Structural Changes in Pellets of Calcium Phosphate Ester Salts after Exposure to Simulated Body Fluid with the Addition of Alkaline Phosphatase

Salts composed of calcium and phosphate esters (SCPEs) have been proposed as potential components for developing next-generation artificial bones, known as bioresponsive materials, due to their ability to transform into hydroxyapatite (HAp) in the physiological environment via enzymatic mediation by alkaline phosphatase (ALP). Previous studies have investigated the reaction behavior of the SCPE powders in simulated body fluid (SBF) modified with ALP. The reaction rates were influenced by the particle size of the SCPEs, specifically, the specific surface area, which in turn affected the morphology of the reaction product, HAp. Therefore, pellet samples, characterized by relatively constant surface areas, are more suitable for elucidating the fundamental reaction mechanisms. In this study, compact powder pellets of calcium salts composed of methyl phosphate (CaMeP), ethyl phosphate (CaEtP), butyl phosphate (CaBuP), and dodecyl phosphates (CaDoP) were utilized to investigate and evaluate their surface reactions in solution. The addition of ALP to SBF accelerated the detection of surface structural changes, even on pellet surfaces. The morphological changes observed on the surfaces of pellets exposed to either SBF or SBF/ALP followed trends similar to those previously reported for powder samples; however, the phase transformation occurred at a slower rate on the pellet surfaces. Although the quantity of the reaction product was insufficient for definite identification of the crystalline phase, the observed morphology suggested the formation of HAp. In the case of CaDoP, the formation of fibrous materials became prominent after 7 d of immersion in SBF/ALP. It was observed that molecules with higher hydrophobicity, generated from the decomposition of phosphate esters in SBF, influenced the development of fibrous materials during the dissolution-precipitation process.