1. Components of bio-medical instrument system

2. 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

3. 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

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

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

6. 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

7. 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

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

9. 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)

10. 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

11. Metal electrode:

12. Metal electrode – electrical equivalent:

13. 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

14. 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

15. Micropipet (Non metallic electrode):
1 micrometre
Filled with electrolyte 3 M KCL

16. Micropipet – electrical equivalent:

17. 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

18. 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

19. Depth electrode:

20. 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

21. Needle electrode:

22. 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

23. 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

24. 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

25. 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

26. Multipoint electrode:
ECG measurement
Nearly 1000 fine active contact points

27. Floating electrode:
Metal does not contact the subject directly but via electrolytic bridge
Also called as liquid junction electrode
Movement artifact is eliminated

28. 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

30. Chemical Electrodes: Hydrogen electrode Practical reference electrode
Used to measure :
Hydrogen electrode
Practical reference electrode
pH electrode
pCO2 electrode
pO2 electrode

31. 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

32. 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

33. 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

34. 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

35. Contd..

36. pH electrode:

37. 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

38. 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

39. 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

40. pO2 electrode: Clark electrode