Brushite (CaHPO4·2H2O) to octacalcium phosphate (Ca8(HPO4)2(PO4)4·5H2O) transformation in DMEM solutions at 36.5 °C

The purpose of this study was to investigate the transformation of brushite (dicalcium phosphate dihydrate, DCPD, CaHPO₄·2H₂O) powders at 36.5 °C in DMEM (Dulbecco's Modified Eagle Medium) solutions. Two sets of brushite powders with different particle shapes were synthesized to use in the above DMEM study. The first of these brushite powders was prepared by using a method which consisted of stirring calcite (CaCO₃) powders in a solution of ammonium dihydrogen phosphate (NH₄H₂PO₄) from 6 to 60 min at room temperature. These powders were found to consist of dumbbells of water lily-shaped crystals. The second one of the brushite powders had the common flat-plate morphology. Both powders were separately tested in DMEM-immersion experiments. Monetite (DCPA, CaHPO₄) powders were synthesized with a unique water lily morphology by heating the water lily-shaped brushite crystals at 200 °C for 2 h. Brushite powders were found to transform into octacalcium phosphate (OCP, Ca₈(HPO₄)₂(PO₄)₄·5H₂O) upon soaking in DMEM (Dulbecco's Modified Eagle Medium) solutions at 36.5 °C over a period of 24 h to 1 week. Brushite powders were known to transform into apatite when immersed in synthetic (simulated) body fluid (SBF) solutions. This study found that DMEM solutions are able to convert brushite into OCP, instead of apatite.     

Mechanochemical-hydrothermal synthesis and characterization of fluoridated hydroxyapatite

Fluoridated hydroxyapatite (FHAp) was successfully synthesized from the starting materials of CaCO₃, CaHPO₄·2H₂O, and CaF₂ via a mechanochemical-hydrothermal route. X-ray diffraction, infrared spectroscopy, surface area measurements, and scanning electron microscopy identified the resultant powders as FHAp nanocrystals with the specific surface areas of up to 114.72 m²/g. The mechanism study revealed that under such mechanochemical-hydrothermal conditions the formation reactions of FHAp were completed in two stages. The starting materials firstly reacted into a poorly crystallized calcium-deficient apatite and the complete incorporation of fluoride ions into apatite occurred in the second stage.     

In vitro effect of magnesium inclusion in sol-gel derived apatite

Magnesium-containing apatite coatings were prepared on Ti6Al4V substrates by sol-gel dip coating method. Standard simulated body fluid (SBF) was used to evaluate the bioactivity of the coatings. A series of the coatings according to the composition (Ca_10−xMg_x)(PO₄)₆(OH)₂, where x = 0 to 2, is synthesized and immersed in the standard SBF for periods of 7 to 35 days for direct deposition of apatite layer from the SBF solution. Scanning electron microscopy (SEM) was used to examine the morphology changes of the SBF apatite layer that occurred during in vitro immersion. X-ray diffractometry, Fourier Transformation Infra-Red Spectroscopy and X-ray Photoelectron Spectroscopy were used to analyse the phases, chemical groups and composition of the sol-gel coating. Results show that as the sol-gel coating contains magnesium, this promotes deposition of apatite layer from SBF. As x ≤ 1, SBF immersion gives rise to a dense apatite layer. However, as x ? 1, selected dissolution of the deposited layer takes place, which results in serious pitting on the surface. Also, Mg ions from the dissolution of the sol-gel coating during immersion in the SBF apparently played a role in the subsequent deposition of apatite o the coating, evidence of Mg was found in the apatite layer.     

In-vitro study of the spontaneous calcification of PHEMA-based hydrogels in simulated body fluid

In-vitro calcification of poly(2-hydroxyethyl methacrylate) (PHEMA)-based hydrogels in simulated body fluid (SBF) under a steady/batch system without agitation or stirring the solutions has been investigated. It was noted that the formation of calcium phosphate (CaP) deposits primarily proceeded through spontaneous precipitation. The CaP deposits were found both on the surface and inside the hydrogels. It appears that the effect of chemical structure or reducing the relative number of oxygen atoms in the copolymers on the degree of calcification was only important at the early stage of calcification. The morphology of the CaP deposits was observed to be spherical aggregates with a thickness of the CaP layer less than 0.5 μ m. Additionally, the CaP deposits were found to be poorly crystalline or to have nano-size crystals, or to exist mostly as an amorphous phase. Characterization of the CaP phases in the deposits revealed that the deposits were comprised mainly of whitlockite [Ca₉MgH(PO₄)₇] type apatite and DCPD (CaHPO₄⋅2H₂O) as the precursors of hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂]. The presence of carbonate in the deposits was also detected during the calcification of PHEMA based hydrogels in SBF solution.     

Bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid

The purpose of this study was to investigate bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid (SBF). Calcium phosphates were electrodeposited on commercially pure titanium substrates in the modified SBF at 60 °C for 1 h maintaining the cathodic potentials of − 1.5 V, − 2 V, and − 2.5 V (vs. SCE). Subsequently, the calcium phosphate coatings were transformed into apatites during immersion in the SBF at 36.5 °C for 5 days. The apatites consisted of needle-shaped crystallites distributed irregularly with different grain sizes. As the coatings were electrodeposited at higher cathodic potential, the crystallite of the apatites got denser and the grain sizes of the apatites became bigger during subsequent immersion in the SBF. However, as the coatings were electrodeposited at higher cathodic potential, the coatings were transformed into apatites with lower crystallinity and the Ca/P atomic ratio of the apatites got higher than 1.67, that of stoichiometric hydroxyapatite, after subsequent immersion in the SBF. In addition, CO₃²⁻ ions contained in the modified SBF were incorporated in the calcium phosphate coating during electrodeposition and had an influence on transforming the calcium phosphate into bonelike apatite during subsequent immersion in the SBF showing that CO₃²⁻ incorporated in the apatites disturbed crystallization of the apatites. These results revealed that the coating electrodeposited at − 2.0 V (vs. SCE) in the modified SBF containing CO₃²⁻ ions was the most bioactive showing transformation into carbonate apatite similar to bone apatite.     

Improved stability of plasma-sprayed dicalcium silicate/zirconia composite coating

Dicalcium silicate/zirconia composite coatings were produced on Ti-6Al-4V substrates using atmospheric plasma spraying. Different weight ratios of zirconia (50 wt.%, 70 wt.%, 90 wt.%) were mechanically blended with dicalcium silicate (C₂S) powders as feedstocks. The composite coatings were immersed in a simulated body fluid (SBF) and a Tris-HCl solution for the in vitro appraisement of stability and long-term performance in a biological environment. The ion concentration changes of Ca, Si, and P in SBF and Tris-HCl solution were monitored using inductively-coupled plasma atomic emission spectroscopy (ICP-AES). Compared to the pure C₂S coating, our results show that the dissolution rate of the composite coatings is effectively reduced and the stability is improved by the addition of zirconia. The high content of zirconia in the coatings ensures the long-term performance in biological environment, while dissolution of C₂S in the coatings results in a higher Ca ion concentration in SBF and rapid precipitation of bone-like apatite on the composite coating surfaces indicating good bioconductivity of the coatings.     

Mechanical modulation and bioactive surface modification of porous Ti–10Mo alloy for bone implants

The objective of this work was to fabricate a suitable porous Ti–10Mo alloy as the human bone replacement implants. The porous Ti–10Mo alloy was fabricated by mechanical alloying and then consolidated by powder metallurgy technique. NH₄HCO₃ powder was used as space-holder. It was indicated that the mean pore size, porosity, compressive strength, and elastic modulus of porous Ti–10Mo alloy could be tailored by the amount of NH₄HCO₃, and then could be matched with those of human bones. Furthermore, porous Ti–10Mo alloy was treated by alkali heat treatment and soaked in the 1.5 times simulated body fluid (1.5SBF). It was observed that the surface and the inside pore wall of porous Ti–10Mo alloy with 25 wt.% NH₄HCO₃ covered with the apatite layer after soaked in 1.5SBF for 28 days. These phenomena indicated that the surface modified porous Ti–10Mo alloy exhibited a high potential for bone-bonding, which was expected to be used as bone tissue implant.     

Gallium-containing phosphosilicate glasses: Functionalization and in-vitro bioactivity

A gallium containing glass 45.7SiO₂·24.1Na₂O·26.6CaO·2.6P₂O₅·1.0Ga₂O₃ (referred to as “Ga1.0”) and a parent Ga-free glass 46.2SiO₂·24.3Na₂O·26.9CaO·2.6P₂O₅ (hereinafter represented as “H”), corresponding to Bioglass® 45S5, were functionalized with Tetraethoxysilane (TEOS) and (3-Aminopropyl)triethoxysilane (APTS) in order to improve their ability to bond with biomolecules, such as drugs, proteins, and peptides. Functionalization with TEOS and APTS promoted the increment in OH groups and formation of NH₂ groups on the glass surface, respectively. The presence of OH or NH₂ groups was investigated by means of IR spectroscopy and elemental analysis. Moreover, in vitro study of these functionalized glasses was performed in simulated body fluid (SBF) so as to investigate the effect of functionalization on the bioactive behavior of H and Ga1.0. The results showed that the functionalization was obtained along with maintaining their bioactivity. The surfaces of both functionalized glasses were covered by a layer of apatite within 30 days of SBF immersion. In addition, CaCO₃ was also identified on the surface of APTS functionalized glasses. However, no gallium release was detected during SBF soaking.     

Mineral phase deposition on pectin microspheres

The nucleation of apatite and calcium phosphates onto natural polysaccharides containing carboxyl groups can be obtained by immersion in biomimetic solutions. The deposition of hydroxyapatite has been recently described also on pectin, and pectin–apatite hybrid gels were proposed as scaffolds for bone tissue regeneration. In this work, injectable calcium phosphate/pectin microspheres were prepared to promote both the natural process of biomineralization and the controlled release of encapsulated cells, genes, enzymes, proteins or drugs in the pathological situ. Pectin microspheres were prepared with different formulations from aqueous solutions using an extrusion system, and incubated in H₂O or SBF for 14 days. The presence of an excess of CaCl₂, deriving from the preparation process, was determinant in promoting the deposition of calcium phosphates from SBF, whereas no mineral deposition was detected on pectin microspheres extensively purified after preparation. The nucleation of calcium phosphates occurred on pectin microspheres incubated in SBF for 14 days and was evaluated through ESEM–EDS analysis and FT-IR spectroscopy.     

Microstructural characteristics and evolution of Ti2AlN/TiAl composites with a network reinforcement architecture during reaction hot pressing process

The microstructural evolution of TiAl matrix composites with a novel network distribution of Ti₂AlN particle reinforcement was studied. The composites were synthesized by reaction hot pressing method using pure Al and nitrided Ti powders as initial materials. Pure Ti powders nitrided at 600 °C for a certain time in an atmosphere of flowing nitrogen turned into new compound Ti(N) powders, which have a shell of titanium nitrides (such as TiN, Ti₂N and TiN_0.3) and a core of Ti–N solid solution. Within the composites synthesized, Ti₂AlN particles, produced by in situ reaction, exhibit a network distribution. The special shell/core structure of the compound Ti(N) powders contributes to this architecture. Nitriding time of the Ti powders greatly affects the microstructure of the composites. Increasing the nitriding time is beneficial to the distribution of Ti₂AlN particles in a continuous network form. However, too long nitriding time can result in the aggregation of Ti₂AlN particles and thus destroy the uniformity of the network structure. The in-situ synthesized Ti₂AlN/TiAl composites with uniform network structure have a superior mechanical property, and their compressive strengths at 800 °C and 1000 °C are 1112 MPa and 687 MPa, respectively.     

Formation of calcium phosphates on low-modulus Ti–7.5Mo alloy by acid and alkali treatments

This study investigated the hydroxyapatite (HA) coating on metal implants in order to enhance their bioactive properties. In this study, HA coatings were formed on the surfaces of commercially pure titanium (c.p. Ti) and Ti–7.5Mo which were acid-etched and subsequently alkali-treated before samples were soaked in simulated body fluid (SBF). Specimens of c.p. Ti and Ti–7.5Mo were etched in either H₃PO₄ or HCl, and subsequently treated in NaOH. The surfaces of acid-etched c.p. Ti showed a porous structure, whereas those of acid-etched Ti–7.5Mo showed some grinding marks, but no porosity. After subsequent alkali treatment in NaOH, the surfaces of both the c.p. Ti and Ti–7.5Mo substrates exhibited microporous network structures. The specimens were then immersed in SBF at 37 °C for 28 days. Apatite began to deposit on acid-etched and NaOH-treated Ti–7.5Mo within 1 day after immersion in the SBF. After 28 days of immersion in the SBF, a dense and uniform layer was produced on the surfaces of all samples. The HA formation rate was the highest for HCl and NaOH-pretreated samples, and the results of EDS and XRD showed that much more intensive peaks of HA appear on the specimens of HCl and NaOH-treated Ti–7.5Mo than on any other sample. Thus, this method of apatite coating Ti–7.5Mo appears to be promising for artificial bone substitutes or other hard tissue replacement materials with heavy load-bearing applications due to their desirable combination of bioactivity, low elastic modulus, and low processing costs.     

Phosphating treatment and corrosion properties of Mg–Mn–Zn alloy for biomedical application

A phosphating treatment was applied to Mg–Mn–Zn alloy in order to improve the corrosion resistance. Surface morphology and phase constitute were observed and identified by SEM, EDS, SAXS, XRD and XPS. SEM observation showed that a rough and crystalline reaction layer was formed on the surface of Mg alloy. With the increasing of phosphating time, the layer became thicker and denser. It has been showed that the reaction layer was mainly composed of brushite (CaHPO₄ · 2H₂O). Small amount of Zn²⁺ was also detected by XPS and EDS. The corrosion resistance of the phosphated samples was measured by the electrochemical polarization and the immersion test in comparison with the bare alloy. The results manifested that the corrosion resistance of Mg alloy was improved by the phosphating treatment, and the corrosion resistance increased with the increase of the phosphating time within 50 min. Immersion tests showed that the phosphate layer could protect magnesium alloy from fast corrosion. The brushite layer has been transformed into hydroxyapatite (HA) during the immersion in the simulated body fluid (SBF) solution, which suggested the brushite layer could provide good biocompatibility.     

Preparation of a bone-like apatite foam cement

The preparation of a porous bone-like calcium deficient apatite implant material was investigated. A novel cement system composed of an equimolar mixture of Ca₄(PO₄)₂O, Ca(H₂PO₄)₂{H₂O, and CaCO₃ was used. At a liquid/powder ratio of 0.83 ml/g low density open framework foam cements were formed due to the rapid evolution of CO₂. The initial product of the reactants was CaHPO₄{2H₂O which then reacted with Ca₄(PO₄)₂O, forming a calcium deficient carbonated apatite, upon soaking of the cement blocks in SBF. Foam-like cements were composed of a plate-like apatite due to epitaxial overgrowth and conversion of the brushite plate precursor. Cylinders of the foam cement were reinforced with an outer layer of a solid apatite cement to form a material suitable for application as a bone-section implant. © 2001 Kluwer Academic Publishers     

Enhanced apatite formation on Ti metal heated in PO2-controlled nitrogen atmosphere

The oxynitridation of biomedical titanium metal under a precisely regulated oxygen partial pressure (P_O2) of 10^? 14 Pa in nitrogen atmosphere at 973 K for 1 h strongly enhanced apatite formation compared with that on Ti heated in air. The factors governing the high apatite-forming ability are discussed from the viewpoint of the surface properties of Ti heated under a P_O2 of 10^? 14 Pa in nitrogen atmosphere determined from X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and zeta potential measurements. Nitrogen (N)-doped TiO₂ (interstitial N) was formed on pure Ti heated under a P_O2 of 10^? 14 Pa in nitrogen atmosphere at 973 K. The XPS O1s main peak shifted toward a lower binding energy upon heating under a P_O2 of 10^? 14 Pa. This shift may be due to the formation of oxygen vacancies. This Ti surface had a positive zeta potential of approximately 20 mV. According to time-of-flight secondary ion mass spectroscopy results, PO₄^3 ? ions were predominantly adsorbed on Ti soaked in simulated body fluid (SBF) after heat treatment, followed by calcium ions. It was concluded that the apatite formation kinetics can be described using the Avrami–Erofeev equation with an Avrami index of n = 2, which implies the instantaneous nucleation of apatite on the surface of Ti soaked in SBF after heat treatment at 973 K under a P_O2 of 10^? 14 Pa.     

Surface modification of Ti–Nb–Zr–Sn alloy by thermal and hydrothermal treatments

Surface modifications by thermal and hydrothermal treatments in solution with calcium ions were investigated with the aim of improving bioactivity and wear resistance of a Ti–Nb–Zr–Sn alloy. The results showed that the first step of thermal treatment at 600 °C significantly increases the surface hardness and energy by forming oxides of Ti and Nb. The second step of hydrothermal treatment in a boiled supersaturated Ca(OH)₂ solution induces a bioactive layer containing CaTiO₃, CaCO₃, Ca(OH)₂ and TiO₂. Using this treatment, a complete Ca–P layer can be formed within 3 days of soaking in simulated body fluid (SBF). The origin of such fast apatite formation was analyzed by comparison with single step thermal or hydrothermal treatment and with thermal plus hydrothermal treatment without calcium ions. The results suggest that the increase of surface energy by thermal treatment and the incorporation of calcium ions by the hydrothermal treatment in calcium ion solution play important roles in the formation of bioactive apatite.     

Fretting wear behavior of calcium phosphate-mullite composites in dry and albumin-containing simulated body fluid conditions

In a recent work, it has been shown that it is possible to achieve a better combination of compressive strength, flexural strength and toughness properties in calcium phosphate (CaP) composites containing 20 and 30 wt% mullite (3Al₂O₃·2SiO₂). In view of their potential application as load bearing implants, the present work reports the friction and wear properties of the newly developed composites against zirconia under dry ambient as well as in simulated body fluid (SBF) containing bovine serum albumin (BSA) protein. For comparison, experiments were also conducted on monolithic hydroxyapatite (HAp, Ca₁₀(PO₄)₆(OH)₂) and mullite under identical conditions. Under the investigated fretting conditions, the mullite-containing composites exhibited higher coefficient of friction (COF) of 0.4–0.6, compared to pure HAp (COF ~ 0.25–0.3). Although the wear resistance of the composites containing 20 or 30 wt% mullite was better in dry conditions, higher wear rate was measured in SBF conditions. The difference in tribological properties has been analyzed in reference to the difference in phase assemblage and mechanical properties. A comparison with some competing biomaterials reveals good potential of the investigated CaP-mullite composites for application as wear resistant implants.     

Preparation and in vitro bioactivity of poly(d,l-lactide) composite containing hydroxyapatite nanocrystals

The nano-sized hydroxyapatite (n-HA) was incorporated into poly(d,l-Lactide) (PDLLA) to form a bioactive and biodegradable composite for application in hard tissue replacement and regeneration. Thin film of PDLLA composite containing 20 mass% of n-HA fillers was successfully developed through integration of solvent co-blending and hot pressing techniques. firstly, n-HA and PDLLA were chemically synthesized, respectively, then mixed together and homogeneously dispersed in N,N-dimethyl formamide(DMF) solvent, finally, the dried blended hybrid containing PDLLA matrix and n-HA fillers was put into the mould and compacted by hot-pressing machine under 8 MPa pressure at 110 °C for 15 min. In vitro studies were conducted using the simulated body fluid(SBF). Composite specimens were soaked in SBF from 1 day to 21 days prior to surface analysis. Results obtained from scanning electron microscopy(SEM) examination, Energy dispersive X-ray detector(EDX) analysis and X-ray diffraction (XRD) analysis showed that a layer of non-stoichiometric apatite formed within 7 days on HA/PDLLA composite surface after its immersion in SBF, demonstrating moderate in vitro bioactivity of n-HA/PDLLA composite, though a moderate rate of apatite formation in SBF was found on initial stage of immersion periods for n-HA/PDLLA composite, compared to the other biomaterial composite. This type of composite film exhibited certain desirable bioactive characteristics, and they are promising bone candidates to develop novel bioactive composites for biomedical application.     

Ceramic–metal and ceramic–polymer composites prepared by a biomimetic process

A biomimetic process was developed to prepare apatite–metal and apatite–polymer composites. A variety of metals and organic polymers incorporated surface functional groups such as Si–OH, Ti–OH or Ta–OH to induce formation of a biologically active bonelike apatite by chemical treatment or physical adsorption. Subsequent immersion in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma or 1.5 SBF led to the formation of a dense and uniform bonelike apatite layer on the surface. Apatite–metal and apatite–polymer composites prepared in this way are believed to be very useful as artificial bone substitutes.     

Apatite formation from simulated body fluid on various phases of TiO2 thin films prepared by filtered cathodic vacuum arc deposition

Hydroxyl (OH⁻)-free TiO₂ thin films with amorphous and crystalline phases were deposited onto (100) silicon substrates using filtered cathodic vacuum arc deposition in order to investigate the in vitro apatite formation in simulated body fluid (SBF). The surface morphology, composition and structure of the TiO₂ thin films were characterized. The X-ray photoelectron spectroscopy results confirmed the presence of calcium and phosphorus on all TiO₂ thin film surfaces after immersion in SBF at 37 °C. Fourier transform infra red results showed the presence of carbonated apatite on the surface of these films. Amorphous structured TiO₂ thin film showed poor ability to form apatite on its surface in SBF. Apatite formation was more pronounced on the surfaces of the anatase films in comparison to those of rutile. The carbonated apatite deposition rate increased significantly when the TiO₂ film was illuminated with UV light prior to immersing in the SBF. In particular, the UV-treated anatase and rutile films showed increased rates of carbonated apatite formation on their surfaces in comparison to samples not treated with radiation. The increase in hydrophilicity due to UV treatment appears beneficial for the apatite growth on these surfaces.     

Growth of calcium phosphate on surface-modified cotton

A study of the growth of amorphous calcium phosphate on surface-modified cotton fibres by a combination of scanning electron microscopy/electron diffraction X-ray analysis, micro-FTIR and X-ray photoelectron spectroscopy is reported. Cotton fibres phosphorylated by the urea/phosphorous acid method and then soaked in saturated Ca(OH)₂ for approximately one week were found to stimulate the growth of a calcium phosphate coating on their surfaces after soaking in 1.5×SBF for as little as 1 day. Ca(OH)₂ soaking of the fibres is found to produce highly crystalline clusters lodged in the fibres which were confirmed by micro-FTIR to be calcium phosphite monohydrate (CaHPO₃·H₂O). In contrast, phosphorylated fibres not subjected to the Ca(OH)₂ treatment did not exhibit calcium phosphate growth upon immersion in 1.5×SBF solution. Soaking of the Ca(OH)₂-treated fibres with time in the 1.5×SBF solution produced progressively thicker layers of calcium phosphate on the fibres as confirmed by scanning electron microscopy and X-ray photoelectron spectroscopy. In general, calcium phosphate coatings formed over 1 1–5 day period soaking in 1.5×SBF solution appeared to consist of agglomerations of a large number of small spherical particles, while coatings formed after 17 days of soaking were distinctly chunky, thick and non-uniform in appearance. Micro-FTIR indicated that CaHPO₃·H₂O clusters were still present in cotton samples even after 4 days of soaking, while after 17 days, only the infrared spectrum typical of calcium phosphate was observed. EDX-measured Ca:P ratios of the coatings, although variable, suggested amorphous calcium phosphate. The mechanism of formation of the coating is believed to involve dissolution of the CaHPO₃.H₂O clusters upon introduction of the Ca(OH)₂-treated phosphorylated cotton into the 1.5×SBF solution which elevates the Ca²⁺ ion concentration in the vicinity of the fibres so stimulating calcium phosphate formation. It is postulated that phosphite groups chemically bound to the cotton fibre surface or a calcium phosphite coating on the fibres act as nucleation sites for calcium phosphate growth in 1.5×SBF solution.