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From a theoretical standpoint, researchers had believed for a long time that if manipulation of individual molecules is indeed possible, then materials with optical, electronic, and other characteristics that are not visualized in bulk could also be designed. Such developments can open new frontiers in consumer products, medicine, and electronics.
Just like how a few amino acids are used by cells to assemble proteins with a broad range of functions and characteristics, nanotechnology can help in designing and engineering materials at the molecular level to have particular properties.
“There is plenty of room at the bottom” is a prophetic quip that is often quoted by the late Caltech physicist Richard A. Feynman in 1959.
After five decades, the promise of nanotechnology is turning out to be a reality—both in the laboratory and in certain commercial consumer products spanning from self-cleaning windows to sunscreens. More interesting are the possibilities provided by targeted cancer treatments, in which a tumor can be removed without causing any side effects to the rest of the body.
Environmental scientists are studying the application of engineered nanoscale materials, also known as engineered nanomaterials, to enhance energy efficiency, to desalinate or purify water, or to clean up harmful wastes.
Undeniably, people are beginning to talk about engineered nanomaterials as an entirely new group of materials, and nanotechnology, being the latest industrial revolution, is as important to the 21st century as the information-technology revolution was to the 20th century and the first industrial revolution was to the 19th century.
However, with such a new, groundbreaking technology, questions are raised about consumers’, occupational, and environmental health and safety. If engineered nanomaterials have physical characteristics that are different from their bulk equivalents, would they also present new risks to human health in their development, use, and disposal?
No one knows this as yet. Existing data essentially indicates that “it depends.” However, scientists both in the private and government sectors are quite interested to find out.
First and foremost, toxicity itself can be very handy. In fact, toxicity is very popular for specific applications, like cancer treatments. (It must also be remembered that toxicity usually relies on dose and administration—even table salt in high doses is said to be toxic.)
Second, if toxicity is identified, then packaging and handling procedures can be developed to reduce the risks of unwanted exposure in manufacturing processes, as is often done in industries utilizing dangerous materials. While considering engineered nanomaterials, safe-handling procedures should be different from those currently employed for larger micrometer-sized particulates—specifically significant for nanomanufacturing workers.
Moreover, questions have been raised regarding the safety of engineered nanomaterials in implantable medical devices or in consumer products, or to animals and plants in the environment following disposal.
Third, developers of nanotechnology are heeding a lesson related to the supposed risk from an unrelated high-tech domain—consumer resistance that emerged at the introduction of products and crops utilizing genetically modified organisms (GMOs).
That resistance partly emerged because GMO products were introduced by biotech companies without any major open discussion of valid questions and concerns in the general public. This caused the public to feel that they had to accept the risks posed both to the environment and health while advantages were restricted to bigger profits for large agribusiness. The outcome was extensive suspicion and mistrust from the general public.
In order to prevent a similar outcome (considering that calls and concerns for regulation have already been expressed in certain quarters), nanotech developers are pursuing the so-called “responsible development.”
That precisely includes encouraging early and candid press coverage of work in evaluating both risks and benefits of engineered nanomaterials, and also upfront regulations developed through transparent processes.
Three purposes are discussed in this article: (1) to outline important basics of the biology and physics of engineered nanomaterials (and, for that matter, also incidental and natural nanoparticles), (2) to emphasize issues and resources, and (3) most significantly, to warn about conflicting pitfalls and findings of logic and to propose perceptive questions for sources.
Disagreement on Classification
As per the National Academies, a distinction is made between three forms of nanoscale particles (usually shortened in the literature as “NSPs”)—engineered, incidental, and natural.
Natural nanoparticles exist in the environment (mineral composites, magnetotactic bacteria, lunar dust, volcanic dust, etc.). Incidental nanoparticles, also known as anthropogenic or waste particles, occur because of man-made industrial processes (welding fumes, coal combustion, diesel exhaust, etc.). Both incidental and natural nanoparticles may have regular or irregular shapes. Usually, engineered nanoparticles have regular shapes, like rings, spheres, tubes, etc.
To produce engineered nanomaterials, a large sample is subjected to milling or lithographic etching to achieve nanosized particles (a method usually known as “top-down”). Alternatively, smaller subunits are assembled through chemical synthesis or crystal growth to grow nanoparticles of the required configuration and size (a method often known as “bottom-up”). Since the particular production method may impact human health risk, sources should be asked to specify.
New questions relating to toxicity are directed at engineered nanomaterials. Nevertheless, the literature about incidental and natural nanoparticles is useful because more details are known about these particles (partly due to research ultrafine aerosols, coal dust, welding fumes, and smog), and because information regarding their behavior can help in interpreting the behavior of engineered nanoparticles.
Moreover, according to the National Academies, nanoscale materials—be it natural or engineered—so far appear to fall into four basic categories.
At present, metal oxides like titanium or zinc oxides are the group with the largest number of commercial nanomaterials. These are employed in sunscreens, cosmetics, scratch-resistant coatings, chemical polishing agents, and ceramics.
Nanoclays, naturally occurring plate-like clay particles, are the second major group. These particles harden or strengthen materials or render them flame-retardant. Nanotubes are the third group and these are utilized in coatings to minimize or dissipate static electricity (for example, in hard disk handling trays, in fuel lines, or in automobile bodies to be coated electrostatically).
Quantum dots are the last group and they are utilized in the self-assembly of nanoelectronic structures or in exploratory medicine. However, it must be noted that not all official sources find the same categorization worthwhile.
For instance, the U.S. Environmental Protection Agency separates engineered nanoparticles into metal-based materials (including both quantum dots and metal oxides), carbon-based materials (fullerenes and nanotubes), composites (including nanoclays), and dendrimers (nano-sized polymers constructed from branched units of unspecified chemistry).
Disagreement on Definition
According to most British and U.S. nanotech experts, NSPs are particles smaller than 100 nm—that is, 0.1 μm—in any one dimension. Hence, a thinner-than-100 nm fiber would be regarded as an NSP, even if it measured several micrometers in length.
However, such a definition is not universal. In Japan, particles below 50 nm in one dimension are regarded as genuine NSPs, and particles ranging between 50 and 100 nm are categorized as “ultrafines.” Having said that, there are also certain U.S. agencies that also use the term “ultrafines” to elucidate particles that measure less than 100 nm (albeit usually in the context of only incidental or natural nanoparticles—rarely referring to engineered nanoparticles).
In order to overcome such confusion, various national and international standards bodies, including IEC, ISO, ASTM, and ANSI, are currently discussing the standardization of characterization, terminology, metrology, and approaches to health and safety.
Until the time everything is decided, sources should be asked to explain assumptions and definitions fundamental to their particular work. The distinctions may be important to the biology and physics being reported.
Just how small is 100 nm? It is around one hundred-thousandth the diameter of a single strand of human hair (which measures 50 to 100 μm). More importantly, 1 μm or 1,000 nm is approximately the size of a bacterium, that is, about the limit of what is seen through a majority of light microscopes. On the other hand, 100 nm is roughly the size of a virus, that is, one-tenth the size of a bacterium.
Similar to viruses, NSPs cannot be seen even through the most perfect light microscope. This is because NSPs are smaller than the wavelengths of light (ranging from approximately 700 nm in the red to 400 nm in the violet); some higher-resolution instruments like a scanning electron microscope will be required to image these nanoparticles. One nanometer is around the size of one sugar molecule.
Four Anticipated Generations
Researchers are already talking in terms of generations of engineered nanomaterials. Passive nanostructures, like coatings, individual particles, etc. are first-generation. These kinds of engineered nanomaterials are already used in certain consumer products. Nanostructures are second-generation, performing an active function, such as sensors or transistors, or that respond in an adaptive manner; many nanostructures are currently being developed.
Third-generation engineered nanomaterials may be three-dimensional (3D) systems that can assemble on their own or can be utilized to target drug delivery to certain parts of the body; they are expected to be developed by 2010. Fourth-generation nanomaterials are expected to be molecular structures by design.
A simple thought experiment reveals why nanoparticles have such remarkable surface area per unit volume. For example, a solid cube of a material measuring 1 cm on a side—approximately the size of one sugar cube—has 6 cm2 of surface area, roughly equal to one side of half a stick of gum.
However, if that volume of 1 cm3 were filled with 1 mm-sized cubes on a side, that would be 1,000 mm-sized cubes (10 mm x 10 mm x 10 mm), each having a surface area of 6 mm2. The overall surface area of the 1,000 cubes totals up to 60 cm2—approximately the same as a single side of two-thirds of a 3 x 5 notecard—because the surface areas of all the millimeter cubes even in the interior of the original volume need to be counted.
However, when that 1 cm3 of volume is filled with 1 nm-sized cubes on a side—totaling 1021, each with an area of 6 nm2—their overall surface area comes to 60 million square centimeters or 6,000 m2. To put this in simple terms, 1 cm3 of cubic nanoparticles has an overall surface area that is one-third larger than a football field!
The Surprising Physics of Engineered Nanomaterials
Fundamental electronic, mechanical, chemical, biological, optical, and other properties at the nanoscale may considerably differ from the properties of bulk materials or micrometer-sized particles.
Surface area can be said to be one reason. It counts because a majority of chemical reactions involving solids occur at the surfaces, where chemical bonds are not complete. In a solid material, the surface area of a cubic centimeter is 6 cm2—approximately the same as one side of half a stick of gum. However, the surface area of a cubic centimeter of 1 nm-sized particles in an ultrafine powder is 6,000 m2—exactly one-third larger than a football field.
Hence, collections of NSPs with their massive surface areas can be remarkably reactive (unless a coating is applied), since over one-third of their chemical bonds are at their surfaces. For instance, silver nanoparticles have been shown to be an effective bactericide—motivating a number of companies to develop reusable water-purification filters with the help of nanoscale silver fibers.
It is not known at what size the properties of a material begin to change. Is there a threshold below which the properties suddenly change? Or is it a slow transformation as one continues from large to small. Actually, both could be true.
Quantum-size effects start to considerably change material properties (like magnetic permeability, electrical conductivity, color of fluorescence, transparency, and other properties) each time they dominate thermal effects, which is about 100 nm for many materials. Quantum-size effects for electronic properties increase inversely with decreasing particle size.
However, for certain materials, other distinct characteristics become marked at specific sizes—for instance, gold nanoparticles have considerably increased catalytic properties at 3 nm. An active area of fundamental research is characterizing material effects at varied sizes.
Carbon and certain other elements such as oxygen, tin, and sulfur are found in numerous structural forms, known as allotropes, which have considerably different traits. For instance, pure carbon in crystalline form is found as diamond (very hard), graphite (very soft), and numerous sizes of Buckminsterfullerenes (based on the number of carbon atoms).
Engineered nanomaterials that have identical chemical compositions can have a wide range of shapes (such as planes, rings, fibers, tubes, and spheres). In addition, the physical properties of each one of these shapes may be different. This is because the pattern of molecular bonds varies, although they are made up of the same atoms.
For instance, until 1985, it was understood that pure carbon exists in just two crystalline forms—diamond (the cubic crystal lattice of this crystalline form extends in all three dimensions) or graphite (the hexagonal crystal lattice of this crystalline form lies in a two-dimensional plane).
The same year, hollow cages of 60 carbon atoms shaped like a soccer ball were initially developed in the lab (and also separately discovered in combustion byproducts and in remote stars)—a novel crystalline form of carbon that was so significant that it received the Nobel Prize in Chemistry in 1996.
The latest form, dubbed buckyball or fullerene after the architect Richard Buckminster Fuller, developer of the geodesic dome of the same shape, was quite stable. Since that time, stable fullerenes of 82, 74, and 70 carbon atoms have also been produced.
In a similar way, titanium dioxide (TiO2) has been produced in NSPs of a minimum of two different crystalline structures as well as shapes, each of which is likely to have different toxicities. Even though TiO2 is usually opaque white—used for making white paints—as an engineered nanoparticle, its optical qualities alter, enabling it to become transparent. However, it can still block ultraviolet light effectively, a combination of traits that appeal to sunscreen and cosmetic makers.
Other properties are also equally important. There are other material properties that may be more significant than just size, and these include surface coatings, crystal structure, charge, preference of individual nanoparticles to combine into larger clumps, and residual contamination based on the method of synthesis.
Hazard, Risk, and Other Terms of Art
If NSPs’ physical properties are very different from other bulk materials, then what would be the risk of human exposure and the implications for toxicology? First, some important definitions:
In the fields of toxicology, risk analysis, or occupational health and safety, many day-to-day words have particular meanings.
“Hazard” is an intrinsic characteristic of a material and has the potential to inflict harm. For instance, sulfuric acid is an unsafe material by virtue of its chemistry and nothing can change this fact, short of changing its chemistry to make it something else.
“Risk” refers to the probability of harm occurring; it is a mixture of a hazard with the possibility of exposure and the frequency and magnitude of doses. Risks are different from hazards and can be minimized and controlled—a hazardous material will present low risk, provided the odds of exposure and the frequency and magnitude of the dose that may be received through that exposure are also low.
In other words, if an unlabeled paper cup of concentrated sulfuric acid is left on a kitchen counter, this would pose high risk because the potential dose and the chances of exposure are high; however, if the same acid is correctly labeled and stored in a chemistry laboratory and is accessed only by trained staff, it would evidently pose minimal risk.
“Exposure” can be described as a combination of a substance concentration in a medium multiplied by the duration of contact. For instance, if a dilute sulfuric acid splashes and is rapidly rinsed off, this can be said to be a low-exposure dose because that would only redden the skin area; however, if concentrated sulfuric acid is allowed to sit on the skin, this would mean a high-exposure dose that may lead to serious burns.
“Dose” refers to the amount of a substance entering a biological system and can be determined as the amount in a specific organ (such as the skin, liver, lung, etc.), or as systemic dose—the overall amount taken up by the biological system. More unanswered questions can be found here.
Questions About Dosimetry
Until now, exposure to toxic doses and dust has been determined in terms of mass per unit volume, typically milligrams per cubic meter. Conversely, even very low concentrations of NSPs—whether engineered, natural, or incidental—in the atmosphere represent a remarkable number of particles, as is well recognized from measurements of ultrafine pollutants.
When lab rats were exposed to 100-nm TiO2 particles, it triggered the same amount of pulmonary inflammation as 10-fold greater mass of larger (1–2.5 μm) particles. As a matter of fact, it was observed in a few cases that the amount of inflammation appears to correspond well with the particle surface area of administered NSPs when compared to their mass. Therefore, some toxicologists are currently speculating the fact whether the surface area would serve as a better measure of dose for NSPs than mass.
Until investigators know which parameter counts most, many researchers are beginning to specify both in their papers.
The Surprising Toxicology of Nanoparticles
Size is likely to have another important biological consequence—where nanoparticles end up in the body.
A range of physical factors like mass, gravity, and aerodynamics makes the largest inhalable dust particles to accumulate mainly in the throat and nose. Any toxic effects can manifest at that site (for instance, nasal cancers crusaded by wood dust).
Tinier particles that deposit in the upper airways are expelled by the mucous lining of the bronchial tubes and trachea and by the “mucosociliary escalator,” the fingerlike cilia, which collectively move the particles up into the throat and nose, where they are sneezed, coughed, swallowed, or blown out. Toxic effects normally result from absorption via the gut (for instance, lead poisoning).
The next tiniest particles enter deeper into the alveolar area (where carbon dioxide and oxygen are exchanged in and out of the blood), and these are often cleared when alveolar macrophages (unique monocytic scavenger cells found in the lungs) ingest and carry these particles away.
However, if NSPs in high concentration are inhaled, a large number of particles—specifically if they do not agglomerate—can upset those clearance mechanisms, and they can enter different parts of the respiratory tract. Normally, toxic effects are caused by the killing of the macrophages, which leads to chronic inflammation that affects lung tissue (silicosis and asbestosis are examples).
Inhaled particles, at sizes below 100 nm, start to act more like gas molecules and they can be deposited anywhere in the respiratory tract through diffusion. Just like gases, NSPs—whether engineered, incidental, or natural—can pass through the lungs and then into the bloodstream due to their “nanoscopic” size. The particles, within hours of reaching potentially sensitive sites like the heart, spleen, kidneys, liver, and bone marrow, are then taken up by cells.
As particles become smaller than the size of a cell, they can start to interact with the cell’s molecular machinery. The olfactory bulb in the central nervous system (where aromatic molecules are identified) appears to absorb less-than-10 nm NSPs from the nasal cavity and these particles can then travel along the dendrites and axons to cross the blood-brain barrier.
Inhalation is not the one and only way into the body. When NSPs are ingested, they can end up in the kidneys, liver, and spleen. When NSPs in the range of 50 nm and smaller are touched, they are likely to enter the skin more easily when compared to larger particles (albeit other aspects like surface coatings and charge of the particles are also significant), at times being engulfed by the lymphatic system and localizing in the lymph nodes.
Similarly, the mucosociliary escalator is not the only route out of the body. Evidence indicates that nanoparticles may be expelled through urine. Conversely, excretion routes for nanoparticles (sweat, feces, and urine) may differ based on charge, size, exposure route, chemical composition, surface coating, and a host of other factors.
While all this uptake of NSPs into internal organs could be a major problem for incidental exposure, it is nevertheless exciting for therapeutic exposure, since it indicates that engineered nanomaterials can be utilized to target treatments to particular organs, including the ones that are usually much difficult to reach (for example, the brain).
To date, results provided by different researchers are more suggestive than definitive. Hence, more studies need to be performed on means of uptake, methods of administration, and on the clearance mechanisms of the body.
Moreover, when particles of nanometer size are produced in combustion processes, most of them crash with other particles; these particles are held together by the powerful surface tension and agglomerate into bigger particles. The density of nanometer particles at the point of generation will decide the distribution of particles sizes.
Gaining a deeper understanding of the particle sizes that might be associated with the synthesis of engineered nanoparticles is one of the early priorities for nanotechnology health research. Nevertheless, size is not the only thing that counts for potential toxicity.
Based on the size, other properties, and different routes of entry, nanoscale particles can end up in various parts of the body. Although several translocation and uptake routes have been illustrated, others are still in the theoretical stage and have to be analyzed. Translocation rates are predominantly unknown and so do retention and accumulation in important target sites and their fundamental mechanisms.
These factors, together with possible adverse effects, are mostly dependent on physicochemical properties of the core and surface of NSPs. Quantitative as well as qualitative changes in NSP biokinetics in a compromised or diseased organism also need to be factored.
The shapes of NSPs provide them special characteristic, but under the Toxic Substances Control Act (TCSA), engineered nanoparticles may not be considered as novel compounds unless they have a special composition. For instance, TiO2 nanoparticles are managed in the same manner with regard to regulation as bulk TiO2, although both forms have different characteristics.
A few studies have demonstrated that materials having the same composition but different sizes and shapes tend to have different toxicities, and also not with a linear association as one might anticipate.
For instance, there was one study that demonstrated that 50 to 130 nm-sized nanoparticles across quartz-crystalline silica (a toxic substance) were found to be less toxic than 1.6-μm particles, but that 10-nm particles were, in fact, more toxic. However, toxicity is also affected by dose and route of entry into the body.
Bulk carbon present in macroscopic components is believed to be medically valuable because it is neither toxic nor rejected by the body. However, some investigators have noticed from experiments that carbon nanotubes (specifically multi-walled carbon nanotubes or single-walled carbon nanotubes) appear to be more toxic when compared to other carbon forms.
Other researchers have questioned that claim because the nanotubes that were used contained trace impurities of solvents or iron. Undeniably, some studies indicate that other forms of nanoscale carbon like C60 fullerenes could prevent toxicity by acting as antioxidants.
The purity of the engineered nanomaterials is possibly at stake here, or in analogous debates over other kinds of engineered nanomaterials. People at this stage absolutely lack repeatable control when it comes to manufacturing processes; today, nanotech production stands more or less where the development of indium gallium arsenide phosphide (InGaAsP) semiconductor lasers was in the early to mid-1980s—comparatively low yield of consistent production.
Hence, buckyball products from a single supplier are not essentially similar to those from another; hence, toxicity is likely to vary.
Careful questions should be asked to sources about the manufacture of particles, their size, experimental techniques, whether they defined the materials themselves during the time when they conducted the experiment or simply assumed the statements made by the supplier, and the comparison of their outcomes with other studies.
With more studies ongoing, there are new and more publications focusing on nanotoxicology. Until more things become certain, the National Institute for Occupational Safety and Health (NIOSH) has declared research requirements as well as interim guidelines for safeguarding workers in the nanotech sectors in its report titled “Approaches to Safe Nanotechnology.”