Transcript Biopotential Electrodes - Iran University of Science and
Biopotential Electrodes (Ch. 5)
Electrode – Electrolyte Interface
Electrode
C Current flow
Electrolyte (neutral charge)
C+, A- in solution e C C A C+ 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)
Electrode – Electrolyte Interface
General Ionic Equations
a) b)
C
C n
ne
A m
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 )
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.
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!
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
Resistance
Current changes resistance of electrolyte and thus, a voltage drop results.
Concentration
Changes in distribution of ions at the electrode electrolyte interface
Activation
The activation energy barrier depends on the direction of current and determines kinetics
V p
V R
V C
V A 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
A
B
C
D
where E 0 : Standard Half Cell Potential
ne
The Nernst equation for half cell potential is
E
E
0
RT nF
ln
a a
C
A a a B
D
Note: interested in ionic activity at the electrode (but note temp dependence
E : Half Cell Potential a : Ionic Activity (generally same as concentration) n : Number of valence electrons involved
Polarizable and Non-Polarizable Electrodes
Perfectly Polarizable Electrodes
Use for 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
Perfectly Non-Polarizable Electrode
Use for 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
Example: Ag-AgCl is used in recording while Pt is use in stimulation
Ag + Cl -
Ag/AgCl Electrode
Relevant ionic equations
Ag
Ag
Cl
Ag
e
AgCl
Cl 2 Governing Nernst Equation
E
E
0
Ag
RT nF
ln
K a Cl
s
Solubility product of AgCl Fabrication of Ag/AgCl electrodes 1. Electrolytic deposition of AgCl 2. Sintering process forming pellet electrodes
Equivalent Circuit
C d R d
: capacitance of electrode-eletrolyte interface : resistance of electrode-eletrolyte interface
R s
: resistance of electrode lead wire
E cell
: cell potential for electrode
Corner frequency Rd+Rs Rs
Frequency Response
100
m
Electrode Skin Interface
E he
Electrode Gel Stratum Corneum Epidermis
100
m
Nerve endings
Dermis and subcutaneous layer
Capillary
C d C e R s E se R e R u R d C
Sweat glands and ducts
E P P R P
Alter skin transport (or deliver drugs) by: Pores produced by laser, ultrasound or by iontophoresis
Skin impedance for 1cm 2 patch: 200kΩ @1Hz 200 Ω @ 1MHz
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
Electrode metal Electrolyte 1. Metal Plate Electrodes (historic) 2. Suction Electrodes (historic interest) 3. Floating Electrodes 4. Flexible Electrodes
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
Commonly Used Biopotential Electrodes
Insulating package Metal disk (a) Double-sided Adhesive-tape ring Snap coated with Ag-AgCl Plastic cup Electrolyte gel in recess (b) External snap Gel-coated sponge Plastic disk
Reusable Disposable
Foam pad Tack Dead cellular material Capillary loops Germinating layer (c)
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 (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
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.
Contacts
Electrode Arrays
Ag/AgCl electrodes Contacts Insulated leads Ag/AgCl electrodes Exposed tip Insulated leads (a) Tines Base (b) Base Examples of microfabricated electrode arrays. (a) One-dimensional plunge electrode array, (b) Two-dimensional array, and (c) Three-dimensional array (c) Base
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
Intracellular Extracellular
Types
– Solid metal -> Tungsten microelectrodes – Supported metal (metal contained within/outside glass needle) – Glass micropipette -> with Ag-AgCl electrode metal
Metal Microelectrodes
C Microns!
Extracellular recording – typically in brain where you are interested in recording the firing of neurons (spikes).
R Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier!
Metal Supported Microelectrodes
(a) Metal inside glass (b) Glass inside metal
Glass Micropipette
heat pull Ag-AgCl wire+3M KCl has very low junction potential and hence very accurate for dc measurements (e.g. action potential) Fill with intracellular fluid or 3M KCl
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 myocyte Need high impedance amplifier…negative capacitance amplifier!
Electrical Properties of Microelectrodes
Metal Microelectrode
Metal microelectrode with tip placed within cell
Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier!
Equivalent circuits
Electrical Properties of Glass Intracellular Microelectrodes
Glass Micropipette Microelectrode
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
Types of stimulating electrodes
1. Pacing
Modern day Pt-Ir and other exotic metal combinations to reduce polarization, improve conductance and long life/biocompatibility
2. Ablation 3. Defibrillation
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
Introduction: neural microsystems Instrumentation for neurophysiology Neural microelectrodes
Neural Microsystems
MEMS Microsystems
Introduction: types of neural microsystems applications
Human
level
Animal
level
Tissue
slice level
Cellular
level External electrodes Subdural electrodes Micro electrodes Microsensors – In vivo applications – – – – In vitro applications
Microelectronic technology for Microelectrodes
Bonding pads SiO 2 insulated Au probes Exposed electrodes Insulated lead vias Silicon probe (a) Si substrate Exposed tips Beam-lead multiple electrode .
(b) Multielectrode silicon probe Miniature insulating chamber Hole Channels Silicon chip Lead via Silicon probe (c) Electrode Multiple-chamber electrode Contact metal film (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 University of Illinois at Urbana-Champaign High-density comb-shape metal microelectrode arrays for recording Fraunhofer Institute of Biomedical (FIBE) Engineering Retina implant
Multi-electrode Neural 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
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)
A B E 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 90°C. C) The samples were then exposed through a mask for 16s using UV light at 365nm and an intensity of 15mW/cm 2.
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.