Multi-walled Carbon Nanotubes and Mesothelioma
An ICON Backgrounder (page 2)
Last Updated on May 21, 2008
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What we have learned from asbestos
Exposure to asbestos, a collection of naturally occurring minerals, has been linked in humans to a type of cancer known as mesothelioma, a disease in which malignant cells develop in the mesothelium, a protective lining that covers most of the body’s internal organs. It occurs both the pleura, which lines the lungs and chest cavity, and the lining of the abdominal cavity and covers the surface of the abdominal organs and heart. Mesothelioma is almost always caused by exposure to certain forms of asbestos and is a response to particles with a high length-to-width (aspect) ratio, that is, particles that are long and thin. Other harmful effects, such as asbestosis and cancer of the lung, larynx and kidney, have also been linked to asbestos exposure. Biopersistence, or the length of time the fibers remain intact in the body, is also a factor in their toxicity. Other factors such as the presence of embedded metals and the fibers’ ability to splinter into many smaller fibers may be important as well. The toxic response to asbestos seems to result partly from the inability of macrophages to surround and isolate the long fibers. Macrophages are special cells that clear foreign material from the body through a process known as phagocytosis. When a macrophage is frustrated in its attempts to surround a particle it initiates a sequence of events that results in injury to the body.
About the Materials
MWCNT are cylindrical concentric tubes of carbon that have many applications, including as electrically conducting additives to plastics, where they are impart mechanical strength and reduce static charge build-up, and as components of flat-panel displays. They are composed of several single-walled nanotubes embedded within one another. (See Figure)
The MWCNT samples used in the J Tox Sci paper had an average width of 100 nanometers (nm) and a distribution of lengths ranging from approximately 1000-20,000 nm. By comparison, the asbestos fibers studied had widths ranging from 30-2500 nm and lengths ranging from 100-35,000 nm with most being less than 10,000 nm long. The aspect ratio was greater than 3 for both MWCNT and asbestos. The study revealed that the MWCNT samples contained both individual MWCNT fibers as well as aggregates of fibers whereas the asbestos fibers do not aggregate. Individual fullerene molecules, soccer ball-shaped molecules composed of sixty carbon atoms measuring approximately 1 nm across, aggregated into clumps of up to 50,000 nm in size.
The MWCNT samples used in the Nature Nano paper can be divided into two groups: long, straight fibers and short tangled fibers. The long straight fibers contained a substantial portion longer than 15,000 nm whereas the short, tangled fibers had no long fibers and were mostly shorter than 5,000 nm. The long straight nanotube samples were individuals or bundles of tubes and the short tangled nanotubes were agglomerated or bundled together.