We need to understand the properties of the materials and how these are connected to the biological effects in order to make them both useful and safe. Indeed, the final common pathways of cellular or organ damage ( e.g., oxidative stress, inflammation, carcinogenesis) may be conserved for different (nano)materials, (12) but it is nevertheless of considerable importance to understand how those pathways are triggered: in essence, toxicology seeks to understand the structure–activity relationship of a chemical or a material. (11) This is not to say that the biological or toxicological effects of a novel material are necessarily “novel”. Thus, while it is evident that important lessons can be learned from previous studies of other engineered nanomaterials, it is equally important to avoid extrapolation from the study of one class of nanomaterials to another-if we acknowledge that new materials have new and useful properties, then we must also accept that such new materials could pose new or unanticipated risks. In fact, carbon nanotubes, if appropriately purified and surface-modified, hold promising prospects in nanomedicine, for instance, for drug or gene delivery and/or imaging.
(6) However, other members of the same family of materials have been found to be nontoxic (7,8) and may even undergo degradation, (9) suggesting that not all carbon nanotubes are asbestos-like (reviewed in ref (10)). (5) We now understand that carbon nanotubes adhere to a certain extent to the so-called pathogenic fiber paradigm, and that some types of multiwalled carbon nanotubes may be considered as potentially carcinogenic to humans. Ten years ago, carbon nanotubes were suggested to display “asbestos-like” pathogenicity in an animal model in the sense that long and rigid, but not short or tangled, carbon nanotubes elicited granuloma formation and inflammation following intraperitoneal injection in mice.
The toxicological evaluation of carbon nanotubes is a case in point. Material characterization, in turn, is a key element of hazard assessment.
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