ELEC4623/ELEC9734: Semester 2 2009ELEC4623/ELEC9734: … · 2009-07-28 · Biomedical...

ELEC4623/ELEC9734: Semester 2 2009ELEC4623/ELEC9734: Semester 2, 2009

Dr Stephen RedmondfSchool of Electrical Engineering & Telecommunications

Email: [email protected]: 9385 6101Rm: 458, ELECENG (G17)

Medical Instrumentation (Webster): Chapter 5Medical Instrumentation (Webster): Chapter 5

Notes: https://subjects.ee.unsw.edu.au/elec4623/

Session 2, 2009 ELEC4623/ELEC9734 1

on http://vista.elearning.unsw.edu.au soon…

Biomedical Instrumentation, Measurement and DesignELEC4623/ELEC9734/

Lectures 3 & 4Biopotential Electrodes and Tissue Equivalent Circuits

Session 2, 2009 2ELEC4623/ELEC9734

Biopotential Electrodes

Biopotentials are generated by nerve or muscle tissueThe most widely used of all biological transducers

l h d h lBioelectric phenomena are associated with almost every organ system


Bioelectric signals sensed by biopotential electrodes

Bioelectric signal Abbreviation Biologic sourceElectrocardiogram ECG (EKG) Heart (body surface)C di l t H t (i t ll d)Cardiac electrogram - Heart (internally sensed)Electromyogram EMG MuscleElectroencephalogram EEG BrainEl t ti EOG E di l fi ldElectrooptigram EOG Eye dipole fieldElectroretinogram ERG Eye retinaAction potential - Nerve or muscleElectrogastogram EGG StomachGalvanic skin response GSR Skin


First impressions

Biopotential electrodes must conduct current to sense voltage (V = I.Zin)Normally current is very small (but non-zero) and Zin very largey y in y gMust have small current (< pA) to prevent instrument from affecting the readingCurrent must flow across the interface between the body and electronic measuring circuitelectronic measuring circuit

Bioelectrode converts ionic current in body to electronic current in circuit via electrode-electrolyte interface


Electrode – Electrolyte Interface

Electrode Electrolyte (neutral charge)Electrode Electrolyte (neutral charge)C+, A- in solution

C C+

Current flow

Electron flow opposite direction

CC

A-

C+

C+e-

e-

To conventional current flow

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. AgCl or polymer coats

Electrode – Electrolyte Interface

−−

−+

+↔

+↔

meAAneCC

m

n1)

2)Oxidation → Reduction ←

1) If electrode is made of atom with same material as cation, then (i) this material gets oxidised and enters the electrolyte ( ) g yas a cation, and (ii) electrons remain at the electrode and flow in the external circuit.

2) If anion can be oxidised at the electrode-electrolyte interface to form a neutral atom, electrons are given to the electrode.

Reactions can go in either directionC t fl f l t d t l t l t O id ti (L f )Current flow from electrode to electrolyte : Oxidation (Loss of e-)Current flow from electrolyte to electrode : Reduction (Gain of e-)Session 2, 2009 7ELEC4623/ELEC9734

Half cell potential (HCP)

When electrode is immersed in electrolyte it (usually) begins to oxidise (reduction could happen too, depending on materials)

Electron remains in metal electrodeElectron remains in metal electrodeCation moves into electrolyte.

As the reaction continues, a polarised double layer is set up at , p y pthe interface

Electrons in electrodeCations in Electrolyte

Eventually this electric field stops any further cations moving to electrolyte

The voltage difference when this equilibrium is reached is calledThe voltage difference when this equilibrium is reached is called the Half Cell Potential (HCP)


Measuring half cell potential

Not possible to measure half cell potentialNot possible to measure half cell potential We try by introducing another material into the electrolyte to complete the circuit (hence the half in the term)But all we now measure is the difference between two differentBut all we now measure is the difference between two different HCPs!

Half cell potentials are thus measured with respect to a hydrogen electrode based on the reaction:hydrogen electrode based on the reaction:

2 2H H +2 2 2H H e+ −↔ +

I b l l d i l i


Its absolute electrode potential is estimated to be 4.44 ± 0.02 V at 25 °C

Standard half cell potentials for common electrode materials (at 25 °C)

Metal and Reaction Potential E°, VAl → Al3+ + 3e- -1.706

Zn → Zn2+ + 2e- -0.763

Cr → Cr3+ + 3e- -0.744Cr → Cr 3e 0.744

Fe → Fe2+ +2e- -0.409

Cd → Cd2+ + 2e- -0.401

Ni → Ni2+ +2e- -0.230

Pb → Pb2+ + 2e 0 126Pb → Pb2+ + 2e- -0.126

H2 → 2H+ + 2e- 0 (by definition)

Ag+Cl- → AgCl + e+ +0.233

2Hg+2Cl- → Hg2Cl2 + 2e- +0.268

Cu → Cu2+ + 2e- +0.340

Cu → Cu+ + e- +0.522

Ag → Ag+ + e- +0.799

Au → Au3+ + 3e- +1.420

Au → Au+ + e+ +`1.680


Nernst equation and ionic availability

Ionic activity is defined as the availability of an ionic species in solution to enter into a reaction

In dilute solutions ionic activity is approximately equal to ionic concentration

At higher concentration, intermolecular effects are significant, so activity of ions is less than their concentration

If ionic activities across a membrane are a1 and a2Nernst equation may be rewritten as:

aRT ⎡ ⎤1

2

ln aRTEnF a

⎡ ⎤= − ⎢ ⎥

⎣ ⎦Session 2, 2009 ELEC4623/ELEC9734 11

Nernst equation and HCP

−++↔+ neDCBA δγβα

For the general oxidation-reduction reaction:

γβ

The Nernst equation for half cell potential is:

⎥⎦

⎤⎢⎣

⎡+= βα

δγ

BA

DC

aaaa

nFRTEE ln0

where E0 : Standard Half Cell Potential

E : Half Cell Potential

I i A i i ( ll i )a : Ionic Activity (generally same as concentration)

n : Number of valence electrons involved


PolarisationStandard HCP measured only when zero current flow across interface WhenStandard HCP measured only when zero current flow across interface. When current flows, difference between observed HCP and equilibrium zero current HCP is known as overpotential:

• V is the total overpotential or polarisation potential of the electrode

ACRp VVVV ++=

Vp is the total overpotential or polarisation potential of the electrode• VR is the ohmic (resistive) overpotential caused by current flow across

electrode/electrolyte resistance (this resistance can vary with current but not necessarily linearly)V i h i i l hi h l f h i h• VC is the concentration overpotential which results from changes in the distribution of ions in the electrolyte in the vicinity of the electrode-electrolyte interface when a current flows.

• VA is the activation overpotential which arises from the fact that reactions in 1) and 2) in slide 7 involve the expenditure of different activation energies which are not identical in both directions (oxidation and reduction different).


Polarisable and nonpolarisable electrodes

Perfectly polarisable electrodes Would behave like capacitors in that no net charge would actually cross the electrode-electrolyte interface. Majority of overpotentialy j y pfrom concentration potential.These are best approximated, by noble or inert metals (e.g. platinum) which do not easily oxidize.

Nonpolarisable electrodes Require that the current passes freely across the interface without any expenditure of energy, thus no overpotentials.any expenditure of energy, thus no overpotentials. The electrode, which closest approaches a nonpolarizableelectrode, is the silver/silver chloride (Ag/AgCl) electrode.


The Silver/Silver Chloride Electrode (Ag/AgCl)Ag electrode with thin layer of AgCl deposit. g y g pElectrolyte commonly contains KCl.

−+ +↔ eAgAg ↓↔+ −+ AgClClAgSAg Cl

a a K+ −× =Under equilibrium

K is called the

AgCl not very soluble and precipitates at rate KsPrecipitation: formation of solid in solution during a reaction

+↔ eAgAg ↓↔+ AgClClAg

⎤⎡ KRT

KS is called the solubility product

⎥⎥⎦

⎤

⎢⎢⎣

⎡+=

−Cl

sAg a

KnFRTEE ln0

RT RT[ ]0 ln lnAg s Cl

RT RTE E K anF nF −⎡ ⎤= + − ⎣ ⎦

First two terms are constants, third term d d i f Cl hi h i ldepends on concentration of Cl- which is also stable due to high concentration in biological fluids, so Ag/AgCl electrode is stable


The electrical noise evident is of such a frequency and magnitude as to seriously

Electrical Noise from Ag/AgCl electrodesFigure shows electrical noise generated for Ag/AgCl electrodes against purely frequency and magnitude as to seriously

interfere with the recording of a high quality ECG signal.

for Ag/AgCl electrodes against purely metallic Ag electrodes.

(a) Normal Ag/AgCl electrode (b) Ag/AgCl Electrode with AgCl rubbed off (c)AgCl redeposited


Electrode-electrolyte interface equivalent circuitThe current voltage relationship are often non linear and dependent on theThe current voltage relationship, are often non-linear and dependent on the magnitude of the current passing through the electrodeElectrode characteristics are different for high and low current densities, are frequency dependent and are also dependent on the current waveformsCapacitance comes from the double layer distribution of charge that occurs at the electrode-electrolyte interfaceQuestion!!!High frequency: f→∞ Zt t→??? 2High frequency: f ∞ Ztot ???Low frequency f→0, Ztot→???.

0.25cm2 Metallic silver electrode impedance

C it f l t d l t l t i t fCd : capacitance of electrode-electrolyte interfaceRd : resistance of electrode-electrolyte interfaceRs : resistance of electrolyte and wiresEhc : half cell potential for electrode


The skin – cross section


The skin electrode interface


Equivalent circuit of skin electrode interface

? ?? ?

The skin: an additional interface!Session 2, 2009 20ELEC4623/ELEC9734


??



The skin: an additional interface!Dominant elements


Impedance vs. frequency


Motion Artifacts

Mechanical disturbance of the distribution of charge at the electrode/electrolyte interface, will momentarily change the HCP until equilibrium is restoredHCP until equilibrium is restored.

Motion artifact mainly affects polarisable electrodes, but minimal for non polarisable electrodes (Why?)minimal for non-polarisable electrodes (Why?)

Mechanical disturbances can also influence Ese (Vep). Removing h ll h l h ff dthe stratum corneum will thus also have an effect on reducing

sensitivity to motion artefacts by short-circuiting this source(don’t abrade too much – irritation!)


Motion artifact(a) Metallic Ag electrodes in(a) Metallic Ag electrodes in agitated physiological saline solution

(b) Same electrodes with an AgCl surface film in agitated physiological saline solution

(c) Output from amplifier used for recordings, when electrodes are replaced by a 1/5-kΩare replaced by a 1/5-kΩresistor

Heavy lines under curvesHeavy lines under curves indicate periods of agitation of saline solution


Motion artifact

Skin stretch is known to be one of the largest generators of motion artifact (Talhouet and Thakor, Physiol. Meas. 17 (1996) 81–93.)


Motion artifact – skin stretch


Research – motion artifact is a big issue in telehealth6

2

4

V)

-2

0

plitu

de (m

V

-6

-4

EC

G A

mp

0 5 10 15 20 25 30-10

-8 Artifact Mask


0 5 10 15 20 25 30Time (s)

Example of movement artifact in a sample ECG recording. Also shown is the artifact detection determinedby the algorithm developed in UNSW.

Typical electrode/skin impedanceDried exudation of the Sterculia Urens tree

F and E are synthetic conductive adhesives,C and D are Karaya gum

and other species of Sterculia (native to India)

C and D are Karaya gum conductive adhesive, A and G are wet paste (gel) electrodes with stainless-steel conductor,G is wet paste Ag/AgClelectrode and

h b hH is the same as G but with a light abrasion of the skin

Typical values of electrode-skin impedance for eight different electrode types measured at 0.05 Hz, 1 minute after application


Metal plate electrodes

Large surfaceAncient (still used!)

l d k h lMetal disk with stainless steel, platinum or gold coating

dFoam pad version is disposable, others may be reused

(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 requiredPrecordial (chest) ECGPrecordial (chest) ECGCan only be used for short periods (why?)S ll h hSmall contact area so high source impedance compared with metal plate electrode


Floating electrodes

Insulating

Metal disk (no direct contact with skin)

Reusable (metal type)

Double sided

Insulatingpackage

Reusable (metal type)

Double-sidedadhesive-tapering

Electrolyte gel in cavity

(a) (b)( ) ( )Snap coated with Ag/AgCl External snap

Plastic cup Plastic diskGel-coated sponge

( )

TackFoam padCapillary loops

Dead cellular material

Germinating layerDisposable Ag/AgCl

Minimize motion artefacts (how?)(c)

g y


Flexible electrodes

Body contours are often irregularRegularly shaped rigid electrodes may not alwayselectrodes may not always work.Example: infants Material :

l l h lPolymer or nylon with silver Carbon filled silicon rubber (Mylar film)

(a) Carbon-filled silicone rubber electrode. (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

Percutaneous electrodePercutaneous = electrode crosses the skin

(a) Insulated needle electrode(b) Coaxial needle electrode(b) Coaxial needle electrode(c) Bipolar coaxial electrode(d) Fine-wire electrode connected

to hypodermic needle, before being inserted

(e) Fine-wire electrode after insertion(e) Fine wire electrode after insertion(f) Cross-sectional view of skin

and muscle, showing coiled fine-wire electrode in place


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.


Standards for pregelled electrodes (AAMI)Direct-current offset voltage. “A pair of electrodes connected gel to g p ggel after 1 min of stabilization must exhibit offset voltages no greater than 100mV.

Combined offset stability and internal noise. “A pair of electrodes connected gel to gel after 1min of stabilization shall generate aconnected gel to gel after 1min of stabilization shall generate a voltage no greater than 150μV P-P in the pass band of 0.15 to 100 Hz.”

Alternating current impedance. “For a pair of electrodes connected gel g p p gto gel the impedance at 10Hz will not exceed 3kΩ. The average for 12 pairs will not exceed 2kΩ.

Defibrillation overload recovery. “The absolute value of polarisation potential of a pair of electrodes connected gel to gel shall not exceedpotential of a pair of electrodes connected gel to gel shall not exceed 100 mV, 5 s after each of four capacitor discharges of 10μF charged to 200V”.

Bias current tolerance. “The observed dc voltage offset change across g gan electrode pair connected gel to gel shall not exceed 100mV when subjected to a continuous 200nA dc current over a period, not exceed 8 hours, as recommended by the manufacturer.”


Practical issuesElectrode and lead wire should ideally be of the same materialElectrode and lead wire should ideally be of the same material

Why? (Hint: Galvani potential)

Use the same material for each electrode, if more than one is used

Use tape to hold electrode onGive some slack to allow for movementLead wire should be VERY flexible

Input impedance of the amplifier must be much higher than the impedance of the source (body) and electrode equivalent impedances, otherwise distortion of the signal will occur. (Geddes et al. 1966, The relationship between input impedance and electrode area)relationship between input impedance and electrode area).


ELEC4623/ELEC9734: Semester 2 2009ELEC4623/ELEC9734: … · 2009-07-28 · Biomedical...

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