Figure 3. Schematic illustration of experimental set-up utilizing a diffusion chamber, which was developed to assimilate the in situ situation as depicted in Fig. 2. The reaction chamber was divided into two (exterior and interior) compartments (each 1.5 mL in volume) by an ultrafiltration membrane (Spectra/por 3, M.W. cut-off 3500). Hydroxyapatite seed crystals, with or without protein coating, were placed in the interior (lower) compartment, which was separated from the exterior compartment by an ultrafiltration membrane. The concentrations of the total PO4 (3 mM) and NaCl (160 mM) as a background electrolyte were the same between the exterior and interior solutions. The pH value of both solutions was adjusted at 7.3 ± 0.2. Fluoride was added to the solution to yield concentrations of 0.05 to 1.0 ppm, as well as 0 ppm by no addition of NaF. The supersaturated solution contained 1 mM CaCl2 and was delivered at constant rates in the exterior compartment. No calcium was added initially to the experimental solution placed in the interior compartment. In this set-up, Ca2+ ions were allowed to diffuse through the membrane into the interior compartment according to the concentration gradient.
