Fate of Carbon Nanotubes Locally Implanted in Mice Evaluated by Near-Infrared Fluorescence Imaging: Implications for Tissue Regeneration

Carbon nanomaterials (CNMs) are used in various functional materials and products as well as in basic research, predominantly in engineering. Because of their high biocompatibility, use of CNMs in bioapplications is widely anticipated. Many researchers have examined the kinetics and toxicity of intravenously administered CNMs. However, little research on the in vivo kinetics of locally implanted CNMs has been reported. In this study, single-walled carbon nanotubes (SWCNTs) were implanted between the periosteum and parietal bone of mice. The in vivo kinetics of the implanted SWCNTs were observed by near-infrared (NIR) fluorescence imaging (excitation wavelength, 730 nm; emission wavelength, >1000 nm), because the body is highly transparent to light in the NIR region, and autofluorescence in this region is low, making NIR fluorescence suitable for bioimaging. SWCNTs were visible in the subcutaneous tissue and medullary cavity. Fluorescence was observed in the cranial region, and its intensity gradually decreased over 24 h. Fluorescence was not observed in other organs, including the liver, spleen, and lung, in whole-body imaging experiments. After 56 days, fluorescence weakened but was still clearly observed in the cranial region. The weakening of fluorescence over time was possibly because SWCNTs became bundled or adsorbed fluorescence-quenching materials in the tissue. These findings suggest that locally implanted SWCNTs remain at the site of implantation and do not accumulate in detectable quantities in other organs. We expect that this proof-of-concept for the biodistribution of SWCNTs after local implantation will promote future studies on the application of CNMs as biomaterials.