Silicon-substituted Hydroxyapatite Coatings Research Articles

The effect of strontium and silicon substituted hydroxyapatite electrochemical coatings on bone ingrowth and osseointegration of selective laser sintered porous metal implants.

Additive manufactured, porous bone implants have the potential to improve osseointegration and reduce failure rates of orthopaedic devices. Substantially porous implants are increasingly used in a number of orthopaedic applications. HA plasma spraying–a line of sight process—cannot coat the inner surfaces of substantially porous structures, whereas electrochemical deposition of calcium phosphate can fully coat the inner surfaces of porous implants for improved bioactivity, but the osseous response of different types of hydroxyapatite (HA) coatings with ionic substitutions has not been evaluated for implants in the same in vivo model. In this study, laser sintered Ti6Al4V implants with pore sizes of Ø 700 μm and Ø 1500 μm were electrochemically coated with HA, silicon-substituted HA (SiHA), and strontium-substituted HA (SrHA), and implanted in ovine femoral condylar defects. Implants were retrieved after 6 weeks and histological and histomorphometric evaluation were compared to electrochemically coated implants with uncoated and HA plasma sprayed controls. The HA, SiHA and SrHA coatings had Ca:P, Ca:(P+Si) and (Ca+Sr):P ratios of 1.53, 1.14 and 1.32 respectively. Electrochemically coated implants significantly promoted bone attachment to the implant surfaces of the inner pores and displayed improved osseointegration compared to uncoated scaffolds for both pore sizes (p<0.001), whereas bone ingrowth was restricted to the surface for HA plasma coated or uncoated implants. Electrochemically coated HA implants achieved the highest osseointegration, followed by SrHA coated implants, and both coatings exhibited significantly more bone growth than plasma sprayed groups (p≤0.01 for all 4 cases). SiHA had significantly more osseointegration when compared against the uncoated control, but no significant difference compared with other coatings. There was no significant difference in ingrowth or osseointegration between pore sizes, and the bone-implant-contact was significantly higher in the electrochemical HA than in SiHA or SrHA. These results suggest that osseointegration is insensitive to pore size, whereas surface modification through the presence of an osteoconductive coating plays an important role in improving osseointegration, which may be critically important for extensively porous implants.

A comparative study on the in-vitro antibacterial behaviour of chemically-modified calcium phosphate coatings

ABSTRACT Drop-on-demand (DoD) micro dispensing technique, which enabled nano apatite crystals including hydroxyapatite (HA), silicon-substituted HA (SiHA), silver-substituted HA (AgHA) droplets to be deposited on glass substrate with precise placement, size, and number of droplets was presented in this work. Multi-material coatings containing SiHA:AgHA = 1:3 (1SiHA-3AgHA) were fabricated for obtaining favourable bioactivity and enhanced antibacterial properties. The antibacterial effect of 1SiHA-3AgHA towards Escherichia coli (E. coli) indicated a delayed exponential growth for up to 24 h, with significantly less bacteria adhesion compared to HA and SiHA coatings. The biological response of adipose-derived mesenchymal stem cells (hASCs) cultured on 1SiHA-3AgHA coatings exhibited enhanced cell attachment, proliferation and differentiation, as compared to HA and AgHA coatings. Hence, DoD micro deposition technique could be potentially used in producing multi-material coatings for biomedical applications.

Deposition of substituted apatite coatings at different coating patterns via drop-on-demand micro-dispensing technique

Silicon-substituted hydroxyapatite (SiHA) and silver-substituted hydroxyapatite (AgHA) potentiated their usages as bone graft materials with favourable bioactivity and enhanced antibacterial properties, respectively. To facilitate bone growth and reduce bacterial adhesion simultaneously, multi-material coatings composed of SiHA and AgHA at ratios of 1:1, 1:2, 1:3 (1SiHA-1AgHA, 1SiHA-2AgHA, 1SiHA-3AgHA), were deposited by different coating patterns via a Drop-on-Demand (DoD) micro-dispensing technique. As an additive manufacturing technique, placement, size, and numbers of droplets could be controlled precisely, and this gave rise to an even distribution of SiHA and AgHA, which integrated functional properties to a thin coating layer of 9·0 ± 0·5 μm. From the antibacterial study, SiHA-AgHA coatings, especially 1SiHA-3AgHA coatings demonstrated an inhibition on the growth of Escherichia coli (E. coli) up to 12 h as compared to pure HA and SiHA coatings.

Preparation of silicon-substituted hydroxyapatite coatings on Ti–30Nb–xTa alloys using cyclic electrochemical deposition method

Silicon-substituted hydroxyapatite coatings on Ti–30Nb–xTa alloys, prepared using a cyclic electrochemical deposition method, have been investigated using a variety of surface analytical experimental methods. The silicon-substituted hydroxyapatite (Si-HA) coatings were prepared by electrolytic deposition in electrolytes containing Ca2+, PO43− and SiO32− ions. The deposited layers were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), and a wettability test. Phase transformation from (α″+β) to largely β occurred with increasing Ta content in the Ti–30Nb–xTa alloys, yielding larger grain size. The morphology of the Si-HA coatings was changed by increasing the number of deposition cycles, with the initial plate-like structures changing to mixed rod-like and plate-like shapes, and finally to a rod-like structure. From the ATR-FTIR spectra, Si existed in the form of SiO44− groups in Si-HA coating layer. The lowest aqueous contact angles and best wettability were found for the Si-HA coatings prepared with 30 deposition cycles.

Silicon-substituted hydroxyapatite coating with Si content on the nanotube-formed Ti–Nb–Zr alloy using electron beam-physical vapor deposition

The purpose of this study was to investigate the electrochemical characteristics of silicon-substituted hydroxyapatite coatings on the nanotube-formed Ti–35Nb–10Zr alloy. The silicon-substituted hydroxyapatite (Si–HA) coatings on the nanotube structure were deposited by electron beam-physical vapor deposition and anodization methods, and biodegradation properties were analyzed by potentiodynamic polarization and electrochemical impedance spectroscopy measurement. The surface characteristics were analyzed by field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction (XRD). The Si–HA layers were deposited with rough features having highly ordered nanotube structures on the titanium alloy substrate. The thickness of the Si–HA coating was less than that of the HA coating. The XRD results confirmed that the Si–HA coating on the nanotube structure consisted of TiO2 anatase, TiO2 rutile, hydroxyapatite, and calcium phosphate silicate. The Si–HA coating surface exhibited lower Icorr than the HA coating, and the polarization resistance was increased by substitution of silicon in hydroxyapatite.

Synthesis and structural properties of sol–gel derived Si-substituted hydroxyapatite coatings

Ceramic coatings have been studied for a long time as a means to improve the performance of orthopedic and dental load bearing implants. Hydroxyapatite (HAP) is the most commonly used calcium phosphate ceramics for this purpose due to its biocompatibility. HAP coatings have been used to promote bone growth and fixation towards implant surfaces to encourage faster recovery times for patients. Sol–gel technique has attracted much attention to produce HAP coatings because of outstanding properties such as a low process temperature, the possibility to tailor microstructure and morphology and convenience for complex-shaped samples. Bone mineral is a CaP-based compound with a structural similarity to HAP and its mineral phase is a multi-substituted HAP. Hence, the usage of stoichiometric HAP has a limited ability to form an interface with bone tissue. Therefore, in this study silicon-substituted hydroxyapatite coatings were synthesized by sol–gel technique and the influences of the Si incorporation on the coatings were discussed. The results showed that HAP coating was successfully synthesized as porous and crack free structure by addition of silica to the precursor solution.

Surface modification of magnetron-sputtered hydroxyapatite thin films via silicon substitution for orthopaedic and dental applications

There have been a significant advances made in the field of bioceramics, particularly hydroxyapatite (HA) during the past 10years. Emphasis has now shifted towards designing HA with enhanced bioactivity for bone tissue repair. The aim of this study was to assess whether surface wettability can be correlated with cellular interactions with silicon-substituted hydroxyapatite (SiHA)-coated titanium (Ti) substrates. SiHA thin coatings of varying Si compositions were deposited on Ti substrates via a magnetron co-sputtering technique. These coatings were then subjected to an in vitro study using primary human ostoeblast (HOB) cells, to evaluate their biological property. HOB cells showed initial poor adhesion and spreading on hydrophobic Ti surface. The application of HA or SiHA thin coatings on Ti substrates by magnetron co-sputtering technique renders the surface more hydrophilic, with water contact angles between 30 and 40°. HOB cells attached, spread and proliferated well on these coatings. Enhanced calcification (formation of calcium phosphate nodules across the collagenous matrices) was observed on SiHA coatings with increasing Si content. This interdisciplinary paper highlighted that enhanced bioactivity was associated with surface wettability. Producing a nanostructured HA coating on a Ti substrate by magnetron sputtering resulted in the promotion of cell proliferation and calcification, and the latter was further enhanced with Si substitution. Hence, SiHA thin coating holds great potential as an alternative dental material.

Evaluation of human osteoblastic cell response to plasma‐sprayed silicon‐substituted hydroxyapatite coatings over titanium substrates

Silicon-substituted hydroxyapatite (Si-HA) coatings have been plasma sprayed over titanium substrates (Ti-6Al-4V) aiming to improve the bioactivity of the constructs for bone tissue repair/regeneration. X-ray diffraction analysis of the coatings has shown that, previous to the thermal deposition, no secondary phases were formed due to the incorporation of 0.8 wt % Si into HA crystal lattice. Partial decomposition of hydroxyapatite, which lead to the formation of the more soluble phases of alpha- and beta-tricalcium phosphate and calcium oxide, and increase of amorphization level only occurred following plasma spraying. Human bone marrow-derived osteoblastic cells were used to assess the in vitro biocompatibility of the constructs. Cells attached and grew well on the Si-HA coatings, putting in evidence an increased metabolic activity and alkaline phosphatase expression comparing to control, i.e., titanium substrates plasma sprayed with hydroxyapatite. Further, a trend for increased differentiation was also verified by the upregulation of osteogenesis-related genes, as well as by the augmented deposition of globular mineral deposits within established cell layers. Based on the present findings, plasma spraying of Si-HA coatings over titanium substrates demonstrates improved biological properties regarding cell proliferation and differentiation, comparing to HA coatings. This suggests that incorporation of Si into the HA lattice could enhance the biological behavior of the plasma-sprayed coating.

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 Ca 10(PO 4) 6 − x (SiO 4) x (OH) 2 − x , having silicon contents up to x = 0.5 (1.4 wt.%), were synthesized by solid-state reaction of Ca 2P 2O 7, CaCO 3, and SiO 2. 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.

Silicon–hydroxyapatite bioactive coatings (Si–HA) from diatomaceous earth and silica. Study of adhesion and proliferation of osteoblast-like cells

The aim of this study consisted on investigating the influence of silicon substituted hydroxyapatite (Si-HA) coatings over the human osteoblast-like cell line (SaOS-2) behaviour. Diatomaceous earth and silica, together with commercial hydroxyapatite were respectively the silicon and HA sources used to produce the Si-HA coatings. HA coatings with 0 wt% of silicon were used as control of the experiment. Pulsed laser deposition (PLD) was the selected technique to deposit the coatings. The Si-HA thin films were characterized by Fourier Transformed Infrared Spectroscopy (FTIR) demonstrating the efficient transfer of Si to the HA structure. The in vitro cell culture was established to assess the cell attachment, proliferation and osteoblastic activity respectively by, Scanning Electron Microscopy (SEM), DNA and alkaline phosphatase (ALP) quantification. The SEM analysis demonstrated a similar adhesion behaviour of the cells on the tested materials and the maintenance of the typical osteoblastic morphology along the time of culture. The Si-HA coatings did not evidence any type of cytotoxic behaviour when compared with HA coatings. Moreover, both the proliferation rate and osteoblastic activity results showed a slightly better performance on the Si-HA coatings from diatoms than on the Si-HA from silica.

Preparation and characterization of silicon-substituted hydroxyapatite coating by a biomimetic process on titanium substrate

A biomimetic method has been used to prepare silicon-substituted hydroxyapatite coatings on titanium substrates. The surface structures of the coatings were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR). Si substituted hydroxyapatite (Si–HA) coatings with different Si contents were deposited successfully on the titanium substrate by immersing the pretreated titanium substrate into silicon containing supersaturated solutions (SSS) with different SiO 3 2− concentrations. The pretreatment of the Ti substrate in a mixed alkaline (NaOH + Ca(OH 2)) followed by a heat treatment produced a 3D porous surface structure with rutile and CaTiO 3 as main phases, which contributed mainly to the fast precipitation and deposition of Si–HA. FTIR results showed that Si in the Si–HA coating existed in the form of SiO 4 4− groups. The cross-section microstructure was observed by scanning electronic microscopy and the shear strength was tested. The coating was about 5–10 μm in thickness and no interval was observed at the interface between the coating and the substrate. Shear strength testing showed that Si–HA/Ti exhibited higher shear strength than HA/Ti due to the existence of the SiO 4 4− group in the 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 SiO 4 unit by substituting for PO 4 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 SiO 4 4− groups, principally displacing carbonate groups off the HA structure.

The Role of Si Substitution into Hydroxyapatite Coatings

Silicon substituted Hydroxyapatite coatings were prepared by Pulsed Laser Deposition from targets made of mixtures of Hydroxyapatite with Si powder at different concentrations. The properties of the Si-HA coatings with several degrees of Si substitution were analyzed by different techniques such as FTIR, XRD, XPS and solid-state NMR. It was found that the Si incorporation causes an amorphization of the structure together with a loss of carbonate groups. Furthermore, the Si atoms are incorporated in the form of SiO4 4- groups, and H(PO4)2- appears as the predominant phosphate group.

Pulsed laser deposition of silicon substituted hydroxyapatite coatings from synthetical and biological sources

Silicon substituted hydroxyapatite (Si–HA) is a new material with an enhanced bioactibity and it can be produced by chemical synthesis. Nevertheless, the coating of metallic substrates with a bioactive material is a common method nowadays to improve its integration with the receptor bone. Si–HA films were deposited by pulsed laser deposition (PLD), using targets composed of mixtures of HA with different Si containing sources such as SiO 2 and diatomaceous earth. The Si–HA films were characterized in terms of structure and chemical composition by spectroscopic techniques (FTIR, XPS), and several ion beam techniques (RBS, PIXE). The analysis revealed that the Si is successfully incorporated into the HA structure, as well as traces of other elements such as Na, Fe or K.

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 degrees 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 H(2)O(2)/NH(3) 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.

Silicon‐substituted hydroxyapatite thin films: Effect of annealing temperature on coating stability and bioactivity

The effect of annealing temperature on the physicochemical and biological characteristics of magnetron cosputtered silicon-substituted hydroxyapatite (SiHA) thin coatings was studied. Annealing is required to transform as-sputtered amorphous films into crystalline coatings. A nanocrystalline, single-phase apatite structure was achieved for coatings heated to 600 or 700 degrees C and, with increasing annealing temperature, the crystallite size increased. Small crystallites were found to be more soluble in the physiological environment but, at the same time, were able to induce early formation of a new apatite layer. A human osteoblast-like (HOB) cell model was used to evaluate the performance of these annealed SiHA coatings. HOB cells attached and grew well on coatings and, after 42 days in culture, a mineralization process was observed to be taking place, with evidence of calcium phosphate minerals throughout the extracellular matrix. Our findings indicated that an annealing temperature of 600 degrees C is sufficient to achieve crystalline SiHA coatings and exhibiting good chemical stability and bioactivity.

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

Hydroxyapatite [Ca-10(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 (similar to 1 mu m 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. (c) 2006 Springer Science + Business Media, Inc.