In addition to in vivo tests on animal models, tests on cells are carried out in vitro for screenings or assessment of mechanistic issues concerning the effects of carbon nanotubes (CNT). To do so, the carbon nanotubes are mostly suspended in solution and are added to the culture medium for exposure of the cell.

Various in vitro studies of that kind have been performed during the recent years. However, the different types of specimen and test methods have been preventing final definite results as to the “toxicity” of CNT. Some conclusions can be drawn from the studies published nevertheless: It was found, for example, that bundling (agglomeration) has an influence on the biological effects of carbon nanotubes ^[5]. In spite of this, there are still doubts whether long or agglomerated CNT can be inhaled at all.
As-produced carbon nanotubes often contain impurities such as catalytic metals (for example iron, nickel, cobalt or molybdenum), amorphous carbon (similar to that occurring in particulate matter) or other additives. Acute effects have been shown to be mainly due to such contaminations ^[1,2,3]. For possible biomedical applications, CNT are often chemically modified with different functional groups causing the nanotubes to become water-soluble and lose their acute-toxicity potential ^[4].
Many of the reported toxic effects of CNT are due to extremely high experimental doses. These could not even occur in the case of accidents or disturbances. To enhance the quality of carbon nanotube studies, characterization of the tested materials should be improved to ensure a better assessment of the causes of the different findings based on their physical-chemical properties. Standardized instruction sheets and internationally accepted reference materials would improve considerably the quality and comparability of in vitro studies.

Literature

1. Donaldson, K et al (2006), Toxicol. Sci. 92, 5-22
   
2. Kagan, VE et al (2006), Toxicol. Lett. 165, 88-100
   
3. Pulskamp, K et al (2007), Toxicol. Lett. 168, 58-74
   
4. Sayes, C. M et al (2006), Toxicol. Lett.161, 135-42
   
5. Wick, P et al (2007), Toxicol. Lett. 168, 121-131
6. Wörle-Knirsch, JM et al (2006) Nano Lett. 6(6), 1261-1268

In view of possible harmful effects, but also regarding potential medical applications, studies of the distribution of carbon nanotubes in the body (in vivo) are of great interest ^[1].
While after injection into the abdominal cavity of the test animal, carbon nanotubes were mainly detected in the kidneys and, in small quantities, in the spleen and lung and were found to be mostly excreted via the urine^[6]. No carbon nanotubes could be detected in the liver and spleen after injection into the veins ^[5]. After intravenous administration, the main part (80 %) of a certain type of water-soluble carbon nanotubes could be detected in the liver where they were mainly taken up by macrophages (scavenger cells). In addition, smaller parts of these carbon nanotubes were found in the lung, the spleen, and the heart. After infusion through the trachea, the same suspension remained almost completely in the lung but was reduced down to 20% within 28 days. Direct administration of the same suspension containing carbon nanotubes lead to direct excretion via the stool of a large part of the nanoparticles within 12 hours. These nanoparticles were found not to have infiltrated the bloodstream. No matter the conditions of administration, acute toxicity was NOT proved in the study ^[2]. After injection into the bloodstream, unmodified carbon nanoparticles were mainly detected in the liver, spleen, and lung, and in the brain in smaller quantities ^[7]. As a matter of fact, not all carbon nanotubes are identical. A large variety of functionalizations, such as combinations with dyes for visualization, change their surface properties and, thus, their behavior (distribution and retention time) in the body ^[6]. Moreover, the purity of individual preparations, the concentration used for administration, and the status of agglomeration (i.e. the degree of agglutination and consequent formation of larger particles) may vary ^[4].
It is due to the immense diverseness in carbon nanotubes as well as to manifest differences in the performance of tests and analyses that consistent results regarding their behavior after uptake in the body could not be obtained so far. Each uptake path should be investigated for each type of carbon nanoparticles (functionalized or unmodified ones) to be able to assess the risks and distribution in the body for each individual case.

Literature

1. Bianco, A et al 2005, Curr. Opin. Chem. Biol. 9, 1-6
   
2. Deng, X et al 2007, Carbon 45, 1419-1424
   
3. Pantarotto, D et al, 2004 Angew. Chem. Int. Edition. 43, 5243-5246
   
4. Qu, G et al 2009, Carbon 47, 2060-2069
   
5. Singh, R et al, 2006 PNAS 103 (9), 3357-3362.
   
6. Wang, H et al, 2004, J. Nanosci. Nanotechnol. 4, 1019-1024
   
7. Yang, S et al, 2007, J. Phys. Chem. C 111, 17761-17764.

The measurement of actual environmental concentrations or expected nanoparticle concentrations is important to assess, if there is a risk for organisms at all. If for example toxic effects only occur at very high concentrations and at the same time environmental concentrations are very low, no hazard is to be expected.
Hence, the distribution in the environment was simulated using computer programs and afterwards concentrations were predicted (PEC-value – predicted environmental concentration). These calculations were based on the production volumes of CNT.
Because CNT are mainly used as composites, only a very small amount reaches air or water directly, whereas the bigger part is disposed in landfills or waste incineration plants. When comparing the calculated environmental concentrations with known hazardous concentrations for environmental organisms, currently there is no risk to be expected from CNT for environmental organisms ^[1,2] . However, in the future these calculated values have to be confirmed by actual values or have to be adjusted to increasing production volumes, respectively.

Literature: 

1. Gottschalk, F. et al. (2009), Environ Sci & Technol 43, 9216-9222.
2. Mueller, N. C.and Nowack, B. (2008), Environ Sci & Technol 42, 4447.