Ferrofluid characterization and cell viability test. a) A TEM image of maghemite nanoparticles. b) Size distribution of the maghemite nanoparticles within ferrofluid (mean = 10.25 nm and standard deviation = 2.96 nm). c) Magnetization versus applied field curve of the ferrofluid. Saturation magnetization of this sample ferrofluid was 1.10 kA m−1, corresponding to a 0.30% volume fraction. d,e) Cell viability test of mouse blood and HeLa cells in four media: HBSS, 0.30%, 0.79%, and 1.03% v/v of customized ferrofluids. After 0, 1, and 2 h incubation, cell viability was determined with Trypan blue exclusion. Viability of blood in ferrofluids up to 2 h was consistently 100%, while viability of HeLa cells was 90%. f) Comparison of viability of HeLa cells in HBSS, EMG 408 and customized ferrofluid (0.30% and 1.03% v/v) as a function of time. EMG 408 shows poor biocompatibility for HeLa cells, resulting in 36% viability after 2 h. Error bars are from three repeats of viability test.