Bioinstrument

Sahand University of Technology

Lecture 3
Biopotential Electrodes

Dr. Shamekhi
Summer 2016
 Introduction
• In order to measure and record potentials (currents) in the
body, it is necessary to provide some interface between the
body and the electronic measuring apparatus.
• Current flows in the measuring circuit for at least a fraction
of the period of time over which the measurement is
made.
• Biopotential electrodes is a transducer that convert the
body ionic current in the body into the traditional
electronic current flowing in the electrode.
• Current is carried in the body by ions, whereas it is carried
in the electrode and its lead wire by electrons.
Electrode change an ionic current into an electronic current
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 Electrode-Electrolyte Interface

Oxidation reaction causes atom to lose electron

Reduction reaction causes atom to gain
electron

Oxidation is dominant when current flow from electrode to

electrolyte, and reduction dominate when the current flow is the
opposite.
Oxidation Reduction

anion

cation

Current flow Current flow

Dr. Shamekhi, Sahand University of Technology n+ − 3
C →C n+
+ ne −
C ←C + ne
 Half-Cell Potential
Half-Cell potential is determined by
-Metal involved
-Concentration of its ion in solution
-Temperature
-And other second order factor

Certain mechanism separate charges at the metal-electrolyte interface results in

one type of charge is dominant on the surface of the metal and the opposite
charge is concentrated at the immediately adjacent electrolyte.
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Half-Cell Potential
Half-cell potential for common
electrode materials at 25 oC

Electrochemists have adopted the Half-Cell

potential for hydrogen electrode to be
zero. Half-Cell potential for any metal
electrode is measured with respect to the
hydrogen electrode. (Why?)

Standard Hydrogen electrode

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 Polarization
Half cell potential is altered when there is current flowing in the electrode.

Overpotential is the difference between the observed half-cell potential with

current flow and the equilibrium zero-current half-cell potential.

Mechanism Contributed to overpotential

-Ohmic overpotential: voltage drop along the path of the current, and current
changes resistance of electrolyte and thus, a voltage drop does not follow ohm’s law.

- Concentration overpotential: Current changes the distribution of ions at the

electrode-electrolyte interface

- Activation overpotential: current changes the rate of oxidation and reduction.

Since the activation energy barriers for oxidation and reduction are different, the net
activation energy depends on the direction of current and this difference appear as
voltage.
V p = VR + VC + VA
Note: Polarization and impedance of the electrode are two of the most important
electrode properties to consider.
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 Half Cell Potential and Nernst Equation
When two ionic solutions of different concentration are separated by
semipermeable membrane, an electric potential exists across the
membrane. RT  a1 
E=− ln  
nF  a2 

a1 and a2 are the activity of the ions on each side of the membrane.
Ionic activity is the availability of an ionic species in solution to enter
into a reaction.
Note: ionic activity most of the time equal the concentration of the ion
For the general oxidation-reduction reaction
αA + βB ↔ γC + δD + ne −
The Nernst equation for half cell potential is
RT  aCγ aDδ 
E=E + 0
ln  α β 
nF  a A aB 
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 Polarizable and Nonpolarizable Electrodes

Perfectly Polarizable Electrodes

Electrodes in which no actual charge crosses the electrode-electrolyte
interface when a current is applied. The current across the interface is
a displacement current and the electrode behaves like a capacitor.
Overpotential is due concentration. Example : Platinum electrode
Perfectly Non-Polarizable Electrode
Electrodes in which current passes freely across the electrode-
electrolyte interface, requiring no energy to make the transition.
These electrodes see no overpotentials. Example: Ag/AgCl Electrode

Example: Ag-AgCl is used in recording while Pt is used in stimulation

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 The Silver/Silver Chloride Electrode
Advantage of Ag/AgCl is that it is stable in liquid that has large
quantity of Cl- such as the biological fluid.
For biological fluid where Cl-
ion is relatively high
aCl − ≈ 1

E=E 0
Ag +
RT
nF
[ ]
ln a Ag +

K s = a Ag + × aCl − = 10 −10
is solubility product
RT  K s 
E = E Ag +
0
ln  
Performance of this Ag ↔ Ag + + e − nF  aCl − 
electrode
Ag + + Cl − ↔ AgCl ↓
constant
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 Electrode Behavior and Circuit Models
Advantages: metal + - Electrolyte
–Low Noise (vs. Metal Electrodes) esp. ECG + -
–Biocompatible + -
The characteristic of an electrode is + -
-Sensitive to current density + -
- waveform and frequency dependent + -

Rd and Cd make up the

impedance associated with
electrode-electrolyte interface
and polarization effects. Rs is
associated with interface effects
and due to resistance in the
electrolyte.
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 The Electrode-Skin Interface

Transparent electrolyte gel containing Cl- is used to maintain good

contact between the electrode and the skin.

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 The Electrode-Skin Interface
For 1 cm2, skin impedance
reduces from approximately
200KΩ at 1Hz to 200Ω at 1MHz.

A body-surface electrode is placed against skin, showing the total

electrical equivalent circuit obtained in this situation. Each circuit
element on the right is at approximately the same level at which the
physical process that it represents would be in the left-hand diagram.
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 Motion Artifact
When polarizable electrode is in contact with an electrolyte, a double
layer of charge forms at the interface. Movement of the electrode will
disturb the distribution of the charge and results in a momentary
change in the half cell potential until equilibrium is reached again.
Motion artifact is less minimum for nonpolarizable electrodes.

Signal due to motion has low frequency so it can be filtered out when
measuring a biological signal of high frequency component such as
EMG or axon action potential. However, for ECG, EEG and EOG whose
frequencies are low it is recommended to use nonpolarizable
electrode to avoid signals due to motion artifact.

Must be considered:
– good adhesive connection to skin
– skin cleaning
– floating electrode
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 Metal-Plate Electrodes
•German silver (a nickel-silver alloy)
•Before it is attached to the body, its
concave surface is covered with electrolyte gel
•Motion Artifacts
•This structure can be used as
a chest electrode for recording the ECG or in
cardiac monitoring for long-term
recordings.
•Electrodes used in monitoring EMGs or EEGs
are generally smaller in diameter than those
used in recording ECGs.
•(b) The thinness of the foil allows it
to conform to the shape of the body surface.
Also, because it is so thin, the
cost can be kept relatively low.

Body-surface biopotential electrodes (a) Metal-

plate electrode used for application to limbs. (b)
Metal-disk electrode applied with surgical tape.

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 Disposable Foam-Pad Electrodes

Disposable foam-pad electrodes, often used with electrocardiograph monitoring apparatus.

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 Suction Electrodes

A metallic suction electrode is often used as a precordial electrode on

clinical electrocardiographs. No need for strap or adhesive and can be
used frequently. Higher source impedance since the contact area is
small
• No straps or adhesives required
• Precordial (chest) ECG
• Can only be used for short periods
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 Floating Electrodes
Metal disk
Insulating
package

Double-sided
Adhesive-tape
Electrolyte gel
ring
in recess
(a) (b)
Snap coated with Ag-AgCl External snap
Gel-coated sponge
Plastic cup Plastic disk

• Swimming in the electrolyte gel

• not in contact with the skin Foam padTack Dead cellular material
• Reduce Motion Artifact Capillary loops Germinating layer
(c)

Examples of floating metal body-surface electrodes: (a) Recessed electrode with

top-hat structure. (b) Cross-sectional view of the electrode in (a). (c) Cross-sectional
view of a disposable recessed electrode of the same general structure shown in (c).
The recess in this electrode is formed from an open foam disk, saturated with
electrolyte gel and placed over the metal electrode. (Minimize motion artifact)
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 Flexible Electrodes

Flexible body-surface
electrodes (a) Carbon-
filled silicone rubber
electrode. (b) Flexible
thin-film neonatal
electrode.
(c) Cross-sectional view of
the thin-film electrode in
(b).

Used for newborn infants.

Compatible with X-ray
Electrolyte hydrogel material is used to hold electrodes to the skin.
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 Internal Electrodes
No electrolyte-skin interface
No electrolyte gel is required

Needle and wire electrodes for

percutaneous measurement of
biopotentials
(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 coiled fine-
wire electrode in place.
For EMG Recording
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 Internal Electrodes (fetal scalp electerod)

Electrodes for detecting fetal electrocardiogram during labor, by means of

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

(a) Wire-loop electrode. (b) Silver-sphere cortical-surface

potential electrode. (c) Multielement depth electrode.
mounted

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 Electrode Arrays
(a) One-dimensional plunge electrode array 10mm long,
0.5mm wide, and 125µm thick, used to measure
potential distribution in the beating myocardium Contacts Insulated leads

(b) Two-dimensional array, used to map epicardial Ag/AgCl electrodes

potential and
(c) Three-dimensional array, each tine is 1,5 mm
Ag/AgCl electrodes
Contacts

Base
Insulated leads Base (b)

(a)
Exposed tip Tines

Base

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(c)
 Microelectrodes

The structure of a metal microelectrode

for intracellular recordings.

Types
1- Solid metal (Tungsten microelectrodes)
2- Supported metal
(metal contained within/outside glass needle)
3- Glass micropipette
(with Ag-AgCl electrode metal)

Structures of two supported metal microelectrodes

(a) Metal-filled glass micropipet.
(b) Glass micropipet or probe, coated with metal film.
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 Microelectrodes

A glass micropipet electrode filled with an electrolytic solution (a) Section of fine-bore
glass capillary. (b) Capillary narrowed through heating and stretching. (c) Final structure of
glass-pipet microelectrode. Dr. Shamekhi, Sahand University of Technology 24
 Metal Microelectrodes
Used in studying the electrophysiology of N = Nucleus
excitable cells by measure potential C = Cytoplasm A
Insulation
differences across the cell membrane. Metal rod B
Cell Cd Tissue fluid
membrane
+ +
Electrode need to be small and strong to + - - - Membrane
potential
Reference
electrode
+ - +
penetrate the cell membrane without + - C -
- +
- +
damaging the cell. + -
+-
N -- ++
-
Tip diameters = 0.05 to 10 µm + - - -
+ + +
- +
- - - -- +
+ + + +

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 Microelectrodes

•Rs :resistance of the metal

•Cd: The metal is coated with an insulating material over all
but its most distal tip
Cd2: outside
Cdi: inside
• Metal-electrolyte interface, Rma, Cma, and Ema
• Reference electrode: Cmb, Rmb, and Emb
• Ri: electrolyte within the cell membrane
• Re: extracellular fluid
•Cw : lead wires Cap.
•Emp: The cell membrane variable potential

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 Glass Intracellular Microelectrodes

Glass Micropipette Microelectrode

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 Electrodes For Electric Stimulation of Tissue
i

(a) Const-Current the voltage response.

voltage pulse is not constant. t

polarization occurs. υ Polarization

The initial rise in voltage corresponding to edge potential
Ohmic Polarization
of the current pulse(voltage -drop across the potential potential
t
resistive components). (a)
The voltage continues to rise with the constant υ
current This is due to the
establishment of a change in the distribution of t
charge concentration
i
Polarization

(b) The current corresponding to the rising edge

of the voltage pulse Polarization t
(b)
jump in a large step and, as the distribution of Current and voltage waveforms seen
the polarization with electrodes used for electric
charge becomes established, stimulation
to fall back to a lower steady-state value
(a) Constant-current stimulation.
(b) Constant-voltage stimulation.
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 Stimulating Electrodes

Points
• Cannot be modeled as a series resistance and capacitance
(there is no single useful model)
• The body/electrode has a highly nonlinear response to
stimulation
• Large currents can cause
– Cavitation
– Cell damage
– Heating

Platinum electrodes:

Applications: neural stimulation

Steel electrodes for pacemakers

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 Types of neural microsystems applications

External Subdural Micro-

Microsensors
electrodes electrodes electrodes

Human
level
–
In vivo
applications
Animal
level

Tissue
slice – –
level In vitro
applications
Cellular
level
– –

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