Silicon-substituted hydroxyapatite composite coating by using vacuum-plasma spraying and its interaction with human serum albumin

The incorporation of silicon can improve the bioactivity of hydroxyapatite (HA). Silicon-substituted HA (Ca₁₀(PO₄)_6−x (SiO₄) _x (OH)_2−x , Si-HA) composite coatings on a bioactive titanium substrate were prepared by using a vacuum-plasma spraying method. The surface structure was characterized by using XRD, SEM, XRF, EDS and FTIR. The bond strength of the coating was investigated and XRD patterns showed that Ti/Si-HA coatings were similar to patterns seen for HA. The only different XRD pattern was a slight trend toward a smaller angle direction with an increase in the molar ratio of silicon. FTIR spectra showed that the most notable effect of silicon substitution was that –OH group decreased as the silicon content increased. XRD and EDS elemental analysis indicated that the content of silicon in the coating was consistent with the silicon-substituted hydroxyapatite used in spraying. A bioactive TiO₂ coating was formed on an etched surface of Ti, and the etching might improve the bond strength of the coatings. The interaction of the Ti/Si-HA coating with human serum albumin (HSA) was much greater than that of the Ti/HA coating. This might suggest that the incorporation of silicon in HA can lead to significant improvements in the bioactive performance of HA.     

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.     

Aerosol deposition of silicon-substituted hydroxyapatite coatings for biomedical applications

Silicon-substituted hydroxyapatite (Si-HA) coatings on commercially pure titanium (Ti) were prepared by aerosol deposition using Si-HA powders. Si-HA powders with the chemical formula Ca₁₀(PO₄)_6 − x(SiO₄)_x(OH)_2 − x, having silicon contents up to x = 0.5 (1.4 wt.%), were synthesized by solid-state reaction of Ca₂P₂O₇, CaCO₃, and SiO₂. The Si-HA powders were characterized by X-ray diffraction (XRD), X-ray fluorescence spectrometry, and Fourier transform infrared spectroscopy. The corresponding coatings were also analyzed by XRD, scanning electron microscopy, and electron probe microanalyzer. The results revealed that a single-phase Si-HA was obtained without any secondary phases such as α- or β-tricalcium phosphate for both the powders and the coatings. The Si-HA coating was about 5 µm thick, had a dense microstructure with no cracks or pores, and showed a high adhesion strength ranging from 28.4 to 32.1 MPa. In addition, the proliferation and alkaline phosphatase activity of MC3T3-E1 preosteoblast cells grown on the Si-HA coatings were significantly higher than those on the bare Ti and pure HA coating. These results revealed the stimulatory effects induced by silicon substitution on the cellular response to the HA coating.     

Diffusivity of silver ions in the low temperature co-fired ceramic (LTCC) substrates

Diffusion of silver inner-electrode occurred during sintering of commercial low temperature co-fired glass ceramic substrate made the dielectric surface become light yellow. The samples added with silicon oxide (SiO₂) powder, however, maintained white color. Silicon-oxide powder was used to modified the sintering behavior and inhibit the silver ions diffusion for the LTCC ceramics. The alumina particles in the LTCC substrates could be regarded as the diffusion barrier of silver ions. The activation energy for silver ions diffusion in the LTCC substrates was 101 kJ/mol. When 5 wt% SiO₂ powder was added into the LTCC substrate, the diffusion activation energy of silver ions became 145 kJ/mol. At sintering temperature of 1180 K, the diffusion coefficient of silver ion in the LTCC ceramic substrates with and without additional SiO₂ were 8.88 × 10⁻¹³ cm²/s and 1.08 × 10⁻¹² cm²/s, respectively.     

Characterization and analysis of CaO–SiO<Subscript>2</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript> ternary system ceramics

The dielectric, thermal and mechanical properties of CaO–SiO₂–B₂O₃ ternary system ceramics by solid-phase method have been carried out and quantitive analysis been examined by X-ray diffraction (XRD) patterns. The results showed that the major crystalline phase of CaO–SiO₂–B₂O₃ ternary system ceramics was wollastonite (about 90 wt%) which existed at the temperature ranging from 950 to 1,100 °C. It is also observed that wollastonite could be transformed to pseudowollastonite at 1,200 °C. In addition, with increase in calcination temperature, the amount of wollastonite increases. When the sintering temperature is at 1,100 °C, the amount of wollastonite has a maximum value of 92.7 wt%. Accordingly, CaO–SiO₂–B₂O₃ ternary system ceramics achieved excellent properties at 1,100 °C, such as dielectric constant of 8.38, dielectric loss of 1.51 × 10⁻³ at 1 MHz, linear thermal-expansion coefficient (300 K) of 6.68 × 10⁻⁶/K, bending strength of 121.75 Mpa. Analysis of the mechanical and dielectric properties showed that the measured bending strength, dielectric constant and loss of CaO–SiO₂–B₂O₃ ternary system ceramics can be substantially modified and improved by controlling the sintering temperature, in particular due to the amount of wollastonite crystalline phase and size of grains.     

Thermal Conductivity and Interfacial Thermal Resistance: Measurements of Thermally Oxidized SiO2 Films on a Silicon Wafer Using a Thermo-Reflectance Technique

This article describes the development of a method to measure the normal-to-plane thermal conductivity of a very thin electrically insulating film on a substrate. In this method, a metal film, which is deposited on the thin insulating films, is Joule heated periodically, and the ac-temperature response at the center of the metal film surface is measured by a thermo-reflectance technique. The one-dimensional thermal conduction equation of the metal/film/substrate system was solved analytically, and a simple approximate equation was derived. The thermal conductivities of the thermally oxidized SiO₂ films obtained in this study agreed with those of VAMAS TWA23 within ± 4%. In this study, an attempt was made to estimate the interfacial thermal resistance between the thermally oxidized SiO₂ film and the silicon wafer. The difference between the apparent thermal resistances of the thermally oxidized SiO₂ film with the gold film deposited by two different methods was examined. It was concluded that rf-sputtering produces a significant thermal resistance ((20 ± 4.5) × 10⁻⁹ m²·K·W⁻¹) between the gold film and the thermally oxidized SiO₂ film, but evaporation provides no significant interfacial thermal resistance (less than ± 4.5 × 10⁻⁹ m²·K·W⁻¹). The apparent interfacial thermal resistances between the thermally oxidized SiO₂ film and the silicon wafer were found to scatter significantly (± 9 × 10⁻⁹ m²·K·W⁻¹) around a very small thermal resistance (less than ± 4.5 × 10⁻⁹ m²·K·W⁻¹).     

Microstructure and microwave dielectric characteristics of CaO–B2O3–SiO2 glass ceramics

CaO–B₂O₃–SiO₂ (CBS) glass powders are prepared by traditional glass melting method, whose properties and microstructures are characterized by Differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the pure CBS glass ceramics possess excellent dielectric properties (ε _r = 6.5, tan δ = 5 × 10⁻³ at 10 GHz), but a higher sintering temperature (>900 °C) and a narrow sintering temperature range (about 10 °C). The addition of a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) could greatly decrease the sintering temperature of CBS glass to 820 °C and significantly enlarge the sintering temperature range to 40 °C. The CBS glass ceramic with 30 wt% LG glass addition sintered at 840 °C exhibits better dielectric properties: ε _r ≈ 6, tan δ < 2 × 10⁻³ at 10 GHz, and the major phases of the sample are CaSiO₃, CaB₂O₄ and SiO₂.     

Corrosion resistance property of micro–arc oxidation coatings on Mg–Li alloy obtained in different systems

通过三种优化工艺体系在Mg--5%Li合金表面上生长陶瓷膜层, 分析了膜层的厚度、显微结构、相组成和耐蚀性. 结果表明, 三种膜层都含有MgO相, 微弧氧化试样的耐蚀性能都明显提高. 使用Na₃PO₄体系制备的膜层含有MgF₂, 膜层最厚、表面有大量裂纹; 使用Na₂SiO₃体系制备的膜层含有橄榄石型Mg₂SiO₄, 耐点蚀性能最好; 使用Na₂SiO₃--Na₃PO₄体系制备的膜层含有MgSiO₃, 致密性最好, 膜层耐均匀腐蚀性能最好.     

Effects of serum protein on ionic exchange between culture medium and microporous hydroxyapatite and silicate-substituted hydroxyapatite

It has been proposed that one of the underlying mechanisms contributing to the bioactivity of osteoinductive or osteoconductive calcium phosphates involves the rapid dissolution and net release of calcium and phosphate ions from the matrix as alternatively a precursor to subsequent re-precipitation of a bone-like apatite at the surface and/or to facilitate ion exchange in biochemical processes. In order to confirm and evaluate ion release from sintered hydroxyapatite (HA) and to examine the effect of silicate substitution into the HA lattice on ion exchange under physiological conditions we monitored Ca²⁺, PO₄ ³⁻ and SiO₄ ⁴⁻ levels in Earl’s minimum essential medium (E-MEM) in the absence (serum-free medium, SFM) or presence (complete medium, C-MEM) of foetal calf serum (FCM), with both microporous HA or 2.6 wt% silicate-substituted HA (SA) sintered discs under both static and semi-dynamic (SD) conditions for up to 28 days. In SFM, variation in Ca²⁺ ion concentration was not observed with either disc chemistry or culture conditions. In C-MEM, Ca²⁺ ions were released from SA under static and SD conditions whereas with HA Ca²⁺ was depleted under SD conditions. PO₄ ³⁻ depletion occurred in all cases, although it was greater in C-MEM, particularly under SD conditions. SiO₄ ⁴⁻ release occurred from SA irrespective of medium or culture conditions but a sustained release only occurred in C-MEM under SD conditions. In conclusion we showed that under physiological conditions the reservoir of exchangeable ions in both HA and SA in the absence of serum proteins is limited, but that the presence of serum proteins facilitated greater ionic exchange, particularly with SA. These observations support the hypothesis that silicate substitution into the HA lattice facilitates a number of ionic interactions between the material and the surrounding physiological environment, including but not limited to silicate ion release, which may play a key role in determining the overall bioactivity and osteoconductivity of the material. However, significant net release of Ca²⁺ and PO₄ ³⁻ was not observed, thus rapid or significant net dissolution of the material is not necessarily a prerequisite for bioactivity in these materials.     

Preparation and properties of low-temperature co-fired ceramic of CaO–SiO2–B2O3 system

Low temperature co-fired ceramic (LTCC) was prepared by sintering a glass selected from CaO–SiO₂–B₂O₃ system, in which 0.5 wt% P₂O₅ and 0.5 wt% ZnO were added to optimize the preparation conditions. The glass powder and sintered bodies were characterized by different analytical techniques such as TG-DTA analysis, X-ray diffraction and Scanning electron microscopy. It was found that the optimal sintering temperature was 820°C based on the microstructure and the properties of sintering bodies, and then the major phases of the LTCC were CaSiO₃, CaB₂O₄ and SiO₂. The obtained products, with dielectric constant about 6.5, dielectric loss about 2 × 10⁻³ at 30 MHz and coefficient of thermal expansion about 8 × 10⁻⁶ °C⁻¹ between 20 and 400°C, are supposed to be suitable for application in wireless communications.     

Electrophoretic deposition of silicon substituted hydroxyapatite coatings from n-butanol–chloroform mixture

Silicon Substituted Hydroxyapatite (Si-HA) coatings were prepared on titanium substrates by electrophoretic deposition (EPD). The stability of Si-HA suspension in n-butanol and chloroform mixture has been studied by electricity conductivity and sedimentation test. The microstructure, shear strength and bioactivity in vitro has been tested. The stability of Si-HA suspension containing n-butanol and chloroform mixture as medium is better than that of pure n-butanol as medium. The good adhesion of the particles with the substrate and good cohesion between the particles were obtained in n-butanol and chloroform mixture. Adding triethanolamine (TEA) as additive into the suspension is in favor of the formation of uniform and compact Si-HA coatings on the titanium substrates by EPD. The shear strength of the coatings can reach 20.43 MPa after sintering at 700 °C for 2 h, when the volume ratio of n-butanol: chloroform is 2:1 and the concentration of TEA is 15 ml/L. Titanium substrates etched in H₂O₂/NH₃ solution help to improve the shear strength of the coatings. After immersion in simulated body fluid for 7 days, Si-HA coatings have the ability to induce the bone-like apatite formation.     

Preparation of titanium-dioxide films by heating titanium/silicon-dioxide structures on silicon in oxygen

 When titanium/silicon-dioxide (Ti/SiO₂) structures prepared by depositing titanium (Ti) on thermally oxidized silicon in vacuum were heated to temperatures of 800–1000°C in flowing oxygen gas, silicon surfaces were covered with a mixture films containing preferentially (110)-oriented Ti0₂ instead of the SiO₂ films. The thickness of the mixture films could be determined by that of the deposited Ti films. Titanium silicide grew only in the region near between the grown mixture film and the silicon substrate. The dielectric constants of the grown mixture films increased exponentially with increasing oxidation temperature and increased slowly with increasing the Ti film thickness, while the breakdown field strength increased slowly with increasing oxidation temperature and increased exponentially with increasing the Ti film thickness. The oxide films prepared at 1000°C had dielectric constants of (15–25)ɛ_o resistivities of 10¹⁰–10¹¹ Ω cm, and breakdown field strengths of about 10⁶ V/cm. Received: 10 February 1998 / Accepted: 10 March 1998     

Studies of early growth mechanisms of hydroxyapatite on single crystalline rutile: a model system for bioactive surfaces

Previous studies have shown that crystalline titanium oxide is in vitro bioactive and that there are differences in the HA formation mechanism depending on the crystalline direction of the titanium oxide surface. In the present study, the early adsorption of calcium and phosphate ions on three different surface directions of the single-crystal rutile TiO₂ substrate has been investigated. A crucial step in the nucleation of HA is believed to be the adsorption of Ca²⁺ and PO₄ ³⁻ from phosphate buffer solutions. The (001), (100) and (110) single crystalline rutile surfaces were soaked in phosphate buffer saline solution for 10 min, 1 h and 24 h at 37°C. The surfaces were then analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). The results show that the adsorption of Ca²⁺ and PO₄ ³⁻ is faster on the (001) and (100) surfaces than on the (110) surface. This study also shows that TOF-SIMS can be used as a tool to better understand the adsorption of calcium and phosphate ions and the growth mechanism of HA. This knowledge could be used to tailor new bioactive surfaces for better biological reaction.     

In situ preparation of self-bonded zeolite MCM-22 bodies by vapor-phase transport method

Self-bonded bodies of zeolite MCM-22 were prepared by vapor-phase transport method. The resultant materials were characterized by means of X-ray diffraction, scanning electron microscope, mercury porosimetry, and nitrogen porosimetry. Self-bonded MCM-22 bodies were in situ prepared at pH 10.0 with the molar composition of 0.05Na₂O:SiO₂:0.033Al₂O₃. It was found that the bodies, prepared by aluminosilicate gel, had been transformed into zeolite MCM-22. The MCM-22 bodies of which the mechanical resistance was 126 N/cm avoided binder accession. By adding auxiliary chemical–PEG20000 to the aluminosilicate gel, the pore size distributions of MCM-22 bodies could be adjusted. The average pore radius of MCM-22 bodies reached in the 149.41–653.64 nm range when AC/SiO₂ ratio was 1.5 × 10⁻⁴–9.0 × 10⁻⁴.     

Effect of SiO 32? /OH? on plasma electrolytic oxidation of Ti-5Mo-4V-3Al

Plasma electrolyte oxidation (PEO) was utilized to produce thick films on titanium and Ti-5Mo-4V-3Al alloys by immersing them in various solutions of Na₂SiO₃ and KOH with different concentrations to investigate the effect of SiO₃²⁻/OH⁻ relations on the morphology and formed phases by utilizing SEM and XRD. Corrosion resistance is evaluated by open circuit potential (OCP) variation of samples in NaCl 3.5% and potentiodynamic polarization. The results show that the unstable film is formed by using more aggressive PEO electrolyte. By increasing this ratio, pore size varied from fine to coarse and the rate of corrosion decreased and OCP became more positive. The best protective film was formed in SiO₃²⁻/OH⁻ ratio of 1.     

Effects of SrO–B2O3–SiO2 glass additive on the microstructure and dielectric properties of CaCu3Ti4O12

The effect of SrO–B₂O₃–SiO₂ glass additive (SBS) on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics was investigated. This SBS–added CCTO ceramics were prepared by the solid state reaction. The undesirable impurity phases Ca₃SiO₅ started appearing in the XRD patterns, suggesting a possible chemical reaction between CaTiO₃ and SiO₂ (the devitrification production of SBS glass). The SBS glass additive promoted the grain growth and densification of CCTO ceramics. Cole–Cole plots of conductance suggested that the resistivity grain boundary decreased with increasing amount of SBS glass (when x = 0–2 wt%), then increased (when x = 2–3 wt%). The addition of SBS glass was desirable to increase the dielectric constants (up to 10⁴) and lowered the dielectric losses of CCTO over the frequency range of 450–40 kHz at the relatively lower sintering temperature for relatively shorter sintering time (1,050 °C, 12 h).     

Dielectric properties of BSTO/MgO ceramic composites

Ba_0.55Sr_0.45TiO₃/MgO ceramic composites with 50wt% MgO content were studied in terms of the phase distribution, microstructure and electric properties, prepared by traditional ceramic process-solid phase synthesis. The results show that a small amount of Mg²⁺ ions have entered BSTO lattices to substitute for Ti⁴⁺ and behave as electron acceptors, contributing to the decrease of the dielectric loss. Ba_0.55 Sr_0.45TiO₃/MgO composites in the paraelectric state at room temperature still have a high tunability because of a field-induced hardening of the soft phonon and the presence of micro- or nano-polar phases in the paraelectric phase. For Ba_0.55Sr_0.45TiO₃/MgO ceramic composites, the microwave loss at 2.5 GHz is 7.1 × 10⁻³ and the value of tunability is 13.8% with the external DC field 4 kV ⋅ mm⁻¹. It can basically meet the requirements of phase shifters working at microwave frequencies. The phase shift of 122^∘ was obtained at 9.2 GHz using the designed phase shifter with the bias voltage 2 kV ⋅ mm⁻¹.     

Calcium and titanium release in simulated body fluid from plasma electrolytically oxidized titanium

The release of titanium and calcium species to a simulated body fluid (SBF) at 37°C has been investigated for titanium treated by dc plasma electrolytic oxidation (PEO) in three different electrolytes, namely phosphate, silicate and calcium- and phosphorus-containing. The average rate of release of titanium over a 30 day period in immersion tests, determined by solution analysis, was in the range ~1.5–2.0 pg cm⁻² s⁻¹. Calcium was released at an average rate of ~11 pg cm⁻² s⁻¹. The passive current densities, determined from potentiodynamic polarization measurements, suggested titanium losses of a similar order to those determined from immersion tests. However, the possibility of film formation does not allow for discrimination between the metal releases due to electrochemical oxidation of titanium and chemical dissolution of the coating.     

Raman temperature dependence analysis of carbon-doped titanium dioxide nanoparticles synthesized by ultrasonic spray pyrolysis technique

Carbon-doped titanium dioxide nanoparticles were prepared by ultrasonic spray pyrolysis technique using titanium tetra-ethoxide as a precursor and glucose as a dopant. The as-synthesized nanoparticles were then characterized using high resolution transmission electron microscopy for the structural properties and temperature dependence Raman spectroscopy for the optical properties. High resolution transmission electron microscopy analysis shows that the ultrasonic synthesized carbon-doped titanium dioxide nanoparticles have an interplanar d-spacing of 0.352 nm, a value close to 0.374 nm of the pure undoped anatase titanium dioxide (bulk). The fast Fourier transform (FFT) of the selected electron diffraction images, of the selected areas, display the fact that only the main diffraction (reflection) plane of Miller indices (101) in titanium dioxide is responsible for diffraction pattern. Raman spectroscopy confirms the titanium dioxide polymorph to be anatase with the intense phonon frequency at 153 cm⁻¹ blue-shifted from 141 cm⁻¹ due to both carbon doping and particle size decrease. The temperature dependence analysis of the spectra shows an initial linear dependence of the Raman shift for the E _g(1) mode at 152.7 cm⁻¹ with increase in temperature up to a critical temperature 450 °C, after which we observe a decrease with increase in temperature. The other Raman modes [B _1g and E _g(3)] exhibit a different temperature dependence in that the B _1g displays a somewhat sinusoidal behavior and the E _g(3) mode shows a linear decrease of Raman shift with an increase in temperature. Temperature dependence analysis of peak width for the E _g(1) indicates the peak width of the as prepared nanoparticle to be 20 cm⁻¹ which is far much larger than that for single crystal of 7 cm⁻¹ at room temperature.     

AC impedance and dielectric spectroscopic studies of Mg<Superscript>2 + </Superscript> ion conducting PVA–PEG blended polymer electrolytes

Polyvinyl alcohol (PVA)–polyethylene glycol (PEG) based solid polymer blend electrolytes with magnesium nitrate have been prepared by the solution cast technique. Impedance spectroscopic technique has been used, to characterize these polymer electrolytes. Complex impedance analysis was used to calculate bulk resistance of the polymer electrolytes. The a.c.-impedance data reveal that the ionic conductivity of PVA–PEG–Mg(NO₃)₂ system is changed with the concentration of magnesium nitrate, maximum conductivity of 9·63 × 10^− 5 S/cm at room temperature was observed for the system of PVA–PEG–Mg(NO₃)₂ (35–35–30). However, ionic conductivity of the above system increased with the increase of temperature, and the highest conductivity of 1·71 × 10^− 3 S/cm was observed at 100°C. The effect of ionic conductivity of polymer blend electrolytes was measured by varying the temperature ranging from 303 to 373 K. The variation of imaginary and real parts of dielectric constant with frequency was studied.