Bio-potential Electrodes

Bio-potential

• Sources of bio-potential Nerve conduction Brain activity Heart Beat Muscle activity• Depends on dimensions Anatomical Bio electric generator

What are biopotentialsBiopotential: An electric potential that is measured between points in living cells,

tissues, and organisms, and which accompanies all biochemical processes. • transfer of information between and within cells• measurement of potentials

Electrodes – Requirement• High-quality bio-potential measurements require

– Good amplifier design– Use of good electrodes and their proper placement on the patient– Good laboratory and clinical practices

• Electrodes should be chosen according to the application

• Basic electrode structure includes:– The body and casing– Electrode made of high-conductivity material– Wire connector– Cavity or similar for electrolytic gel– Adhesive rim

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.

Some half cell potentials

Standard Hydrogen electrode

Note: Ag-AgCl has low junction potential & it is also very stable -> hence used in ECG electrodes!

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 over potentials. Example : Platinum electrode

Use for recording

Use for stimulation

Motion ArtifactWhy

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

2. Suction Electrode

3. Floating Electrodes

4. Flexible Electrodes

– Large surface: Ancient, therefore still used, ECG

– Metal disk with stainless steel; platinum or gold coated

For EMG, EEG electrodes

– smaller diameters

– motion artifacts

– Disposable foam-pad:

(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

Metal plate electrodes

- No straps or adhesives required- precordial (chest) ECG- used for short periods

Suction Electrode

Suction electrodes

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 pad Capillary loops

Dead cellular material

Germinating layer

Gel-coated sponge

Floating Electrodes

Reusable

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

(a) Carbon-filled silicone rubber electrode. (b) Flexible thin-film neonatal electrode.(c) Cross-sectional view of the thin-film electrode in (b).

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

Flexible electrodes

Ag-AgCl, Silver-Silver Chloride Electrodes

• The most commonly used electrode type• Silver is interfaced with its salt silver-chloride• Choice of materials helps to reduce junction potentials

– Junction potentials are the result of the dissimilar electrolytic interfaces

• Electrolytic gel enhances conductivity and also reduces junction potentials– Typically based on sodium or potassium chloride, concentration in

the order of 0.1 M weak enough to not irritate the skin

• Relatively low-cost and general purpose electrode• Particularly suited for ambulatory or long term use

Gold Electrodes

• Very high conductivity suitable for low-noise .• Inertness suitable for reusable electrodes• Body forms cavity which is filled with electrolytic gel• Compared to Ag-AgCL: greater expense, higher

junction potentials and motion artifacts• Often used in EEG, sometimes in EMG

Needle electrodes

• Obviously invasive electrodes• Used when measurements have to be taken from the organ itself• Small signals such as motor unit potentials can be measured• Needle is often a steel wire with hooked tip

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.

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

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

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)

Preparation for Electrodes

• Skin preparation by abrasion or cleansing• Placement close to the source being measured• Placement above bony structures where there is less muscle mass• Distinguishing features of different electrodes:

– How secure? The structure and the use of strong but less irritant adhesives– How conductive? Use of noble metals vs. cheaper materials– How prone to artifact? Use of low-junction-potential materials such as Ag-AgCl– If electrolytic gel is used, how is it applied? High conductivity gels can help reduce the

junction potentials and resistance but tend to be more allergenic or irritating

Baseline drift due to thechanges in junctionpotential or motion artifactsChoice of electrodes Muscle signal

interference Placement Electromagnetic

interference Shielding

Electrical Properties of electrodes

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

100

100

Nerve endings Capillary

The Instrumentation Amplifier• Potentially combines the best features desirable for biopotential

measurements– High differential gain, low common mode gain, high CMRR, high input resistance

• Simple and cheap, although high-quality OpAmps with high CMRR should be used

Gain control

1

21 21

R

RG

3

42 R

RG

CMRR fine tuning

Application-specific requirements• ECG amplifier

– Lower corner frequency 0.05 Hz, upper 100Hz– Safety and protection: leakage current below safety standard limit of 10 uA– Electrical isolation from the power line and the earth ground– Protection against high defibrillation voltages

• EEG amplifier– Gain must deal with microvolt or lower levels of signals– Components must have low thermal and electronic noise @ the front end– Otherwise similar to ECG

• EMG amplifier– Slightly enhanced amplifier BW suffices– Post-processing circuits are almost always needed (e.g. rectifier +

integrator)

Electro cardiogram