3. OBJECTIVES:
Origin of bio signals
Propagation of bioelectric signal
Electrodes
Recording of bioelectric signals
Analysis of recorded bio-electric signal
complications
10. Microelectrodes
Measure potential difference across cell membrane
Requirements
Small enough to be placed into cell
Strong enough to penetrate cell membrane
Typical tip diameter: 0.05 – 10 microns
11. Stimulating Electrodes
Features
– Net current across electrode-electrolyte interface
is not zero
– The body/electrode has a highly nonlinear
response to
stimulation Platinum electrodes:
– Large currents can cause Applications: neural
– Chemical reaction stimulation
– Cavitation
– Cell damage Steel electrodes for
– Heating pacemakers and
Applications of stimulating electrodes defibrillators
1. Pacing
2. Ablation
3. Defibrillation
12. Practical Hints for using an
electrode .....
Electrode and lead wire (exposed to
the electrolyte) must be of the same
material.
Use two similar electrodes when
measuring differentials.
Lead electrode interface failure.
Electrode insulation material.
Input impedance of Amplifier.
13. WHAT ARE THE BASIC
ELECTRODE REQUIRED FOR
RECORDING OF A BIO-
POTENTIAL
?
ACTIVE ELECTRODE
REFERANCE ELECTRODE
GROUND ELECTRODE
14. Active electrode:
Actually this is the only
electrode which pick the signal
from the subjects body.
BUT
HOW ?
17. Electrode – Electrolyte Interface
Electrode Electrolyte (neutral charge)
C C+, A- in solution
Current flow
C C+
e- C
A- C+
e-
A-
C+ : Cation A- : Anion e- : electron
Fairly common electrode materials: Pt, Carbon, …, Au, Ag,…
Electrode metal is use in conjunction with salt, e.g. Ag-AgCl, Pt-Pt
black, or polymer coats (e.g. Nafion, to improve selectivity)
18. Electrode – Electrolyte Interface
General Ionic Equations
a) n
C C ne
b) m
A A me
a) If electrode has same material as cation, then this material gets oxidized
and enters the electrolyte as a cation and electrons remain at the electrode
and flow in the external circuit.
b) If anion can be oxidized at the electrode to form a neutral atom, one
or two electrons are given to the electrode.
The dominating reaction can be inferred from the following :
Current flow from electrode to electrolyte :
Oxidation (Loss of e-)
Current flow from electrolyte to electrode :
Reduction (Gain of e-)
19. Half Cell Potential
A characteristic potential difference established by the
electrode and its surrounding electrolyte which depends on
the metal, concentration of ions in solution and
temperature (and some second order factors) .
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. Other half cell potentials are
expressed as a potential difference with this electrode.
Reason for Half Cell Potential : Charge Separation at Interface
Oxidation or reduction reactions at the electrode-electrolyte
interface lead to a double-charge layer, similar to that which exists
along electrically active biological cell membranes.
20. Measuring Half Cell Potential
Note: Electrode material is metal + salt or polymer selective membrane
22. Biopotential Electrodes – The Basics
The interface between the body and electronic
measuring devices
Conduct current across the interface
Current is carried in the body by ions
Current is carried in electronics by electrons
Electrodes must change ionic current into electronic
current
This is all mediated at what is called the Electrode-
Electrolyte Interface or the Electrode-Tissue Interface
24. Current Flow at the Electrode-Electrolyte Interface
Ion- flow Electrons move in opposite
Electron flow direction to current flow
Ion+ flow
Cations (C+ ) move in same
direction as current flow
Anions (A– ) move in opposite
direction of current flow
Chemical oxidation (current
flow right) - reduction
(current flow left) reactions at
the interface:
+ Current flow C C++e– (5.1)
The current crosses it from left to right. A– A + e– (5.2)
The electrode consists of metallic atoms
C. The electrolyte is an aqueous solution No current at equilibrium
containing cations of the electrode metal
C+ and anions A-.
25. Half-Cell Potential
When metal (C) contacts electrolyte, oxidation (C C + + e –
) or reduction (A- A + e –) begins immediately.
Local concentration of cations at the surface changes.
Charge builds up in the regions.
Electrolyte surrounding the metal assumes a different electric
potential from the rest of the solution.
This potential difference is called the half-cell potential ( E0 ).
Separation of charge at the electrode-electrolyte interface
results in a electric double layer (bilayer).
Measuring the half-cell potential requires the use of a second
reference electrode.
By convention, the hydrogen electrode is chosen as the
reference.
27. Nernst Equation
When two aqueous ionic solutions of different concentration are
separated by an ion-selective semi-permeable membrane, an
electric potential exists across the membrane.
For the general oxidation-reduction reaction
Note: interested in
A B C D ne ionic activity at the
electrode
The Nernst equation for half cell potential is (but note temp
dependence
RT a a
E E0 ln C D
nF a A aB
where E0 : Standard Half Cell Potential E : Half Cell Potential
a : Ionic Activity (generally same as concentration)
n : Number of valence electrons involved
28. Equivalent Circuit
Cd : capacitance of electrode-eletrolyte interface
Rd : resistance of electrode-eletrolyte interface
Rs : resistance of electrode lead wire
Ecell : cell potential for electrode
Corner frequency
Rd+Rs
Rs
Frequency Response
29. Electrode Skin Interface
Ehe Alter skin
transport
Electrode Cd Rd
(or deliver
drugs) by:
Sweat glands
Gel Rs and ducts Pores
100 produced
Ese EP by laser,
ultrasound
Stratum Corneum
Epidermis Ce Re CP RP or by
iontophores
100
is
Dermis and
subcutaneous layer
Ru
Nerve Skin impedance for 1cm2 patch:
endings Capillary 200kΩ @1Hz
200 Ω @ 1MHz
30. Polarization
If there is a current between the electrode and electrolyte, the observed half
cell potential is often altered due to polarization.
Overpotential
Difference between observed
and zero-current half cell
potentials
Activation
Resistance Concentration
The activation energy
Current changes resistance Changes in distribution
barrier depends on the
of electrolyte and thus, of ions at the electrode-
direction of current and
a voltage drop results. electrolyte interface
determines kinetics
V p VR VC VA
Note: Polarization and impedance of the electrode are two of the most important electrode
properties to consider.
31. Polarizable and Non-Polarizable
Electrodes
Use for recording
Perfectly Polarizable Electrodes
These are 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. Example : Ag/AgCl Electrode
Perfectly Non-Polarizable Electrode
These are electrodes where current passes freely across the electrode-
electrolyte interface, requiring no energy to make the transition. These
electrodes see no overpotentials. Example : Platinum electrode
Use for stimulation
Example: Ag-AgCl is used in recording while Pt is use in
stimulation
32. Motion Artifact
Why
When the electrode moves with respect to the electrolyte, the
distribution of the double layer of charge on polarizable electrode
interface changes. This changes the half cell potential temporarily.
What
If a pair of electrodes is in an electrolyte and one moves with respect
to the other, a potential difference appears across the electrodes
known as the motion artifact. This is a source of noise and
interference in biopotential measurements
Motion artifact is minimal for non-polarizable electrodes
33. But one thing that should not
be stopped is questioning
