Biopotential Electrodes


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Chapter 5 Biopotential Electrodes by Michael R. Neuman:

Chapter 5 Biopotential Electrodes by Michael R. Neuman in John G. Webster (Editor) Medical Instrumentation: Application and Design John Wiley & Sons, 1998 ISBN 0-471-15368-0

Biopotential Electrodes Outline:

Biopotential Electrodes Outline The Electrode-Electrolyte Interface Polarization Polarizable and Nonpolarizable Electrodes Electrode Behavior & Circuit Models The Electrode-Skin Interface & Motion Artifact Body-Surface Recording Electrodes Internal Electrodes Electrode Arrays Microelectrodes Electrodes for Electric Stimulation of Tissue Practical Hints in Using Electrodes

Biopotential Electrodes – The Basics:

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:

Figure 5.1 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 - . Current Flow at the Electrode-Electrolyte Interface Electrons move in opposite 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: C C + + e – (5.1) A – A + e – (5.2) No current at equilibrium  Electron flow  Ion - flow + Current flow  Ion + flow 

Half-Cell Potential:

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 ( E 0 ). 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.

Half-Cell Potentials of Common Metals at 25 ºC:

Half-Cell Potentials of Common Metals at 25 ºC Metal Potential E 0 (volts) Al - 1.706 Zn - 0.763 Cr - 0.744 Fe - 0.409 Cd - 0.401 Ni - 0.230 Pb - 0.126 H 0.000 AgCl + 0.223 Hg 2 Cl 2 + 0.268 Cu + 0.522 Ag + 0.799 Au + 1.680 By definition: Hydrogen is bubbled over a platinum electrode and the potential is defined as zero.

Electrode Polarization:

Electrode Polarization Standard half-cell potential ( E 0 ): Normally E 0 is an equilibrium value and assumes zero-current across the interface. When current flows, the half-cell potential, E 0 , changes. Overpotential ( V p ): Difference between non-zero current and zero-current half-cell potentials; also called the polarization potential (V p ) . Components of the overpotential ( V p ): Ohmic ( V r ) : Due to the resistance of the electrolyte (voltage drop along the path of ionic flow). Concentration ( V c ): Due to a redistribution of the ions in the vicinity of the electrode-electrolyte interface (concentration changes). Activation ( V a ): Due to metal ions going into solution (must overcome an energy barrier, the activation energy) or due to metal plating out of solution onto the electrode (a second activation energy). V p = V r + V c + V a (5.4)

Nernst Equation:

Nernst Equation Governs the half-cell potential: where E – half-cell potential E 0 – standard half-cell potential (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 n – valence of the electrode material F – Faraday constant [ 96,500 C/(mol/valence) ] – ionic activity of cation C n+ (its availability to enter into a reaction) (5.6)

Polarizability & Electrodes:

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 electrodes: 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.

The Classic Ag/AgCl Electrodes:

The Classic Ag/AgCl Electrodes Figure 5.2 A silver/silver chloride electrode, shown in cross section. Features: Practical electrode, easy to fabricate. Metal (Ag) electrode is coated 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 – (5.10) Reaction 2: silver cations combine with chloride anions Ag + + Cl – Ag Cl (5.11) AgCl is only slightly soluble in water so most precipitates onto the electrode to form a surface coating.

Ag/AgCl Electrodes:

Ag/AgCl Electrodes Solubility product ( K s ): The rate of precipitation and of returning to solution. At equilibrium: K s = a Ag + x a Cl - (5.12) The equation for the half-cell potential becomes E = E 0 Ag + ln ( K s ) - ln ( a Cl - ) (5.15) Determined by the activity of the chloride ion. In the body, the activity of Cl – is quite stable. RT nF RT nF constant

Ag/AgCl Fabrication:

Ag/AgCl Fabrication Electrolytic process 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 resistor limits the current. A milliammeter measures the plating current. Reaction has an initial surge of current. When current approaches a steady state (about 10 µA), the process is terminated. Cathode Anode A Electrochemical Cell

Sintered Ag/Ag Electrode:

Sintered Ag/Ag Electrode Figure 5.3 Sintering Process 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 in the electrolytically generated electrodes). Silver powder is added to increase conductivity since AgCl is not a good conductor.

Calomel Electrode:

Calomel Electrode Calomel is mercurous chloride (Hg 2 Cl 2 ). Approaches perfectly non-polarizing behavior Used as a reference in pH measurements. Calomel paste is loaded into a porous glass plug at the end of a glass tube. Elemental Hg is placed on top with a lead wire. Tube is inserted into a saturated KCl solution in a second glass tube. A second porous glass plug forms a liquid-liquid interface with the analyte being measured. Hg 2 Cl 2 Hg K Cl Electrolyte being measured porous glass plug Lead Wire

Electrode Circuit Model:

Electrode Circuit Model E hc is the half-cell potential C d is the capacitance of the electric double layer (polarizable electrode properties). R d is resistance to current flow across the electrode-electrolyte interface (non-polarizable electrode properties). R s is the series resistance associated with the conductivity of the electrolyte. At high frequencies: R s At low frequencies: R d + R s Figure 5.4

Ag/AgCl Electrode Impedance:

Ag/AgCl Electrode Impedance Figure 5.5 Impedance as a function of frequency for Ag electrodes coated with an electrolytically deposited AgCl layer. The electrode area is 0.25 cm 2 . Numbers attached to curves indicate the number of mA  s for each deposit.

Nichel- & Carbon-Loaded Silicone:

Nichel- & Carbon-Loaded Silicone Figure 5.6 Electrode area is 1.0 cm 2

Skin Anatomy:

Skin Anatomy Figure 5.7

Electrode-Skin Interface Model:

Electrode-Skin Interface Model Figure 5.8 A body-surface electrode is placed against skin, showing the total electrical equivalent circuit obtained in this 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. Sweat glands and ducts Electrode Epidermis Dermis and subcutaneous layer R u R e E se E he R s R d C d E P R P C P C e Gel Motion artifact: Gel is disturbed, the charge distribution is perturbed changing the half-cell potentials at the electrode and skin. Minimized by using non-polarizable electrode and mechanical abrasion of skin. Skin regenerates in 24 hours.

Metal Electrodes:

Metal Electrodes Figure 5.9 Body-surface biopotential electrodes (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 electrocardiograph monitoring apparatus.

Metal Suction Electrodes:

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 Figure 5.10

Floating Metal Electrodes:

Floating Metal Electrodes 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. Double-sided Adhesive-tape ring Insulating package Metal disk Electrolyte gel in recess (a) (b) (c) Snap coated with Ag-AgCl External snap Plastic cup Tack Plastic disk Foam pad Capillary loops Dead cellular material Germinating layer Gel-coated sponge Mechanical technique to reduce noise. Isolates the electrode-electrolyte interface from motion artifacts.

Flexible Body-Surface Electrodes:

Flexible Body-Surface Electrodes Figure 5.12 Carbon-filled silicone rubber (b) Flexible Mylar film with Ag/AgCl electrode (c) Cross section of the Mylar electrode

Percutaneous Electrodes:

Percutaneous Electrodes Figure 5.13 Insulated needle (b) Coaxial needle (c) Bipolar coaxial needle (d) Fine wire, ready for insertion (e) Fine wire, after insertion (f) Coiled fine wire, after insertion

Fetal Intracutaneous Electrodes:

Fetal Intracutaneous Electrodes Figure 5.14 Suction needle electrode Suction electrode (in place) Helical electrode (attached by corkscrew action)

Implantable Electrodes:

Implantable Electrodes Figure 5.15 Multielement depth electrode array Wire-loop electrode Cortical surface potential electrode

Microfabricated Electrode Arrays:

Microfabricated Electrode Arrays Figure 5.16 (a) One-dimensional plunge electrode array (b) Two-dimensional array, and (c) Three-dimensional array Contacts Insulated leads (b) Base Electrodes Electrodes Base Insulated leads (a) Contacts (c) Tines Base Exposed tip

Intracellular Recording Electrode:

Intracellular Recording Electrode Figure 5.17 Metal needle with a very fine tip (less than 1.0 µm) Prepared by electrolytic etching Metal needle is the anode of an electrolytic cell, and is slowly drawn out of the electrolyte solution (difficult to produce) Metal must have great strength: stainless steel, platinum-iridium, tungsten, tungsten carbide.

Supported Metal Electrodes:

Supported Metal Electrodes Figure 5.18 (a) Metal-filled glass micropipet. (b) Glass micropipet or probe, coated with metal film.

Glass Micropipet Electrodes:

Glass Micropipet Electrodes 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.

Microfabricated Microelectrodes:

Microfabricated Microelectrodes Figure 5.20 Bonding pads Silicon probe Exposed electrodes Insulated lead vias Lead via Electrode Silicon probe Miniature insulating chamber Hole Si substrate Exposed tips SiO 2 insulated Au probes (a) Beam-lead multiple electrode (b) Multielectrode silicon probe (c) Multiple-chamber electrode Channels Silicon chip Contact metal film (d) Peripheral-nerve electrode

Microelectrode Electrical Model:

Microelectrode Electrical Model Figure 5.21 (a) Electrode with tip placed within a cell, showing origin of distributed capacitance N = Nucleus C = Cytoplasm Metal rod Tissue fluid Membrane potential N C Insulation Cell membrane + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - N = Nucleus C = Cytoplasm Shank Capacitance: Submerged Shaft Capacitance: e r , e 0 = dielectric const. D = avg. dia. of shank d = dia. of electrode t = thickness of insulation layer L = length of shank & shaft, respectively (5.16) (5.17) + + + + + + + - - - - - - - Shaft Submerged shaft Shank in tissue fluid Shank inside cell Electrode tip inside cell Tissue fluid C

Microelectrode Electrical Model:

Microelectrode Electrical Model (a) Electrode with tip placed within a cell N = Nucleus C = Cytoplasm Metal rod Tissue fluid Membrane potential N C B A Reference electrode Insulation C d Cell membrane + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - (b) Equivalent circuit Figure 5.21 B A R mb R ma E mb E ma E mp R i R e C mb C ma C di C d2 R s C w C d1 Metal- electrolyte interface Reference electrode model Lead wire capacitance Shaft capacitance Electrode resistance Tissue fluid resistance Shank capacitance Cell membrane Cytoplasm resistance To amplifier

Microelectrode Electrical Model:

Microelectrode Electrical Model E mp Membrane and action potential C ma R ma C d + C w E ma - E mb E 0 A B B A R mb R ma E mb E ma E mp R i R e C mb C ma C di C d2 R s C w (c) Simplified equivalent circuit (b) Equivalent circuit Figure 5.21 R s , R i , R e , and R mb are very small compared to R ma .

Glass Micropipet:

Glass Micropipet Figure 5.22 (a) Electrode with its tip placed within a cell, showing the origin of distributed capacitance. (b) Equivalent circuit. E ma R ma R t R i R e (b) E mp E mb R mb C mb E j E t C ma C d A B Cell membrane Tip + - + - + + + - - + - - + - + - + + + + + - - - - - + + + - - - + - - + + + + - - - Taper Internal electrode Glass A B To amplifier Electrolyte in micropipet Stem (a) Reference electrode Cell membrane Cytoplasm N = Nucleus N Environmental fluid C d R t = electrolyte resistance in shank & tip C d = capacitance from micropipet electrolyte to environmental fluid E j = liquid-liquid junction potential between micropipet electrolyte & intracellular fluid E t = tip potential generated by the thin glass membrane at micropipet tip R i = intracellular fluid resistance E mp = cell membrane potential R e = extracellular fluid resistance Internal electrode Reference electrode

Glass Micropipet:

Glass Micropipet Figure 5.22 (b) Equivalent circuit. (c) Simplified equivalent circuit. E ma R ma R t R i R e (b) E mp E mb R mb C mb E j E t C ma C d A B R t E m A B Membrane and action potential (c) E mp E m = E j + E t + E ma - E mb C d = C t 0 R t = all the series resistance lumped together (ranges from 1 to 100 M W ) C t = total distributed capacitance lumped together (total is tens of pF) E m = all the dc potentials lumped together Behaves like a low-pass filter.

Figure 5.23 (a) Constant-current stimulation (b) Constant-voltage stimulation Charge transfer characteristics of the electrode are very important. Platinum black and Iridium oxide are very good stimulating electrode materials.:

(a) Polarization potential Polarization Polarization v v i i t t t t Ohmic potential (b) Polarization potential Figure 5.23 (a) Constant-current stimulation (b) Constant-voltage stimulation Charge transfer characteristics of the electrode are very important. Platinum black and Iridium oxide are very good stimulating electrode materials. Stimulating Electrodes

Practical Hints in Using Electrodes:

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.

Practical Hints in Using Electrodes:

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. Use a tapered region of insulation that gradually increases in diameter from that of the wire towards that of the electrode as one gets closer and closer to 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.

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