This chapter addresses biomedical sensors, which are important factors for the quality of measurement of biological and physiological variables, and thus deserve close attention both with respect to the medical functions and the technical requirements. There are special demands for biomedical sensors, particularly for invasive sensors that must pass sterilization by temperature or radiation at much higher levels than the rest of the medical equipment. On the other hand, miniaturization, mechanical strength, electrical safety, and sterility are important elements of the sensor design process. The sensor determines the accessibility, spatial resolution, accuracy, sensitivity, selectivity, and response time of the measurement. In order to convert the parameter of interest into an electrical signal suited for medical use, a transducing element is necessary. Knowledge of the complete chain of transducer materials, their electrical properties, and integration with bio-amplifiers are therefore necessary to design high-quality biomedical sensors.
Top9.1. Chapter Objectives
This chapter enables the reader to:
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Describe the most common biomedical sensors and their clinical applications.
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Classify the different generic sensor types and explain advantages and disadvantages of each different type.
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Identify suitable amplifier circuits for each sensor type.
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Analyze biomedical sensor modes of operation and limitations.
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Identify and analyze sources of error for each clinical sensor application.
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Describe different sterilization methods and their suitability for different sensor types.
Top9.3. A Comprehensive Definition Of Biomedical Sensors
The sensor concept is widely used in literature and everyday speech and, therefore, often not very well defined. In this chapter, we define the most important terms as unambiguous as possible but still within the generally accepted understanding of the concepts. In a generalized model, a physical variable of interest called the measurand is to be measured and recorded. An example of a well-known biomedical measurand is body temperature, which is directly recordable by a thermocouple (section 9.10). Very few physical processes are directly recordable, and a mediator is necessary to convert the measurand into a readable signal suitable for processing, presentation and recording.
Sensor is a mediator able to convert one or more measurands or physical variables into an equivalent signal variable of another type of quantity within a frame of a given unity (Pallàs-Areny & Webster, 2001, pp. 3-4). A sensor measures a selected parameter and comprises a transducing element converting the selected parameter into a signal; e.g., electric, light or mechanical pointer position.
Biosensor is a sensor applied on a biological material. Example: blood pressure sensor.
Transducer is in most cases a synonym to sensor. However, while a sensor comprises a sensing element and necessary signal processing elements in order to facilitate further post-processing of the measured quantity, a transducer is limited to the elements needed for the conversion process in the transducing element.
Transducing element is based upon a material like a piezoelectric crystal, piezoresistive semiconductor, steel wire, thermoelement, temperature sensitive resistor, pH-sensitive glass membrane, semiconductor radiation detector et cetera.
Biological sensor uses a biological transducing element. The biomaterial may be a part of living tissue; e.g., the electric field cell morphology biosensor (Giaever & Keese, 1993). It may also be dead tissue; e.g., as in a hair hygrometer.