A new core-shell nanostructure consisting of inorganic hydroxyapatite (HAP) nanoparticles as the core and organic alginate as the shell (denoted as HAP@Alg) was successfully synthesized by a pre-gel method and applied to pH-responsive drug delivery systems (DDS). HAP@Alg nanoparticles have the advantages of hydroxyapatite and alginate, where hydroxyapatite provides pH-responsive degradability, and alginate provides excellent biocompatibility and COOH functionality. Through the subsequent addition of CaCl(2) and phosphate solutions to the alginate solution, HAP@Alg nanoparticles with controllable particle sizes (ranging from 160 to 650 nm) were obtained, and their core-shell structure was confirmed through transmission electron microscopy (TEM) observation. Rhodamine 6G (R6G), a positively charged dye, was selected as a model drug for pH-sensitive DDS. R6G was encapsulated in the HAP/Alg nanoparticles upon synthesis, and its loading efficiency could reach up to approximately 63.0%. The in vitro release behavior of the loaded R6G at different pH values was systematically studied, and the results indicated that more R6G molecules were released at lower pH conditions. For example, after releasing for 8 h, the release amount of R6G at pH 2.0 was 2.53-fold the amount at pH 7.4. We attributed this pH-sensitive release behavior to the dissolution of the HAP core in acidic conditions. The results of the MTT assay and confocal laser scanning microscopy indicated that the HAP@Alg were successfully uptaken by liver cancer cells (HepG2) without apparent cytotoxicity. The synthesized HAP@Alg nanoparticles show great potential as drug nanovehicles with high biocompatibility, enhanced drug loading, and pH-responsive features for future intracellular DDS.

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A composite cement made of strontium containing tricalcium silicate and hydroxyapatite (SrC3S/HAp) was prepared and studied through a two-stage sol–gel method. The elemental components are 75 wt% of strontium-doped tricalcium silicate (SrC3S) and 25 wt% of hydroxyapatite (HAp). According to previous studies, the SrC3S has good mechanical properties and hydraulic conductivity, and the HAp has good biocompatibility. Therefore, the 75SrC3S-25HAp powder with 10 wt% of NaH2PO4 solution was mixed to prepare the slurry material. This slurry was measured for the working time and setting time at 37 °C under saturated vapor pressure. The material properties were evaluated in terms of crystal structures, surface morphologies, and mechanical properties. The in vitro testing was conducted to determine the ion release rate and curing behavior under a simulated body fluid environment. Finally, the L929 murine fibroblast cells were cultured to study the biocompatibility of the material. The results indicated that the operating time was 15.2 min, and the setting time was 43.6 min. After being soaked into simulated body fluid for14 days of the experiment, the compressive strength of 75SrC3S-25HAp remained 22.1 MPa, and the release of calcium and strontium ions was 501.4 ppm and 49.5 ppm, respectively. The cell viabilities were higher than 70% in various concentrations. These results suggest the 75SrC3S-25HAp has excellent potential for bone cement application.

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