Key Engineering Materials Vols. 529-530

Abstract: The addition of silicon ions to hydroxyapatite (HA) provides a more inorganic bone-like chemical composition compared to stoichiometric HA. It is known to aid the bioactivity of the material and to improve the rates of osseointegration, osteoconduction and bone mineralisation. The literature, however, lacks detailed information regarding each step of the aqueous precipitation procedure to produce silicon-substituted HA (Si-HA). The current work utilised Raman spectroscopy at each stage of the aqueous precipitation method to determine how the silicate is incorporated into the HA structure when producing Si-HA. Raman spectra indicated that at the initial stages of the reaction the disilicate ion (Si₂O₇^6-) formed with the orthosilicate (SiO₄^4-) ion becoming more dominant after sintering. The results demonstrated that the form of silicate in the Si-HA aqueous precipitation method can be tracked using Raman spectroscopy.

Abstract: The main goal of this work is to prepare carbon and silicon co-substituted calcium hydroxyapatite (C_x-Si_y-HA) for bone tissue engineering application. This study includes the synthesis of pure powders with a controlled amount of carbonate (x) and silicate (y) ions within the apatite structure, their characterization with the establishment of database for different compositions, and the manufacture of dense bioceramics. Carbon-silicon co-substituted hydroxyapatite (C_0.5-Si_0.5-HA) powders are synthesized by aqueous precipitation. According to structural, spectroscopic and elemental characterizations, silicate and carbonate are included in the apatite lattice and their stoichiometries are controlled. The heat treatments under CO₂ atmosphere allow the sintering of pellets without decomposition of the apatite structure.

Abstract: Tricalcium phosphate (TCP) ceramics are useful biodegradable bone-repairing materials. Silicate-containing TCP ceramics are expected to be useful as biodegradable bone-repairing materials which promote the bone regeneration because it has been reported that the silicate promotes bone formation. In the present study, silicate-containing TCP ceramics were prepared through a wet chemical process at the starting compositions from 0 to 0.05 in the Si/(P+Si) molar ratio. The prepared silicate-containing TCP ceramics were characterized and evaluated in vitro. The crystal phase of the products was α-TCP, and the tendency that the lattice constants linearly shifted from 0 to 0.05 in the starting Si/(P+Si) molar ratio was observed. It is speculated that the added silicate was incorporated in the crystal structure of TCP. The pellets were prepared by a sintering process, and soaked in a simulated body fluid (SBF) to estimate their bone-bonding ability. The addition of silicate to TCP promoted to hydroxyapatite formation on the TCP ceramics in SBF. This result implies the high possibility that the silicate addition would promote the bone-bonding ability of the TCP ceramics.

Abstract: Boron-containing hydroxyapatite (BAp) powders with Ca/(P+B) molar ratio of 1.67 (x=0.4, 0.5, and 0.6) were prepared by an ultrasonic spray-pyrolysis (USSP) technique. The resulting BAp powders were uniaxially compressed to fabricate the green compacts. The BAp ceramics were fabricated by firing the compacts at 1200 °C for 5 h. The crystalline phases of the BAp (x=0.4) ceramics were bipahses of apatite and small amounts of α-tricalcium phosphate (a-Ca₃(PO₄)₂; α-TCP). Meanwhile, HAp and BAp (x=0.5 and 0.6) ceramics were apatite single phase. The dense BAp ceramics with relative density of 90% over were fabricated from the powder of x=0.5 and 0.6. The static contact angles of the BAp ceramics (x=0.5 and 0.6) increased with an increase of pH; meanwhile, the surface potential decreased with an increase of pH. These changes in surface properties may be due to the increase of OH group in B(OH)₃ formed via dissociation of BO₂ group in BAp.

Abstract: Mg substituted hydroxyapatite (Mg-HA) has been reported to promote activity of osteoblast and inhibit function of osteoclast in vitro. Mg-HA was synthesized by ion implantation of Ca²⁺, P²⁺ and Mg²⁺ beams by using an electrostatic medium energy accelerator. Oxygen injection had a major role in the formation of HA on Ti substrate. The osteoblast cells spread and formed lamellae on the coating surfaces. But Mg²⁺ ion implanted HA had shown higher osteoblast cell count and higher protein activity. The presence of Mg in the coating had better osteoblast activity and differentiation. Implantation of Ca and P ions into Ti substrate to successfully form HA and subsequent Mg ion incorporation in this layer shows that medium energy ions are very helpful in increasing the tensile strengths drastically.

Abstract: Zinc is an essential trace element in body and has an important role of bone formation. Osteoporosis occurs by imbalance of osteoclasts and osteoblasts activity. The osteoclasts activity lowers the pH of peripheral body environment. Therapeutic agents release material in response to the osteoclasts activity is expected to be a controlled released material. In this study, Zn-containing amorphous calcium phosphate (ACP) powder was synthesized by wet synthesis. Zn content of obtained powders was higher than that of beginning content. ACP powders could be easy to take Zn at wet synthesis. After soaking in phosphate buffered saline for 24 hours, all of synthetic powders were transformed into low crystalline apatite. On the other hand, after soaking in acetic acid buffer for 24 hours, all these powders dissolved. Zn-containing ACP powders are expected to be a Zn controlled released material.

Abstract: The properties of hydroxyapatite can be improved by substitution of biologically relevant ions, such as magnesium (Mg) and strontium (Sr), into its structure. Previous work in the literature has not reached agreement as to site preferences in these substitutions, and there are suggestions that these may change with differing levels of substitution. The current work adopted a quantum mechanical approach based on density functional theory using the CRYSTAL09 code to investigate the structural changes relating to, and site preferences of, magnesium and strontium substitution (to 10 mol%) in hydroxyapatites and also to predict the corresponding vibrational spectra in the harmonic approximation. The structures underwent full geometrical optimisation within the P6₃ space group, indicating an energetic site preference for the Ca (2) site in the case of Mg substitution, and the Ca (1) site in the case of Sr. Shrinkage of the unit cell was observed in the case of Mg substitution, and expansion in the case of Sr substitution, in agreement with the corresponding ionic radii. Thermodynamic properties of the structures obtained from the harmonic vibrational frequency calculations confirmed that the structures were minima on the potential energy surface. Isotopic substitutions indicated that the main contribution of Sr and Mg to vibrational modes is at frequencies < 400 cm^-1.

Abstract: Pure β-TCP scaffolds with an aligned, lamellar, open and interconnected porosity were fabricated by the ice-templating process. The morphology of the scaffold was analysed and the mechanical properties of the different scaffolds were tested by compression tests. The structural sizes of the scaffolds (pore width and ceramic cell wall thickness) were adjusted by the onset of a constant ice front velocity. For this purpose a freezing device was developed and a specific solution of the non-steady heat equation with a disturbing factor (phase transformation energy from water) was used to model the process parameters. It was found that increasing the ice front velocity decreases the structural sizes and consequently increases the compression strength of the scaffold.

Abstract: The most important characteristic of biomaterial as bone-repairing material, in addition to biocompatibility and appropriate porosity, is providing mechanical strength complying with injured tissue. In the present work, slurry with 15 vol% HA prepared from calcinated hydroxyapatite. The prepared slurry freeze casted unidirectionally with the cooling rate of 8°C/min from the ambient temperature. Then, green bodies freeze-dried for 72h following with sintering at different temperatures of 1250-1350°C with intervals of 25°C. The results showed that lamella space and porosity decreases with temperature while compressive strength and shrinkage goes up. Total porosity has a range of 75-83% while has a compressive strength of ~2-8 MPa. The sintered sample at 1350°C, with 75% porosity, which has a ~ 8 MPa compressive strength, chose to be the optimum. Also, some dendritic branch like structure and bridges can be seen on the internal walls of lamellae which can improve mechanical properties. These features may improve adhesion and growth of osseous cells.

Abstract: The paper presented here deals with the investigations of orthophosphates (Q₀) containing none or differing amounts of meta-(Q₂) and diphosphate phases (Q₁) for the use of 3-dimensional printing process in order to create porous, bioactive, nonloadbearing bone replacement scaffolds. The main ceramic phase in all cases is Ca₁₀[K/N(PO₄)₇ hereinafter called 401545(100) consisting of 99,9% Q₀ and 0,1% Q₁-phase. The other phosphate ceramics i) 401545(40) consists of 75% Q₀-phase, 22% Q₁-phase and 4% Q₂-phase ii) 401545(15) consists of 65% Q₀-phase, 33% Q₁-phase and 2% Q₂-phase iii) 401545consists of 56% Q₀-phase, 40% Q₁-phase and 4% Q₂-phase. The in-house produced ceramics where crushed and sieved to achieve particles of irregular shape in the range of 45-90µm. These powders show a quite good flowability and were used to generate cylindrical samples with a diameter of 5,5mm and a height of 11mm via 3-dimensional printing using a R1 printer from ProMetal company (USA). After drying the samples at T=125°C for 48 hours they were sintered at temperatures according to the thermal analysis results in the range of 900°C up to 1300°C. Afterwards the porosity, the linear shrinkage and the compressive strength were determined.