Lecture 7 Electrodes CH 5
Lecture 7 Electrodes CH 5
Lecture 7 Electrodes CH 5
Biopotential
Electrodes
(Ch. 5)
Electrode – Electrolyte Interface
Electrode Electrolyte (neutral charge)
C C+, A- in solution
Current flow
C C+
e- C
A- C+
e-
A-
a) C ↔ C n + + ne −
b) Am − ↔ 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.
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.
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.
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
αA + βB ↔ γC + δD + ne − Note: interested
in ionic activity
The Nernst equation for half cell potential is at the electrode
RT a γ δ
(but note temp
C aD
E=E +
0
ln α β dependence
nF a A a B
where E0 : Standard Half Cell Potential E : Half Cell Potential
a : Ionic Activity (generally same as concentration)
n : Number of valence electrons involved
Polarizable and Non-Polarizable
Electrodes
Use for
Perfectly Polarizable Electrodes recording
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
Use for
Perfectly Non-Polarizable Electrode stimulation
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
Corner frequency
Rd+Rs
Rs
Frequency Response
Electrode Skin Interface
Ehe
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
Electrolyte
Think of the
1. Metal Plate Electrodes construction of
(historic) electrosurgical
electrode
2. Suction Electrodes
And, how does
(historic interest) electro-surgery
work?
1. Floating Electrodes
2. Flexible Electrodes
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
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
Commonly Used Biopotential
Electrodes Insulating
Metal disk
package
Double-sided
Adhesive-tape
ring Electrolyte gel
in recess
(a) (b)
Reusable
Snap coated with Ag-AgCl External snap
Gel-coated sponge
Plastic cup Plastic disk Disposable
Floating Electrodes
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(a) Carbon-filled silicone rubber electrode.
(Mylar film) (b) Flexible thin-film neonatal electrode.
(c) Cross-sectional view of the thin-film
electrode in (b).
Internal Electrodes
Needle and wire electrodes for
percutaneous measurement of
biopotentials
(c)
Microelectrodes
Why
Measure potential difference across cell membrane
Requirements
– Small enough to be placed into cell Intracellular
– Strong enough to penetrate cell membrane Extracellular
– Typical tip diameter: 0.05 – 10 microns
Types
– Solid metal -> Tungsten microelectrodes
– Supported metal (metal contained within/outside glass needle)
– Glass micropipette -> with Ag-AgCl electrode metal
Metal Microelectrodes
C
Microns!
R
Extracellular recording – typically in brain where you
are interested in recording the firing of neurons
(spikes).
Glass Micropipette
KCl has very low
junction potential
and hence very
heat accurate for dc
measurements
pull (e.g. action
potential)
Instrumentation for
neurophysiology
Neural MEMS -
Microsystems Microsystems
Neural
microelectrodes
Introduction: types of neural microsystems applications
Human
level
–
In vivo
applications
Animal
level
Tissue
slice – –
level In vitro
applications
Cellular
level
– –
Microelectronic technology
for Microelectrodes Bonding pads
Silicon probe
Si substrate
Exposed tips
(a) Beam-lead multiple electrode . (b) Multielectrode silicon probe
Miniature
insulating Channels Silicon chip
chamber Hole
Lead via
Silicon probe
Contact
Electrode metal film
(c) Multiple-chamber electrode (d)
Peripheral-nerve electrode
Different types of microelectrodes fabricated using microfabrication/MEMS
technology
Michigan Probes for Neural
Recordings
Neural Recording
Microelectrodes
Reference :
http://www.acreo.se/acreo-rd/IMAGES/PUBLICATIONS/PROCEEDINGS/ABSTRACT-
KINDLUNDH.PDF
In vivo neural microsystems: 3 examples
University of Michigan
Smart comb-shape microelectrode arrays for
brain stimulation and recording
Reference :
http://www.cyberkineticsinc.com/technology.htm
Reference :
http://www.nottingham.ac.uk/neuronal-networks/mmep.htm
WPI’s Nitric Oxide
Nanosensor
Nitric Oxide Sensor
• Developed at Dr.Thakor’s Lab, BME, JHU
• Electrochemical detection of NO
B
F
C G
D H
So, you are an inventor who has a better idea. Describe an improvement
• to make the electrode cheaper
• more suitable for lower noise measurement for EEG
• circumvent patents that are based on plastic/foam electrode body
• attractive to consumers for use with their ECG machines at home
• reduce artifact (minimize the motion of skin/electrode) in ambulatory recording
• How would you detect bacteria or other microorganisms in water supply? Make sure
that your method distinguishes inert particulate matter from living cellular matter.
• Draw the equivalent circuit model of the skin and an ECG electrode. Identify the key
sources of electrical interference and otherwise the elements that would likely
contribute to the poor quality of recordings.
• Design an amplifier interface for the following two applications: Patch clamp ion
channel current amplifier: Your goal is to amplify pA level current to produce 1 Volt
output.
• Strain gauge sensor amplifier: Your goal is to convert 10 ohm change in resistance of
a strain gauge to produce 1 volt output.
• You are asked to design a laboratory set up for a Professor who is interested in
making very low level ion channel current measurements from single cardiac cells
using the patch clamping technique. What are the likely sources of interference? What
would you do to ensure that there is minimal noise in the laboratory set up?
• Draw the equivalent circuit of a patch clamp glass pipette. This electrode differs
slightly from the conventional microelectrode that penetrates the cell and obtains
intracellular potentials, in that it seals to the cell membrane and generally measures the
whole cell current. Show all the equivalent circuit elements of the electrode and the
cell.
• Design a very simple, small circuit to measure/transduce the whole cell current from
the patch clamp electrode and convert into the amplified voltage signal.
• For far too long the microelectrodes that have been used in the laboratory fall into
two categories: glass or metal microelectrodes. These record from a single cell at a
time. What is the current technology for recording from sites in the tissue from
multiple cells at once (extracellularly OR intracellularly). Draw a schematic of such an
electrode array.
• List some other types of electrodes or microelectrodes that have been developed for
laboratory and research use.
5. Electrodes and Microelectrodes
Contrast the glass microelectrode that penetrates the cell versus patch clamp electrode.
Which measures what (current/voltage) and of what magnitude? Which one is
bigger/smaller? What is the impedance of microelectrode vs. patch electrode? Which one
could be used to record from a single sub-micron sized ion channel?
For a research application, a scientist comes up with the idea of optically measuring potential
on cell membrane. His basic idea is to use a dye that binds to cell surface. When the dye is
excited by a bright light (superluminscent LED), it gives out fluorescence proportional to cell
membrane voltage. The optical signal is picked up by a photo detector. Draw the circuit to
pass a very large (about 100 mA) pulse of current through the LED to intensely illuminate
the cell for very brief duration and then detect nA ampere level photo current produced by
the fluorescence signal
You are asked to measure the impedance of the skin. In fact, lie detectors use changes in
skin impedance (as a measure of autonomic reflex) to indicate whether a person is lying.
Draw the equivalent circuit model of human skin and electrode. Based on reasonable
estimates of the skin properties, sketch a rough frequency response of the skin (from dc to
100 kHz)
Now design a circuit to measure the impedance, taking care not to violate any safety
consideration.
6. Neural electrodes/microelectrodes
You want to record from neurons in the brain. However, you want to record from
dozens of neurons all at once from several closely spaced microelectrodes. What
material and process would you use to make the microelectrode array?
•What metal would you prefer to use to make electrode arrays of about 10 micron
square size to make electrical contacts with dozens of neurons?
•What metal would you prefer to use to stimulate dozens of neurons in a deep brain
microelectrode based stimulator?
•(which metal provides good recording vs stimulating properties – and at the same
time not be toxic to brain tissue)?
• You are asked to develop an experimental set up to record from rat brain cells using
microelectrodes. What precautions would you take to minimize the electrical
interference in your recording set up?
Question/ideas!
• Make a better electrode
• Research different electrode technologies
– Ion selective, immunosensors, ISFET,
electrochemical
– MEMS microelectrode technologies