*2.4. Cell Adhesion and Cell Counting*

Paraformaldehyde was intended to freeze the cell structures in a state as close as possible to their living state. For each configuration and donor, the cells of three samples of each pair of material were fixed at 3, 7, 14 and 21 days after the start of the cell culture. The samples were placed in a new 12-well plate and then 1 mL of 4% paraformaldehyde (PAF) was added in each. The PAF acted for 10 min, then it was removed and the samples were washed with phosphate buffered saline (PBS).

A 0.1% solution of 4', 6-diamidino-2-phenylindole (DAPI), i.e., 1 μL of DAPI per 999 μL of PBS, was deposited in each well. The DAPI inserted into the DNA of the nucleus cells and emitted a 455 nm blue light when excited by 345 nm light. This allowed us to observe the cells under a fluorescence optical microscope and to count them on each section of the chemical gradient.
