Fig. 6: Viscosity near spheroid.

a 3D rendering of a section of a spheroid of MCF-7 cells expressing td-Tomato (relative dimensions: 200 µm × 400 µm × 100 µm (150–250 µm from the coverslip)). The inset shows one of the areas where viscosity (relative to the solvent) measurements were acquired at incremental distances from the spheroid surface—three measurements (4 µm, 12 µm and 30 µm from the spheroid surface) are depicted as spheres (not to scale). The colour gradient for each sphere represents the relative viscosity sampled by angle at each measurement position. A fourth position (6 µm) has not been shown to aid visualisation. b Relative viscosity measurements in a plane perpendicular to the spheroid surface, passing through the highest and lowest measured viscosity at the probe position n = 18. c Relative viscosity measurements of the nutrient media at each position for a direction perpendicular to the spheroid surface (blue with jitter applied to data) and glass (red), showing increased apparent viscosity at positions closer to the surface but lower than values predicted from Faxén’s law (black dashed line).