Rowan ECE 09.404 - Lecture 6: Biopotential Electrodes

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This Week in PBS&DBiomedical SensorsBiopotential ElectrodesElectrode – Electrolyte InterfaceHalf-Cell PotentialsSilver – Silver Chloride (Ag/AgCl)ElectrodesIonic ActivityRelative half-Cell PotentialsElectrode Behavior & Circuit ModelsElectrode Behavior & Circuit ModelsFrequency DependencyElectrode – Skin InterfaceSkinDermisYep…You guessed it Right(The electric Model…)Motion ArtifactsCommonly Used Biopotential ElectrodesBody-surface biopotential electrodesSuction ElectrodeFloating ElectrodesFlexible ElectrodesInternal ElectrodesFetal ECG ElectrodesElectrode ArraysMicroelectrodesMetal MicroelectrodesMetal Supported MicroelectrodesGlass MicropipetMicroelectronic technologyfor MicroelectrodesPrinciples of Biomedical Systems & DevicesPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Principles of Biomedical Systems & DevicesLecture 6: Biopotential ElectrodesThis Week in PBS&D Electrodes and sensors used to detect biopotentialsª Half-cell potentialª Ag/AgCl Electrodes Electrode Behavior and Circuit Models Electrode Skin Interface Motion Artifacts Commonly used body surface electrodesª Metal plate electrodesª Suction electrodesª Floating electrodesª Flexible electrodesBiomedical Sensors In vivo and in vitro measurements Î biomedical sensorsª Physicalª Electricalª Chemicalª Biological  Transducer: Convert biological information into a measurable andquantifiable electrical / optical / physical signal. It converts one form of energy into another Sensor: Convert a physical parameter into an electrical output Actuator: Convert electrical signal into a physical output Things we measure: dimensional / mechanical changes, chemical changes, electrical changes, thermal changes Biopotentials: Electrical changesBiopotential Electrodes Provide the interface necessary between the electronic changes in the body and the recording / measuring device. It is not a mere metal that conducts the electrical changes in the body, and hence it is not as trivial as it may seem, why…?ª The current in the body is carried by ______________, whereas the current in wires are carried by ____________. Biopotential electrodes must have a transducer function! The interface required for the charge transfer is the electrode –electrolyte interface:ª Electrode: some sort of metal with desired propertiesª Electrolyte: a conducting medium, specifically formulated for the electrode metal being usedElectrode – Electrolyte InterfaceEhc−−−++⇔+⇔meAAneCCmnElectrode-electrolyte interface The current crosses it from left to right. The electrode consists of metallic atoms C. The electrolyte is an aqueous solution containing cations of the electrode metal C+and anions A-.Half-Cell PotentialsHow would you measure the half-cell potential of an electrode?You can’t !Standard half-cell potentials, with reference to Hydrogen, at 25 ºCSilver – Silver Chloride (Ag/AgCl)Electrodes)()(reductionAgClClAgoxidationeAgAg⇔++⇔−+−+Ionic ActivityRelative half-Cell Potentials()++=ncanFRTEE ln0sClAgKaa=×−++=−ClsAgAgaKnFRTEE ln0()()−−+=ClAgAganFRTKsnFRTEE lnln0Why is this important…?Electrode Behavior & Circuit Models What good is an electrode if we cannot come up with a network model of it? Empirical resultsª Nonlinearª Frequency dependentª Reactive behavior Î Capacitance at the electrode – electrolyte interfaceª Half-cell potential Î Voltage sourceª Metal electrode Î Resistive lossElectrode Behavior & Circuit ModelsEquivalent circuit for a biopotential electrode in contact with an electrolyteEhcis the half-cell potential, Rdand Cdmake up the impedance associated with the electrode-electrolyte interface, and Rsis the series resistance associated with interface effects and due to resistance in the electrolyte.Frequency DependencyExperimentally determined magnitude of impedance as a function of frequency for electrodes.Electrode – Skin Interface Apart from the electrode – electrolyte interface, there is also the skin interface. The skin consists of three layers, Epidermis, dermis and subcutaneous layer. We are most interested at epidermis, as that is the main contact with the electrodeª Stratum corneum (I see dead cells…!)ª Stratum granulosumª Stratum (basale) germinativum (where new skin cells form) Deep layers of skin consist of vascular and nervous components, as well as sweat glands, sweat ducts and hair folliclesª With the exception of sweat glands, no particular characteristics affecting the electrode performanceDirection of cell aging& movementSkinElectrode goes here !DermisYep…You guessed it Right(The electric Model…)ElectrodeEpidermisDermis andsubcutaneous layerReCdCeGelStratum CorneumEheRdSweat glandsand ductsEPRPIf only we could getrid of these ….RsEseCPRuSkin impedance for 1cm2 patch:200kΩ @1Hz200 Ω @ 1MHzMotion Artifacts Electrode moves Î Charge distribution gets screwed up! Charge distribution gets screwed up… Î Signal goes wild ! Signal goes wild Î Measurement goes off the window Measurement goes off the window Î patient dies ! Using non-polarizable electrodes (Ag/AgCl) reduce the low frequency noise created by motion artifacts Î ECG, EEG should use such electrodes (it can be filtered out of EMG) However, the gel-skin contact also creates motion artifacts, and the Ag/AgCl electrodes can’t do damn anything about it…! What to do…?Commonly Used Biopotential Electrodes Metal plate electrodesª Large surface: Ancient, therefore still used, ECGª Metal disk: EMG, EEG smaller diameters, w/stainless steel, platinum or gold coated Æ motion artifactsª Disposable foam-pad: Cheap!, minimum prep time Æ Hospitals, ECG Suction electrodesª No straps or adhesives required, precordial (chest) ECG, can only be used for short periods only Floating electrodesª Their metal disk is recessed, swimming in the electrolyte gel, so it is not in contact with the skin Æ reduces motion artifact Flexible electrodesª Since body contours are often irregular, regularly shaped rigid electrodes may not always work, particularly with infants. Polymer or nylon with silver deposits, or carbon filled silicon rubber in the form of a thin film (Mylar film) Internal electrodes ª Trust me, you don’t wanna know!Body-surface biopotential electrodes(a) Metal-plate electrode used for application to limbs. Metal-disk electrode applied with surgical tape.


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Rowan ECE 09.404 - Lecture 6: Biopotential Electrodes

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