The Bioeffects of Carbon Nanotubes (CNTs)

Since I was in Biarritz, I sneaked in to hear some talks at the annual world conference on all things related to carbon, Carbon 2009, being held here this week. As you may know, a hot topic these days in the field of carbon chemistry is carbon nanotubes (CNTs). Unsurprisingly, a large proportion of the conference papers centered on the synthesis of CNTs and their (potential) applications. As a non-chemist (I’m more into physics and biology), I was more interested in the few presentations that addressed the topic of biological interactions and possible bioffects of carbon nanotubes. As is the case for the whole budding domain of nanotechnologies, bioeffects are my prime concern. Unlike most, I’m not concerned at all about the H1N1 virus (there’s Tamiflu/Relenza, and we can always make a vaccine), nor genetically modified organisms (there is no human safety issue whatsoever, it’s a big misunderstanding; the only issue is the possible impact on plant biodiversity in the wild). No, what really scares me is the possible dangers of all things “nano”. Nano-products/materials/devices are all very new and, given their infinitesimal size (visible only by electron microscopy), all too easily absorbed by our bodies, potentially right through cell membranes. Because of the infancy of the work done at this size regime, there has not been any long-term studies on in vivo effects on humans. For carbon nanotubes, researchers are only starting now to study potential effects in vitro, and generally still only chemically, without any cells in the mix.

At Carbon 2009, I appreciated one talk in particular given by Xinyuan Liu (from the Laboratory for Environmental and Health Nanoscience at Brown University) which outlined the inconsistencies in CNT bioeffects studies published to date. She also outlined the three potential categories of risk related to carbon nanotubes:

1. Metal impurities left after the synthesis of carbon nanotubes:
   It turns out that many of the bioeffects reported to date are likely attributable to impurities that remain in the sample after the complex chemical synthesis processes used to generate CNTs. In particular, metal catalysts such as nickel or ytirium. For example, ytirium has been shown to inhibit calcium ion channels (important in the transmission of signals from one nerve to the next, or from nerve to muscle).
2. The effect of carbon nanotubes on reactive oxygen species (ROS):
   CNTs apparently deplete natural antioxidants (in simulated biological environments) in a dose-dependent fashion. For example, folic acid adheres to the hydrophobic surface of CNTs.
3. The biophysical implications of the geometry of carbon nanotubes:
   Given the long length of many CNTs, they are too large for single macrophages to absorb. Eventually, when multiple macrophages would be recruited to this hopeless task, there would be the potential to develop granulomas. This is perhaps why some researchers have reported asbestos-like pathology related to CNTs (as the problem with asbestos is analogous). The other biophysical implication of carbon nanotubes is their biopersistence. It is likely that the different types of CNTs vary in persistence, so more studies need to be done. For now, we must consider the possibility that carbon nanotubes are bioaccumulative (like heavy metals).

What worries me is not the carbon nanotubes that will be chemically integrated into other materials (and never to be near humans in their raw form), such as improved airplane wings, or those that will form the basis of new medical treatments, such as drug-delivering mechanisms (as these will have to go through the typical battery of clinical trials). My concern is the carbon nanotubes that were not purposely intended for human exposure, such as those in an industrial manufacturing process. If there ever was an accidental release of CNTs in the environment, not only would we not know their long-term effects (on humans, or on the biosphere), we would be hard-pressed to find and measure their levels, let alone “clean up” the invisible mess.