To measure and record potentials and currents in the body an electrode can be used as biomedical sensor [11]. This seems to be a very simple function, but in fact an electrode recording biopotentials is actually a transducer, converting ionic currents in the body into electronic currents in the electrode. This transduction function greatly complicates electrode design. Figure 4 shows an electrode-electrolyte interface. The electrode only has one type of charge carrier (electron), whereas the electrolyte has two types of charge carriers (cation and anion). The electrolyte is an aqueous solution containing cations of the electrode metal C+ and anions A-. The electrode consists of metallic atoms C and the current crosses the interface from left to right. At the interface, charge is exchanged through chemical reactions, which can be generally represented as:where n is the valence of C and m is the valence of A.


Figure 4. Electrode-electrolyte interface

A potential difference know as the half-cell potential is determined by the metal involved, the concentration of its ions in solution, and the temperature. The standard half cell potential, E0, is the potential for 1M concentration solution at 25°C when no current flows across the interface. When a circuit is constructed to allow current to flow across an electrode-electrolyte interface, the observed half-cell potential is often altered. The difference between the observed half-cell potential for a particular circuit and the standard half cell potential is known as the overpotential. Three basic mechanisms contribute to the overpotential: ohmic, concentration, and activation.
1. The ohmic overpotential is the voltage drop across the electrolyte itself due to the finite resistivity of the solution. Overall, this is usually not a big voltage in high concentration solutions.
2. The concentration overpotential results from changes in ionic concentration near the electrode-electrolyte interface when current flows. Oxidation-reduction reaction rates at the interface change with excess charge due to a finite current. This modifies the equilibrium concentration of ions changing the half-cell potential.
3. Charge transfer in the oxidation-reduction reaction at the interface is not entirely reversible. For metal ions to be oxidized, they must overcome an energy barrier. If the direction of current flow is one way, then either oxidation or reduction dominates, and the height of the barrier changes. This energy difference produces a voltage between the electrode and the electrolyte, known as the activation overpotential.
The overpotential of an electrode is then given by the sum of these three polarization mechanisms:

 Vp = Vr + Vc + Va

where Vr is the ohmic overpotential, Vc is the concentration overpotential, and Va is the activation overpotential. Note that overpotentials impede current flow across the interface. A way to minimize Vp is to use nonpolarizable electrodes. These allow conduction current to flow across the interface with no energy exchange and there are no overpotentials for this type of electrode. The best electrode to use for all possibilities for biological electrode system is the silver/silver chloride (Ag/AgCl) electrode. This is made of a silver metal base with attached insulated lead wire coated with a layer of the ionic compound AgCl. The electrode is then immersed in an electrolyte bath in which the principle anion of the electrolyte is Cl-. For the best results, the electrolyte solution should also be saturated with AgCl so there is little chance for any of the surface film on the electrode to dissolve. Cl- is an attractive anion for electrode applications with mammals since these animals (including humans) have an excess of chloride ions in solution. The electrode-electrolyte interaction is described by the reaction

 Ag ⇔ Ag+ + e-

 Ag+ + Cl- ⇔ AgCl(precipitate)

And the Nerst equation for the reaction can be written as:

   RT               RT

 E = E0 + —  ln(Ks)- —— ln(acl-)

   nF              nF

The first two terms on the right side of this last expression are constants - only the third is related to ionic activity. In biological systems, the large chlorine ion concentration makes its activity fairly constant. This means that the half-cell potential for this electrode is quite stable for biological systems. In this article we will only consider low current densities, and consequently the electrode-electrolyte interface can be modeled as a linear system with an equivalent circuit composed exclusively of linear components (i.e., voltage/current sources, resistors, capacitors and inductors). The terminal characteristics of an electrode have both resistive and reactive components.

 

Figure 5. Equivalent circuit for a biopotential electrode in contact with an electrolyte.

Ehc is the half-cell potential, Rd and Cd make up the impedance associated with the electrode-electrolyte interface and polarization effects, and Rs is the series resistance associated with interface effects and due to resistance in the electrolyte

Fig. 5 shows the equivalent circuit of the electrode-electrolyte interface. In this circuit Rd and Cd represent the resistance (i.e., conduction currents) and the capacitance (i.e., displacement currents) respectively resulting from the double-layer of ionic charge at the electrode-electrolyte interface. The resistance Rs is the series resistance associated with equivalent losses in the electrolyte itself.
There are three primary layers in the skin. The outermost layer, or epidermis, plays the most important role in the electrode-skin interface. It is a constantly changing layer, the outer surface of which consists of dead material on the skin’s surface with different electrical characteristics from live tissue. The deeper layers of skin contain the vascular and nervous components of the skin as well as the sweat glands, ducts, and hair follicles. These layers are similar to others in the body, and with the exception of the sweat glands, can be modeled as equivalent to the electrical characteristics of the rest of the viscera. Given this anatomy, a general equivalent circuit describing the characteristics of both the electrode-electrolyte interaction and the connection to the skin can be developed, as illustrated in figure 6.
Figure 6 Total electrical equivalent circuit obtained for a body-surface electrode placed against skin
The epidermis can be considered a semipermeable membrane to ions, so a potential given by the Nernst equation, can be developed if there is a difference in ionic concentrations across this membrane. The dermis and subcutaneous layer under it behave in general as pure resistances. They generate negligible DC potentials. Finally, the electrical characteristics of the sweat glands must also be taken into account for a complete model of a skin electrode. The fluid secreted by sweat glands contains Na+, K+, and Cl- ions, the concentrations of which differ from those in extracellular fluid. This produces a potential between the lumen of the sweat duct and the dermis and subcutaneous layers. There is also a parallel RpCp combination with this potential representing the wall of the sweat gland and duct. This equivalent model has been used by many authors to simulate the electrical behaviour of the electrode-skin interaction.