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## Chemical and biological sensors and actuators

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The tongue and the nose: Two of our most important chemical sensors, the tongue and the nose not only share a close and connected space but also cooperate in determining taste. Both may also be called biosensors. The tongue is a multifunction muscle, perhaps the most flexible in the body. Taste, the chemical analysis of substances that come in contact with the tongue, is processed by taste buds or sensors and can detect five distinct flavors: salty, sour, bitter, sweet, and savory. Although taste buds are found mostly on the tongue, some can be found as well on the soft palate, upper esophagus, and epiglottis (the area in the back of the mouth between the tongue and the larynx). Most taste buds reside in protrusions on the surface of the tongue and open toward its upper surface, an opening through which food elements come in contact with it (gustatory pore). The human tongue may contain upwards of 8,000 taste buds or as few as 2,000, depending on individual variations and on age. Taste is transmitted through nerves to the gustatory section of the brain. The tongue has other functions as well. In humans it is an integral part of processing food and cleaning the mouth and, significantly, of speech. As such it serves as a mechanical organ. In some animals it is part of the heat regulation mechanism (as, e.g., in dogs). In many animals it serves as an indispensable hygienic function in cleaning fur or drinking (e.g., in cats) and the cleaning of soft organs (such as cleaning the eyes in some reptiles or the muzzle in bovines). Specialized functions of the tongue can be found, examples being the prehensile tongue of the chameleon, the split tongue of snakes or the elongated tongue of the giraffe serving as a hook for feeding purposes. The second chemical organ is the nose. It consists of a relatively simple structure with its external, visible protrusion and its two nostrils. Internally it has a number of functions. Immediately behind the nostrils are three bony surfaces called conchae that force and regulate the airflow downward toward the lungs. These also warm the air and, together with a mucous surface and hairs, filter the air of debris and dust. Soft tissue on their sides also controls the amount of air and its speed by constricting or enlarging the opening. Above, in the upper part of the nose cavity and out of the main airstream, a separate cavity contains the olfactory organ, the cells that are responsible for smell. This cavity is open toward the airstream, sampling the air, but because air does not flow through it, the molecules linger in it long enough to accomplish the smelling function. It is for this reason that smells sometimes seem to linger long after their causes have disappeared. The olfactory cells are connected to the olfactory section of the brain. The sense of smell is usually not considered as critical as that of sight or hearing, but it is somehow connected with long-term memory. Long after the sights or sounds of an event have faded, the odors of a place or a situation linger in the brain, still vivid and evoking. The nose also has certain adaptations. In most mammals the nose has a secondary olfactory bulb called the vomeronasal organs that sense certain chemical messages associated with social and sexual conditions. These organs bypass the cerebral cortex and link to sections in the brain responsible for reproduction and maternity and also affect aggressiveness in males. Another adaptation in some reptiles (snakes, lizards) is the combination of a forked tongue that samples the air and deposits molecules into an organ (called the Jacobson organ) on the roof of the mouth to chemically sense the environment.

Chapter Contents:

• 8.1 Introduction—chemistry and biochemistry
• 8.2 Chemical units
• 8.3 Electrochemical sensors
• 8.3.1 Metal oxide sensors
• 8.3.2 Solid electrolyte sensors
• 8.3.3 The metal oxide semiconductor chemical sensor
• 8.4 Potentiometric sensors
• 8.4.1 Glass membrane sensors
• 8.4.2 Soluble inorganic salt membrane sensors
• 8.4.3 Polymer-immobilized ionophore membranes
• 8.4.4 Gel-immobilized enzyme membranes
• 8.4.5 The ion-sensitive field-effect transistor
• 8.5 Thermochemical sensors
• 8.5.1 Thermistor-based chemical sensors
• 8.5.2 Catalytic sensors
• 8.5.3 Thermal conductivity sensors
• 8.6 Optical chemical sensors
• 8.7 Mass sensors
• 8.7.1 Mass humidity and gas sensors
• 8.7.2 SAW mass sensors
• 8.8 Humidity and moisture sensors
• 8.8.1 Capacitive moisture sensors
• 8.8.2 Resistive humidity sensor
• 8.8.3 Thermal conduction moisture sensors
• 8.8.4 Optical humidity sensor
• 8.9 Chemical actuation
• 8.9.1 The catalytic converter
• 8.9.2 The airbag
• 8.9.3 Electroplating
• 8.9.4 Cathodic protection
• 8.10 Problems

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