Geometry of carbon nanotubes and mechanisms of phagocytosis and toxic effects

Our plain-English summary of Geometry of carbon nanotubes and mechanisms of phagocytosis and toxic effects, by V.M. Harik (Toxicology Letters, 2017).

The study

• Author: V.M. Harik
• Journal: Toxicology Letters
• Published: 2017;273:69-85
• Type of article: Review
• Cited by: 27 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

A review of in vivo and in vitro toxicological studies of the potential toxic effects of carbon nanotubes is presented along with the analysis of experimental data and a hypothesis about the nanotube-asbestos similarity. Developments of the structure-activity paradigm have been reviewed along with the size effects and the classification of carbon nanotubes into eleven distinct classes (e.g., the high aspect ratio nanotubes, thick multi-wall nanotubes and short nanotubes). Scaling analysis of similarities between different classes of carbon nanotubes and asbestos fibers in the context of their potential toxicity and the efficiency of phagocytosis has been reviewed. The potential toxic effects of carbon nanotubes have been characterized by their normalized length, their aspect ratio and other parameters related to their inhalability, engulfment by macrophages and the effectiveness of phagocytosis. Geometric scaling parameters and the classification of carbon nanotubes are used to develop an updated parametric map for the extrapolation of the potential toxic effects resulting from the inhalation of long and short carbon nanotubes. An updated parametric map has been applied to the evaluation of the efficiency of phagocytosis involving distinct classes of carbon nanotubes. A critical value of an important nondimensional parameter characterizing the efficiency of phagocytosis for different nanotubes is presented along with its macrophage-based normalization. The present evaluation of the potential toxicological effects of the high aspect ratio carbon nanotubes is found to be in the agreement with other available studies and earlier scaling analyses.

Copyright © 2017 Elsevier B.V. All rights reserved.

In plain English

This was a single-author review and theoretical analysis of published laboratory and animal studies of carbon nanotubes, a man-made fiber, and of the idea that some behave like asbestos. It reported no new experiments. The author sorted carbon nanotubes into eleven classes by size and shape. The author used Mph, a ratio of macrophage (lung clearing cell) size to nanotube length, to describe how well macrophages engulfed nanotubes. Values above 1 were the criterion for efficient engulfment and below 1 for deficiency, but the author treated this as graded, calling 1 to 2 nearly total deficiency. With macrophages varying from 10 to 20 micrometres, the author put the lower value of the critical fiber length at about 10 micrometres. Five classes of long, high aspect ratio nanotubes were judged potentially toxic because of their length. Using asbestos fiber-length thresholds from other researchers, the author linked ranges of the ratio to asbestosis, mesothelioma and lung cancer. The author concluded that the nanotube-asbestos similarity hypothesis had been proven. The author also described the proofs as still developing and called for further study.

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: Review of published in vivo and in vitro toxicology studies of carbon nanotubes, combined with the author's own theoretical scaling (dimensional) analysis; no new experimental data
• Who was studied: No study population of its own; the article reviewed published toxicological studies of carbon nanotubes (in mice, rats and cells in the laboratory) and compared them with asbestos fibers
• Exposure: Single-wall and multi-wall carbon nanotubes (SWCNTs and MWCNTs) of different lengths, diameters and aspect ratios, grouped into eleven classes and compared with asbestos fibers
• Main results:
  • The author defined a ratio, Mph, of macrophage size to nanotube length. Values greater than 1 were the criterion for efficient phagocytosis (engulfment by macrophages), and different values above 1 corresponded to various degrees of efficiency. Values less than 1 indicated inefficiency.
  • The author stated that effective phagocytosis can be achieved for values of Mph much higher than 10.
  • Because typical macrophage size varies from 10 to 20 micrometres, the critical fiber length depends on macrophage size. The author placed the lower value of the critical fiber length, at which phagocytosis begins to be limited (Mph = 1), at about 10 micrometres.
  • Of the eleven nanotube classes, the five classes of long, high aspect ratio nanotubes were judged to have potential toxic effects because of their length.
  • The author's parametric map placed long, high aspect ratio carbon nanotubes in the region of the most ineffective phagocytosis.
  • The author took asbestos fiber-length thresholds from other researchers (Esmen and Lippmann) and linked ranges of Mph to diseases. An Mph from 5 to 10 was associated with asbestosis, 2 to 4 with mesothelioma, and 1 to 2 with lung cancer.
  • The author stated that this evaluation of high aspect ratio carbon nanotubes agreed with available experimental studies and earlier analyses.
• The authors' conclusion: The author concluded that the hypothesis of similarity between asbestos fibers and carbon nanotubes had been proven, both by scaling analysis and by experimental studies.
• Limitations the authors state:
  • The author stated that further studies of the efficiency of phagocytosis are needed.
  • The author noted that in several reviewed animal studies the nanotubes were instilled rather than inhaled.
  • In the final paragraph the author described the proofs of the nanotube-asbestos similarity hypothesis as still developing.

Topics

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

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