Enhancement effect on antibacterial property of gray titania coating by plasma-sprayed hydroxyapatite-amino acid complexes during irradiation with visible light.

The aim of this study was to reveal the mechanism of enhancement of antibacterial properties of gray titania by plasma-sprayed hydroxyapatite (HAp)-amino acid fluorescent complexes under irradiation with visible light. Although visible-light-sensitive photocatalysts are applied safely to oral cavities, their efficacy is not high because of the low energy of irradiating light. This study proposed a composite coating containing HAp and gray titania. HAp itself functioned as bacteria catchers and gray titania released antibacterial radicals by visible-light irradiation. HAp-amino acid fluorescent complexes were formed on the surface of the composite coating in order to increase light intensity to gray titania by fluorescence, based on an idea bioinspired by deep-sea fluorescent coral reefs. A cytotoxicity assay on murine osteoblastlike cells revealed that biocompatibility of the HAp-amino acid fluorescent complexes was identical with the that of HAp. Antibacterial assays involving Escherichia coli showed that the three types of HAp-amino acid fluorescent complexes and irradiation with three types of light-emitting diodes (blue, green, and red) significantly decreased colony-forming units. Furthermore, kelvin probe force microscopy revealed that the HAp-amino acid fluorescent complexes preserved the surface potentials even after irradiation with visible light, whereas those of HAp were significantly decreased by the irradiation. Such a preservative effect of the HAp-amino acid fluorescent complexes maintained the bacterial-adhesion performance of HAp and consequently enhanced the antibacterial action of gray titania.

Effects of compression on orientation of ligands in fluorescent complexes between hydroxyapatite with amino acids and their optical properties.

This study aims to reveal the effects of pressure during cold isostatic pressing (CIP) on the microstructure and optical properties of fluorescent HAp complexes. Although the microsturucture-dependent properties of fluorescent HAp complexes have been reported to improve the antibacterial properties of photocatalyst coating layers, the mechanism behind the changes in the fluorescence properties of highly compressed HAp complexes has not yet been unveiled. CIP was successfully used to fabricate fluorescent HAp - amino acid complexes, and their fluorescence intensities increased with increasing fabrication pressure. Peak wavelength of fluorescence emitted by the HAp - amino acid complexes exhibited yellow to red shift. Although the thickness of the amino acid layer was saturated in higher pressure cases, the concentration of amino acids increased proportionally with pressure, which suggests changes in the packing structures of the ligands in the HAp- amino acid complexes. Polarized Raman spectroscopy measurements clearly detected ligands normally arranged to the HAp layer under high pressure fabrication conditions, which can provide the tightly packed ligand structure in the HAp- amino acid complexes. These tightly packed ligand structure in the HAp- amino acid complexes could emit stronger fluorescence owing to the increased density of complexations. This newly found pressure dependency in the optical properties of HAp-amino acid complexes is beneficial for developing biocompatible fluorescence materials or enhancement agents for antibacterial coating layers.

Fatigue Strength Estimation Based on Local Mechanical Properties for Aluminum Alloy FSW Joints.

Overall fatigue strengths and hardness distributions of the aluminum alloy similar and dissimilar friction stir welding (FSW) joints were determined. The local fatigue strengths as well as local tensile strengths were also obtained by using small round bar specimens extracted from specific locations, such as the stir zone, heat affected zone, and base metal. It was found from the results that fatigue fracture of the FSW joint plate specimen occurred at the location of the lowest local fatigue strength as well as the lowest hardness, regardless of microstructural evolution. To estimate the fatigue strengths of aluminum alloy FSW joints from the hardness measurements, the relationship between fatigue strength and hardness for aluminum alloys was investigated based on the present experimental results and the available wide range of data from the references. It was found as: σa (R = -1) = 1.68 HV (σa is in MPa and HV has no unit). It was also confirmed that the estimated fatigue strengths were in good agreement with the experimental results for aluminum alloy FSW joints.

Prediction of cyclic delamination lives of plasma-sprayed hydroxyapatite coating on Ti-6Al-4V substrates with considering wear and dissolutions.

This study aims at developing the prediction model of cyclic delamination lives of plasma-sprayed HAp coating on Ti-6Al-4V substrate by considering wear by interface contacts and dissolution effect by Simulated Body Fluid (SBF). Delamination of HAp coating can lead to loosening of implants stem and final failure in vivo. In the fracture mechanism of interfaces between HAp coating with Ti substrates, only adhesive strength (interracial tensile strength) or fatigue behavior by longitudinal cracking have been observed. Cyclic delamination mechanism by considering various loading modes and corrosion effect has not been revealed yet. The interface delamination rates by cyclic loading were much higher than those by static loading tests. The result clearly demonstrated that the interface demalination behaviors are dominated not by maximum stress, but by stress range. Surface profile measurement and SEM observation also demonstrated damages by interface contact or third body wear at delamination tips of HAp coating only in the cases of compressions. The mechanisms of acceleration on the delaminations are third-body wear or wedge effect by worn particles which increased mean stress level during cyclic loading. Cyclic loading tests under SBF also revealed that cyclic delamination lives were shortened probably due to crevice corrosion at interfaces. Dissolutions at the tips of delaminations were observed by SEM images under tensile loading condition in SBF. Linearly adding the effects of wear and dissolutions into Paris law could successfully predict the delamination lives of HAp coating for various loading ratios in SBF.

Cyclic delamination behavior of plasma-sprayed hydroxyapatite coating on Ti-6Al-4V substrates in simulated body fluid.

This study aimed to clarify the effect of a simulated body fluid (SBF) on the cyclic delamination behavior of a plasma-sprayed hydroxapatite (HAp) coating. A HAp coating is deposited on the surfaces of surgical metallic materials in order to enhance the bond between human bone and such surfaces. However, the HAp coating is susceptible to delamination by cyclic loading from the patient's gait. Although hip joints are subjected to both positive and negative moments, only the effects of tensile bending stresses on vertical crack propagation behavior have been investigated. Thus, the cyclic delamination behavior of a HAp coating was observed at the stress ratio R=-1 in order to determine the effects of tensile/compressive loading on the delamination behavior. The delamination growth rate increased with SBF immersion, which decreased the delamination life. Raman spectroscopy analysis revealed that the selective phase dissolution in the HAp coating was promoted at interfaces. Finite element analysis revealed that the energy release rate Gmax showed a positive value even in cases with compressive loading, which is a driving force for the delamination of a HAp coating. A prediction model for the delamination growth life was developed that combines a fracture mechanics parameter with the assumed stress-dependent dissolution rate. The predicted delamination life matched the experimental data well in cases of lower stress amplitudes with SBF.

Formation of stacked luminescent complex of 8-hydroxyquinoline molecules on hydroxyapatite coating by using cold isostatic pressing.

Cold isostatic pressing successfully formed a chelate complex of 8-hydroxyquinoline (8 Hq) molecules on plasma-sprayed hydroxyapatite (HAp) coating by solid-state reaction. The complex emits a fluorescence peak at approximately 500 nm by UV irradiation. The red shift of the fluorescence was newly observed in the cases of highly compressed complex due to π - π stacking of aromatic ring in the molecular structure of 8 Hq. The immersed complex coating in Simulated Body Fluid (SBF) demonstrated amorphous apatite precipitation and kept its fluorescence property.

Effect of dissolution/precipitation on the residual stress redistribution of plasma-sprayed hydroxyapatite coating on titanium substrate in simulated body fluid (SBF).

The residual stress distributions in hydroxyapatite (HAp) coating with and without mixed hydroxyapatite/titanium (HAp/Ti) bond coating on commercially pure Titanium substrate (cp-Ti) were evaluated by Raman piezo-spectroscopy analysis. The Raman shifted position 962cm(-1), which is the symmetrical stretching of surrounded oxygen atoms with phosphorous atom ( [Formula: see text] ), was referred to analyses of stress dependency. The piezo-spectroscopic coefficient, which is a Raman shift value per stress (cm(-1)/GPa), was fitted from the result of four-points bending test of rectangular HAp bar and as-sprayed HAp on Zn plate. The calculated values were 3.89cm(-1)/GPa for the former and 7.11cm(-1)/GPa for the latter. By using these calibrations, the compressive residual stress in HAp coating with HAp/Ti bond coating (HA-B) has been found to be distributed in the range of -137MPa to -75MPa. For the heat-treated HAp coating (HA-B-HT) specimen, the compressive residual stresses placed in the range of -40--22MPa. The changes in the values of residual stress of the HAp coating after immersion in SBF were also evaluated. The residual stress in HA-WB specimens tend to change from compressive to tensile after 30 days immersion. The HA-B-HT specimens exhibited similar behavior and reached to zero stress after the immersion. The mechanism of the changes in residual stress would be the effect of stress redistribution around melted calcium phosphate particles to remained HAp splats.

Failure behavior of plasma-sprayed HAp coating on commercially pure titanium substrate in simulated body fluid (SBF) under bending load.

Four point bending tests with acoustic emission (AE) monitoring were conducted for evaluating failure behavior of the plasma-sprayed hydroxyapatite (HAp) top coat on commercially pure titanium (cp-Ti) plate with and without mixed HAp/Ti bond coat. Effect of immersion in simulated body fluid (SBF) on failure behavior of the coated specimen was also investigated by immersing the specimen in SBF. The AE patterns obtained from the bending test of the HAp coating specimens after a week immersion in SBF clearly showed the earlier stage of delamination and spallation of the coating layer compared to those without immersion in SBF. It was also found that the bond coating improved failure resistance of the HAp coating specimen compared to that without the bond coat. Four point bend fatigue tests under ambient and SBF environments were also conducted with AE monitoring during the entire fatigue test for investigating the influence of SBF environment on fatigue failure behavior of the HAp coating specimen with the mixed HAp/Ti bond coat. The specimens tested at a stress amplitude of 120 MPa under both ambient and SBF environments could survive up to 107 cycles without spallation of HAp coating layer. The specimens tested under SBF environment and those tested under ambient environment after immersion in SBF showed shorter fatigue life compared to those tested under ambient environment without SBF immersion. Micro-cracks nucleated in the coating layer in the early stage of fatigue life and then propagated into the cp-Ti substrate in the intermediate stage, which unstably propagated to failure in the final stage. It was found from the XRD analysis that the dissolution of the co-existing phases and the precipitation of the HAp phase were taken place during immersion in SBF. During this process, the co-existing phases disappeared from the coating layer and the HAp phase fully occupied the coating layer. The degradation of bending strength and fatigue life of the HAp coating specimens tested under SBF environment would be induced by dissolution of the co-existing phases from the coating layer during immersion in SBF.

Acoustic emission and fatigue damage induced in plasma-sprayed hydroxyapatite coating layers.

In order to improve the adhesive strength of hydroxyapatite (HAp) coatings, grit blasting with Al(2)O(3) powder and then wet blasting with HAp/Ti mixed powders was carried out on a commercially pure Ti (cp-Ti) substrate. Subsequently, an HAp/Ti bond coat layer and HAp top coat layer were deposited by plasma spraying. Fatigue tests of the HAp-coated specimens were carried out under four-point bending. Acoustic emission (AE) signals during the entire fatigue test were monitored to investigate the fatigue cracking behavior of the HAp-coated specimens. The HAp-coated specimens could survive up to 10(7) cycles without spallation of the HAp coating layers at the stress amplitude of 120 MPa. The HAp-coated specimens without HAp/Ti bond coat layer showed shorter fatigue life and easy crack nucleation compared to the HAp-coated specimens with HAp/Ti bond coat layer. The delamination and spallation of the HAp top coat with HAp/Ti bond coat on cp-Ti was not observed until the crack propagated into the cp-Ti during the final fracture stage of the fatigue cycle. Therefore, the HAp/Ti bond coat layer was found to greatly improve the fatigue damage resistance of the HAp coating layer. Three stages of the fatigue failure behavior of the HAp top coat with HAp/Ti bond coat on a cp-Ti substrate can be clearly estimated by the AE monitoring technique. These stages are cracks nucleating and propagating in the coating layer, cracks propagating in the substrate, and cracks propagating unstably to final fracture.

Characterization and fatigue damage of plasma sprayed HAp top coat with Ti and HAp/Ti bond coat layers on commercially pure titanium substrate.

The surface of commercially pure Ti (cp-Ti) substrate was grit-blasted with Al(2)O(3) powders and then wet-blasted with HAp/Ti mixed powders at room temperature. Then plasma spraying with Ti powders or HAp/Ti mixed powders on the blasted surface was carried out to form a bond coat layer, denoted as T50 and T100 bond coat for the former and HT100 bond coat for the later. The HAp top coat was subsequently sprayed with 100 mum thickness. The XRD patterns showed that the as-sprayed HT100 bond coat layer was mainly composed of HAp with minor components of Ti and TiO(2). EDS analysis also showed there co-existed HAp and Ti without reaction in the HT100 bond coat layer. Some cracks were observed in the bond coat and the top coat layers after compression-compression and tension-tension fatigue tests. The HT100 bond coat specimen produced less AE signal and a small amount of debonding and cracking in compression-compression fatigue test. The HT100 specimen could survive up to 10 million cycles at stress amplitude of 200 MPa, which is high enough compared to the maximum stress in bones: the order of 100 MPa. The degree of damage (debonding and cracking) in tension-tension fatigue test was more severe than that in compression-compression fatigue testing.

Formation of hydroxyapatite layer on bioactive Ti and Ti-6Al-4V by simple chemical technique.

Bioactive coatings on cp-Ti and Ti-6Al-4V were prepared by a simple chemical technique. Specimens of cp-Ti and Ti-6Al-4V were initially immersed in a 5 M NaOH solution at 60 degrees C for 24 h which resulted in the formation of a porous network structure composed of Na2Ti5O11 and TiO2. The specimens were then immersed in a Ca-rich solution either at 60 degrees C or at 36.5 degrees C for 24 h. During this treatment Na+ was released and Ti-OH groups were formed. Subsequently, TiO2 dissociated from the Ti-OH group and combined with calcium ions to form calcium titanate (CaTiO3), which was embedded in a titania gel layer during the immersion period. The specimens were then immersed in r-SBF at 36.5 degrees C for 1-30 days. After immersion in r-SBF for 3 days, HAp (hydroxyapatite) spheroids began to deposit on the substrates, and within a week the surfaces were covered. The HAp spheroids were 5 microm in size with a Ca/P ratio of 1.68 which was close to bone-like apatite (1.67). The average thicknesses of HAp layer after immersion in r-SBF for 3 days, 1 week, and 2 weeks were 3.8, 5.6, and 6.4 microm, respectively. A scratch test, used to evaluate the adhesive strength of the HAp layer, showed that the HAp layer was not scraped off until the applied load reached 26 N.