Evaluating the mineral maturation pathways of carbonated hydroxyapatite based on pH

Abstract

Bones are composed of an organic framework (70%) and calcium phosphate (30%). The calcium phosphate phase providing bone strength is hydroxyapatite. Bone growth is a complex process as many factors can encourage or inhibit this process. For instance, pyrophosphate is known to block hydroxyapatite binding sites. The aim is to study the effect of pH on hydroxyapatite formation and its precursor phases, and to see what factors inhibit or encourage the growth of this phase. Four sets of experiments were prepared using the double decomposition method with 0.1 M calcium chloride and 0.12 M sodium phosphate, one set with no additives, carbonate, pyrophosphate and citrate were made. Buffered experiments were conducted at pH 7 to 9 as pH 5 and 6 produced limited precipitate in the unbuffered preliminary run. Samples were analyzed using the Fourier-transform infrared spectroscopy and Raman spectroscopy. Overall, crystallinity increased over time. With increasing pH, the rate of hydroxyapatite growth increased, as phosphate becomes the dominant species which is needed for formation. Pyrophosphate and citrate showed no inhibition, unless in excess concentration. Carbonate was present in all samples suggesting atmospheric carbon dioxide interaction, therefore, trends observed may also be due to changing carbonate content. With increasing pH, more bands were produced with greater changes in band parameters demonstrating higher reactivity. This was seen as the unbuffered spectra produced initially more crystalline material. In a buffered setting, like the body, the body will maintain a less crystalline material by slowing recrystallization to keep this reactivity for bone remodeling