Despite many studies, no final judgement can be made on the toxicity of carbon nanotubes, as the observed effects are discussed very controversially in the literature.
Studies on Living Organisms – in vivo
In view of possible adverse health effects as well as with regard to potential medical applications, studies of the distribution of carbon nanotubes inside the body (in vivo) is of great interest .
Different suspensions of carbon nanotubes. © Reprinted & adapted from Toxicology Letters, 168(2), Wick P., Manser P., Limbach L.K., Dettlaff-Weglikowska U., Krumeich F., Roth S., Stark W.J., Bruinink A., The degree and kind of agglomeration affect carbon nanotube cytotoxicity, pp 121-131, Copyright (2007), with permission from Elsevier.
Carbon nanotubes (CNTs) are usually delivered into the lungs of laboratory animals by inhalation, instillation or aspiration. No systemic toxicity of multi-walled carbon nanotubes (MWCNTs) could be observed in an inhalation study of several weeks with rats . In case of installation and aspiration, the nanoparticles are first suspended in a liquid and then administered as droplets either directly into the lungs (instillation) or applied dropwise on the tongue followed by an inhalation step (aspiration).
However, the physiological relevance of both methods is questionable one the one hand because the nanoparticles aggregate within the liquid and on the other hand because the complete dosage is applied at once. A final judgement concerning the risks of exposure to carbon nanotubes under realistic workplace conditions is not possible now due to the limited amount of inhalation studies from independent laboratories.
In different studies, carbon nanotubes were injected either in the abdominal cavity, the veins or into the lungs of laboratory animals [3-5]. In these cases, CNTs were mostly found in kidneys, spleen and liver or in the lungs. Once within the organs, they are frequently internalised by residential macrophages.
When carbon nanotubes were administered through the stomach, they were excreted within 12 hours without getting into the blood stream. Acute toxicity could NOT be detected in this study regardless of the application method . However, up to now there are only few studies available regarding the influence of carbon nanotubes on internal organs .
Furthermore, carbon nanotubes are a very heterogeneous class of materials. Through various functionalisations, e.g. binding of a fluorescence dye for visualisation in microscopy, the surface properties of CNTs are altered which in turn also changes their behaviour in the body (distribution and retention time) . It is possible to "control" the risk potential of carbon nanotubes by specific surface modification .
But due to the enormous variation of the material (purity, surface properties, applied dose and their agglomeration state, i.e. to which extent single carbon nanotubes stick together and form bigger particles) also the outcome of these studies is differing .
Due to the huge heterogeneity of carbon nanotubes and the differences in test designs and analytics, it remains unclear, how CNTs behave after their uptake into the body. The standardisation of test protocols in the future is absolutely necessary to solve this problem.
- Bianco, A et al. (2005), Curr Opin Chem Biol, 9(6): 674-679.
- Pauluhn, J (2010), Toxicol Sci, 113(1): 226-242.
- Singh, R et al. (2006), PNAS, 103(9): 3357-3362.
- Qu, GB et al. (2009), Carbon, 47(8): 2060-2069.
- Yang, ST et al. (2007), J Phys Chem C, 111(48): 17761-17764.
- Deng, X et al. (2007), Carbon, 45(7): 1419-1424.
- Johnston, HJ et al. (2010), Nanotoxicology, 4(2): 207-246.
- Wang, H et al. (2004), J Nanosci Nanotechnol, 4(8): 1019-1024.
Studies Outside the Organism – in vitro
Brightfield image of A549 cells treated with single-walled carbon nanotubes (SWCNTs, highlighted with white arrows). © Karin Pulskamp / Forschungszentrum Karlsruhe, 2009.
As an alternative to in vivo animal tests, the effects of carbon nanotubes (CNTs) are often investigated in cell culture systems (in vitro). Here the CNTs are transferred into a liquid (suspension), then added to the cell culture medium thus bringing the cells into contact with the CNTs (exposure). Despite the plethora of such performed in vitro studies over the last decade, it is not possible to make a final statement on the toxicity of carbon nanotubes due to the differences of the tested CNT materials and applied test methods.
However, some conclusions can be drawn from the published work: bundling (agglomeration) of the carbon nanotubes potentially increases their toxicity . Nevertheless, inhaling of such long or agglomerated CNTs is very limited and due to their large size, they cannot reach the deeper regions of the lung. Furthermore, many of the reported toxic events of carbon nanotubes can be attributed to the high-applied doses, which in reality will not even be achieved with malfunctions during the production.
TEM picture of carbon nanotubes (CNTs). © Used with the permission of Woerle-Knirsch, JM et al. (2006). Oops they did it again! Carbon nanotubes hoax scientists in viability assays. Nano Lett, 6(6): 1261-12. Copyright (2013) © American Chemical Society.
Even after production, CNTs are often contaminated with various residues like catalyst metals (in most cases iron, but also nickel, cobalt or molybdenum), amorphous carbon (similar to its appearance in fine dust) or other additives. It has been proven that the observed acute effects were mainly caused by the contaminants [2,3,4].
Carbon nanotubes are considered as very stable and biopersistent. However, experiments demonstrated that an enzymatic degradation of functionalised and non-functionalised single-walled CNTs is possible [5,6]. Likewise, scavenger cells (macrophages) were able to degrade single-walled carbon nanotubes in vitro.
Using carbon nanotubes in medical applications, e.g. as vehicle for active substances or drugs, requires often some chemical modifications (cross cutting – coatings for nanomaterials). Such modified carbon nanotubes are then e.g. water-soluble and therefore cause no acute toxicity [1,8].
In order to deduce reliable conclusions on the toxicity of carbon nanotubes it is necessary to not only thoroughly characterise the materials but also to apply environmentally relevant doses and to use adjusted pharmaceutical dosage forms together with realistic and suitable testing methods .
For the future it is essential to develop suitable models, which on the one hand will analyse the results of comparative in vitro and in vivo studies and which on the other hand will help to measure and characterise the genetic answer (switching-on and –off of genes or genetic programmes) of cells or even the whole organism. With these models in hand, it should be possible to predict the toxicity of a substance or a nanomaterial . Equally, the usage of standardised operating procedures (SOPs) together with internationally accepted reference materials will significantly improve the quality and comparability of in vitro studies.
- Kaiser, JP et al. (2011), Curr Med Chem, 18(14): 2115-2128.
- Donaldson, K et al. (2006), Toxicol Sci, 92(1): 5-22.
- Kagan, VE et al. (2006), Toxicol Lett, 165(1): 88-100.
- Pulskamp, K et al. (2007), Toxicol Lett, 168(1): 58-74.
- Allen, BL et al. (2008), Nano Lett, 8(11): 3899-3903.
- Allen, BL et al. (2009), J Am Chem Soc, 131(47): 17194-17205.
- Kagan, VE et al. (2010), Nat Nanotechnol, 5(5): 354-359.
- Sayes, CM et al. (2006), Toxicol Lett, 161(2): 135-142.
- Wick, P et al. (2011), ChemSusChem, 4(7): 905-911.
- Snyder-Talkington, BN et al. (2012), J Toxicol Environ Health B Crit Rev, 15(7): 468-492.
- Woerle-Knirsch, JM et al. (2006), Nano Lett, 6(6): 1261-1268.