Electrodes: Types:»3 types Micro electrode Depth & needle electrodes Surface electrodes
Micro electrodes: Intra cellular electrodes Used to measure the potential near or within cell Features: Types: Smaller dimension (0.5 to 5 microns)»metallic»non metallic (Micropipet)
Metal microelectrode: Formed by electrolytically etching the tip of fine Tungsten or stainless steel Electropointing Coated with insulating material Chloriding the tip : reduce the impedance
Metal electrode:
Metal electrode electrical equivalent:
Equivalent: E A metal electrode-electrolyte potential at the microelectrode tip E B Reference electrode electrolyte potential E C variable cell membrane potential R A resistance of connecting wire R B resistance of wire connected to reference electrode
Contd. Impedance of microelectrode is inversely proportional to the area of the tip & frequency. Z in of the amplifier should be high : if not, behaves like a high pass filter
Micropipet (Non metallic electrode): 1 micrometre Filled with electrolyte 3 M KCL
Micropipet electrical equivalent:
Equivalent: E A metal electrode-electrolyte potential at the microelectrode tip E B Reference electrode electrolyte potential E C variable cell membrane potential E D potential existing at the tip due to different electrolytes present in the pipet and the cell R A resistance of connecting wire R B resistance of wire connected to reference electrode R T resistance of electrolyte filled
Depth & needle electrode: Depth electrode: Used to study the electrical activity of the neurons in superficial layers of the brain & also the oxygen tension Bundle of Teflon insulated platinum (90%) & Iridium (10%) alloy wires End of supporting wire is rounded Active area : 0.5 mm 2
Depth electrode:
Needle electrode: Electroneurography Resembles medicine dropper or hypodermic needle Bend at one end and is inserted through the lumen and is advanced into the muscle Needle is withdrawn and the bent wire is resting inside the muscle
Needle electrode:
Surface electrodes: Larger area electrodes : ECG potentials Smaller area electrodes: EEG & EMG potentials Types:»Metal plate electrodes»suction cup electrode»adhesive tape electrode»multipoint electrode»floating electrode
Metal plate electrodes: Rectangular (3.5 cm x 5 cm) & circular ( 4.75 diameter) in shape German silver, nickel silver, nickel plated steel ECG measurements
Suction Cup electrode: Flat surfaces of the body and to regions where the underlying tissue is soft Physically large but only rim (smaller area) is in contact to the skin
Adhesive tape electrode: The pressure of the surface electrode against the skin may squeeze the electrode paste out So, this type of electrodes are used Light metallic screen backed by a pad for electrode paste
Multipoint electrode: ECG measurement Nearly 1000 fine active contact points
Floating electrode: Metal does not contact the subject directly but via electrolytic bridge Also called as liquid junction electrode Movement artifact is eliminated
Distortion in signals: Ag-Agcl electrode uses shielded cable to reduce interference Johnson noise (random movement of charge carriers) or ohmic noise ( ohmic component of the electrode impedance) : micro electrodes Movement artifacts : equal half cell potential and high impedance will minimize the artifacts (floating electrodes) Distortion : movement of low current density
Chemical Electrodes: Used to measure : Hydrogen electrode Practical reference electrode»ph electrode»pco2 electrode»po2 electrode
Hydrogen electrode: Reference electrode (potential = 0) Used to measure ph of body fluids Platinum is electrolytically coated with finely divided platinum to increase the surface area This electrode with absorbed hydrogen is inserted into a catheter and is placed in pulmonary artery Monitored with a fluoroscope
Drawback of hydrogen electrode: Not sufficiently stable to serve as a reference electrode During measurement hydrogen gas should be supplied So, calomel & silver silver chloride electrodes are used
Practical reference electrode: Ag-AgCl : smaller & stable half cell potential Ag-AgCl is kept at the central position and potassium chloride solution is filled around the electrode Calomel or mercurous chloride electrode is another popular reference electrode
ph electrode: ph = log 10 1/[H + ] = - log 10 [H + ] 7 neutral solution <7 acidic solution >7 basic solution Human blood is slightly basic; venous blood : 7.35 & atrial :7.40
Contd..
ph electrode:
Contd.. Spherical bulb : 0.5 cm in diameter Permits the passage of only hydrogen ions in the form of H 3 O + Inside the glass bulb silver-silver chloride non polarisable electrode is immersed in chloride buffer solution usually of ph = 1 Other side is glass bulb is exposed to the solution of unknown ph Adv: independent of oxidation reduction potentials
pco2 electrode: Standard glass ph electrode covered with rubber membrane permeable to CO 2 Between the glass and membrane, water is there ph is measured and interpreted in terms of pco2 on the basis of the linear relationship between log pco 2 and ph of the solution
po2 electrode: Piece of platinum wire embedded in an insulating glass holder Other end is exposed to electrolyte where oxygen is allowed to diffuse through the membrane Ag-AgCl : reference electrode 0.7 V is applied between platinum(-ve) and reference electrode(+ve) Reduction takes place at the platinum wire
po 2 electrode: Clark electrode