Apatite Coating on Organic Polymers by a Biomimetic Process

Dense and uniform layers of a biologically active carbonate-containing hydroxyapatite can be formed on various kinds of organic polymers by the following biomimetic method. First, a substrate is set in contact with particles of CaO–SiO₂-based glass soaked in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma for forming the apatite nuclei on the substrate. Second, the substrate is soaked in another solution highly supersaturated with respect to the apatite, e.g, with ion concentrations 1.5 times those of SBF (1.5SBF) for making the apatite nuclei grow on the substrate in situ. The induction period for the apatite nucleation, which is defined as the time of the first treatment required for forming enough of the apatite nuclei to make the continuous layer after the second treatment, was almost 24 h for most of the examined polymers. The adhesive strength of the formed apatite layer to the polymers was as high as 3 to 4 M Pa for poly(ethylene terephthalate), poly-ether sulfone, and poly (vinyl alcohol) hydrogel. This type of apatite–organic polymer composite is expected to be useful for repairing not only living hard tissues but also soft ones.     

Interconnected Hydroxyapatite Nanofibers in the Biomimetic Coating of a Bioinert Ceramic Substrate

A bioinert ceramic substrate, α-Al₂O₃, has been coated with hydroxyapatite (HAp) by the biomimetic route using simulated body fluid (SBF) solution at room temperature. The substrate was incubated at 37°C in SBF for 6 days with a periodic replacement with freshly prepared SBF at 48-h intervals. After 6 days, continuous nanofiber-like structures of HAp (5–35 μm in length, 0.05 μm diameter) were obtained, connecting the intra- and interglobular clusters, within the coated mineral layer on the substrate surface. This is a unique and new observation, and this phenomenon has been demonstrated by a simple fractal growth model.     

In-vitro bioactivity of wollastonite materials derived from limestone and silica sand

This paper describes the behaviour of bioactive wollastonite materials containing Malaysian limestone and silica sand. Wollastonite, which is also known as calcium silicate (CaSiO₃), is an industrial mineral composed of calcium, silicon and oxygen. Pseudowollastonite, which is a primary crystal of wollastonite, was synthesised via a solid-state reaction at a temperature of 1450 °C. The in-vitro bioactivity of wollastonite was examined by soaking it in simulated body fluid (SBF) solution for 1–7 days at 36.5 °C. The soaked wollastonite samples were characterised using XRD, SEM-EDX, FTIR and ICP analyses. Apatite particles precipitated on the surface of the wollastonite sample after the sample was soaked in the SBF. The XRD analysis indicated the presence of an increasing amount of the hydroxyapatite phase as the soaking time increased. The SEM and EDX analyses indicated the formation of granules of agglomerated apatite particles on the surface of the soaked wollastonite sample. During the formation of apatite, phosphate ions from the SBF solution were consumed. This process was confirmed by ICP, which revealed a decrease in ion concentration after the soaking process. The FTIR analysis indicated that the peaks of the phosphate ions increase when the apatite layer forms on the surface of the wollastonite sample. After the soaking process, a calcium deficient hydroxyapatite layer was observed on the wollastonite sample. The study concludes that wollastonite produced from Malaysian limestone and silica sand is bioactive and may be used as an implantable biomaterial.     

Bonelike Apatite Formation Induced on Zirconia Gel in a Simulated Body Fluid and Its Modified Solutions

Formation of bonelike apatite on zirconia gel in a simulated body fluid (SBF) with ion concentrations almost equal to those in human blood plasma, in modified SBF solutions to have increased pH values, and modified SBF solutions to have increased concentrations of calcium and phosphate ions has been investigated. The zirconia gel forms apatite on its surface in SBF, indicating that Zr-OH groups, abundant on the gel, act as effective apatite nucleation centers. Apatite formation is accelerated by increases in pH and in the concentration of calcium and phosphate ions, which is explained by an increase in the ionic activity product of the apatite in the SBF. These results suggest that zirconia ceramics may exhibit a bone-bonding ability by forming an apatite layer on their surfaces in the living body when they are modified to have many Zr-OH groups on their surfaces.     

Apatite-Forming Ability of Sodium-Containing Titania Gels in a Simulated Body Fluid

An essential condition for an artificial material to bond to living bone is the formation of bonelike apatite on its surface in the living body. The bonelike apatite can be reproduced on the bone-bonding material even in an acellular simulated body fluid (SBF) with ion concentrations almost equal to those of human blood plasma. In the present study, the dependence of the apatite-forming abilities of sodium-containing titania gels in a SBF on composition and structure is examined. The sodium-containing titania gels are model substances produced on the surface layer of bioactive titanium metal prepared by sodium hydroxide solution and heat treatments. When sodium-containing titania gels are immersed in the SBF, Na⁺ ions incorporated in the gels are exchanged with the H₃O⁺ ions in the SBF. This ion exchange causes an accompanying increase in the pH of the SBF and increases its ionic activity product, thus providing favorable conditions for apatite nucleation on the surfaces of the gels. Nevertheless, sodium-containing titania gels that do not contain anatase do not form apatite on their surfaces. Independent of the composition, the gels form apatite on their surfaces in the SBF, specifically when they contain anatase. These results imply that the Ti-OH groups on titania, which have been proposed to be responsible for the apatite formation, are effective for apatite nucleation when they are arranged in a specific structural unit based on the anatase structure.     

Evaluation of ascorbic acid-loaded calcium phosphate bone cements: Physical properties and in vitro release behavior

In this study, different concentrations of ascorbic acid (50, 100 and 200 µg/mL) were added to the liquid phase of a calcium phosphate cement (CPC). The cements were immersed in simulated body fluid (SBF) for different intervals and physical, physicochemical and mechanical properties of them were evaluated. The release of added ascorbic acid from CPCs into the SBF solution was also studied. From the results, both setting time and injectability of CPC decreased by adding ascorbic acid, however the compressive strength was sharply increased before soaking in SBF solution. But, the compressive strength values of all cements (with or without ascorbic acid) soaked in SBF solution for more than 7 d duration were comparable. The X-ray diffractometry results showed that in vitro apatite formation ability of cement reactants did not change by adding ascorbic acid. The scanning electron microscopy images indicated that morphology of the formed apatite crystals was nano-needlelike and needle diameter was less than 100 nm. The loaded ascorbic acid was slowly released from CPC into the SBF solution so that about 10% and 20% of the loaded drug was released after 504 h for the cements containing 100 and 200 µg/mL ascorbic acid, respectively. The release rate was increased when the amount of added ascorbic acid decreased by 50 µg/mL.     

Effect of Texture on the Rate of Hydroxyapatite Formation on Gel-Silica Surface

An apatite layer can be formed on pure gel-silica soaked in simulated body fluid. The rate of formation depends on solution parameters and sintering temperature of the gelsilica. In this study, the effect of the texture of the gel-silica on the rate of hydroxyapatite formation was investigated. The apatite formation was monitored by means of Fourier-transform infrared reflection spectroscopy as well as by the measurement of changes in the ion concentration of the fluid. The induction time for apatite nucleation on the gel silica decreased as pore size and pore volume increased. The substrate parameters that affect nucleation are discussed and a mechanism that assumes pores as nucleation sites for hydroxyapatite is proposed.     

Investigation of Apatite Deposition onto Charged Surfaces in Aqueous Solutions Using a Quartz-Crystal Microbalance

Hydroxyapatite (HAp) deposition onto positively charged surfaces (i.e., self-assembled monolayers (SAMs) terminated with NH₂ head groups) and negatively charged surfaces (i.e., OH-SAMs (weak) and COOH-SAMs (strong)) soaked at 50°C in aqueous supersaturated solutions (1.5 SBF, pH 7.0–7.6; SBF = simulated body fluid) was investigated using a quartz-crystal microbalance. The results revealed that the solution conditions greatly influenced the formation of HAp on the charged surfaces. In a stable supersaturated solution of simulated body fluid (1.5 SBF, pH <7.4), more strongly negative surfaces had a more powerful induction capability for the heterogeneous nucleation of HAp (COOH > OH), whereas nucleation was obviously prohibited on a positive surface (NH₂-SAM). On the other hand, after the calcium phosphate particles had nucleated homogeneously in an unstable soaking solution (1.5 SBF, pH ≧7.4), adhesion of the HAp microparticles to the NH₂-SAM was observed. A two-step interaction is conceivable to describe the formation of HAp on the positive NH₂-SAM: At the first stage, electrostatic interaction dominates the adhesion of HAp microparticles; at the second stage, hydrogen bonds possibly form between the HAp microparticles and the amino head groups of the NH₂-SAM, for a firm bonding with the substrate, and the microparticles grow progressively into a thin film. The electrophoretic behaviors of the HAp microparticles confirmed this hypothesis.     

Bioactive and degradable organic–inorganic hybrids

CaO–SiO₂–poly(vinyl alcohol) (PVAL) and CaO–P₂O₅–SiO₂–PVAL organic–inorganic hybrids were obtained as monoliths and characterized before and after be soaked in a solution mimicking human plasma. The hybrids were obtained by adding PVAL (0.9, 1.8 and 3.6 wt.%) to three CaO–(P₂O₅)–SiO₂ gel glasses with 25 mol% of CaO and 0, 2.5 and 5 mol%, respectively of P₂O₅. The influence of PVAL and P₂O₅ on the monoliths obtaining and on their textural properties and in vitro behavior was analyzed. Additions of PVAL favored the synthesis of cracked-free monoliths able to be coated with bone-like apatite after be soaked in Kokubo's simulated body fluid (SBF), i.e. to present in vitro bioactivity. Increasing P₂O₅ contents made the hybrids syntheses difficult and decreased their in vitro bioactivity. In addition, the in vitro degradation of hybrids increased with the increasing of PVAL and P₂O₅. Thus, hybrids with the highest amounts of both components showed so high degradation in SBF that the apatite layer formation was impeded. Organic–inorganic hybrids in these systems could be clinically used as bone defect fillers in non load bearing applications or as matrices in controlled release systems.     

Fabrication and In Vitro Bioactivity of Poly (ϵ-caprolactone) Composites Filled with Silane-Modified Nano-Apatite Particles

Silane coupling agents were firstly employed to modify the surfaces of nano-apatite (n-HA) particles, and then thin films of the silanized n-HA/PCL composites were successfully developed by incorporating solvent dispersion and thermal co-blending with hot-pressing methods. In vitro studies were conducted using the 2-time simulated body fluid (2SBF). Composite specimens were soaked in 2SBF from 3 to 14. Results showed that a layer of bone-like apatite was formed within 7 days on the surfaces of all composites, after its immersion in 2SBF, demonstrating moderate in vitro bioactivity of these composites.     

Hydrolysis of α-Tricalcium Phosphate in Simulated Body Fluid and Dehydration Behavior During the Drying Process

The hydrolysis of α-tricalcium phosphate (α-TCP) in a simulated body fluid (SBF) at 37°C was investigated. The hydration rate was found to be slower in SBF than that in deionized water. The concentration of ions in SBF was monitored by ICP. The hydrolysis product, which was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infra red, and X-ray photoelectron spectroscopy, was determined to be carbonate-containing, calcium-deficient hydroxyapatite (CO₃−CDHAp) with Mg²⁺, Na⁺, and Cl⁻ impurities similar to the biological apatite. An amorphous layer on the α-TCP surface was found to be the precursor of the apatite phase, which may either form crystalline apatite or may decompose back to α-TCP at a lower temperature.     

碳/碳复合材料表面软复合磷酸钙层

为改进碳／碳复合材料的生物活性，发展了表面软复合磷酸钙层的制备工艺。首先在碳／碳复合材料表面通过离子束辅助沉积技术形成的钛镀层，然后经浓碱液处理后呈多孔网状，该网状结构可在模拟体液（SBF）中诱导沉积出生理磷灰石层，从而在碳／碳复合材料表面形成软复合磷酸钙层。所获得的软复合磷酸钙层厚度约为8μm ，结晶结构为羟基磷灰石，但其Ca,P摩尔比n(Ca)/n(P)低于1．67。     

Surface Potential of Poly(lactic acid) Composites Containing Calcium Carbonates in Simulated Body Fluid

The surface of a poly(lactic acid) composite (CCPC) containing calcium carbonate was reported to show apatite formation in simulated body fluid (SBF) within 24 h at 37°C. In the present work, ζ potentials of CCPC were examined to determine the rapid apatite formation mechanism at an early stage after soaking in SBF. The ζ potentials of CCPC showed negatively charged values in SBF. The potential decreased immediately after soaking, and subsequently increased during 1–6 h of soaking. After 6–9 h of soaking, no change in ζ potentials was observed. Laser Raman spectroscopy results suggested that the change in the ζ potential is closely related to the amount of Ca²⁺-coordinated carboxy groups on the CCPC surface. The concentration of phosphate ion in SBF decreased after 6 h. Apatite formation was suggested to begin after 6–9 h of soaking via Ca²⁺ coordination to carboxy groups and the subsequent adsorption of phosphate ions.     

Chemical Processing of CaHPO4·2H2O:

The aim of this paper is to develop a robust chemical process to synthesize Na- and K-doped brushite (DCPD: dicalcium phosphate dihydrate, CaHPO₄·2H₂O), a potential starting material for bone substitutes. The powders were synthesized by using sodium phosphate and potassium phosphate and aqueous solutions containing calcium chloride at room temperature, followed by drying at 37°C. DCPD powders thus formed were found to contain 460 ppm K and 945 ppm Na. On calcination in air, these powders readily transformed into monetite (DCPA: dicalcium phosphate anhydrous, CaHPO₄) first, and then into Ca₂P₂O₇ (calcium pyrophosphate). Na- and K-doped DCPD powders were shown to completely transform, in less than 1 week, into poorly crystalline carbonated apatite on immersion in an acellular simulated/synthetic body fluid (SBF) solution at 37°C. The Tris (i.e., tris(hydroxymethyl)aminomethane) buffered SBF solution used in this study had a carbonate ion concentration of 27 m M equal to that of human plasma. DCPD powders of this study displayed a notable apatite-inducing ability. This finding suggests the use of these DCPD powders as the starting materials for potential bone substitutes, which can be easily manufactured in aqueous solutions friendly to living tissues, at temperatures between room temperature and 37°C.     

单晶硅阳极氧化层表面骨状磷灰石的体外诱导沉积

在磷酸钠碱性电解质中对单晶硅(100)进行阳极氧化处理,并将阳极氧化处理后的样品在模拟体液(simulated body fluid,SBF)中浸泡以考察其体外诱导骨状磷灰石沉积能力。用扫描电子显微镜观测体外浸泡前后样品的表面形貌,用电子能谱仪和X射线衍射仪研究阳极氧化后与体外浸泡不同时间后样品表面的成分。结果表明:单晶硅在10%的磷酸钠电解质中于5～20mA/cm2阳极氧化后,其表面原位形成火山口状结构,在SBF中浸泡1d后有磷灰石在单晶硅的阳极氧化层表面成核,浸泡6d后诱导形成了纳米骨状缺钙磷灰石层,体现出良好的体外诱导活性,表明阳极氧化处理是一种有希望应用于改善生物学与医学用单晶硅表面生物相容性的有效途径。     

Characterization of Insoluble Layers Formed by NaOH Attack on the Surface of a ZrO2-Containing Silicate Glass

The durability of an alkali-resistant glass containing ZrO₂ in NaOH solution was studied by scanning electron microscopy. Insoluble Zrrich reaction-product layers ∼1 to 3 μm thick were observed on the surface of glass which had been soaked for 25 d in 2N NaOH at 95°C. In some cases the layer seems to be an adherent film, but in others it adheres loosely to the glass surface and does not seem to effectively block alkali attack .     

Effect of magnesium content in simulated body fluid on apatite forming ability of wollastonite ceramics

Abstract

Three simulated body fluids were prepared by varying the magnesium ion content. Ceramic specimens, obtained by uniaxial pressing of wollastonite powders followed by sintering, were immersed in each simulated body fluid for different periods of time at physiological conditions of pH and temperature. In all the cases, an apatite layer was formed on the ceramic specimens. However, the morphology of the layer changed substantially. Furthermore, the crystallinity of the apatite layer decreased as the magnesium content in SBF was increased.     

Microscopic and spectroscopic evidences for multiple ion-exchange reactions controlling biomineralization of CaO.MgO.2SiO2 nanoceramics

This study is focused on the mechanism of in vitro biomineralization on the surface of CaO.MgO.2SiO₂ (diopside) nanostructured coatings by scanning electron microscopy, energy-dispersive X-ray spectroscopy and inductively coupled plasma spectroscopy assessments. A homogeneous diopside coating of almost 2 µm in thickness was deposited on a medical-grade stainless steel by coprecipitation, dipping and sintering sequences. After soaking the sample in a simulated body fluid (SBF) for 14 days, a layer with the thickness of 8 µm is recognized to be substituted for the primary diopside deposit, suggesting the mineralization of apatite on the surface. Investigations revealed that the newly-formed layer is predominantly composed of Ca, P and Si, albeit with a biased accumulations of P and Si towards the surface and substrate, respectively. The variations in the ionic composition and pH of the SBF due to the incubation of the sample were also correlated with the above-interpreted biomineralization. In conclusion, the multiple ion-exchange reactions related to Ca, Mg, Si and P were found to be responsible for the in vitro bioactivity of nanodiopside.     

Hydroxyapatite Coating on a Collagen Membrane by a Biomimetic Method

The coating of a carbonate-containing hydroxyapatite (HAp) on a nonbioactive collagen membrane via a biomimetic method has been investigated. The collagen membranes were soaked in a simulated body fluid (SBF) solution with and without citric acid, and carbonate-containing HAp formed only in the SBF solution that contained citric acid. The results were explained in terms of the strong chelation ability of citric acid with the calcium ion. Practical application may involve the inclusion of citric acid in the SBF solution to promote the formation of HAp on previously nonbioactive collagen membranes.     

Effect of carbonate content on the in vitro bioactivity of carbonated hydroxyapatite

The effect of carbonate content on the apatite-forming ability of carbonated hydroxyapatite (CHA) in simulated body fluid (SBF) has been investigated. Five different nanocrystalline B-type CHA with carbonate content ranged from 2.01 to 5.25 wt% were prepared, sintered, and assessed for their in vitro bioactivity in SBF solution for 7-weeks under static conditions at 36.5 °C. The formation of the apatite layer and the surface morphology of CHA were examined by using a scanning electron microscope (FESEM) at week 1, 3, and 7 of SBF immersion, respectively. The Ca/P molar ratio of the CHA was determined by X-ray fluorescence (XRF). In addition, the sample weight changes and the pH of the SBF solution were measured. The results show that the formation of apatite layer depends on the carbonate content of CHA. Increasing the carbonate content caused significant increases in the surface area of CHA and the rate of apatite formation. Weight loss was observed for all CHA samples during the first week of SBF immersion, and thereafter followed by weight regain weekly until week 7. The changes in the pH of SBF and the Ca/P molar ratio were proportional to the carbonate content of CHA. This study thus highlights the importance of determining carbonate content aspect that govern the bioactivity of CHA.