In Vitro Study of Octacalcium Phosphate Behavior in Different Model Solutions

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.