Synthesis of Hydroxyapatite/Bioglass Composite Nanopowder Using Design of Experiments

Composite scaffolds of hydroxyapatite (HAp) nanoparticles and bioactive glass (BG) were applied as an appropriate selection for bone tissue engineering. To this end, HAp/BG composite was synthesized by a hydrothermal method using Design of Experiments (DOE) with a combined mixture–process factor design for the first time. The input variables were hydrothermal temperature at three levels (i.e., 100, 140, 180 °C) as a process factor and two mixture components in three ratios (i.e., HAp 90, 70, 50; BG 50, 30, 10). The degree of crystallinity and crystal size in the composite were the output variables. XRD showed that only a small fraction of BG was crystallized and that a wollastonite phase was produced. The XRD results also revealed that incorporation of Si into the HAp structure inhibited HAp crystal growth and restricted its crystallization. The FTIR results also showed that the intensity of the hydroxyl peak decreased with the addition of silicon into the HAp structure. DOE results showed that the weight ratio of the components strongly influenced the crystal size and crystallinity. SEM and FTIR results identified the greatest bioactivity and apatite layer formation in the Si-HAp sample with an HAp70/BG30 ratio after 14 days immersion in simulated body fluid (SBF) solution, as compared to other ratios and HAp alone. Therefore, the combination of HAp and BG was able to yield a HAp/BG composite with significant bioactivity.

Preparation of Silicon Hydroxyapatite Nanopowders under Microwave-Assisted Hydrothermal Method

The synthesis of partially substituted silicon hydroxyapatite (Si-HAp) nanopowders was systematically investigated via the microwave-assisted hydrothermal process. The experiments were conducted at 150 °C for 1 h using TMAS (C₄H₁₃NO₅Si₂) as a Si⁴⁺ precursor. To improve the Si⁴⁺ uptake in the hexagonal structure, the Si precursor was supplied above the stoichiometric molar ratio (0.2 M). The concentration of the TMAS aqueous solutions used varied between 0.3 and 1.8 M, corresponding to saturation levels of 1.5–9.0-fold. Rietveld refinement analyses indicated that Si incorporation occurred in the HAp lattice by replacing phosphate groups (PO₄³⁻) with the silicate (SiO₄⁻) group. FT-IR and XPS analyses also confirmed the gradual uptake of SiO₄⁻ units in the HAp, as the saturation of Si⁴⁺ reached 1.8 M. TEM observations confirmed that Si-HAp agglomerates had a high crystallinity and are constituted by tiny rod-shaped particles with single-crystal habit. Furthermore, a reduction in the particle growth process took place by increasing the Si⁴⁺ excess content up to 1.8 M, and the excess of Si⁴⁺ triggered the fine rod-shaped particles self-assembly to form agglomerates. The agglomerate size that occurred with intermediate (0.99 mol%) and large (12.16 mol%) Si contents varied between 233.1 and 315.1 nm, respectively. The excess of Si in the hydrothermal medium might trigger the formation of the Si-HAp agglomerates prepared under fast kinetic reaction conditions assisted by the microwave heating. Consequently, the use of microwave heating-assisted hydrothermal conditions has delivered high processing efficiency to crystallize Si-HAp with a broad content of Si⁴⁺.

Low-Cost Deposition of Antibacterial Ion-Substituted Hydroxyapatite Coatings onto 316L Stainless Steel for Biomedical and Dental Applications

Substitutions of ions into an apatitic lattice may result in antibacterial properties. In this study, magnesium (Mg)-, zinc (Zn)-, and silicon (Si)-substituted hydroxyapatite (HA) were synthesized using a microwave irradiation technique. Polyvinyl alcohol (PVA) was added during the synthesis of the substituted HA as a binding agent. The synthesized Mg-, Zn-, and Si-substituted HAs were then coated onto a 316L-grade stainless-steel substrate using low-cost electrophoretic deposition (EPD), thereby avoiding exposure to high temperatures. The deposited layer thickness was measured and the structural, phase and morphological analysis were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The bacterial adhesion of Staphylococcus aureus was characterized at 30 min, 2 h and 6 h. The results showed homogeneous, uniform thickness (50–70 µm) of the substrate. FTIR and XRD showed the characteristic spectral peaks of HA, where the presence of Mg, Zn and Si changed the spectral peak intensities. The Mg–HA coating showed the least bacterial adhesion at 30 min and 2 h. In contrast, the Si–HA coating showed the least adhesion at 6 h. EPD showed an effective way to get a uniform coating on bio-grade metal implants, where ionic-substituted HA appeared as alternative coating material compared to conventional HA and showed the least bacterial adhesion.