Waiting for a new form of telecommunication.
RECALL THE BOOM IN POPULARITY that classical music underwent in the late nineteenth century, when anyone who had access to a telephone could appreciate the virtuosity of the great performers. At the time the transmission of visual images seemed a utopian dream, but only a few years after Alexander Graham Bell patented his famous device Paul Nipkow was awarded a patent in Germany for an apparatus that would transmit such images. This time the beneficiaries were the followers of Polymnia, Terpsichore, Erato, Melpomene, and Thalia.
What realms of communication are left to conquer? The transmission of tactile stimuli is now being mastered, thanks to the design of special gloves fitted with piezoelectric crystals that register or exert pressure. But smells? Flavors? The delay in developing teleolfaction and telegustation is a source of frustration for gourmets.
Olfactory Stimulation
Olfactory and gustatory sensations arise from the binding of odorant and sapid molecules with receptor cells in the nose and mouth. Two possible methods for transmitting such sensations at a distance may be entertained. One could imagine encoding the electrical activity produced in the brain by smell and taste in a series of signals that would stimulate the brain of the receiver by means of electrodes. This is what André Holley and Anne-Marie Mouly at the University of Lyon have been trying to do, hoping to be able to condition rats through excitation of the olfactory bulb.
Alternatively, it might be possible to analyze mixtures of odorant and taste molecules in the same fashion as colors, associating them with arbitrarily selected stimuli. Then, once the electronically encoded information has been transmitted and received, the basic molecules of these stimuli would be combined to reproduce the initial sensations.
Sensors called artificial noses might be useful components for the realization—still a remote prospect—of such systems. Originally developed by the food industry, such sensors are already used in certain processing plants to provide an objective evaluation of the volatile compounds emitted by food. Several types exist.
Artificial Noses
At the Institut National de la Recherche Agronomique (INRA) station in Theix, Jean-Louis Berdagué and his colleagues developed a mass spectrometry system capable of analyzing all volatile compounds in a given sample. Samples are first thermically decomposed in a heated cup. After fragmentation and ionization, the smoke molecules pass into a mass spectrometer, where magnetic and electric fields bend the trajectories of the various molecules in proportion to their mass and electrical charge. Finally, a sensor identifies the quantity of each kind of fragment. In this way the Theix researchers were able to obtain a unique electronic signature for any given mixture. The results are truly wonderful: Such a signature makes it possible to pinpoint the origin of a particular oyster along the coastline of France. What gastronome could match this feat?
Another method involves the use of doped semiconductors on which volatile compounds are reversibly adsorbed, diminishing the electrical resistance of the semiconductor. At the INRA laboratory in Dijon, Patrick Mielle and his colleagues are studying networks of sensors composed of a ceramic substrate, heated by a resistant element, that is coated with a semiconductor such as tin oxide and doped with zinc, iron, nickel, or cobalt oxides. These sensors react differently to the various adsorbed molecules. Whatever the exact mixture of volatile compounds, contact with the sensor network produces a distinctive electrical signature that is then processed by a computer.
The practical application of these networks remains problematic. First, because the signals of the various sensors slowly drift over time, the Dijon researchers devised a rapid transfer system in which compounds are measured and characterized by the sensors and their signatures fed into the network with only a slight delay. Second, the sensors were observed to react quite differently, depending on the temperature. This contingency, which must be controlled for during the measuring process, turns out to be an advantage because measurements made at several temperatures make it possible to obtain different reactions from a single sensor, in effect multiplying the number of sensors in the network.
Finally, because recording the sensors’ signals and recognizing the signatures of the various compounds make heavy demands on the system’s limited processing power, in practice the number of sensors must be limited to fewer than a dozen. Mielle and his colleagues therefore proposed recording the signals at various instants after the injection of the samples into the measuring cell, proportionally increasing the number of data points generated.
The measuring cells being tested today capture 80% of the gaseous phase molecules, and networks consisting of six sensors that record measurements at four temperatures are able to characterize mixtures of volatile compounds in about ten seconds. The capacities of the human nose are no longer quite as unrivaled as they once were.