3.2 Types of Biomedical Sensors and Transducers

Biomedical sensors and transducers are crucial in modern healthcare. They convert physiological signals into electrical outputs, enabling accurate monitoring and diagnosis. From resistive to optical sensors, each type has unique advantages and limitations in measuring vital parameters.

Selecting the right sensor for specific biomedical applications is key. Whether it's ECG electrodes for heart activity or accelerometers for movement disorders, understanding sensor mechanisms helps healthcare professionals choose the best tools for patient care and research.

Types and Operating Principles of Biomedical Sensors and Transducers

Types of biomedical sensors

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• Resistive sensors detect changes in resistance due to applied force (strain gauges) or temperature variations (thermistors)
• Capacitive sensors measure displacement, pressure, or acceleration by detecting changes in capacitance between plates or dielectric properties
• Inductive sensors determine displacement or position through variations in inductance caused by the movement of a magnetic core or conductive target
• Piezoelectric sensors generate electrical charge when subjected to mechanical stress, enabling measurement of pressure, acceleration, or force
• Optical sensors utilize properties such as absorption, reflection, or fluorescence to measure various physiological parameters using light
• Electrochemical sensors rely on the interaction between analytes and electrodes to measure chemical concentrations (glucose, lactate) or pH

Mechanisms of common sensors

• Electrodes convert ionic currents in the body (ECG, EEG, EMG) to electrical currents in the measurement circuit
• Strain gauges consist of a conductive material whose resistance changes when subjected to strain, often used in pressure sensors and force transducers
• Pulse oximetry utilizes the difference in absorption of red and infrared light by oxygenated and deoxygenated hemoglobin to measure blood oxygen saturation
• Photoplethysmography (PPG) detects variations in light absorption due to pulsatile blood flow to measure changes in blood volume

Advantages vs limitations of sensors

• Resistive sensors: simple, low-cost, and easy to integrate but sensitive to temperature variations and may require amplification
• Capacitive sensors: high sensitivity, low power consumption, and non-contact measurement but sensitive to electromagnetic interference and may require complex signal conditioning
• Inductive sensors: robust, long-term stability, and suitable for harsh environments but limited miniaturization and may require a power source for excitation
• Piezoelectric sensors: high sensitivity, wide frequency range, and self-generating signal but sensitive to temperature variations and may require charge amplifiers
• Optical sensors: non-invasive, high spatial resolution, and immune to electromagnetic interference but sensitive to ambient light and motion artifacts
• Electrochemical sensors: high selectivity, fast response time, and suitable for continuous monitoring but limited lifetime, drift, and may require frequent calibration

Selection for biomedical applications

• Cardiovascular monitoring
  1. ECG electrodes for measuring heart electrical activity
  2. Pressure transducers for measuring blood pressure (arterial, venous)
• Respiratory monitoring
  1. Thermistors or thermocouples for measuring respiratory airflow (inhaled, exhaled)
  2. Strain gauges for measuring chest wall movement (expansion, contraction)
• Neurological monitoring
  1. EEG electrodes for measuring brain electrical activity (alpha, beta, theta waves)
  2. Accelerometers for measuring tremor or movement disorders (Parkinson's, essential tremor)
• Biomechanical monitoring
  1. Strain gauges for measuring force or pressure distribution (gait analysis, prosthetics)
  2. Accelerometers and gyroscopes for measuring motion and orientation (balance, posture)
• Biochemical monitoring
  1. Electrochemical sensors for measuring glucose, lactate, or pH (diabetes management, sepsis detection)
  2. Optical sensors for measuring oxygen saturation (pulse oximetry) or hemoglobin concentration (anemia screening)