An Analysis of Hydroxyapatite Surface Layer Formation

Abstract

A mechanism for the formation of hydroxyapatite (HAp) surface layers compositionally different from the bulk compositions is described. Surface layer formation is a consequence of HAp being a compound of variable composition: Ca 10 (PO 4 ) 6 (OH) 2 to ∼Ca 9 HPO 4 (PO 4 ) 5 OH, existing over Ca/P ratio from 1.67 for stoichiometric to ∼1.5 for fully calcium-deficient HAp. One consequence of this compositional variability is that only one HAp composition (Ca/P ratio) dissolves congruently (the solution and solid have the same Ca/P ratio). HAp compositions having Ca/P ratios different from the congruently dissolving composition will form surface layers. Equilibration of stoichiometric HAp with water will result in the formation of a surface layer having a Ca/P ratio of less than 1.67. In general, for solid HAp compositions having Ca/P ratios greater than that of the congruently dissolving composition, a nonstoichiometric surface layer will form and the Ca/P ratio in solution will exceed that of the surface layer. For solid compositions having lower Ca/P ratios than the congruently dissolving composition, the opposite will be true.

The importance of amino acid interactions in the crystallization of hydroxyapatite

“…HA particles at pH 7.4 are negatively charged, owing to the presence of Ca vacancies and to the acidic pK a of HA surfaces [10,24,58,59]. Therefore, a stronger electrostatic interaction should occur between HA growing nuclei and the positively charged AA, Arg.…”

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…Systematic studies of defect constellations in CDHA are available in literature (Zahn and Hochrein 2008;Liou et al 2004). As a first approximation, CDHA may be considered as HA with some ions missing (ionic vacancies) (Brown and Martin 1999). The more amount of Ca is deficient, the more disorder, imperfections and vacancies are in the CDHA structure (Honghui et al 2007).…”

Combined effect of magnesium and amino glutamic acid on the structure of hydroxyapatite prepared by hydrothermal method

“…Under the same conditions, the apatite surface is considered as negatively charged, therefore some authors consider that the electrostatic interactions between the surface and the aminoacid are very weak [58,59]. In the present study, glutamic molecules are present in the form of carboxylate ions, which can lead to calcium complexes that participate in the formation of the crystalline edifice.…”