Modelling of Electrode Electrolytic Interface

Bio-potentials: Ionic voltages produced as a result of the electrochemical activity of excitable cells.

Electrodes: Transducers to convert ionic potentials into electrical potentials

Electrode Electrolytic Interface

1. Charge separation occurs which leads to electrical double layer.
2. Half cell potential is generated
3. Electrolyte offers some resistance
4. DC offset of the electrode presents some resistance.

Human body electrode Interface:

Resistance offered by epidermis layer needs to be considered

Hence to reduce the effect of body resistance

1. use of Gel between electrode and skin
2. Preparing the skin surface by removal of stratum corneum

Important aspects:

Electrodes – Basics

• High-quality biopotential measurements require
• Good amplifier design
• Use of good electrodes and their proper placement on the patient
• Good laboratory and clinical practices
• Electrodes should be chosen according to the application
• Basic electrode structure includes:
• The body and casing
• Electrode made of high-conductivity material
• Wire connector
• Cavity or similar for electrolytic gel
• Adhesive rim
• The complexity of electrode design often neglected

Electrodes - Basics

• Skin preparation by abrasion or cleansing
• Placement close to the source being measured
• Placement above bony structures where there is less muscle mass
• Distinguishing features of different electrodes:
• How secure? The structure and the use of strong but less irritant adhesives
• How conductive? Use of noble metals vs. cheaper materials
• How prone to artifact? Use of low-junction-potential materials such as Ag-AgCl
• If electrolytic gel is used, how is it applied? High conductivity gels can help reduce the junction potentials and resistance but tend to be more allergenic or irritating

Ag-AgCl, Silver-Silver Chloride Electrodes

• The most commonly used electrode type
• Silver is interfaced with its salt silver-chloride
• Choice of materials helps to reduce junction potentials
• Junction potentials are the result of the dissimilar electrolytic interfaces
• Electrolytic gel enhances conductivity and also reduces junction potentials
• Typically based on sodium or potassium chloride, concentration in the order of 0.1 M weak enough to not irritate the skin
• The gel is typically soaked into a foam pad or applied directly in a pocket produced by electrode housing
• Relatively low-cost and general purpose electrode
• Particularly suited for ambulatory or long term use

References:

• http://www.ece.queensu.ca/undergraduate/ugradcourses/elec408/ELEC408_ElectrodeTheoryNotes.pdf
• http://faculty.ksu.edu.sa/ElBrawany/Courses/Med202Files/W5-.ppt
• http://en.wikipedia.org/wiki/Action_potential