Multi-functional P(3HB) microsphere/45S5 Bioglass®-based composite scaffolds for bone tissue engineering

Novel multi-functional P(3HB) microsphere/45S5 Bioglass®-based composite scaffolds exhibiting potential for drug delivery were developed for bone tissue engineering. 45S5 Bioglass®-based glass–ceramic scaffolds of high interconnected porosity produced using the foam-replication technique were coated with biodegradable microspheres (size < 2 μm) made from poly(3-hydroxybutyrate), P(3HB), produced using Bacillus cereus SPV. A solid-in-oil-in-water emulsion solvent extraction/evaporation technique was used to produce these P(3HB) microspheres. A simple slurry-dipping method, using a 1 wt.% suspension of P(3HB) microspheres in water, dispersed by an ultrasonic bath, was used to coat the scaffold, producing a uniform microsphere coating throughout the three-dimensional scaffold structure. Compressive strength tests confirmed that the microsphere coating slightly enhanced the scaffold mechanical strength. It was also confirmed that the microsphere coating did not inhibit the bioactivity of the scaffold when immersed in simulated body fluid (SBF) for up to 4 weeks. The hydroxyapatite (HA) growth rate on P(3HB) microsphere-coated 45S5 Bioglass® composite scaffolds was very similar to that on the uncoated control sample, qualitatively indicating similar bioactivity. However, the surface topography of the HA surface layer was affected as shown by results obtained from white light interferometry. The roughness of the surface was much higher for the P(3HB) microsphere-coated scaffolds than for the uncoated samples, after 7 days in SBF. This feature would facilitate cell attachment and proliferation. Finally, gentamycin was successfully encapsulated into the P(3HB) microspheres to demonstrate the drug delivery capability of the scaffolds. Gentamycin release kinetics was determined using liquid chromatography–mass spectrometry. The release of the drug from the coated composite scaffolds was slow and controlled when compared to the observed fast and relatively uncontrolled drug release from the bone scaffold (without microsphere coating). Thus, this unique multifunctional bioactive composite scaffold has the potential to enhance cell attachment and to provide controlled delivery of relevant drugs for bone tissue engineering.     

Nucleation and growth of biomimetic apatite layers on 3D plotted biodegradable polymeric scaffolds: Effect of static and dynamic coating conditions

Apatite layers were grown on the surface of newly developed starch/polycaprolactone (SPCL)-based scaffolds by a 3D plotting technology. To produce the biomimetic coatings, a sodium silicate gel was used as nucleating agent, followed by immersion in a simulated body fluid (SBF) solution. After growing a stable apatite layer for 7 days, the scaffolds were placed in SBF under static, agitated (80 strokes min^?1) and circulating flow perfusion (Q = 4 ml min^?1; t_R = 15 s) for up to 14 days. The materials were characterized by scanning electron microscopy/energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and thin-film X-ray diffraction. Cross-sections were obtained and the coating thickness was measured. The elemental composition of solution and coatings was monitored by inductively coupled plasma spectroscopy. After only 6 h of immersion in SBF it was possible to observe the formation of small nuclei of an amorphous calcium phosphate (ACP) layer. After subsequent SBF immersion from 7 to 14 days under static, agitated and circulating flow perfusion conditions, these layers grew into bone-like nanocrystalline carbonated apatites covering each scaffold fiber without compromising its initial morphology. No differences in the apatite composition/chemical structure were detectable between the coating conditions. In case of flow perfusion, the coating thickness was significantly higher. This condition, besides mimicking better the biological milieu, allowed for the coating of complex architectures at higher rates, which can greatly reduce the coating step.     

Functionally graded hydroxyapatite coatings doped with antibacterial components

A series of functionally graded hydroxyapatite (FGHA) coatings incorporated with various percentages of silver were deposited on titanium substrates using ion beam-assisted deposition. The analysis of the coating’s cross-section using transmission electron microscopy (TEM) and scanning transmission electron microscopy equipped with energy dispersive X-ray spectroscopy has shown a decreased crystallinity as well as a distribution of nanoscale (10–50 nm) silver particles from the coating/substrate interface to top surface. Both X-ray diffraction and fast Fourier transforms on high-resolution TEM images revealed the presence of hydroxyapatite within the coatings. The amount of Ag (wt.%) on the outer surface of the FGHA, as determined from X-ray photoelectron spectroscopy, ranged from 1.09 to 6.59, which was about half of the average Ag wt.% incorporated in the entire coating. Average adhesion strengths evaluated by pull-off tests were in the range of 83 ± 6 to 88 ± 3 MPa, which is comparable to 85 MPa for FGHA without silver. Further optical observations of failed areas illustrated that the dominant failure mechanism was epoxy failure, and FGHA coating delamination was not observed.     

Polyelectrolyte/silver nanocomposite multilayer films as multifunctional thin film platforms for remote activated protein and drug delivery

We demonstrate a nanoparticle loading protocol to develop a transparent, multifunctional polyelectrolyte multilayer film for externally activated drug and protein delivery. The composite film was designed by alternate adsorption of poly(allylamine hydrochloride) (PAH) and dextran sulfate (DS) on a glass substrate followed by nanoparticle synthesis through a polyol reduction method. The films showed a uniform distribution of spherical silver nanoparticles with an average diameter of 50 ± 20 nm, which increased to 80 ± 20 nm when the AgNO₃ concentration was increased from 25 to 50 mM. The porous and supramolecular structure of the polyelectrolyte multilayer film was used to immobilize ciprofloxacin hydrochloride (CH) and bovine serum albumin (BSA) within the polymeric network of the film. When exposed to external triggers such as ultrasonication and laser light the loaded films were ruptured and released the loaded BSA and CH. The release of CH is faster than that of BSA due to a higher diffusion rate. Circular dichroism measurements confirmed that there was no significant change in the conformation of released BSA in comparison with native BSA. The fabricated films showed significant antibacterial activity against the bacterial pathogen Staphylococcus aureus. Applications envisioned for such drug-loaded films include drug and vaccine delivery through the transdermal route, antimicrobial or anti-inflammatory coatings on implants and drug-releasing coatings for stents.     

In vitro degradation and mechanical integrity of Mg–Zn–Ca alloy coated with Ca-deficient hydroxyapatite by the pulse electrodeposition process

The key to manufacturing magnesium-based alloys that are suitable as biodegradable orthopaedic implants is how to adjust their degradation rates and mechanical integrity in the physiological environment. In this study, to solve this challenge, a soluble Ca-deficient hydroxyapatite (Ca-def HA) coating was deposited on an Mg–Zn–Ca alloy substrate by pulse eletrodeposition. This deposition can be demonstrated by X-ray diffractometry and energy dispersion spectroscopy analyses, and the Ca/P atomic ratio of as-deposited coating is about 1.33 (within the range from 1.33 to 1.65). By regulating the appropriate pulse amplitude and width, the Ca-def HA coating shows better adhesion to Mg–Zn–Ca alloy, whose lap shear strength is increased to 41.8 ± 2.7 MPa. Potentiodynamic polarization results in Kokubo’s simulated body fluid (SBF) indicate that the corrosion potential of Mg alloy increases from ?1645 to ?1414 mV, while the corrosion current density decreases from 110 to 25 μA/cm², which illustrates that the Ca-def HA coating improves the substrate corrosion resistance significantly. Since orthopaedic implants generally serve under conditions of stress corrosion, the mechanical integrity of the Mg–Zn–Ca alloy was measured using the slow strain rate tensile (SSRT) testing technique in SBF. The SSRT results show that the ultimate tensile strength and time of fracture for the coated Mg–Zn–Ca alloy are higher than those of the uncoated one, which is beneficial in supporting fractured bone for a longer time. Thus Mg–Zn–Ca alloy coated with Ca-def HA is be a promising candidate for biodegradable orthopaedic implants, and is worthwhile to further investigate the in vivo degradation behavior.     

A bioactive coating of a silica xerogel/chitosan hybrid on titanium by a room temperature sol–gel process

A bioactive coating consisting of a silica xerogel/chitosan hybrid was applied to Ti at room temperature as a novel surface treatment for metallic implants. A crack-free thin layer (<2 μm) was coated on Ti with a chitosan content of >30 vol.% through a sol–gel process. The coating layer became more hydrophilic with increasing silica xerogel content, as assessed by contact angle measurement. The hybrid coatings afforded excellent bone bioactivity by inducing the rapid precipitation of apatite on their surface when immersed in a simulated body fluid (SBF). Osteoblastic cells cultured on the hybrid coatings were more viable than those on a pure chitosan coating. Furthermore, the alkaline phosphate activity of the cells was significantly higher on the hybrid coatings than on a pure chitosan coating, with the highest level being achieved on the hybrid coating containing 30% chitosan. These results indicate that silica xerogel/chitosan hybrids are potentially useful as room temperature bioactive coating materials on titanium-based medical implants.     

In vitro cytotoxicity evaluation of porous TiO2–Ag antibacterial coatings for human fetal osteoblasts

Implant-associated infections (IAIs) may be prevented by providing antibacterial properties to the implant surface prior to implantation. Using a plasma electrolytic oxidation (PEO) technique, we produced porous TiO₂ coatings bearing various concentrations of Ag nanoparticles (Ag NPs) (designated as 0 Ag, 0.3 Ag and 3.0 Ag) on a Ti–6Al–7Nb biomedical alloy. This study investigates the cytotoxicity of these coatings using a human osteoblastic cell line (SV-HFO) and evaluates their bactericidal activity against methicillin-resistant Staphylococcus aureus (MRSA). The release of Ag and the total amount of Ag in the coatings were determined using a graphite furnace atomic absorption spectrometry technique (GF-AAS) and flame-AAS, respectively. Cytotoxicity was evaluated using the AlamarBlue assay coupled with the scanning electron microscopy (SEM) observation of seeded cells and by fluorescence microscopy examination of the actin cytoskeleton and nuclei after 48 h of incubation. Antibacterial activity was assessed quantitatively using a direct contact assay. AlamarBlue viability assay, SEM and fluorescence microscopy observation of the SV-HFO cells showed no toxicity for 0 Ag and 0.3 Ag specimens, after 2, 5 and 7 days of culture, while 3.0 Ag surfaces appeared to be extremely cytotoxic. All Ag-bearing surfaces had good antibacterial activity, whereas Ag-free coatings showed an increase in bacterial numbers. Our results show that the 0.3 Ag coatings offer conditions for optimum cell growth next to antibacterial properties, which makes them extremely useful for the development of new antibacterial dental and orthopedic implants.     

Fluoridated hydroxyapatite coatings on titanium obtained by electrochemical deposition

Hydroxyapatite (HA) and fluoridated hydroxyapatite (FHA) coatings were deposited on titanium substrates using an electrochemical technique. Different concentrations of F^? ions were incorporated into the apatite structure by adding NaF into the electrolyte. Typical apatite structures were obtained for all the coatings after electrodeposition and subsequent post-treatment, including alkaline immersion and vacuum calcination. The coatings were uniform and dense, with a thickness of ～5 μm. When the F-concentration was higher than 0.012 M in the electrolyte, a saturation of F in the coating occurred and the F/Ca ratio in the coatings became almost constant (F/Ca ratio = 0.125). The FHA coatings showed higher bonding strength and lower dissolution rate than HA coating, particularly for those with a fluoridation level of 0.5–0.625. Compared with pure Ti, FHA and HA coatings exhibited higher biological affinity like cell proliferation and alkaline phosphatase activity. Regarding clinical application, it is suggested that a moderate content of F, such as Ca₅(PO₄)₃(OH)_0.375?0.5F_0.5?0.625, be most suitable as a compromise among cell attachment, cell proliferation, apatite deposition and dissolution resistance.     

Mechanical properties and in vitro response of strontium-containing hydroxyapatite/polyetheretherketone composites

Strontium-containing hydroxyapatite/polyetheretherketone (Sr-HA/PEEK) composites were developed as alternative materials for load-bearing orthopaedic applications. The amount of strontium-containing hydroxyapatite (Sr-HA) incorporated into polyetheretherketone (PEEK) polymer matrix ranged from 15 to 30 vol% and the composites were successfully fabricated by compression molding technique. This study presents the mechanical properties and in vitro human osteoblast-like cell (MG-63) response of the composite material developed. The bending modulus and strength of Sr-HA/PEEK composites were tailored to mimic human cortical bone. PEEK reinforced with 25 and 30 vol% Sr-HA exhibited bending modulus of 9.6 and 10.6 GPa, respectively; alternatively, the bending strengths of the composites were 93.8 and 89.1 MPa, respectively. Based on the qualitative comparison of apatite formation in SBF and quantitative measurement of MG-63-mediated mineralization in vitro, the Sr-HA/PEEK composite was proven to outperform HA/PEEK in providing bioactivity. However, no difference was found in the trend of cell proliferation and alkaline phosphatase activity between different composites. Strontium, in the form of strontium-containing hydroxyapatite (Sr-HA), was confirmed to enhance bioactivity in the PEEK composites.     

In vitro bioactivity and phase stability of plasma-sprayed nanostructured 3Y-TZP coatings

In this work, plasma-sprayed nanostructured zirconia coatings stabilized with 3 mol.% yttria (3Y-TZP) were deposited on Ti substrates. The microstructure and phase composition of coatings were characterized using scanning electron microscopy and X-ray diffraction. The in vitro bioactivity of coatings was evaluated by examining the formation of bone-like apatite on its surface in simulated body fluid. MG63 cell lines were cultured on the coating to investigate its cytocompatibility. The crystalline phase of the as-sprayed coating was tetragonal zirconia, and no monoclinic zirconia was detected. The size of the grains on the as-sprayed coating surface was less than 100 nm. The apatite could precipitate on the surface of the coating immersed in simulated body fluid for 28 days while no apatite was formed on the surface of 3Y-TZP ceramic control, indicating that the bioactivity of the coating is superior to the ceramic with the same composition. It also revealed that the polished coating whose nanostructural outmost layer was removed was bioinert, implying the significance of the nanosized grains for its bioactivity. MG63 cells could adhere, grow and proliferate well on the coating surface, indicating that the coating had good cytocompatibility. Phase stability of plasma-sprayed 3Y-TZP coating was evaluated under hydrothermal conditions at 134 °C. It revealed that the plasma-sprayed nanostructured zirconia coating was more sensitive to aging than that of zirconia ceramics.     

The effect of polyelectrolyte multilayer coated titanium alloy surfaces on implant anchorage in rats

Advances have been achieved in the design and biomechanical performance of orthopedic implants in the last decades. These include anatomically shaped and angle-stable implants for fracture fixation or improved biomaterials (e.g. ultra-high-molecular-weight polyethylene) in total joint arthroplasty. Future modifications need to address the biological function of implant surfaces. Functionalized surfaces can promote or reduce osseointegration, avoid implant-related infections or reduce osteoporotic bone loss. To this end, polyelectrolyte multilayer structures have been developed as functional coatings and intensively tested in vitro previously. Nevertheless, only a few studies address the effect of polyelectrolyte multilayer coatings of biomaterials in vivo. The aim of the present work is to evaluate the effect of polyelectrolyte coatings of titanium alloy implants on implant anchorage in an animal model. We test the hypotheses that (1) polyelectrolyte multilayers have an effect on osseointegration in vivo; (2) multilayers of chitosan/hyaluronic acid decrease osteoblast proliferation compared to native titanium alloy, and hence reduce osseointegration; (3) multilayers of chitosan/gelatine increase osteoblast proliferation compared to native titanium alloy, hence enhance osseointegration. Polyelectrolyte multilayers on titanium alloy implants were fabricated by a layer-by-layer self-assembly process. Titanium alloy (Ti) implants were alternately dipped into gelatine (Gel), hyaluronic acid (HA) and chitosan (Chi) solutions, thus assembling a Chi/Gel and a Chi/HA coating with a terminating layer of Gel or HA, respectively. A rat tibial model with bilateral placement of titanium alloy implants was employed to analyze the bones’ response to polyelectrolyte surfaces in vivo. 48 rats were randomly assigned to three groups of implants: (1) native titanium alloy (control), (2) Chi/Gel and (3) Chi/HA coating. Mechanical fixation, peri-implant bone area and bone contact were evaluated by pull-out tests and histology at 3 and 8 weeks. Shear strength at 8 weeks was statistically significantly increased (p < 0.05) in both Chi/Gel and Chi/HA groups compared to the titanium alloy control. No statistically significant difference (p > 0.05) in bone contact or bone area was found between all groups. No decrease of osseointegration of Chi/HA-coated implants compared to non-coated implants was found. The results of polyelectrolyte coatings in a rat model showed that the Chi/Gel and Chi/HA coatings have a positive effect on mechanical implant anchorage in normal bone.     

Electrically polarized HAp-coated Ti: In vitro bone cell–material interactions

Electrically polarized bulk sintered hydroxyapatite (HAp) compacts have been shown to accelerate mineralization and bone tissue ingrowth in vivo. In this work, a comprehensive study has been carried out to investigate the influence of surface charge and polarity on in vitro bone cell adhesion, proliferation and differentiation on electrically polarized HAp-coated Ti. Uniform and crack free sol–gel derived HAp coatings of 20 ± 1.38 μm thickness were polarized by application of an external d.c. field of 2.0 kV cm^?1 at 400 °C for 1h. In vitro bioactivity of polarized HAp coatings was evaluated by soaking in simulated body fluid, and bone cell–material interactions were studied by culturing with human fetal osteoblast cells (hFOB) for a maximum period of 11 days. Scanning electron microscopic observation showed that accelerated mineralization on negatively charged surfaces favored rapid cell attachment and faster tissue ingrowth over non-polarized HAp coating surfaces, while positive charge on HAp coating surfaces restricted apatite nucleation with limited cellular response. Immunochemistry and confocal microscopy confirmed that the cell adhesion and early stage differentiation were more pronounced on negatively charged coating surfaces as hFOB cells expressed higher vinculin and alkaline phosphatase proteins on negatively charged surface compared to cells grown on all other surfaces. Our results in this study are process independent and potentially applicable to any other commercially available coating techniques.     

Electron storage mediated dark antibacterial action of bound silver nanoparticles: Smaller is not always better

Size tunable silver nanoparticles (Ag NPs) are synthesized and incorporated into titanium oxide coatings (TOCs) by manipulating the atomic-scale heating effect of silver plasma immersion ion implantation (Ag PIII). The resulting Ag NPs/TOC composite coatings possess electron storage capability that gives rise to both controlled antibacterial activity and excellent compatibility with mammalian cells. The precipitation behavior of these Ag NPs is qualitatively constrained by the classical nucleation theory. Both photoluminescence (PL) spectra and fluorescence microscopy results demonstrate that larger Ag NPs (5–25 nm) are better at reserving electrons than smaller ones (～4 nm). The antibacterial activities of the as-sprayed and Ag PIII treated TOCs show that Ag NPs with a different size act distinctively to bacteria: large particles induce serious cytosolic content leakage and lysis of both Staphylococcus aureus and Escherichia coli cells while small ones do not. The excellent activity of larger Ag NPs against bacteria is highly related to their stronger electron storage capability, which can induce accumulation of adequate valence-band holes (h⁺) at the titanium oxide side, arousing oxidation reactions to bacterial cells in the dark. Moreover, the in vitro cell culture assay (using both MG63 and MC3T3 cells) reveals no significant cytotoxicity and even good cytocompatibility on the Ag PIII treated samples. Our results show that, by taking advantage of the boundary property between Ag NP and titanium oxide, the antibacterial activity of Ag NPs can be accurately controlled. This study provides a distinct criterion for the design of nanostructured surfaces such that their osteoblast functions and antibacterial activity are perfectly balanced.     

Evaluation of antibacterial and remineralizing nanocomposite and adhesive in rat tooth cavity model

Antibacterial and remineralizing dental composites and adhesives were recently developed to inhibit biofilm acids and combat secondary caries. It is not clear what effect these materials will have on dental pulps in vivo. The objectives of this study were to investigate the antibacterial and remineralizing restorations in a rat tooth cavity model, and determine pulpal inflammatory response and tertiary dentin formation. Nanoparticles of amorphous calcium phosphate (NACP) and antibacterial dimethylaminododecyl methacrylate (DMADDM) were synthesized and incorporated into a composite and an adhesive. Occlusal cavities were prepared in the first molars of rats and restored with four types of restoration: control composite and adhesive; control plus DMADDM; control plus NACP; and control plus both DMADDM and NACP. At 8 or 30 days, rat molars were harvested for histological analysis. For inflammatory cell response, regardless of time periods, the NACP group and the DMADDM + NACP group showed lower scores (better biocompatibility) than the control group (p = 0.014 for 8 days, p = 0.018 for 30 days). For tissue disorganization, NACP and DMADDM + NACP had better scores than the control (p = 0.027) at 30 days. At 8 days, restorations containing NACP had a tertiary dentin thickness (TDT) that was five- to six-fold that of the control. At 30 days, restorations containing NACP had a TDT that was four- to six-fold that of the control. In conclusion, novel antibacterial and remineralizing restorations were tested in rat teeth in vivo for the first time. Composite and adhesive containing NACP and DMADDM exhibited milder pulpal inflammation and much greater tertiary dentin formation than the control adhesive and composite. Therefore, the novel composite and adhesive containing NACP and DMADDM are promising as a new therapeutic restorative system to not only combat oral pathogens and biofilm acids as shown previously, but also facilitate the healing of the dentin–pulp complex.     

A quantitative in vitro method to predict the adhesion lifetime of diamond-like carbon thin films on biomedical implants

A quantitative method using Rockwell C indentation was developed to study the adhesion of diamond-like carbon (DLC) protective coatings to the CoCrMo biomedical implant alloy when immersed in phosphate-buffered saline (PBS) solution at 37 °C. Two kinds of coatings with thicknesses ranging from 0.5 up to 16 microns were investigated, namely DLC and DLC/Si-DLC, where Si-DLC denotes a 90 nm thick DLC interlayer containing Si. The time-dependent delamination of the coating around the indentation was quantified by means of optical investigations of the advancing crack front and calculations of the induced stress using the finite element method (FEM). The cause of delamination for both types of coatings was revealed to be stress-corrosion cracking (SCC) of the interface material. For the DLC coating a typical SCC behavior was observed, including a threshold region (60 J m^?2) and a “stage 1” crack propagation with a crack-growth exponent of 3.0, comparable to that found for ductile metals. The DLC/Si-DLC coating exhibits an SCC process with a crack-growth exponent of 3.3 and a threshold region at 470 J m^?2, indicating an adhesion in PBS at 37 °C that is about eight times better than that of the DLC coating. The SCC curves were fitted to the reaction controlled model typically used to explain the crack propagation in bulk soda lime glass. As this model falls short of accurately describing all the SCC curves, limitations of its application to the interface between a brittle coating and a ductile substrate are discussed.     

Effect of chitosan scaffold microstructure on mesenchymal stem cell chondrogenesis

Although numerous biomaterials have been investigated as scaffolds for cartilage tissue engineering, the effect of their microstructure on final construct characteristics remains unclear. The biocompatibility of chitosan and its similarity with glycosaminoglycans make it attractive as a scaffold for cartilage engineering. Our objective was to evaluate the effect of chitosan scaffold structure on mesenchymal stem cell proliferation and chondrogenesis. Chitosan fibrous scaffolds and chitosan sponges were seeded with mesenchymal stem cells in a chondrogenic medium. Constructs were analyzed 72 h after seeding via scanning electron microscopy (SEM), weight measurements and DNA quantification. Constructs were cultured for 10 or 21 days prior to confocal microscopy, SEM, histology, quantitative analysis (weight, DNA and glycosaminoglycan (GAG)), and quantitative real-time polymerase chain reaction. Mesenchymal stem cells maintained a viability above 90% on all chitosan scaffolds. The cell numbers in the constructs were similar at 72 h, 10 days and 21 days. However, matrix production was improved in chitosan fibrous constructs based on the GAG quantification and collagen II mRNA expression. Chondrogenesis on chitosan scaffolds is superior on microfibers compared to macroporous sponges.     

The effect of alkaline phosphatase coated onto titanium alloys on bone responses in rats

The enzyme alkaline phosphatase (ALP) was recently proposed as an implant coating material in order to improve the biological performance of orthopedic and dental implants. The present study evaluated the in vivo bone response to electrosprayed coatings, consisting of ALP, calcium phosphate (CaP) or a combination thereof (composite coating: ALP + CaP) compared to non-coated controls (gritblasted and acid etched). A total of 80 implants (n = 10) with a gap of 1.0 mm, was implanted intramedullary and bilaterally into the femurs of 80 rats. After 1 and 4 weeks, bone response was evaluated qualitatively (histology) and quantitatively (histomorphometry). The results of this study show that all electrosprayed coatings (ALP, CaP, ALP + CaP) significantly improve osteoconduction compared to non-coated controls after 4 weeks of implantation, without significant differences among these coated groups. Consequently, the results indicate that ALP-coatings improve the osteogenic response to a comparable extent as CaP-coatings or an ALP + CaP composite coating. In conclusion, the current study proofs that ALP-coatings have potential as bone implant coatings, though long-term data remain to be obtained. From a clinical perspective, it was observed that the process of osteoconduction is related to positional determinants, which needs to be taken into account when analyzing data on bone response.     

Synthesis and properties of hydroxyapatite-containing porous titania coating on ultrafine-grained titanium by micro-arc oxidation

Equal channel angular pressing results in ultrafine-grained (～200–500 nm) Ti with superior mechanical properties without harmful alloying elements, which benefits medical implants. To further improve the bioactivity of Ti surfaces, Ca/P-containing porous titania coatings were prepared on ultrafine-grained and coarse-grained Ti by micro-arc oxidation (MAO). The phase identification, composition, morphology and microstructure of the coatings and the thermal stability of ultrafine-grained Ti during MAO were investigated subsequently. The amounts of Ca, P and the Ca/P ratio of the coatings formed on ultrafine-grained Ti were greater than those on coarse-grained Ti. Nanocrystalline hydroxyapatite and α-Ca₃(PO₄)₂ phases appeared in the MAO coating formed on ultrafine-grained Ti for 20 min (E20). Incubated in a simulated body fluid, bone-like apatite was completely formed on the surface of E20 after 2 days, thus evidencing preferable bioactivity. Compared with initial ultrafine-grained Ti, the microhardness of the E20 substrate was reduced by 8% to 2.9 GPa, which is considerably more than that of coarse-grained Ti (～1.5 GPa).     

Microstructure,bioactivity and osteoblast behavior of monoclinic zirconia coating with nanostructured surface

A monoclinic zirconia coating with a nanostructural surface was prepared on the Ti–6Al–4V substrate by an atmospheric plasma-spraying technique, and its microstructure and composition, as well as mechanical and biological properties, were investigated to explore potential application as a bioactive coating on bone implants. X-ray diffraction, transmission electron microscopy, scanning electron microscopy and Raman spectroscopy revealed that the zirconia coating was composed of monoclinic zirconia which was stable at low temperature, and its surface consists of nano-size grains 30–50 nm in size. The bond strength between the coating and the Ti–6Al–4V substrate was 48.4 ± 6.1 MPa, which is higher than that of plasma-sprayed HA coatings. Hydrothermal experiments indicated that the coating was stable in a water environment and the phase composition and Vickers hardness were independent of the hydrothermal treatment time. Bone-like apatite is observed to precipitate on the surface of the coating after soaking in simulated body fluid for 6 days, indicating excellent bioactivity in vitro. The nanostructured surface composed of monoclinic zirconia is believed to be crucial to its bioactivity. Morphological observation and the cell proliferation test demonstrated that osteoblast-like MG63 cells could attach to, adhere to and proliferate well on the surface of the monoclinic zirconia coating, suggesting possible applications in hard tissue replacements.     

Transmission electron microscopy of coatings formed by plasma electrolytic oxidation of titanium

Transmission electron microscopy and supporting film analyses are used to investigate the changes in composition, morphology and structure of coatings formed on titanium during DC plasma electrolytic oxidation in a calcium- and phosphorus-containing electrolyte. The coatings are of potential interest as bioactive surfaces. The initial barrier film, of mixed amorphous and nanocrystalline structure, formed below the sparking voltage of 180 V, incorporates small amounts of phosphorus and calcium species, with phosphorus confined to the outer ～63% of the coating thickness. On commencement of sparking, calcium- and phosphorus-rich amorphous material forms at the coating surface, with local heating promoting crystallization in underlying and adjacent anodic titania. The amorphous material thickens with increased treatment time, comprising almost the whole of the ～5.7-μm-thick coating formed at 340 V. At this stage, the coating is ～4.4 times thicker than the oxidized titanium, with a near-surface composition of about 12 at.% Ti, 58 at.% O, 19 at.% P and 11 at.% Ca. Further, the amount of titanium consumed in forming the coating is similar to that calculated from the anodizing charge, although there may be non-Faradaic contributions to the coating growth.