2 Electrode – Electrolyte Interface Electrode Electrolyte (neutral charge)C+, A- in solutionCCurrent flowCC+e-CA-C+e-A-C+ : Cation A- : Anion e- : electronFairly common electrode materials: Pt, Carbon, …, Au, Ag,…Electrode metal is use in conjunction with salt, e.g. Ag-AgCl, Pt-Pt black, or polymer coats (e.g. Nafion, to improve selectivity)
3 Electrode – Electrolyte Interface General Ionic Equationsa)b)a) If electrode has same material as cation, then this material gets oxidized and enters the electrolyte as a cation and electrons remain at the electrode and flow in the external circuit.b) If anion can be oxidized at the electrode to form a neutral atom, one or two electrons are given to the electrode.The dominating reaction can be inferred from the following :Current flow from electrode to electrolyte : Oxidation (Loss of e-)Current flow from electrolyte to electrode : Reduction (Gain of e-)
4 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.
5 Measuring Half Cell Potential Note: Electrode material is metal + salt or polymer selective membrane
6 Some half cell potentials Standard Hydrogen electrodeNote: Ag-AgCl has low junction potential & it is also very stable -> hence used in ECG electrodes!
7 PolarizationIf there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization.Note: Polarization and impedance of the electrode are two of the most important electrode properties to consider.
8 Nernst EquationWhen two aqueous ionic solutions of different concentration are separated by an ion-selective semi-permeable membrane, an electric potential exists across the membrane.For the general oxidation-reduction reactionNote: interested in ionic activity at the electrode(but note temp dependenceThe Nernst equation for half cell potential iswhere E0 : Standard Half Cell Potential E : Half Cell Potentiala : Ionic Activity (generally same as concentration)n : Number of valence electrons involved
9 Polarizable and Non-Polarizable Electrodes Use for recordingPerfectly Polarizable ElectrodesThese 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 ElectrodePerfectly Non-Polarizable ElectrodeThese are electrodes where current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition. These electrodes see no overpotentials. Example : Platinum electrodeUse for stimulationExample: Ag-AgCl is used in recording while Pt is use in stimulation
10 Ag/AgCl Electrode Relevant ionic equations Governing Nernst Equation Solubility product of AgClAg+Cl-Fabrication of Ag/AgCl electrodesElectrolytic deposition of AgClSintering process forming pellet electrodes
11 Equivalent Circuit Rd+Rs Rs Frequency Response Cd : capacitance of electrode-eletrolyte interfaceRd : resistance of electrode-eletrolyte interfaceRs : resistance of electrode lead wireEcell : cell potential for electrodeCorner frequencyRd+RsRsFrequency Response
12 Electrode Skin Interface EheAlter skin transport (or deliver drugs) by:Pores produced by laser, ultrasound or by iontophoresisElectrodeCdRdSweat glandsRsand ductsGel100 mReEseEPRPCPCeStratum CorneumEpidermis100 mDermis andsubcutaneous layerRuNerve endingsSkin impedance for 1cm2 patch:200 1MHzCapillary
13 Motion ArtifactWhyWhen 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.WhatIf 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 measurementsMotion artifact is minimal for non-polarizable electrodes
14 Body Surface Recording Electrodes Electrode metalElectrolyteThink of the construction of electrosurgical electrodeAnd, how does electro-surgery work?Metal Plate Electrodes (historic)Suction Electrodes(historic interest)Floating ElectrodesFlexible Electrodes
15 Commonly Used Biopotential Electrodes Metal plate electrodesLarge surface: Ancient, therefore still used, ECGMetal disk with stainless steel; platinum or gold coatedEMG, EEGsmaller diametersmotion artifactsDisposable foam-pad: Cheap!(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
16 Commonly Used Biopotential Electrodes Suction electrodesNo straps or adhesives requiredprecordial (chest) ECGcan only be used for short periodsFloating electrodesmetal disk is recessedswimming in the electrolyte gelnot in contact with the skinreduces motion artifactSuction Electrode
17 Commonly Used Biopotential Electrodes Metal diskInsulatingpackageDouble-sidedAdhesive-taperingElectrolyte gelin recess(a)(b)ReusableSnap coated with Ag-AgClExternal snapGel-coated spongeDisposablePlastic cupPlastic diskTackDead cellular materialFoam padCapillary loopsGerminating layer(c)Floating Electrodes
18 Commonly Used Biopotential Electrodes Flexible electrodesBody contours are often irregularRegularly shaped rigid electrodesmay not always work.Special case : infantsMaterial :- Polymer 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).
19 Internal ElectrodesNeedle 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.The latest: BION – implanted electrode for muscle recording/stimulationAlfred E. Mann Foundation
20 Fetal ECG ElectrodesElectrodes 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.
22 Microelectrodes Why Measure potential difference across cell membrane RequirementsSmall enough to be placed into cellStrong enough to penetrate cell membraneTypical tip diameter: 0.05 – 10 micronsTypesSolid metal -> Tungsten microelectrodesSupported metal (metal contained within/outside glass needle)Glass micropipette -> with Ag-AgCl electrode metalIntracellularExtracellular
23 Metal Microelectrodes Microns!RExtracellular recording – typically in brain where you are interested in recording the firing of neurons (spikes).Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier!
24 Metal Supported Microelectrodes (a) Metal inside glass (b) Glass inside metal
25 Glass Micropipette heat pull Ag-AgCl wire+3M KCl has very low junction potential and hence very accurate for dc measurements (e.g. action potential)heatpullA 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.Fill with intracellular fluid or 3M KClIntracellular recording – typically for recording from cells, such as cardiac myocyteNeed high impedance amplifier…negative capacitance amplifier!
26 Electrical Properties of Microelectrodes Metal MicroelectrodeMetal microelectrode with tip placed within cellUse metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier!Equivalent circuits
27 Electrical Properties of Glass Intracellular Microelectrodes Glass Micropipette Microelectrode
28 Stimulating Electrodes Features– Cannot be modeled as a series resistance and capacitance(there is no single useful model)– The body/electrode has a highly nonlinear response tostimulation– Large currents can cause– Cavitation– Cell damage– HeatingPlatinum electrodes:Applications: neural stimulationModern day Pt-Ir and other exotic metal combinations to reduce polarization, improve conductance and long life/biocompatibilitySteel electrodes for pacemakers and defibrillatorsTypes of stimulating electrodesPacingAblationDefibrillation
29 Intraocular Stimulation Electrodes Reference : Lutz Hesse, Thomas Schanze, Marcus Wilms and Marcus Eger, “Implantation of retina stimulation electrodes and recording of electrical stimulation responses in the visual cortex of the cat”, Graefe’s Arch Clin Exp Ophthalmol (2000) 238:840–845
38 In vivo neural microsystems: 3 examples University of MichiganSmart comb-shape microelectrode arrays for brain stimulation and recordingUniversity of Illinois at Urbana-ChampaignHigh-density comb-shape metal microelectrode arrays for recordingFraunhofer Institute of Biomedical (FIBE) EngineeringRetina implant
41 Nitric Oxide Sensor Developed at Dr.Thakor’s Lab, BME, JHU Electrochemical detection of NOLeft: Schematic of the 16-electrode sensor array. Right: Close-up of a single site. The underlying metal is Au and appears reddish under the photoresist. The dark layer is C (300µm-x-300µm)
42 Cartoon of the fabrication sequence for the NO sensor array GDHCartoon of the fabrication sequence for the NO sensor arrayA) Bare 4” Si wafer B) 5µm of photoresist was spin-coated on to the surface, followed by a pre-bake for 1min at 90°C. C) The samples were then exposed through a mask for 16s using UV light at 365nm and an intensity of 15mW/cm2. D) Patterned photoresist after development. E) 20nm of Ti, 150nm of Au and 50nm of C were evaporated on. F) The metal on the unexposed areas was removed by incubation in an acetone bath. G)A 2nd layer of photoresist, which serves as the insulation layer, was spun on and patterned. H) The windows in the second layer also defined the microelectrode sites.