More Evidence that the Moon Came from the Earth

Will et al. Indigenous noble gases in the Moon’s interior. Science Advances, Aug 2022.

Humankind has always been fascinated with the Moon and studying it for nearly five centuries since Galileo. A recent discovery now adds new evidence to the currently favored “Giant Impact” theory which hypothesizes that the Moon was formed by a massive collision between Earth and another Mars-sized celestial body around 4.5 billion years ago. A group of researchers examined six samples of lunar meteorites collected in Antarctica using an exceptionally sensitive mass spectrometer and found that the meteorites contained noble gases like Neon and Helium, consistent with those found in the Earth’s mantle. Researchers proposed two possible scenarios for how the noble gases became trapped in the Moon’s interior. In the first scenario, impactors got mixed with the lunar mantle during cooling of the magma oceans to solidify over few million years of the Moon’s formation. In the second scenario, the Moon has been formed from a debris field surrounding the Earth where noble gases were directly mixed into the Moon’s interior mass. Discovery of noble gases on the moon may also inform us about its water content, too. If these gases are still there, then water could also been present in the Moon’s interior. Such water resources could be an invaluable resource for future human missions. More broadly, this study suggests that a wide variety of life-forming material can survive giant impacts early in a planet’s life. We now could make more reliable models of how planets and solar systems form and even how life is originated on the Earth.

Restoring cell functions after death?

Andrijevic et al. Cellular recovery after prolonged warm ischaemia of the whole body. Nature, August 2022.

Organ transplantation is an extremely complicated medical process. There is a massive shortage of donor organs. Waiting lists are long. Even if a patient is lucky to match with a donor organ, getting that organ before it dies through cell damage has been a big challenge. A new technology may offer a solution to extend the time that donor organs survive. A group of researchers has recently developed a technology called OrganEx, which can restore cellular activity even after death. Very shortly after the death of an organism, all cells start to die and organs begin to fail. The researchers worked with one hundred pigs to see whether cellular structures could be saved, or cell damage could be reversed, when OrganEx is applied after death. OrganEx has two major components. First is a device that simulates the heart and lung function by pushing a mix of blood and a drug cocktail to the organs. Second is the drug cocktail made of 13 chemical compounds. One hour after death, the pigs were hooked up to the OrganEx machine which pumped the cocktail to the animal’s organs for six hours. The results were striking; OrganEx could restore critical cell functions after death. While this is a huge step for organ preservation, researchers still have to make more tweaks for the technology to be used in humans. Once fully developed, OrganEx is expected to keep organs outside the body for long-term or transported longer distances.

Sweat-powered wearable electronic devices

Liu et al. Microbial biofilms for electricity generation from water evaporation and power to wearables. Nature Communications, July 2022.

Researchers have developed a biofilm that sticks to the skin like a Band-Aid to harness sweat for electricity that could power wearable devices. The biofilm is made using a type of bacteria called “geobacter sulfurreducens” known for its ability to produce electricity. In this biofilm design, bacteria convert energy from evaporation into electricity by using the moisture on a person’s skin. Most strikingly, researchers found that the biofilm bacteria do not need to be fed because they are dead! They do not need to be alive to produce electricity. The biofilm consists of thin sheets of bacteria colonies (thickness less than 0.1 millimeter) that is sandwiched between two mesh electrodes and sealed with a soft, sticky biopolymer to enable it to grip to the skin. Sticking this biofilm on your skin is like plugging in a battery. This technology has potential to revolutionize wearable electronics by solving the major problem of power supply. Moreover, this is a real green energy-driven device made naturally by the microbes and devoid of any unsustainably produced materials and toxic waste byproducts. The current version of the biofilm can produce enough energy to power small devices such as medical sensors or personal electronics, but the researchers hope to explore larger films that can power even more sophisticated devices.

Radiation, which refers to emissions of light or particles, is a type of energy transfer. Radiation takes place at any given moment in the environment or the body. Among sources of radiation to which humans are exposed daily are sun rays, radio waves coming from devices such as mobile phones and TV sets, appliances such as ovens or irons that emit heat, and medical machines such as ultrasounds. The radiation emitted from devices and machines do not cause ionization. Ionizing radiation is made up of high-energy wavelengths or particles, and this is the kind of radiation we get from x-ray, CT, and nuclear imaging. This is used to penetrate tissue to reveal the body’s internal organs and structures. Ionizing radiation can damage DNA, and when our cells cannot fully repair the damage, this may result in DNA mutations.[1] The radiation which poses real danger to humans and has the power to ionize is when radioactive—or unstable—atoms decay and emit alpha (α), beta (β), and gamma (γ) rays.

The earth, air, water, and all living things are more or less radioactive because radioactive atoms are everywhere. The average person is annually exposed to radiation levels of 2.6 – 10 mSv (millisievert), which is not that alarming. The maximum limit recommended for people exposed to radiation for occupational reasons is 100 mSv. The lungs of a person who smokes one pack of cigarettes a day are exposed to an annual radiation of 106 mSv.

How can we protect ourselves?

It is recommended by the World Health Organization that children younger than 16 should not use mobile phones; when they do, their calls should not exceed 10 minutes. When purchasing devices, you should also take into account its SAR (Specific Absorption Rate). Prefer devices with a SAR<1 W/kg. It is also recommended to unplug electrical devices when you are not using them, to keep electrical appliances as far away from your head as possible, use the hairdryer for short periods and in intervals, and to avoid using mobile phones for long conversations (or use headphones!).

It’s also worth reconsidering whether using radiation-emitting devices such as mammography, x-rays, or ultrasounds are absolutely necessary. In 2010, the British Department of Health and Social Care banned using tomography for screening purposes. Another study in the US found that one in ten people are exposed to high levels of radiation because of medical tests.

The average radiation rates (mSv) a person was exposed to during use of certain imaging devices is as follows:

Researchers also found that employees in nuclear power plants were exposed to amounts of radiation that far exceeded allowable amounts.

Beware of radon

The natural radiation humans are exposed to most is the gas radon. Some matter with radioactive atoms such as uranium and thorium – both present in the earth since its birth – emit radon, which seeps through the earth and into the walls of houses and through gaps in plumbing. It is recommended to air houses at least 15 minutes every 24 hours as the only way to be protected from radon.

The resistance of living things

Creatures have been created with different forms of resistance to the elements, including radiation. For example, dogs have a lower resistance than humans, while many other creatures such as rabbits, tortoises, and fruit flies have a higher resistance. And then there is the cockroach, which can survive even a nuclear attack. The lethal radiation dose for cockroaches is an incredible 670- 1000 Sv, whereas it is 6-8 Sv for humans.

Scorpions are also much more radiation-resistant than humans. They can withstand up to 1500 Sv, an amount that is 250 times the maximum dose humans can take. Studies have found a correlation between the strength of a scorpion’s venom and their resistance to radiation. The greater the amount of venom, the greater the resistance they have. The presence of the neural transmitter serotonin supports this view.

Are humans radioactive too?

Humans contain trace amounts of radioactive atoms, namely uranium (²³⁸U), potassium (⁴⁰K), and carbon (¹⁴C). An 80 kg human has natural radiation of 8000 becquerel every second, which is equal to 100Bq per kilogram. This amount is not high enough to cause any worry. The human body has 40 trillion cells on average, and every cell has about 100 trillion atoms. The proportion of the radiating atoms in the body is about 8000/4×10²¹.

Precision protection

The radioactive atoms in the body with the highest probability for carcinogenic effects are potassium (⁴⁰K) and carbon (¹⁴C) atoms. The decomposition that leads to cancer stems from mutations in genes, but the molecules that are the building blocks of genes do not have potassium atoms. The likelihood that a cell gets harmed is very low: it is necessary that the particles emitted from the radioactive potassium atom crash into the DNA molecule and harm it, which is as unlikely as threading a needle when blindfolded. The DNA is precisely protected inside the nucleus located at the center of the cell. If we consider the fact that the average diameter of a cell is about 10 microns (1 micron is one-thousandth of a millimeter), we can better appreciate how little space DNA occupies.

Radiocarbon atoms (¹⁴C), on the other hand, might be present in DNA molecules, and they are more dangerous because the emitted particles are more likely to find the target despite having weaker radioactive properties than potassium. A radioactive carbon atom turns into a nitrogen (¹⁴N) atom and may thus cause a chemical change in the DNA. In other words, the carbon atom is possibly to blame for the unexpected development of cancer.

The likelihood of harmful radioactive particles hitting a person’s DNA is low, and the protective system provided for it lowers the likelihood of developing cancer even more. New DNA molecules that form during DNA coupling are repeatedly checked by inspector enzymes. If there is an error, it is detected and then corrected. The broken code is taken out to be replaced with the correct version. Meanwhile, all these steps are checked by other enzymes assigned to the task. More errors might be made in the newly produced DNA molecule because of external factors. Yet ribosomes in the cell start to produce repair enzymes, as per the instructions from the DNA.

When thinking about all the protective factors that have been coded into the DNA for our survival against the 8000 radioactive activities that occur in our body every second, one cannot help but feel awe for the infinite mercy and wisdom that operate in our lives.

References

• http://time.com/5069317/california-mobile-phone-radiation/
• https://www.health.harvard.edu/cancer/radiation-risk-from-medical-imaging
• Choppin, G. et al., Radiochemistry and Nuclear Chemistry, Oxford: Elsevier Science & Technology, 1995.
• www.physics.isu.edu/radinf/natural.htm

[1] https://www.health.harvard.edu/cancer/radiation-risk-from-medical-imaging

Heart Failure

The heart is basically a pump that provides the force needed to circulate blood and its contents to the body. It consists of four chambers: left ventricle, left atrium, right ventricle, and right atrium. The right ventricle and atrium collect the blood from the whole body and pump it to the lungs for removal of carbon dioxide and replenishment of oxygen. On the other hand, the left ventricle and atrium are responsible for collecting the blood from the lungs and pumping it to the body through the aorta (main artery). Non-stop blood circulation requires life-long and unfailing heart muscle power. Common symptoms of heart failure include waking up at the middle of night with shortness of breath and decreased ability to walk a few steps upstairs. Heart failure could arise due to any condition that decreases efficiency of the myocardium (heart muscle – Figure 1) through myocardial infarctions (death of muscles due to lack of oxygen, also known as heart attack) or overloading, such as hypertension, that requires increased contraction force. Loss of function in the ventricles (lower chamber of the heart) may require the use of ventricular assist devices (VAD).

Ventricular assist devices

A VAD is a mechanical pump that is implanted into the chest of patients to support the heart function through bridging the blood flow from the lower chamber to the aorta (Figure 2).^2,3 A VAD is usually useful during or after cardiac surgeries until recovery of the heart or while waiting for a heart transplant. VADs could also be used long term if the patient is not eligible for a heart transplant due to other complications. A VAD has several components, including a tube carrying blood out of the heart into a pump, a pump with another tube carrying blood to the aorta, and a power supply connecting to a control unit that monitors the VAD`s functionality.

There are different designs of VADs such as HeartMate, HeartWare, DuraHeart, and so on. Some VADs pump like the heart does, using a pumping action, and others use continuous blood flow. Intriguingly, VADs with continuous flow lead to a loss of normal pulse, but this has been found to decrease complications and increase survival. Two types of VADs include left ventricular assist device (LVAD) or right VAD (RVAD). LVADs are the most commonly used ones due to higher incidence of left ventricular function loss. If both LVAD and RVADs are used together, they are called a biventricular assist device (BVAC). VADs nowadays could be used not only for people with end stage heart failure, but also earlier stages of heart failure, including children. A VAD takes ninety percent of the pumping function of the heart. When using a VAD, your heart will still beat and have a rhythm. If none of these works for a patient, this requires the use of a mechanical or artificial heart, also known as total artificial heart (TAH).

Total artificial hearts

An artificial heart is a mechanical device that substitutes for the failing heart.^4,5,6 They are commonly used during heart transplantations to bridge the blood flow temporarily or to replace the heart permanently during a shortage of transplantable hearts. Early studies with artificial heart trials go back to the 1940s. Since then, various groups worldwide have invested in development of artificial heart prototypes and performed animal and human trials. Total artificial heart prototypes include, but are not limited to, SynCardia, ABIOMed (AbioCor), and Carpentier (CARMAT).6

SynCardia is developed from the Jarvik-7. It was first implanted in 1982. Dr. Robert Jarvik originally designed the Jarvik-7 (Figure 3). Barney Clark underwent the first artificial heart implantation at the University of Utah and survived for 112 days. This was followed by other implants. The longest survival with the Jarvik-7 is 620 days. However, the device is more commonly used on patients as a bridge during heart transplants. It has two pumps that resemble the two ventricles of the heart and is pneumatically (air) powered. The pump of SynCardia is covered with polyurethane. A pneumatic driver used in the US is a non-portable console, and requires patients to stay in the hospital. However, a portable version has been developed in Europe that could be carried a backpack while a patient waits for a donor heart.

The ABIOMed (AbioCor) is a completely self-contained, total artificial heart, which avoids the need for an external console or having wires or tubes piercing the skin to power the device. It uses a wireless energy transfer system, also known as a transcutaneous energy transmission system. This decreases the risk of developing infections due to implants.

CARMAT, on the other hand, is designed by Alain F. Carpentier.⁷ It is a fully implantable artificial heart with embedded biomaterials that make the device more biocompatible. In addition, the CARMAT includes valves made from cow heart tissue and has internal pressure sensors. This allows a person to adjust the flow rate in response to increased demand, such as during exercise. This feature distinguishes the CARMAT from other artificial hearts that provides a constant flow rate.

Biological artificial hearts

Synthetic replacement of organs is one of the long-sought dreams of modern medicine. To this end, there are efforts to develop biological artificial hearts in the laboratory. One recent study took the approach of producing a decellurized (empty from cells) scaffold of a mouse heart and recellurized it with human cardiac cells, and showed that lab-grown human heart tissue can beat on its own (about 40-50 beats per minute) in as short as a few weeks (Figure 4).⁸ They took the advantage of induced pluripotent cell (iPS cells) technology to produce multi-potential cardiovascular progenitor (MCP) cells, which could give rise to all three types of cardiac cells found in the heart. This area of research is still in its infancy but in the future, at least, it may provide tools to generate patches of heart tissue to replace damaged parts of the hearts. Given the success of a mouse heart cellurized with human cardiac cells, it’s possible that scientists will also try to decellurize the heart of a monkey or another animal and then cellurize it with human cardiac cells to produce a beating human heart in the laboratory as an alternative source to a full heart transplant.

Another approach to a biological artificial heart is to genetically modify animals in a way that their hearts will be compatible with a human body. Genetic engineering is a rapidly evolving field that one day could provide such tools for scientist to grow necessary organs in monkeys, dogs, or maybe even horses. Genetic modification may overcome tissue rejection issues when they are transplanted into a human body. For instance, one study tested the possibility of a heart transplant from genetically modified pigs into monkeys and showed the applicability of heart transplants between different species.^9,10

Until the development of biological artificial hearts, ventricular assist devices and mechanical artificial hearts seem to the best options. However, artificial heart implants have had various complications, including infections, pneumonia, high fevers, and multiple organ failures with variable survival rates based on the type of device and materials used. Another issue is the necessity of artificial heart to meet requirements of the body in terms of heart flow rate. For instance, the required flow rate of someone walking or exercising is different than someone at rest. In addition, the possibility of mechanical or computerized systems to fail may be a source of distress in patients implanted with artificial hearts or assist devices. This reminds us of that as long as we take care of our heart’s health, we won’t have worry whether its battery could fail – and what a mercy that is.

It is stunning that even with so much need and effort we are still not able to develop something that completely replaces all the functions of a heart. This clearly points that the heart is a marvelous gift granted to us. We are counting on every beat of the heart for our survival, and we should give thanks, with every beat, for what an incredible gift we’ve been given.

References

1. Bui, A. L., Horwich, T. B. & Fonarow, G. C. Epidemiology and risk profile of heart failure. Nat Rev Cardiol 8, 30-41 (2010).

2. What Is a Ventricular Assist Device? NHLBI, NIH. Retrieved from www.nhlbi.nih.gov/health/health-topics/topics/vad/ on 1/2/14.

3. Ventricular assist devices (VADs) Definition – Tests and Procedures – Mayo Clinic. Retrieved from www.mayoclinic.org/tests-procedures/ventricular-assist-devices/basics/definition/PRC-20020578 on 1/2/14.

4. Artificial Hearts. Retrived from www.umasswiki.com/wiki/Artificial_Hearts on 1/2/14.

5. Artificial heart. Retrieved from en.wikipedia.org/wiki/Artificial_heart on 1/2/14.

6. Heart Assist Devices. Texas Heart Institute. Retrieved from http://texasheart.org/Research/Devices/index.cfm on 1/12/14

7. Carmat artificial heart patient in good condition: hospital. Retrieved from www.reuters.com/article/2013/12/30/us-carmat-patient-idUSBRE9BS07O20131230 on 1/2/14.

8. Lu, Tung-Ying, Bo Lin, Jong Kim, Mara Sullivan, Kimimasa Tobita, Guy Salama, and Lei Yang. “Repopulation of decellularized mouse heart with human induced pluripotent stem cell-derived cardiovascular progenitor cells.” Nature communications 4 (2013).

9. Heart of genetically modified pig ‘successfully transplanted into monkey’, South Korea scientists claim. Retrieved from http://www.dailymail.co.uk/news/article-2164964/South-Korea-scientists-successfully-transplant-heart-genetically-modified-pig-monkey.html on 12/1/14

10. Pig to human transplants. Retrieved from http://www.theguardian.com/world/2002/jan/03/qanda.simonjeffery on 12/1/14

The human ear and hearing

The human ear can be divided into several functional sections: the outer ear, the middle ear, the inner ear, and the auditory nerve. Sound goes through a series of changes as it travels through these sections until reaching the brain. The outer ear picks up sound pressure waves, amplifies them and then converts them into mechanical vibrations on the ear drum, which is connected to a series of small bones in the middle ear. These small bones further amplify or diminish the mechanical vibrations in the ear drum and transfer them to the cochlea, a snail-shaped cavity filled with fluid which is located in the inner ear. Change in fluid pressure caused by vibrations within the cochlea lead to changes in the flexible membrane, called the basilar membrane. These changes contain information about the frequency and strength of the sound that has entered the ear. Attached to the basilar membrane are mechanical receptor cells, called hair cells, which are bent according to the deflections of the basilar membrane.The hair cells have hair-like structures. The bending of these hairs assists the release of an electrochemical substance that causes neurons to send electrical signals to the brainstem through the auditory nerve. These signals are in the form of a message (or a code) that the brain understands.

Cochlear Implant (CI) devices

If there is a broken link in any part of the auditory pathway, the brain does not receive any coded signals, and hearing impairment occurs. If a large number of hair cells or auditory neurons in the cochlea have been damaged, then the person is diagnosed as profoundly deaf. The hair cells can be damaged by certain diseases (e.g., meningitis, Meniere’s disease), by congenital disorders, by certain drug treatments, or by other causes. One negative outcome of damaged hair cells is that they can subsequently lead to the degeneration of adjacent auditory neurons. Research has indicated that the most common cause of deafness is the loss of hair cells (>95%) rather than the loss of auditory neurons. This has encouraged scientists to try implanting a device inside the iner ear or cochlea, bypassing the normal hearing mechanism of the ear, to stimulate the remaining auditory neurons directly through electrical signals. These are called Cochlear Implant (CI) devices, which can restore partial hearing in profoundly deaf people . A standard CI system, shown in Figure 1, composes of and performs the following functions: a microphone picks up sound pressure waves and converts these into electrical signals. The signals are sent to the speech processor that is worn by the patient. The speech processor analyzes and encodes these sound signals, sending them back to the external pick-up coil . After passing through a wireless radio link that lies between the external and implanted coils and an implanted electronic devise, coded signals are sent to the implanted array of electrodes in the cochlea to electrically stimulate the remaining auditory neurons , and the brain receives what it interprets to be sound.

Electrical stimulation of the ear, or CI research, can be traced back to the 1800s. The Italian scientist Alessandro Volta used a battery as a research instrument to demonstrate that electric stimulation could result in a number of human sensations . After connecting a 50- volt battery to his ears, he noted that “…at the moment when the circuit was completed, I received a shock in the head, and some moments after I began to hear a sound, or rather noise in the ears, which I cannot well define: it was a kind of crackling with shocks, as if some paste or tenacious matter had been boiling…”. That electric stimulation of the auditory nerve provides hearing sensation in deaf people was reported more than 100 years after Volta . Electric stimulation in two deaf patients resulting in hearing was reported in 1957. These successes resulted in intensive research into helping deaf people hear in the 1960s and 1970s. One of the early successful single-channel CI devices was developed in the early 1970’s (3MCorp/House) and became the first commercially available CI device approved in the United States in 1984. The University of Utah developed a six electrode implant called the Ineraid or the Symbion device in the early 1990s. It was followed by other devices in Europe, the United States, and Australia.

The present status of Cochlear Implants

Today, around 10% of the population in developed countries suffers from hearing impairment. At present, the number of CI users has reached more than 100,000 worldwide, and is still growing rapidly. Functionally, CI has evolved from the single-electrode device that was used as an aid for lip-reading and

sound awareness to a modern, multielectrode device that can allow an average user to talk on the telephone. Even though significant technological progress has been achieved in the last 50 years, there are still many mysteries about the human hearing process and the parts of the ear. Here, we will compare some aspects of the healthy human ear and CI devices, looking to the future. The human ear operates over a range of sound pressures (its dynamic range) which is greater than one million to one (120dB), with as many as 200 discrete steps in the range. In contrast, today’s CI devices typically provide a dynamic range of three to one (10dB) to ten to one (20dB) with 20 discrete steps. This major difference is mainly due to the fact that the human ear is very adaptive in noisy environments, and is able to suppress noisy background, while picking up and processing appropriate sound signals for better perception. CIs do not differentiate between sounds, but amplify all sounds, which results in poor sound perception. Today, a typical multi-channel CI system uses 16 to 24 electrodes implanted in the cochlea with 8 to 22 signal processing channels. A potential shortcoming of having so many electrodes and channels in current CI technology is the electrical interference of electrodes during simultaneous electrode stimulation. These electrical interactions can disrupt the stimulus waveform prior to neural activity and degrade sound perception. The normal ear contains roughly 3,500 inner hair cells in the cochlea that are tuned to different frequencies from 20 to 20,000 Hz. They are connected to about 35,000 auditory nerves. Hair cells work as signal processing channels, yet each of the inner hair cells has also been wired in a sophisticated and little-understood fashion to 10-20 auditory nerve fibers that carry information to the central nervous system. Since they work in the chemical domain, they do not have the gross interference issues of CI electrodes. While good speech understanding has been achieved by users of modern multi-electrode CIs operating in quiet environments with 70–80% sentence recognition, allowing users to talk on the telephone, the CI devices do not discriminate between noise and the meaningful signals, only achieving speech understanding at between 70% and 80%, which falls to 10% or lower in noisy environments. It is a great challenge for CI users to appreciate music. Some CI listeners reported that they can enjoy music and are able to recognize melodies, but most described musicas sounding unpleasant and noisy, and performance could not be increased with current CI technology. CI users have difficulty in identifying differences in frequencies. Typically, they cannot discriminate any frequency difference for frequencies higher than 500 Hz, while the normal ear can hear up to 20,000 Hz with frequency discrimination between 2 to 3Hz at best. This gross difference is related to the issues surrounding signal processing strategies and electrodes of current CI systems. Predicting post-surgical performance based on presurgical conditions and tests of a CI candidate is still a problem for the physician. The cost of surgery is still high; in the United States, for example, a typical cost is between $40,000 and $75,000. Beyond these issues, the moral, cultural and ethical issues related to CIs are very complex. They are still debated, and are an important part of CI development in the world today. The hair cells in the human ear naturally deteriorate and die as we grow older. This process is typically sped up with exposure to loud noise. In common with all mammals, new hair cell generation in human ears stops right after the birth. However, in fish and amphibians, very similar cells are present and reproduce throughout life. Recently, it was found that hair cells of birds are repaired after being damaged by exposure to noise or ototoxic agents. It was also discovered that hair cells in the mammalian vestibular (balance) organ, very similar to those in the hearing system, can regenerate. These findings, along with other advancements in medical fields, lead to long-term research into different aids for hearing- impaired people. Despite the fact that hearing loss is usually permanent, scientists are optimistic that it may eventually be possible to reverse the damage in the ear by repairing or regenerating the sensory hair cells through gene therapy, stem cell transplantation, or ultimately by replacing the human cochlea with an artificial one. Today, Auditory Brainstem Implants are also being tried on humans for direct brainstem stimulation, bypassing the ears and the auditory nerves. Human beings and most animals on earth are born and equipped with a pair of ears for a good reason: having two ears enhances hearing and sound localization. Scientists are examining whether this is also true for deaf children who receive not one, but two CIs.

Conclusion

The sense of hearing is a gift for human beings which they hold dear and are grateful for, as much as for any of the other senses with which they have been equipped. It is important to strive to find cures for all kind of diseases, yet, more important than the cure is prevention of harm to our body and its amazing senses. Here, we have tried to open a small window onto human hearing, to examine how related impairments are being dealt with through cochlear implant (CI) devices, as well as looking at the issues related to these devices and the future directions of research for restoring hearing to deaf people. It is obvious that we have learned much about human hearing and ear in the past century; yet, this may well be just the tip of the iceberg.

References

1. S.U. Ay, F.-G. Zeng, B.J. Sheu, “ Hearing with bionic ear,” IEEE Circuits & Devices Magazine, Vol. 13, No. 3, pp.18-23, May 1997.

2. F.-G. Zeng, “Trends in cochlear implants,” Trends in Amplification, Vol. 8(1), pp.1-34, 2004.

3. A. Volta, “On the electricity excited by mere contact of conducting substances of different kinds,” Royal Soc. Philos.Trans., vol. 90, pp.403 431, 1800.

4. A.M. Andreev, G.V. Gersuni, A.A.Volokhov, “On the electrical excitability of the human ear: On the effect of alternating currents on the affected auditory apparatus,” Journal of Physiology USSR, Vol. 18, pp.250-265, 1935.

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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