Structural characterization and biological fluid interaction of Sol-Gel-derived Mg-substituted biphasic calcium phosphate ceramics

Influence of Magnesium Source on the Mechanochemical Synthesis of Magnesium-Substituted Hydroxyapatite

Magnesium, as one of the most abundant cations in the human body, plays an important role in both physiological and pathological processes. In this study, it was shown that a promising biomedical material, Mg-substituted hydroxyapatite (Mg-HA), can be synthesized via a fast mechanochemical method. For this method, the nature of magnesium-containing carriers was shown to be important. When using magnesium oxide as a source of magnesium, the partial insertion of magnesium cations into the apatite structure occurs. In contrast, when magnesium hydroxide or monomagnesium phosphate is used, single-phase Mg-HA is formed. Both experimental and theoretical investigations showed that an increase in the Mg content leads to a decrease in the lattice parameters and unit cell volume of Mg-HA. Density functional theory calculations showed the high sensitivity of the lattice parameters to the crystallographic position of the calcium site substituted by magnesium. It was shown experimentally that the insertion of magnesium cations decreases the thermal stability of hydroxyapatite. The thermal decomposition of Mg-HA leads to the formation of a mixture of stoichiometric HA, magnesium oxide, and Mg-substituted tricalcium phosphate phases.

Alkaline earth metals for osteogenic scaffolds: From mechanisms to applications

Regeneration of bone defects is a significant challenge today. As alternative approaches to the autologous bone, scaffold materials have remarkable features in treating bone defects; however, the various properties of current scaffold materials still fall short of expectations. Due to the osteogenic capability of alkaline earth metals, their application in scaffold materials has become an effective approach to improving their properties. Furthermore, numerous studies have shown that combining alkaline earth metals leads to better osteogenic properties than applying them alone. In this review, the physicochemical and physiological characteristics of alkaline earth metals are introduced, mainly focusing on their mechanisms and applications in osteogenesis, especially magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Furthermore, this review highlights the possible cross-talk between pathways when alkaline earth metals are combined. Finally, some of the current drawbacks of scaffold materials are enumerated, such as the high corrosion rate of Mg scaffolds and defects in the mechanical properties of Ca scaffolds. Moreover, a brief perspective is also provided regarding future directions in this field. It is worth exploring that whether the levels of alkaline earth metals in newly regenerated bone differs from those in normal bone. The ideal ratio of each element in the bone tissue engineering scaffolds or the optimal concentration of each elemental ion in the created osteogenic environment still needs further exploration. The review not only summarizes the research developments in osteogenesis but also offers a direction for developing new scaffold materials.

Biodegradable magnesium phosphates in biomedical applications

As an essential element, magnesium is involved in a variety of physiological processes. Magnesium is the second most abundant cation in cells and the fourth most abundant cation in living...

Biocompatible magnesium-doped biphasic calcium phosphate for bone regeneration

Autologous bone grafting remains the gold standard for almost all bone void-filling orthopedic surgery. However, autologous bone grafting has several limitations, thus scientists are trying to identify an ideal synthetic material as an alternative bone graft substitute. Magnesium-doped biphasic calcium phosphate (Mg-BCP) has recently been in the spotlight and is considered to be a potential bone substitute. The Mg-BCP is a mixture of two bioceramics, that is, hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), doped with Mg2+ , and can be synthesized through chemical wet-precipitation, sol-gel, single diffusion gel, and solid state reactions. Regardless of the synthesis routes, it is found that the Mg2+ preferentially accommodates in β-TCP lattice instead of the HA lattice. The addition of Mg2+ to BCP leads to desirable physicochemical properties and is found to enhance the apatite-forming ability as compared to pristine BCP. In vitro results suggest that the Mg-BCP is bioactive and not toxic to cells. Implantation of Mg-BCP in in vivo models further affirmed its biocompatibility and efficacy as a bone substitute. However, like the other bioceramics, the optimum physicochemical properties of the Mg-BCP scaffold have yet to be determined. Further investigations are required regarding Mg-BCP applications in bone tissue engineering.

Synthesis, Characterization and Process Optimization of Bone Whitlockite

Whitlockite, being the second most abundant bio-mineral in living bone, finds huge applications in tissue regeneration and implants and its synthesis into its pure form has remained a challenge. Although precipitation of whitlockite phase has been reported recently in many publications, effects of various parameters to control such phase as well as conditions for the bulk preparation of this extremely important bio-mineral have not been investigated so far. In this work, we report the precipitation of pure whitlockite phase using common precursors. As reported in the literature, whitlockite is stable in a narrow pH range, therefore; optimization of pH for the stabilization of whitlockite phase has been investigated. Additionally, in order to narrow down the optimum conditions for the whitlockite precipitation, effect of temperature and heating conditions has also been studied. The obtained solids were characterized using powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). From PXRD analysis, it was observed that heating the precursor's mixture at 100 °C with subsequent aging at the optimized pH resulted in the precipitation of pure whitlockite phase. These results were further confirmed by TGA, SEM and Raman spectroscopy analysis and it was confirmed that the conditions reported here favor whitlockite precipitation without formation of any secondary phase. These reaction conditions were further confirmed by changing all the parameters like aging, heating time, feed rate of precursors one by one. From PXRD analysis of these samples, it was concluded that not only pH but temperature, heating time, aging time and feed rate effect simultaneously on the precipitation of pure whitlockite phase and a subtle change in any of these parameters could lead to the formation of undesired stable secondary calcium phosphate phases.

Healing of osteoporotic bone defects by micro-/nano-structured calcium phosphate bioceramics

The micro-/nano-structured calcium phosphate bioceramic exhibited a higher new bone substitution rate in an osteoporotic bone defect rat model.

Nano-biphasic calcium phosphate/polyvinyl alcohol composites with enhanced bioactivity for bone repair via low-temperature three-dimensional printing and loading with platelet-rich fibrin

Background and aim

As a newly emerging three-dimensional (3D) printing technology, low-temperature robocasting can be used to fabricate geometrically complex ceramic scaffolds at low temperatures. Here, we aimed to fabricate 3D printed ceramic scaffolds composed of nano-biphasic calcium phosphate (BCP), polyvinyl alcohol (PVA), and platelet-rich fibrin (PRF) at a low temperature without the addition of toxic chemicals.

Methods

Corresponding nonprinted scaffolds were prepared using a freeze-drying method. Compared with the nonprinted scaffolds, the printed scaffolds had specific shapes and well-connected internal structures.

Results

The incorporation of PRF enabled both the sustained release of bioactive factors from the scaffolds and improved biocompatibility and biological activity toward bone marrow-derived mesenchymal stem cells (BMSCs) in vitro. Additionally, the printed BCP/PVA/PRF scaffolds promoted significantly better BMSC adhesion, proliferation, and osteogenic differentiation in vitro than the printed BCP/PVA scaffolds. In vivo, the printed BCP/PVA/PRF scaffolds induced a greater extent of appropriate bone formation than the printed BCP/PVA scaffolds and nonprinted scaffolds in a critical-size segmental bone defect model in rabbits.

Conclusion

These experiments indicate that low-temperature robocasting could potentially be used to fabricate 3D printed BCP/PVA/PRF scaffolds with desired shapes and internal structures and incorporated bioactive factors to enhance the repair of segmental bone defects.

Cu-doping of calcium phosphate bioceramics: From mechanism to the control of cytotoxicity

In this study, the Cu-doping mechanism of Biphasic Calcium Phosphate (BCP) was thoroughly investigated, as was its ionic release behavior, in order to elucidate cytotoxicity features of these bioceramics. BCP are composed of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) and β-TCP (Ca₃(PO₄)₂). The two phases present two different doping mechanisms. Incorporation into the β-TCP structure is achieved at around 700 °C thanks to a substitution mechanism leading to the Cu-doped Ca_3-xCu_x(PO₄)₂ compound. Incorporation into the HAp structure is achieved thanks to an interstitial mechanism that is limited to a Cu-poor HAp phase for temperatures below 1100 °C (Ca₁₀Cu_x(PO₄)₆(OH)_2-2xO_2x with x < 0.1). Above 1100 °C, the same interstitial mechanism leads to the formation of a Cu-rich HAp mixed-valence phase (Ca₁₀Cu²⁺_xCu⁺_y(PO₄)₆(OH)_2-2x-yO_2x+y with x + y ∼ 0.5). The formation of both high-temperature Cu-doped α-TCP and Cu₃(PO₄)₂ phases above 1100 °C induces a transformation into the Cu-rich HAp phase on cooling. The linear OCuO oxocuprate entity was confirmed by EXAFS spectroscopy, and the mixed Cu⁺/Cu²⁺ valence was evidenced by XPS analyses. Ionic releases (Cu⁺/Cu²⁺, Ca²⁺, PO₄^2- and OH^-) in water and in simulated body media were investigated on as-synthesized ceramics to establish a pretreatment before biological applications. Finally the cytotoxicity of pretreated disks was evaluated, and results confirm that Cu-doped BCP samples are promising bioceramics for bone substitutes and/or prosthesis coatings.

STATEMENT OF SIGNIFICANCE

Biphasic Calcium Phosphates (BCP) are bioceramics composed of hydroxyapatite (HAp, Ca₁₀(PO₄)₆(OH)₂) and beta-Tricalium Phosphate (β-TCP, Ca₃(PO₄)₂). Because their chemical and mineral composition closely resembles that of the mineral component of bone, they are potentially interesting candidates for bone repair surgery. Doping can advantageously be used to improve their biological behaviors; however, it is important to describe the doping mechanism of BCP thoroughly in order to fully appraise the benefit of the doping process. The present paper scrutinizes in detail the incorporation of copper cation in order to correctly interpret the behavior of the Cu-doped bioceramic in biological fluid. The understanding of the copper doping mechanism, related to doping mechanism of others 3d-metal cations, makes it possible to explain the rates and kinetic of release of the dopant in biological medium. Finally, the knowledge of the behavior of the copper doped ceramic in biological environment allowed the tuning of its cytotoxicity properties. The present study resulted on pre-treated ceramic disks which have been evaluated as promising biocompatible ceramic for bone substitute and/or prosthesis coating: good adherence of bone marrow cells with good cell viability.

Atomic scale modeling of iron-doped biphasic calcium phosphate bioceramics

Biphasic calcium phosphates (BCPs) are bioceramics composed of hydroxyapatite (HAp, Ca₁₀(PO₄)₆(OH)₂) and beta-Tricalcium Phosphate (β-TCP, Ca₃(PO₄)₂). Because their chemical and mineral composition closely resembles that of the mineral component of bone, they are potentially interesting candidates for bone repair surgery, and doping can advantageously be used to improve their biological behavior. However, it is important to describe the doping mechanism of BCP thoroughly in order to be able to master its synthesis and then to fully appraise the benefit of the doping process. In the present paper we describe the ferric doping mechanism: the crystallographic description of our samples, sintered at between 500°C and 1100°C, was provided by Rietveld analyses on X-ray powder diffraction, and the results were confirmed using X-ray absorption spectroscopy and ⁵⁷Fe Mössbauer spectrometry. The mechanism is temperature-dependent, like the previously reported zinc doping mechanism. Doping was performed on the HAp phase, at high temperature only, by an insertion mechanism. The Fe³⁺ interstitial site is located in the HAp hexagonal channel, shifted from its centre to form a triangular three-fold coordination. At lower temperatures, the Fe³⁺ are located at the centre of the channel, forming linear two-fold coordinated O-Fe-O entities. The knowledge of the doping mechanism is a prerequisite for a correct synthesis of the targeted bioceramic with the adapted (Ca+Fe)/P ratio, and so to be able to correctly predict its potential iron release or magnetic properties.

STATEMENT OF SIGNIFICANCE

Biphasic calcium phosphates (BCPs) are bioceramics composed of hydroxyapatite (HAp, Ca₁₀(PO₄)₆(OH)₂) and beta-Tricalium Phosphate (β-TCP, Ca₃(PO₄)₂). Because their chemical and mineral composition closely resembles that of the mineral component of bone, they are potentially interesting candidates for bone repair surgery. Doping can advantageously be used to improve their biological behaviors and/or magnetic properties; however, it is important to describe the doping mechanism of BCP thoroughly in order to fully appraise the benefit of the doping process. The present paper scrutinizes in detail the incorporation of ferric cation in order to correctly interpret the behavior of the iron-doped bioceramic in biological fluid. The temperature dependent mechanism has been fully described for the first time. And it clearly appears that temperature can be used to design the doping according to desired medical application: blood compatibility, remineralization, bactericidal or magnetic response.

First-Row Transition Metal Doping in Calcium Phosphate Bioceramics: A Detailed Crystallographic Study

Doped calcium phosphate bioceramics are promising materials for bone repair surgery because of their chemical resemblance to the mineral constituent of bone. Among these materials, BCP samples composed of hydroxyapatite (Ca10(PO₄)₆(OH)₂) and β-TCP (Ca₃(PO₄)₂) present a mineral analogy with the nano-multi-substituted hydroxyapatite bio-mineral part of bones. At the same time, doping can be used to tune the biological properties of these ceramics. This paper presents a general overview of the doping mechanisms of BCP samples using cations from the first-row transition metals (from manganese to zinc), with respect to the applied sintering temperature. The results enable the preparation of doped synthetic BCP that can be used to tailor biological properties, in particular by tuning the release amounts upon interaction with biological fluids. Intermediate sintering temperatures stabilize the doping elements in the more soluble β-TCP phase, which favors quick and easy release upon integration in the biological environment, whereas higher sintering temperatures locate the doping elements in the weakly soluble HAp phase, enabling a slow and continuous supply of the bio-inspired properties. An interstitial doping mechanism in the HAp hexagonal channel is observed for the six investigated cations (Mn2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+) with specific characteristics involving a shift away from the center of the hexagonal channel (Fe3+, Co2+), cationic oxidation (Mn3+, Co3+), and also cationic reduction (Cu⁺). The complete crystallochemical study highlights a complex HAp doping mechanism, mainly realized by an interstitial process combined with calcium substitution for the larger cations of the series leading to potentially calcium deficient HAp.

Biphasic calcium phosphates bioceramics (HA/TCP): Concept, physicochemical properties and the impact of standardization of study protocols in biomaterials research

Biphasic calcium phosphates (BCP) bioceramics have become the materials of choice in various orthopedic and maxillofacial bone repair procedures. One of their main advantages is their biodegradation rate that can be modified by changing the proportional ratio of the composition phases. For enhanced bone tissue regeneration, the bioactivity of BCP should be increased by optimizing their physicochemical properties. To date, the ideal physicochemical properties of BCP for bone applications have not been defined. This is mostly related to lack of standard study protocols in biomaterial science especially with regards to their characterizations and clinical applications. In this paper we provided a review on BCP and their physicochemical properties relevant to clinical applications. In addition, we summarized the available literature on their use in animal models and evaluated the influences of different composition ratios on bone healing. Controversies in literature with regards to ideal composition ratio of BCP have also been discussed in detail. We illustrated the discrepancies in study protocols among researchers in animal studies and emphasized the need to develop and follow a set of generally accepted standardized guidelines. Finally; we provided general recommendations for future pre-clinical studies that allow better standardization of study protocols. This will allow better comparison and contrast of newly developed bone substitute biomaterials that help further progress in the field of biomaterial science.

Magnesium-containing mixed coatings on zirconia for dental implants: mechanical characterization and in vitro behavior

An important challenge in the field of dental and orthopedic implantology is the preparation of implant coatings with bioactive functions that feature a high mechanical stability and at the same time mimic structural and compositional properties of native bone for a better bone ingrowth. This study investigates the influence of magnesium addition to zirconia-calcium phosphate coatings. The mixed coatings were prepared with varying additions of either magnesium oxide or magnesium fluoride to yttria-stabilized zirconia and hydroxyapatite. The coatings were deposited on zirconia discs and screw implants by wet powder spraying. Microstructure studies confirm a porous coating with similar roughness and firm adhesion not hampered by the coating composition. The coating morphology, mechanical flexural strength and calcium dissolution showed a magnesium content-dependent effect. Moreover, the in vitro results obtained with human osteoblasts reveal an improved biological performance caused by the presence of Mg(2+) ions. The magnesium-containing coatings exhibited better cell proliferation and differentiation in comparison to pure zirconia-calcium phosphate coatings. In conclusion, these results demonstrate that magnesium addition increases the bioactivity potential of zirconia-calcium phosphate coatings and is thus a highly suitable candidate for bone implant coatings.

Preferential occupancy of strontium in the hydroxyapatite lattice in biphasic mixtures formed from non-stoichiometric calcium apatites

Hydroxyapatite unit cell reflecting the preferential occupancy of Sr²⁺ at Ca²⁺ (2) site. Crystal framework represents bonding and orientation of PO₄³⁻ with Ca²⁺ and Sr²⁺ and their relative OH⁻ bonding, which proposes the thermodynamically stable configuration.

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.

MC3T3-E1 proliferation and differentiation on biphasic mixtures of Mg substituted β-tricalcium phosphate and amorphous calcium phosphate

A low temperature aqueous approach was used to synthesize nanocrystalline, high surface area Mg(2+) substituted β-tricalcium phosphate (β-TCMP) to assess its potential use as a synthetic bone graft substitute. X-ray diffraction indicated that β-TCMP was the predominant crystalline phase formed. However, thermal analysis revealed the presence of a secondary amorphous phase which increased with increasing Mg(2+) concentration. Further analysis by Rietveld refinement indicated that the level of ionic substitution of Ca(2+) by Mg(2+) was significantly lower than the amount of Mg(2+) measured using elemental analysis, confirming the formation of a Mg(2+) rich secondary amorphous phase. MC3T3-E1 proliferation on substrates prepared using β-TCMP was assessed using the MTT assay. In comparison to commercially available β-TCP, increased proliferation was observed on samples prepared with 50% Mg, despite elevated Mg(2+) and PO4(3-) concentrations in culture media. Alkaline phosphatase (ALP) activity and qRT-PCR were used to study the effect of varying Mg(2+) substitution on osteogenic differentiation. Cells cultured on β-TCMP substrates prepared with increased Mg(2+) concentrations expressed significantly increased levels of ALP activity and osteogenic genes such as, osteocalcin, collagen-1, and Runx2, in comparison to those cultured on commercially available β-TCP.

Synthesis of Mg2+ Incorporated Hydroxyapatite by Ion Implantation

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.

On the effect of temperature on the insertion of zinc into hydroxyapatite

Rietveld analysis of X-ray powder diffraction patterns recorded from 28 hydroxyapatite (HAp) samples containing various amounts of zinc (0, 1.6, 3.2 and 6.1wt.% Zn) and heat treated at various temperatures (between 500°C and 1100°C) has enabled the Zn insertion mechanism into the HAp crystal structure to be finely characterized. The formation of Zn-doped HAp was achieved above 900°C only. Zn-doped HAp has the Ca(10)Zn(x)(PO(4))(6)(OH)(2-2)(x)O(2)(x) (0<x⩽0.25) chemical composition with a constant Ca/P ratio of 1.67 due to the insertion mechanism into the hexagonal channel (partial occupancy of the 2b Wyckoff site with the formation of linear O-Zn-O entities). Samples heat treated at 500°C were almost single phase, HAp did not incorporate Zn and about half of the Zn atoms incorporated during the synthesis are not observable by X-ray powder diffraction (contained in an amorphous compound or physisorbed at the HAp surface). The reversible formation of Zn-doped β-TCP phase was observed at 600°C, reached its maximum content at 900°C and had almost vanished at 1100°C. The results presented here strengthen the recently described mechanism of Zn insertion in the interstitial 2b Wyckoff position of the HAp structure, and explain the origin of the contradictory reports in the corresponding literature.

Biphasic, triphasic and multiphasic calcium orthophosphates

Biphasic, triphasic and multiphasic (polyphasic) calcium orthophosphates have been sought as biomaterials for reconstruction of bone defects in maxillofacial, dental and orthopedic applications. In general, this concept is determined by advantageous balances of more stable (frequently hydroxyapatite) and more resorbable (typically tricalcium orthophosphates) phases of calcium orthophosphates, while the optimum ratios depend on the particular applications. Therefore, all currently known biphasic, triphasic and multiphasic formulations of calcium orthophosphate bioceramics are sparingly soluble in water and, thus, after being implanted they are gradually resorbed inside the body, releasing calcium and orthophosphate ions into the biological medium and, hence, seeding new bone formation. The available formulations have already demonstrated proven biocompatibility, osteoconductivity, safety and predictability in vitro, in vivo, as well as in clinical models. More recently, in vitro and in vivo studies have shown that some of them might possess osteoinductive properties. Hence, in the field of tissue engineering biphasic, triphasic and multiphasic calcium orthophosphates represent promising biomaterials to construct various scaffolds capable of carrying and/or modulating the behavior of cells. Furthermore, such scaffolds are also suitable for drug delivery applications. This review summarizes the available information on biphasic, triphasic and multiphasic calcium orthophosphates, including their biomedical applications. New formulations are also proposed.

Thorough analysis of silicon substitution in biphasic calcium phosphate bioceramics: a multi-technique study

Four samples of composition Ca(10)(PO(4))(6-x)(SiO(4))(x)(OH)(2-x), with x=0.0, 0.1, 0.2 and 0.5, were prepared and characterized using powder X-ray and neutron powder diffraction, and (1)H, (31)P and (29)Si nuclear magnetic resonance (NMR) spectroscopy. The composition of the Si-substituted HAp phases was determined by joint Rietveld refinements from powder X-ray and powder neutron diffraction data. Taking into account electroneutrality, a chemical formula for the Si-substituted HAp phases with indication of the incorporated silicate amount is proposed. Solid-state (29)Si NMR confirms the presence of only Q(0) species, in good agreement with the presence of substituted HAp and beta-TCP phases only. Thanks to NMR spectroscopy, two types of protons in the Si-substituted HAp phase were identified, the new site corresponding to species engaged in hydrogen bonding with silicate anions. This allowed further refinement of the formulae for these phases with very good quantitative agreement for populations derived from the refinement and integration of NMR data.