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.     

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.     

Pulsed laser deposition of silicon-substituted hydroxyapatite coatings

Thin films of Si-substituted hydroxyapatite (Si-HA) were deposited on Si and Ti substrates by pulsed laser deposition (PLD), in the presence of a water vapour atmosphere. The PLD ablation targets were made with different mixtures of commercial carbonated HA and Si powder, in order to produce the Si-HA thin films. The physicochemical properties of the coatings and the incorporation of the Si into the HA structure was studied by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The Si atoms were successfully incorporated into the HA structure, and were found to be in the form of SiO₄⁴⁻ groups, principally displacing carbonate groups off the HA structure.     

Synthesized silicon-substituted hydroxyapatite coating on titanium substrate by electrochemical deposition

Silicon-substituted hydroxyapaptite (Si-HA) coatings were prepared on titanium substrates by electrolytic deposition technique in electrolytes containing Ca²⁺, PO₄ ³⁻ and SiO₃ ²⁻ ions with various SiO₃ ²⁻/(PO₄ ³⁻ + SiO₃ ²⁻) molar ratios(η_si). The deposition was all conducted at a constant voltage of 3.0 V, with titanium substrate as cathode and platinum as anode, for 1 h at 85°C. The coatings thus prepared were characterized with inductively coupled plasma (ICP), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), field-emission-type scanning electron microscope (FSEM). The results show that the silicon amount in the coatings increases linearly to about 0.48 wt% at first with increasing η_si between 0 and 0.03, then increases slowly to about 0.55 wt% between 0.03 and 0.10 and finally maintains almost at a level around 0.55 wt% between 0.10 and 0.30. The tree-like Si-HA crystals are observed in the coatings prepared in the electrolyte of η_si = 0.20. And the presence of silicon in electrolytes decreases the thickness of the coatings, with effect being more significant as η_si increased. Additionally, the substitution of Si causes some OH⁻ loss and changes the lattice parameters of hydroxyapatite (HA).     

Nano-structural bioactive gradient coating fabricated by computer controlled plasma-spraying technology

The poor mechanical property of hydroxyapatite was the major problem for load bearing and implant coating in clinical applications. To overcome this weakness, a bioactive gradient coating with a special design composition of hydroxyapatite (HA), ZrO₂, Ti, bioglass was developed. This 120 μm coating with an upper layer of 30–50 μm porous HA produced by computer controlled plasma spraying which maintained energy level of the plasma which ensure proper melting of powder. The crystal size of the coating was 18.6–26.2 nm. Transformation of t-ZrO₂ to m-ZrO₂ reduced the thermal stress that weakened the coating and lowered down interfacial strength of the coating and metal substrate. Thermal stress of sprayed coating was 16.4 MPa which was much smaller than the sample without thermal treatment of 67.1 MPa. Interfacial strength between the coating and metal substrate was 53 MPa which is much higher than conventional Hydroxyapatite coating. Based on XRD analysis crystallinity of HA approached 98%. Therefore, high temperature treatment improved long term stability of the coating through improved crystallinity of hydroxyapatite and reduced other impure calcium phosphate phase.     

Comparison of the coatings deposited using Ti and B4C powder by reactive plasma spraying in air and low pressure

The coatings were deposited by reactive plasma spraying (RPS) in air and low-pressure plasma spraying (LPPS) based on the reaction between Ti and B₄C powder, respectively. The thermal spray powder of Ti and B₄C added with powder Cr (metallic binder) in air is compared with that without powder Cr addition in the low pressure. (Prior to deposition, the powder was screened and separated for RPS whereas spray drying, sintering and sieving were done for LPPS.) The phase composition and the microstructure of coatings were studied by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The anti-corrosion property of coatings was also investigated. It is found that the coating prepared by RPS, which is more densification, is composed of TiN, TiB₂, and a small phase fraction of titanium oxides. The composition of the coating deposited by reactive LPPS is TiB₂, Ti(C, N), Ti₄N_3−x and impurity phase of Ti₅Si₃. There is no appearance of titanium oxides in low pressure. The coatings have the typical lamellar structure and adhere to the bond coating well. The mean Vickers microhardness value of the coating deposited by RPS is higher than that of the coating deposited by LPPS. Furthermore, the corrosion resistance of the coating deposited by RPS is superior to that of the coating prepared by LPPS in near neutral 3.5 wt% NaCl electrolyte.     

Formation of highly oriented hydroxyapatite in hydroxyapatite/titanium composite coating by radio-frequency thermal plasma spraying

Highly oriented hydroxyapatite (HA) coatings with excellent adhesion were successfully obtained on titanium (Ti) and titanium alloy through a radio-frequency thermal plasma spraying method. The ratio of HA and Ti powders supplied into the plasma was precisely controlled by two microfeeders so as to change the composition from Ti-rich to HA-rich toward the upper layer of the formed coatings. The bond (tensile) strength of the HA/Ti composite coatings was ca. 40–50 MPa. XRD patterns showed that the topmost HA layer of the coatings had an apatite structure with (00l) preferred orientation. The degree of this orientation showed a tendency to increase with an increase in the substrate temperature during spraying.     

Effect of the preparation methodology on some physical and electrochemical properties of Ti/Ir<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>O<Subscript>2</Subscript> materials

The aim of this work was to prepare electrodes based on the Ti/Ir _x Sn_(1−x)O₂ composition, as well as test their stability toward the chlorine evolution reaction (ClER). To this end, two different preparation routes were investigated: thermal decomposition of polymeric precursors (DPP) and standard decomposition using isopropanol as solvent (SD/ISO). A systematic investigation of the structural, morphological, and electrochemical properties of the anodes with a nominal composition of Ti/Ir _x Sn_(1−x)O₂, prepared through the two different methodologies, was carried out. The oxide layer surface morphology, microstructure, and composition were investigated by Energy Dispersive X-ray Spectroscopy (EDS) and Scanning Electron Microscopy (SEM) techniques prior to and after accelerated life tests. EDS analyses following total deactivation of the electrode gave evidence of a relatively large content of Ir in the coating. XRD results showed there was formation of solid solution between IrO₂ and SnO₂, and the degree of miscibility of these solutions is controlled by the preparation method. Thus, the DPP method led to phase separation and large interval of immiscibility between the oxides analyzed. On the other hand, the SD/ISO method led to formation of solid solution for all the investigated compositions. The SD/ISO method produced materials rich in Ir, so the electrode lifetime was much longer if compared with the DPP counterparts.     

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.     

A new way of incorporating silicon in hydroxyapatite (Si-HA) as thin films

Bioactive silicon-containing hydroxyapatite (Si-HA) thin films that can be used as coatings for bone tissue replacement have been developed. A magnetron co-sputtering technique was used to deposit Si-HA films up to 700 nm thick on titanium substrates, with a silicon level up to 1.2 wt%. X-ray diffraction demonstrated that annealing transformed the as-deposited Si-HA films which were amorphous, into a crystalline HA structure. A human osteoblast-like (HOB) cell model was used to determine the biocompatibility of these films. HOB cells were seen to attach and grow well on the Si-HA films, and the metabolic activity of HOB cells on these films was observed to increase with culture time. Furthermore, mineralisation of the cell layers was observed after 8 weeks of culture. Based on the present findings, Si-HA of different film compositions demonstrate bioactive properties in-vitro, and indicate the potential as biocoatings for a wide variety of medical implants including load-bearing applications such as the femoral stem of hip replacement implants.     

Silicon-substituted hydroxyapatite (SiHA): A novel calcium phosphate coating for biomedical applications

Hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂], (HA) is similar in composition to bone mineral and has been found to promote new bone formation when implanted in a skeletal defect. However, its use in biomedical applications is limited by its relatively slow rate of biological interaction, and there is also a requirement to improve the success rate of HA implants in younger active patients, particularly where implants will be in place long-term. The addition of silicon (Si) into HA has been demonstrated to enhance the speed, and quality of the bone repair process. This paper describes the synthesis and detailed characterisation of nanocrystalline silicon-substituted hydroxyapatite (SiHA) thin coatings applied to a titanium substrate via a magnetron co-sputtering process. Amorphous SiHA coatings (∼1 μm thick) with varying Si content up to 4.9 wt% were produced before being transformed into crystalline films by heat-treatment. The crystalline coating was characterised by X-ray diffraction (XRD) and infrared (IR) analysis, and confirmed to be a single-phase apatite. The substitution of Si into HA resulted in an increase in both the a- and c-axes of the unit cell parameters, but a decrease in the crystallite size, with increasing Si substitution. This substitution also caused a decrease in the intensities of both the O–H and P–O bands in the IR spectra. Hence, these findings confirmed that the crystal structure of HA was altered with Si substitution. In vitro cell culture work showed that these SiHA thin coatings exhibited enhanced bioactivity and biofunctionality. An increase in the attachment and growth of human osteoblast-like (HOB) cells on these coatings was observed throughout the culture period, with the formation of extracellular matrix. In addition, confocal microscopy revealed that HOBs developed mature cytoskeletons with clear evidence of actin stress fibres, along with defined cell nuclei.     

Structural characterization of silicon-substituted hydroxyapatite synthesized by a hydrothermal method

Silicon-substituted hydroxyapatite (Si-HA) was prepared successfully by hydrothermal method. The crystalline phase, microstructure, chemical composition, morphology and thermal stability of Si-HA were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results show that the substitution of the silicate groups for the phosphate groups causes some OH^- loss to maintain the charge balance and changes the lattice parameters of HA. Furthermore, the substitution of the silicate groups restrains the growth of Si-HA crystal. DSC analysis shows that the small amount of silicon incorporates into HA lattice does not influence the thermal stability of HA.     

Hydroxyapatite formation from cuttlefish bones: kinetics

Highly porous hydroxyapatite (Ca₁₀(PO₄)₆·(OH)₂, HA) was prepared through hydrothermal transformation of aragonitic cuttlefish bones (Sepia officinalis L. Adriatic Sea) in the temperature range from 140 to 220°C for 20 min to 48 h. The phase composition of converted hydroxyapatite was examined by quantitative X-ray diffraction (XRD) using Rietveld structure refinement and Fourier transform infrared spectroscopy (FTIR). Johnson–Mehl–Avrami (JMA) approach was used to follow the kinetics and mechanism of transformation. Diffusion controlled one dimensional growth of HA, predominantly along the a-axis, could be defined. FTIR spectroscopy determined B-type substitutions of CO₃ ²⁻ groups. The morphology and microstructure of converted HA was examined by scanning electron microscopy. The general architecture of cuttlefish bones was preserved after hydrothermal treatment and the cuttlefish bones retained its form with the same channel size (~80 × 300 μm). The formation of dandelion-like HA spheres with diameter from 3 to 8 μm were observed on the surface of lamellae, which further transformed into various radially oriented nanoplates and nanorods with an average diameter of about 200–300 nm and an average length of about 8–10 μm.     

Hydroxyapatite-anatase-carbon nanotube nanocomposite coatings fabricated by electrophoretic codeposition for biomedical applications

In order to eliminate micro-cracks in the monolithic hydroxyapatite (HA) and composite hydroxyapatite/carbon nanotube (HA/CNT) coatings, novel HA/TiO₂/CNT nanocomposite coatings on Ti6Al4V were attempted to fabricate by a single-step electrophoretic codeposition process for biomedical applications. The electrophoretically deposited layers with difference contents of HA, TiO₂ (anatase) and CNT nanoparticles were sintered at 800°C for densification with thickness of about 7–10 μm. A dense and crack-free coating was achieved with constituents of 85 wt% HA, 10 wt% TiO₂ and 5 wt% CNT. Open-circuit potential measurements and cyclic potentiodynamic polarization tests were used to investigate the electrochemical corrosion behavior of the coatings in vitro conditions (Hanks’ solution at 37°C). The HA/TiO₂/CNT coatings possess higher corrosion resistance than that of the Ti6Al4V substrate as reflected by nobler open circuit potential and lower corrosion current density. In addition, the surface hardness and adhesion strength of the HA/TiO₂/CNT coatings are higher than that of the monolithic HA and HA/CNT coatings without compromising their apatite forming ability. The enhanced properties were attributed to the nanostructure of the coatings with the appropriate TiO₂ and CNT contents for eliminating micro-cracks and micro-pores.     

Elaboration of Monophasic and Biphasic Calcium Phosphate Coatings on Ti6Al4V Substrate by Pulsed Electrodeposition Current

Calcium phosphate coatings on Ti6Al4V substrates are elaborated by pulsed electrodeposition. The surface morphology and chemical composition of the coatings are characterized by SEM–EDS. The obtained results are systematically confirmed at the nanometre scale using TEM. Moreover, XRD is performed in order to identify the coatings phases. The results show that pulsed electrodeposition allows uniform coatings to be obtained without the holes and craters usually observed with classical electrodeposition. After appropriate heat treatment, these coatings have a biphasic composition of stoichiometric hydroxyapatite and β‐tricalcium phosphate. Moreover, the addition of 9% H₂O₂ to the electrolyte leads to monophasic coatings made of stoichiometric hydroxyapatite. As an indication of the passive nature of the electrodeposited coating, electrochemical potentiodynamic tests are performed in physiological solution in order to determine the corrosion behaviour of these coatings.     

Kinetics and mechanisms of converting bioactive borate glasses to hydroxyapatite in aqueous phosphate solution

Borate bioactive glasses are receiving increasing attention as scaffold materials for bone repair and regeneration. In this study, the kinetics and mechanisms of converting three groups of sodium–calcium–borate glasses with varying CaO:B₂O₃ ratio to hydroxyapatite (HA) in 0.25 M K₂HPO₄ solution were investigated at 10–70 °C. Glass disks with the composition 2Na₂O·(2 − x)CaO·(6 + x)B₂O₃ (x = 0, 0.5, and 1.0) were immersed for up to 8 days in the potassium phosphate solution. The conversion kinetics to HA were monitored by measuring the weight loss of the glass, while X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy were used to study structural and compositional changes. All three groups of glasses formed HA on their surfaces, showing that the glasses were bioactive. At 10–37 °C, the conversion kinetics was well fitted by the contracting sphere model. Also, the contracting sphere model has a good fit for the early stage of conversion at 70 °C, whereas a three-dimensional (3D) diffusion model provided a good fit to the data of the later stage. The results of this study provide kinetic and structural data for the design of borate bioactive glasses for potential applications in bone tissue engineering.     

Biomineralization capability of adherent bio-glass films prepared by magnetron sputtering

Radiofrequency magnetron sputtering deposition at low temperature (150°C) was used to deposit bioactive glass coatings onto titanium substrates. Three different working atmospheres were used: Ar 100%, Ar + 7%O₂, and Ar + 20%O₂. The preliminary adhesion tests (pull-out) produced excellent adhesion values (~75 MPa) for the as-deposited bio-glass films. Bioactivity tests in simulated body fluid were carried out for 30 days. SEM–EDS, XRD and FTIR measurements were performed. The tests clearly showed strong bioactive features for all the prepared films. The best biomineralization capability, expressed by the thickest chemically grown carbonated hydroxyapatite layer, was obtained for the bio-glass coating sputtered in a reactive atmosphere with 7% O₂.     

Effect of titanium and titania on chemical characteristics of hydroxyapatite plasma-sprayed into water

HA and its composite particles (HA/Ti, HA/TiO₂) were plasma-sprayed into water as well as on the Ti substrate, respectively. The microstructure and phase compositions of the sprayed HA and its composite particles before and after impinging on the substrate were studied by using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The results showed that the HA in the composite particles sprayed into water had a higher crystallinity than that in the composite coating. The addition of Ti or TiO₂ could both influence the decomposition of HA, but no chemical reacting product between them was formed before and after impinging on the substrate. However, EDS analyses showed the occurrence of interdiffusion of elements between HA and TiO₂, which was favorable to enhance the cohesive strength of particles in the composite coating. The post heat treatment at 650 °C for 2 h can effectively improve the crystallinity of coating by transforming amorphous phases into HA.     

Preparation of an electrodeposited hydroxyapatite coating on titanium substrate suitable for in-vivo applications

In this paper the porous hydroxyapatite coating on Ti implant materials was prepared by the process of electrodeposition, hydrothermal and sinter. The surface morphology, bond strength and thickness of HA coatings were investigated by SEM, AFM, and its biocompatibility was evaluated by cytotoxicity experiments and implant experiments, respectively. Results showed that (1) The HA coatings was 50 μm thickness and adhered on the Ti substrate strongly, which bond strength reached 38MPa. AFM analysis showed that the HA coating was porous structure, in which the mean pore size was 236.5 μm, (2) Cytotoxicity experiments and implant experiments showed that HA-coated Ti implant materials has little cytotoxicity in vitro and little inflammatory reaction in vivo, and there were no statistically disparity between HA-coated Ti implant and titanium implant materials of clinical application (p > 0.05), which demonstrated that HA-coated Ti has a good biocompatibility.     

Interaction of ZrFe<Subscript>2</Subscript> doped with Ti and Al with hydrogen

The influence of doping with Ti and Al on the structure and hydrogen sorption properties of ZrFe₂ was studied by XRD, XRSMA, and measurement of hydrogen absorption and desorption isotherms at pressure up to 300 MPa. The hydrogen capacity and equilibrium desorption pressures of hydrides decrease with increasing Al content at a constant ratio of Ti and Zr. The increase in the Ti content at a constant content of Al in alloys also leads to a decrease in hydrogen capacity; however, the equilibrium desorption pressures of hydrides increase considerably. Zr_1−x Ti _x (Fe_1−y Al _y )₂ (x= 0.2–0.8; y = 0.05–0.4) alloys were investigated.