Biopotential Electrodes Electrode Electrolyte Interface Electrode C Electrolyte

Biopotential Electrodes

Electrode – Electrolyte Interface Electrode C Electrolyte (neutral charge) C+, A- in solution Current flow ee. C+ : Cation C C C+ A- C+ AA- : Anion e- : electron Fairly common electrode materials: Pt, Carbon, …, Au, Ag, … Electrode metal is use in conjunction with salt, e. g. Ag-Ag. Cl, Pt-Pt black, or polymer coats (e. g. Nafion, to improve selectivity)

Electrode – Electrolyte Interface General Ionic Equations a) 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-)

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 Interface Oxidation 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.

Measuring Half Cell Potential Note: Electrode material is metal + salt or polymer selective membrane

Some half cell potentials Standard Hydrogen electrode Note: Ag-Ag. Cl has low junction potential & it is also very stable -> hence used in ECG electrodes!

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 Resistance Current changes resistance of electrolyte and thus, a voltage drop results. Concentration Changes in distribution of ions at the electrodeelectrolyte interface Activation The activation energy barrier depends on the direction of current and determines kinetics Note: Polarization and impedance of the electrode are two of the most important electrode properties to consider.

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 The Nernst equation for half cell potential is where E 0 : Standard Half Cell Potential Note: interested in ionic activity at the electrode (but note temp dependence E : Half Cell Potential a : Ionic Activity (generally same as concentration) n : Number of valence electrons involved

Polarizable and Non-Polarizable Electrodes Perfectly Polarizable Electrodes Use for recording These are electrodes in which no actual charge crosses the electrodeelectrolyte interface when a current is applied. The current across the interface is a displacement current and the electrode behaves like a capacitor. Example : Ag/Ag. Cl Electrode Use for stimulation Perfectly Non-Polarizable Electrode These are electrodes where current passes freely across the electrodeelectrolyte interface, requiring no energy to make the transition. These electrodes see no overpotentials. Example : Platinum electrode Example: Ag-Ag. Cl is used in recording while Pt is use in stimulation

Ag/Ag. Cl Electrode Relevant ionic equations Cl 2 Governing Nernst Equation Ag+Cl- Fabrication of Ag/Ag. Cl electrodes 1. Electrolytic deposition of Ag. Cl 2. Sintering process forming pellet electrodes Solubility product of Ag. Cl

Equivalent Circuit Cd Rd Rs Ecell Rd+Rs : capacitance of electrode-eletrolyte interface : resistance of electrode lead wire : cell potential for electrode Corner frequency Rs Frequency Response

Electrode Skin Interface Ehe Electrode Cd Rd Gel 100 m Stratum Corneum Epidermis 100 Dermis and subcutaneous layer m Nerve endings Capillary Ce Rs Sweat glands and ducts Ese EP Re CP RP Alter skin transport (or deliver drugs) by: Pores produced by laser, ultrasound or by iontophoresis Ru Skin impedance for 1 cm 2 patch: 200 kΩ @1 Hz 200 Ω @ 1 MHz

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

Body Surface Recording Electrodes Electrode metal Electrolyte 1. Metal Plate Electrodes (historic) 2. Suction Electrodes 1. (historic interest) 3. Floating Electrodes 4. Flexible Electrodes Think of the construction of electrosurgical electrode And, how does electro-surgery work?

Commonly Used Biopotential 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

Commonly Used Biopotential Electrodes Suction electrodes - No straps or adhesives required - precordial (chest) ECG - can only be used for short periods Floating electrodes - metal disk is recessed - swimming in the electrolyte gel - not in contact with the skin - reduces motion artifact Suction Electrode

Commonly Used Biopotential Electrodes Insulating package Double-sided Adhesive-tape ring Metal disk Electrolyte gel in recess (a) (b) Snap coated with Ag-Ag. Cl Plastic cup Foam pad External snap Gel-coated sponge Plastic disk Reusable Disposable Dead cellular material Tack Capillary loops Germinating layer (c) Floating Electrodes

Commonly Used Biopotential 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(a) Carbon-filled silicone rubber electrode. (Mylar film) (b) Flexible thin-film neonatal electrode. (c) Cross-sectional view of the thin-film electrode in (b).

Internal 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. The latest: BION – implanted electrode for muscle recording/stimulation Alfred E. Mann Foundation

Fetal ECG 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.

Electrode Arrays Contacts Ag/Ag. Cl electrodes Contacts Insulated leads Ag/Ag. Cl electrodes Base Insulated leads (a) Tines Exposed tip Base (b) Base (c) Examples of microfabricated electrode arrays. (a) One-dimensional plunge electrode array, (b) Two-dimensional array, and (c) Three-dimensional array

Microelectrodes Why Measure potential difference across cell membrane Requirements Intracellular – Small enough to be placed into cell – Strong enough to penetrate cell membrane Extracellular – Typical tip diameter: 0. 05 – 10 microns Types – Solid metal -> Tungsten microelectrodes – Supported metal (metal contained within/outside glass needle) – Glass micropipette -> with Ag-Ag. Cl electrode metal

Metal Microelectrodes C Microns! R Extracellular 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!

Metal Supported Microelectrodes (a) Metal inside glass (b) Glass inside metal

Glass Micropipette heat pull Fill with intracellular fluid or 3 M KCl Ag-Ag. Cl wire+3 M KCl has very low junction potential and hence very accurate for dc measurements (e. g. action potential) 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. Intracellular recording – typically for recording from cells, such as cardiac myocyte Need high impedance amplifier…negative capacitance amplifier!

Electrical Properties of Microelectrodes Metal Microelectrode Metal microelectrode with tip placed within cell Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier! Equivalent circuits

Electrical Properties of Glass Intracellular Microelectrodes Glass Micropipette Microelectrode

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 to stimulation Platinum electrodes: – Large currents can cause Applications: neural – Cavitation stimulation – Cell damage – Heating Modern day Pt-Ir and other exotic Types of stimulating electrodes 1. Pacing 2. Ablation 3. Defibrillation metal combinations to reduce polarization, improve conductance and long life/biocompatibility Steel electrodes for pacemakers and defibrillators

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

In vivo neural microsystems (FIBE): challenge

In vivo neural microsystems (FIBE): biocompatibility - variant

In vivo neural microsystems (FIBE): state of the art

Introduction: neural microsystems Instrumentation for neurophysiology Neural Microsystems Neural microelectrodes MEMS Microsystems

Introduction: types of neural microsystems applications External electrodes Subdural electrodes Human level Microelectrodes Microsensors – In vivo applications Animal level Tissue slice level – Cellular level – – In vitro applications –

Microelectronic technology for Microelectrodes Bonding pads Insulated lead vias Si. O 2 insulated Au probes Exposed electrodes Silicon probe Si substrate Exposed tips (a) Beam-lead multiple electrode Miniature insulating chamber Hole . (b) Multielectrode silicon probe Channels Silicon chip Lead via Silicon probe Electrode (c) Multiple-chamber electrode Contact metal film (d) Peripheral-nerve electrode Different types of microelectrodes fabricated using microfabrication/MEMS technology

Michigan Probes for Neural Recordings

Neural Recording Microelectrodes Reference : http: //www. acreo. se/acreo-rd/IMAGES/PUBLICATIONS/PROCEEDINGS/ABSTRACTKINDLUNDH. PDF

In vivo neural microsystems: 3 examples University of Michigan Smart comb-shape microelectrode arrays for brain stimulation and recording University of Illinois at Urbana-Champaign High-density comb-shape metal microelectrode arrays for recording Fraunhofer Institute of Biomedical (FIBE) Engineering Retina implant

Multi-electrode Neural Recording Reference : http: //www. cyberkineticsinc. com/technology. htm Reference : http: //www. nottingham. ac. uk/neuronal-networks/mmep. htm

WPI’s Nitric Oxide Nanosensor

Nitric Oxide Sensor • Developed at Dr. Thakor’s Lab, BME, JHU • Electrochemical detection of NO Left: 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)

A E B F C G D H Cartoon of the fabrication sequence for the NO sensor array A) Bare 4” Si wafer B) 5µm of photoresist was spin-coated on to the surface, followed by a prebake for 1 min at 90°C. C) The samples were then exposed through a mask for 16 s using UV light at 365 nm and an intensity of 15 m. W/cm 2. D) Patterned photoresist after development. E) 20 nm of Ti, 150 nm of Au and 50 nm of C were evaporated on. F) The metal on the unexposed areas was removed by incubation in an acetone bath. G)A 2 nd 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.

NO Sensor Calibration

NO Sensor Calibration

Multichannel NO Recordings