Octacalcium phosphate (OCP), a new-generation bone substitute material, is a considered precursor of the biological bone apatite. The two-layered structure of OCP contains the apatitic and hydrated layers and is intensively involved in ion-exchange surface reactions, which results in OCP hydrolysis to hydroxyapatite and adsorption of ions or molecular groups presented in the environment. During various in vitro procedures, such as biomaterial solubility, additive release studies, or the functionalization technique, several model solutions are applied. The composition of the environmental solution affects the degree and rate of OCP hydrolysis, its surface reactivity, and further in vitro and in vivo properties. The performed study was aimed to track the structural changes of OCP-based materials while treating in the most popular model solutions of pH values 7.2−7.4: simulated body fluid (SBF), Dulbecco's phosphate-buffered saline (DPBS), supersaturated calcification solution (SCS), normal saline (NS), and Dulbecco's modified Eagle's medium (DMEM). Various degrees of OCP hydrolysis and/or precipitate formation were achieved through soaking initial OCP granules in the model solutions. Detailed data of X-ray diffraction, Fourier-transform infrared spectroscopy, atomic emission spectrometry with inductively coupled plasma, and scanning electron microscopy are presented. Cultivation of osteosarcoma cells was implemented on OCP pre-treated in DMEM for 1−28 days. It was shown that NS mostly degraded the OCP structure. DPBS slightly changed the OCP structure during the first treatment term, and during further terms, the crystals got thinner and OCP hydrolysis took place. Treatment in SBF and SCS caused the precipitate formation along with OCP hydrolysis, with a larger contribution of SCS solution to precipitation. Pre-treating in DMEM enhanced the cytocompatibility of materials. As a result, on performing the in vitro procedures, careful selection of the contact solution should be made to avoid the changes in materials structure and properties and get adequate results.

Octacalcium phosphate granules were subsequently immersed for different lengths of time from 1 day to 44 days at constant temperature 37±0.4°C in solutions, simulating the extracellular body fluids: Supersaturated Calcification Solution (SCS), Simulated Body Fluid (SBF), based on the ISO-10993 “Biological Evaluation of Medical Devices” standard. The role of buffer`s systems pH-value, time of the material exposure and calcium and phosphorus ionic concentrations were analysed with the main focus on kinetics of degradation, changes in the morphological characteristics and phase composition of octacalcium phosphate samples.

Octacalcium phosphate is the precursor of biological apatite, that is why it is of special interest in bone regeneration due to it’s biocompatibility and biodegradability. This study was performed to examine the in vitro stability of octacalcium phosphate granules in physiological solutions. Powder X-ray diffraction (XRD), infrared spectroscopy (IR), plasma atomic emission spectroscopy (AES) and scanning electron microscopy (SEM) data were obtained before and after granules immersion. The variation of Ca2+ and HPO4 2- concentration in the soaking solutions suggests that the material transformation occurs through octacalcium phosphate dissolution followed by hydroxyapatite precipitation.

In this work was considered the influence of the buffer solutions, modeling biogenic extracellular liquid, including Simulated Body Fluid and also physiological solution for comparison, on microstructure and properties of octacalcium phosphate granules. The research was conducted, according to Russian state standard specification № 10993-14-2001. The correlation of pH - value and concentration of calcium and phosphorus ions in solutions on a degradation kinetics and recrystallization of the surface layer were analyzed.

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