Researchers are continuing to investigate how the shape and size of carbon nanotubes (CNT) affect the pulmonary systems of mice over time. To answer the question, researchers are depositing CNT of varying sizes, length, and thickness into the lungs of mice and assessing the cellular interactions at varying intervals. Several recent papers have been published suggesting that the scientific community is progressing toward some answers.
CNT are long thin nanoparticles that are composed of a single wall (SWCNT) or multiple walls (MWCNT) of graphene sheets rolled into cylinders. Pulmonary exposure to both SWCNT and MWCNT reportedly causes early inflammation and fibrosis; however, the inflammatory response following MWCNT exposure appears to be more prominent than the response following SWCNT. One study suggests a possible explanation: enzymes produced during inflammatory states in the human lung are actually able to biodegrade SWCNT.
Upon initial inhalation or aspiration, another study measured the interactions of the MWCNT in the lung at 1, 3, and 28 days. The larger MWCNT were better able to escape the cellular structures compared to the smaller MWCNT. The larger MWCNT also caused an increased reaction by the white blood cells when compared to the smaller MWCNT. The authors in that study hypothesize that the larger MWCNT may be more challenging for the white blood cells to attack, resulting in an influx. Yet, all of the MWCNT examined in the study, large and small, followed the same progression over the 28-day test: they entered the cells either by diffusion or absorption, they gathered within the cell into vesicles, and then they were able to escape the vesicle enclosure.
After long term exposure, however, MWCNT have been found to encourage the growth of cells with DNA damage that could result in the development of a neoplastic process. One study sought to emulate potential workplace exposure and, after injecting either a carcinogenic initiator or corn oil into the mice, exposed them to MWCNT for a total of 15 days for 5 hours per day, and 5 days per week. Seventeen months after exposure, their lungs were examined and the data indicated that 90% of the mice that had both the carcinogenic initiator and MWCNT exposure had some form of lung cancer.
Additional studies suggest that thin MWCNT with high crystallinity were more likely to pierce cell membranes whereas, thick or tangled MWCNT were less toxic, less carcinogenic, and less likely to produce inflammation.
These works suggest that altering the shape, dimension, length, and structure of CNT can impact their effect on the body in both the long- and short-term. Medical and scientific researchers are not the only constituencies following these developments closely – – the regulatory, environmental, and legal communities are as well.