Incubation period of cells in the presence of external magnetic field. Cell aggregates following washes with a nonparamagnetic medium after diamagnetic 3D cell printing of (a) 5000 MCF-7 cells on a ULA surface (n=3) and (b) 3000 MCF-7 cells on a TCT surface (n=3) in 25 mM Gd-DTPA for 1, 3, 6, and 24 hours. Incubations periods indicate durations for exposure to the paramagnetic medium and the externally applied magnetic field after which the medium is replaced by 0 mM Gd-DTPA removed to prevent overexposure of Gd-DTPA and the magnetic field. At 1 and 3 hours of incubation, the cells are successfully concentrated to the zones of low magnetic field strength, which are determined by the arrangements of the magnets. However, following medium changes to remove Gd-DTPA, the 3D structures do not maintain their aggregated structures. Only for 6 hours of exposure do the cells remain as a 3D structure following medium changes. Therefore, a minimum of 6 hours is sufficient for producing a single cell structure for cell suspensions in both ULA and TCT surfaces. Scale bar = 200 μm.