Key Engineering Materials Vol. 631

Abstract: Nanometer scale Ca-deficient hydroxyapatite (nanoapatite) is a potential candidate as artificial bone substitute materials owing to its similarity to the bone with respect to composition, morphology and osteoclastic degradation or adsorbent materials for blood purification therapy to remove pathogenic substances. The initial biodegradation behaviors, the initial cell-material interaction and the protein adsorption properties of nanoapatite must depend on the microstructure. The purpose of this study is the preparation of nanoapatite particles and their structural characterization by using X-ray diffraction (XRD) and solid-state NMR spectroscopy. The nanoapatite particles were prepared by precipitation processing method, and the effects of magnesium ions on the precipitation of calcium phosphate were examined, because Mg ions are well-known to play a role of inhibition of crystal growth. The addition of Mg ions led to the precipitation of nanometer scale Ca-deficient apatite crystals having 1.33-1.63 of the molar ratio (Mg+Ca)/P. NMR analyses showed that the microstructure of Mg•HAp particles can be explained by a crystalline HAp core covered with a thin amorphous hydrated calcium phosphate layer.

Abstract: The main goal of this study was to evaluate the behavior of Sr- and/or Mg-containing hydroxyapatite (HAp) bioceramics in simulated body fluid (SBF). Sr-and/or Mg-containing HAp powders were synthesized by modified wet chemical precipitation method. Sr-and/or Mg-containing HAp bioceramics were prepared by uniaxial pressing of the precipitated powders and subsequent sintering at 1100 °C for 1 h. The synthesis products were characterized in terms of chemical, phase and molecular composition. Influence of the substitutions on thermal stability, morphology and microstructure of the HAp products were evaluated. Results suggest that incorporation of Sr (up to 1.45 wt.%) in HAp structure induced an increasing of particle sizes, but incorporation of Mg (up to 1.05 wt.%) led to a reduction of particle sizes of the HAp powders. The ability to simultaneously release bioactive ions and the apatite-formation ability of the Sr-and/or Mg-containing HAp bioceramics were evaluated through immersing the samples in SBF for different time periods. Ca ions release and apatite-formation ability on the surfaces of the Sr-and/or Mg-containing HAp bioceramics in SBF depends on Sr and/or Mg concentration in the samples.

Abstract: In the investigation of mechanical properties, calcium phosphate cements exhibit large sample-to-sample deviation due to its porous nature, possibility of unhomogenous distribution and small specimen size. This situation generates difficulties for obtaining accurate results and creates an obstacle for testing different composition where only a small batch size is available. In this respect, specimen shape, whether being injected, porosity ratio, surface quality, bearing support design have significant matter on variability in terms of three-and four-point bending test. Therefore, different methods have been studied to reduce variability with a simpler material preparation than common methods on injected and moulded cement. The entire comparison is made with the consideration of three-and four-point bending testing, the eccentric loading error calculation with engineering calculation software “Mathcad 15”, porosity measurement with Archimedes method, microstructure investigation on Scanning Electron Microscope (SEM), macro-porosity distribution measurement by Micro CT Scanner.

Abstract: Solid-state transformation of CDA at high temperature has been investigated using TEM microscopy and diffraction from sintered biphasic calcium phosphate (hydroxyapatite-HA, and beta-tricalcium phosphate-TCP). Microcrystals, between 200nm and 800nm approximately, separated by grain boundaries were found to be either HA-HA or TCP-TCP, but not HA-TCP, suggesting that heteroepitaxial growth is very unlikely between these two orthophosphates. TEM-correlated EDX elemental analysis also demonstrated a higher ionic substitution (Na, Mg) of TCP phase.

Abstract: α-tricalcium phosphate (α-TCP), which shows higher solubility than β-TCP, is bioabsorbable and receives special attention for its ability of turning into hydroxyapatite (HAp) in a physiological condition. Problems occasionally occur as α-TCP porous body is too brittle to be handled. Compressive strengths of β-TCP and HAp dense sintered blocks are reported to be comparable to that of human cortical bone. However, α-TCP dense body has rarely been reported. For fabrication of dense sintered body, two-step sintering (TSS) was applied in this study. The TSS generally has firing processes at the lower temperatures, following that at the higher temperatures. TSS is known as one of most effective processing to prevent grain growth comparing with conventional sintering (CS). Dense body of α-TCP was fabricated by both CS and TSS. TSS processing involves heating specimens to T₁ temperature, followed by holding at a relatively lower T₂ temperature for 12 h. Microstructures of the sintered bodies were characterized and mechanical properties were also evaluated. The specimen prepared by TSS processing with T₁ 1400 °C, T₂ 1300 °C showed the lowest porosity (2.7%) and highest compressive strength (714 MPa) among the prepared specimens. TSS processing might be applicable on densification of calcium phosphate powders to fabricate dense body.

Abstract: The present study focuses on the physico-chemical and structural properties of composite scaffolds composed of biopolymer matrices (collagen or polysaccharide) loaded with calcium phosphate granules. A systematic three-dimensional analysis method was used to quantitatively characterize a series of plugs, strips and putties in terms of percentage of inorganic filler particles, size of the loaded granules, and spatial homogeneity of the calcium phosphate granules distribution. It appears clearly that each biomaterial currently available on the market offers specific properties. As a consequence, surgeons have to choose the medical device that best suits their needs depending on the clinical constraints but also should be aware of the mineral properties which remains key to bone reconstruction.

Abstract: Peroxide ions in apatite provides an additional resource for imparting an antibacterial capability in apatite. A hydrothermal process has been developed for including peroxide ions into the apatite lattice. Three oxygen generation compounds, hydrogen peroxide, ammonium persulphate and paracetic acid were investigated for peroxyapatite generation. Hydrogen peroxide provides the highest peroxide containing apatite. Both the oxygen generation and the apatite lattice formation represented the two critical factors for producing peroxyapatite. Unlike with high temperature processing, the cooling rate did not influence the retained peroxide content. This new process provides a building block for investigating antibacterial properties of peroxyapatite in a low temperature process.

Abstract: Tetracalcium phosphate (TTCP) requires the highest synthesis temperatures of all the calcium phosphates, but now a new process is available at 400 °C lower than previously, at 900 °C. Instead of ball-milling reactants for a homogeneous mix, the reactants were included in an amorphous phase. Heating produced hydroxyapatite, oxyapatite and then TTCP. Amorphous nanoparticles were synthesized and heated in air or in vacuum. The sequence of solid-state reactions were tracked with X-ray diffraction and Fourier transform infra-red spectroscopy. Heating in air stabilized the carbonate containing apatite, thereby requiring higher temperatures for decomposition, as per previous studies. Heating in vacuum promoted oxyapatite; a critical step for reaction with calcium oxide to generate TTCP. This faster process enables production at a lower temperature and reduces the use of ball milling for producing fine TTCP powders.

Abstract: Amorphous calcium phosphate (ACP) plays an important role in the body and can be used as an intermediate phase for forming calcium phosphates. All ACPs are thermodynamically unstable compounds, unless stored in dry conditions or at low temperature (-18^oC), and spontaneously undergo transformation to crystalline calcium phosphates (CaP). This work will investigate the influence of drying on the stability of ACP. ACPs powders were prepared by wet synthesis; mixing solution made of Ca (NO₃)₂∙4H₂O and 30% ammonia with (NH₄)₂HPO₄ and (NH₄)₂CO₃ solution at room temperature. The suspension was stirred, filtered and washed several times with deionized water containing ammonia. ACP samples were dried at different conditions and with different drying agents (DA). XRD and FTIR spectra showed poorly crystallinity powders after drying. Some FTIR spectra indicated residual organic compounds from drying. The Rietveld’s method and Schrrer’s relationship estimated the particle size (0.5 – 20 nm) of ACP. Thermogravimetry (TG) revealed that the moisture (7% – 25%) is released upon drying, and the drying agents have no significant effect on. The drying methods are ordered to show which the most effective for removing moisture. By changing the drying conditions, it is a possible to obtain poorly crystalline ACPs with different particle size and moisture content.

Abstract: We have previously synthesized silver-containing hydroxyapatite (Ag-HAp) powders by an ultrasonic spray-pyrolysis (USSP) technique. On the other hand, we have successfully fabricated novel calcium-phosphate cements (CPCs) composed of mainly β-tricalcium phosphate (β-TCP) phase with anti-washout property (hereafter, β-TCP cement), which was set on the basis of chelate-bonding ability of inositol phosphate (IP6). In this study, we developed novel CPCs with both anti-bacterial and anti-washout properties by adding the Ag-HAp powder into the above β-TCP cements, and examined their anti-bacterial property and cytotoxicity. The Ag-HAp powders with Ag contents of 0, 2, and 5 mol% as a nominal composition were synthesized by an USSP technique. The raw powder for β-TCP cement was prepared by ball-milling the commercially-available β-TCP powder in the IP6 solution. The Ag-HAp/β-TCP powders were prepared by mixing Ag-HAp powder and β-TCP cement powder at a ratio of 25:75 in mass. The Ag-HAp/β-TCP cement was fabricated by mixing the above-mentioned Ag-HAp/β-TCP powder and 2.5 mass% Na₂HPO₄ solution at a powder/liquid ratio of 1/0.3 [g/cm³]. The anti-bacterial property of resulting cements was evaluated using Staphylococcus aureus by biofilm formation test. The Ag-HAp/β-TCP cements containing 2 and 5 mol% Ag showed strong anti-bacterial property among examined specimens. Furthermore, the cytotoxicity of Ag⁺ ion eluted from these cements was also examined using osteoblastic MC3T3-E1 cells and Transwell^® kit. The relative cell viability cultured on each Ag-containing cement specimen was over 80 %, compared with the control (polystyrene plate). These results demonstrate that the present Ag-HAp/β-TCP cements containing 2 mol% Ag are promising one of the candidates as CPCs with both anti-bacterial property and biocompatibility.