Bone is an organic-inorganic composite with the ability to regenerate itself. Thus, several studies based on artificial organic-inorganic interface sciences have been tried to develop capable materials for effective regenerative bone tissues. Hydroxyapatite nanoparticles (HAp NPs) have extensively been researched in bone tissue engineering due to the compositional and shape similarity to the mineral bone and excellent biocompatibility. However, HAp alone has low mechanical strength, which limits its applications. Therefore, HAp NPs have been deposited on the biocompatible polymer matrix, obtaining composites with the enhanced mechanical, thermal, and rheological properties and with higher biocompatibility and bioactivity. For developing new biomedical applications, polymer-HAp interfacial interactions that provide biofusion should be investigated. This paper reviewed common coating techniques for obtaining HAp NPs/polymer fusion interfaces as well as in vitro studies of interfacial interactions with proteins and cells, demonstrating better biocompatibility. Studies based on interfacial interactions between biomolecules and HAp NPs were highlighted, and how these interactions can be affected by specific protein preadsorption was also summarized.

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The control of the hydration and protein adsorption states on hydroxyapatite surface was systematically discussed, which is very important for the proper understanding of the controllable interfacial interactions between cells and bioceramics.

The synthesized elliptical hydroxyapatite (E-HAp) and needle-like HAp (N-HAp) nanoparticles (NPs) were electrophoretically deposited on a gold (Au) substrate. A comparative study of the hydration layers on E-HAp, N-HAp, and Au films was achieved to investigate the interfacial effect of the hydration layers on the conformation of the adsorbed fibrinogen (Fgn) and fibroblast adhesion properties. As a result, the ratios of three types of hydration layer states (free water, intermediate water, nonfreezing water) analyzed by a Fourier transform infrared (FT-IR) spectral deconvolution of the O–H stretching absorption band were investigated. The ratio of the bonding water state (i.e., intermediate and nonfreezing water molecules) is almost the same between two HAp films, and the E-HAp film with an elliptical shape and smaller particle size exhibited the smallest ratio of nonfreezing water, which can suppress the denaturation of the adsorbed protein. Subsequently, FT-IR spectral deconvolution results of the amide I band of the adsorbed Fgn on the E-HAp film indicated the higher proportion of α-helix and β-sheet structures as compared with those on the N-HAp and Au films, suggesting that the smaller proportion of nonfreezing waters would play a significant role in the stereoscopic Fgn conformation. In the culture of fibroblasts, FT-IR spectra of the adhered cells on the E-HAp, N-HAp, and Au films exhibited different absorbance intensities of the amide A, I, II, and III bands, suggesting a different amount of collagen-producing states by the cells, which were also supported by immunostaining results of the collagen type I. Therefore, the different hydration structures on the films clearly influenced the conformation of the adsorbed protein, and the preferential conformation was found at the interfaces between the fibroblasts and the underground E-HAp films.

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In this review, the current status of the influence of added ions (i.e., SiO44−, CO32−, etc.) and surface states (i.e., hydrated and non-apatite layers) on the biocompatibility nature of hydroxyapatite (HA, Ca10(PO4)6(OH)2) is discussed. It is well known that HA is a type of calcium phosphate with high biocompatibility that is present in biological hard tissues such as bones and enamel. This biomedical material has been extensively studied due to its osteogenic properties. The chemical composition and crystalline structure of HA change depending on the synthetic method and the addition of other ions, thereby affecting the surface properties related to biocompatibility. This review illustrates the structural and surface properties of HA substituted with ions such as silicate, carbonate, and other elemental ions. The importance of the surface characteristics of HA and its components, the hydration layers, and the non-apatite layers for the effective control of biomedical function, as well as their relationship at the interface to improve biocompatibility, has been highlighted. Since the interfacial properties will affect protein adsorption and cell adhesion, the analysis of their properties may provide ideas for effective bone formation and regeneration mechanisms.