Dissolution and re-crystallization processes in multiphase silicon stabilized tricalcium phosphate

Development and performance analysis of Si-CaP/fine particulate bone powder combined grafts for bone regeneration

Background

Although autogenous bone grafts as well as several bone graft substitute material have been used for some time, there is high demand for more efficient and less costly bone-substitute materials. Silicon-substituted calcium phosphates (Si-CaP) and fine particulate bone powder (FPBP) preparations have been previously shown to individually possess many of the required features of a bone graft substitute scaffold. However, when applied individually, these two materials fall short of an ideal substitute material. We investigated a new concept of combining Si-CaP with FPBP for improved performance in bone-repair.

Methods

We assessed Si-CaP/FPBP combined grafts in vitro, by measuring changes in pH, weight loss, water absorption and compressive strength over time.

Results

Si-CaP/FPBP combined grafts was found to produce conditions of alkaline pH levels compared to FPBP, and scaffold surface morphology conducive to bone cell adhesion, proliferation, differentiation, tissue growth and transport of nutrients, while maintaining elasticity and mechanical strength and degradation at a rate closer to osteogenesis.

Conclusion

Si-CaP/FPBP combined grafts was found to be superior to any of the two components individually.

The use of physiological solutions or media in calcium phosphate synthesis and processing

This review examined the literature to spot uses, if any, of physiological solutions/media for the in situ synthesis of calcium phosphates (CaP) under processing conditions (i.e. temperature, pH, concentration of inorganic ions present in media) mimicking those prevalent in the human hard tissue environments. There happens to be a variety of aqueous solutions or media developed for different purposes; sometimes they have been named as physiological saline, isotonic solution, cell culture solution, metastable CaP solution, supersaturated calcification solution, simulated body fluid or even dialysate solution (for dialysis patients). Most of the time such solutions were not used as the aqueous medium to perform the biomimetic synthesis of calcium phosphates, and their use was usually limited to the in vitro testing of synthetic biomaterials. This review illustrates that only a limited number of research studies used physiological solutions or media such as Earle's balanced salt solution, Bachra et al. solutions or Tris-buffered simulated body fluid solution containing 27mM HCO3(-) for synthesizing CaP, and these studies have consistently reported the formation of X-ray-amorphous CaP nanopowders instead of Ap-CaP or stoichiometric hydroxyapatite (HA, Ca10(PO4)6(OH)2) at 37°C and pH 7.4. By relying on the published articles, this review highlights the significance of the use of aqueous solutions containing 0.8-1.5 mMMg(2+), 22-27mM HCO3(-), 142-145mM Na(+), 5-5.8mM K(+), 103-133mM Cl(-), 1.8-3.75mM Ca(2+), and 0.8-1.67mM HPO4(2-), which essentially mimic the composition and the overall ionic strength of the human extracellular fluid (ECF), in forming the nanospheres of X-ray-amorphous CaP.

Cell responses to two kinds of nanohydroxyapatite with different sizes and crystallinities

INTRODUCTION

Hydroxyapatite (HA) is the principal inorganic constituent of human bone. Due to its good biocompatibility and osteoconductivity, all kinds of HA particles were prepared by different methods. Numerous reports demonstrated that the properties of HA affected its biological effects.

METHODS

Two kinds of nanohydroxyapatite with different sizes and crystallinities were obtained via a hydrothermal treatment method under different temperatures. It was found that at a temperature of 140°C, a rod-like crystal (n-HA1) with a diameter of 23 ± 5 nm, a length of 47 ± 14 nm, and crystallinity of 85% ± 5% was produced, while at a temperature of 80°C, a rod-like crystal (n-HA2) with a diameter of 16 ± 3 nm, a length of 40 ± 10 nm, and crystallinity of 65% ± 3% was produced. The influence of nanohydroxyapatite size and crystallinity on osteoblast viability was studied by MTT, scanning electron microscopy, and flow cytometry.

RESULTS

n-HA1 gave a better biological response than n-HA2 in promoting cell growth and inhibiting cell apoptosis, and also exhibited much more active cell morphology. Alkaline phosphatase activity for both n-HA2 and n-HA1 was obviously higher than for the control, and no significant difference was found between n-HA1 and n-HA2. The same trend was observed on Western blotting for expression of type I collagen and osteopontin. In addition, it was found by transmission electron microscopy that large quantities of n-HA2 entered into the cell and damaged the cellular morphology. Release of tumor necrosis factor alpha from n-HA2 was markedly higher than from n-HA1, indicating that n-HA2 might trigger a severe inflammatory response.

CONCLUSION

This work indicates that not all nanohydroxyapatite should be considered a good biomaterial in future clinical applications.

Silicon-stabilized α-tricalcium phosphate and its use in a calcium phosphate cement: characterization and cell response

α-Tricalcium phosphate (α-TCP) is widely used as a reactant in calcium phosphate cements. This work aims at doping α-TCP with silicon with a twofold objective. On the one hand, to study the effect of Si addition on the stability and reactivity of this polymorph. On the other, to develop Si-doped cements and to evaluate the effect of Si on their in vitro cell response. For this purpose a calcium-deficient hydroxyapatite was sintered at 1250°C with different amounts of silicon oxide. The high temperature polymorph α-TCP was stabilized by the presence of silicon, which inhibited reversion of the β→α transformation, whereas in the Si-free sample α-TCP completely reverted to the β-polymorph. However, the β-α transformation temperature was not affected by the presence of Si. Si-α-TCP and its Si-free counterpart were used as reactants for a calcium phosphate cement. While Si-α-TCP showed faster hydrolysis to calcium-deficient hydroxyapatite, upon complete reaction the crystalline phases, morphology and mechanical properties of both cements were similar. An in vitro cell culture study, in which osteoblast-like cells were exposed to the ions released by both materials, showed a delay in cell proliferation in both cases and stimulation of cell differentiation, more marked for the Si-containing cement. These results can be attributed to strong modification of the ionic concentrations in the culture medium by both materials. Ca-depletion from the medium was observed for both cements, whereas continuous Si release was detected for the Si-containing cement.

The influence of silicon substitution on the properties of spherical- and whisker-like biphasic α-calcium-phosphate/hydroxyapatite particles

In this work, the influence of the morphology of hydroxyapatite particles on silicon substitution through hydrothermal synthesis performed under the same conditions was investigated. Spherical- and whisker-like hydroxyapatite particles were obtained starting from calcium-nitrate, sodium dihydrogen phosphate, disodium-ethylenediaminetetraacetic acid and urea (used only for the synthesis of whisker-like particles) dissolved in aqueous solutions. Silicon was introduced into the solution using tetraethylorthosilicate. X-ray diffraction, infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy indicate that silicon doping induce different phase compositions and bioactivity of spherical- and whisker-like hydroxyapatite particles obtained under the same hydrothermal conditions. Silicon-substituted, spherical hydroxyapatites particles showed greater phase transformation to silicon-substituted α- calcium-phosphate compared with whiskers-like hydroxyapatite particles synthesized with the same amount of added silicon. Metabolic activity assay performed with SaOs2 osteosarcoma cells showed better biocompatibility of annealed biphasic spherical-like particles compared with annealed whiskerlike particles while dried spherical-like particles induce high cytotoxicity effect.

Mechanical properties, electronic structure and bonding of alpha- and beta-tricalcium phosphates with surface characterization

The mechanical properties and electronic structure of alpha- and beta-tricalcium phosphate (TCP) crystals are studied by using two ab initio density functional methods, the Vienna Ab initio Simulation Package (VASP) and the orthogonalized linear combination of atomic orbitals method. Based on the VASP optimized crystal structures, the elastic constants of alpha- and beta-TCP are obtained using an effective stress-strain computational scheme. From the calculated elastic constants, the bulk modulus, shear modulus, Young's modulus and Poisson's ratios are obtained. The results show that the mechanical properties of the two crystals are comparable and that alpha-TCP is somewhat softer than beta-TCP. Comparison with experimental extrapolations of the elastic constants shows significant differences, which attest to the difficulty of obtaining single crystal samples. The calculated electronic structure results show that both crystals are large gap insulators with a direct band gap of 4.89 eV for alpha-TCP and 5.25 eV for beta-TCP. Effective charge calculations show that, on average, beta-TCP has slightly less charge transfer per Ca than alpha-TCP. The (010) ((001)) surface model for alpha-TCP (beta-TCP) is studied using a supercell slab geometry and fully relaxed to obtain the optimized structures. The estimated surface formation energies are 0.777 and 0.842 J m(-2) for alpha-TCP and beta-TCP, respectively. The electronic structures of the two surface models are compared with the bulk models. Charge density analysis shows that the surfaces of both TCP crystals are positively charged overall owing to the presence of Ca ions near the surfaces.

In vitro structural changes in porous HA/beta-TCP scaffolds in simulated body fluid

Porous scaffolds of biphasic calcium phosphate (hydroxyapatite/beta-tricalcium phosphate (beta-TCP)) have been fabricated and changes induced both in phase composition and porous architecture by immersion in simulated body fluid (SBF) under static and orbital stirring conditions have been studied. The starting porous scaffolds exhibit a low and randomized micro- and mesoporosity, an interconnected macroporosity centered at 100 and 0.6microm, a fractal connectivity of D=2.981 and total percent porosity of ca. 80%. After immersion for up to 60days the micro- and mesoporosity increase slightly, which could be attributed to dissolution of the beta-TCP phase confirmed by transmission electron microscopy. The effects of the change in the porous framework with SBF immersion time favor the bioactive behavior of the tested materials, inducing a nucleation and growth of a nanocrystalline apatite phase as the interconnected macroporosity centered at 0.6microm is reduced. The macroporosity centered at 100microm is still stable after 60days in SBF. Therefore, these biphasic calcium phosphate porous scaffolds combine bioactive behavior with the stability of interconnected macroporosity over large periods of soaking time in SBF under static and orbital stirring conditions.

Silicon substitution in the calcium phosphate bioceramics

Silicon (Si) substitution in the crystal structures of calcium phosphate (CaP) ceramics such as hydroxyapatite (HA) and tricalcium phosphate (TCP) generates materials with superior biological performance to stoichiometric counterparts. Si, an essential trace element required for healthy bone and connective tissues, influences the biological activity of CaP materials by modifying material properties and by direct effects on the physiological processes in skeletal tissue. The synthesis of Si substituted HA (Si-HA), Si substituted alpha-TCP (Si-alpha-TCP), and multiphase systems are reviewed. The biological performance of these Si substituted CaP materials in comparison to stoichiometric counterparts is discussed. Si substitution promotes biological activity by the transformation of the material surface to a biologically equivalent apatite by increasing the solubility of the material, by generating a more electronegative surface and by creating a finer microstructure. When Si is included in the TCP structure, recrystallization to a carbonated HA is mediated by serum proteins and osteoblast-like cells. Release of Si complexes to the extracellular media and the presence of Si at the material surface may induce additional dose-dependent stimulatory effects on cells of the bone and cartilage tissue systems.