Specific nanotubes not toxic in the intestines

With the promising applications that carbon nanotubes might have for our daily lives, there exists the possibility that they may yet be harmful to us in some manner. Scientists at the Hopkins Bloomberg School of Public Health, School of Medicine, Department of Chemistry and Kennedy Krieger Institute have recently found that multiwalled carbon nanotubes, which are toxic, are not taken up by enterocytes, the absorptive cells lining your intestines.

Carbon nanotubes are allotropes, one of the possible physical states, of carbon with a cylindrical nanostructure, and exist in two main varieties: single-walled and multi-walled. Since their inception in the early 1990s, carbon nanotubes have been closely studied for their unique properties and potential applications in nanotechnology, electronics, optics and other fields of materials science.

Due to the range of prospective applications, availability of carbon nanotubes in the market is expected to rapidly increase in the next few years. It will then become likely that carbon nanotubes may be used in occupational settings, or even in medical technology, creating the possibility that humans will be exposed to the material. For this reason, it is important to evaluate the risks of carbon nanotubes for human health.

Information on their toxicity is limited, although previous in vitro studies on lung cells have reported inflammation, increased oxidative stress, deleterious action on a cell's genetic material and cell death. In vivo studies have found increase granulomas, fibrosis, or inflammation in lung cells exposed to carbon nanotubes.

Their toxicity and uptake via the intestine, however, is not a thoroughly explored subject. The aim of this study, published online in the Journal of Toxicology and Environment Health on November 2, was to shed some light on this matter by characterizing interactions between the human intestine and two types of multi-walled carbon nanotubes: pristine (p-MWCNT) and oxidized (o-MWCNT).

The p-MWCNT are hydrophobic and unstable in aqueous solutions, and the researchers hypothesized that this might enhance their ability to interact with cell membranes. The o-MWCNT have a higher surface oxygen content and are more easily dispersible in water.

The study made use of the Caco-2 cell line as a model for human intestinal cells. The Caco-2 cell line is a human intestinal cancer cell line, and when fully differentiated, the cells exist as a single layer and express features similar to those of human intestinal enterocytes.

Researchers obtained the Caco-2 and RAW 264.7 cells from the American Type Culture Collection and exposed them to the multi-walled carbon nanotubes for transmission micron electroscopy analysis. They exposed the cells apically to the oxidized or pristine nanotubes for 24 hours. The researchers decided not to use artificial surfactants to increase dispersion of nanotubes for uniform cell exposure, as this would not be similar to typical intestinal cell exposure. The experiments were repeated in the presence of natural organic matter, which has surfactant-like effects on multi-walled nanotubes.

Cells were prepared and examined on an FEI Technai 12 TWIN or a Hitachi 7600 transmission electron microscope. Cells displayed many of characteristic properties of enterocytes, such as microvilli, basolateral basement membrane and glycogen. The cell monolayers also had developed tight junctions and electrical resistance.

Researchers evaluated cell viability using cytotoxicity assays, which measured lactate dehydrogenase activity. The did not observe significant differences in activity between control Caco-2 cells and those exposed to the nanotubes, both with or without organic matter. No significant cytotoxicity was observed in the RAW 264.7 cells exposed to multi-walled carbon nanotubes using the lactate dehydrogenase activity assay either.

Examination of the cells using transmission electron microscopy also showed that even when the nanotubes were visible at the cell surface, they did not appear to penetrate the cell surface or be engulfed by the cells. The only difference was revealed using ultrastructural analysis: Microvilli colocalized with the nanotubes were slightly damaged or even missing. However, neither oxidized nor pristine nanotubes were observed to be taken up by Caco-2 cells.

The study found little to no evidence of multiwalled carbon nanotubes being taken up by human intestinal enterocytes. On the other hand, the nanotubes were taken up by the macrophage RAW 264.7 line. While the findings of the study do not support the idea that multi-walled carbon nanotubes are toxic to intestinal enterocytes, further research into the effects of these nanotubes on living cells throughout the body is necessary before they can be deemed safe for a wide range of uses in human settings.