measuring recording and monitoring bioelectric signal

bioelectricity, electric potentials and currents produced by or occurring within living organisms. Bioelectric potentials are generated by a variety of biological processes and generally range in strength from one to a few hundred millivolts. In the electric eel, however, currents of one ampere at 600 to 1,000 volts are generated.ioelectric potentials are identical with the potentials produced by devices such as batteries or generators. In nearly all cases, however, a bioelectric current consists of a flow of ions (i.e., electrically charged atoms or molecules), whereas the electric current used for lighting, communication, or power is a movement of electrons. If two solutions with different concentrations of an ion are separated by a membrane that blocks the flow of the ions between them, the concentration imbalance gives rise to an electric-potential difference between the solutions. In most solutions, ions of a given electric charge are accompanied by ions of opposite charge, so that the solution itself has no net charge. If two solutions of different concentrations are separated by a membrane that allows one kind of ion to pass but not the other, the concentrations of the ion that can pass will tend to equalize by diffusion, producing equal and opposite net charges in the two solutions. In living cells the two solutions are those found inside and outside the cell. The cell membrane separating inside from outside is semipermeable, allowing certain ions to pass through while blocking others. In particular, nerve- and muscle-cell membranes are slightly permeable to positive potassium ions, which diffuse outward, leaving a net negative charge in the cell. bioelectric potential across a cell membrane is typically about 50 millivolts; this potential is known as the resting potential. All cells use their bioelectric potentials to assist or control metabolic processes, but some cells make specialized use of bioelectric potentials and currents for distinctive physiological functions. Examples of such uses are found in nerve and muscle cells. Information is carried by electric pulses (called action potentials) passing along nerve fibres. Similar pulses in muscle cells accompany muscular contraction. In nerve and muscle cells, chemical or electrochemical stimulation results in temporary changes in the permeability of cell membranes, allowing the electric potential between inside and outside to discharge as a current that is propagated along nerve fibres or that activates the contractile mechanism of muscle fibres. The transport of sodium ions is involved in the production of action potentials. Among other cells in which specialized functions are dependent on the maintenance of bioelectric potentials are the receptor cells sensitive to light, sound, and touch and many of the cells that secrete hormones or other substances.

Various fishes, both marine and freshwater, have developed special organs that are capable of generating substantial electric discha

2. Source of Bio electric Signals
Biomedical signals are those signals which are used primarily for extracting information of
biological system under study. Biomedical signals are of different types.
•
Bioelectric Signals:
•
They are generated by nerve cells and muscle cells.
•
Their basic source is the cell membrane potential.
•
The electric field generated by many cells constitutes the bio-electric signal.
•
Eg: ECG (electrocardiographic) and EEG (electroencephalographic)signals.
•
BioacousticSignals, Biomechanical Signals, Biochemical Signals, BiomagneticSignals, Bio-
optical Signals, Bio-impedance Signals,

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BIOPOTENTIALELECTRODES
•Tomeasureandrecordpotentialsandcurrentsinthebody,itisnecessarytoprovidesomeinterface
betweenthebodyandtheelectronicmeasuringapparatus.
•Theinterfacefunctioniscarriedoutbybiopotentialelectrodes.
•Theelectrodemustservesasatransducertochangeanioniccurrentintoanelectroniccurrent.
•Whenelectrodesintheirsimplestformmadeofpiecesofmetal,areplacedonorinsidethebody,they
comeincontactwithbodyfluidswhichmaybeconsideredaselectrolytes.
•Duetothiscontactbetweenametalandanelectrolytesolution,anelectrochemicalreactionproducesa
differenceofpotentialbetweenthemetalandsolution.
•Thechemicalreactionsthatoccurbetweenmetalsandelectrolytesinfluencetheperformanceofbio
potentialelectrodes.

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•Whenelectrodeisplacedontheskinsurface,thereissomeelectricalresistanceattheelectrode-skininterface.
•Sincetheskin’snaturalresistanceishighcomparedtotheresistanceofthebodyfluids,theselectedskinsiteistobe
wellpreparedbycleaningwithalcoholoracetoneandbyapplyingacommerciallyavailableconductivegel.
•Thisensuresalowvalueofelectrode-skininterfaceresistance.
•Whenanelectrodemakescontactwiththeskinviaanelectrolytepaste,theequivalentcircuitisshownbelowinfigure.
•Rsbecomestheeffectiveresistanceofthepaste(gel)betweentheelectrodeandtheskin.
•Theepidermisoftheskinmaybeconsideredasasemipermeablemembraneandthepotential differenceacrossit is
representedbyEsc.
•Theepidermiclayerhasalsoanelectricalimpedance,whichisrepresentedbytheparallelcircuit Ceand Re.
•The dermisandsubcutaneouslayerunderitbehavesingeneralaspureresistanceRde.
BIOPOTENTIALELECTRODES

20. Equivalentcircuitof skinelectrodeinterface
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5

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ELECTRODETYPES
•Awidevarietyofelectrodescanbeusedtomeasurebioelectricevents,butnearlyallcanbeclassifiedas
belongingtooneofthesethreebasictypes:
–1.SkinsurfaceElectrodes:TheyareusedtomeasureECG,EEGandEMGpotentialsonthesurfaceofthe
skin.
–2.NeedleElectrodes:TheyareusedtopenetratetheskintorecordEEGpotentialsfromalocalregionof
thebrain,orEMGpotentialsfromaspecificgroupofmuscles.
–3.MicroElectrodes:Theyareusedtomeasurebioelectricpotentialsnearasinglecell
•Allthreetypesofbiopotentialelectrodeshavethemetal-electrolyteinterfacedescribedintheprevious
section.Ineachcase,anelectrodepotentialisdevelopedacrosstheinterface,proportionaltothe
exchangeofionsbetweenthemetalandtheelectrolytesofthebody.Thedoublelayerofchargeatthe
interfaceactsasacapacitor.Thus,theequivalentcircuitofbiopotentialelectrodeincontactwiththe
bodyconsistsofavoltageinserieswitharesistance-capacitancenetworkofthetypeshownin
previousfigure.

22. •
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SKINSURFACEELECTRODES
•Manydifferenttypesofelectrodesforrecordingvariouspotentialsonthebodysurfacehave
beendevelopedovermanyyears.
•Varioustypesofskin surfaceelectrodesare:
–Metal-plateelectrodes,Suctionelectrodes,Floatingelectrodes,Spray-on electrodes,disposable
electrodes,ear-clipelectrode,scalpsurfaceeletrodes
1.METALPLATEELECTRODES
•Mostfrequentlyusedformofbiopotentialsensingelectrodeisthe metal-plateelectrode.
•Inits simplestform,itconsistsofmetallicconductorincontactwith theskin.
•An electrolytesoakedpadorgelisusedtoestablishandmaintaingoodcontact.

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Metal-plateelectrodes
•Threedifferenttypesofmetalplateelectrodesareavailable:
–a)Metal-plateelectrodeisusedforapplicationoflimbs
–b)Metal-diskelectrodeisusedappliedwithsurgicaltape
–c)Disposablefoam-padelectrodes
1.a:Metal-plateelectrodeisusedforapplicationoflimbs
•Itconsistsofaflatmetalplatethathasbeenbentintoacylindersegment.
•Aterminalisplacedonitsoutsidesurfacenearoneend;thisterminalisusedtoattachthelead wiretothe
electrocardiograph.
•Apost,placedonthissamesidenearthecenterisusedtoconnectarubbertraptotheelectrode andholdit
inplaceonanarmor leg.
•TheelectrodeistraditionallymadeofGermansilver(anickel-silveralloy).
•Beforeitis attachedto thebody,itsconcavesurfaceiscoveredwithelectrolytegel.

24. MetalPlateElectrodes
Figure:Metalplateelectrode:Theseplatesare
usuallymadeoforplatedwith,silver,nickel,or
somesimilaralloy.
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25. ImmersionElectrode
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Metal PlateElectrodes
1.b:Metal-diskelectrodeisusedappliedwithsurgicaltape
•Secondcommonvarietyofmetal-plateelectrodeisthemetaldisk,whichhasaleadwiresolderedorweldedtothe
backsurface.Theycanbemadeofseveraldifferentmaterials.
•Attimestheconnectionbetweenleadwireandelectrodeisprotectedbyalayerofinsulatingmaterialsuchasepoxy
orpolyvinylchloride.
1.c:Disposablefoam-padelectrodes
•TheycanbeusedasachestelectrodeforrecordingECGorincardiacmonitoringforlongtermrecordings.
•Itconsistsofarelativelylargediskofplasticfoammaterialwithasilverplateddiskononesideattachedtoasilver
platessnapsimilartothatusedonclothinginthecenterofotherside.
•Aleadwireisconnectedontotheelectrodeandused.The silverplateddiskservesastheelectrodeandmaybe
coatedwithAgCllayer.Alayerofelectrolytegelcoversthedisk.
•Theelectrodesideofthefoamiscoveredwithanadhesivematerialthatiscompatiblewiththeskin.
•Aprotectivecoverorstripofpaperorcottonorclothplacedovertheelectrodeside.

27. Pavithra.K.N_gec_palakkad
2.SUCTIONELECTRODES
•Amodificationofthemetalplateelectrodethatrequiresnostrapsoradhesivesforholdingitinplaceisthesuction
electrodewhichisshowninfigure4.
•Suchelectrodesarefrequentlyusedelectrocardiographyastheprecordial(chest)leads,becausetheycanbeplaced
atparticularlocationsandusedthatmakescontactwiththeskinatitsbase.
•Anappropriateterminalfortheleadwireistobeattachedtothemetalcylinder,andarubbersuctionbulbfitsover
itsotherbase.
•Theelectrolytegelisplacedoverthecontactingsurfaceoftheelectrode,thebulbissqueezed,andtheelectrodeis
thenplacedonthechestwall.
•Thebulbisreleasedandappliessuctionagainsttheskinholdingtheelectrodeassemblyinplace.
•Suchelectrodecanbeusedonlyforshortperiodsoftime;thesuctionandthepressureofthecontactsurfaceagainst
theskincancauseirritation.
•Althoughtheelectrodeitselfisquietlarge.Itisobservedthattheactualcontactingareaisrelativelysmall.
•Thiselectrodethustendstohavehighersourceimpedancethantherelativelylargesurfaceareametalplate
electrodesusedforECGlimbelectrodes.

28. SUCTIONCUPELECTRODE
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29. Pavithra.K.N_gec_palakkad
3. FLOATINGELECTRODES
•Anothertypeofskinsurfaceelectrodeisfloatingelectrodeorliquidjunctionelectrodeshowninfigure5.
•Theprinciplefeatureisthattheactualmetaldiskisrecessedinacavitysothatitdoesnotmakedirectcontactwiththe
skin.
•Theelectrical contactisestablishedthroughtheelectrolytepastefilledinthecavity.
•Theelectrodeelementismade ofsilver andisoftencoatedwithsilver chloride(AgCl).
4.Spray-onelectrodes
•SpecialproblemsencounteredinthemonitoringoftheECGofastronautsduringlongperiodsoftime,and
underconditionsofperspirationandconsiderablemovement,ledtothedevelopmentofspray-onelectrodes,
inwhichasmallspotofconductiveadhesiveissprayedorpaintedovertheskin ,whichhadpreviouslybeen
treatedwithanelectrolytecoating.

30. FLOATINGELECTRODES
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31. Pavithra.K.N_gec_palakkad
5. disposableelectrodes
•Varioustypesofdisposableelectrodeshavebeenintroduced inrecentyearstoeliminatetherequirement
forcleaningandcareaftereachuse.AnexampleisshowninFigure4.10.
•PrimarilyintendedforECGmonitoring,theseelectrodescanalsobeusedforEECandEMGaswell.In
general,disposableelectrodesareofthefloatingtypewithsimplesnapconnectors bywhichtheleads,
whicharereusable,areattached.Althoughsomedisposableelectrodescanbereusedseveraltimes,their
costisusuallylowenoughthatcleaningforreuseisnotwarranted.
6.Ear-clipelectrodes:ear-clipelectrode(Figure4.11)wasdevelopedforuseasareferenceelectrodeforEEG
measurements.
7.Scalpsurfaceelectrodes:ScalpsurfaceelectrodesforEEGareusuallysmalldisksabout7mmindiameteror
smallsolderpelletsthatareplacedonthecleanedscalp,usinganelectrolytepaste.Thistypeofelectrode
isshowninFigure4.12.

32. Pavithra.K.N_gec_palakkad

33. 2.NEEDLEELECTRODES
•Toreduceinterfaceimpedanceand,consequently,movementartifacts,someelectroencephalographers
usesmallsubdermalneedlestopenetratethescalpforEEGmeasurements.Theseneedleelectrodes,
showninFigure4.13,arenotinsertedintothebrain;theymerelypenetratetheskin.Generally,theyare
simplyinsertedthroughasmallsectionoftheskinjustbeneaththesurfaceandparalleltoit.
•InNeedleelectrode,theelectrodeortheleadwirepenetratestheskin,ortheymaybeimplanted
internallyandconnectedtoanimplantedelectroniccircuit.Toevaluateindividualmotorunitswithina
muscle,aneedleelectrodemustbeplacedintothemuscleitself.
•NeedleelectrodesforEMG-consistmerelyoffineinsulatedwires,placedsothattheirtips,whichare
bare,areincontactwiththenerve,muscle,orothertissuefromwhichthemeasurementismade.The
remainderofthewireiscoveredwithsomeformofinsulationtopreventshorting.Wireelectrodesof
copperorplatinumareoftenusedforEMGpickupfromspecificmuscles.Thewiresareeither
surgicallyimplantedorintroducedbymeansofahypodermicneedlethatisla
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34. •Withthistypeofelectrode,themetal-electrolyteinterfacetakesplacebetweentheun
insulatedtipofthewireandtheelectrolytesofthebody,althoughthewireisdippedinto an
electrolytepastebeforeinsertioninsomecases.
•Thehypodermicneedleis sometimes a partofthe
electrodeconfigurationandisnotwithdrawnInstead,the
wiresforming theelectrodesarecarriedinsidetheneedle,
whichcreatestheholenecessaryforinsertion,protectsthewires,andactsasa grounded
shield.
•Asinglewireinsidetheneedleservesasa unipolarelectrode,whichmeasuresthe
potentialsatthepointofcontactwithrespecttosomeindifferentreference.
•Iftwowiresareplacedinsidetheneedle,themeasurementiscalled bipolarandprovides
averylocalizedmeas
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NEEDLEELECTRODES
19

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3.MICROELECTRODES
•Microelectrodesareelectrodeswithtipssufficientlysmalltopenetrateasinglecellinordertoobtain
readingsfromwithinthecell.Thetipmustbesmallenoughtopermitpenetrationwithoutdamagingthecell.
Thisactionisusuallycomplicatedbythedifficultyofaccuratelypositioninganelectrodewithrespecttoa
cell.
•Microelectrodesaregenerallyoftwotypes:metalandmicropipet.
•Suchelectrodesmustbesmallwithrespecttothecelldimensionstoavoidcausingserious cellularinjuryand
therebychangingthecell’sbehaviour.
•Inadditiontobeingsmall,theelectrodeusedformeasuringintracellularpotentialmustalsobe strongsothatitcan
penetratethe cellmembraneandremainmechanicallystable.
•Theelectrodesthatmeettheserequirementsareknownasmicroelectrodes.
•Theyhavetipdiameterrangingfromapproximately0.05to10μm.
•Microelectrodescanbeformedfromsolid-metalneedles,frommetalcontainedwithinoronthesurfaceofaglass
needle,orfromaglassmicropipettehavingalumenfilledwithanelectrolytesolution.

36. Pavithra.K.N_gec_palakkad
Metalmicroelectrodes:
•Metalmicroelectrodesareformedbyelectrolyticallyetchingthetipofafinetungstenor stainless-steelwire
tothedesiredsize.
•Thenthewireiscoatedalmosttothetipwithaninsulatingmaterial.Someelectrolytic processingcanalso
beperformedonthetiptolowertheimpedance.
•Themetal-ioninterfacetakesplacewherethemetaltipcontactstheelectrolyteseither insideoroutsidethe
cell.
Micro pipetmicroelectrodes:
•Themicropipettypeofmicroelectrodeisaglassmicropipetwiththetipdrawnouttothedesiredsize
[usuallyabout1micron(nowmorecommonlycalledmicrometer)indiameter].Themicropipetisfilledwith
anelectrolytecompatiblewiththecellularfluids.
•Thistypeofmicroelectrodehasadualinterface.Oneinterfaceconsistsofametalwireincontactwiththe
electrolytesolutioninsidethemicropipet,whiletheotheristheinterfacebetweentheelectrolyteinsidethe
pipetandthefluidsinside
orimmediatelyoutsidethecell.

37. •AcommercialtypeofmicroelectrodeisshowninFigure4.3.Inthiselectrodeathinfilmofpreciousmetalis
bondedtotheoutsideofadrawnglassmicroelectrode.Themanufacturerclaimssuchadvantagesaslower
impedancethanthemicropipetelectrode,infiniteshelflife,repeatableandreproducibleperformance,and
easycleaningandmaintenance.
•Themetalelectrolyteinterfaceisbetweenthemetalfilmandtheelectrolyteofthecell.
•Microelectrodes,becauseoftheirsmallsurfaceareas,haveimpedanceswellupintothemegaohms.Forthis
reason,amplifierswithextremelyhighimpedancesarerequiredtoavoidloadingthecircuitandtominimizethe
effectsofsmallchangesininterfaceimpedance
.
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