Nanomaterial Safety

Abstract

Cancer research employing nanotechnology demonstrates that even slight variations in nanoparticles can drastically affect health risks.

As nanotech components–materials roughly 1-100 nanometers in dimension–are engineered into household, industrial, and medical products, scientists and the public need to better understand their interactions with biological systems and with the environment. The small size, high reactivity, and unique properties of nano-materials–the same properties that drive interest in their commercial applications–have raised concern about their implications for health and safety. Though the overwhelming majority of engineered nanoscaled particles (nanoparticles) have been shown to be less toxic than household cleaning products (ranging from slightly toxic to moderately toxic on the Hodge and Sterner scale), most of this data comes from single-exposure acute toxicity studies. The chronic risks associated with repeat-exposure over extended periods of time are less well understood.

Understanding the long-term impact of these materials is important because researchers are developing nanotechnologies for a range of uses that put them in direct contact with the human body and the environment. For instance, nearly all modern sunscreens contain nanoscale titanium dioxide or zinc oxide, which are colorless and scatter light more efficiently than larger particles of the same material; nanoscale silver, which has antimicrobial properties, appears in commercial products ranging from wound dressings to the liner in your washing machine; nanoscale solar photovoltaic cells efficiently convert absorbed light into electrical energy by layering and using a wider spectrum of sunlight; and membranes made of nests of “superhydrophobic” nanowires can absorb pollutants like oil or heavy metals without absorbing water, making them ideal for use in environmental remediation.

Most germane to the National Cancer Institute's Nanotechnology Characterization Laboratory (NCL), nanotechnology is enabling breakthroughs in medical applications. Nanotech-based transducers (devices that convert one type of energy into another) make use of nanocantilever, nanowire, or carbon nanotube sensors that allow for near-single-molecule detection of disease markers, bacteria, viruses, or chemical toxins. And scientists are employing nanoparticles as drug carriers for chemotherapeutics, delivering strong medicines to tumors while reducing adverse effects on healthy tissue.

Attaching drugs that are toxic to cells to a nanoparticle reduces systemic exposure and concentrates the drug in tumor tissue, increasing efficacy and decreasing toxicity compared to legacy chemotherapeutic drugs. To improve tumor imaging, nanoparticles made of materials such as metal oxides can enhance contrast in MRI, CT, or PET scans. Nanoparticles can also be coated with antibodies or ligands to actively target tumors, in contrast with “passive” targeting where the size and surface properties of nanoparticles cause them to accumulate in tumor tissue. In total more than 50 ongoing clinical trials for cancer treatments involve nanoparticles.

Much of the discussion about nanorisks to human health and the environment is focused on carbon nanotubes and other carbon-based nanoparticles, such as fullerenes. In 2003, in vitro studies indicated that single-walled nanotubes were toxic to cells.1 This toxicity was later shown to be caused by an impurity in the synthesis process, and when tested again, the purified carbon nanotubes did not have the same effect.2

Between 2003 and 2006, a number of studies reported that instilling a liquid solution of nanotubes directly into the lungs of mice caused inflammation.3 These results raised concerns that nanotubes might be an inhalation risk in occupational settings where nanotubes are manufactured. Then two new studies with seemingly contradictory results appeared in back-to-back 2008 issues of Nature Nanotechnology.4 The first showed that mice injected intravenously with carbon nanotubes showed no signs of chronic or acute toxicity. The second study seemed to show that mice injected with nanotubes (this time in their abdominal cavities) developed symptoms similar to asbestos exposure. The mainstream media only published the results of the latter paper, hyping the potential health risks associated with nanotubes. Two more recent inhalation studies also reported conflicting results: one showed toxicity, the other found none.5 This ambiguity further complicates scientists' abilities to assess nanomaterials safety.

To clarify this uncertainty, labs around the country, including the NCL, have undertaken a series of experiments. The preliminary results of NCL experiments point to a surprising conclusion: Unlike traditional “small molecules” and particles that have been used in consumer products and industrial processes for decades, every nanotech-based product is unique. A seemingly small change in the surface chemistry, coating, synthesis, or formulation of a nanoparticle can dramatically alter in vivo results. For example, NCL conducted an animal study to determine the safety of a polymer-coated gold nanoparticle intended for cancer therapy. We injected laboratory rats with the particles and found that the animals developed lung lesions (a manifestation of toxicity) after two weeks. These results contradicted the manufacturer's previous studies, but when we repeated the same experiment with a freshly synthesized batch of sample, the rats had no lesions and showed no significant toxic response. A fairly rigorous battery of testing found the two batches of nanoparticles to be essentially indistinguishable–they were produced in the same synthetic process, had equivalent size and surface charge, and appeared similar under electron microscopy. Finally, we looked at the particles' polymer coatings. A sample of the fresh batch had a higher molar ratio of polymer on its surface than the previous batch, and some of the polymer in the earlier batch had been displaced by ions while kept in its storage solution. This small difference in the concentration of coating caused a large disparity in the in vivo results.

Unlike traditional “small molecules” and particles that have been used in consumer products and industrial processes for decades, every nanotech-based product is unique. A seemingly small change in the surface chemistry, coating, synthesis, or formulation of a nanoparticle can dramatically alter in vivo results.

If such small changes can so drastically influence in vivo results, researchers need to better understand which properties are important for making nanoparticles biocompatible and to develop protocols for engineering around toxicities and/or predictive tools (i.e. modeling and simulation) for the nanotech industry. In its 3 years of operation, the NCL has assessed more than 140 types of nano-particles, including liposomes, dendrimers and other polymers, quantum dots, gold colloids, metal oxides, and fullerenes. Out of the many physicochemical properties that define nanoparticles, we have found that size, surface charge, and tendency to repel water (hydrophobicity) are the attributes that most influence the bio-compatibility of nanoparticles intended for medical applications.

For example, the degradation of the polymer coating on the gold nanoparticle cited above caused the first sample batch to be more hydrophobic (less soluble) than the freshly made particles. This, in turn, affected their tendency to bind blood proteins and ultimately determined their accumulation in, and toxicity to, the lungs. As a rule of thumb, we find that hydrophobic nanoparticles have very short half-lives in vivo, on the order of seconds to minutes. The attacking white blood cells in an immune system quickly engulf hydrophobic particles, removing them from circulation. This process is not always detrimental, however, since some treatments that incorporate nanoparticles rely on these white blood cells to carry the particles to lymph nodes or regions of inflammation. Bayer HealthCare Pharmaceutical's Feridex, an agent that increases the contrast displayed by an MRI of the liver, employs this type of passive targeting.

With respect to size, the majority of experimental data for nano-sized particles indicates that they are not inherently dangerous or toxic.6 The data, including that generated by NCL, suggest that the ability of the lung, gastrointestinal tract, and skin to impede larger molecules and particles remains effective against systemic exposure to most nanosized particles. For example, studies conducted in national labs, including ours, have shown that nanosized titanium dioxide and polystyrene do not penetrate normal human skin.7 Very small nanoparticles may be absorbed by the gut if ingested, but they are confined to the lymphoid tissue. When deliberately introduced into the bloodstream, the size of nanoparticles seems to influence the rate and route of clearance from the body. For example, particles less than about 8 nanometers in dimension can be excreted by the kidneys and those larger than about 200 nanometers get trapped by the liver and spleen.8 By identifying these limits, researchers have been able to develop nanoparticles between a few dozen to a few hundred nanometers that passively accumulate in cancerous tumors. This phenomenon, called the enhanced permeation and retention effect, is due to the “leaky” vasculature and decreased drainage in tumor tissues.

Charge is also important to nanoparticle biocompatibility. Our lab has conducted a series of structure activity studies, where we synthesized nanoparticles with relatively constant size and molecular weight–but with a wide range in zeta potential (the difference between the charge on the nanoparticle's hydration layer and that of the bulk solvent). When evaluating the effect of these nano-particles using in vitro assays, we found that particles with positive charges are more toxic to cells than neutral or negatively charged nanoparticles. Positively charged particles also tend to rupture red blood cells and induce clotting in proportion to increasing surface charge. Animal studies in other labs have shown that this trend holds true for in vivo models as well. The NCL is working closely with government agencies, such as the Food and Drug Administration and the National Institutes of Standards and Technology to develop methods and identify trends such as these, which can be used to evaluate nanoparticle efficacy and toxicity.

The potential of nanotechnology to increase material strength, to reduce pollution and energy consumption, and to help diagnose and treat disease, will fall short if these types of evaluation methods are not developed. To responsibly develop and commercialize products containing nanomaterials requires that scientists also measure and monitor the environmental, health, and safety ramifications of these applications. No two nanomaterials are exactly alike, and the safety or toxicity of one particular nanoparticle type cannot be extrapolated to other particles. Rather, the biocompatibility of these materials depend on their individual physical traits.

Reading List

For more information on the ideas and issues discussed in this essay, the Bulletin recommends reading:

Nanotechnology Safety Concerns Revisited Stephan T. Stern and Scott E. McNeil, Toxicological Sciences, vol. 101, no. 1. An article summarizing published findings related to nanotech safety risks, including the potential for nanomaterial exposure, the relative hazard nanomaterials pose to humans and the environment, and the present deficits in our understanding of risk.

Nanotechnology for Cancer Therapy Mansoor M. Amiji (2006) A general text focused on promising nanoscience and nanotechnology strategies for imaging and treating cancer.

How Meaningful are the Results of Nanotoxicity Studies in the Absence of Adequate Material Characterization? David B. Warheit, Toxicological Sciences, vol. 101, no. 2. An article by a well-respected toxicologist emphasizing the importance of adequately characterizing nanomaterials prior to toxicological experimentation.

The National Nanotechnology Initiative Strategy for Nanotech-nology-Related Environmental, Health, and Safety Research www.nano.gov/NNI_EHS_Research_Strategy.pdf. This document describes the National Nanotechnology Initiative's strategy for addressing priority research on environmental, health, and safety aspects of nanomaterials.