Biopotential Electrodes (Ch. 5)

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Biopotential Electrodes(Ch. 5)
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Electrode Electrolyte Interface
Electrode Electrolyte
(neutral charge)
C, A- in solution
C
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)
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Electrode Electrolyte Interface
General Ionic Equations
a)
b)
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-)
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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.
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Measuring Half Cell Potential
Note Electrode material is metal salt or
polymer selective membrane
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Some half cell potentials
Standard Hydrogen electrode
Note Ag-AgCl has low junction potential it is
also very stable -gt hence used in ECG electrodes!
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Polarization
If there is a current between the electrode and
electrolyte, the observed half cell potential is
often altered due to polarization.
Note Polarization and impedance of the electrode
are two of the most important electrode
properties to consider.
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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 ionic activity at the
electrode (but note temp dependence
The Nernst equation for half cell potential is
where E0 Standard Half Cell Potential E
Half Cell Potential a Ionic
Activity (generally same as concentration)
n Number of valence electrons involved
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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
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Ag/AgCl Electrode
Relevant ionic equations
Cl2
Governing Nernst Equation
Solubility product of AgCl
AgCl-

• Fabrication of Ag/AgCl electrodes
• Electrolytic deposition of AgCl
• Sintering process forming pellet electrodes

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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
RdRs
Rs
Frequency Response
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Electrode Skin Interface
Ehe
Alter skin transport (or deliver drugs) by Pores
produced by laser, ultrasound or by iontophoresis
Electrode
Rd
Cd
Sweat glands
and ducts
Rs
Gel
100 m
Stratum Corneum
Epidermis
100 m
Dermis and
subcutaneous layer
Ru
Skin impedance for 1cm2 patch 200kO _at_1Hz 200 O _at_
1MHz
Nerve endings
Capillary
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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
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Body Surface Recording Electrodes
Electrode metal
Electrolyte
Think of the construction of electrosurgical
electrode And, how does electro-surgery work?

• Metal Plate Electrodes (historic)
• Suction Electrodes
• (historic interest)
• Floating Electrodes
• Flexible Electrodes

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Commonly Used Biopotential Electrodes

• Metal plate electrodes
• Large surface Ancient, therefore still used, ECG
• Metal disk with stainless steel platinum or gold
  coated
• EMG, EEG
• smaller diameters
• motion artifacts
• Disposable foam-pad Cheap!

(a) Metal-plate electrode used for application to
limbs. (b) Metal-disk electrode applied with
surgical tape. (c)Disposable foam-pad
electrodes, often used with ECG
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Commonly Used Biopotential Electrodes

• Suction electrodes
• No straps or adhesives required
• precordial (chest) ECG
• can only be used for short periods

• Floating electrodes
• metal disk is recessed
• swimming in the electrolyte gel
• not in contact with the skin
• reduces motion artifact

Suction Electrode
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Commonly Used Biopotential Electrodes
Metal disk
Insulating package
Double-sided Adhesive-tape ring
Electrolyte gel in recess
Reusable
(a)
(b)
External snap
Snap coated with Ag-AgCl
Gel-coated sponge
Disposable
Plastic cup
Plastic disk
Dead cellular material
Tack
Foam pad
Capillary loops
Germinating layer
(c)
Floating Electrodes
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Commonly Used Biopotential Electrodes

• Flexible electrodes
• Body contours are often irregular
• Regularly shaped rigid electrodes
• may not always work.
• Special case infants
• Material
• - Polymer or nylon with silver
• - Carbon filled silicon rubber
• (Mylar film)

(a) Carbon-filled silicone rubber electrode. (b)
Flexible thin-film neonatal electrode.(c)
Cross-sectional view of the thin-film electrode
in (b).
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Internal Electrodes
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.
The latest BION implanted electrode for muscle
recording/stimulation Alfred E. Mann Foundation
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Fetal ECG Electrodes
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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Electrode Arrays
Examples of microfabricated electrode arrays.
(a) One-dimensional plunge electrode array, (b)
Two-dimensional array, and (c) Three-dimensional
array
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Microelectrodes

• Why
• 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
• Types
• Solid metal -gt Tungsten microelectrodes
• Supported metal (metal contained within/outside
  glass needle)
• Glass micropipette -gt with Ag-AgCl electrode metal

Intracellular Extracellular
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Metal Microelectrodes
C
Microns!
R
Extracellular recording typically in brain
where you are interested in recording the firing
of neurons (spikes). Use metal
electrodeinsulation -gt goes to high impedance
amplifiernegative capacitance amplifier!
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Metal Supported Microelectrodes
(a) Metal inside glass (b) Glass inside metal
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Glass Micropipette
Ag-AgCl wire3M KCl has very low junction
potential and hence very accurate for dc
measurements (e.g. action potential)
heat
pull
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.
Fill with intracellular fluid or 3M KCl
Intracellular recording typically for recording
from cells, such as cardiac myocyte Need high
impedance amplifiernegative capacitance
amplifier!
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Electrical Properties of Microelectrodes
Metal Microelectrode
Metal microelectrode with tip placed within cell
Equivalent circuits
Use metal electrodeinsulation -gt goes to high
impedance amplifiernegative capacitance
amplifier!
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Electrical Properties of Glass Intracellular
Microelectrodes
Glass Micropipette Microelectrode
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Stimulating Electrodes
Features
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 Modern day Pt-Ir and other exotic
metal combinations to reduce polarization,
improve conductance and long life/biocompatibility
Steel electrodes for pacemakers and
defibrillators

• Types of stimulating electrodes
• Pacing
• Ablation
• Defibrillation

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Intraocular Stimulation Electrodes
Reference Lutz Hesse, Thomas Schanze, Marcus
Wilms and Marcus Eger, Implantation of retina
stimulation electrodes and recording of
electrical stimulation responses in the visual
cortex of the cat, Graefes Arch Clin Exp
Ophthalmol (2000) 238840845
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In vivo neural microsystems (FIBE) challenge
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In vivo neural microsystems (FIBE)
biocompatibility - variant
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In vivo neural microsystems (FIBE) state of the
art
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Introduction neural microsystems
Instrumentation for neurophysiology
MEMS - Microsystems
Neural Microsystems
Neural microelectrodes
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Introduction types of neural microsystems
applications
In vivo applications
In vitro applications
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Microelectronic technologyfor Microelectrodes
Beam-lead multiple electrode.
Multielectrode silicon probe
Multiple-chamber electrode
Peripheral-nerve electrode
Different types of microelectrodes fabricated
using microfabrication/MEMS technology
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Michigan Probes for Neural Recordings
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Neural Recording Microelectrodes
Reference http//www.acreo.se/acreo-rd/IMAGES/PU
BLICATIONS/PROCEEDINGS/ABSTRACT-KINDLUNDH.PDF
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In vivo neural microsystems 3 examples
University of Michigan Smart comb-shape
microelectrode arrays for brain stimulation and
recording University of Illinois at
Urbana-Champaign High-density comb-shape metal
microelectrode arrays for recording Fraunhofer
Institute of Biomedical (FIBE) Engineering Retina
implant
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Multi-electrode Neural Recording
Reference http//www.cyberkineticsinc.com/techno
logy.htm
Reference http//www.nottingham.ac.uk/neuronal-n
etworks/mmep.htm
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WPIs Nitric Oxide Nanosensor
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Nitric Oxide Sensor

• Developed at Dr.Thakors Lab, BME, JHU
• Electrochemical detection of NO

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A
E
B
F
C
G
D
H
Cartoon of the fabrication sequence for the NO
sensor array A) Bare 4 Si wafer B) 5µm of
photoresist was spin-coated on to the surface,
followed by a pre-bake for 1min at 90C. C) The
samples were then exposed through a mask for 16s
using UV light at 365nm and an intensity of
15mW/cm2. D) Patterned photoresist after
development. E) 20nm of Ti, 150nm of Au and 50nm
of C were evaporated on. F) The metal on the
unexposed areas was removed by incubation in an
acetone bath. G)A 2nd layer of photoresist, which
serves as the insulation layer, was spun on and
patterned. H) The windows in the second layer
also defined the microelectrode sites.
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NO Sensor Calibration
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NO Sensor Calibration
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Multichannel NO Recordings