Effect of chemical composition and state of the surface on the toxic response to high aspect ratio nanomaterials

Our plain-English summary of Effect of chemical composition and state of the surface on the toxic response to high aspect ratio nanomaterials, by B. Fubini, I. Fenoglio, M. Tomatis, et al (Nanomedicine, 2011).

The study

• Authors: B. Fubini, I. Fenoglio, M. Tomatis, F. Turci
• Journal: Nanomedicine
• Published: 2011;6(5):899-920
• Type of article: Review
• Cited by: 53 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Nanomaterials often act as a double sword. On the one hand they offer exceptional new properties, but on the other hand show signs of toxicity. High aspect ratio nanomaterials (HARNs) cause more concern than isometric nanoparticles owing to their physical similarity with asbestos. Many compounds may be prepared in fibrous shape with nano-sized diameter differing one from the other in various ways. This article reports a comparative picture of the chemical features and related toxic responses to a variety of HARNs, namely carbon nanotubes, asbestos, carbon nanofibers, oxide and metal wires and rods. In spite of similarities in form, durability and several biological responses elicited in vitro and in vivo, carbon nanotubes - opposite to asbestos - quench radicals, are hydrophobic and may be fully purified from metal impurities. Most of the other HARNs produced so far are metal or metal oxide compounds, less biopersistent than carbon nanotubes.

In plain English

This review compared the chemistry and reported toxic effects of fiber-shaped nanomaterials (carbon nanotubes, carbon nanofibers, and metal and metal oxide nanowires and nanorods) with asbestos. The authors said carbon nanotubes resemble asbestos in shape and durability but differ chemically. Asbestos attracts water and generates free radicals. Carbon nanotubes repel water unless oxidized or functionalized, and studies reported them both producing and scavenging reactive oxygen species, partly depending on metal content. Iron-free synthetic chrysotile nanotubes did not show the cell damage natural chrysotile caused in human lung cells. In laboratory tests, synthetic chrysotile with 0.6% iron by weight did cause damage, including DNA strand breaks. Animal studies of carbon nanotubes and mesothelioma conflicted. Mesotheliomas followed injection into the abdominal cavity of cancer-prone mice or into the scrotum of rats, while another rat study found none. The authors called this evidence still weak. Estimated lung clearance half-times were years to decades for amosite and crocidolite. Most chrysotile fibers cleared within months, though some may clear slowly. The authors concluded that carbon nanotubes were hazardous in most forms examined, and noted that data were still scarce.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Narrative review of published studies comparing the chemical features and toxic responses of high aspect ratio nanomaterials with asbestos
• Who was studied: No people were studied. The review covered materials: carbon nanotubes, asbestos, carbon nanofibers, and oxide and metal nanowires and nanorods. Other researchers had tested these in cell, cell-free and animal studies.
• Exposure: Carbon nanotubes (single-walled and multiwalled) compared with asbestos, meaning chrysotile and the amphiboles, including amosite and crocidolite. This included synthetic chrysotile nanotubes with and without iron, natural chrysotile (UICC A, from Rhodesia), and other fiber-shaped nanomaterials.
• Main results:
  • Asbestos fibers are all very hydrophilic (water-attracting)
  • Carbon nanotubes are highly hydrophobic (water-repellent) unless oxidized or functionalized
  • In the authors' comparison table, asbestos is listed as generating free radicals and reactive oxygen species, while carbon nanotubes are listed as scavenging them
  • Studies of carbon nanotubes described both production and scavenging of reactive oxygen species. The authors linked this to the variable amount of bioavailable iron from sample to sample.
  • Pure carbon nanotubes did not generate reactive oxygen species in cultured cells
  • In cited studies on human lung epithelial A549 cells, natural chrysotile (UICC A) was cytotoxic after 24 hours of incubation, but iron-free synthetic chrysotile nanotubes were not
  • In laboratory tests, synthetic chrysotile nanotubes made with 0.6% and 0.9% (w/w) iron caused DNA strand breaks, lipoperoxidation, inhibition of redox metabolism and alterations of cell integrity, even at the lowest iron loading
  • Animal studies on whether carbon nanotubes induce mesothelioma gave conflicting results: some reported mesothelioma induction and one reported no carcinogenic response
  • In one cited study, malignant mesotheliomas followed injection of multiwalled carbon nanotubes, in a large volume, into the abdominal cavity of p53+/– heterozygous mice, a genetically engineered strain with an increased propensity to toxicity-induced cancer
  • In another cited study, mesothelioma followed a single intrascrotal administration of multiwalled carbon nanotubes in male Fischer 344 rats
  • In a further cited study, multiwalled carbon nanotubes did not induce any mesothelioma in male Wistar rats
  • The authors judged the evidence for mesothelioma after carbon nanotube exposure to be still weak. They said it needs study by different administration routes and in long-term animal experiments.
  • Estimated lung clearance half-times for amosite and crocidolite were years to decades. Most chrysotile fibers were cleared within months, although some may be sequestered and slowly cleared.
  • Unpublished data from the authors' laboratory indicated that natural chrysotile nanofibers had lower toxic potency than micron-sized fibers
• The authors' conclusion: The authors concluded that carbon nanotubes, both multiwalled and single-walled, were hazardous in most of the forms examined
• Limitations the authors state:
  • The available data on high aspect ratio nanomaterials were too scarce for a systematic toxicity comparison like the one made for chrysotile substitutes
  • The animal mesothelioma experiments used carbon nanotube preparations that differed in form, length and level of contaminants, which might explain their different outcomes
  • Little was known about the potential toxicity of nanowires

Topics

Medical terms in this study, explained in our Lexicon: Aspect Ratio (Length-to-diameter ratio).

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