|The Dog’s Nose Knows…Or Does It?
Explosives Detection by Mechanical and Electrical “Noses”
Explosives have been a major threat to airports and military personnel for centuries, but with the endless amounts of information available on the Internet, and technology advancing at an incredibly rapid rate, dangerous weapons have never been so easy to make. Not only are newly made explosives a concern to the world, but unexploded landmines from decades-past wars are still killing thousands of people each year. Rigorously trained canines are key to explosives detection, but technological detection devices may serve as a much less expensive method for preventing deadly blasts.
Prepared by Michelle Sivak
Michelle is a junior majoring in Biomedical Engineering at the University of Southern California. She will be graduating in May of 2013, and hopes to be employed at a leading biotechnical company, or to pursue her Ph. D at a professional physical therapy or veterinary school.
Paper Submitted November 6, 2011
Marc Aubertin, Writing 340 Professor
USC Viterbi School of Engineering
The canine nose contains 220 million olfactory receptors. This number may not sound very high, considering the trillions of cells that compose the bodies of higher organisms like dogs or humans, but when compared to the mere 5 million olfactory receptors that humans possess, the reason that dogs are so essential to rescue teams, police forces, and defense units around the world becomes evident. Dogs can detect seemingly endless types of odors and can even identify odors at concentrations down to parts per trillion, meaning they can identify a scent even if the source of odor is diluted by as much as 1:108 ! Intricate nasal anatomy and the highly developed olfactory lobe of their brains gives dogs a sense of smell 1,000 times greater than that of humans. Since the mid-1800s, dogs’ noses have been put to work tracking missing people and detecting drugs, explosives, and most recently, some cancers . Sniffer dogs are usually used in airports for explosive detection in luggage and on passengers, as well as in the military for rescue and landmine discovery. Training dogs can be costly, however – for example, the Transportation Security Administration (TSA) provides Pittsburgh International Airport with $50,000 per dog per year to cover training and related expenses. Additionally, employing these talented animals comes as a package deal, for the dogs work as a team with their handlers, who typically work with the dogs for a time of 6-8 years . Although some security specialists remain firm in their beliefs that no technology can “perform as well as the canines do,” the dogs can mistake clean luggage or cargo as hazardous, which can result in evacuation of an entire terminal, a lengthy and costly process . As terrorists and militia gain access to more sophisticated and dangerous chemicals or weapons, more civilians’ lives become endangered. Canines serve as essential aids in the detection of explosives and the prevention of many deaths, but the need for more efficient and sensitive security measures in airports and on the battlefield is constantly growing.
The number of trained sniffer dogs in the world is limited – thus, the development of electrical and mechanical detection devices is important for the safety and security of people around the globe. Currently, several different types of detection technologies exist that have the potential to change explosives detection methods, especially for military and security purposes. Although some of these devices clearly mimic the physiological processes of the canine nose, others rely more substantially on mechanical and electrical fundamentals for operation. Regardless, engineers that design explosive detection technologies are inspired by the canine’s impeccable sense of smell and ability to identify harmful substances. Before one can truly recognize and appreciate how these devices work, however, a look at common explosive compounds is imperative.
A Look at the Common Explosive TNT
2,4,6 trinitrotoluene, better known as TNT, remains one of the most widely employed explosives around the globe. TNT is a crystalline compound and is considered safe and simple to manufacture and handle, but has high explosive power when detonated . TNT has been a major component of homemade bombs and land mines since WWI, and is present in most of the unrecovered land mines in existence today. Annually, approximately 15,000-20,000 people are killed by landmines, many of which were planted dozens of years ago. Afghanistan consistently reports the highest number of casualties from land mines each year, the majority of which are children. Although approximately 100,000 mines are excavated around the world yearly, if mine recovery continues at this rate, removal of the rest of the world’s mines would take hundreds of years – and that is if no more mines are implanted in the meantime . Most of the countries burdened with vast mine fields cannot afford adequate detection devices, so simple metal detectors serve as the most common method for mine investigation. Trained canines are still one of the best mine detectors around, but the availability of the best trained dogs for mine detection is low and transporting explosive-sniffing dog teams from one country to another is very expensive. Furthermore, just as humans sometimes die in the process of searching for mines, dogs may lose their lives as well. There exists a tremendous need for inexpensive and accurate mechanical or electrical devices that can detect explosive compounds, particularly TNT because of its common presence in landmines and homemade bombs .
Fido – A Fitting Name for an Electronic Nose
Dogs recognize odors due to their unique physiological smelling processes, or more simply, sniffing. Sniffing is different from normal breathing patterns because the dog will take in a series of very short, fast breathes, which forces vapors into the nostrils and over the olfactory epithelium. When an odor is initially unidentifiable, the nasal pocket, created by the bony structures within the dog’s snout, permits the odorous molecules to collect and stimulate the transmission of a nerve impulse to the brain . Scientists understand how a dog recognizes an odor, and develop detection devices based on this physiological process. One such device inspired by the canine nose and odor detection via vapor detection is a chemical sensor called “Fido,” created by ICx TechnologiesTM. 
Figure 1: Chromophore reaction in “traditional” sensors, where only the single molecule becomes quench, compared to AFP molecules, in which a “molecular wire” forms after binding of explosive molecule .
Fido began as a research project for the Defense Advanced Research Projects Agency, a program launched to inspire the production of devices with mine and explosive detecting abilities comparable to that of a well-trained canine. Fido is comprised of a small glass capillary, called the Sensing Element, coated with amplifying fluorescent polymer (AFP) . Ambient air is drawn into the Sensing Element, and vapors are passed over the coating of AFP, a process similar to inhaled air passing over a dog’s olfactory epithelium towards their embedded sensory receptors. When the AFP molecules, called chromophores, are activated by light at a designated wavelength, the molecules fluoresce. However, when explosive molecules enter the tube and bind to the chromophores, they become rigid and bind to surrounding chromophores, forming a “molecular wire,” as seen in Figure 1 . This process reduces, or quenches, the AFP molecule’s fluorescence by suppressing their natural, excited state – the state in which they emit light. Other electronic “sniffers” use fluorescent polymers to detect odors or explosives, but are less indicative of the presence of explosive material because only the single fluorescent molecule to which an explosive molecule binds is quenched. In Fido’s case, when a molecule of AFP is bound by the attacking molecule, a chain reaction occurs, resulting in the joining of several chromophores to form a rigid “wire” that no longer produce visible fluorescence. Thus, the number of chromophores in Fido’s Sensing Element that can be quenched by one explosive molecule is greatly amplified (compared to the one single chromophore that is quenched by the binding of an explosive molecule in detection devices other than Fido) . This reduction of light – acting as a stimulus like the binding of molecules to an olfactory receptor in the canine nose – then causes the device’s photonic detector to send a signal to Fido’s electronic interface, alerting the user of danger. Despite ICx TechnologiesTM’s original intention to create a device to detect explosives chemically similar to TNT, studies have shown that Fido can also recognize other dangerous compounds including the often difficult to detect chemical PETN. Fido provides a cost-effective method for landmine detection, can be used for screening people and vehicles for explosive chemicals, and has been adapted for underwater detection in the device called the Fido SeaPup. Fido has even detected explosives at concentrations as miniscule as 1 part per quadrillion !
Fido’s chemical sensing abilities have obvious parallels to an actual canine’s olfactory system. Other explosive detection technologies have been developed, unquestionably inspired by dogs and their impeccable abilities, but that operate with less similarity to the canine nose’s physiological processes, relying more heavily on mechanical and electrical systems.
Using MEMS to Detect Explosives
Figure 2: Sequence of events that lead to adsorption of explosive onto sensory layer of cantilever beam. Reaction forces to different molecules are memorized, creating “fingerprints” so that explosives may be recognized in the field .
A different type of technological nose that applies a type of memory sensor to its central design uses a microelectromechanical system, commonly abbreviated as MEMS. MEMS is a system that combines elements of simple mechanics, sensors, and electronics on a silicon substrate, or a layer of material onto which the MEMS elements are fabricated . One such “nose” that operates based on a MEMS system uses nanomechanical cantilever sensors, where the sensors are extremely small – only several hundred micrometers in length and one micrometer thick – silicon beams, pictured in Figure 2, supported at one of their ends so that a force can be applied to the other end and measured. The tiny beams are coated with a sensory layer, onto which the explosive molecules adsorb. When a molecule attaches to the beam, the beam bends due to the newly applied stress. The cantilever beam will bend in a different manner with every new type of molecule that adsorbs to its sensor layer, so that an individual compound will have its own unique reaction force pattern in the device. Thus, the beams can be tested with explosive molecules such as TNT, the response pattern signals can be stored in a computer, enabling the device to recognize the molecule’s “fingerprint” when used in the field . This type of device can be very useful for identifying a wide range of chemicals or explosive compounds, because the beams are not sensitive to any particular type of compound, the way that a device like Fido is most sensitive to chemicals that resemble TNT. This MEMS detection device can potentially be programmed to detect the elusive explosive compounds PETN, but this substance is in fact so hazardous and worrisome to airports and military troops, that it warrants its own detective device.
PETN and its Nemesis Nanosensor
Figure 3: When PETN molecules flow over the nanotubes at the sensor’s center, thetubes’ electrical current becomes disrupted and a signal will be raised .
Pentaerythritol tetranitrate, or PETN, is an extremely dangerous and difficult to detect plastic explosive. Because PETN is plastic, it cannot be seen on x-ray machines or metal detectors utilized for security in airports. Furthermore, PETN is only slightly volatile, which creates problems for bomb-sniffing dogs who depend on odors present in the air to identify explosives . These qualities make PETN an ideal material for terrorists, and was the explosive of choice for the infamous “underwear bomber” in late 2009 . Presently, this chemical can only be found using an ion-mobility spectrometer, or IMS, which identifies molecules based on their ionic drift velocities – each velocity unique to a certain explosive molecule, thus acting like a fingerprint, similar to the identification process employed in the MEMS detection device – through an applied electric field. Unfortunately, IMS devices can only be used on individuals for spot-checking . However, an efficient and inexpensive technology specifically designed to identify this deadly plastic material has been developed and is currently awaiting large-scale production. Scientists from Germany’s Technische Universität Darmstadt have developed a nanosensor (seen in Figure 3) that contains a series of electrical nanotubes over which ambient air flows. One PETN molecule in the midst of roughly 1 billion air molecules will change the conductivity of the tubes, as the PETN molecule’s nitrogen groups seal themselves to the surfaces of the nanotubes, and alter their electrical current, or flow of electrons. This sensor is so new that the university is still “seeking industrial collaboration partners,” and hopes are that the nanosensors can be installed in airport metal detectors or x-ray machines to discreetly check passengers for traces of PETN . The nanosensors would channel the air surrounding passengers as they walked through security, thus checking every single passenger in an extremely quick, accurate, and unobtrusive manner. The fact that such an added secuirty measure would have the ability to screen each and every passenger that walks through an airport is a phenomenal acheivement, especially when the screening would take no extra time. Individually swiping passengers with an IMS device would necessitate travelers to arrive at the airport much earlier than they already do, and there are certainly not enough bomb-sniffing dogs to search the millions of people that pass through the world’s 43,982 total airports each day ! Although the exact cost of manufacturing Technische Universität Darmstadt’s nanosensor could not be determined for this article, one of the nanosensor’s lead engineers Mario Boehme “fancies using them at sports events or in other types of security checks” because they have such a small size and can be made at considerably “low cost” .
Although many engineered explosives detection devices are still in their testing phases, once they become more available for use in the field, military and security operations will become tremendously more efficient and successful. As technologies continue to advance and become less expensive, creating smaller and more sophisticated explosives becomes a much more achievable task for terrorists. The United Nations estimates that 120 million landmines in 70 countries remain underground and still active . Thousands of people are unexpectedly killed each year by landmine explosions, but millions of lives could be saved if a safe and affordable detection method were available for the countries in dire need of landmine excavation. For centuries, canines and their extraordinarily sharp sense of smell have prevented disasters at airports and in mine-ridden lands around the world. Dogs provide a significant help to military units and villages seeking to detect explosives, but there are not enough well-trained dogs in the world to assist every community in need of their help. Additionally, certain scenarios can be just as dangerous, if not more, for the dogs as for people; in such cases, an inexpensively produced electrical or mechanical machine – something that is not alive, and can be sacrificed while working in the field – would be an ideal tool in dangerous life-saving operations across the globe. The special relationships formed between the working partnership of man and dog can never be replaced by an electronic or mechanical device. However, these technological “sniffers” have the power to detect explosives as well as – sometimes even better than – dogs, while at the same time removing our furry best friends from situations of harm, and sometimes even death.
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