Preparation of Octacalcium Phosphate Thin Film with Exposing Reactive Crystalline Plane in Biological Fluid

Octacalcium phosphate (OCP) has been used as a bone replacement material due to its higher bone affinity. However, the mechanism of affinity has not been clarified. Since the 100 crystalline plane of OCP is closely involved in the biological reactions during osteogenesis, it is important to expose the 100 crystalline plane of OCP to the biological fluid to precisely measure the interfacial reactions. In this study, the OCP plate-like crystals were fixed on a conductive substrate in the form of single-particle deposition, and the thin films with exposing 100 crystalline planes were fabricated. Then, the characteristics of hydration layers in the OCP crystals were enhanced by the exposure of 100 crystalline planes through the thin film formation, and the bioreactivity was found to be associated with the swelling and dissolution of the hydration layer in the biological fluid. Specifically, the OCP crystals were deposited on the gold sensor by electrophoretic deposition (OCP/Au-1). The results showed that the OCP plate-like crystals were selectively deposited on the gold sensor by electrophoresis. Subsequently, it was found that the ultrasonication of OCP/Au-1 resulted in the formation of an OCP crystalline thin film (m-OCP/Au-1) with the single-particle thickness on the gold sensor with exposing 100 crystalline planes. Moreover, the FT-IR spectra of m-OCP/Au-1 showed that the structure of the phosphate ions was rearranged by ultrasonication in the hydration layer, resulting in the regulation of the layered nanostructures, promoting higher crystallinity. Furthermore, the XPS spectra of m-OCP/Au-1 indicated that the hydrogen phosphate ions in the hydration layer were exposed on the 100 crystalline plane of the topmost surface. The prepared m-OCP/Au-1 was stable in citrate buffer, whereas it showed very high reactivity in phosphate buffer as the hydration layer gradually dissolved after the swelling, which was measured by the QCM-D technique. Therefore, the OCP crystalline thin films in this study were found to have higher surface reactivity due to the enhanced exposure of the hydration layer, which is assumed to be the cause of their bone-regeneration-promoting effect (i.e., higher bone affinity). The films in this study were stable at gastric acid pH and dissolved at neutral pH, which could make them useful as the orally administered drug carrier.