Effect of Laser Irradiation on the Dissolution Kinetics of Hydroxyapatite Preparations

Abstract

This research investigated the effects of a laser irradiation treatment on the dissolution characteristics of hydroxyapatite (HAP), and the results provide an insight into the relationship between the effects of laser treatment and the two-site dissolution kinetics of HAP samples. The HAP samples prepared by aqueous precipitation and digestion at ~100°C were irradiated with a CO2 laser (20–50 W) with a beam diameter of 14mm for a total of 10–400s. Dissolution rates of the laser-treated HAP samples were subsequently determined in acetate buffer (pH = 4.5, μ = 0.50) at various levels of partial saturation (0–24% with respect to the HAP thermodynamic solubility of pKsp = 116). The following were the important findings. The X-ray diffraction and the IR spectroscopy results suggested that the HAP crystalline structure was not changed by laser treatment. Laser treatment of HAP powder at 50W for 400 s, however, caused an ~3.5-fold reduction in the specific surface area of HAP and reduced the initial dissolution rate of HAP in acetate buffer by a factor of ~22.9. Also, this laser treatment appeared to reduce the dissolution rate of HAP in 16 and 24% partially saturated acetate buffer from substantial levels to essentially zero. These results may be summarized as follows. Laser treatment of HAP results in a reduction in the dissolution rate and also a reduction in the specific surface area of this material. However, the dissolution rate reduction is significantly greater than the reduction in the specific surface area. Also, the percent reduction in the dissolution rate of laser-treated HAP in partially saturated buffers appears to be much greater than that for non-laser-treated HAP. In order to independently evaluate the effects of laser irradiation on the ion activity product (IAP) of HAP, a rotating disk dissolution study (based on Levich theory) of lased HAP pellets (50W for 400 s, 13 kJ/cm2) was performed. The ion activity product (KHAP) governing the dissolution of the lased HAP pellet was found to be 1 × 10−129.2 ± 0.7. This value for KHAP, together with the HAP powder results (above), strongly suggests that laser irradiation converts HAP possessing a two-site character to a HAP of a lower effective solubility (i.e., site #2 only).