Ch5_BME420_Biopotential Electrodes.pdf

Biomedical Instrumentation (BME420 ) Chapter 5: Bio-potential Electrodes John G. Webster, 4th Edition Dr. Qasem Qananwah 4/25/2023 BME420: Biomedical Instrum...

Biomedical Instrumentation (BME420 ) Chapter 5: Bio-potential Electrodes John G. Webster, 4th Edition Dr. Qasem Qananwah 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 1  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Outline The Electrode-Electrolyte Interface Polarization: Polarizable and Nonpolarizable Electrodes Electrode Behavior & Circuit Models The Electrode-Skin Interface & Motion Artifact Electrode Types 1. Body-Surface Recording Electrodes 2. Internal Electrodes 3. Electrode Arrays Practical Hints in Using Electrodes 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 2  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics 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 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 3  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Current Flow at the Electrode-Electrolyte Interface - Electrons move in opposite  Ion flow  Electron flow Ion+ flow  direction to current 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–  Figure 5.1 The current crosses it from left to right. The A– ⇌ A + e– electrode consists of metallic atoms C. The electrolyte is an aqueous solution containing cations of the electrode metal C+ No current at equilibrium and anions A-. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 4  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Electrode-Electrolyte Interface metal electrolyte M+ - I To sense a signal e a current I must flow ! A- 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 5  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Electrode-Electrolyte Interface metal electrolyte M+ - I e A- But no electron e- is passing the interface! 4/25/2023 ? BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 6  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Electrode-Electrolyte Interface Metal Cation: leaving into the electrolyte No current What’s going on? 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 7  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Electrode-Electrolyte Interface Metal Cation: leaving into the electrolyte Oxidization Left to Right M ⇌ M++e– No current One atom M out of the metal is oxidized to form one cation M+ and giving off one free electron e- to the metal. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 8  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Electrode-Electrolyte Interface Metal cation: joining the metal No current What’s going on? 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 9  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Electrode-Electrolyte Interface Metal cation: joining the metal Reduction Right to Left M ⇌ M++e– One cation M+ No current out of the electrolyte becomes one neutral atom M taking off one free electron from the metal. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 10  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics No current Electrode-Electrolyte Interface Oxidation or reduction reactions at the electrode-electrolyte interface lead to a double-charge layer 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 11  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Electrode-Electrolyte Interface a) If electrode has same material as cation, then this material gets oxidized and enters the electrolyte as a cation General Ionic Equations and electrons remain at the electrode and flow in the external circuit. a) C  C n   ne b) If anion can be oxidized at the electrode to form a b) Am   A  me  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-) 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 12  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics 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. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 13  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics 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 other factors like pressure). This potential difference is called the half-cell potential ( E0 ). Half cell potential cannot be measured without a second electrode. By convention, the hydrogen electrode is chosen as the reference. Other half cell potentials are expressed as a potential difference with this electrode. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 14  Chapter 5: BIO-POTENTIAL ELECTRODES Biopotentials Electrodes Basics Half-Cell Potentials of Common Metals at 25 ºC Metal Potential E0 (volts) Al - 1.706 Zn - 0.763 Cr - 0.744 Fe - 0.409 Cd - 0.401 Ni - 0.230 Pb - 0.126 By definition: Hydrogen is bubbled H 0.000 over a platinum electrode and the AgCl + 0.223 potential is defined as zero. Hg2Cl2. + 0.268 Cu + 0.522 Ag + 0.799 Au + 1.680 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 15  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Polarization Standard half-cell potential ( E0 ): Normally E0 is an equilibrium value and assumes zero-current across the interface. If there is a current between the electrode and electrolyte, the observed half cell potential ( E0 ) is often altered due to polarization. Overpotential or Polarizable Potential ( Vp ) Difference between non-zero and zero-current half cell potentials Activation Resistance Concentration The activation energy Current 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 Vp  VR  VC  VA Note: Polarization and impedance of the electrode are two of the most important electrode properties to consider. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 16  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Polarization Nernst Equation Governs the half-cell potential: RT EE  0 ln(aC n ) n – valence of the electrode material nF F – Faraday constant [ 96,500 where C/(mol/valence) ] E– half-cell potential aC–n ionic activity of cation Cn+ E0– standard half-cell potential (its availability to enter into a reaction)  (the electrode in an electrolyte with unity activity at standard temperature) R– universal gas constant [ 8.31 J/(mol K) ] T– absolute temperature in K 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 17  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Polarization Polarizability & Electrodes Perfectly Polarizable Electrodes No charge crosses the electrode when current is applied Noble metals are closest (like platinum and gold); they are difficult to oxidize and dissolve. Current does not cross, but rather changes the concentration of ions at the interface. Behave like a capacitor. Perfectly Non-Polarizable Electrode All charge freely crosses the interface when current is applied. No overpotential is generated. Behave like a resistor. Silver/silver-chloride is a good non-polarizable electrode. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 18  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Behavior & Model metal + - Electrolyte Ehc is the half-cell potential + - + - Cd is the capacitance of the electric + - double layer (polarizable electrode + - properties). + - + - Rd is resistance to current flow across + - the electrode-electrolyte interface (non-polarizable electrode properties). Rs is the series resistance associated with the conductivity of the electrolyte. At high frequencies: Rs Electrode Circuit Models At low frequencies: Rd + Rs 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 19  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Behavior & Model The characteristics of an electrode are: − Sensitive to current density − Waveform and frequency dependent Electrode Circuit Models 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 20  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Behavior & Model Example 5.4 0.572 - + Rd+Rs Corner frequency Ex 0.233 V Rs Frequency Response Test Electrode Reference Electrode 1 cm2 nickel-and carbon-loaded silicone rubber electrode 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 21  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Skin Interface Skin Anatomy 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 22  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Skin Interface Electrode-Skin Interface Model Ehe For 1 cm2, skin impedance reduces from approximately 200K at 1Hz to 200 at Electrode Cd Rd 1MHz. Sweat glands Gel and ducts Rs Transparent electrolyte gel containing Ese EP Cl- is used to maintain good contact Epidermis between the electrode and the skin. Ce Re CP RP Figure 5.8 A body-surface electrode is placed against skin, Dermis and showing the total electrical equivalent circuit obtained in this subcutaneous layer Ru situation. Each circuit element on the right is at approximately the same level at which the physical process that it represents would be in the left-hand diagram. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 23  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Skin Interface 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 measurements Motion artifact is minimal for non-polarizable electrodes 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 24  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Skin Interface Motion Artifact Signal due to motion has low frequency so it can be filtered out when measuring a biological signal of high frequency component such as EMG or axon action potential (AAP). However, for ECG, EEG and EOG whose frequencies are low it is recommended to use nonpolarizable electrode to avoid signals due to motion artifact. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 25  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Skin Interface Electrode noise characteristics Extracellular action potentials have amplitude in the range of 50-500µV ! Very low-level input signals Total system input-referred noise should be < 20µVrms. System noise= Electrode noise + Preamplifier noise Main source of electrode noise is thermal noise: RN is noise resistance (real part of probe impedance magnitude). f is recording bandwidth. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 26  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types The Classic Ag/AgCl Electrodes Features: – Practical electrode, easy to Approach the characteristic of a perfectly nonpolarizable electrode fabricate. Advantage of Ag/AgCl is that it is stable in liquid that has large – Metal (Ag) electrode is coated quantity of Cl- such as the biological fluid. with a layer of slightly soluble ionic compound of the metal and a suitable anion (Cl). Reaction 1: silver oxidizes at the Ag/AgCl interface Ag ⇌ Ag + + e – Reaction 2: silver cations combine with chloride anions Ag + + Cl – ⇌ Ag Cl AgCl is only slightly soluble in water so most precipitates onto the electrode Figure 5.2 A silver/silver chloride to form a surface coating. electrode, shown in cross section. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 27  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types The Classic Ag/AgCl Electrodes Solubility product ( Ks ): The rate of precipitation and of returning to solution. At equilibrium: Ks = aAg+ x aCl - The equation for the half-cell potential becomes E = E0Ag + RT ln ( Ks ) - RT ln ( aCl - ) nF nF constant Determined by the activity of the chloride ion. In the body, the activity of Cl – is quite stable. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 28  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types The Classic Ag/AgCl Electrodes Silver chloride’s rate of precipitation and of returning to solution is a constant Ks know as the solubility product. 10 K s  a Ag   aCl   10 For biological fluid where Cl- ion is relatively high aCl   1 EE 0 Ag  RT nF   ln a Ag  RT  K s  EE  ln  0 Ag  nF  aCl   RT RT E  E Ag 0  ln K s  ln aCl  nF nF 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 29  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types The Classic Ag/AgCl Electrodes Advantage of Ag/AgCl electrodes: Approach the characteristic of a perfectly nonpolarizable electrode It is stable in liquid that has large quantity of Cl- such as the biological fluid. Ag/AgCl exhibits less electric noise than the equivalent metallic Ag electrode. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 30  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types The Classic Ag/AgCl Electrodes 1. Electrolytic process Ag/AgCl Fabrication Large Ag/AgCl electrode serves as the cathode. Smaller Ag electrode to be chloridized serves as the anode. A 1.5 volt battery is the energy source. A A resistor limits the current. A milliammeter measures the plating current. Ag  Ag   e Reaction has an initial surge of Ag   Cl   AgCl  Cathode Anode current. When current approaches a steady Electrochemical Cell state (about 10 µA), the process is terminated. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 31  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types The Classic Ag/AgCl Electrodes 2. Sintering Process Ag/AgCl Fabrication A mixture of Ag and AgCl powder is pressed into a pellet around a silver lead wire. Baked at 400 ºC for several hours. Known for great endurance (surface does not flake off as Figure 5.3 in the electrolytically generated electrodes). Silver powder is added to increase conductivity since AgCl is not a good conductor. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 32  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Metal Electrodes Metal plate electrodes Large surface: Ancient, therefore still used, ECG Metal disk with stainless steel; platinum or gold coated EMG, EEG smaller diameters motion artifacts Disposable 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 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 33  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Metal Suction Electrodes A paste is introduced into the cup. The electrodes are then suctioned into place. Ten of these can be with the clinical electrocardiograph – limb and precordial (chest) electrodes 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 34  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Metal disk Insulating package Floating Metal Electrodes Double-sided Adhesive-tape Electrolyte gel Mechanical technique ring in recess to reduce noise. (a) (b) Isolates the electrode- Snap coated with Ag-AgCl External snap electrolyte interface Plastic cup Gel-coated sponge Plastic disk from motion artifacts. (c) Foam padTack Dead cellular material Capillary loops Germinating layer Figure 5.11 (a) Recessed electrode with top-hat structure. (b) Cross-sectional view of the electrode in (a). (c) Cross- sectional view of a disposable recessed electrode of the same general structure shown in Figure 5.9(c). The recess in this electrode is formed from an open foam disk, saturated with electrolyte gel and placed over the metal electrode. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 35  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Flexible Body-Surface Electrodes Flexible electrodes - Body contours are often irregular - Regularly shaped rigid electrodes may not always work. - Special case : infants - Material : - 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). 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 36  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Percutaneous Electrodes Needle 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. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 37  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Fetal ECG Electrodes Electrodes 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. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 38  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Insulated leads Contacts Microfabricated Electrode Arrays Electrodes Contacts Electrodes Base Insulated leads Base (b) (a) Exposed tip Tines Figure 5.16 (a) One-dimensional plunge electrode array (b) Two-dimensional array, and (c) Three-dimensional array Base (c) 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 39  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Microelectrodes Metal Electrodes Supported-Metal Microelectrodes Micropipette Electrodes 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 40  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Metal Electrodes The metal Microelectrode is essentially a fine needle of a strong metal that is insulated with an appropriate insulator up to its tip. Figure 5.17 The structure of a metal microelectrode for intracellular recordings. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 41  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Supported-Metal Microelectrodes The properties of two different materials are used to advantage in supported metal microelectrodes. A strong insulating material makes the basic support. A metal with good electrical conductivity constitutes the contacting portion of the electrode. Glass for mechanical support and insulator. Figure 5.18 Structures of two supported metal microelectrodes (a) Metal-filled glass micropipet. (b) Glass micropipet or probe, coated with metal film. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 42  Chapter 5: BIO-POTENTIAL ELECTRODES Electrode Types Micropipette electrodes Glass micropipette electrodes are fabricated from glass capillaries. Electrolyte solution. A cap containing a metal electrode is then sealed to the pipette. The metal electrode contacts the electrolyte within the pipette. Figure 5.19 A 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. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 43  Practical Hints in Using Electrodes Ensure that all parts of a metal electrode that will touch the electrolyte are made of the same metal. Dissimilar metals have different half-cell potentials making an electrically unstable, noisy junction. If the lead wire is a different metal, be sure that it is well insulated. Do not let a solder junction touch the electrolyte. If the junction must touch the electrolyte, fabricate the junction by welding or mechanical clamping or crimping. For differential measurements, use the same material for each electrode. If the half-cell potentials are nearly equal, they will cancel and minimize the saturation effects of high-gain, dc coupled amplifiers. Electrodes attached to the skin frequently fall off. Use very flexible lead wires arranged in a manner to minimize the force exerted on the electrode. Tape the flexible wire to the skin a short distance from the electrode, making this a stress-relief point. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 44  Practical Hints in Using Electrodes A common failure point in the site at which the lead wire is attached to the electrode. Repeated flexing can break the wire inside its insulation. Prove strain relief by creating a gradual mechanical transition between the wire and the electrode. Match the lead-wire insulation to the specific application. If the lead wires and their junctions to the electrode are soaked in extracellular fluid or a cleaning solution for long periods of time, water and other solvents can penetrate the polymeric coating and reduce the effective resistance, making the lead wire become part of the electrode. Such an electrode captures other signals introducing unwanted noise. Match your amplifier design to the signal source. Be sure that your amplifier circuit has an input impedance that is much greater than the source impedance of the electrodes. 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 45  And this concludes… 4/25/2023 BME420: Biomedical Instrumentation Biomedical Systems and Informatics Engineering Department 46