The dispersal of scent takes place via the thermal and light-like behavior of the fragrant particles in the air. Thousands of points, letters, and words are positioned in each and every air particle. Each molecule is created in a form to carry sounds, sights, and odor. Millions of sound waves, scents and images are being transported and translocated into each of the trillions of air particles without deforming or mixing. As our knowledge pertaining to this transport grows, new technological products that will enable the transport of odors will be offered to the service of mankind.

Our sense of smell occurs in the brain. The chemical molecules exiting a lemon peel stimulate the odor receptors in the nose, which are then transmitted to brain to be interpreted as electric signals. Our olfactory system can easily distinguish more than ten thousand scents. This has inspired scientists to design similar devices. These models are called “electronic noses.”

A series of chemical receptors are utilized in the electronic nose instead of the receptor proteins of the human nose. Each of these is designed to sense various scents. These devices are difficult to produce, as the cost grows for a more sensitive device. The signals that sensors collect from the environment are converted into binary codes via electronic systems and then sent to a computer. The role of human nerve cells in charge of sensing odor is replaced by the electronic systems of a computer.

Mostly in their early phases, electronic noses are beginning to be used in various sectors, primarily those involving foods and perfumes, as well as the medical and chemical industries.

How does odor transportation take place?

As I already mentioned, the aromatic molecules transported via air particles in their gaseous state are detected by the smell sensor system and converted into electric signals. Quite a few different materials are used as conductors: conductive polymers, semi-conductive metal oxides, a quartz-crystal micro-balance (QMB), surface acoustic wave (SAW) sensors, pellistors, and infrared sensors.

Once the electric signals are converted into binary, the odor information is determined via a software program in which algorithms such as artificial nerve networks and support vector machines are employed. This information is then transmitted to a remote medium via lines of communication, such as a computer network, the internet, or another form of mobile communication. The odor type is received in the target computer. This detection stage can be completed in the target PC when necessary.

Today’s technology can only permit the transmission of odor data. In order to perceive the transmitted information at the target location as smell, the scents must be present as stored in containers and must be triggered via received odor data to be dispersed. The research in this field is limited, with ongoing pilot studies.

How can diseases be diagnosed with odors?

The natural functions of the human body, such as sweat, blood, urine, and feces, can be used to help diagnose diseases. The odor of the gases in human breath holds significant information regarding body health. There are between two hundred and four hundred different gases found in human breath. Furthermore, the number of gas types detected and described in the breath can exceed three thousand. While blood gets cleaned in the lungs, the gases of the used blood pass to the breath via the alveoli. Therefore, many critical pieces of bodily information are present in the breath.

The gases exhaled through our breath are composed of various alkaline and aromatic compounds. Each of these is a potential indicator that provides information about a disease. The gases and their ratio in the breath of a healthy person are well established. Since the ratio of the gases in the breath gets altered depending on the cause of an illness – such as diabetes (Type I and II), cancer of the ear-nose-throat, tuberculosis, and women’s reproductive diseases – can be diagnosed by utilizing the electronic nose.

There are other uses for the technology, too. NASA is developing a highly sensitive artificial nose for space research. This device will almost be able to distinguish every type of chemical compound, making more sensitive measurements than a human nose. With this device, the detection of harmful substances in the space station will be possible.

Google has announced that significant progress has been made regarding the “Google nose” which helped revolutionize searching for aromas. The Google Aroma database (http://www.google.com.tr/intl/tr/landing/nose/) stores more than 15 million kinds of scent. The days when we will be able to smell the scent of any product through our internet based devices do not seem to be too distant. To make this possible, sound waves would be converted into odor signals. There is a partially-imaginary video prepared to show how this can feel.

New technologies will change our relationship with smell, which has always been deeply important to humanity. Reference is made of this in the Qur’an, especially when the Prophet Jacob of Canaan sensed the fragrance of his son, Joseph, who was hundreds of miles away. The verse, from the chapter of Joseph, reads, “Surely, I sense the fragrance of Joseph, unless you would consider me a dotard. “It shows how valuable scent is to us, as anyone who has had a long lost memory triggered by an unexpected smell understands. As the verse suggests, losing our sense of smell is akin to losing our minds. Research has borne this out, as one of the first symptoms of Alzheimer’s disease is the loss of smell. In fact, monitoring loss of scent has helped with the early detection and prevention of Alzheimer’s. This is yet another way that our body has been perfectly calibrated to cue us in to its messages. In this regard, as with many, technology is still trying to catch up to nature.

Less well known is that research is also continuing into the scent of repellents to deter pests like insects and ticks. Various plants are known for the effects of their aroma, and are in widespread use for protection, especially in very hot and humid climates where there are a great number of arthropods; for instance, it is known that mosquitoes will not approach a house that has basil plants on the windowsill.

Another plant which is well known and used commonly for its scent is “patchouli” (pogostemon cablin), the scented essential oil of which is obtained by steaming the plant and collecting the oil which emerges. The plant is a member of the mint family, and its actual origin is India, where its scent can be found even in the famous Indian ink. Patchouli leaves used to be placed between carpets and rugs made in Iran and Turkey to protect them from any harmful pests or insects before they were sent to Europe. During the Victorian period carpets, shawls and rugs exported from India were also sprinkled with the fragrance of patchouli to protect them from moth, In fact any carpets, rugs or shawls that did not have the scent of patchouli were not favored because they were believed to have been manufactured in Europe. The fragrance of the plant, which is longer lasting than most other scents, is believed by the Chinese, Japanese and Arabs to prevent the spread of infectious diseases and also used frequently in perfumes and soaps. Widely used in Europe in the eighteen hundreds, patchouli became the most popular fragrance of the generation in America in the sixties.

The research into patchouli has been limited so far and scientific observations are insufficient; nevertheless, the chemical composition of the various scents found in the oil of this plant have been identified, and the long human experiences of using this plant and its fragrance may hold great significance for scientific research in the future.

The plant contains patchouli alcohol, pogostone, friedelin, epifriedelinol, pachypodol, retusin, oleanolic asid, beta-sitosterol and daucosterol, most of which prevent nausea. It has also been found to contain alpha-bulnesene, which prevents the clotting of blood. In addition, according to an article in the 2008 February edition of the Phytotherapy research journal, since the oil obtained from patchouli essence is an effective fly killer, the oil could also be an effective as a component of insect repellents and might even be an effective arthropod and tick repellent.

Looking at these studies we clearly see other ways in which we could benefit from this substance; for instance, by adding a few drops to water we could use it in household cleaning and therefore get rid of unwanted odors at the same time as preventing insects in the home without the use of carcinogenic chemicals. The oil is also known to be used to prevent fungus, to reduce perspiration and eliminate unwanted body odors and for dietary purposes, due to its effectiveness in reducing the appetite.

Moreover, patchouli has also long been a fragrance very much sought and used as a form of treatment for its soothing qualities and positive effect on spiritual health. Martin Henglein, who was one of the founders of aromatherapy and developer of the theory of the curative aspect of the fragrances of plants, recognizes geranium, rosemary, bergamot, and patchouli as the four primary aromas, and he emphasizes that these four fragrances perform various functions. Geranium can prevent addictions from progressing and even assist people in abandoning addictions and bad habits. For instance, the role geranium plays in giving up smoking is indisputable: when the desire to smoke increases and becomes unbearable the aroma of geranium temporarily eliminates the desire to smoke. According to Henglein, rosemary improves memory while bergamot increases activity in the brain and the ability to understand; patchouli activates the mechanism which motivates a person’s energy.

Robert Tisserand, owner of a treatment center in England, believes that certain fragrances can also cure psychological illnesses, Tisserand says that these aromas have a positive effect on the signal molecules (neurotransmitters) that provide communication with the nerve cells and can help to cure psychological problems. Fragrances encourage the body to release endorphin, a substance which resembles morphine (a pain reliever) and this is why rose oil, jasmine, sage, cananga (ylang ylang), patchouli, and grapefruit are recommended for depression, to increase confidence, and to help with abnormalities of sexual function. If patchouli is used in excess, it allegedly may cause a sedative effect or may reduce sleep. Otherwise, it is claimed to have a balancing effect on the body’s energy and psychological condition, inspire a sense of calmness, eliminate laziness, support treatment of addictions, and relieve feelings of fear and depression.

Everyone knows that charming fragrances enhance positive thought and feelings, and we also know the negative aspects of bad odors. It is reported that the Prophet Muhammad, peace be upon him, said, “I have been made to love perfume,” drawing attention to the importance of pleasant fragrances and reminding us that Jacob received the glad tidings that his son Joseph was still alive because of the scent of his shirt.

References

• Guan L, Quan LH, Xu LZ, Cong PZ. (1994): Chemical constituents of Pogostemon cablin (Blanco) Benth. Zhongguo Zhong Yao Za Zhi. 1994 Jun;19(6):355–6, 383.
• Yang Y, Kinoshita K, Koyama K, Takahashi K, Tai T, Nunoura Y, Watanabe K.(1999): Anti-emetic principles of Pogostemon cablin (Blanco) Benth. Phytomedicine. 1999 May, 6 (2): 89–93.
• Luo J, Guo X, Feng Y. (2002): Constituents analysis on volatile oil of Pogostemon cablin from different collection time cultivated in Hainan. Zhong Yao Cai. 2002 Jan. 25 (1): 21–3
• Hsu HC, Yang WC, Tsai WJ, Chen CC, Huang HY, Tsai YC. (2006): Alpha-bulnesene, a novel PAF receptor antagonist isolated from Pogostemon cablin. Biochem Biophem Biophys Res Commun. 2006 Jul 7;345(3):1033–8.
• Tsai YC, Hsu HC, Yang WC, Tsai WJ, Chen CC, Watanabe T. (2007): Alpha-bulnesene, a PAF inhibitor isolated from the essential oil of Pogostemon cablin. Fitoterapia. 2007 Jan. 78 (1):7–11.
• Pavela R. (2008): Insecticidal properties of several essential oils on the house fly (Musca domestica L.). Phytother Res. 2008 Feb;22(2):274–8.
• Jantan, I. and Zaridah M. Z.(1999): Development of Environment-Friendly Insect Repellents From The Leaf Oils of Selected Malaysian Plants. ASEAN Review of Biodiversity and Environmental Conservation (ARBEC) November–December 1999, p.1–7.

Even though garden cultivation of roses began some 5,000 years ago, possibly in China, the fossil record indicates that roses have existed for millions of years. Wherever roses are found, they become a symbol of something: love, beauty, royalty, death, war, and politics. Sometimes, they even represent famous names in history.

During the Roman Empire, roses were grown extensively in the Middle East. Those roses, the most fragrant in the world, were used to make perfume. Their strong scent also caused people to regard them as having a deep mystical significance. However, roses grown in China do not have such a scent. Perhaps this is one reason why roses are not nearly so beloved in China.

Roses seem to make their first literary appearance in the eighth century bce. In The Iliad, an epic poem of ancient Greece, Hectares body was anointed with rose oil. Roses are often mentioned in ancient Greek myths in a wide variety of different forms. For example, a white raised symbolized Aphrodites purity and innocence. Furthermore, her desire and passion for Adonis, her wounded love, was represented by a few drops of her blood changing a white rose into a red rose. In another myth, Zeus scattered roses across the land as a wedding gift to Eros and Psyche.

The Romans also embellished their myths with roses. They used roses at funerals as a sign of resurrection, for example, and Flora was the goddess of spring and flowers. Despite being credited with creating the rose, other Greek deities helped Flora give it its life, sweet scent, and beautiful shape.

Presumably, a correlation exists between ancient Greek and Roman mythology and Christianity with respect to roses. Harpcrates, who was bribed into secrecy with a white rose by Eros, became the god of silence. This was carried over into Christianity, for a rose was often carved onto a meeting rooms plastered ceilings to warn that the discussions were sub rosa (under the rose, or secret).

The rose and religion

Christians used the red rose as a symbol of Jesus passion, martyrdom, and resurrection, as well as for Marys motherhood and purity. Mary is mostly linked with the white rose. The roses design also implies the liturgy. For instance, Mary is surrounded with brilliant roses in this beautiful poem by Wordsworth (1807):

In trellised shed with clustering roses gay,
And, Mary! Oft beside our blazing fire,
When yeas of wedded life were as a day
Whose current answers to the hearts desire.(1)

Also, Mary is often reported as being accompanied by showers of fragrant rose petals. Many stained-glass window scenes of the Blessed Mother and other saints show them holding roses. Underlying all of this is the idea that fragrance is good.

In the seventh century, Prophet Muhammad is symbolized with a rose. It seems that every opening bloom points to another wonderful person in the garden of humanity. The roses reputation increased a thousandfold with the coming of the Prophet. His great personality illuminated the roses color, and his perfect morality is the source of its fragrant sense.

Farid ad-Din Attar (d. 1230), one of the greatest Sufi mystic poets of all time, wrote in his The Rose Garden:

In the rose bed, mystery glows.
The secret is hidden in the rose.(2)

Another Sufi mystic, Hazrat Inayat Khan (1882-1927), notes that just as the rose consists of many petals held together, so a Sufis soul shows many different qualities. These qualities emit fragrance in the form of a spiritual personality. As the rose has a beautiful structure, the Sufi has a fine structure, manner, dealing with others, speech, action, and so on. Just as the roses beautiful perfume penetrates the entire room, the Sufi penetrates society and helps to heal its problems.

Furthermore, the Quran honors the red rose by mentioning only it out of all flowers: When the sky is torn apart, so it was (like) a red rose like ointment. Then which of your Lords favors will you deny? (55:37-38). The beauty of a single rose is just one small creation among all of Gods bestowed bounties. The rose has a distinct shape, and whoever created it and tore the sky apart, just like we might tear apart a red rose, created it so that we could understand that the rose was created for significant reasons.

In the Islamic tradition, wearing perfume is a food for the soul and the spirit. One hadith relates that one of the Prophets favorite things was good fragrance. That is why Muslims often wear pure essence of rosewater, especially before prayers. Rosewater is also the preferred soak for the miswak (a traditional toothbrush taken from the branches of a fibrous tree).

Possessing one of the most heavenly scents, a roses essence has several aromatherapeutic benefits as well. Valerie Worwood, in her The Complete Book of Aromatherapy, writes that a roses essence is a powerful and uplifting anti-depressant, aids in digestion, and soothes frayed nerves. These benefits can be enjoyed just by sprinkling some rose water or essence on your body, adding a few drops to your bathwater, or burning the scent in an incense burner in your home.

Associating roses with religion is as old as religion itself. How nice it would be if the veneration that all Jews, Christians, and Muslims have for the rose could make all of us understand that we are bound together with each other and many others in this very way! Moreover, these three monotheistic religions trace their origins to the Middle East, the ancestral home of modern roses.

The rose in other cultures

Many other cultures feature things related to the rose. For example, different sects within Hinduism worship deities through such household and temple offerings as rose attar (perfume). Ancient Persian legends speak of a nightingale that so dearly loved the white rose that it grasped it tightly. The thorns, which pierced its breast and caused its blood to fall on the rose, turned it red. Hence, one type of rose is known as the red damask. Interestingly, Rose is one of the most popular names for girls in many languages. The Turkish word gul, for instance, means rose and smile and has many popular derivations in Turkey.

Around the twelfth or thirteenth century, knights returning from the Crusades brought the rose home to Europe. By the middle of the fifteenth century, the rose was used as a symbol for the factions fighting to control England. The white rose symbolized York, and the red rose symbolized Lancaster. As a result, the conflict became known by historians as the War of the Roses. The red rose is still the emblem of England, since Lancaster won the war. Roses were in such high demand during the seventeenth century that royalty considered roses or rose water as legal tender. Both were often used as barter and for payment. Napoleons wife Josephine established an extensive collection of roses in western Paris during the 1800s.

A new interpretation

In the twentieth century, a splendid new interpretation of the rose and nightingale story was presented by Bediuzzaman Said Nursi. His famous The Words contains an addendum to the discussion on the nightingale. The nightingales small wage is the delight it experiences from gazing on smiling, beautiful roses, and the pleasure it receives from conversing with them and pouring out its woes. In other words, its sorrowful song is not a complaint arising from animal grief; rather, it is the birds thanks for the gifts of the Most Merciful.

Moreover, according to Said Nursi, every sort of being–even stars–has a nightingale. But the most excellent and noble nightingale, whose voice was the most ringing, whose recitation was the most complete, whose thanks were the most universal, who brought all beings of the heavens and Earth in the universes garden to rapture through his poetry was Prophet Muhammad, the nightingale of the luminous Quran. Here, the rose is associated with divine knowledge and wisdom.

Conclusion

Today, people prefer to buy roses for special occasions as precious gifts. As Shakespeare said, there is a meaning kept in the rose that we have to understand, just as the rose keeps its scent within itself. On Valentines Day roses express our love, on Mothers Day they tell our mothers that we remember their mercy toward us, and on Teachers Day they show a students respect for his or her teachers.

Most modern-day roses can be traced back to this symbolic ancestry. The popularity of roses seems to rise and fall according to gardening trends of the time. Gardeners realize that roses fit the lifestyle of every century. With the rise of emphasis on spirituality in the twenty-first century, roses are once again enjoying a resurgence in popularity.

Roses have a special hold upon us and, even though it is hard to capture in words, there is something about roses that evokes a response beyond the virtue of great beauty. This article has presented some historical traces of roses in several civilizations. Perhaps you will associate them with a favorite meaning or figure. So, would you like to finish the following sonnet of Shakespeare in your own way?

I have seen roses damasked, red and white,
But no such roses see I …?(3)
(the original says in her cheeks)

Footnotes

1. www.poetry.com
2. Omar Ali-Shah, tr. The Rose Garden (Gulistan) of Saadi (Reno, NV: Tractus, 1997).
3. From My Mistress Eyes Are Nothing Like The Sun, Sonnet 130, www.poets.org.

New Findings in Biology

Olfaction was long considered a uniquely mammalian trait. Scientists have disproven this by showing that many intertebrates can smell. For example, birds were thought to be unable to smell, although they have nostrils in their bills. John Audubon, a famous nineteenth-century bird artist, reached this mistaken conclusion by observing vultures confronted with a covered and an uncovered animal corpse, he concluded that they could not smell. The minute weight of the birds’ olfactory bulb consolidated this widespread misconception. Recent research shows that birds use smell when finding and distinguishing food, choosing proper nesting sites and mates, and following avian navigation routes. Ken Stager, an orinthogist at Los Angeles County Natural History Museum, used turkey vultures to disprove Audubon’s vulture experiment. Marine biologist Betsy Bang, who measured the olfactory bulbs and tissues in the brains of 151 bird species, calculated the olfactory bulb’s mass as being between 3% to 37% of the brain’s entire mass. This shows that the ratio, and not the weight, determines a bird’s ability to smell.

Other examples are as follows:

•Pigeons perceive small amounts of odorants. If their olfactory bulbs are blocked, they become lost.

•Certain seabirds (e.g., white chinned petrels) are sensitive to the chemical emitted by their main food (plankton), and so follow an olfactory path over the sea.

•European starlings smell the best region for their nesting site.

•Chickens detect inedible bugs (e.g., bright-colored bad-tasting caterpillars) through smell and sight.

•Salmon return to their hatching sites years later by using the unique olfactory memory of these sites left in their brains.

Smelling is far more developed in mammals, especially dogs and cats, which can sense parts per billion or trillion and can identify millions of different odorants. Science would benefit greatly if such abilities could be reproduced in sensors. But first, how does the human nose smell?

Perceiving Odors

Scientists divide human olfaction into steps. First, a potential odorant emits an odor’s basic elements: volatile organic compounds (VOCs). We perceive an odor when molecules are transformed into an odor by binding the receptor proteins.(1) After binding with certain types of VOCs, these receptor proteins cause depolarization. The electrical charges produce unique signals, which the epithelium’s sensory cells transmit to our neural network (axons).

These signals then are carried to a cluster of neural networks in the brain (glomeruli).(2) Ultimately, the impulse reaches the hypothalamus and describes the scent through a process of classification and identification.

Quantifying Scents

Human odor panels or gas chromatography and mass spectroscopy (GC/MS) are used to identify odors. Such quantification is problematic, however, because it is hard to quantify the VOC’s perception in the nose as a unit of odor. Quantifying mass, volume, temperature, light intensity, and the molecular concentration of a soluble substance in a solution are reasonably objective and can be measured as a multiple of a standard unit.

But a standard olfactory measure does not exist, for it varies according to time and environment. Odor concentration is expressed as a multiple of a threshold: 50% of human “sniffers” must detect-not necessarily identify-it. This threshold is defined by the American Society for Testing and Materials (ASTM), and is accepted as the absolute threshold of odor perception. It takes 5 or 10 odor units for the human panel to identify the odor. GC/MS also can identify the odor’s chemical composition.

Electronic Nose

After developments in electronic sight and hearing, scientists sought similar progress in odor perception. Research began at the University of Warwick (Coventry, England) in the 1980s. Its participants coined the term “electronic nose,” now commonly known as “e-nose.”(3) Their progress made it a commercial commodity with many applications.

E-noses have moved from being metal oxide devices, to conducting polymers, and now to laptop-size or pocket-size odor sensors. The Swiss Federal Institute of Technology (Zurich) has made one the size of a wrist-watch. However, current e-nose use is largely restricted to labs and military applications. Scientists are trying to match or surpass the human sense of smell’s accuracy and sensitivity, after which they will work on surpassing that of the canine species.

Uses and Advantages

E-noses have a wide application in agriculture. Since they can detect minute differences, an e-nose using polymer materials can determine whether a tomato is sun-ripened, picked green, or internally damaged, and whether apple juice comes from a concentrate or is authentic but pasteurized.

Volunteers often test such products. But who wants to determine if corn oil is rancid or canola oil is oxidized? E-noses, having no such “qualms,” detect changed odors in oil samples and provide far more accurate reports.

In animal science and poultry, e-noses provide detailed reports about spoiled food. Judy Arnold, a microbiologist in Athens, GA, researches food quality for the Agricultural Research Service (ARS).

In 1998, researchers discovered that e-noses can detect gases produced by spoiled poultry products. They claim that an e-nose can determine freshness, period of time in a refrigerator, and the amount of fat in white meat. Such an objective evaluation benefits poultry farmers and producers by eliminating returns of “funny-smelling” poultry. E-noses also can detect meat’s decay rate and bacteria, overall quality and freshness, the composition of mixed meat-part products (e.g., processed meat), and how long a ham has been dry-cured. Given this, the e-nose’s ability to examine a bundle of scents makes it very useful. It can perform hundreds of preliminary assessments that would occupy a chemist for months.

The military uses e-noses to detect land mines and traces of chemical-biological weapons. This is important, for over 100 million land mines litter 62 war-torn countries. Although dog-sniffers are useful, this practice is inhumane (dogs are often injured) and impractical (they need lots of training).

E-noses also are better than metal detectors and ground-penetrating radar and infrared imaging-the former detects even tiny pieces of metal, whereas the latter often images pebbles. As e-noses can identify traces of TNT or similar explosives to the 100 parts per quadrillion level, their detection rate is far more accurate and efficient. Nomadics, a Still-water, OK-based company, produces a cigar-box-sized e-nose for this purpose. Tufts University produces an optical e-nose that is designed and functions much like a mammalian nose.

Environmentalists use e-noses to analyze air. For instance, e-noses can report the chemical makeup of odors emitted by a farm’s store of manure, detect the compounds causing that odor, help minimize leaks, and determine a new diet that will decrease such odors. With their ability to detect toxic VOCs and compounds leaking from a factory’s or waste site’s storage areas, e-noses will help environmentalists force industry to change its practices. The major difficulty here remains sampling, as concentrations vary with time and place.

Caltech has used Department of Defense funding to develop a device that identifies odors in seconds. Its 32 components swell like sponges when exposed to a particular vapor, and its resistance (hence conductivity) changes accordingly. As it can detect any type of odor, doctors at the Children’s Hospital in Los Angeles are studying medical applications. Currently, it is applied to patients’ breath to help diagnose upper respiratory infections.

The major advantages of e-noses over human noses in these areas are objectivity; ability to measure odors over long real-time periods; and immunity to fatigue, infection, mental state, hazardous material, and adaptation (gradual loss of sensitivity).

How E-noses Work

E-noses have three functional components: a sample handler, a gas sensor array, and a signal processing system. Its output identifies the odorant, estimates its concentration, and relates its characteristic properties. A sensor recognizes different types and concentrations of odors through its arrays, each of which has a different sensitivity. The resulting combination provides the response pattern that enables the e-nose to identify odorants.

In a typical e-nose, a vacuum pump pulls the first air sample into the tube housing the electronic sensor arrays. The air sampling unit exposes the odorant to the sensor, after which VOCs interact with the surface and the sensor’s active material until reaching a steady state. The sensor’s response is recorded and transmitted to the signal-processing unit. When completed, a washing gas cleanses the sensor. After the reference gas is applied to the unit, the sensor is ready to measure again.

E-nose Technologies

The sensor is the e-nose’s key element, and the sensor type is its defining characteristic. There are 5 types of e-nose sensors, as follows:

Optical sensors: Optical fiber sensors work through fluorescence and chemoluminescence. The tube’s glass fibers contain a thin encoated active material in their sides and at both ends. As VOCs interact with the organic matrix’s chemical dyes, the dye’s fluorescent emission changes the spectrum. These changes then are measured and recorded for different odorous particles.

Fiber arrays with different dye mixtures can be used as sensors. These are fabricated by dipcoating (binding a plastic solution to a substrate), micro electromechanical system (MEMS), and precision machining. The main advantage is that this adjustable tool can filter out noise. Also, since many dye forms are available in biological research, sensors are cheap and easy to fabricate. But the instrumentation control systems are complex, which adds to the cost, and have a limited lifetime due to photo bleaching (the sensing process slowly consumes the fluorescent dyes).

Optical sensors are sensitive and can measure low ppb (parts per billion); however, they are still in the researach stage of development.

Spectrometry-based Sensors: This group consists of a molecular spectrum-based gas chromatography (GC), an atomic mass spectrum-based mass spectrometry (MS), and a transmitted light spectrum-based light spectrum (LS). The first two can analyze the odor’s components accurately, which is a plus. However, their use of a vapor trap to increase concentration can alter the odor’s characteristics. LS devices do not consume the sample, but do require tunable quantum-well devices. GC and MS devices are commercially available, while LS devices are only at the research stage. All spectrometry-based sensors are fabricated by MEMS and precision machining, and can measure odors to a low ppb level.

The GC tube decomposes the odorant into its molecular constituents, and MS forms a mass spectrum for each peak. The spectra then is compared to a large precompiled database of spectral peaks to classify and identify odorants.

MOSFET (Metal-oxide-silicon field-effect-transistor): The basic principle here is capacitive charge coupling. In other words, VOCs react with the catalytic metal and thereby alter the device’s electrical properties. The device’s selectivity and sensitivity can be fine-tuned by varying the metal catalyst’s thickness and composition. MOSFETs are micro-fabricated and commercially available, but can measure only parts per million. They can be manufactured by electronic interface circuits, which minimizes batch-to-batch variation. However, the gas produced by the VOC-metal reaction must penetrate the MOSFET’s gate.

Conductivity Sensors: The sensor types used here are metal oxide or conducting polymer. Both operate on the principle of conductivity, for their resistance changes as they interact with VOCs. Metal oxide sensors are common, commercially available, inexpensive, and easy to produce (they are micro-fabricated). Their sensitivity ranges from 5-500 ppm. However, they only operate at high temperatures (200°C to 400°C).

In conducting polymer sensors, VOCs bond with the polymer backbone and change the polymer’s conductivity (resistance). They are micro-fabricated together with electroplating and screen printing, are commercially available, and can measure from .1 to 100 ppm. They operate at room temperature, yet are very sensitive to humidity. Moreover, it is hard to electropolymerize the active material, which makes batch-to-batch variation inevitable. Sometimes VOCs penetrate the polymer chain, which means that the sensor must be returned to its neutral and reference state-a very time-consuming process.

Piezoelectric Sensors: These devices, which measure any change in mass, come in two varieties: quartz crystal microbalance (QCM) and surface acoustic wave (SAW) devices.

QCM sensors have a resonating disk and metal electrodes on each side. While applying the gas sample to the resonator’s surface, the polymer surface absorbs VOCs from the environment. Thus its mass increases, which increases resonance frequency. As the U.S. Navy has long used QCMs, this technology is familiar, developed, and commercially available. A QCM sensor is fabricated by screen-printing, wire bonding, and MEMS. Althoug it can measure a 1.0 Ng mass change, its MEMS fabrication and interface electronics is a major disadvantages. QCM sensors are quite linear in mass changes, their sensitivity to temperature can be adjusted, and their response to water can vary for the material used.

MEMS techniques should be handled carefully, for the surface-to-volume ratio increases drastically as dimensions approach the micrometer levels. Measurement accuracy is lost when the increasing surface-to-volume ratio begins to degrade the signal-to-noise ratio. This problem occurs in most micro-fabricated devices. SAW devices have much higher frequencies. Since 3-D MEMS processing is unnecessary, SAW devices are cheaper. As with QCM devices, many polymer coatings are available. The differential devices can be quite sensitive. However, interface electronics require more complex electronics than those of conductivity sensors for both QCM and SAW sensors. Also, as the active membrane ages, resonance frequencies can drift and so must be detected for frequency by time. SAW devices are commercially available and sensitive to mass changes at the 1.0 pg level.

Pattern Recognition

Any e-nose’s primary task is to identify an odorant and perhaps measure its concentration. After the signal processing step comes the crucial step of pattern recognition: preprocessing, feature extraction, classification, and decision-making. A database of odors must be formed for comparison purposes.

Preprocessing accounts for sensor drifts and reduces sample-to-sample variation. This can be done by normalizing sensor response ranges, manipulating sensor baselines, and compressing sensor transients.

Feature extraction involves dimensionality reduction, a crucial step for statistical data analysis, since the database’s examples usually are subject to financial constraints. The higher dimensionality caused by sensor arrays is reduced to relevant pattern-recognition information and thus extracts only significant data. As most dimensions are correlated and dependent, it is better to reduce dimensionality to a few informative axes.

Feature extraction usually is accomplished by classical principal component analysis (PGA) or linear discriminant analysis (LDA). PCA is a linear transformation that finds the maximum variance projections and the most widely used technique for feature extraction. But as PCA ignores class labels, it is not an optimal technique for odor recognition.

LDA seeks to maximize the distance between class label examples and minimize the within distance, and thus is a more appropriate approach. LDA is also a linear transformation. For instance, LDA might better discriminate subtle but crucial odor projections, whereas PCA can remove the high variance random noise in a projection.(4)

The classification stage identifies odors. Classical classification techniques are KNN (k nearest neighbors), Bayesian classifiers, and ANN (artificial neural networks]. KNN with, say, 5 nearest points will find the 5 closest matches from the precompiled database. The closest match will be assigned as the tested material’s odorant class.

Bayesian classifiers first assign a posterior probability to the classes in the lower dimension and then pick the class that maximizes the predetermined probability distribution. ANN is closer to biological odor recognition. After being trained by the odor database, it is exposed to the unknown odorant in order to recognize the largest applicable response odorant class. The classifier estimates the class and places a confidence level on it.

In decision-making, risks and application-specific knowledge are considered in order to modify the classification. All decisions are reported-even a nonmatch.

Conclusion

As this article indicates, we can expect great progress in this area. And with each step forward, science and technology will continue to point toward the Greatest Artist’s most subtle designs and allow us to appreciate them better.

Footnotes

1. There are over 100 million receptor proteins of about 1,000 different types.
2. A human olfactory bulb contains approximately 2,000 glomeruli.
3. The terms “electronic nose” and “e-nose” are incorrect, for these devices cannot be considered “real” noses. The correct terminology should be “electronic arrays for chemical sensory and identification.” However, “e-nose” has gained wide acceptance in the literature since it first appeared during a 1991 NATO workshop in Reykjavik, Iceland.
4. Such nonlinear transformations as Sammon nonlinear maps and Kohonen self organizing maps also are used in feature extraction. These preserve the distance between pairs of examples when reducing dimensionality to 2 or 3.

References

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• Wolfgang, Gopel, and Tilo Weiss. “Design for Smelling.” IEEE Spectrum (Sept. 1998): 32-34.
• www.planetee.com/planetee/servlet/DisplayDocument?ArticleID=6899.
• www.sfn.org/briefings/smell.html.