Plasma spraying of zirconia-reinforced hydroxyapatite composite coatings on titanium: Part II Dissolution behaviour in simulated body fluid and bonding degradation

The change of phase, morphology and bond strength of plasma sprayed hydroxyapatite (HA) coating and ZrO2/HA composite coatings immersed in simulated body fluid (SBF) for various periods of time was studied. X-ray diffractometry (XRD) and scanning electron microscopy (SEM) were used to identify the phase and observe the morphology of the coating surface before and after immersion. In addition, inductively coupled plasma emission spectroscopy (ICP) was used to measure the ion release rate of coatings in SBF for various periods of time. Observation of the morphology by SEM shows that the composite coating with the addition of ZrO2 in HA significantly reduced the dissolution rate of impurity phases in simulated body fluid. The argument was supported by measurement of Ca2+ ion concentration in SBF. During plasma spraying, less OH- ions were lost in a ZrO2-containing composite coating. This factor, together with the reduced effective surface of the ZrO2-containing HA coating, were attributed to the reduced dissolution rate of the composite coatings. All the plasma sprayed coatings degraded after immersion in SBF owing to dissolution of constituents in the coating, however, the addition of ZrO2 in HA improved the bonding strength of HA coating after immersion in SBF.     

Plasma-sprayed hydroxyapatite coating: effect of different calcium phosphate ceramics

Three kinds of calcium phosphate ceramic powders, namely commercial hydroxyapatite (CHA), self-made hydroxyapatite (SMHA) and synthesized hydroxyapatite (SHA), are employed as starting materials for plasma-sprayed coating onto a stainless steel (316L) substrate. Results show a mixture of hydroxyapatite (HA), tricalcium phosphate (TCP), and tetracalcium phosphate (TeCP) phases appearing in the CHA and SHA-derived coatings and a primary of a HA with trace contents of tricalcium phosphate phases resulting in the SMHA-derived coating. The HA appears to be the only observable crystalline phase present in the SMHA-derived coating after 7 days of incubation with a simulated body fluid (SBF); however, part of the impurities, i.e. TCP and TeCP, remain in the other coatings. No apparent microcracks can be found on the coated surfaces when SMHA and SHA are used. The poor packing density of SHA reflects its weakness in bonding strength to the substrate surface compared with that obtained using CHA and SMHA powders. The surface morphology of the coatings was found to alter significantly after a sufficient period of incubation.     

等离子喷涂制备HA/ZrO2复合涂层

采用等离子喷涂技术,在Ｔｉ－６Ａｌ－４Ｖ基体上成功地制备了羟基磷灰石／氧化锆（ＨＡ／ＺｒＯ２）复合涂层,对涂层的微观结构,相组成和结合强度进行了研究,并以模拟体液试验评估涂层的生物活性,结果表明,复合涂层较有较为微观结构,ＨＡ／ＺｒＯ２复合涂层的结合强度明显高于ＨＡ涂层,ＨＡ／６０ｗｔ％ＺｒＯ２涂层的结合强度高达２８．５ＭＰａ,为ＨＡ涂层的２．２倍,复合涂层在模拟体液中浸泡一段时间后,表面覆盖一层     

等离子喷涂制备HA/ZrO2复合涂层

采用等离子喷涂技术,在Ｔｉ－６Ａｌ－４Ｖ基体上成功地制备了羟基磷灰石／氧化锆（ＨＡ／ＺｒＯ_２）复合涂层,对涂层的微观结构、相组成和结合强度进行了研究,并以模拟体液试验评估涂层的生物活性．结果表明,复合涂层具有较为均匀的微观结构．ＨＡ／ＺｒＯ_２复合涂层的结合强度明显高于 ＨＡ涂层, ＨＡ／６０ ｗｔ％ ＺｒＯ_２涂层的结合强度高达 ２８．５ＭＰａ,为 ＨＡ涂层的 ２．２倍．复合涂层在模拟体液中浸泡一段时间后,表面覆盖一层碳酸磷灰石（ｃａｒｂｏｎａｔｅ－ａｐａｔｉｔｅ）,表明涂层具有良好的生物活性．     

Bond degradation at the plasma-sprayed HA coating/Ti-6AI-4V alloy interface: an in vitro study

The successful use of the plasma-sprayed HA-coated Ti-6AI-4V system requires strong adhesion between the ceramic coating and the underlying metal substrate. The aim of this study was to evaluate the bond strength at the HA coating (HAC)/Ti-6AI-4V interface, for specimens that had and had not been subjected to immersion in a pH-buffered, serum-added simulated body fluid (SBF). Moreover, coating characteristics affecting the mechanical stability after having been immersed in SBF were clarified. The results showed that bonding degradation of approximately 25–33% of the original strength was measured after immersion in SBF, and that this predominantly depended on the characteristics of the HAC and the period of immersion. Since the surface morphologies of HACs have dissolved in the SBF, it is suggested that the interlamellar structure of the HAC was weakened and, therefore, the bond strength degraded. As both the crystallinity and impurity phases of the HAC increased with immersion time, it can be concluded that the dissolution of the HAC resulting from the initial microstructure has overtaken that of the coating crystallinity and phase purity. A denser microstructure is required to ensure a satisfactory HAC/Ti-6AI-4V interface.     

Biological responses of neonatal rat calvarial osteoblasts on plasma-sprayed HA/ZrO2 composite coating

Plasma-sprayed hydroxyapatite (HA) coating, applied to metal substrates, can induce a direct chemical bond with bone and hence achieve a biological fixation of the implant. However, the poor bonding strength between the HA coating and the substrate has been a concern for the orthopedists. In a previous study, the zirconia-reinforced hydroxyapatite composite coatings (HA/ZrO₂) could significantly improve the mechanical strength before and after soaking in simulated body fluid. This study aims to investigate the biological responses of osteoblasts on plasma-sprayed HA/ZrO₂ coating. The osteoblasts derived from neonatal rat calvarial were cultured in Dulbeccos modified Eagle medium (DMEM) with fetal bovine serum (FBS) on the surface of plasma-sprayed HA coating, HA/ZrO₂ coating, and ZrO₂ coating, respectively. The biological responses were investigated by the cell growth (1, 3, 5, and 10 days) and the cell morphology under scanning electron microscopy (SEM) (3, 6, 12, 24 and 48 h). Examination by SEM revealed that osteoblasts on HA coatings exhibit less spreading during the medium phase (6 and 12 h), while, better morphologies were observed at the latter phases (24 and 48 h). This should be derived by the dissolution of HA coating in the culture medium. On HA/ZrO₂ coating, the cells showed the poor morphologies at the latter phases (24 and 48 h). This could be explained by the no apatite formed at the surface HA/ZrO₂ coating after soaking in simulated body fluid. The lower contents of ZrO₂ coating in HA coating and the addition of other solid solution (ZrO₂–MgO, CaO–ZrO₂, ZrO₂–CeO₂) in HA coating are the two possible methods to improve the cytocompatibility of HA/ZrO₂ coating.     

三维钛网表面双生物陶瓷涂层的制备及其性能

采用浸渍涂敷-烧结法首次在医用三维钛网表面制备出双生物陶瓷涂层,利用X射线衍射、场发射扫描电子显微镜对HA-BG/BG/Ti复合材料进行了微观表征,拉伸法测量了Ti基体与BG涂层的结合强度,模拟人体体液(SBF)评价复合材料的生物相容性.研究表明:该双生物陶瓷涂层的内层为生物玻璃(BG)涂层,外层为多孔结构的羟基磷灰石-生物玻璃(HA-BG)复合涂层.Ti基体被致密的BG涂层包覆,由于在BG/Ti界面发生化学反应,界面的结合强度提高,平均结合强度达27 MPa.生物相容性实验表明,HA-BG/BG/Ti复合材料表面会被一层整齐、致密的HA覆盖,具有良好的生物相容性.     

Plasma spraying of zirconia-reinforced hydroxyapatite composite coatings on titanium: Part I Phase, microstructure and bonding strength

Plasma-sprayed hydroxyapatite (HA) coatings applied to metal substrates can induce a direct chemical bond with bone and hence achieve biological fixation of the implant. However, the poor bonding strength between HA and substrate has been of concern to orthopaedists. In this study, two submicrometre ZrO2 powders stabilized with both 3 and 8 mol% Y2O3 (TZ3Y and TZ8Y, respectively) were incorporated in a plasma-sprayed HA coating on Ti-6Al-4V substrate to investigate the change in phase, microstructure and bonding strength. The results show that ZrO2 composite coatings contain more unmelted particles and greater porosity. During plasma spraying, ZrO2 reacts with the CaO in HA to form CaZrO3 and accelerates HA decomposition to -TCP and Ca4P2O9. Nevertheless, bonding strength increases with increase of ZrO2 content in the range 0 to 10 wt% studied. The higher Y2O3-containing TZ8Y apparently exerts a greater strengthening effect than the lower Y2O3-containing TZ3Y.     

Fabrication of nano-structured HA/CNT coatings on Ti6Al4V by electrophoretic deposition for biomedical applications

In order to improve the bone bioactivity and osteointegration of metallic implants, hydroxyapatite (HA) is often coated on their surface so that a real bond with the surrounding bone tissue can be formed. In the present study, cathodic electrophoretic deposition (EPD) has been attempted for depositing nanostructured HA coatings on titanium alloy Ti6Al4V followed by sintering at 800 degrees C. Nano-sized HA powder was used in the EPD process to produce dense coatings. Moreover, multiwalled carbon nanotubes (CNTs) were also used to reinforce the HA coating for enhancing its mechanical strength. The surface morphology, compositions and microstructure of the monolithic coating of HA and nanocomposite coatings of HA with different CNT contents (4 to 25%) on Ti6Al4V were investigated by scanning-electron microscopy, energy-dispersive X-ray spectroscopy and Xray diffractometry, respectively. Electrochemical corrosion behavior of the various coatings in Hanks' solution at 37 degrees C was investigated by means of open-circuit potential measurement and cyclic potentiodynamic polarization tests. Surface hardness, adhesion strength and bone bioactivity of the coatings were also studied. The HA and HA/CNT coatings had a thickness of about 10 microm, with corrosion resistance higher than that of the substrate and adhesion strength higher than that of plasma sprayed HA coating. The properties of the composite coatings were optimized by varying the CNT contents. The enhanced properties could be attributed to the use of nano-sized HA particles and CNTs. Compared with the monolithic HA coating, the CNT-reinforced HA coating markedly increased the coating hardness without deteriorating the corrosion resistance or adhesion strength.     

Microstructures and bond strengths of plasma-sprayed hydroxyapatite coatings on porous titanium substrates

Hydroxyapatite (HA) coating was carried out by plasma spraying on bulk Ti substrates and porous Ti substrates having a Young’s modulus similar to that of human bone. The microstructures and bond strengths of HA coatings were investigated in this study. The HA coatings with thickness of 200–250 μ m were free from cracks at interfaces between the coating and Ti substrates. XRD analysis revealed that the HA powder used for plasma spraying had a highly crystallized apatite structure, while the HA coating contained several phases other than HA. The bond strength between the HA coating and the Ti substrates evaluated by standard bonding test (ASTM C633-01) were strongly affected by the failure behavior of the HA coating. A mechanism to explain the failure is discussed in terms of surface roughness of the plasma-sprayed HA coatings on the bulk and porous Ti substrates.     

Improved stability of plasma-sprayed dicalcium silicate/zirconia composite coating

Dicalcium silicate/zirconia composite coatings were produced on Ti-6Al-4V substrates using atmospheric plasma spraying. Different weight ratios of zirconia (50 wt.%, 70 wt.%, 90 wt.%) were mechanically blended with dicalcium silicate (C₂S) powders as feedstocks. The composite coatings were immersed in a simulated body fluid (SBF) and a Tris-HCl solution for the in vitro appraisement of stability and long-term performance in a biological environment. The ion concentration changes of Ca, Si, and P in SBF and Tris-HCl solution were monitored using inductively-coupled plasma atomic emission spectroscopy (ICP-AES). Compared to the pure C₂S coating, our results show that the dissolution rate of the composite coatings is effectively reduced and the stability is improved by the addition of zirconia. The high content of zirconia in the coatings ensures the long-term performance in biological environment, while dissolution of C₂S in the coatings results in a higher Ca ion concentration in SBF and rapid precipitation of bone-like apatite on the composite coating surfaces indicating good bioconductivity of the coatings.     

Surface characterization of colloidal-sol gel derived biphasic HA/FA coatings

Hydroxyapatite (HA) powders are ultrasonically dispersed in the precursor of fluoridated hydroxyapatite (FHA) or fluorapatite (FA) to form a “colloidal sol”. HA/FA biphasic coatings are prepared on Ti6Al4V substrate via dip coating, 150 °C drying and 600 °C firing. The coatings show homogenous distribution of HA particles in the FA matrix. The relative phase proportion can be tailored by the amount of HA in the colloidal sol. The surfaces of the coatings consist of two kinds of distinct domains: HA and FA, resulting in a compositionally heterogeneous surface. The biphasic coating surface becomes increasingly rougher with HA powders, from around 200 nm of pure FA to 400–600 nm in Ra of biphasic coatings. The rougher biphasic HA/FA surfaces with chemically controllable domains will favor cell attachment, apatite layer deposition and necessary dissolution in clinical applications.     

Coatings of needle/stripe-like fluoridated hydroxyapatite on H2O2-treated carbon/carbon composites prepared by induction heating and hydrothermal methods

A hydroxyapatite (HA) coating was achieved on H₂O₂-treated carbon/carbon (C/C) composite through hydrothermally treating and induction heating deposited CaHPO₄ coating in an ammonia solution under ultrasonic water bath. Then, this HA coating was placed in a NH₄F solution and hydrothermally treated again to fabricate fluorinated hydroxyapatite (FHA) coatings for 24 h at 353, 373, 393 and 413 K, respectively. The structure, morphology and chemical composition of the HA and FHA coatings were characterized by SEM, XRD, EDS and FTIR, and the adhesiveness and chemical stability of these FHA coatings were examined by a scratch test and an immersion test, respectively. The results showed that the as-prepared FHA coatings contained needle-like or stripe-like crystals, different from those of the HA coating. As the fluoridation temperature rose, the adhesiveness of the FHA coating first increased from 34.8 to 40.9 N at a temperature between 353 and 393 K, and then decreased to 24.2 N at 413 K, while the dissolution rate of the FHA coating decreased steadily. The reasons for the property variation of the FHA coatings were proposed by analyzing the morphology, composition and structure of the coatings.     

Development of a composite based on hydroxyapatite and magnesium and zinc‐containing sol–gel-derived bioactive glass for bone substitute applications

In the present study, a bioceramic-based composite was prepared by sintering compacts made up of mixtures of hydroxyapatite (HA) and sol–gel-derived bioactive glass (64SiO₂-26CaO-5MgO-5ZnO) (based on mol%) powders. HA powder was mixed with different concentrations of the glass powders up to 30 wt.%. The effect of adding bioactive glass powder to HA matrix, on the mechanical properties of the composite was assessed by compression test. The specimen with the highest compressive strength was chosen to be immersed in simulated body fluid (SBF) to study apatite forming ability and dissolution behavior. It was found that compressive strength of the specimen was decreased 65% after maintaining in the SBF for 14 days. X-ray diffraction (XRD) showed prevalence of HA and β-TCP related peaks. Also, the surface morphology of the composite was observed using scanning electron microscopy (SEM). The study of degradation behavior revealed Si release capability of this composite. Biological evaluations in vitro confirmed the composite studied could induce osteoblast-like cells' activities.     

Cytotoxicity investigations of plasma sprayed calcium phosphate coatings

One potential alternative material to replace hydroxyapatite (HAp) as a coating material for plasma-sprayed coatings on implants for hip replacement is fluorapatite (FAp). FAp has advantages over HAp regarding the capability of being chemically stable during the coating process. This leads to surface coatings containing high apatite rates with a mechanical stability (bond strength, microhardness) comparable to HAp. From the technical point of view the production of FAp coatings is well investigated, although studies on biocompatibility of FAp coatings are fewer. This paper reports the production of HAp and FAp coatings with varying solubilities by plasma spraying and their in vitro cytotoxicity. Varying solubilities were realized by using modified plasma-spray parameters in common with suitable apatite powders with different crystallinities. Coating solubilities were evaluated by immersing the plasma-sprayed coatings in deionized water and electrolyte solution. Afterwards, cytotoxicity tests were performed using a modified half-slide technique. Cell attachment and cell morphology were evaluated. Neither HAp nor FAp coatings exhibited cytoxic influence on cells in culture. Results suggest that HAp coatings stimulate cell growth and FAp coatings do not. This could be explained by a negative effect on cell growth of the dissolved fluoride ions.     

Structure and properties of hydroxyapatite-bioactive glass composites plasma sprayed on Ti6Al4V

Hydroxyapatite (HA)-coated Ti6Al4V has recently been used as a bone substitute in orthopaedic and dental applications because of its favourable bioactivity and mechanical properties. Studies in the literature have shown that the bioactivity of calcium phosphate bioactive glass (BG) is higher than that of HA. In an attempt to increase the bioactivity of Ha-coated Ti6Al4V and enhance the bonding strength between coating and substrate, in the present study, HA/BG composites are applied onto Ti6Al4V using a plasma spraying technique. Microstructure and phase changes of the composite coating after plasma spraying are studied. The coating-substrate bonding strength is evaluated using an Instron, following the ASTM C633 method. Results indicate that the average bonding strengths of BG, HA/BG and HA coatings are 33.0±4.3, 39.1±5.0, and 52.0±11.7 MPa, respectively. Open pores with sizes up to 50 m are found in both BG and HA/BG coatings, which are probably advantageous in including mechanical interlocking with the surrounding bone structure, once implanted. These HA/BG composites could provide a coating system with sufficient bonding strength, higher bioactivity, and a significant reduction in cost in raw materials. The future of this HA/BG composite coating system seems pretty bright.     

Plasma-sprayed zirconia bond coat as an intermediate layer for hydroxyapatite coating on titanium alloy substrate

This study aims to strengthen the bonding at HA coating/Ti–6Al–4V interface by adding an intermediate ZrO₂ bond coat between them. The bonding strength of the HA/ZrO₂ coating was evaluated with the separately prepared HA coating as control. The phase, microstructure and chemistry, and surface roughness of the plasma-sprayed two-layer HA/ZrO₂ coating on Ti–6A1–4V substrate were investigated by X-ray diffractometry, scanning electron microscopy, and surfcorder, respectively. Experimental results indicate that the bonding strength increases from 28.6±3.22 MPa for HA coating to 36.2±3.02 MPa for HA/ZrO₂ composite coating. Elemental analysis employed on the surface of ZrO₂ bond coat, on which the HA top coat was first dissolved completely in HCl acid, reveals the sign of diffusion of calcium ions from HA to ZrO₂ bond coat. In addition, rougher surface morphology provided by ZrO₂ bond coat is also considered to aid in the bonding at HA/ZrO₂ interface. Similar coating system done by other researchers is compared and discussed.     

等离子喷涂HA／Ti复合涂层研究

对等离子喷涂HA／Ti复合涂层进行模拟体液和体外细胞试验,以考察涂层的生物学性能．结果指出,涂层经模拟体液浸泡后,表面覆盖一层碳酸磷灰石(carbonate-apatite),这表明涂层具有良好的生物活性．粗糙的涂层表面易于形成碳酸磷灰石．模拟体液的浓度太小或pH值太大,均会导致碳酸磷灰石层不能在涂层表面形成．体外细胞试验显示,成骨细胞能在涂层表面紧密贴壁并正常生长,显示涂层具有优良的生物相容性．     

等离子喷涂HA/Ti复合涂层研究 Ⅱ.生物学性能

对等离子喷涂ＨＡ／Ｔｉ复合涂层进行模拟体液和体外细胞试验,以考察涂层的生物学性能。结果指出,涂层经模拟体液浸泡后,表面覆盖一层碳酸磷灰石（ｃａｒｂｏｎａｔｅ－ａｐａｔｉｔｅ）,这表明涂层具有良好的生物活性,粗糙的涂层表面易于形成碳酸磷灰石,,模拟体液的浓度太小或ｐＨ值太大,均会导致碳酸磷灰石层不能在涂层表面形成,体外细胞试验显示,成骨细胞能在涂层表面紧密贴壁并正常生长,显示涂层具有优良的生物相容性     

Microstructure of Bioglass/Hydroxynpatite Coatings on Ti Substrate

Composite coatings of bioglass and hydroxyapatite (briefly named HA/BG) with different hydroxyapatite contents on titanium substrate were successfully fabricated. The fabricated coatings are characterized by rough and poriform surface.The densities of the coatings decrease with the increase of HA content. There is a transition layer with a 5 μm thickness between the BG coating and the substrate. During heat-treatment, hydroxyapatite crystals with hexastyle shape have precipitated from the BG.