TABLE 114.1 Classification of TABLE 114.2 Physical Variables Sensed by Biomedical Sensors According Biomedical Sensors to Their Interface with the Host lacement, velocity, acceleration(linear and angular erature Noninvasive Noncontacting (weight and m nvasive Implanted gy(optical) involve some tissue damage associated with their installation. For example, a needle electrode for picking up electromyographic signals directly from muscles; a blood pressure sensor placed in an artery, vein, or the heart itself; or a blood flow transducer positioned on a major artery are all examples of invasive sensors. We can also classify sensors in terms of the quantities that they measure. Physical sensors are used neasuring physical quantities such as displacement, pressure, and flow, while chemical sensors are used to determine the concentration of chemical substances within the host. A subgroup of the chemical sensors that re concerned with sensing the presence and the concentration of biochemical materials in the host are known as bioanalytical sensors, or sometimes they are referred to as biosensors In the following paragraphs we will look at each type of sensor and present so ome of the important issues surrounding these types of sensors. 114.2 Physical Sensors Physical variables associated with biomedical systems are measured by a group of sensors known as physical sensors. A list of typical variables that are frequently measured by these devices is given in Table 114.2. These quantities are similar to physical quantities measured by sensors for nonbiomedical applications, and the devices used for biomedical and nonbiomedical sensing are, therefore, quite similar. There are, however, two principal exceptions: pressure and flow sensors The measurement of blood pressure and blood flow in humans and other animals remains a difficult problem in biomedical sensing. Direct blood pressure measurement refers to evaluation of the blood pressure using a ensor that is in contact with the blood being measured or contacts it through an intermediate fluid such as physiologic saline solution. Direct blood pressure sensors are invasive. Indirect blood pressure measurement involves a sensor that does not actually contact the blood. The most familiar indirect blood pressure measure- ment is the sphygmomanometer cuff that is usually used in most medical examinations. It is a noninvasive instrument. Until recently, the primary sensor used for direct blood pressure measurement was the unbonded strain gauge pressure transducer shown in Fig. 114.1. The basic principle of this device is that a differential pressure seen across a diaphragm will cause that diaphragm to deflect. This deflection is then measured by a displacement transducer In the unbonded strain gauge sensor a closed chamber is covered by a flexible diaphragm. This diaphragm is attached to a structure that has four fine gauge wires drawn between it and the chamber walls. a dome with the appropriate hardware for coupling to a pressure source covers the diaphragm on the side opposite the chamber such that when the pressure in the dome exceeds the pressure in the chamber, the diaphragm is deflected into the chamber. This causes two of the fine wires to stretch by a small amount while the other two wires contract by the same amount. The electrical resistance of the wires that are stretched increases while that of the wires that contract decreases. By connecting these wires, or more correctly these unbonded strain gauges, into a Wheatstone bridge circuit, a voltage proportional to the deflection of the diaphragm can be obtained In recent years semiconductor technology has been applied to the design of pressure transducers. Silicon strain gauges that are much more sensitive than their wire counterparts are formed on a silicon chip, and micromachining technology is used to form this portion of the chip into a diaphragm with the strain gauges integrated into its surface. This structure is then incorporated into a plastic housing and dome assembly. The entire sensor can be fabricated and sold inexpensively so that disposable, single-use devices can be made. These e 2000 by CRC Press LLC© 2000 by CRC Press LLC involve some tissue damage associated with their installation. For example, a needle electrode for picking up electromyographic signals directly from muscles; a blood pressure sensor placed in an artery, vein, or the heart itself; or a blood flow transducer positioned on a major artery are all examples of invasive sensors. We can also classify sensors in terms of the quantities that they measure. Physical sensors are used in measuring physical quantities such as displacement, pressure, and flow, while chemical sensors are used to determine the concentration of chemical substances within the host. A subgroup of the chemical sensors that are concerned with sensing the presence and the concentration of biochemical materials in the host are known as bioanalytical sensors, or sometimes they are referred to as biosensors. In the following paragraphs we will look at each type of sensor and present some examples as well as describe some of the important issues surrounding these types of sensors. 114.2 Physical Sensors Physical variables associated with biomedical systems are measured by a group of sensors known as physical sensors. A list of typical variables that are frequently measured by these devices is given in Table 114.2. These quantities are similar to physical quantities measured by sensors for nonbiomedical applications, and the devices used for biomedical and nonbiomedical sensing are, therefore, quite similar. There are, however, two principal exceptions: pressure and flow sensors. The measurement of blood pressure and blood flow in humans and other animals remains a difficult problem in biomedical sensing. Direct blood pressure measurement refers to evaluation of the blood pressure using a sensor that is in contact with the blood being measured or contacts it through an intermediate fluid such as a physiologic saline solution. Direct blood pressure sensors are invasive. Indirect blood pressure measurement involves a sensor that does not actually contact the blood. The most familiar indirect blood pressure measure￾ment is the sphygmomanometer cuff that is usually used in most medical examinations. It is a noninvasive instrument. Until recently, the primary sensor used for direct blood pressure measurement was the unbonded strain gauge pressure transducer shown in Fig. 114.1. The basic principle of this device is that a differential pressure seen across a diaphragm will cause that diaphragm to deflect. This deflection is then measured by a displacement transducer. In the unbonded strain gauge sensor a closed chamber is covered by a flexible diaphragm. This diaphragm is attached to a structure that has four fine gauge wires drawn between it and the chamber walls. A dome with the appropriate hardware for coupling to a pressure source covers the diaphragm on the side opposite the chamber such that when the pressure in the dome exceeds the pressure in the chamber, the diaphragm is deflected into the chamber. This causes two of the fine wires to stretch by a small amount while the other two wires contract by the same amount. The electrical resistance of the wires that are stretched increases while that of the wires that contract decreases. By connecting these wires, or more correctly these unbonded strain gauges, into a Wheatstone bridge circuit, a voltage proportional to the deflection of the diaphragm can be obtained. In recent years semiconductor technology has been applied to the design of pressure transducers. Silicon strain gauges that are much more sensitive than their wire counterparts are formed on a silicon chip, and micromachining technology is used to form this portion of the chip into a diaphragm with the strain gauges integrated into its surface. This structure is then incorporated into a plastic housing and dome assembly. The entire sensor can be fabricated and sold inexpensively so that disposable, single-use devices can be made. These TABLE 114.1 Classification of Biomedical Sensors According to Their Interface with the Biologic Host Noninvasive Noncontacting Body surface Invasive Indwelling Implanted TABLE 114.2 Physical Variables Sensed by Biomedical Sensors Displacement, velocity, acceleration (linear and angular) Temperature Force (weight and mass) Pressure Flow Radiant energy (optical)