Hydroxyapatite/Bioactive Glass Films Produced by a Sol–Gel Method: In Vitro Behavior

Hydroxyapatite (HA) and HA/bioactive glass (49S) films were deposited on Si(100) substrates by a sol–gel dip‐coating method. The microstructure and in vitro bioactivity of the films were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X‐ray photoelectron spectroscopy (XPS). Polycrystalline HA and amorphous bioactive glass films were obtained after annealing at 600 and 400 °C, respectively. The crystallization temperature of HA was determined to be around 568 °C. The surfaces of the HA films were covered with an apatite layer consists of spherulites formed by nanosized needle‐like aggregates after the soaking in simulated body fluid (SBF) for 10 days, while amorphous HA/bioactive glass surface was covered with larger spherical crystallites. Both XPS and EDS results obtained from HA/bioactive glass film, after soaking in SBF, showed increasing P amounts on the surface at the expense of Si. The higher density of the newly formed layer on HA/bioactive glass surface than that of the HA surface after 10 days of soaking was evidence of increased reaction rate and apatite forming ability when bioactive glass layer is present on the HA films.     

Preparation of layered bioceramic hydroxyapatite/sodium titanate coatings on titanium substrates using a hybrid technique of alkali-heat treatment and electrochemical deposition

Bioceramic hydroxyapatite/sodium titanate coating on sandblasted titanium substrate was fabricated by a three-step process. At first, the sandblasted titanium substrate was coated with a flake-like sodium titanate layer by alkali-heat treatment. In the second step, the alkali-heat treated titanium substrate was hydrothermal treated at 180 °C for 4 h with calcium solutions. In the third step, the hydroxyapatite (HA) coating was deposited onto the hydrothermal treated layer via electrochemical deposition method. The surface topography and roughness of the coatings were determined by field emission scanning electron microscope (FESEM) and a mechanical contact profilometer, respectively. The surface compositions were evaluated by X-ray diffraction (XRD), energy-dispersive X-ray spectrum (EDS), and X-ray photoelectron spectroscopy (XPS). The EDS, XPS, and XRD analysis confirm the presence of element Ca, Ca²⁺, and CaTiO₃ on sodium titanate layer after hydrothermal treatment with Ca(NO₃)₂ solution, respectively. FESEM micrograph shows the rod/needle-shaped crystallites are highly densely packed on the calcium-ion-containing layer with an average size of ~50 nm in diameter. The results indicate that the sodium titanate layer containing Ca²⁺ ions possesses higher ability to induce HA formation compared with the pure sodium titanate layer. It is revealed that surface composition plays an important role in the electrochemical deposition of HA. The calcium-ion-containing layer probably makes the nucleation of HA easy and effectively promotes orientated growth of HA on flake-like sodium titanate surface. The sodium titanate layer possesses a lower corrosion current density and a higher corrosion potential than sandblasted-Ti substrate. The sodium titanate layer should act as a barrier to the release of metal ions from metallic substrate to physiological solutions and thus reducing the electrochemical reaction rate.     

Hydroxyapatite coating by biomimetic method on titanium alloy using concentrated SBF

This article reports a biomimetic approach for coating hydroxyapatite on titanium alloy at ambient temperature. In the present study, coating was obtained by soaking the substrate in a 5 times concentrated simulated body fluid (5XSBF) solution for different periods of time with and without the use of CaO-SiO₂ based glass as a possible source of nucleating agent of apatite formation. Optical microscopic and SEM observations revealed the deposition of Ca-P layer on the titanium alloy by both the methods. Thickness of coating was found to increase with the increase in immersion time. The use of glass did not help the formation of apatite nuclei on the substrate and the coating obtained by this method was also not uniform. EDX analysis indicated that the coating consisted of Ca-P based apatite globules, mostly in agglomerated form, and its crystallinity was poor as revealed by XRD.     

Topographical characterization and microstructural interface analysis of vacuum-plasma-sprayed titanium and hydroxyapatite coatings on carbon fibre-reinforced poly(etheretherketone)

In the present study, topographical characterization and microstructural interface analysis of vacuum-plasma-sprayed titanium and hydroxyapatite (HA) coatings on carbon fibre-reinforced polyetheretherketone (CF/PEEK) was performed. VPS-Ti coatings with high roughness values (Ra=28.29±3.07 m, Rz=145.35±9.88 m) were obtained. On this titanium, intermediate layer HA coatings of various thicknesses were produced. With increasing coating thickness, roughness values of the HA coatings decreased. A high increase of profile length ratio, Lr, of the VPS-Ti coatings (Lr=1.45) compared to the grit-blasted CF/PEEK substrate (Lr=1.08) was observed. Increasing the HA coating thickness resulted in a reduction of the Lr values similar to the roughness values. Fractal analysis of the obtained roughness profiles revealed that the VPS-Ti coatings showed the highest fractal dimension of D=1.34±0.02. Fractal dimension dropped to a value of 1.23–1.25 for all HA coatings. No physical deterioration of the CF/PEEK substrate was observed, indicating that substrate drying and the used VPS process parameter led to the desired coatings on the composite material. Cross-section analysis revealed a good interlocking between the titanium intermediate layer and the PEEK substrate. It is therefore assumed that this interlocking results in suitable mechanical adhesive strength. From the results obtained in this study it is concluded that VPS is a suitable method for manufacturing HA coatings on carbon fibre-reinforced PEEK implant materials.     

Investigation on the effect of collagen and vitamins on biomimetic hydroxyapatite coating formation on titanium surfaces

This study uses an in vitro experimental approach to investigate the roles of collagen and vitamins in regulating the deposition of hydroxyapatite layer on the pure titanium surface. Titanium implants were coated with a hydroxyapatite layer under biomimetic conditions by using a supersaturated calcification solution (SCS), modified by adding vitamins A and D₃, and collagen. The hydroxyapatite deposits on titanium were investigated by means of scanning electron microscopy (SEM) coupled with X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Fourier transformed infrared (FTIR) spectroscopy. The results obtained have shown that hydroxyapatite coatings were produced in vitro under vitamins and collagen influence.     

Biomimetic coatings on titanium: a crystal growth study of octacalcium phosphate

The biomimetic approach allows the coating of metal implants with different calcium-phosphate (Ca-P) phases. Films elaborated at physiological conditions exhibited structures closely resembling those of bone mineral. For instance, octacalcium phosphate (OCP, Ca₈(HPO₄)₂(PO₄)₄ · 5H₂O) crystals have been deposited on titanium through a two-step procedure. After cleaning and etching, Ti6Al4V plates were immersed for 24 h into a simulated body fluid (SBF1). A thin amorphous carbonated Ca-P layer precipitated on the metal substrate. Secondly, these thinly Ca-P coated titanium substrates were immersed for 48 h into another simulated body fluid (SBF2). The thin amorphous carbonated Ca-P layer induced the fast precipitation of a second Ca-P layer of 55 m in thickness composed of OCP crystals. The measurements of Ca and P concentrations versus soaking time in SBF2 showed that the carbonated Ca-P layer partially dissolved before the deposition of the OCP coating. X-ray diffraction (XRD) revealed that OCP crystals grew epitaxially on the substrate. OCP is known to be one of the precursors during the bone mineralization process, thereby, this new generation of biomimetic coatings are promising for orthopedic surgery. © 2001 Kluwer Academic Publishers     

Bone-like apatite coating on Mg-PSZ/Al2O3 composites using bioactive systems

A biomimetic method was used to promote bioactivity on zirconia/alumina composites. The composites were composed of 80 vol% Mg-PSZ and 20 vol% Al₂O₃. Samples of these bioinert materials were immersed in simulated body fluid (SBF) for 7 days on either a bed of wollastonite ceramics or bioactive glass. After those 7 days, the samples were immersed in a more concentrated solution (1.4 SBF) for 14 days. Experiments were also performed without using a bioactive system during the first stage of immersion. A bone-like apatite layer was formed on the surface of all the materials tested, using wollastonite the bioactive layer was thicker and its morphology was close to that observed on the existing bioactive systems. A thinner apatite layer consisting of small agglomerates was obtained using bioactive glass. The thickness of the ceramic layers was within the range of 15 to 30 μm.     

Comparison of Surface Treatment Methods for Promoting the Adhesion of Glass on Titanium

A glass allowing the wetting of titanium has previously been developed. Two surface treatments promoting glass adhesion on titanium are compared: preoxidation and phosphatation combined with preoxidation. After preoxidation at 700 °C, a 1 m-thick TiO2 layer covers titanium. The phosphated titanium substrates are oxidized at 600 °C to obtain a 20 m-thick oxifluoride layer. After firing, glass adhesion is obtained with both surface treatments, but a critical time of firing appears with preoxidized titanium. WDS analysis suggests that diffusion of TiO2 into the glass is responsible for adhesion on preoxidized titanium, while a complex oxifluoride layer allows redox phenomena in the case of phosphated titanium. Pull-off tests have measured a maximum strength from 1.5 to 3 MPa whatever the surface treatment. Measurement of transverse crack densities in the vitreous coatings gives a higher value for phosphated than for preoxidized titanium. This confirms that better adhesion is obtained after phosphatation treatment.     

Thin hydroxyapatite layers formed on porous titanium using electrochemical and hydrothermal reaction

In our previous study, it was found that hydroxyapatite (HA) microcrystals were precipitated by hydrothermal treatment on an anodic titanium oxide film containing calcium and phosphorus (AOFCP) with an equivalent Ca/P ratio to HA, which was formed on a titanium metal anode in an aqueous electrolytic solution of dissolved calcium acetate and -glycerophosphate. In this study, the formation mechanism of the AOFCP has been clarified. Spark discharges, which occur on titanium surface with a large amount of heat generation, cause crystallization of the TiO₂ matrix of the AOFCP and incorporation of calcium and phosphorus into the matrix from these electrolytes simultaneously. The calcium and phosphorus in the matrix seem to exist as ions rather than as calcium phosphate. Also, thin HA layers consisting of the many precipitated microcrystals can be uniformly formed even on titanium with complex shapes or surface geometries such as the mesh, roughened surfaces and bead-coated porous coating by the present method.     

Morphological study by XPS and AFM of wide band gap β-ln2S3 thin films synthesized by a dry physical process

Wide band gap -ln₂S₃ thin films have interesting properties to substitute CdS as the buffer layer in thin film solar cells. They have n-type conductivity and their optical band gap is about 2.8 eV. In this paper, -ln₂S₃ thin films deposited on smooth glass and on rough SnO₂-coated glass substrates have been morphologically studied by X-ray photoelectron spectroscopy (XPS) and by atomic force microscopy (AFM). The results obtained show that on each substrate the films are continuous without any pinholes and cracks. Films deposited on glass cover homogenously the whole surface of the substrate. In the case of SnO₂ substrates, the material deposited preferentially fills the hollows of the rough surface inducing a decrease of its roughness value. It is shown that these morphological properties are very promising for buffer layer application.     

Growth and characterization of hydroxyapatite nanorice on TiO2 nanofibers

Hydroxyapatite (HA) coating with nanoparticles like nanorice is fabricated on chemically pretreated titanium (Ti) surface, through an electrochemical deposition approach, for biomaterial applications. The Ti surface was chemically patterned with anatase TiO₂ nanofibers. These nanofibers were prepared by in situ oxidation of Ti foils in a concentrated solution of H₂O₂ and NaOH, followed by proton exchange and calcinations. Afterward, TiO₂ nanofibers on Ti substrate were coated with HA nanoparticles like nanorice. The obtained samples were annealed at high temperature to produce inter diffusion between TiO₂ and HA layers. The resultant layers were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Infrared Spectroscopy (FTIR), corrosion tests in SBF solution, and Electron Probe Micro Analysis (EPMA). It was found that only Ti from the titanium substrate diffuses into the HA coating and a good corrosion resistance in simulated body fluid was obtained.     

Near net-shape fabrication of hydroxyapatite glass composites

Near net-shape fabrication of hydroxyapatite (HA) glass composites has been attempted by infiltrating a glass into porous HA performs. Main efforts were put to develop glasses that are chemically compatible with HA at elevated temperatures. After extensive investigations in the phosphate and borosilicate systems, glasses of (50-55)SiO₂-(20-25)B₂O₃-(10-20)Li₂O-(0-6)CaO (wt%) composition were successfully developed. The glass shows good chemical compatibility with HA at elevated temperatures. Dense HA/glass composites can be fabricated at 850–950 C by the melt infiltration process. Investigations demonstrated a good near net-shape capability of the process, where the linear shrinkage induced by the infiltration process is less than 0.1%. Preliminary mechanical tests showed that the fracture strength and toughness of the infiltrated HA/glass composite are comparable with dense HA.     

The sintering and mechanical behavior of hydroxyapatite with bioglass additions

There is increasing interest in the potential of composites of hydroxyapatite with phosphate- or silicate-based bioactive glasses, and certain of these glass additions have been found, in previous work, to aid densification and form a mechanically-reinforced, bioactive material; in particular, large improvements in flexural strength and fracture toughness were obtained through the addition of small amounts of phosphate glass. Less is known about the mechanical behavior of HA/bioglass composites, although in vivo studies by other workers have shown encouraging biological results. In this investigation, the sintering behavior, mechanical properties, and microstructure of composites of HA with up to 50 wt % glass, were analyzed. X-ray diffraction showed the phase composition of sintered composites with up to 5 wt % added bioglass to be non-stoichiometric HA with -TCP or -TCP. Phase analysis of composites containing higher glass additions was impracticable due to peak broadening and overlap, although reaction products, at the highest glass additions and sintering temperatures, may include wollastonite-2M and -Na₂Ca₄(PO₄)₂SiO₄. Sintered density, and mechanical properties other than fracture toughness, showed no significant improvement over HA.     

Electrical conductivity of field-assisted ion-exchanged glass with molten PbCl2 salt

Field-assisted ion-exchange was carried out between soda-lime-silica glass tubes and molten PbCl₂ at 823 K for 2 h under constant potential of 12 V. The field-assisted ion-exchange leads to the formation of a lead-exchanged layer underneath the glass surface. The influence of the exchange on the bulk d.c. and a.c. electrical conductivity was investigated and compared with that of the glass before ion-exchange. The d.c. conductivity of the exchanged glass showed an activation energy higher than that of the glass before ion-exchange. The higher activation energy was attributed to the lower mobility of lead cations in the exchanged layer in comparison with that of sodium cations. The a.c conductivity of the exchanged glass showed an increase in the dielectric constant and a decrease in the dielectric loss. These findings suggest that lead cations enter the glass network in modifying positions in the exchanged glass. The results also support the data previously obtained that one lead cation from the molten PbCl₂ replaced two sodium cations from the glass.     

Biomimetic apatite formation on a CoCrMo alloy by using wollastonite,bioactive glass or hydroxyapatite

A biomimetic method was used to promote a bioactive surface on a CoCrMo alloy (ASTM F75). To enhance the nucleation of apatite on the metallic substrate, wollastonite ceramics (W), bioactive glass (BG) or hydroxyapatite (HA) were used in the biomimetic method. Metallic samples were chemically treated and immersed for 7 days in SBF on a bed of bioactive material (W, BG or HA) followed by an immersion in 1.5SBF for 7 or 14 days without bioactive system.A bonelike apatite layer was formed on the surface of all the samples tested. The samples treated with wollastonite showed a higher rate of apatite formation and the morphology of the layer was closer to that of the existing bioactive systems. A higher crystallinity of the apatite layer was also observed by using wollastonite. The pH of the SBF, the Ca/P ratio and the thickness of the layer on the samples treated with wollastonite and bioactive glass increased as increasing the immersion time. The thickness of the layer on the samples treated with hydroxyapatite also increased with time, but the pH of the SBF and the Ca/P ratio changed with no a defined trend.     

Experimental hydration of two synthetic glassy blast furnace slags in water and alkaline solutions (NaOH and KOH 0.1 N) at 40° C: structure,composition and origin of the hydrated layer

The hydration of blast furnace slags has been modelled using two synthetic (CaO, SiO₂, Al₂O₃, MgO) glasses with different Al₂O₃/MgO values. Experiments (duration: 16 h to 150 d) were performed at 40° C in deionized water (pH 6.5) and in NaOH and KOH (0.1 N) solutions (pH=12.9). The hydrated layer was characterized from a combination of several techniques at different scales: surface analysis by XPS and SEM; TEM of ultrathin diamond-cut sections including electron microdiffraction and EDS analysis; X-ray diffraction of scratched powders. In water, the hydrated zone is only about 0.5m thick after 150 d with a leached layer covered by a thin siliceous film on which are scattered rare amorphous lamellae. In alkali media, the hydrated zone is composed of three parts: an inner layer made of modified residual glass, calcium-depleted and richer in magnesium and aluminium than the initial glass; an intermediate lamellar of constant thickness (O.3m) after 15 d with magnesium and aluminium as major components (hydrotalcite type composition) and labelling the initial solid-solution interface, an outer layer with initially abundant C-S-H more or less carbonated after reaction with atmospheric CO₂. The hydrotalcite-type layer separates the inner domain dominated by the formation and evolution of a leached glass layer from an outer one, where the precipitation of C-S-H and other amorphous or crystalline compounds, followed by carbonation, are the major processes.     

Production and evaluation of hydroxyapatite reinforced polysulfone for tissue replacement

A variety of bioactive composites have been developed for tissue replacement over the last two decades. In this investigation, a new material consisting of hydroxyapatite (HA) and polysulfone (PSU) was produced and evaluated for potential medical applications. The HA/PSU composite containing up to 20 vol % of HA was studied at the initial stage. It was manufactured via a standardized procedure which included drying, blending, compounding and injection/compression molding. Defect-free composite samples (rectangular bars, discs and dumbbell specimens) could be obtained by injection molding. Thick composite plates could be made by compression molding. Both compounded materials and molded parts were assessed using a variety of techniques. It was found through scanning electron microscopy (SEM) that HA particles were well dispersed in the PSU matrix. Thermogravimetric analysis (TGA) verified the amount of HA in the composite. Differential scanning calorimetry (DSC) results indicated that the glass transition temperature (T_g) of the polymer matrix was not affected by the incorporation of HA. Rheological analysis revealed that PSU and the composite exhibited pseudoplastic behavior. For unfilled PSU, its viscosity decreased with an increase in temperature. The viscosity of HA/PSU composite increased with an increase in the HA volume fraction. It was shown through dynamic mechanical analysis (DMA) that the storage modulus of the composite was increased with an increase in HA volume percentage below T_g of the polymer, while tan was maintained at nearly the same level. It was established that water uptake reached an equilibrium after 7 days' immersion in distilled water for PSU and HA/PSU composite. After 7 days' immersion in distilled water, the storage modulus of the composite was decreased less than that of PSU. © 2001 Kluwer Academic Publishers     

Surface microstructure and cell biocompatibility of silicon-substituted hydroxyapatite coating on titanium substrate prepared by a biomimetic process

Silicon-substituted hydroxyapatite (Si-HA) coatings with 0.14 to 1.14 at.% Si on pure titanium were prepared by a biomimetic process. The microstructure characterization and the cell compatibility of the Si-HA coatings were studied in comparison with that of hydroxyapatite (HA) coating prepared in the same way. The prepared Si-HA coatings and HA coating were only partially crystallized or in nano-scaled crystals. The introduction of Si element in HA significantly reduced P and Ca content, but densified the coating. The atom ratio of Ca to (P + Si) in the Si-HA coatings was in a range of 1.61–1.73, increasing slightly with an increase in the Si content. FTIR results displayed that Si entered HA in a form of SiO₄ unit by substituting for PO₄ unit. The cell attachment test showed that the HA and Si-HA coatings exhibited better cell response than the uncoated titanium, but no difference was observed in the cell response between the HA coating and the Si-HA coatings. Both the HA coating and the Si-HA coatings demonstrated a significantly higher cell growth rate than the uncoated pure titanium (p < 0.05) in all incubation periods while the Si-HA coating exhibited a significantly higher cell growth rate than the HA coating (p < 0.05). Si-HA with 0.42 at.% Si presented the best cell biocompatibility in all of the incubation periods. It was suggested that the synthesis mode of HA and Si-HA coatings in a simulated body environment in the biomimetic process contribute significantly to good cell biocompatibility.     

Hydroxyapatite coating of poly(2-hydroxyethyl methacrylate) hydrogel by biomimetic method

The recently proposed biomimetic method has been performed on samples of poly(2-hydroxyethyl methacrylate) p(HEMA) hydrogel in order to improve their interface with bone. Following the biomimetic process it was possible to obtain a bioactive hydroxyapatite coating on the p(HEMA) substrates. FTIR analysis showed that characteristic peaks of p(HEMA) progressively disappear, owing to the formation of a surface coating. In fact new peaks appear corresponding to the ones of P-O stretching (1116 and 1035 cm–1) and P-O bending (580 cm–1) vibration modes, thus suggesting that the method is effective in promoting the formation of a surface phosphate layer. The formation of hydroxyapatite crystals is confirmed by SEM and EDS results. The adhesive strength was measured and turned out to be higher than the one reported for PMMA substrate. The experimental results suggest that, as reported in the literature for other supports, the silicate ions released from the glass in the first stage bind themselves to the polymeric support.     

Effect of N<Subscript>2</Subscript> gas annealing and SiO<Subscript>2</Subscript> barrier on the optical transmittance and electrical resistivity of a transparent Sb-doped SnO<Subscript>2</Subscript> conducting film

During the fabrication process of transparent conducting thin films of ATO (antimony-doped tin oxide) on a soda lime glass substrate by a sol-gel dip coating method, the effects of the SiO₂ buffer layer formed on the substrate and N₂ annealing treatment were investigated quantitatively. The deposited ATO thin film was identified as a crystalline SnO₂ phase and the film thickness was about 100 nm/layer at a withdrawal speed of 50 mm/min. Optical transmittance and electrical resistivity of the 400 nm-thick ATO thin film that was deposited on SiO₂ buffer layer/soda lime glass and then annealed under nitrogen atmosphere were 84% and 5.0 × 10^–3cm, respectively. The XPS analysis confirmed that a SiO₂ buffer layer inhibited Na ion diffusion from the substrate, preventing the formation of a secondary phase such as Na₂SnO₃ and SnO and increasing Sb ion concentration and ratio of Sb⁵⁺/Sb³⁺ in the film. It was found that N₂ annealing treatment leads to the reduction of Sn⁴⁺ as well as Sb⁵⁺, however the reduction of Sn⁴⁺ is more effective, and consequently results in a decrease in the electrical resistivity to produce excellent electrical properties in the film. © Springer Science + Business Media, Inc.