Biomedical Electronics & Bioinstrumentation Biopotential Electrodes

Contents Principles of Electrode-electrolyte Interface Polarization Phenomena Polarizable & Non-Polarizable Electrodes Ag/AgCl Electrode Skin-electrode Interface Body-surface & Internal Electrodes

Biopotential Electrodes Provide means of interface between the body and electronics measuring apparatus. Biopotential electrodes transduce ionic conduction to electronic conduction so that biopotential signals can be recorded. They generally consist of metal contacts packaged so that they can be easily attached to the skin or other body tissues.

Electrode-Electrolyte Interface Observe the following diagram: C C+ e- A- I Electrode Electrolyte

Electrode-Electrolyte Interface The net current that crosses the interface, from the electrode to the electrolyte consists of: Electrons moving in the opposite direction to that of the currents in the electrode. Cations moving in the same direction as the current. Anions moving in the opposite direction to that of current in the electrolyte.

Electrode-Electrolyte Interface For charge to cross the interface, There are no free electrons in the electrolyte. There are no free cations or anions in the electrode. Something must occur at the interface that transfers the charge between these carriers.

Electrode-Electrolyte Interface What happens are actually chemical reaction at the interface: Where, n : Valence of C m : Valence of A

Electrode-Electrolyte Interface Reactions involving cations: Electrode is made of some atoms of the same material as the cations. Material in the electrode at the interface can oxidize to form cations and more free electrons. Cations discharged into the electrolyte. Electrons remain as charge carrier in the electrode.

Electrode-Electrolyte Interface Reactions involving anions: Anions coming to electrode-electrolyte interface can be oxidized to neutral atoms. This gives one or more free electrons to the electrode. Both reactions are reversible but not entirely. When no current crosses, these reactions still occur.

Electrode-Electrolyte Interface Rate of oxidation equals to rate of reduction reaction. So net transfer of charge across the interface is zero. When current flows from electrode to electrolyte, oxidation reaction dominates. When current flows from electrolyte to electrode, reduction reaction dominates.

Half-Cell Potential Half-cell potential determined by: Metal involved. Concentration of its ions in the solution. Temperature. Other second-order factors. Knowledge of half-cell potential is important to understand the behavior of biopotential electrode.

Half-Cell Potential Let us consider placing a piece of metal into solution containing ion of that metal. The ions are cations. To maintain equilibrium, the solution needs to have equal concentration of anions. When the metal comes into contact with the solution, reaction begins immediately.

Half-Cell Potential Initially, the reaction depends on: Concentration of cations in the solution. Equilibrium condition for the particular reaction. Local concentration in the solution at the interface changes, which affects the anion concentration as well. The neutrality is then not maintained in the region.

Half-Cell Potential Thus, different electric potential exists between the electrolyte surrounding the metal than the rest of the solution. The phenomena explained assumes no electrical current exists between the electrode and the electrolyte.

Half-Cell Potential Metal & Reaction Potential Eo (V) Al  Al3+ + 3e- -1.706 Zn  Zn2+ + 2e- -0.763 Cr  Cr3+ + 3e- -0.744 Fe  Fe2+ + 2e- -0.409 Cd  Cd2+ + 2e- -0.401 Ni  Ni2+ + 2e- -0.230 Pb  Pb2+ + 2e- -0.126 H2  2H+ + 2e- 0.000 (by definition) Ag + Cl-  AgCl + e- +0.223 2Hg + 2Cl-  Hg2Cl2 + 2e- +0.268 Cu  Cu2+ + 2e- +0.340 Cu  Cu+ + e- +0.522 Ag  Ag+ + e- +0.799 Au  Au3+ + 3e- +1.420 Au  Au+ + e- +1.680 The metal undergoing the reaction shown has the sign and potential Eo when referenced to the hydrogen electrode at 25°C.

Polarization If there is a current, the observed half-cell potential will be altered. Difference is due to polarization of the electrodes. The observed difference between the observed half-cell potential and the zero-current half-cell potential is known as overpotential. Components of overpotentials: Ohmic overpotential Concentration overpotential Activation overpotential

Ohmic Overpotential Direct result of the resistance of the electrolyte. When electrodes are immersed in the electrolyte, voltage drop exist along the path of the current in the electrolyte. Resistance between the electrodes vary as a function of current.

Concentration Overpotential Results from change of ionic concentration in the electrolyte at the vicinity of the electrode- electrolyte interface. When current flows, the equilibrium of reactions does not exist. Thus, the ionic concentration is expected to change and affects the half-cell potential observed.

Activation Overpotential Charge-transfer process involving redox reaction is not entirely reversible. For metal atoms to oxidize into metal ions that are capable of going into the solution, the atoms must overcome an energy barrier. This barrier (activation energy) governs the kinetics of the reaction.

Polarization Potential These 3 components are additive. Where, Vp : Polarization potential Vr : Ohmic overpotential Vc : Concentration overpotential Va : Activation overpotential

Polarizable and Non-Polarizable Theoretically: Perfectly polarizable electrode Perfectly non-polarizable electrode Classification refers to what happen at the electrode when current passes between it and the electrolyte. In reality however, it is impossible to fabricate these perfect electrodes.

Polarizable Electrode No equal charge crosses the electrode-electrolyte interface when current is applied. In reality, current exists but it is only a displacement current where the electrode behaves like a capacitor. In practical, platinum comes close to behaving as perfectly polarizable electrode.

Polarizable Electrode The current passing between the electrodes are primarily changes the ionic concentration at the interface. Majority of the overpotential observed are from concentration overpotential, Vc. These electrode shows strong capacitive effect.

Non-Polarizable Electrode Current passes freely across the electrode- electrolyte interface, requiring no energy to make the transition. Thus no overpotential exists for a perfectly non- polarizable electrode. In practical, the Ag/AgCl electrode behaves closest to perfectly non-polarizable electrode.

Ag/AgCl Electrode A member of a class of electrode which consists of a metal coated with a layer of slightly soluble ionic compound of that metal with a suitable anion. Easily fabricated in laboratories. Structure immersed in electrolyte of anions with relatively high concentration.

Ag/AgCl Electrode

Ag/AgCl Electrode Silver metal base attached with insulated lead wire is coated with ionic compound AgCl. AgCl is very slightly soluble in water, so it remains stable. Electrode immersed in electrolyte bath containing Cl-.

Ag/AgCl Electrode For best result, AgCl in solution should be highly concentrated. Governed by the following chemical reactions: Where, Ag+ : Silver ion Cl- : Chloride ion

Ag/AgCl Electrode First, oxidation of silver atoms at the surface to silver ions in the solution at the interface. Second, the ions combine with Cl- to form ionic compound AgCl. AgCl is slightly soluble, thus are deposited on the electrode surface.

Skin Structure

Skin Structure Epidermis consist of 3 layers: Stratum germinativum Stratum granulosum Stratum corneum Dead cells have different electrical properties than live cells. Deeper layer contains vascular and nervous components.

Electrode-Skin Interface Cl- electrolyte gel or cream used to maintain good skin contact with the electrode. Rs exist due to interface effect of the gel between the electrode and the skin. Stratum corneum is a semipermeable membrane to ions. If there is difference in ionic concentration, there is potential Ese.

Electrode-Skin Interface Epidermal layer behaves as a parallel RC circuit. For 1cm2, skin impedance reduces from 200kΩ at 1Hz to 200Ω at 1MHz. The dermis and subcutaneous layer acts as pure resistance Ru. DC potentials are also generated which are neglible. The impedance created by the epidermal layer can be reduced through abrasion of the skin, which theoretically shorts the parallel RC circuit.

Electrode-Skin Interface Equivalent Electrical Circuit

Motion Artifact In polarizable electrode, the ionic layer distribution at the interface changes when the electrode moves with respect to the electrolyte. Results in momentary change of half-cell potential until equilibrium is established. However, motion artifact is minimal for non- polarizable electrode.

Motion Artifact In addition, the motion artifact can also be caused by variations in the electrolyte gel-skin potential, Ese. This can be reduced by abrasion of the stratum corneum. This also reduces the skin impedance of the epidermal layer.

Body-Surface Electrodes Metal-plate electrodes

Body-Surface Electrodes Suction electrode

Body-Surface Electrodes Floating electrodes

Body-Surface Electrodes Flexible electrodes

Internal Electrodes Percutaneous electrodes

Internal Electrodes Percutaneous electrodes

Internal Electrodes Fetal electrodes

Further Reading… Webster, J.G. (1997). Medical Instrumentation: Application and Design. 3rd Ed., Wiley. Chapter 5