Morphology and immersion behavior of plasma-sprayed hydroxyapatite/bioactive glass coatings

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.

Adhesion and mechanical properties of nanocrystalline hydroxyapatite coating obtained by conversion of atomic layer-deposited calcium carbonate on titanium substrate

The purpose of this study was to evaluate the mechanical properties of nanocrystalline hydroxyapatite coating by tensile adhesion testing and scratch testing. The coating was manufactured on titanium substrate by converting atomic layer-deposited (ALD) calcium carbonate thin film in dilute phosphate solution. The tensile adhesion testing was performed with hydraulic testing device in accordance with ISO 4624 and ISO 16276-1. Scratch testing was done according to SFS-EN 13523-12 with spherical 10 µm scratching tip. Characterization of the samples was done with light and electron microscopy after which they were stained with alizarin red and the failure modes and loadings were analyzed. The highest obtained tensile adhesion value was 6.71 MPa produced with 4000 ALD cycles, converted to hydroxyapatite in alkaline solution, and annealed for 30 min in 700 °C. The annealing improved the adhesion values by approximately 0.8 MPa, but examining the samples with electron microscopy showed intact coating in both annealed and non-annealed samples. Samples produced with 4000 cycles performed better in testing than 2000 cycle samples, and better adhesion was also achieved with alkaline conversion solution compared to neutral solution.

Bioglass implant-coating interactions in synthetic physiological fluids with varying degrees of biomimicry

Synthetic physiological fluids are currently used as a first in vitro bioactivity assessment for bone grafts. Our understanding about the interactions taking place at the fluid-implant interface has evolved remarkably during the last decade, and does not comply with the traditional International Organization for Standardization/final draft International Standard 23317 protocol in purely inorganic simulated body fluid. The advances in our knowledge point to the need of a true paradigm shift toward testing physiological fluids with enhanced biomimicry and a better understanding of the materials' structure-dissolution behavior. This will contribute to "upgrade" our vision of entire cascades of events taking place at the implant surfaces upon immersion in the testing media or after implantation. Starting from an osteoinductive bioglass composition with the ability to alleviate the oxidative stress, thin bioglass films with different degrees of polymerization were deposited onto titanium substrates. Their biomineralization activity in simulated body fluid and in a series of new inorganic-organic media with increasing biomimicry that more closely simulated the human intercellular environment was compared. A comprehensive range of advanced characterization tools (scanning electron microscopy; grazing-incidence X-ray diffraction; Fourier-transform infrared, micro-Raman, energy-dispersive, X-ray photoelectron, and surface-enhanced laser desorption/ionization time-of-flight mass spectroscopies; and cytocompatibility assays using mesenchymal stem cells) were used. The information gathered is very useful to biologists, biophysicists, clinicians, and material scientists with special interest in teaching and research. By combining all the analyses, we propose herein a step forward toward establishing an improved unified protocol for testing the bioactivity of implant materials.

Synthesis and characterization of novel multiphase bioactive glass-ceramics in the CaO-MgO-SiO(2) system

Three new chemical compositions based on the CaO-MgO-SiO(2) system were designed first, and then three novel glass-ceramics (M1, M2, and M3) were prepared by sol-gel method. X-ray diffraction analysis confirmed that they were predominantly composed of akermanite, wollastonite, and dicalcium silicate crystalline phases. The coefficient of thermal expansion (CTE) of M2 was 10.79 x 10(-6) degrees C(-1), closest to that of Ti-6Al-4V alloy, and the Young's modulus of M2 was 29.73 GPa, similar to that of the cortical bone. The bioactivity in vitro of M2 was evaluated by investigating its bonelike hydroxyapatite (HA)-formation ability in simulated body fluid (SBF), and the biocompatibility in vitro was detected by osteoblast proliferation, differentiation, and adhesion assay. The results revealed that M2 possessed bonelike carbonated hydroxyapatite (CHA)-formation ability in SBF and could significantly stimulate cell proliferation and differentiation. Furthermore, osteoblasts adhered and spread well on M2, indicating good bioactivity and biocompatibility in vitro.

Formation of hydroxyapatite coating using novel chemo-biomimetic method

A novel chemo-biomimetic method was developed to deposit hydroxyapatite (HA), simulating the porous nano-scale structure and chemical composition of natural bone. Electrochemical activation in NaOH solution, a prerequisite process to heterogeneously nucleate hydroxyapatite in this investigation, creates nano-scale porous structure on the surface of Ti6Al4V alloy. XPS analysis confirmed that the surface of activated Ti6Al4V substrate converted to TiO2 during activation, existing in the form of hydrated TiO2. Benefiting from the biocompatible top-layer of hydrated TiO2 and the favorable alkaline surface chemistry created through the electrochemical activation, the HA coating nucleates heterogeneously and grows continuously on the activated substrate resembling the nano-scale porous bone-like structure. The coating was characterized using XRD, SEM/FESEM/EDX, TEM and FTIR, and was confirmed as pure hydroxyapatite. A coating thickness of 50 microm was achieved, which is preferable and acceptable for medical implant application to promote bone ingrowth, thus enhancing fixation and biocompatibility of implant surface.

Characterization of functionally graded hydroxyapatite/titanium composite coatings plasma-sprayed on Ti alloys

Bioceramic coatings like hydroxyapatite (HA) have shown promising bioactive properties in load-bearing implant applications. The aim of this work is to deposit functionally graded HA/Ti layers consisting of an underlying Ti bond coat, the alternating layer, and an HA top-layer on Ti6Al4V substrates using plasma spray to improve the coating-substrate interface properties. The alternating layers were created by means of changing the feeding rate and input power of Ti and HA powders, which gradually decrease Ti content with increasing depth from the Ti bond-coat. The major consideration is to examine the stability of the graded coatings. Experimental results indicated that surface chemistry and morphology of the graded coatings were similar to those of monolithic HA coatings. The bond strength values of the as-sprayed graded coatings were much superior to those of monolithic HA coatings. The cyclic fatigue did have a statistically significant effect on bond strength of monolithic HA coatings, with a decrease of 23%. However, the graded coatings were able to survive 1 million cycles of loading in air without significantly reduced bond strength. The in vitro electrochemical measurement results also indicated that the graded coatings had a more beneficial and desired behavior than monolithic HA coatings after fatigue.

In vitro bioactivity of laser ablation pseudowollastonite coating

Pseudowollastonite (psW) coatings on titanium alloys substrates were prepared by laser ablation and immersed in simulated body fluid (SBF) for different periods in order to investigate the nucleation and growth of hydroxyapatite (HA)-like formation on their surface. The structure of the coatings before soaking was analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The interfacial reactions product was examined by thin-film XRD, SEM and transmission electron microscopy at low and high resolution level, both fitted with energy-dispersive X-ray spectroscopy. Additional changes in ionic concentration, using inductively couple plasma atomic emission spectroscopy, were determined as well as pH right at the psW-coatings/SBF interface using an ion-sensitive field effect transistor. The solution composition changes, increasing the Ca(2+) and Si(4+) concentration and pH as a function of the soaking time while HPO(4)(2-) decreased. The results obtained showed that the coating surfaces were covered by HA-like, which indicated that the psW-coating possesses good bioactivity and also suggested that the mechanism of HA-like layer formation in SBF was similar to that showed in in vitro test by other silica-based materials.

In vitro behavior of sintered powder injection molded Ti-6Al-4V/HA

This article reports the morphology and mechanical properties of sintered powder injection molded Ti-6Al-4V/HA parts in a simulated physiological environment. Sintered Ti-6Al-4V/HA parts were immersed in a simulated body fluid (SBF) with ion concentrations that were comparable to those of human blood plasma for a total period of 12 weeks. At intervals of 2 weeks, the immersed Ti-6Al-4V/HA parts were analyzed with the use of scanning electron microscopy (SEM), X-ray diffractometry (XRD), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Mechanical properties such as flexural strength, flexural modulus, compressive strength, and compressive modulus were also evaluated. Results showed that complete dissolution of the more soluble phases such as tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), and calcium oxide (CaO) were found after 2 weeks of immersion in SBF. ICP analysis showed that high calcium concentration release of around 200 ppm was observed in the SBF solution after 2-4 weeks of immersion, indicating that dissolution has taken place. Next, a gradual decrease in calcium concentration release in the SBF solution was observed after immersion for 4-6 weeks, with increasing amounts of calcium phosphate precipitates being observed on the Ti-6Al-4V/HA surface. Mechanical properties such as strength and modulus were found to deteriorate during 2-4 weeks of immersion, followed by gradual increment as the immersion period increased. This study also showed that parts sintered at 1150 C exhibited faster dissolution and precipitation rates than parts sintered at 1050 C in a physiological environment.

Apatite formed on the surface of plasma-sprayed wollastonite coating immersed in simulated body fluid

Wollastonite coatings on titanium alloys substrates were prepared by plasma spraying and incubated in simulated body fluids for different periods to investigate the nucleation and growth of apatite on their surface. Surface structural changes of the specimens were analyzed by XRD and IR technologies. SEM and EDS were used to observe surface morphologies and determine the composition of wollastonite coatings before and after immersion in simulated body fluid. The changes in the concentrations of calcium, silicon and phosphorus in the simulated body fluids due to the immersion of the specimens were measured by inductively coupled plasma atomic emission spectroscopy. The results obtained showed that hydroxycarbonate apatite can be formed on the surface of the coating soaked in SBF for 1 day. With longer immersion periods, the coating surface was covered by hydroxycarbonate apatite, which indicated that the wollastonite coating possesses good bioactivity.

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

The microstructure and properties of a series of plasma-sprayed coatings from sinter-granulated powders fabricated from SiO2, CaO, P2O5 and Na2O-containing HA composite powders on Ti-6Al-4V substrate were reported. The immersion behavior of these coatings in a simulated body fluid (SBF) was also investigated. The results showed that sinter-granulated apatite-matrix powders were irregularly shaped and appeared quite similar. XRD patterns showed that during fabrication of the powders, P2O5 and SiO2 enhanced the decomposition of HA structure, while CaO and Na2O did not. Reasonably high bond strengths (45-50 MPa) were obtained from all coatings. The plasma spray process itself enhanced the decomposition of apatite and chemical reactions among different phases. When immersed in SBF, the intensities of such phases as alpha- and beta-TCP in all coatings decreased with immersion time and an apatite precipitation took place on all coating surfaces. The immersed SiO2- and CaO-containing HA (HSC) coating had the highest rate of apatite precipitation among all coatings. The variations in calcium ion concentration in simulated body fluid indicated that the HSC-immersed solution reached its maximal Ca concentration the earliest, while the HSCP (HA, SiO2, CaO and P2O5)-immersed solution reached its maximum the latest.