114 Biomedical sensors 114.1 Introduction 114.2 Physical Sensors 114.3 Chemical Sensors 114.4 Bioanalytical Sensors Michael R Neuman 114.5 Applicat 114.6 Summary 114.1 Introduction Any instrumentation system can be described as having three fundamental components: a sensor, a signal processor, and a display and/or storage device. Although all these components of the instrumentation system are important, the sensor serves a special function in that it interfaces the instrument with the system being measured In the case of biomedical instrumentation a biomedical sensor(which in some cases may be referred to as a biosensor) is the interface between the electronic instrument and the biologic system. There are some general concerns that are very important for any sensor in an instrumentation system regarding its ability to effectively carry out the interface function. These concerns are especially important for biomedical sensors, since the sensor can affect the system being measured and the system can affect the sensor performance. Sensors must be designed so that they minimize their interaction with the biologic host. It is important that the presen of the sensor does not affect the variable being measured in the vicinity of the sensor as a result of the interaction between the sensor and the biologic system. If the sensor is placed in a living organism, that organism will probably recognize the sensor as a foreign body and react to it. This may in fact change the quantity being sensed in the vicinity of the sensor so that the measurement reflects the foreign body reaction rather than a central characteristic of the host Similarly, the biological system can affect the performance of the sensor. The foreign body reaction might cause the host to attempt to break down the materials of the sensor as a way to remove it. This may, in fact, degrade the sensor package so that the sensor can no longer perform in an adequate manner. Even if the foreign body reaction is not strong enough to affect the measurement, just the fact that the sensor is placed in a warm, aqueous environment may cause water to eventually invade the package and degrade the function of the sensor. Finally, as will be described below, sensors that are implanted in the body are not accessible for calibration Thus, such sensors must be extremely stable so that frequent calibrations are not necessary Biomedical sensors can be classified according to how they are used with respect to the biologic system. Table 114.1 shows that sensors can range from noninvasive to invasive as far as the biologic host is concerned. The most noninvasive of biomedical sensors do not even contact the biological system being measured. Sensors of radiant heat or sound energy coming from an organism are examples of noncontacting sensors. Noninvasive sensors can also be placed on the body surface Skin surface thermometers, biopotential electrodes, and strain gauges placed on the skin are examples of sive sensors. Indwelling sensors are those which can be placed into a natural body cavity that communicates with the outside. These are sometimes referred to as minimally invasive sensors and include such familiar sensors as oral-rectal thermometers, intrauterine pressure transduc- ers,and stomach pH sensors. The most invasive sensors are those that need to be surgically placed and that c 2000 by CRC Press LLC© 2000 by CRC Press LLC 114 Biomedical Sensors 114.1 Introduction 114.2 Physical Sensors 114.3 Chemical Sensors 114.4 Bioanalytical Sensors 114.5 Applications 114.6 Summary 114.1 Introduction Any instrumentation system can be described as having three fundamental components: a sensor, a signal processor, and a display and/or storage device. Although all these components of the instrumentation system are important, the sensor serves a special function in that it interfaces the instrument with the system being measured. In the case of biomedical instrumentation a biomedical sensor(which in some cases may be referred to as a biosensor) is the interface between the electronic instrument and the biologic system. There are some general concerns that are very important for any sensor in an instrumentation system regarding its ability to effectively carry out the interface function. These concerns are especially important for biomedical sensors, since the sensor can affect the system being measured and the system can affect the sensor performance. Sensors must be designed so that they minimize their interaction with the biologic host. It is important that the presence of the sensor does not affect the variable being measured in the vicinity of the sensor as a result of the interaction between the sensor and the biologic system. If the sensor is placed in a living organism, that organism will probably recognize the sensor as a foreign body and react to it. This may in fact change the quantity being sensed in the vicinity of the sensor so that the measurement reflects the foreign body reaction rather than a central characteristic of the host. Similarly, the biological system can affect the performance of the sensor. The foreign body reaction might cause the host to attempt to break down the materials of the sensor as a way to remove it. This may, in fact, degrade the sensor package so that the sensor can no longer perform in an adequate manner. Even if the foreign body reaction is not strong enough to affect the measurement, just the fact that the sensor is placed in a warm, aqueous environment may cause water to eventually invade the package and degrade the function of the sensor. Finally, as will be described below, sensors that are implanted in the body are not accessible for calibration. Thus, such sensors must be extremely stable so that frequent calibrations are not necessary. Biomedical sensors can be classified according to how they are used with respect to the biologic system. Table 114.1 shows that sensors can range from noninvasive to invasive as far as the biologic host is concerned. The most noninvasive of biomedical sensors do not even contact the biological system being measured. Sensors of radiant heat or sound energy coming from an organism are examples of noncontacting sensors. Noninvasive sensors can also be placed on the body surface. Skin surface thermometers, biopotential electrodes, and strain gauges placed on the skin are examples of noninvasive sensors. Indwelling sensors are those which can be placed into a natural body cavity that communicates with the outside. These are sometimes referred to as minimally invasive sensors and include such familiar sensors as oral-rectal thermometers, intrauterine pressure transduc￾ers, and stomach pH sensors. The most invasive sensors are those that need to be surgically placed and that Michael R. Neuman Case Western Reserve University