Biomedical Engineering Reference
In-Depth Information
the lungs and dermis. The HiPco process, for instance, forms aerosols that easily
break down into smaller particles, leading to higher airborne concentrations.
In contrast, laser vaporization forms compact aerosols that are more difficult to
break down into smaller particles. 85,133 Another hazard of CNTs is the accumula-
tion along the food chain. Oberdorster et al. studied the possible accumulation of
SWNTs ingested by round worms ( Caenorhabditis elegans ). In this model, SWNTs
were shown to move through the digestive tract and were not absorbed by the
animals. 134 Another study, in compliance with these results, reported detection
of unsolubilized SWNTs, ingested by fish, in fecal material. 135 However, even
though these studies indicate that SWNTs are not absorbed by these animals,
there is still a possibility of CNTs moving up the food chain.
4.4.3.1 In Vitro Studies on Carbon Nanotubes
Initial studies examining the toxicity of CNTs involved mainly unpurified and
nonfunctionalized CNTs. One of the first studies reported exposure of human
epidermal keratinocytes to pristine SWNTs to result in accelerated oxidative
stress, as indicated by the formation of free radicals and accumulation of per-
oxidative products, depletion of total antioxidant reserves, an increase of lipid
peroxidation products, and loss of cell viability. 136 In another study, addition
of unpurified MWNTs to dermal keratinocytes resulted in reduced cell viabil-
ity and an undesirable increase in interleukin-8 (IL-8) up to six times. 137 When
added to skin fibroblasts, cell cycle arrest was induced, and apoptosis and
necrosis were increased. 138 Also, nonfunctionalized aggregated SWNTs and
(raw) aggregated MWNTs incubated with murine lung alveolar macrophages
resulted in severe toxic effects, even comparable with those of asbestos, within
a short term of 48 h. 139 According to studies by other research groups, this tox-
icity, however, was indicated to be mainly resulting from the metal catalysts
present in the used unpurified CNTs. Metal catalysts are known to lead to the
formation of hazardous reactive oxygen species. 137,138,140 -142
In time, knowledge of not only purification but also of functionaliza-
tion of CNTs increased. Owing to their extreme hydrophobic nature, CNTs
aggregate almost immediately, which makes them difficult to handle and
increases their toxicity on cells. 143,144 Therefore, CNT functionalization, by
surface treatments and coatings, is being investigated with great interest. In
a biological model system, it was shown that well-dispersed SWNTs were
less cytotoxic than micrometer-sized aggregates. 144 However, the effect of
functionalization on cell behavior is difficult to predict, since these treat-
ments do not only alter their solubility, but also their cell interaction. In most
cases, as reported in several studies, functionalization increases the solu-
bility of CNTs, and therefore decreases their toxicity, as mainly the hydro-
phobic character is associated with their toxicity. 145 For instance, addition to
neurons of functionalized MWNTs with an increased hydrophilic character
increased the number of branches grown per neuron, compared with pristine
MWNTs. 146 A study by Nimmagadda et al. 147 hypothesized that the toxicity
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