Biomedical Electrodes, Sensors, and Transducers from: Chaterjee, Biomedical Instrumentation, chapter 6 © D. J. McMahon 2014 rev 140928

Definition of Biomedical Electrodes, Sensors, and Transducers Solid electric conductor Detects electrical activity Transducer: Converts energy types Sensor: Detects change in physical stimulus Turns it into a signal

FIGURE 6-1 Various sensors

FIGURE 6-3 A block diagram of human-machine interface

Biomedical Electrodes Conductor Makes contact with a nonmetal Electrochemical cell Electrode is anode or cathode FIGURE 6-4 Charge density in the muscle-electrode interface

Theory of Electrodes Electrochemical cell is divided into: Voltaic cell Generates electrical current Chemical reaction: two half reactions Electrolytic cell Uses the electrical current

FIGURE 6-7 An electrolytic cell FIGURE 6-6 A voltaic cell

FIGURE 6-8 A table of half-cell potentials

Silver/Silver Chloride Reference Electrode Stable reference electrode FIGURE 6-12a Silver/silver chloride electrode

FIGURE 6-12c A photo of silver/silver chloride button electrodes FIGURE 6-12b A view of a silver/silver chloride electrode

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Provides a low-offset, low impedance interface with the monitor with minimal effect on the patient’s skin.

Types of Electrodes Include: Polarizable or nonpolarizable Electrode shapes: button or bar types Electrode configurations: monopolar or bipolar Skin-surface electrodes Needle electrodes Cup electrodes

FIGURE 6-13a A surface electrode FIGURE 6-13b A needle electrode. FIGURE 6-13c A cup electrode

Classification of Sensors and Transducers If based upon stimulus: Mechanical Electric Magnetic Optical Thermal Acoustic

Performance Characteristics of Electrodes, Sensors, and Transducers Sensitivity Output change divided by input change Static and dynamic error Difference between measured and actual value Static error: measured value doesn’t change Dynamic error: measured value changes

Performance Characteristics (cont’d.) Accuracy How close output is to true value Hysteresis Unable to repeat data in opposite direction Frequency response Range of frequencies Bandwidth

Performance Characteristics (cont’d.) Reproducibility Ability to produce identical output values At different times under the same conditions Resolution Smallest step of meaningful output Measurable over range

Pressure Transducers Pressure Types of pressure transducers: Force per unit area Types of pressure transducers: Metal strain gauge Linear variable differential transformer (LVDT)

FIGURE 6-15a A simple metal gauge FIGURE 6-15b Backing material of a metal gauge

FIGURE 6-16a A simple LVDT transducer FIGURE 6-15c A balanced bridge circuit with a metal gauge

FIGURE 6-16c LVDT in an amplifier circuit FIGURE 6-16b Output voltage in a LVDT transducer

Flow Transducers Measure flow of gases, liquids, or solids Given as: Q = (area) (velocity) FIGURE 6-17 A simple flow transducer

Blood flow detection by ultrasound & Doppler shift:

Temperature Transducers Types include: Resistance temperature detector (RTD) Metal wire resistance increases with temperature Thermistor Semiconductor device Negative or positive temperature coefficient Thermocouple Converts temperature reading into voltage reading Uses two dissimilar metals: Seebeck effect

FIGURE 6-19a A simple thermocouple transducer FIGURE 6-19b A thermocouple with ice bath

Optical Transducers Don’t require direct physical contact Avoid disturbances Photoconductive cell Common type Uses reflection of light Detects distance or level

Summary Acquiring biomedical signals Electrode Sensor senses stimulus Electrode or transducer is essential Electrode Solid electric conductor Sensor senses stimulus Strain gauge and LVDT Pressure sensors RTD, thermistor, and thermocouple Temperature sensors

Summary (cont’d.) Strain gauge LVDT RTDs or thermistors Thermocouples Determines amount of strain Most common: metal foil gauges LVDT Provides output voltage proportional to displacement RTDs or thermistors Resistance varies with temperature Thermocouples Voltage varies with temperature