Cytotoxicity of MWNTs suspended in F-68, F-127 or BSA as a function of bath sonication time at 37 kHz and 120 W. After sonication, MWNT suspensions were diluted with non-sonicated F-68, F-127 or BSA solution to an equal MWNT concentration of 200 μg/mL. After dilution with an equal volume of 2X concentrated cell culture media, NRK cells were incubated for 24 h with the various MWNT suspensions at a final MWNT concentration of 100 μg/mL or in control media that contained no MWNTs. Cytotoxicity was assessed by cell proliferation, quantified by the crystal violet assay. Each data point is the mean of three independent experiments ± SD.