Types of electrodes
Surface: EEG/EMG/ECG potentials
measured on skin
Needle: penetrates skin; EEG from
local region of brain / EMG
potential from a specific
group of muscles
Microelectrodes: bioelectric potential near /
within a single cell
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Body Surface Recording Electrodes
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Metal-plate electrodes
1. Rectangular / Circular plates
rectangular plate area: 3.5 x 5 cm; circular dia: 5cm
German Silver (Ni-Ag alloy/Ni-plated steel/chrome
electrolyte gel (inside concave surface) – electrical contact
Rubber strap/belt to attach to jelly rubbed area
Metal-plate electrode used for application to limbs
Preferred for use during surgery; not suitable for long-term
patient monitoring
Reusable for several years
Lead wire to the eelectrocardiograph
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2. Metal disc with lead wire
for EEG/EMG/ECG
Ag/Platinum/Ag/AgCl
for long-time ECG chest recordings on skin site applied with
electrode paste
smaller diameter for EMGs and EEGs than for ECGs
can be fabricated from thin Ag foil single-use disposable
electrodes
lead wire soldered to back surface
and covered with PVC
metal disc electrode
applied with surgical tape 4
3. Pre gelled – disposable
disc of plastic foam material
Ag-plated disc on one side attached to a Ag - plated snap on the
other side
Bottom: Layer of electrolyte on the metal disc
Electrode side: covered with adhesive material, already in place,
ready to be applied
Release-paper strip over the adhesive side
Packed in foil envelope to prevent evaporation of water content
of gel; used for ECG
Time saving and cheap
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4. Suction electrode
Modified metal plate electrode with no straps/adhesives
Hollow, metallic, cylindrical electrode - contact with the skin at its
base
Rubber bulb for suction which fits over its top
Can be removed easily from one location to another
Routinely used as precordial/chest electrode for ECG
Used for short period of time suction and pressure of contact on
skin can cause irritation
electrolyte
gel
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5. Floating/liquid junction
Metal plate electrodes cannot be used under dynamic conditions , i.e.
patient performing a physical task (motion artifacts)
Features:
Actual metal disk recessed into a cavity: no direct contact with the
skin
Electrolytic contact: electrolyte paste filled in the cavity
Fixed on the skin with a double-sided adhesive-tape ring
Electrode element: Ag
Since cavity does not move w.r.t. electrode element/ metal disk
no mechanical movement of charge double layer
top-hat electrode
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6. Flexible electrodes
Body contours often irregular regularly shaped rigid electrodes
may not always work motion artifacts
woven stretchable nylon impregnated with silver particles and
lead wire bonding achieved by epoxy OR
C filled Si rubber compound in the form of a thin film/disk as
active element
For premature infants: flexible thin-film electrodes
(i) to prevent skin ulceration (ii) to conform to shape of chest
Mylar film with Ag/AgCl film deposited
Transparent to x-rays (Ag very thin)
Flexible
thin-film
neonatal
electrode
cross-section
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C-filled Si rubber electrode
Internal Electrodes
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1. Needle electrodes
(a) Insulated needle
o SS tip exposed; shank insulated with a varnish coating
o joint (wire + shank) in plastic hub (protection)
o can obtain EMG activity from a particular muscle; also to
monitor ECG continuously during surgery (placed sub-
cutaneously on each limb; no need for gel)
(b) Co-axial needle
insulated fine wire running down the center of the lumen of a
hypodermic needle; remainder filled with epoxy resin
tip filed to
oblique c-s
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(c) Bipolar coaxial
-- multiple active electrodes (wires) in a single needle
-- differential measurement of electric activity in the immediate
vicinity of the tip
Needle electrodes only for acute measurements; uncomfortable
for long-term implantation due to
• stiffness
• size
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2. Wire electrodes
for chronic recordings
fine wire 25-125 µm dia (SS); either coiled or straight
placed in lumen of the needle inserted through skin into
muscle at desired position & depth
needle slowly withdrawn electrode left in place; bent over
portion serves as a barb
removal of wire small force straightens out the barb
c-s view of skin and muscle, showing in place:
non-insulated tip bent back
on itself to form a J shaped
structure
fine-wire electrode coiled fine wire electrode (150µ 12
dia)
before insertion
3. Fetal ECG Electrodes
not adequate ECG data using surface electrodes (amniotic fluid
places entire body of fetus at a common potential)
for detecting fetal electrocardiogram during labor, by means of
intracutaneous needles
electrode to penetrate fetus skin for fetal ECG
Suction electrode: after suction the skin is drawn into the cap &
pierced; 50-700 µV ECG can be recorded
pierces stratum
corneum
contacts amniotic fluid
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Helical electrode
o stainless steel needle
o shaped like one screw of a helix
o mounted on a plastic hub
o corkscrew type action for attachment
additional SS
reference electrode
helical electrode, which is attached to
fetal skin during advanced labour by
rotating it so that needle twists just
beneath surface of the skin
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Some Advanced Electrodes
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Microelectrodes
Measures p.d. across cell membrane
Small enough to be placed into cell
Strong enough to penetrate cell membrane
0.05 – 10 m (tip diameter)
Microelectrodes can be made from:
-- solid metal needles
-- metal contained within/ on the surface of a glass needle
-- a glass micropipette (lumen filled with an electrolytic
solution)
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1. Metal microelectrodes
Miniature version of needle electrode
Insulated metallic shaft supports the metallic needle
(SS/Pt-Ir alloy/Tungsten) by polymeric material
Small and robust: suitable for neurophysiologic studies
Fabrication difficult
MWCNT–PPy coating strongly reduces the
impedance of extracellular metal
microelectrodes. (A) SEM images at low
resolution show the tips of (i) non-coated and
(ii) coated microelectrodes
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2. Metal - supported microelectrodes
Glass tube drawn to a micropipette structure & filled with metal
Glass gives mechanical support & insulation
Ag alloy/Pt-Ag etc.
metal - filled glass
micropipette
3. Micropipette electrodes glass micropipette coated with metal
film (thickness ~ 1/10th of a m)
glass capillary drawn to a very narrow point
(fraction of a micron) & filled with electrolyte
solution (KCl)
electrolytic solution in contact b/w the cell
: 0.1-10 m
and the metal electrode (SS/Pt/Ag) in the
shank of the pipette
KCl
sealed with a cap, fabrication difficult
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4. Electrode arrays
Microfabrication technology
10mm long,0.5mm wide, 125 thick
for measuring
transmural
potential distn
in the beating
myocardium
6 pairs of electrodes
mapping electric potential over a region of an organ
1mm ;
can fit over
heart
Molybdenum substrate 19
Acute brain slices
Acute slices from cerebellum
(e.g. from hippocampus)
Micromachining: electrodes with diameter of 10 μm arranged in a
distance of only 100 μm resulting in a square recording area of only
700 μm length
Titanium nitride (TiN): excellent signal to noise ratio, very sturdy
material
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Applications
Cardiac myocytes and whole-heart preparations
Circadian rhythm, e. g. long-term recordings of supra-chiasmatic
nucleus (SCN) neurons in culture
Synaptic plasticity, e. g. paired-pulse facilitation (PPF), long-term
potentiation (LTP) and depression (LTD)
Micro-EEGs
Neuroregeneration
Ion channel screening
Safety pharmacology, e. g. screening for pro-arrhythmogenic
effects of compounds
Acute brain and retina slices, cultured slices, and single cell
cultures
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Electrical properties of microelectrodes
(i) Rs: series resistance due to the metal itself (major
contribution from metal in the tip & shank
portion 22
(ii)Cd: distributed capacitance - expressed in
lumped form by separating:
-- shank and tip
-- shaft
Shank and tip region:
microelectrode coaxial cylinder capacitor (approx):
C/length (F/m): Cd1/L = 2r0/ln(D/d) (1)
where
r: relative dielectric constant of insulation material
0: dielectric constant of free space
D: diameter of cylinder (electrode + insulation)
d: diameter of electrode
L: length of shank
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Shaft portion:
parallel plate capacitor with d = diameter of electrode and t =
thickness of the insulation layer
C/length (F/m): Cd2/L = r0d/t (2)
Cd2 : only that portion of the electrode shaft that is
submerged in the extracellular fluid
Cd when only shank is submerged?
(iii) significant contributions by metal-electrolyte
interface components:
(a) associated with microelectrode: Rma Cma Ema
(b) associated with reference electrode: Rmb Cmb Emb
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Rma : resistance associated with metal - electrolyte
interface
Cma : polarization effects at metal-electrolyte interface
Ema : half-cell potential of microelectrode
Rmb : resistance associated with reference electrode -
electrolyte interface
Cmb : polarization effects at metal-electrolyte interface
for reference electrode
Emb : half-cell potential of reference electrode
(impedance effects of reference electrode much lower
than microelectrodes)
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(iv) Ri: a series resistance associated with the
electrolyte within the cell membrane
(v) Re: series resistance due to extracellular fluid
(vi) Emp: cell membrane variable potential
(vii) Cw : capacitance associated with lead wires
(using these 7 parameters - equivalent electrical circuit
of metal microelectrode is obtained)
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Equivalent circuit of a microelectrode
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Simplified equivalent circuit
Simplification of equivalent circuit
(i) Neglect impedance of reference electrode
(ii) Neglect Ri and Re
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(iii) Lump all distributed capacitance together
Micropipette electrodes
(i) Rma/Cma/Ema:
contributions from
metal (internal
electrode) electrolyte
interface
(ii) Rt: series resistance of
electrolyte in shank &
tip region of the
microelectrode
(iii) Cd: capacitance
corresponding to
glass in this region
distributed C
due to
shaft region
neglected (glass wall
of the electrode
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much thicker in the shaft region; capacitive contribution quite
(iv) Two potentials associated with the tip of micropipette
electrode:
o Ej (liquid junction potential)
o Et (tip potential)
Ej : liquid junction set up b/w the electrolyte in the
micropipet and the intracellular fluid
Et : thin glass wall surrounding the tip region of
the micropipet behaves like a glass membrane
and has an associated membrane potential
(v) Ri & Re: resistances corresponding to intra-cellular
and extracellular fluids, respectively
(vi) Rmb/Cmb/Emb : reference electrode characteristics
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Equivalent circuit of micropipette electrode
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Simplified equivalent circuit
• Overall series resistance lumped together (Rt)
• Lump all distributed capacitance together (Ct)
• All associated dc potentials lumped together (Em)
• Neglect impedance of reference electrode
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Stimulating Electrodes
Follow: same design as electrodes used for recording
biopotentials
Differ: currents ~ mA cross the electrode - electrolyte interface
Examples: cardiac defibrillators; electric stimulators
Equivalent circuit at interface: changes as stimulus progresses
Nature of Stimulus
Constant current stimulus pulse
Voltage Response: Resulting voltage pulse not constant:
1. Initially a rise in V corresponding to the leading edge of the
current pulse
2. Constant current V still continues to rise
3. Current falls back to its low value - Voltage drops but not to its
initial value; instead, after the initial steep fall a slower
decay
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Constant current stimulation
due to change in distribution
due to V drop across the resistive
(polarization) of charge
components of the electrode-
3 concentration at electrode-
electrolyte interface
electrolyte interface
corresponding to dissipation
of polarization charges at
the interface
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Constant voltage stimulus pulse
Current Response: Resulting current pulse not constant
1. Current corresponding to the rising edge of the voltage pulse
jumps in a large step
2. Current falls back to a lower steady-state value
3. Current changes direction when voltage pulse falls
4. Current slowly returns to its initial zero value
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Constant voltage stimulation
1 2
current jumps as 3
as distribution of polarization
polarization starts
charge gets established falls
back to lower steady-state value
due to dissipation of polarization
charges built up at the electrode-
electrolyte interface
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Choosing Materials for Stimulating electrodes
If a stimulating current causes the material of the electrode to be
oxidized (electrode is consumed)
(i) lifetime of the electrode gets limited
(ii) toxic to tissue due to concentration of electrode
material ions in the vicinity of the electrode
If Ag/AgCl electrodes used formation of excess Cl- ions
(characteristics of electrode may be changed)
Best stimulating electrodes: noble metals (at least SS) minimal
chemical reaction; limitation polarization effects
e.g.: C filled Si rubber for transcutaneous stimulation in clinical
pain management
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Using Electrodes
1. Same material – (i) electrode + any part of lead wire (that may
be exposed to the electrolyte) (ii) when a pair of electrodes
used
2. Solder not to be used - to connect electrode to its lead wire
unless material not to be in contact with the electrolyte
(mechanical bond/welding preferred)
3. Lead wires – flexible and strong: for better results, lead wire to
be taped to the skin a few cm from the electrode (some slack in
the wire b/w tape and electrode)
4. Point of frequent failure - point at which lead wire enters the
electrodes (even though insulation appears intact wire within
may be broken due to severe repeated flexing at this point)
5. High-humidity environment - insulation of polymeric material –
often absorbs water conductive/ corrosive.; correct choice of
insulation material desirable
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