Biopotential Electrodes
Electrode – Electrolyte Interface
Electrode Electrolyte (neutral charge)
C+, A- in solutionC
C
C
A-
A-
C+
C+e-
e-
Current flow
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)
Electrode – Electrolyte Interface
meAA
neCCm
n
General Ionic Equations
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.
a)
b)
Current flow from electrode to electrolyte : Oxidation (Loss of e-)Current flow from electrolyte to electrode : Reduction (Gain of e-)
The dominating reaction can be inferred from the following :
Half Cell PotentialA 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 InterfaceOxidation 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.
Measuring Half Cell Potential
Note: Electrode material is metal + salt or polymer selective membrane
Some half cell potentials
Standard Hydrogen electrode
Note: Ag-AgCl has low junction potential & it is also very stable -> hence used in ECG electrodes!
PolarizationIf there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization.
OverpotentialDifference between observed and zero-current half cell potentials
ResistanceCurrent changes resistance
of electrolyte and thus, a voltage drop results.
ConcentrationChanges in distributionof ions at the electrode-
electrolyte interface
ActivationThe activation energy barrier depends on the
direction of current and determines kinetics
ACRp VVVV Note: Polarization and impedance of the electrode are two of the most important electrode properties to consider.
Nernst Equation
BA
DC
aa
aa
nF
RTEE ln0
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 neDCBA
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
Note: interested in ionic activity at the electrode(but note temp dependence
Polarizable and Non-Polarizable Electrodes
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
Example: Ag-AgCl is used in recording while Pt is use in stimulation
Use for recording
Use for stimulation
Ag/AgCl Electrode
eAgAg
AgClClAg
Ag+Cl-
Cl2
Relevant ionic equations
Governing Nernst Equation
Cl
sAg a
K
nF
RTEE ln0
Solubility product of AgCl
Fabrication of Ag/AgCl electrodes
1. Electrolytic deposition of AgCl
2. Sintering process forming pellet electrodes
Equivalent Circuit
Cd : capacitance of electrode-eletrolyte interfaceRd : resistance of electrode-eletrolyte interfaceRs : resistance of electrode lead wireEcell : cell potential for electrode
Frequency Response
Corner frequency
Rd+Rs
Rs
Electrode Skin Interface
Sweat glandsand ducts
Electrode
Epidermis
Dermis andsubcutaneous layer Ru
Ehe
Rs
RdCd
Gel
Re
Ese EP
RPCPCe
Stratum Corneum
Skin impedance for 1cm2 patch:200kΩ @1Hz
200 Ω @ 1MHz
Alter skin transport (or deliver drugs) by:
Pores produced by laser, ultrasound or by iontophoresis
100
100
Nerve endings Capillary
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
Body Surface Recording Electrodes
1. Metal Plate Electrodes (historic)
2. Suction Electrodes
(historic interest)
3. Floating Electrodes
4. Flexible Electrodes
Electrode metal
Electrolyte
Think of the construction of electrosurgical electrode
And, how does electro-surgery work?
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
Double-sidedAdhesive-tapering
Insulatingpackage
Metal disk
Electrolyte gelin recess
(a) (b)
(c)
Snap coated with Ag-AgCl External snap
Plastic cup
Tack
Plastic disk
Foam padCapillary loops
Dead cellular material
Germinating layer
Gel-coated sponge
Commonly Used Biopotential Electrodes
Floating Electrodes
Reusable
Disposable
(a) Carbon-filled silicone rubber electrode. (b) Flexible thin-film neonatal electrode.(c) Cross-sectional view of the thin-film
electrode in (b).
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)
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/stimulationAlfred E. Mann Foundation
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.
Electrode Arrays
Examples of microfabricated electrode arrays. (a) One-dimensional plunge electrode array, (b) Two-dimensional array, and (c) Three-dimensional array
ContactsInsulated leads
(b)Base
Ag/AgCl electrodes
Ag/AgCl electrodes
BaseInsulated leads
(a)
Contacts
(c)
Tines
Base
Exposed tip
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 -> Tungsten microelectrodes– Supported metal (metal contained within/outside glass needle)– Glass micropipette -> with Ag-AgCl electrode metal
Intracellular
Extracellular
Metal Microelectrodes
Extracellular recording – typically in brain where you are interested in recording the firing of neurons (spikes).
Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier!
Microns!
R
C
Metal Supported Microelectrodes
(a) Metal inside glass (b) Glass inside metal
Glass Micropipette
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.
Intracellular recording – typically for recording from cells, such as cardiac myocyteNeed high impedance amplifier…negative capacitance amplifier!
heat
pull
Fill with intracellular fluid or 3M KCl
Ag-AgCl wire+3M KCl has very low junction potential and hence very accurate for dc measurements (e.g. action potential)
Electrical Properties of Microelectrodes
Metal microelectrode with tip placed within cell
Equivalent circuits
Metal Microelectrode
Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier!
Electrical Properties of Glass Intracellular Microelectrodes
Glass Micropipette Microelectrode
Stimulating Electrodes
– 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
Types of stimulating electrodes1. Pacing2. Ablation3. Defibrillation
Features
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
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”, Graefe’s Arch Clin Exp Ophthalmol (2000) 238:840–845
In vivo neural microsystems (FIBE): challenge
In vivo neural microsystems (FIBE): biocompatibility - variant
In vivo neural microsystems (FIBE): state of the art
Neural microelectrodes
MEMS - Microsystems
Instrumentation for neurophysiology
Neural Microsystems
Introduction: neural microsystems
–
– –
– –
External electrodes
Subdural electrodes
Micro-electrodes
Microsensors
Human level
Animal level
Tissue slice level
Cellular level
Introduction: types of neural microsystems applications
In vivo applications
In vitro applications
Microelectronic technologyfor Microelectrodes
Bonding pads
Si substrateExposed tips
Lead viaChannels
Electrode
Silicon probe
Silicon chip
Miniatureinsulatingchamber
Contactmetal film
Hole
SiO2 insulatedAu probes
Silicon probe
Exposedelectrodes
Insulatedlead vias
(b)
(d)
(a)
(c)
Different types of microelectrodes fabricated using microfabrication/MEMS technology
Beam-lead multiple electrode. Multielectrode silicon probe
Multiple-chamber electrode Peripheral-nerve electrode
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 MichiganSmart comb-shape microelectrode arrays for brain stimulation and recording
University of Illinois at Urbana-ChampaignHigh-density comb-shape metal microelectrode arrays for recording
Fraunhofer Institute of Biomedical (FIBE) EngineeringRetina implant
Multi-electrode Neural Recording
Reference :http://www.nottingham.ac.uk/neuronal-networks/mmep.htm
Reference :
http://www.cyberkineticsinc.com/technology.htm
WPI’s Nitric Oxide Nanosensor
Nitric Oxide Sensor• Developed at Dr.Thakor’s Lab, BME, JHU
• Electrochemical detection of NO
Left: Schematic of the 16-electrode sensor array. Right: Close-up of a single site. The underlying metal is Au and appears reddish under the photoresist. The dark layer is C (300µm-x-300µm)
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 90°C. 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.
A
B
C
D H
G
F
E
NO Sensor Calibration
NO Sensor Calibration
Multichannel NO Recordings