Blood sensor

Table 15.1 Performance goals for intravascular blood sensors. (lABG values from the... 

For measurements of partial pressures in tissues and in liquids such as serum or blood, sensors according to Severinghaus are used. A membrane separates the external liquid or gas phase from the electrolyte surrounding a glass electrode, the potential of which is measured against an Ag/AgCl electrode (see Sections 23.3 and 23.4). 

One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors to be developed was for dissolved O2 in blood, which was developed in 1956 by L. C. Clark. The design of the amperometric sensor is shown in Figure 11.38 and is similar to potentiometric membrane electrodes. A gas-permeable membrane is stretched across the end of the sensor and is separated from the working and counter electrodes by a thin solution of KCl. The working electrode is a Pt disk cathode, and an Ag ring anode is the... 

Amperometry is a voltammetric method in which a constant potential is applied to the electrode and the resulting current is measured. Amperometry is most often used in the construction of chemical sensors that, as with potentiometric sensors, are used for the quantitative analysis of single analytes. One important example, for instance, is the Clark O2 electrode, which responds to the concentration of dissolved O2 in solutions such as blood and water. 

A small (25-kg), portable apheresis system, available in 1993, is designed to meet a wide variety of blood cell separation needs. The role of the apheresis system is to control the behavior, separation, and collection of blood components from the bowl while maintaining maximum donor safety. The system controls the flow rates of blood and components through variable pump speeds. It directs the flow of components out of the bowl, by fully automatic opening and closing of valves based on the output of the system sensors. The system monitors the separation of blood components in the bowl by an optics system that aims at the shoulder of the bowl. A sensor on the effluent line monitors the flow of components out of the bowl. 

The special design of the Latham bowl allows for a specific blood cell separation known as SURGE. This technique makes use of the principle of critical velocity. The Latham bowl is filled until the huffy coat, ie, layer of platelets and white cells, moves in front of the bowl optics. At this point the machine starts to recirculate plasma through the bowl at increasing rates. The smallest particles, ie, platelets, ate the first to leave the bowl. Their high number causes the effluent line to turn foggy. The optical density of the fluid in the effluent line is monitored by the line sensor. A special algorithm then determines when to open and close the appropriate valves, as well as the optimum recirculation rate. 

Fouling of the pH sensor may occur in solutions containing surface-active constituents that coat the electrode surface and may result in sluggish response and drift of the pH reading. Prolonged measurements in blood, sludges, and various industrial process materials and wastes can cause such drift. Therefore, it is necessary to clean the membrane mechanically or chemically at intervals that are consistent with the magnitude of the effect and the precision of the results requited. 

Electrodes may also be rendered selective to more complex analytes using enzyme or other overcoats (see Biopolymers, analytical techniques Biosensors). The enzyme converts the analyte into a detectable ion or gas. Glucose and blood urea nitrogen sensors can be made in this way. 

Body temperatures are primarily sensed by temperature sensors in the hypothalamus near the center of the brain. Arterial blood flowing over and near the hypothalamus gives it information about the average thermal condition of... 

Miniaturized catheter-type ISE sensors, such as the implantable probe shown in Figure 5-20 represent the preferred approach for routine clinical in-vivo monitoring of blood electrolytes. For these intravascular measurements the reference electrode is placed outside die artery (in die external arm of die catheter), tints obviating biocompatability and drift problems associated with its direct contact with the blood. 

FIGURE 5-20 Miniaturized ISE catheter sensor for continuous monitoring of blood electrolytes. (Reproduced with permission from reference 58.)... 

A Nemstian response of 59 mV per decade change in concentration is commonly observed (at 25°C). Relation to the partial pressure carbon dioxide is accomplished by the use of Henry s law. A catheter sensor configuration has been developed for in-vivo monitoring of blood carbon dioxide (61). 

FIGURE 6-21 A silicon-based sensor array for monitoring various blood electrolytes, gases, and metabolites. (Courtesy of i-STAT Co.)... 

Enzyme sensors are based primarily on the immobilization of an enzyme onto an electrode, either a metallic electrode used in amperometry (e.g., detection of the enzyme-catalyzed oxidation of glucose) or an ISE employed in potentiometry (e.g., detection of the enzyme-catalyzed liberation of hydronium or ammonium ions). The first potentiometric enzyme electrode, which appeared in 1969 due to Guilbault and Montalvo [140], was a probe for urea with immobilized urease on a glass electrode. Hill and co-workers [141] described in 1986 the second-generation biosensor using ferrocene as a mediator. This device was later marketed as the glucose pen . The development of enzyme-based sensors for the detection of glucose in blood represents a major area of biosensor research. 

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