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


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


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).


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

Multichannel NO Recordings

Biopotential Electrodes

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