Durability and inflammogenic impact of carbon nanotubes compared with asbestos fibres

Our plain-English summary of Durability and inflammogenic impact of carbon nanotubes compared with asbestos fibres, by M.J. Osmond-McLeod, C.A. Poland, F. Murphy, et al (Particle and Fibre Toxicology, 2011).

The study

• Authors: M.J. Osmond-McLeod, C.A. Poland, F. Murphy, L. Waddington, H. Morris, S.C. Hawkins, S. Clark, R. Aitken, M.J. McCall, K. Donaldson
• Journal: Particle and Fibre Toxicology
• Published: 2011;8:15
• Cited by: 64 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: It has been suggested that carbon nanotubes might conform to the fibre pathogenicity paradigm that explains the toxicities of asbestos and other fibres on a continuum based on length, aspect ratio and biopersistence. Some types of carbon nanotubes satisfy the first two aspects of the fibre paradigm but only recently has their biopersistence begun to be investigated. Biopersistence is complex and requires in vivo testing and analysis. However durability, the chemical mimicking of the process of fibre dissolution using in vitro treatment, is closely related to biopersistence and more readily determined. Here, we describe an experimental process to determine the durability of four types of carbon nanotubes in simulated biological fluid (Gambles solution), and their subsequent pathogenicity in vivo using a mouse model sensitive to inflammogenic effects of fibres. The in vitro and in vivo results were compared with well-characterised glass wool and asbestos fibre controls.

Results: After incubation for up to 24 weeks in Gambles solution, our control fibres were recovered at percentages consistent with their known in vitro durabilities and/or in vivo persistence, and three out of the four types of carbon nanotubes tested (single-walled (CNTSW) and multi-walled (CNTTANG2, CNTSPIN)) showed no, or minimal, loss of mass or change in fibre length or morphology when examined by electron microscopy. However, the fourth type [multi-walled (CNTLONG1)] lost 30% of its original mass within the first three weeks of incubation, after which there was no further loss. Electron microscopy of CNTLONG1 samples incubated for 10 weeks confirmed that the proportion of long fibres had decreased compared to samples briefly exposed to the Gambles solution. This loss of mass and fibre shortening was accompanied by a loss of pathogenicity when injected into the peritoneal cavities of C57Bl/6 mice compared to fibres incubated briefly. CNTSW did not elicit an inflammogenic effect in the peritoneal cavity assay used here.

Conclusions: These results support the view that carbon nanotubes are generally durable but may be subject to bio-modification in a sample-specific manner. They also suggest that pristine carbon nanotubes, either individually or in rope-like aggregates of sufficient length and aspect ratio, can induce asbestos-like responses in mice, but that the effect may be mitigated for certain types that are less durable in biological systems. Results indicate that durable carbon nanotubes that are either short or form tightly bundled aggregates with no isolated long fibres are less inflammogenic in fibre-specific assays.

© 2011 Osmond-McLeod et al; licensee BioMed Central Ltd.

In plain English

This study tested how four types of carbon nanotubes held up in simulated biological fluid, compared with glass wool, long fibre amosite and long fibre chrysotile asbestos. After 24 weeks, about 75% of the amosite and 30% of the chrysotile remained by weight. Three nanotube types showed no meaningful loss. One multi-walled type (CNTLONG1) kept about 70% of its weight from week 3; by week 10, its share of measured fibres over 15 μm had roughly halved. CNTLONG1, a single-walled type and the three controls were then injected into the abdominal cavities of female mice. Amosite caused inflammation and fibrotic plaques regardless of incubation; chrysotile incubated for 10 weeks caused no fibrotic response. Unincubated CNTLONG1 caused inflammation and strong fibrosis, both reduced after 10 weeks' incubation. The single-walled type caused no inflammatory response. The authors concluded that, in this fibre-specific test, the adverse response depended on durability and on long, discrete nanotubes or fibre-shaped nanotube clumps. They noted that handling losses could cause up to 20% variation in recovered weight, and that lungs were not studied.

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: Laboratory experiment: in vitro durability testing in simulated biological fluid, followed by an animal experiment (injection into the peritoneal cavity of mice)
• Who was studied: Four types of carbon nanotubes (one single-walled, three multi-walled) and three control fibres. In the animal tests, only CNTSW, CNTLONG1 and the three controls were injected. The animals were female C57Bl/6 mice aged 8 weeks, four per sample type per time-point, plus three vehicle-only control mice, housed at the University of Edinburgh.
• Exposure: Long fibre amosite (LFA), long fibre chrysotile (LFC), glass wool X607 and four carbon nanotube types were incubated in Gambles solution at 37°C for up to 24 weeks. Samples of X607, LFA, LFC, CNTSW and CNTLONG1 incubated for 0 or 10 weeks were injected into the mouse peritoneal cavity at a presumed mass of 50 μg. Mice were examined at 24 hours and 7 days.
• Main results:
  • After 24 weeks in Gambles solution, the amosite (LFA) was recovered at about 75% of its original weight (75.43 ± 1.33% in Table 2).
  • After 24 weeks in Gambles solution, the chrysotile (LFC) was recovered at about 30% of its original weight (28.23 ± 3.03% in Table 2).
  • Among the carbon nanotubes, only CNTLONG1 showed a significant loss of mass; it was recovered at about 70% of its original weight from week 3 onward. The authors attributed the variation seen in the other three nanotube types to experimental error.
  • In CNTLONG1 incubated for 10 weeks, the proportion of measured fibres longer than 15 μm was approximately half that in CNTLONG1 incubated for 0 weeks. The authors said the size measurements indicate trends only.
  • In mice, 10 weeks of incubation did not change the response to amosite (LFA), which caused inflammation and fibrotic plaques on the diaphragm.
  • Chrysotile (LFC) incubated for 10 weeks did not cause a fibrotic response at 7 days in mice. Unincubated chrysotile, and amosite at either 0 or 10 weeks, did cause one.
  • Chrysotile incubated for 10 weeks gave a reduced response at 24 hours. However, at 7 days the total cell count in these mice was higher than in mice given unincubated chrysotile or amosite.
  • Unincubated CNTLONG1 caused an acute inflammatory response at 24 hours that did not subside by 7 days, and a strong fibrotic response at 7 days.
  • CNTLONG1 incubated for 10 weeks caused reduced inflammatory and fibrotic responses compared with unincubated CNTLONG1.
  • The single-walled nanotube sample (CNTSW) did not cause an inflammatory response in the mice.
  • The glass wool X607 did not cause a statistically significant inflammatory response, except for the total cell count at 7 days, regardless of incubation time.
  • The authors suggested that the loss of harmful effect may have been linked more to the reduced proportion of long fibres than to the loss of mass.
• The authors' conclusion: The authors concluded that three of the four nanotube types were close to fully durable. In the fibre-specific mouse assay, the adverse response depended on both durability and the presence of discrete, long nanotubes or fibre-shaped clumps of nanotubes. Unincubated CNTLONG1 caused an asbestos-like response, which was reduced after incubation lowered the proportion of long fibres.
• Limitations the authors state:
  • Sample preparation by different operators and sample losses during solution refreshing and filtration may account for up to 20% variation in sample recovery, so differences of that size may not be real.
  • Fibre size measurements from electron microscope images indicate trends only and are not absolute values.
  • Doses were based on an assumption of no mass loss. The 10-week CNTLONG1 sample had actually lost 30% of its mass, so the actual dose may have been smaller.
  • The lack of response to CNTSW in the peritoneal cavity does not mean it would be harmless in the lung, which was not investigated.
  • CNTSPIN could not be tested in mice because it could not be dispersed well enough.
  • The fibre measurement rules differed from World Health Organisation guidelines.

Topics

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

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