- Biomedical Transducers Defined: Biomedical transducers are devices that convert physical quantities into measurable electrical signals used in medical diagnostics and monitoring.
- Piezoelectric Transducers: Employed in devices like phonocardiography to detect and record heart sounds by converting pressure into electrical signals.
- Thermoresistive Applications: These transducers measure temperature changes by observing resistance variations, useful in monitoring body and skin temperatures.
- Photoelectric Sensing: Utilized to determine blood oxygen saturation and other physiological parameters by detecting light intensity changes.
- Clinical Applications: Transducers are essential in various medical procedures, from monitoring blood pressure to assessing respiratory functions.
Application of Piezoelectric Transducer
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- Crystal microphone measures and records the Heart sounds (Phonocardiography). The microphone operates in the flat frequency range of 20 to 1000 Hz, which suppresses the breathing sounds and room noise. Sounds from heart chambers are measured with Catheter-tip piezoelectric sensors.
- A piezoelectric crystal detects Korotkoff sounds, crucial for measuring systolic and diastolic blood pressure, as illustrated in the figure.
- A piezoelectric sensor senses Radial pulse
- In ultrasonic scanning devices, piezoelectric transducers are used. When an ultrasonic pulse transmits through the human body, it reflects an echo signal of different frequency. The received is displayed on Cathode Ray Oscilloscope.
- In hospitals, continuous medication delivery requires monitoring the dosage. This is achieved through drop-counting using a piezoelectric crystal, where a mesh attached to the transducer generates a pulse with each medicine drop.
Application of Thermoresistive Transducer
In metals and semiconductors, the value of resistance changes constantly. This change in resistance is the basis for all Thermoresistive temperature sensors. Temperature coefficient relates temperature and resistance. In positive type, resistance raises with temperature and in negative type resistance decreases with increase in temperature. In metals, temperature coefficient is positive and in semiconductors, temperature coefficient is negative.
- A thermoresistive transducer measures skin and body temperature.
- Blood flow measurement in the human body employs a thermoresistive transducer. Enclosed at the catheter’s tip, a heated thermistor loses heat to the blood as it passes through blood vessels. The resultant cooling effect, which is proportional to blood flow, alters the thermistor’s resistance, thereby indicating blood flow rates.
- A thermistor measures Respiratory rate. Place a glass bead thermistor directly in the path of nasal airflow. Pass an amount of current through the thermistor. For each cycle of expiration, the cooling effect of nasal airflow causes the resistance of thermistor to increase. Cathode Ray Oscilloscope records the resistance increase.
Application of Photoelectric Transducers
Photoemissive Tube
A photoemissive tube consists of a gas-filled tube with two electrodes, one cathode, and another anode. Cathode acts as one electrode, and it has a specially coated material around it. When light falls on the cathode, it releases electrons. The release in electron produces a current, which is proportional to the light intensity. Usually, antimony, silver, bismuth is employed as coating materials.
Photovoltaic Cell
A photovoltaic cell has an outer layer of selenium, coated with a transparent metal film. The metal film and selenium layer are insulated from each other, and this forms the barrier layer. Light with high intensity illuminates the barrier layer. As the light falls on it, it releases electrons, which causes a potential difference in the barrier layer. After the occurrence of potential difference, the metal film turns into positive, and selenium layer turns into negative. This is an example for the active transducer.
A photovoltaic cell consists of a selenium layer covered by a transparent metal film, creating an insulated barrier layer. When illuminated by intense light, this layer releases electrons, generating a potential difference that polarizes the metal film positively and the selenium layer negatively, exemplifying an active transducer.
Applications
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- We can measure pulsatile blood volume change with a Photodetector. To detect the pulse, we can either use Transmittance or Reflectance techniques as shown in the figure below. In transmittance technique, pulsating blood flow modifies the optical density. In reflectance technique, blood flow changes the intensity of reflected light. The changes in blood flow are seen immediately with these methods.
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- We can measure changes around the circumference of the chest with a pneumograph that has a photodiode as seen in the figure below. Wrap the chest with a rubber bellow. Inside the bellows, the movable metal bar is attached. When the chest expands during breathing, the amount of light that falls on the photodiode varies due to the metal bar. Calibrate the obtained result to get the respiratory volume.
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- Blood pressure can be measured with Photodetectors as shown in the figure below. At the free end of a bourdon tube between lamp and photodiode, a shade is attached. Bourdon tube is filled using a saline solution. Pressure is created inside the tube due to blood pressure. As the blood pressure increases, the pressure inside the tube displaces the shade. This displacement is proportional to the output from the phototube.
- Oximetry, the determination of blood oxygen saturation, utilizes photoelectric transducers, especially during open-heart surgery. Illuminating earlobes rich in vascular beds, two photovoltaic detectors capture the reflected light: one in the red spectrum (640mµ) and another in the IR spectrum (800mµ). The red output correlates with blood oxygen levels, while the IR output does not. The differential between these outputs quantifies oxygen saturation.
