vr : Ohmic overpotential
Vc : Concentration overpotential
Va : Activation overpotential
a
c
r
p V
V
V
V 



36. 1.3 Polarizable and Non-Polarizable
 Theoretically:
i. Perfectly polarizable electrode (STIMULATION)
ii. Perfectly non-polarizable electrode (RECORDING)
 Classification refers to what happen at the electrode
when current passes between it and the electrolyte.
 In reality however, it is impossible to fabricate these
perfect electrodes
Example: Ag-AgCl is used in recording while Pt is use in stimulation

37. i. Polarizable Electrode
 No equal charge crosses the electrode-electrolyte
interface when current is applied.
 In reality, current exists but it is only a displacement
current where the electrode behaves like a capacitor.
 In practical, platinum electrode comes close to
behaving as perfectly polarizable electrode.
 STIMULATION

38.  The current passing between the electrodes are
primarily changes the ionic concentration at the
interface.
 Majority of the overpotential observed are from
concentration overpotential, Vc.
 These electrode shows strong capacitive effect.

39. ii. Non-Polarizable Electrode
 Current passes freely across the electrode-electrolyte
interface, requiring no energy to make the transition.
 Thus no overpotential exists for a perfectly non-
polarizable electrode.
 In practical, the Ag/AgCl electrode behaves closest to
perfectly non-polarizable electrode.
 RECORDING

40. 1.4 Electrode behavior and circuit
models

41. Ehc is the half-cell potential,
Rd and Cd make up the impedance associated with the electrode-electrolyte
interface and polarization effects,
Rs is the series resistance associated with interface effects and due to
resistance in the electrolyte.
Equivalent circuit for a biopotential electrode in
contact with an electrolyte
43

42. Ag/AgCl Electrode
 A member of a class of electrode which consists of a
metal coated with a layer of slightly soluble ionic
compound of that metal with a suitable anion.
 Easily fabricated in laboratories.
 Structure immersed in electrolyte of anions with
relatively high concentration

43. Ag/AgCl Electrode

44.  Silver metal base attached with insulated lead wire is
coated with ionic compound AgCl.
 AgCl is very slightly soluble in water, so it remains
stable.
 Electrode immersed in electrolyte bath containing Cl-.

45.  For best result, AgCl in solution should be highly
concentrated.
 Governed by the following chemical reactions:
Where,
Ag+ : Silver ion
Cl- : Chloride ion



 e
Ag
Ag
AgCl
Cl
Ag 
 


46.  First, oxidation of silver atoms at the surface to silver
ions in the solution at the interface.
 Second, the ions combine with Cl- to form ionic
compound AgCl.
 AgCl is slightly soluble, thus are deposited on the
electrode surface.

47. 1.5 The electrod skin surface
Skin Structure

48.  Epidermis consist of 3 layers:
i. Stratum germinativum
ii. Stratum granulosum
iii. Stratum corneum
 Dead cells have different electrical properties than live
cells.
 Deeper layer contains vascular and nervous
components.

49. Electrode-Skin Interface
 Cl- electrolyte gel or cream used to maintain good skin
contact with the electrode.
 Rs exist due to interface effect of the gel between the
electrode and the skin.
 Stratum corneum is a semipermeable membrane to
ions.
 If there is difference in ionic concentration, there is
potential Ese.

50.  Epidermal layer behaves as a parallel RC circuit.
 For 1cm2, skin impedance reduces from 200kΩ at 1Hz
to 200Ω at 1MHz.
 The dermis and subcutaneous layer acts as pure
resistance Ru.
 DC potentials are also generated which are neglible.
 The impedance created by the epidermal layer can be
reduced through abrasion of the skin, which
theoretically shorts the parallel RC circuit.

52. 1.5 Motion Artifact
 In polarizable electrode, the ionic layer distribution at
the interface changes when the electrode moves with
respect to the electrolyte.
 Results in momentary change of half-cell potential until
equilibrium is established.
 However, motion artifact is minimal for non-polarizable
electrode.

53.  In addition, the motion artifact can also be caused by
variations in the electrolyte gel-skin potential, Ese.
 This can be reduced by abrasion of the stratum
corneum.
 This also reduces the skin impedance of the epidermal
layer.

54. 1.6 Body-Surface Electrodes

55. Body-Surface Electrodes
Suction electrode

56.  Floating electrodes

57.  Flexible electrodes

58. 1.7 Internal Electrodes
 Neadle & wired for
Percutaneous electrodes
(a) Insulated needle electrode,
(b) Coaxial needle electrode,
(c) Bipolar coaxial electrode,
(d) Fine-wire electrode connected
to hypodermic needle, before
being inserted,
(e) Cross-sectional view of skin
and muscle, showing fine-wire
electrode in place,
(f) Cross-sectional view of skin
and muscle, showing coiled
fine-wire electrode in place

59.  Fetal electrodes
(a) Suction electrode,
(b) Cross-sectional view of
suction electrode in
place, showing
penetration of probe
through epidermis,
(c) Helical electrode, that is
attached to fetal skin by
corkscrew-type action.

60. Implantable electrodes
(a) Wire-loop electrode,
(b) platinum-sphere cortical-surface
potential electrode,
(c) Multielement depth electrode

61. One-dimensional plunge electrode
array (after Mastrototaro et al.,
1992),
Two-dimensional array, and
Three-dimensional array (after
Campbell et al., 1991).
Examples of microfabricated electrode arrays

62. Capacitance per unit length
0 = dielectric constant of free space
r = relative dielectric constant of insulation material
D = diameter of cylinder consisting of electrode plus
insulation
D = diameter of electrode
L = length of shank
64
The structure of a metal microelectrode for
intracellular recordings

63. (a) Metal-filled glass micropipet.
(b) Glass micropipet or probe, coated with metal film.
65
Structures of two supported metal microelectrodes

64. (a) Section of fine-bore glass capillary,
(b) Capillary narrowed through heating and stretching,
(c) Final structure of glass-pipet microelectrode.
66
A glass micropipet electrode filled with an electrolytic
solution

65. (a) Beam-lead multiple electrode. (Based on Figure 7 in K. D. Wise, J. B. Angell, and A.
Starr, “An Integrated Circuit Approach to Extracellular Microelectrodes.” Reprinted with
permission from IEEE Trans. Biomed. Eng., 1970, BME-17, pp. 238–246.)
(b) Multielectrode silicon probe after Drake et al.
(c) Multiple-chamber electrode after Prohaska et al.
(d) Peripheral-nerve electrode based on the design of Edell.
67
Different types of microelectrodes fabricated using microelectronic
technology

66. (a) Electrode with tip placed within
a cell, showing origin of
distributed capacitance,
(b) Equivalent circuit for the
situation in (a),
(c) Simplified equivalent circuit.
(From L. A. Geddes, Electrodes and the
Measurement of Bioelectric Events,
Wiley-Interscience, 1972. Used with
permission of John Wiley and Sons,
New York.)
68
Equivalent circuit of metal microelectrode

67. (a) Electrode with its tip placed within a
cell, showing the origin of distributed
capacitance,
(b) Equivalent circuit for the situation in
(a),
(c) Simplified equivalent circuit. (From L.
A. Geddes, Electrodes and the Measurement of
Bioelectric Events, Wiley-Interscience, 1972.
Used with permission of John Wiley and Sons,
New York.)
69
Equivalent circuit of glass micropipet microelectrode

68. (a) Constant-current
stimulation,
(b) Constant-voltage
stimulation.
70
Current and voltage waveforms seen with electrodes used for electric
stimulation

69. 71
Simplified equivalent circuit of a Needle type EMG electrode
pair and equivalent circuit of the input stage of an amplifier
Needle type EMG electrode

70. EXAMPLES 1
Figure shows equivalent circuit of a biopotential electrode. A pair of these electrodes
are tested in a beaker of physiological saline solution. The test consists of measuring
the magnitude of the impedance between the electrodes as a function of frequency via
low-level sinusoidal excitation so that the impedances are not affected by the current
crossing the electrode–electrolyte interface. The impedance of the saline solution is
small enough to be neglected. Sketch a Bode plot (log of impedance magnitude versus
log of frequency) of the impedance between the electrodes over a frequency range of 1
to 100,000 Hz.

71. Assume Figure in previous slide models
both electrodes of the pair.
The low corner frequency is
Fc = 1/(2RC) = 1/(2·20 kW·100 nF)
= 80 Hz.
The high corner frequency is
Fc = 1/(2 RC)
= 1/(2·20 kW||300 W·100 nF)
= 5380 Hz.
The slope between the two corner
frequencies is –1 on a log-log plot

72. Examples 2
We want to develop an electrical model for a specific biopotential
electrode studies in the laboratory. The electrode is characterized by
placing it in a physiological saline bath in the laboratory, along with
an Ag/AgCl electrode having a much greater surface area and a
known half-cell potential of 0.233 V. The dc voltage between the two
electrodes is measured with a very-high-impedance voltmeter and
found to be 0.572 V with the test electrode negative The magnitude of
the impedance between two electrodes is measured as a function of
frequency at very low currents; it is found to be that given in Figure
in slide 12. From these data, determine a circuit model for the
electrode.

73. Half cell potential of the test electrode Ehc = 0.223 V – 0.572 = -0339 V
At frequencies greater than 20 kHz Cd is short circuit. Thus Rs = 500 Ω = 0.5 kΩ,
At frequencies less than 50 Hz Cd is open circuit. Thus Rs + Rd = 30 kΩ. Thus
Rd = 30 kΩ - Rs = 29.5 kΩ
Corner frequency is 100 Hz. Thus
Cd = 1/(2πf Rd) = 1/(2π100×29500) = 5.3×10-8 F = 0.53×10-9 F = 0.53 nF

74. DRAW THE SIGNAL

75. A pair of biopotential electrodes are implanted in an animal to measure the electrocardiogram for a radiotelemetry
system. One must know the equivalent circuit for these electrodes in order to design the optimal input circuit for the
telemetry system. Measurements made on the pair of electrodes have shown that the polarization capacitance for the
pair is 200 nF and that the half-cell potential for each electrode is 223 mV. The magnitude of the impedance between
the two electrodes was measured via sinusoidal excitation at several different frequencies. The results of this
measurement are given in the accompanying table. On the basis of all of this information, draw an equivalent circuit
for the electrode pair. State what each component in your circuit represents physically, and give its value.
77
Example

76. Solution
78
The 600 W is the tissue impedance plus the electrode/electrolyte high-frequency interface
impedance.
The 19400 W is the electrode/electrolyte low-frequency interface impedance.
The 200 nF is the electrode/electrolyte interface capacitance.
The 223 mV is the electrode/electrolyte polarization voltage.

77. Design and Control of Devices for Human-Movement Assistance
Recapping
Nonpolarized electrodes – allow to current to pass freely in electrode-electrolyte
interface. Their HCP is near to 0.
Silver-silver chloride electrodes (Ag/AgCl)
Equivalent circuit of a
biopotential electrode:
Scheme of the contour
“electrode-amplifier”
The input signal to the amplifier consists of 5 components: (1)Desired biopotential, (2) Undesired
biopotentials, (3) A power line interference signal and its harmonics, (4) Interference signals
generated by the tissue-electrode interface, (5) Noise.

78. Transmission of the signals between the ECG transmitter and the relay transmitter (transmission of the
signal in the human body):
Tissue equivalent circuit

79. Equivalent circuit of the tissue-electrodes contour

80. Further Reading…
1. Webster, J.G. (1997). Medical Instrumentation:
Application and Design. 3rd Ed., Wiley.
 Chapter 5

81. Sample question

82. Summary
 Electrodes in Biomedical Instrumentation
 Electrodes are devices that convert ionic potentials into electronic
potentials. The type of electrode used for the measurements depends on the
anatomical location of the bioelectric event to be measured. In order to
process the signal in electronic circuits, it will be better to convert ionic
conduction into electronic conduction. So simply bio-electrodes are a class of
sensors that transdues ionic conduction into electronic conduction. The
purpose of bio-electrodes is to acquire bioelectrical signals such as ECG, EMG,
EEG etc.
Electrodes are mainly classified into two. They are perfectly polarized
electrodes and perfectly non-polarized electrodes. There are a wide variety
of electrodes which can be used to measure bioelectric events. The three
main classes of electrodes are Microelectrodes, Body Surface electrodes and
Needle electrodes.