Water Sorptivity of Hydroxyapatite Discs and Demineralized Enamel of Varying Densities

Objectives: To obtain the water sorption of hydroxyapatite (HA) discs and artificially demineralized enamel (DE) slabs of varying densities as preliminary data for clinical categorization of enamel demineralization.
Methods: Eleven hydroxyapatite discs (4 mm in diameter and ~2 mm in thickness) were prepared through compaction of hydroxyapatite powder under pressure ranging from 31.8 to 286.5 MPa and at a speed of 1mm/min, to produce samples of densities ranging from 1.409 to 1.893 g/cm³. Samples were placed in a desiccator overnight to remove moisture before testing. Enamel slabs were obtained by sectioning and demineralized by immersion in a solution of 0.05M acetate buffer, 1.12mM calcium and 0.77mM phosphate (pH=4) for 1 month. Both HA discs and DE slabs were then tested using a contact-angle meter (Drop Master, KYOMA, Japan) through the dispensing of 1.5 µL water droplet and dynamically recording the contact angle over time. The absorbed volume was automatically calculated by the software FAMAS, and plotted against square root of time to determine the sorptivity of each sample.
Results: For HA discs, The densest sample showed the lowest sorptivity (0.065 mL/s^1/2) while the least dense sample showed the highest sorptivity (0.884 mL/s^1/2). The relationship between sorptivity and density appeared to be linear within the range of values tested (R² = 0.95). For DE slabs, measured sorptivity values ranged from 0.003 to 1.06 mL/s^1/2). Using the linear regression equation, DE densities were estimated to range from 1.33 to 1.91 g/cm³.
Conclusions: Within the range of the measured values, hydroxyapatite’s density seems to have a linear relationship with its water sorptivity, where increasing density (a less porous structure) results in a decrease in sorptivity and vice versa. Demineralized enamel slabs seem to follow a similar pattern.