Components of bio-medical instrument system

Half cell potential (or) Electrode potential: Voltage developed at an electrode – electrolyte interface The passage of ions from the metal into the solution The combination of metallic ions in solution with electrons in the metal to form atoms of the metal. Electrical double layer

Half cell potential (or) Electrode potential: Perfectly polarised electrodes: Electrodes in which no net transfer of charge occurs across the metal electrolyte interface Perfectly Non-polarised electrodes: Unhindered exchange of charge occurs

Half cell potential (or) Electrode potential: Ehc = half cell potential Cd = electrode capacitance Rd = leakage resistance Rs = series electrolyte and skin resistance

Half cell potential : Z = Rs + Rd / (1+j2ΠfRdCd) Nernst equation: Ehc = -2.303 RT/nF (log10 C1f1 / C2f2) Where, C = concentration, f = activity coefficients R = gas constant = 8.314 kJ/k mol/ K T = absolute temperature in Kelvin F = no. of coulombs transferred (or) Faraday constant = 96500 columbs n = valency of the ion

Purpose of the electrode paste: Dry outer skin is highly non-conductive Washed throughly Rubbed briskly to remove some of outer cells Coated with consuctive paste – electrode paste (reduces the impedance and reduces artifacts) Electrode is applied ECG : R: 100 kΩ to 10kΩ C: 0.01 μF to 0.1 μF

Electrode material: To reduce polarisation effect, the electrodes are coated. Reduces noise voltage Increases stability Silver coated with silver chloride (widely used) : Half cell potential : 2.5 mV

Electrodes: 3 types Types: Micro electrode Depth & needle electrodes Surface electrodes

Micro electrodes: Intra cellular electrodes Used to measure the potential near or within cell Features: Smaller dimension (0.5 to 5 microns) Types: 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: EA – metal electrode-electrolyte potential at the microelectrode tip EB – Reference electrode – electrolyte potential EC – variable cell membrane potential RA – resistance of connecting wire RB – resistance of wire connected to reference electrode

Contd. Impedance of microelectrode is inversely proportional to the area of the tip & frequency. Zin 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: EA – metal electrode-electrolyte potential at the microelectrode tip EB – Reference electrode – electrolyte potential EC – variable cell membrane potential ED – potential existing at the tip due to different electrolytes present in the pipet and the cell RA – resistance of connecting wire RB – resistance of wire connected to reference electrode RT – 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 mm2

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: Hydrogen electrode Practical reference electrode 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 = log10 1/[H+] = - log10 [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 H3O+ 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 CO2 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 pCO2 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

pO2 electrode: Clark electrode