Understanding Bio-potential Electrodes in Biomedical Instrumentation

1. Bio-potential Electrodes and its Applications Ms.J.Rajeswari Assistant Professor, Department of Electrical and Electronics Engineering, Velammal College of Engineering and Technology

2. Biomedical engineering is the application of knowledge and technologies to solve the problem of the living system. • It involves diagnosis, treatment and prevention of disease in human. Biomedical engineering

3. It involves measurement of biological signals like ECG, EMG, or any electrical signalsgenerated in the human body. • Biomedical Instrumentationhelpsphysicians to diagnose the problem and provide treatment. Biomedical Instrumentation

4. Components of Biomedical Instrumentation System

5. Measurand: The measurand is the physical quantity. • Human body acts as the source for measurand, and it generates bio-signals. • Example: body surface or blood pressure in the heart Components of Biomedical Instrumentation System

6. Sensor / Transducer: The transducerconverts one form of energy to another form usually electrical energy. • For example, the piezoelectric signal which converts mechanical vibrations into the electrical signal • The transducer produces a usable output depending on the measurand. Components of Biomedical Instrumentation System

7. Signal Conditioner: Signal conditioning circuits are used to convert theoutput from the transducer into an electrical value. • Generally, signal conditioning process includesamplification, filtering, analogue to digital and Digital to analogue conversions. • Signal conditioning improvesthe sensitivity of instruments. Components of Biomedical Instrumentation System

8. Display: It is used to provide a visual representation of the measured parameter or quantity. • Example: Chart recorder, Cathode Ray oscilloscope (CRO). • Sometimes alarms are used to hear the audio signals. • Example: Signals generated in Doppler Ultrasound Scanner used for Fetal Monitoring. Components of Biomedical Instrumentation System

9. Data Storage and Data Transmission: Data storage is used to store the data and can be used for future reference. • Recently Electronic Health records are utilized in hospitals. • Data transmission is used in Telemetric systems, where data can be transmitted from one location to another remotely. Components of Biomedical Instrumentation System

10. The human body generates electrical signals on the body surface. • Recording electrodes picks the bioelectric events produced in the body. • The picked up signals are given to the amplifier and then to display. • Electrodestransfer ionic conduction in the tissueto electronic conduction to measure the values. Bio Electrode Potential

11. Generally, there are two types of electrodes namely surface electrodes and needle electrodes. • A surface electrode picks the potential difference from the tissue surface without damaging the live tissue. • Deep electrode indicates the electric potential value from inside the cell. Electrodes

12. Electric potential generated in the body are ionic potential. • Transducersconvert the ionic current in the body into an electronic current that flows through the electrode. • It conducts small current across the interface between body and measuring circuit. Biopotential Electrodes: Significance

13. A net volume of current passes across the interface from the electrode to electrolyte. • At the electrode-electrolyte interface, current flows from left to right Biopotential Electrodes: Significance

14. Half-cell potential is the voltagedeveloped at the electrode-electrolyte interface. • In a metal – solution interface, electrode potential arises at two conditions • i) when ions travel from metal into the solution • ii) when ions in solution combine with electrons in the metal they form the atom of metal Half Cell Potential

15. Electrode is a solid electric conductor through which an electric current enters or leaves an electrolytic cell. • It converts ionic potentials to electronic potentials. • Different types of electrodes used for biological measurements depend on the anatomical locations, from where the bioelectric signals are measured. • Bioelectric electrodes acquire the signals like ECG, EEG, EMG, etc. Electrodes

16. Types of Electrodes

17. Surface electrode measures the potential available from the surface of the skin. • It senses the signal from heart, brain and nerves. • Larger surface electrodessense the ECG signals. • Smaller surface electrodes sense the EMG and EEG signals Surface Electrodes

18. ECG measurement technique uses either rectangular or circular shaped plate electrodes made of nickel, silver or German silver materials. • It has a smaller contact area and do not seal completely on the patient. Surface Electrodes : Metal Plate Electrodes

19. Electrodes are pasted on the skin using electrolyte paste. • The electrode slippage and plate displacement are the two major disadvantages of this electrode type. • They are very sensitive, leading to measurement errors. • Since it is suitable for application on four limbs of the body, they called limb electrodes. Surface Electrodes : Metal Plate Electrodes

20. During surgical procedure since patient’s legs are immobile, limb electrodes are preferred. • Chest electrodes interfere with the surgery, so not used for ECG measurement. • At the same time for a long-term patient monitoring limb-electrodes are not used. Surface Electrodes : Metal Plate Electrodes

21. To measureECGfrom various positions on the chest, Suction cup electrodes are used. • It suits well to attach electrodes on flat surface of the body and on soft tissue regions. • They have a good contact surface. Surface Electrodes : Suction Cup Electrodes or Welsh Cup Electrodes

22. Physically they are large but the skin contacts only the electrode rim. • It has high contact impedance. • They have a plastic syringe barrel, suction tube and cables. • Recently, due to infection and cleaning procedures, these electrodes are not used. Surface Electrodes : Suction Cup Electrodes or Welsh Cup Electrodes

23. Suction Cup Electrodes

24. In the surface electrode, the pressure of surface electrode against the skin squeezes out the electrode paste. • To avoidthis problem, adhesive electrodes are used. • It has a lightweight metallic screen. • They have a pad at behind for placing electrode paste. • This adhesive backinghold the electrode on place and tight. • It also helps to avoid evaporation of electrolyte present in the electrode paste. Surface Electrodes : Adhesive Type Electrodes

25. Surface Electrodes : Adhesive Type Electrodes

26. In metal plate or limb electrodes, the major disadvantage is the movement errors. • Motion artefactoccursdue to the motion at the interface between electrode and electrolyte. • The interface gets stabilized using Floating electrodes. Floating Electrodes

27. The floating electrodes do not contact the humansubject directly. • They contact the subject via electrolytic paste or jelly. • The advantage of this type is the mechanical stability. Floating Electrodes

28. Microelectrodemeasures the electric potential from within a single cell. • It has very small diameter tips that can penetrate deep into the cell without damaging the human cell. • The functions of microelectrodes are potential recording to inject medicines. Microelectrodes

29. Generally, when microelectrode is inside cell, reference electrode is outside the cell. • It has high impedances in range of mega ohm due to their small size. • Two types of microelectrode are • Metal Microelectrode • Non- Metallic (Micropipette) Microelectrode

30. The tungsten filament or stainless steel wire made into minute structure forms the tip of the microelectrode. • This technique is electropointing. • The insulating material covers the entire electrode for safety purpose. • Few electrolytic processing is done to reduce the impedance. Metal Microelectrode

31. Measurement of bioelectric potentialsrequires two electrodes. • Resulting voltage potential is the difference betweenpotential of microelectrode and reference electrode. Metal Microelectrode

32. The total sum of three potentials is as follows E=EA+EB+EC • Where, EA – metal electrode – electrolyte potential at microelectrode tip. • EB – Reference electrode – electrolyte potential. • EC – Variable cell membrane potential. Metal Microelectrode

33. This electrode uses Non - metallic material to measure the potential from a single cell. • It consists of glass micropipette of diameter 1 micrometer. • Micropipet filled with electrolyte solution that is compatible with cellular fluids. Non - Metal Microelectrode (Micropipet)

34. Stem of Micropipethas a thin flexible wire made out of chloride silver, stainless steel or tungsten. • One end of the Micropipet attaches to the rigid support and other free end rests on the cell. Non - Metal Microelectrode (Micropipet)

35. The potential voltage generated is as follows. • E=EA+EB+EC+ED • EA – potential voltage between the metal wire and an electrolyte filled inside Micropipet. • EB – potential between the reference electrode and extracellular fluid. • EC – variable cell membrane potential. • ED – potential at the tip due to electrolytes present inside pipet and the cell Non - Metal Microelectrode (Micropipet)

36. When electrode gets closer to the bioelectric generator, it penetrates into the skin. • Therefore, the electrode should be sharp for penetration to obtain and record the bioelectric events. Depth and Needle Electrodes

37. Depth electrode studies the electrical activity of the neurons in the surface of the brain. • This type of electrode consists of bundle of Teflon insulated platinum and iridium alloy wires. • For easy insertion of the electrodes into the brain, the end of supporting wire is round shaped. Depth Electrodes

38. Number of individual electrodes forms the electrode array or bundle. • In the bundle of electrodes, end of each individual wires has the individual electrode Depth Electrodes

39. Needle electroderecords the peripheral nerve action potential. • It resembles a medicinal syringe. • At one end a short insulated wire is bent. • The bent portion passes through the lumen of the needle. Needle Electrodes

40. This setupgoes into the muscle. • Now the needle is withdrawn. • The bent wire remains inside the muscle. • Two type of needle electrodes namely • Mono-polar Electrode: This type uses single reference electrode placed on the skin. Bi - polar Electrode: This type has one reference electrode and one active electrode. Needle Electrodes

41. Generally, biological/bioelectricsignals have low amplitude and low frequency. • Therefore, to increase the amplitude level of biosignals,amplifiers are designed. • The outputs from these amplifiers are used for further analysis and they appear as ECG, EMG, or any bioelectric waveforms. • Such amplifiers are defined as Bio Amplifiers or Biomedical Amplifier Biomedical Amplifier

42. The biological amplifier should have a high input impedance value. • The range of value lies between 2 MΩ and 10 MΩ depending on the applications. • Higher impedance value reducesdistortion of the signal. • When electrodespick up biopotentials from the human body, the input circuit should be protected. • Every bio-amplifier should consist of isolation and protection circuits, to prevent the patients from electrical shocks. Basic Requirements for Biological Amplifiers

43. Since the output of a bioelectric signal is in millivolts or microvolt range, the voltage gain value of the amplifier should be higher than 100dB. • Throughout the entire bandwidth range, a constant gain should be maintained. • A bio-amplifier should have a small output impedance. Basic Requirements for Biological Amplifiers

44. A good bio-amplifier should be free from drift and noise. • Common Mode Rejection Ratio (CMRR) value of amplifier should be greater than 80dB to reduce the interference from common mode signal. • The gain of the bio-amplifier should be calibrated for each measurement. Basic Requirements for Biological Amplifiers

45. Differential Amplifier • Instrumentation Amplifier • Chopper Amplifier • Isolation Amplifier Types of Bio Amplifiers

46. Must be a low-noise device • Its output is amplified many times, so any noise injected here also gets amplified many times! • Should be dc coupled to the electrodes • Include no series capacitors in the input leads (input bias currents build charge on series input capacitors). • To preserve low frequency content of the input signals. Preamplifier

47. Use relatively low gain for the preamplifier • Input bias currents can build charge on polarizable electrodes, creating a dc offset in the input signals. • Use a high-input impedance OpAmp to reduce these charging effects. • High gain will saturate the output of the preamplifier. • Employ capacitive coupling for later stages of the amplifier circuit to avoid saturation effects. Preamplifier

48. Gain: 800 • DC stage: G=25 (input signals can be 300 mV) • AC coupled band-pass stage: G=32 • With µA 776 OpAmps • CMRR: 86 dB at 100 Hz • Noise: 40 mV p-p • Frequency response • .05 to 150 Hz • Flat over 4 - 40 Hz for bias compen-sation Low pass An ECG PreAmplifier Common-mode adjustment Coupling capacitor (high pass)

49. Differential Amplifiers R2 • Infinite Input impedance thus current passes from R3 to R4 and from R1 to R2 R1 - E1 A Vinput R3 + Voutput E2 R4 Book Assumes: Vinput = E2-E1 And R1 =R3 and R2=R4 R1 R2 A E1 I1 I2 Voutput E2 A R4 R3 I4 I3

50. In differential mode noise common to both input signals is cancelled R2 1V R1 - E1 A 2V R3 + 3V Voutput E2 Advantages of Differential Amplifier R4