“Only when the last tree has died and the last river has been poisoned and the last fish has been caught will we realize we cannot eat money.” Cree Indian Proverb

The table I have under my laptop while writing this article, the materials used for my laptop, the cover case for my phone, the pen I have by my phone, the package for the mail I have received, the dividers I have in my notebook, the hair dryer I have for drying my samples before performing FT-IR on my samples, the FT-IR machine itself … They are all made up of plastics. I could go on and on, giving examples of what I observe in my immediate environment made of plastics. It would not be exaggerated to say that after the Stone Age, Bronze Age, and Iron Age, we are now living in the “Plastic Age” given the fact that the production of plastics has increased from 1.5 million tons per year in the 1950’s to 260 million tons per year in 2007.1 The majority of plastics we use in our daily life are petroleum-based plastics. What that means is, the starting materials of these plastics are chemicals derived from crude oil. There are some major concerns related with these petroleum based plastics – the Earth may run out of oil one day, or the questionable durability of how these plastics biologically degrade. Further environmental concerns exist, such as the toxic additives these plastics contain, including plasticizers like adipates and phthalate. Burning these plastics can release billions of tons of toxic pollutants every year; moreover, most plastic production reactions are done in toxic solvents, so the disposal of these solvents becomes a problem.2 Reflecting on it, it’s an incredible mercy that we have been able to get away with all the waste we have produced up to this point. But the question is: how much longer can we get away with such wasteful behavior?

One of Paulo Coelho’s passages from his book The Winner Stands Alone exactly describes my attitude and desire to “go green.” My heart pounds as I read the sentences that so touched me:

It seems now that-despite wars, famine in Africa, terrorism, the violation of human rights, and the arrogant attitude of certain developed countries-our main preoccupation is saving poor planet Earth from the many threats created by human society. “Ecology. Save the planet. How ridiculous.”

Hamid knows, however, that there’s no point in fighting the collective unconscious. The colors, the accessories, the fabrics, the so-called charity events attended by the Superclass, the books being published, the music being played on the radio, the documentaries made by ex-politicians, the new films, the material used to make shoes, the new bio-fuels, the petitions handed in to members of parliament and congressmen, the bonds being sold by the largest of the world banks, everything appears to focus on one thing: saving the planet. Fortunes are made overnight; large multinationals are given space in the press because of some completely irrelevant action they are taking; unscrupulous NGOs place advertisements on the major TV channels and receive hundreds of millions of dollars in donations because everyone seems obsessed with the fate of the Earth. Whenever he reads articles in newspapers or magazines written by politicians using global warming or the destruction of the environment as a platform for their electoral campaigns, he thinks:

“How can we be so arrogant? The planet is, was, and always will be stronger than us. We can’t destroy it; if we overstep the mark, the planet will simply erase us from its surface and carry on existing. Why don’t they start talking about not letting the planet destroy us? Because ‘saving the planet’ gives a sense of power, action, and nobility. Whereas ‘not letting the planet destroy us’ might lead to feelings of despair and impotence, and to a realization of just how very limited our capabilities are.” 3

On that note I would like to share some amazing facts I found while searching articles written on bacterial biopolymers, but first of all I would like to introduce some definitions on the concepts I will be writing about.

Plastics have many definitions, but usually, in a daily conversation, plastics mean “anything that can be molded or shaped.” Scientifically, a plastic is a sub category of a polymer. Poly- meaning “more than one” and -mer meaning “member of a particular group.”[4] Basically, a polymer is a naturally occurring or synthetic compound made of many relatively simple repeating units that are linked together in the same fashion, forming a carbon rich backbone in most cases. For example, PVC is a well known synthetic polymer, in which the monomer (the repeating unit) as seen in Figure 1 is repeated several times. A well known natural polymer is cellulose, in which the monomer as seen in Figure 2 is repeated several times.

Here it is important to note the difference between a polymer and a plastic. All plastics are polymers, as in the example of PVC, whereas not all polymers are plastics, as in the example of cellulose. The combination of the chemicals, and the type of bonds these chemicals are linked to each other by, determines the properties and applications of the polymers. The molecular weight of the polymer depends on how many times the monomer repeats itself. The molecular weight of polymers can be controlled during production with chemical techniques. One significant difference between natural vs. synthetic polymers is the molecular weight distribution. When the polymer is synthesized in the lab, the polymer product is a combination of different molecular weight chains. In other words, when a polymerization reaction takes place, lots of polymer chains are produced and one chain is never the same length or weight as another. Instead, there is a molecular weight distribution as seen in Figure 3, where most of the polymer chains in the solution have a molecular weight close to the value of Mw. So in the solution, we will have polymer chains that have molecular weights close to each other, and some extreme short or long polymer chains. It is impossible to synthesize a polymeric solution where all the polymer chains are of identical length and weight; therefore, we speak about the average molecular weight when the case is synthetic polymers. However, when we look at any polymer produced in nature, we see that the polymer chain length and molecular weight are the same every time the polymer is produced. So instead of a molecular weight distribution, natural polymers have a molecular weight value. This is important because the narrower the molecular weight distribution is, the better.

When talking about bio plastics, it is important to make the differentiation between bio-derived plastics and bio-based plastics. As Dr. R. Narayan explained in his talk at Johnson County Community College[5] , bio-derived plastics means that the plastic is isolated from a living organism, meaning that the living organism performs the polymerization reaction and then you extract the polymer from the organism.

On the other hand, bio-based plastics mean that the starting material of the plastic is derived from a living organism instead of a petroleum-based material, but it is polymerized into a plastic by humans. Therefore, not all bio-based plastics are biodegradable; however, the fact that the starting material is from a plant that can be replaced in a couple of years rather than a petroleum-based product which can only be replaced after a couple million years, drives motivation for their usage. There is the ethical concern that bio-based plastics are usually made from food sources, such as corn, however Dr. R. Narayan, who is one of the leaders in the field, argues that if the situation is handled appropriately, this should not be a problem. He argues that one up-side of the situation would be to increase values of crops and the prevention of mass migration to big cities. It’s your call to decide which side you favor more.

What is more interesting to me is the polymers being created in nature. A chemistry doctorate, Dr. Lon J. Mathias, writes that “We humans make nylons in tons per day in huge chemical plants where simple molecules are joined together in large quantities to give products that we need or want. Nature is much more careful and concise in how she does things. For a living organism to make an enzyme, another enzyme or active species must be involved. The synthesis always involves a template, or recording, of how the individual amino acids are to be joined together to give the final polymer. The enzyme adds a single amino acid, one at a time, as indicated by the mRNA. This is a slow and tedious process and takes a long time. Sometimes the enzyme gets frustrated, waiting for the right amino acid to come along, and slaps a wrong one on instead. To compensate for this, the enzyme is made to back up occasionally to check its work. If it has made a mistake, it has a process for clipping out the wrong amino acid and inserting the right one. We humans never do this. If we make a mistake, we simply grind it up and throw it away.”6

Dr. Mathias goes on, comparing the manufacturing conditions between nature’s form of polymerization and humanity’s. He says polypeptides in nature are synthesized in water, whereas we synthesize our polypeptides in toxic organic solvents. “This leads us to a problem: what do we do with the organic solvents when we’re through? Sometimes we burn them, but more commonly we try to recycle these materials, which not only are getting more expensive to buy in the first place (compared to cheap water, which is everywhere, or almost everywhere) but are also a responsibility for their recycling, purification, and final disposal. An example of how nature uses water in this way, and one which we still haven’t figured out, is the production of spider silk. Spiders spin their webs from solutions of polypeptides in water. These solutions are squeezed through the spider’s tiny spinneret and elongated quickly to form the spider webs which we’ve all seen and sometimes become tangled in. What’s really weird is that, once these spider webs form, they are no longer soluble in water. If we could just figure out how spiders first make spider silk in water and then spin their webs from it, we could make nylon the same way. This might save us a lot of waste disposal problems, and money.”6

Another spectacular creation in nature is polymers produced in bacteria which can be used as plastics once isolated from the bacteria. A wide range of biopolymers that are synthesized in bacteria serve diverse biological functions and have material properties suitable for numerous industrial and medical applications.7 Different carbon sources are efficiently converted into a diverse range of polymers with varying chemical and material properties.7 To be a little more specific, four major classes of polymers are produced by bacteria: polysaccharides, polyesters, polyamides and inorganic polyanhydrides (such as polyphosphates).7 These polymers serve various biological functions, for example, as reserve material or as part of a protective structure, and can provide a substantial advantage for bacteria under certain environmental conditions.7 Some of these biopolymers can be isolated from bacteria and can be used as plastic. Biopolymers are, by definition, biodegradable, and so their application as commodity products becomes increasingly attractive in view of the desire to avoid the use of recalcitrant oil based polymers that will accumulate in the environment.7 Biodegradable means that when exposed to the microbial flora present in a given environment (for example, in soil or water), biopolymers are fully degraded and mineralized to CO2 and H2O.5 The reason biopolymers are 100% degradable is, as they are produced in bacteria as storage material, they have sites where bacterial enzymes could attack to break them down when they search for nutrients. Whereas other polymers – even bio based polymers – will not have these enzymatic sites, so they are not always biodegradable.

One popular class of polymers produced by bacteria which can be used as plastics is called polyhydroxyalkanoates (PHA’s). PHA’s are a class of polymers produced in nature by the bacterial fermentation of sugar or lipids. They are produced by bacteria to store carbon and energy when there is a nutrient lacking from the environment. Many kinds of bacteria are able to produce PHA’s, such as soil inhabiting bacteria, and many bacteria in activated sludge, high seas, or extreme environments. 8 As we store fats in our bodies, the bacterium store PHA’s. In an environment that contains all of the necessary nutrients, bacteria grow and reproduce – in other words they produce biomass. However, when subjected to specific nutrient depletion (nutrients such as nitrogen or phosphorus) and excess amount of carbon resources, the bacterium starts storing PHA granules (Picture 3). The moment the missing nutrient is introduced back into the environment, the bacterium starts degrading the PHA granules and continues to produce biomass. Therefore, by manipulating the nutrient resources in the environment and providing optimum conditions, bacterium can be pushed to produce PHA’s.[9]

There are metabolic pathways involving various enzymes for the conversion of carbon sources to polymers. Scientists have been trying to genetically engineer bacteria for the increased production of these polymers. In some cases it is possible to over-express the key enzymes in the pathways to achieve increased production of PHA. However, this kind of research takes a lot of time and effort because altering biological activity is a very complicated process and in most cases, cells give unpredictable responses to alterations. By feeding the bacterium with different carbon sources at different conditions, it is also possible to alter the composition of the polymers. Moreover, different strains of bacterium produce different types of polymers; therefore, the range of biopolymer research is very wide. With over 150 different PHA monomers (the repeating unit of polymers) being reported, PHA with flexible thermal and mechanical properties have been developed. 7 Such diversity has allowed the development of various applications.

During his speech at the “2nd International PLASTiCE Conference Trends in Bioplastics” in Slovenia, 9 Dr. Martin Koller explained that there are two types of PHA’s that a microorganism produces. The first type are short length PHA’s (3-5 carbons in the backbone) and the second type are medium chain length PA’s (6-12 carbons in the backbone). While the medium chain length PHA’s can be used for biodiesel production, the short chain length PHA’s can be used as thermoplastics (plastics that can melt with heat, and can therefore be processed with the help of heat). These thermoplastics can be isolated from the organisms they are produced in by solvent extraction, mechanical disruption, or by using hypotonic media (having the lower osmotic pressure of two fluids) for cells that have high intracellular osmotic pressure.9 In the last case, the cells will explode due to the pressure difference and release the PHA’s; deionized water can be used as the hypotonic media. However, only specific strains can be treated with this method. At the moment, the most common technique used for extraction is solvent extraction. These solvents – such as chloroform or dichloromethane – are generally toxic, therefore creating a contradiction with the point of producing biopolymers.

Although not mainstream, some of these bacterial plastics are produced in the industrial world.8 The simplest and widest application for bacterial plastics is for packaging purposes. They can also be used in therapeutic applications, as they are generally biocompatible. Drugs can be incorporated into them, therefore as they biodegrade, they release the drug in a controlled time frame.9 For example, Dr. Martin Koller and his group have just finalized a project called “BRIC – BioResorbable Implants for Children,” funded by the Austrian Research Promotion Agency (FFG).10 Their purpose was to isolate a biocompatible polymer produced from bacterium which could be degraded and removed from the body within a certain time. The point of this project is based on the fact that in contrast to the traditional implants that need to be removed from the body after a certain amount of time, such as plates, screws or pins, the newly developed implants could be degraded and removed from the body naturally, preventing the need for a second surgery. This is a great advantage, especially for children, who would suffer greatly from additional surgeries.

Bacterial bioplastics have many other applications; however the biggest obstacle for their usage is the cost of production. During his speech, Dr. Keller stated the production of bacterial bioplastics is around five times more costly than petroleum based plastics. Most of the cost is related with the bioreactors needed to grow the bacterium and the solvents used to extract the polymers. The scientists are hoping to develop new techniques to reduce the cost of the polymers.

It is breathtaking that these creatures we cannot even see with the naked eye have been synthesizing polymers as well as we do, if not even better, and for a lot longer than us. The polymers they synthesize are completely biodegradable, have a constant molecular weight, and do not require toxic chemicals for their production, unlike the synthetic polymers we produce in the lab. They don’t harm nature as we do. And THAT is powerful.

References

1- Simon, Tristan (2007). “Experience Curves in the World Polymer Industry” Utrecht University, Netherlands.

2- Lei Pei, Markus Schmidt and Wei Wei (2011). “Conversion of Biomass into Bioplastics and Their Potential Environmental Impacts, Biotechnology of Biopolymers.” InTech.

3- Coelho Paulo(2008), “The Winner Stands Alone.” pg: 139.

4- http://dictionary.reference.com/

5- Narayan, Ramani (2013)”Bioplastics and Reducing Carbon Footprint.” JCCC Video. Johnson County Community College, USA.

6- Mathias, Lon J. (2005).”Natural Polymers.” Polymer Science Learning Center. The University of Southern Mississippi, USA.

7- Rehm, Bernd H.A.(2010). “Bacterial polymers: biosynthesis, modifications and applications” Nature Reviews Microbiology. Massey University, New Zealand.

8- Chen, Guo-Qiang (2010). “Plastics Completely Synthesized by Bacteria: Polyhydroxyalkanoates”. Plastics from Bacteria: Natural Functions and Applications, Microbiology Monographs, Springer. Tsinghua University, China.

9- Koller, Martin (2012). “Polyhydroxyalkanoates: Biodegradable polymeric materials from renewable resources” Plastice Project Video. 2nd International PLASTiCE Conference Trends in Bioplastics, Slovenia.

10- No name (2013).”Plastics from Renewable Raw Materials:Body automatically breaks down implants” Graz University of Technology, Austria.

11- Nishiyama, Yoshiharu; Langan, Paul; Chanzy, Henri (2002). “Crystal Structure and Hydrogen-Bonding System in Cellulose Iβ from Synchrotron X-ray and Neutron Fiber Diffraction”. J. Am. Chem.The University of Tokyo, Japan.

12- Ritter, Stephen(2005). “Green Success.” Science and Technology. pg: 40-43.

13- Waters Co. (2013). “GPC-Gel Permeation Chromatography”. Web.

Many people probably would agree that the transistor was the greatest technological invention of the twentieth century. The first transistor, invented by Jack Kilby and Robert Noyce in 1958, was made from silicon. Even though integrated circuit technology has advanced to the level of putting millions of transistors on a fingernail-sized chip, transistors are still made from silicon. Now, however, both the scientific community and the high-tech industry are very excited about something new: plastic electronics. For many of us, this might sound like an oxymoron, for we know plastic only as an insulator. So how can a protective substrate or a carriage box to the actual electronic device be converted into an electronic device?

The story of plastic electronics started late 1970s with Alan Heeger, Alan Macdiarmid, and Hideki Shirakawa. These scientists demonstrated that the molecular structure of certain polymers (plastics) can be manipulated and then used as conductors. The Swiss Academy of Sciences was somewhat slow in recognizing their work, for they were awarded the Nobel prize in chemistry only in 2000. Nevertheless, the scientific community did not wait for the Nobel committee’s recognition of plastics. Since 1977, plastic has become probably the most common material in our daily lives.

Silicon versus plastic

All computer chips are made out of silicon (semiconductor) and aluminum (metal). Silicon is a great material for integrated circuits, because it is available, can be acquired in an extremely pure state (single crystal or amorphous), and has a very high mobility (the speed that electrons can travel through material). This mobility, in turn, determines the device’s switching speed. However, silicon has one important drawback: It is not easy to process.

Integrated circuit technology can deposit millions of silicon transistors on a single chip. However, the procedure for making these devices usually requires facilities worth billions of dollars, for silicon has to go through complicated photolithography procedures under clean room conditions before it can be incorporated into a device. But is it really worthwhile to spend billions of dollars on such facilities? The answer probably looks obvious, since Intel remains one of the world’s largest companies. However, we do not really need such a high quality in many of the applications for which silicon is used. So if there is something cheaper that can do the job perfectly, why not use it? Plastic is far cheaper, but not so sophisticated an alternative.

Plastic is cheap because billion-dollar facilities are not required to convert it into a device. In fact, the technology needed to process plastic into an electronic device is only slightly more advanced than an ink-jet printer. Electronic circuits are printed on an insulating polymer, and then certain parts of the polymer are exposed to UV light in order to convert the insulating polymer into a conducting polymer. The result is a device in which only the parts that we want to conduct are conducting. Moreover, these conducting parts sit on a protective insulating sheet of plastic. That is pretty much all we need for many applications.

The case for plastic

Plastic has two advantages over silicon: price and flexibility. Like other inorganic elements, silicon has strong covalent bonds between its atoms. As these bonds are rigid, they cannot bend or stretch. Plastic has very loose molecular bonds that can tolerate a significant amount of bending and stretching. Flexibility combined with electronics implies applications like flexible displays that can be rolled up and taken somewhere else, reloadable electronic newspapers that can be bent like paper, disposable mobile phones, and many others.

So if plastic is that good, why is it not the electronics industry’s standard material? For one simple reason: Plastic’s loose molecular bonds, which make the material so flexible, make it more difficult for the electrons to travel through it. Thus, plastic devices are slower than silicon devices. Until several years ago, the mobility of a typical conducting plastic used to be around 0.1 cm2/volts, whereas crystalline (best) silicon could reach 1000 cm2/volts at room temperature. Recently, a new class of polymers (pentacene) has been found in which molecules tend to self-organize. As a result, the mobility has been pushed up to 3 cm2/volts. Scientist working on pentacene estimate a number close to 50 cm2/volts as the limit of achievable mobility for this special polymer.

These expectations are not just wishful thinking of some optimistic scientists. In fact, even now there are some significant outcomes of plastic technology.

Current developments

John Rogers and coworkers from Bell Labs (Lucent Technologies) have patterned 256 polymer transistors on the back-plane of a flexible optical display. Richard Friend and coworkers from University of Cambridge have produced thin film transistor circuits using a high-resolution inkjet printing. Dago de Leeuw and colleagues at Philips Research Laboratories in Eindhoven, The Netherlands, have developed a new technique called photochemical patterning. In photochemical patterning, a light sensitive-polymer is exposed to ultraviolet light through a mask shaped in the form of the desired circuit. The ultraviolet light changes the polymer from a conducting state to a non-conducting state. In this process, the polymer’s resistance can increase as much as 11 orders of magnitude (100000000000). The advantage of this technique over the patterning techniques used for silicon is that it does not need any vacuum and can be used on flexible substrates. The problems with photochemical patterning are that it is not significantly cheaper than photolithography and etching used for silicon, and it can be used only for light-sensitive polymers. Different research groups have developed various techniques that have pros and cons compared to photochemical patterning. However, many of the techniques cannot print features that are small enough for electronic circuits. The critical length is the distance between the transistor’s source and drain, typically 0.01 mm. This is the distance that the field-induced charges have to travel. As the drive current and switching speed of the device depend on this distance, having too large of a distance reduces the capabilities of the device. Another technique that pursues quite a different approach is microcontact printing. Developed by the Bell Labs group, microcontact printing with rubber-like stamps can make small enough features. Scientists have used this technique to make a flexible display in which a transistor controls each pixel. The key point in developing this technique was using gold pads, instead of a polymeric material, to deposit the transistor’s source and drain. Even though this device is not completely plastic, it is a step toward that goal. When a special kind of ink was applied to a thin film of gold, it formed some sort of self-assembled layer on the gold, which then produced well-defined patterns and sharp edges. This provided the required resolution to make small enough features necessary for an electronic device having a reasonable speed. These two techniques show two important aspects of the problem. In photochemical printing, we have a device that is completely plastic and so has the important advantage of flexibility. However, it is not as cheap as it could be and does not have the required resolution. In the second technique, the outcome is not a 100 percent plastic device, so it is not as flexible as a purely plastic circuit. However, it can be manufactured very cheaply and has a better resolution (and thus a higher switching speed).

Conclusion

Many other approaches are being employed to develop this new and exciting technology. If plastic electronics does become standard for at least some applications, it probably will be a hybrid of these different techniques. If the optimistic group of scientists working on plastic electronics prove to be right, one day we might see TV screens curling around the walls of our rooms and even reloadable electronic newspapers that can be folded and carried like regular newspapers. Who knows what new inventions will come with this new technology?

References

• Garnier, F., (et al). Science 265 (1994): 1684-86.
• Gelinck, G., T. Geuns, and D. de Leuw. Applied Physics Letters 77 (2000): 406-8.
• Levi, Barbara G. ‘New Printing Technologies Raise Hopes for Cheap Plastic Electronics.’ Physics Today (February 2001). Online at: www.physicstoday.org/pt/vol-54/iss-2/p20.html.
• Nobel Focus: Electricity through Plastic.’ Physical Review Focus (24 October 2000). Online at: http://focus.aps.org/v6/st18.html.
• Scott, Campbell. ‘Electronics Put It on Plastics.’ Physics in Action. (October 1998). Online at: www.physicsweb.org/article/world/11/30/3/1.
• Voss, David. ‘Cheap and Cheerful Circuits.’ Nature 407 (28 September 2000).

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

• Baltes, Henry, Dirk Lange, and Andreas Koll. “The Electronic Nose in Lilliput.” IEEE Spectrum (Sept. 1998): 35-38.
• Barinaga, Marcia. “Salmon Follow Watery Odor Home.” Science 286 (22 Oct. 1999): 705-6.
• http://csmt.jpl.nasa.gov/enose.html.
• http://faculty.washington.edu/chudler/nosek.html.
• Malakoff, David. “Following the Scent of Avian Olfaction.” Science 286 (22 Oct. 1999): 704-5.
• Mamberts, Peter. “Seven-Transmembrane Proteins as Odorant and Chemosensory Receptors.” Science 286 (22 Oct. 1999): 707-10.
• Perkins, Sid. “Eau, Brother! Electronic Noses Provide a New Sense of the Future.” Science News 157 (19 Feb. 2000): 125-27.
• Schiffmann, Susan, and H. Troy Nagle. “The How and Why of Electronic Noses.” IEEE Spectrum (Sept. 1998): 22-32.
• Stern, Peter and Jean Marx. “Making Sense of Scents.” Science 286 (22 Oct. 1999): 703.
• 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.