Surface: Metal PlateElectrodes (a)Metal-plate electrode used for application to limbs. (b)Disposable foam-pad electrodes, often used with ECG
Surface: Suction Electrodes• No straps or adhesives• Precordial (chest) ECG• Can only be used for short periods
MicroelectrodesMeasure potential difference across cell membraneRequirements Small enough to be placed into cell Strong enough to penetrate cell membrane Typical tip diameter: 0.05 – 10 microns
Stimulating Electrodes Features – Net current across electrode-electrolyte interface is not zero – The body/electrode has a highly nonlinear response to stimulation Platinum electrodes: – Large currents can cause Applications: neural – Chemical reaction stimulation – Cavitation – Cell damage Steel electrodes for – Heating pacemakers and Applications of stimulating electrodes defibrillators 1. Pacing 2. Ablation 3. Defibrillation
Practical Hints for using anelectrode ..... Electrode and lead wire (exposed to the electrolyte) must be of the same material. Use two similar electrodes when measuring differentials. Lead electrode interface failure. Electrode insulation material. Input impedance of Amplifier.
WHAT ARE THE BASICELECTRODE REQUIRED FORRECORDING OF A BIO-POTENTIAL ?ACTIVE ELECTRODEREFERANCE ELECTRODEGROUND ELECTRODE
Active electrode: Actually this is the only electrode which pick the signal from the subjects body. BUT HOW ?
WE HAVE TOUNDERSTANDELECTRODE JELLYINTERFACE 1ST
Electrode – Electrolyte Interface Electrode Electrolyte (neutral charge) C C+, A- in solution Current flow C C+ e- C A- C+ e- 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. Ag-AgCl, Pt-Pt black, or polymer coats (e.g. Nafion, to improve selectivity)
Electrode – Electrolyte Interface General Ionic Equations a) n C C ne b) m A A me 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-)
Half Cell Potential A 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-electrolyteinterface lead to a double-charge layer, similar to that which existsalong electrically active biological cell membranes.
Measuring Half Cell PotentialNote: Electrode material is metal + salt or polymer selective membrane
Biopotential Electrodes – The Basics The interface between the body and electronic measuring devices Conduct current across the interface Current is carried in the body by ions Current is carried in electronics by electrons Electrodes must change ionic current into electronic current This is all mediated at what is called the Electrode- Electrolyte Interface or the Electrode-Tissue Interface
Current Flow at the Electrode-Electrolyte Interface Ion- flow Electrons move in opposite Electron flow direction to current flow Ion+ flow Cations (C+ ) move in same direction as current flow Anions (A– ) move in opposite direction of current flow Chemical oxidation (current flow right) - reduction (current flow left) reactions at the interface: + Current flow C C++e– (5.1) The current crosses it from left to right. A– A + e– (5.2) The electrode consists of metallic atoms C. The electrolyte is an aqueous solution No current at equilibrium containing cations of the electrode metal C+ and anions A-.
Half-Cell Potential When metal (C) contacts electrolyte, oxidation (C C + + e – ) or reduction (A- A + e –) begins immediately. Local concentration of cations at the surface changes. Charge builds up in the regions. Electrolyte surrounding the metal assumes a different electric potential from the rest of the solution. This potential difference is called the half-cell potential ( E0 ). Separation of charge at the electrode-electrolyte interface results in a electric double layer (bilayer). Measuring the half-cell potential requires the use of a second reference electrode. By convention, the hydrogen electrode is chosen as the reference.
Standard Hydrogen electrodeNote: Ag-AgCl has low junctionpotential & it is also very stable ->hence used in ECG electrodes!
Nernst Equation When 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 reaction Note: interested in A B C D ne ionic activity at the electrode The Nernst equation for half cell potential is (but note temp dependence RT a a E E0 ln C D nF a A aB where E0 : Standard Half Cell Potential E : Half Cell Potential a : Ionic Activity (generally same as concentration) n : Number of valence electrons involved
Equivalent Circuit Cd : capacitance of electrode-eletrolyte interface Rd : resistance of electrode-eletrolyte interface Rs : resistance of electrode lead wire Ecell : cell potential for electrode Corner frequency Rd+Rs Rs Frequency Response
Electrode Skin Interface Ehe Alter skin transport Electrode Cd Rd (or deliver drugs) by: Sweat glands Gel Rs and ducts Pores100 produced Ese EP by laser, ultrasound Stratum Corneum Epidermis Ce Re CP RP or by iontophores100 is Dermis and subcutaneous layer Ru Nerve Skin impedance for 1cm2 patch: endings Capillary 200kΩ @1Hz 200 Ω @ 1MHz
Polarization If there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization. Overpotential Difference between observed and zero-current half cell potentials Activation Resistance Concentration The activation energyCurrent changes resistance Changes in distribution barrier depends on the of electrolyte and thus, of ions at the electrode- direction of current and a voltage drop results. electrolyte interface determines kinetics V p VR VC VA Note: Polarization and impedance of the electrode are two of the most important electrode properties to consider.
Polarizable and Non-Polarizable Electrodes Use for recording Perfectly Polarizable Electrodes These 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 Electrode Perfectly Non-Polarizable Electrode These 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 electrode Use for stimulationExample: Ag-AgCl is used in recording while Pt is use instimulation
Motion Artifact Why When 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. What If 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
But one thing that should notbe stopped is questioning