Hydroxyapatite is the main constituent of mammalian hard tissues. Basic applications of synthetic hydroxyapatites include bone and dental implantology and drug delivery systems. The study of hydroxyapatite surface properties could give greater insight into the processes of bone mineralization and degradation. Nitroxide radicals are stable radicals that exhibit anticancer and antioxidative properties and are often used as spin probes to study the dynamics of complex biological systems. In this work, we attempted to adsorb the stable 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) on two hydroxyapatites (HAs) differing in specific surface area and the degree of hydration. The adsorption was carried out from cyclohexane, 1-chlorobutane and water. The solutions after adsorption were studied spectrophotometrically, while the obtained composites were characterized via NMR and EPR spectroscopy. The results show that it is possible to reproducibly obtain fairly stable composites, where the main factors influencing the adsorbed amount of the radical are solvent polarity and specific surface area of hydroxyapatite. The Langmuir isotherm was determined to be the most suitable adsorption model. The analysis of EPR and NMR spectra allowed us to determine the distribution of the TEMPO molecules on the hydroxyapatite surface, as well as a probable adsorption mechanism. The HA/TEMPO composites could potentially be used to study certain properties of hydroxyapatite surfaces with EPR spectroscopy. They could also be used as fillers after hard tissue surgery, as well as metal-free MRI contrasts.

Efficient production of nanocrystalline hydroxyapatite with desirable properties is a key focus in research on nanomaterials based on this mineral. It has been shown that, in the widely used wet synthesis method, fundamental factors such as the choice of calcium source and the type of post-reaction suspension maturation can play a crucial role. This study examines the influence of these two factors on parameters including synthesis yield, crystal size and morphology, material crystallinity and specific surface area, variations in OH group content, and the Ca/P molar ratio. The effect of these factors on drug release from tablets containing the synthesized hydroxyapatites was also evaluated. The results clearly demonstrate that calcium acetate decreases the synthesis yield of hydroxyapatite nanocrystals, particularly under dynamic maturation conditions. Moreover, this calcium source exerts the strongest influence on the drug release profile from tablets incorporating the obtained hydroxyapatite.