Figure 5.

Diagrammatic model showing the difference of mass transports between the 3D (a) and 2D (b) cultures. In this model, due to the constantly free rolling of cells with the matrix in the medium for the 3D mass transport (a), the stagnant region could not be created. As contrast, the 2D mass transport (b) encounters two barriers: the stagnant region and the cell membrane. The stagnant region is a spontaneous barrier for mass transport adjacent to any membrane or catalyst surface. There would occur a concentration drop no sooner than the cells start to consume the medium.