A February 2008 study published by six Stanford scientists examined the long term fate of intravenously injected carbon nanotubes in mice.   The scientists’ goal was to measure the circulation of SWCNTs in the bloodstream and to determine whether they accumulate in organs and/or tissues.  At the same time, the scientists also studied the effects of functionalizing SWCNTs with polymers.  They found that functionalizing the SWCNTs with polyethyleneglycol enabled full blood circulation in 1 day, yet there was little uptake by the liver and spleen.  Additionally, near complete clearance from main organs occurred in about two months through the excretion of urine and feces.  No toxic side effects were observed.   The authors further found that the SWCNTs became more biologically inert as they increased the number of functionalized polymer branches. 

The scientists concluded that this and other studies "provide a strong indication of the lack of toxicity of well functionalized SWNTs in mice before clearance from the body. In contrast to a previous study of nonfunctionalized pristine carbon nanotubes causing fiber toxicity to mice, our well functionalized SWNTs are highly biocompatible for in vivo applications."

Z. Liu, et al., "Circulation and long-term fate of functionalized, biocompatible single-walled carbon nanotubes in mice probed by Roman spectroscopy," PNAS, Vol. 105, No. 5 at 1410-1415 (February 5, 2008).