Formation of 3D cellular structures. 3D cellular structures composed of MDA-MB-231 (monotypic), 12.5, 25, and 50% fibroblast (cocultures) and fibroblast (monotypic) cells (a) printed with magnetic assistance and (b) formed through gravitational settling are imaged at 3, 6, 9, 12, 24, and 48 hours. A monotypic 3D structure containing MDA-MB-231 is formed at 24 hours with magnetic assistance; in contrast, a similar formation at 48 hours when gravity is used alone. As the proportion of fibroblast cells increases, the structure formation time decreases. Again, magnetically assisted printing reduces structure formation time as compared to formation under the influence of gravity alone for all cell populations. The panels with red borders indicate the time it takes to form a 3D cellular structure. The images are taken after the cell medium is replaced with fresh medium for both methods. The scale bar is equal to 100 μm.