electrodes_ch 5.ppt

7/25/2019 Electrodes_Ch 5.ppt

1/45

Biopotential

Electrodes(Ch. 5)


2/45

Electrode Electrolyte Interface

Electrode Electrolyte (neutral charge)

C+, A- in solutionC

CC

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 olymer coats !e.g. "a#ion, to imro$e selecti$ity%


3/45

Electrode Electrolyte Interface

+

+

+

meAA

neCC

m

n

General Ionic Equations

a) If electrode has sa!e !aterial as cation, then this !aterial "ets

o#idi$ed and enters the electrolyte as a cation and electrons re!ain

at the electrode and flow in the e#ternal circuit.

%) If anion can %e o#idi$ed at the electrode to for! a neutral ato!,

one or two electrons are "i&en to the electrode.

a)

%)

Current flow fro! electrode to electrolyte '#idation (oss of e-)

Current flow fro! electrolyte to electrode eduction (*ain of e-

)

he do!inatin" reaction can %e inferred fro! the followin"


4/45

Half Cell Potential

A characteristic potential difference esta%lished %y the electrode and its

surroundin" electrolyte which depends on the !etal, concentration of ions

in solution and te!perature (and so!e second order factors) .

Half cell potential cannot be measured without a second electrode

he half cell potential of the standard hydro"en electrode has %een ar%itrarily

set to $ero. 'ther half cell potentials are e#pressed as a potential difference

with this electrode.

!eason for Half Cell Potential " Charge #eparation at Interface'#idation or reduction reactions at the electrode-electrolyte interface

lead to a dou%le-char"e layer, si!ilar to that which e#ists alon"

electrically acti&e %iolo"ical cell !e!%ranes.


5/45

$easuring Half Cell Potential

"ote: Electrode material is metal & salt or olymer selecti$e membrane


6/45

#ome half cell potentials

tandard ydro"en electrode

"ote: Ag-AgCl has low

junction otential ' it is

also $ery stable -( hence

used in EC) electrodes*


7/45

Polari%ation

If there is a current %etween the electrode and electrolyte, theo%ser&ed half cell potential is often altered due to polari$ation.

&'erpotentialifference %etween o%ser&ed and $ero-current half cell potentials

!esistance

Current chan"es resistance

of electrolyte and thus,

a &olta"e drop results.

Concentration

Chan"es in distri%ution

of ions at the electrode-

electrolyte interface

cti'ation

he acti&ation ener"y

%arrier depends on the

direction of current and

deter!ines /inetics

AC+ ,,,, ++=

"ote: Polariation and imedance o# the electrode are two o# the

most imortant electrode roerties to consider.


8/45

ernst Equation

+=

A

/C

aa

aa

nF

+0

EE ln0

1hen two a2ueous ionic solutions of different concentration areseparated %y an ion-selecti&e se!i-per!ea%le !e!%rane, an electric

potential e#ists across the !e!%rane.

3or the "eneral o#idation-reduction reaction

+++ ne/CA

he 4ernst e2uation for half cell potential is

where E0 tandard alf Cell otential E alf Cell otential

a Ionic Acti&ity ("enerally sa!e as concentration)

n 4u!%er of &alence electrons in&ol&ed

"ote: interested

in ionic acti$ity

at the electrode

!but note tem

deendence


9/45

Polari%able and on*Polari%able

Electrodes

Perfectly Polari%able Electrodes

hese are electrodes in which no actual char"e crosses the electrode-

electrolyte interface when a current is applied. he current across theinterface is a displace!ent current and the electrode %eha&es li/e a

capacitor. E#a!ple A"6A"Cl Electrode

Perfectly on*Polari%able Electrode

hese are electrodes where current passes freely across the electrode-

electrolyte interface, re2uirin" no ener"y to !a/e the transition. hese

electrodes see no o&erpotentials. E#a!ple latinu! electrode

E1amle: Ag-AgCl is used in recording while Pt is use in stimulation

2se #or

recording

2se #or

stimulation


10/45

g+gCl Electrode

+ + eAgAg

+ + AgClClAg

A"+Cl-

Cl7

ele&ant ionic e2uations

*o&ernin" 4ernst E2uation

+=

Cl

sAg

a

3

nF

+0EE ln0

olu%ility

product of A"Cl

3a%rication of A"6A"Cl electrodes

8. Electrolytic deposition of A"Cl

7. interin" process for!in" pellet electrodes


11/45

Equi'alent Circuit

Cd capacitance of electrode-eletrolyte interface

+d resistance of electrode-eletrolyte interface

+s resistance of electrode lead wire

Ecell cell potential for electrode

3re2uency esponse

Corner #re4uency

+d&+s

+s


12/45

Electrode #,in Interface

weat "landsand ducts

Electrode

Epidermis

-ermis andsubcutaneous layer

Ru

Ehe

Rs

RdCd

Gel

Re

Ese EP

RPCPCe

#tratum Corneum

/in i!pedance for 8c!7patch

700/9 :8$

700 9 : 8;$

Alter skin

transort !or

deli$er drugs%

by:Pores

roduced by

laser,

ultrasound or

by

iontohoresis

566

566

"er$e

endings Caillary


13/45

$otion rtifact

.hy

1hen the electrode !o&es with respect to the electrolyte, the

distri%ution of the dou%le layer of char"e on polari$a%le electrode

interface chan"es. his chan"es the half cell potential te!porarily.

.hat

If a pair of electrodes is in an electrolyte and one !o&es with

respect to the other, a potential difference appears across the

electrodes /nown as the motion artifact.his is a source of

noise and interference in %iopotential !easure!ents

;otion artifact is !ini!al for non-polari$a%le electrodes


14/45

/ody #urface !ecording Electrodes

8. ;etal late Electrodes

(historic)

7. uction Electrodes

(historic interest)


15/45

Commonly 0sed /iopotential

Electrodes

$etal plate electrodes

ar"e surface Ancient,

therefore still used, EC*

;etal dis/ with stainless steel>platinu! or "old coated

E;*, EE*

s!aller dia!eters

!otion artifacts isposa%le foa!-pad Cheap?

(a) ;etal-plate electrode used for application to li!%s.

(%) ;etal-dis/ electrode applied with sur"ical tape.

(c)isposa%le foa!-pad electrodes, often used with EC*


16/45


Electrodes

#uction electrodes

-4o straps or adhesi&es

re2uired

-precordial (chest) EC*-can only %e used for short

periods

1loating electrodes

-!etal dis/ is recessed-swi!!in" in the electrolyte "el-not in contact with the s/in-reduces !otion artifact

uction Electrode


17/45

ou%le-sided

Adhesi&e-tape

rin"

Insulatin"pac/a"e

;etal dis/

Electrolyte "el

in recess(a) (%)

(c)

nap coated with A"-A"Cl E#ternal snap

lastic cup

ac/

lastic dis/

3oa! pad

Capillary loops

ead cellular !aterial

*er!inatin" layer

*el-coated spon"e


Electrodes

1loating Electrodes

+eusable

/isosable


18/45

(a) Car%on-filled silicone ru%%er electrode.

(%) 3le#i%le thin-fil! neonatal electrode.

(c) Cross-sectional &iew of the thin-fil!

electrode in (%).


Electrodes

1le2ible electrodes

-Body contours are often

irre"ular

-e"ularly shaped ri"idelectrodes

!ay not always wor/.-pecial case infants

-;aterial - oly!er or nylon with sil&er

- Car%on filled silicon ru%%er

(;ylar fil!)


19/45

Internal Electrodes

4eedle and wire electrodes for

percutaneous !easure!ent of

%iopotentials

(a) Insulated needle electrode.(%) Coa#ial needle electrode.

(c) Bipolar coa#ial electrode.

(d) 3ine-wire electrode connected

to hypoder!ic needle, %efore

%ein" inserted.(e) Cross-sectional &iew of s/in

and !uscle, showin" coiled

fine-wire electrode in place.

0he latest: 89" imlanted electrode #or muscle recording;stimulationAl#red E.


20/45

1etal ECG Electrodes

Electrodes for detectin" fetal electrocardio"ra! durin" la%or, %y !eans

of intracutaneous needles (a) uction electrode. (%) Cross-sectional &iew of

suction electrode in place, showin" penetration of pro%e throu"h epider!is.

(c) elical electrode, which is attached to fetal s/in %y cor/screw type action.


21/45

Electrode rrays

E#a!ples of !icrofa%ricated electrode arrays.

(a) 'ne-di!ensional plun"e electrode array,

(%) wo-di!ensional array, and

(c) hree-di!ensional array

Contacts Insulated leads

(b)

Base

Ag/AgCl electrodes

Ag/AgCl electrodes

BaseInsulated leads

(a)

Contacts

(c)

Tines

Base

Exposed tip


22/45

$icroelectrodes

.hy

;easure potential difference across cell !e!%rane

!equirements

!all enou"h to %e placed into cell

tron" enou"h to penetrate cell !e!%rane

ypical tip dia!eter 0.05 80 !icrons

3ypes

olid !etal -@ un"sten !icroelectrodes

upported !etal (!etal contained within6outside "lass needle)

*lass !icropipette -@ with A"-A"Cl electrode !etal

8ntracellular

E1tracellular


23/45

$etal $icroelectrodes

E1tracellular recording tyically in brain where you

are interested in recording the #iring o# neurons

!sikes%.

2se metal electrode&insulation -( goes to high

imedance amli#iernegati$e caacitance amli#ier*


24/45

$etal #upported $icroelectrodes

(a) ;etal inside "lass (%) *lass inside !etal

Ag AgCl wire&=


25/45

Glass $icropipette

A "lass !icropipet electrode filled

with an electrolytic solution(a) ection of fine-%ore "lass

capillary.

(%) Capillary narrowed throu"h

heatin" and stretchin".

(c) 3inal structure of "lass-pipet

!icroelectrode.

8ntracellular recording tyically #or recording #rom cells, such as

cardiac myocyte

"eed high imedance amli#iernegati$e caacitance amli#ier*

heat

ull

Fill with

intracellular #luidor =< 3Cl

Ag-AgCl wire&=

electrodes_ch 5.ppt

Documents