Application of Transducers in Biomedical Instrumentation

Application of Piezoelectric Transducer

• • In biomedical instrumentation, piezoelectric transducers convert pressure and vibration into electrical signals. A crystal microphone measures and records heart sounds (phonocardiography). That microphone is often specified for a flat band of 20 to 1000 Hz so breathing and room noise sit outside the passband more than the heart sounds. Sounds from heart chambers are measured with catheter-tip piezoelectric sensors.
  • A piezoelectric crystal can detect Korotkoff sounds used with a cuff to read systolic and diastolic blood pressure, as illustrated in the figure. Auscultation with a stethoscope is still the reference method.

• A piezoelectric sensor senses Radial pulse
• In ultrasonic scanning devices, piezoelectric transducers are used. When an ultrasonic pulse travels through the body, tissue interfaces return an echo. Some teaching setups display the received echo on a Cathode Ray Oscilloscope; clinical scanners use a digital monitor.
• In hospitals, continuous medication delivery requires monitoring the dosage. Some drip monitors do this by drop-counting with 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 with temperature. That change is the basis for thermoresistive temperature sensors. The temperature coefficient relates temperature and resistance. In the positive type, resistance rises with temperature. In the negative type, resistance falls as temperature rises. In metals the coefficient is positive. In many semiconductor thermistors the coefficient is negative.

• A thermoresistive transducer measures skin and body temperature.
• Blood flow measurement in the human body can employ a thermoresistive transducer. Enclosed at the catheter’s tip, a heated thermistor loses heat to the blood as it passes through blood vessels. The cooling, which increases with flow, alters the thermistor’s resistance and is used as a flow indication.
• A thermistor can measure respiratory rate. Place a glass bead thermistor directly in the path of nasal airflow. Pass a small current through the thermistor. For each expiration, the cooling from nasal airflow causes the resistance of the thermistor to increase. A Cathode Ray Oscilloscope can record that resistance change.

Application of Photoelectric Transducers

Photoemissive Tube

A photoemissive tube consists of a gas-filled tube with two electrodes, one cathode and another anode. The cathode has a specially coated material around it. When light falls on the cathode, it releases electrons. The released electrons produce a current, which is proportional to the light intensity. Antimony, silver and bismuth are typical coating materials.

Photovoltaic Cell

A historic 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 of 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 that potential difference appears, the metal film turns positive and the selenium layer turns negative. This is an example of an active transducer. Clinical pulse sensors more often use a silicon photodiode.

The same selenium photovoltaic cell uses a transparent metal film over the selenium to form the insulated barrier layer. Intense light releases electrons and generates a potential difference that polarizes the metal film positively and the selenium layer negatively, which is an active transducer.

Applications

• • We can measure pulsatile blood volume change with a photodetector. To detect the pulse, we can use Transmittance or Reflectance techniques as shown in the figure below. In the transmittance technique, pulsating blood flow modifies the optical density. In the reflectance technique, blood flow changes the intensity of reflected light. The waveform follows those optical changes beat by beat.

• • 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.

• • Blood pressure can be measured with a photodetector as shown in the figure below, as one laboratory arrangement rather than the usual clinical cuff. At the free end of a Bourdon tube between lamp and photodiode, a shade is attached. The Bourdon tube is filled using a saline solution. Blood pressure raises the pressure inside the tube and displaces the shade. That displacement sets the output from the phototube.

• Oximetry, the determination of blood oxygen saturation, can use photoelectric transducers, including during open-heart surgery. Finger probes are now more common than earlobe pickups. Two detectors capture reflected or transmitted light: one in the red spectrum (640mµ) and another in the IR spectrum (800mµ). The red output varies with blood oxygen. The IR output is used as a reference. The differential between these outputs estimates oxygen saturation.