Structure and immersion behavior of plasma-sprayed apatite-matrix coatings

The synergistic promotion of osseointegration by nanostructure design and silicon substitution of hydroxyapatite coatings in a diabetic model

Accumulating evidence indicates much higher failure rates for biomedical titanium implants in diabetic patients. This phenomenon is attributed to impaired osteoblastic function, suppressed angiogenesis capacity, and abnormal osteoclast activation in diabetic patients. Our previous study demonstrated that titanium implants coated with highly crystalline nanostructured hydroxyapatite (nHA) promoted the osteogenic differentiation of bone marrow stromal cells (BMSCs) and bone-implant osseointegration under healthy conditions. Furthermore, recent studies showed that silicon-substituted biomaterials exhibited excellent osteogenesis and angiogenesis performance while repressing osteoclastogenesis. Hence, we proposed that a combination of nanostructural modification and Si substitution might produce synergetic effects to mitigate the impaired osseointegration of bone implants under diabetes mellitus (DM) conditions. To confirm this hypothesis, titanium implants coated with highly crystalline Si-substituted nHA (Si-nHA) were successfully fabricated via atmospheric plasma spraying combined with hydrothermal treatment. An in vitro study demonstrated that compared to the original HA coating, the nHA coating improved osteogenic and angiogenic differentiation and altered the OPG/RANKL ratio of DM-BMSCs. In addition, the Si-nHA coating further enhanced cell proliferation, improved osteogenic and angiogenic differentiation, and repressed osteoclastogenesis in DM-BMSCs. An in vivo study confirmed that the titanium implants coated with nHA or Si-nHA effectively promoted bone formation and bone-implant osseointegration in a diabetic rabbit model, with the Si-nHA coating exhibiting the best stimulatory effect. Collectively, the results suggest that the nanostructured topography and Si substitution act synergistically to ameliorate the poor bone regeneration and osseointegration associated with DM. Thus, the results provide a promising coating method for dental and orthopedic applications under diabetic conditions.

Strontium substituted biomimetic calcium phosphate system derived from cuttlefish bone

Biomimetic triphasic strontium-substituted calcium phosphate (CaP) powders were prepared by wet precipitation method at 50°C, using CaCO₃ , (NH₂ )₂ COH₃ PO₄ , and Sr(NO₃ )₂ as reagents. Calcite was prepared from biogenic source (cuttlefish bone). The synthesized powders have been characterized by elemental analysis, Fourier transform infrared spectrometry, X-ray diffraction, Rietveld refinement studies and cell viability test. Phase transformation and ion release were analyzed during 7 days of incubation in simulated body fluid at 37°C. The raw precipitated powders were composed of calcium deficient carbonated hydroxyapatite (HA), octacalcium phosphate (OCP), and amorphous calcium phosphate (ACP). After heat treatment at 1200°C β-tricalcium phosphate (β-TCP) was detected. Strontium substitution for calcium results in an increase of lattice parameters in HA, OCP, and β-TCP. Sr²⁺ occupy the Ca(1) site in HA, Ca(3,4,7,8) sites in OCP and Ca(1,2,3,4) sites in β-TCP. Along with Sr²⁺ substitution, presence of Mg²⁺ and Na⁺ ions was detected as a result of using biogenic calcium carbonate. The culture of human embryonic kidney cells indicated noncytotoxicity of the prepared CaP powders with emphasis on the cell proliferation during 3 days of culture.

Component effects of bioactive glass on corrosion resistance and in vitro biological properties of apatite-matrix coatings

BACKGROUND

Surface modification of metallic implants is critical for improving the clinical performance of the dental and orthopedic devices. Bioactive glasses exhibit different levels of cellular function and physicochemical behavior; however, there have been few previous studies on the effect of constituents of the bioactive glasses on the in vitro osteogenic activity and corrosion resistance of apatite-based coatings.

OBJECTIVE

The objective of this work was to investigate the effect of SiO2, CaO, Na2O, and P2O5 on plasma-sprayed apatite coatings on Ti alloy substrates for tailoring the properties of implants making them suitable for clinical applications.

METHODS

The corrosion potential and corrosion current of various coatings in simulated body fluid (SBF) were examined. MG63 cell proliferation, differentiation, and mineralization of plasma-sprayed apatite-matrix coatings were evaluated.

RESULTS

The SiO2 and CaO-containing HA (HSC) coating had a higher corrosion potential than the other three coatings, while SiO2-containing HA (HS) coating displayed the highest corrosion current among all coatings. The effect of the oxides on cell functions followed the order SiO2 > CaO > P2O5 > Na2O in terms of cell attachment, proliferation, differentiation, and mineralization.

CONCLUSIONS

The flexibility in oxide doping may allow for the tunable biological properties and corrosion-resistant ability of the apatite coatings.

In Vitro Characterizations of Si4+ and Zn2+ Doped Plasma Sprayed Hydroxyapatite Coatings Using Osteoblast and Osteoclast Coculture

Osteoporosis is one of the most commonly identified bone disorders, which leads to an enhanced risk of bone fracture, especially for the older population. Hydroxyapatite (HA) coated titanium (Ti) alloys have been used widespread for load bearing applications like hip or knee replacements owing to their compositional similarity to natural bone; however, incorporation of osteoinductivity is still a challenge. The objective of this study is to evaluate the effects of SiO₂ and ZnO as dopants in HA coated Ti alloys on cellular osteoporotic conditions mimicked by an in vitro osteoblast and osteoclast coculture model. HA, Si-HA, and Zn-HA coatings showed adhesive bond strengths of 25.7 ± 1.9 MPa, 23.8 ± 2.3 MPa, and 22.9 ± 3.5 MPa, respectively. To study the effects of doped HA coatings on the simulated osteoporotic cellular condition, human mesenchymal stem cells (hMSCs) and monocytes (THP-1) were seeded simultaneously in a ratio of 1:4, respectively. Gene expressions studies were carried out with marker genes showing that the presence of the dopants in the HA coating enhanced osteoblast proliferation along with diminishing cell growth of osteoclasts. This study demonstrates the promising effects of SiO₂ and ZnO in plasma sprayed HA coatings on alleviating osteoporosis cellular conditions, which can potentially be used for load-bearing implants in aging patients whose bone resorption is more dominant than bone formation.

Influence of substrate metal alloy type on the properties of hydroxyapatite coatings deposited using a novel ambient temperature deposition technique

Hydroxyapatite (HA) coatings are applied widely to enhance the level of osteointegration onto orthopedic implants. Atmospheric plasma spray (APS) is typically used for the deposition of these coatings; however, HA crystalline changes regularly occur during this high-thermal process. This article reports on the evaluation of a novel low-temperature (<47°C) HA deposition technique, called CoBlast, for the application of crystalline HA coatings. To-date, reports on the CoBlast technique have been limited to titanium alloy substrates. This study addresses the suitability of the CoBlast technique for the deposition of HA coatings on a number of alternative metal alloys utilized in the fabrication of orthopedic devices. In addition to titanium grade 5, both cobalt chromium and stainless steel 316 were investigated. In this study, HA coatings were deposited using both the CoBlast and the plasma sprayed techniques, and the resultant HA coating and substrate properties were evaluated and compared. The CoBlast-deposited HA coatings were found to present similar surface morphologies, interfacial properties, and composition irrespective of the substrate alloy type. Coating thickness however displayed some variation with the substrate alloy, ranging from 2.0 to 3.0 μm. This perhaps is associated with the electronegativity of the metal alloys. The APS-treated samples exhibited evidence of both coating, and significantly, substrate phase alterations for two metal alloys; titanium grade 5 and cobalt chrome. Conversely, the CoBlast-processed samples exhibited no phase changes in the substrates after depositions. The APS alterations were attributed to the brief, but high-intensity temperatures experienced during processing.

A novel nano-porous alumina biomaterial with potential for loading with bioactive materials

Nano-porous alumina, with the potential for being loaded with bioactive materials, has been proposed as a novel material for coating implants. In this study, the shear strength of the interface between such nano-porous anodic aluminium oxide (AAO) coatings and titanium substrates, their biocompatibility, and their potential for pore loading have been investigated. An interface shear strength in excess of 29 MPa was obtained which is comparable with that of conventional plasma sprayed hydroxyapatite implant coatings. The viability and differentiation of MG63 osteoblastic cells co-cultured on the coating was found to be broadly comparable to that of similar cells co-cultured on conventional bioinert implant materials such as titanium and fully dense alumina. Extensive pore loading with silica nano-particles of different sizes and in different combinations was demonstrated throughout the thickness of AAO layers 1 microm and 60 microm thick. This work has demonstrated, that with suitable choice of pore filling materials, this novel coating might simultaneously combat infection, encourage bone regeneration, and secure fixation of the implant to bone.

In vitro studies of plasma-sprayed hydroxyapatite/Ti-6Al-4V composite coatings in simulated body fluid (SBF)

The bioactivity of plasma-sprayed hydroxyapatite (HA)/Ti-6Al-4V composite coatings was studied by soaking the coatings in simulated body fluid (SBF) for up to 8 weeks. This investigation was aimed at elucidating the biological behaviour of plasma-sprayed HA/Ti-6Al-4V composite coatings by analyzing the changes in chemistry, and crystallinity of the composite coating in a body-analogous solution. Phase composition, microstructure and calcium ion concentration were analyzed before, and after immersion. The mechanical properties, such as tensile bond strength, microhardness and Young's modulus were appropriately measured. Results demonstrated that the tensile bond strength of the composite coating was significantly higher than that of pure HA coatings even after soaking in the SBF solution over an 8-weeks period. Dissolution of Ca-P phases in SBF was evident after 24h of soaking, and, a layer of carbonate-apatite covered the coating surface after 2 weeks of immersion. The mechanical properties were found to diminish with soaking duration. However, slight variation in mechanical properties was found after supersaturation of the calcium ions was attained with the precipitation of the calcium phosphate layers.