Phage single-stranded DNA-binding protein or host DNA damage triggers the activation of the AbpAB phage defense system.

IMPORTANCE: Although numerous phage defense systems have recently been discovered in bacteria, how these systems defend against phage propagation or sense phage infections remains unclear. The Escherichia coli AbpAB defense system targets several lytic and lysogenic phages harboring DNA genomes. A phage-encoded single-stranded DNA-binding protein, Gp32, activates this system similar to other phage defense systems such as Retron-Eco8, Hachiman, ShosTA, Nhi, and Hna. DNA replication inhibitors or defects in DNA repair factors activate the AbpAB system, even without phage infection. This is one of the few examples of activating phage defense systems without phage infection or proteins. The AbpAB defense system may be activated by sensing specific DNA-protein complexes.

A finite element study on the interactive effect between the damage of the cup-bone interface and the bone strain of hip implants under various fixation conditions.

Interfacial damage has a high impact on the loosening of the acetabular cup. However, monitoring this damage induced by the variations in loading conditions, such as the angle, amplitude, and frequency in vivo, is challenging. In this study, we evaluated the risk of loosening of the acetabular cup due to interfacial damages induced by the deviation in loading conditions and amplitudes. A three-dimensional model of the acetabular cup component was developed, and the interfacial crack growth between the cup and the bone was modeled using a fracture mechanics approach, which simulated the extent of interfacial damage and associated cup displacement. The interfacial delamination mechanism changed with the increasing inclination angle, wherein a fixation angle of 60° exhibited the largest area of contact loss. The compressive strain of embedding the simulated bone at the remaining bonding area accumulated as the lost contact area widened. Such interfacial damages, namely, the growth of the lost contact area and accumulated compressive strain in the simulated bone, promoted both embedding and rotational displacement of the acetabular cup. In the worst case of a fixation angle of 60°, the total displacement of the acetabular cup exceeded the limit of the modified safe zone, suggesting a quantitative risk of dislocation of the acetabular cup induced by the accumulated interfacial damage. Furthermore, nonlinear regression analyses between the degree of displacement of the acetabular cup and the extent of the two types of interfacial damage demonstrated that the interactive effect of the fixation angle with the loading amplitude showed a significant effect on increasing cup displacement. These findings suggest that proper control of the fixation angle during operation is useful in preventing the loosening of the hip joint.

Generation of equatorial plasma bubble after the 2022 Tonga volcanic eruption.

Equatorial plasma bubbles are a phenomenon of plasma density depletion with small-scale density irregularities, normally observed in the equatorial ionosphere. This phenomenon, which impacts satellite-based communications, was observed in the Asia-Pacific region after the largest-on-record January 15, 2022 eruption of the Tonga volcano. We used satellite and ground-based ionospheric observations to demonstrate that an air pressure wave triggered by the Tonga volcanic eruption could cause the emergence of an equatorial plasma bubble. The most prominent observation result shows a sudden increase of electron density and height of the ionosphere several ten minutes to hours before the initial arrival of the air pressure wave in the lower atmosphere. The propagation speed of ionospheric electron density variations was ~ 480-540 m/s, whose speed was higher than that of a Lamb wave (~315 m/s) in the troposphere. The electron density variations started larger in the Northern Hemisphere than in the Southern Hemisphere. The fast response of the ionosphere could be caused by an instantaneous transmission of the electric field to the magnetic conjugate ionosphere along the magnetic field lines. After the ionospheric perturbations, electron density depletion appeared in the equatorial and low-latitude ionosphere and extended at least up to ±25° in geomagnetic latitude.

Localized mesospheric ozone destruction corresponding to isolated proton aurora coming from Earth's radiation belt.

Relativistic electron precipitation (REP) from the Earth's radiation belt plays an important role in mesospheric ozone loss as a connection between space weather and the climate system. However, the rapid (tens of minutes) destruction of mesospheric ozone directly caused by REP has remained poorly understood due to the difficulty of recognizing its location and duration. Here we show a compelling rapid correspondence between localized REP and ozone destruction during a specific auroral phenomenon, the called an isolated proton aurora (IPA). The IPA from the Earth's radiation belt becomes an important spatial and temporal proxy of REP, distinct from other auroral phenomena, and allowing visualizing micro-ozone holes. We found ozone destruction of as much as 10-60% within 1.5 h of the initiation of IPA. Electromagnetic ion cyclotron waves in the oxygen ion band observed as the driver of REP likely affect through resonance with mainly ultra-relativistic (> 2 mega-electron-volts) energy electrons. The rapid REP impact demonstrates its crucial role and direct effect on regulating the atmospheric chemical balance.

Surface modification of hydroxyapatite nanoparticles for bone regeneration by controlling their surface hydration and protein adsorption states.

Autogenous bone and metallic implant grafting has been used to repair and regenerate bone defects. However, there are still many unresolved problems. It is suggested that bioceramic nanoparticles should be developed and designed to promote effective bone regeneration. In addition, it is necessary to synthesize bioceramic nanoparticles that can support proteins related to bone repair and regeneration such as collagen and albumin. As the protein-interactive bioceramic, hydroxyapatite (HA) deserves to be mentioned and has several attractive properties that are useful in biomedical fields (e.g., biocompatibility, protein adsorption capacity and stability in the physiological environment). In order to prepare novel HA nanoparticles with high biocompatibility, it can be considered that human bones are mainly composed of HA and contain a small amount of silicate, and therefore, the design of coexistence of HA with silicate can be focused. Moreover, it is proposed that the state of the hydration layer on the nanoparticle surfaces can be controlled by introducing heteroelements and polymer chains, which have a great influence on the subsequent protein adsorption and cell adhesion. In this perspective, in order to develop novel bioceramic nanoparticles for the treatment of bone defect, the design of highly biocompatible HA nanoparticles and the control of the hydration layer and protein adsorption states on the surfaces were systematically discussed based on their surface modification techniques, which are very important for the proper understanding of the interface between cells and bioceramics, leading to the further application in biomedical fields.

Effects of composition of hydroxyapatite/gray titania coating fabricated by suspension plasma spraying on mechanical and antibacterial properties.

While several metallic implants with bioactive coatings have been developed thus far for treating bone deformations or deterioration, a multifunctional coating with the desired mechanical and antibacterial properties has not been demonstrated. This study aimed to reveal the effect of the composition of hydroxyapatite (HAp)/gray titania coatings on the mechanical and antibacterial properties for biomedical applications. Suspension plasma spray (SPS) aided successful deposition of HAp/gray titania coatings on the surface of titanium substrates. The microstructure of coatings with different compositions was then characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy to identify the crystal structure. All results consistently demonstrated that SPS could transform Ti2O3 into TiO2 with mixed Magneli phases, such as Ti4O7 and Ti3O5, which could typically demonstrate photocatalytic activity. Hardness, Young's modulus, and interfacial strength of composite coatings commonly increased with an increase in the weight percentage of TiO2. A multi-modal damage assessment combining acoustic emission (AE), infrared ray camera (IR), and digital-image-correlation (DIC) was performed to monitor the damage process of HAp composite coating, which successfully revealed initiations of microcracks and nonlinear deformation at interface until fracture. Antibacterial test performed for examining the cytotoxic effects against E. coli under LED light irradiation conditions revealed that SPS HAp/gray titania coating could significantly enhance the antibacterial properties. Enhanced antibacterial properties can be attributed to an increase in the number of Magneli phases and better bacterial adhesion was attributed to hydrophilic properties conferred by submicron-sized particles. Hence, SPS can help fabricate visible light-responsive antibacterial coating, which can be used for medical devices.

The hokW-sokW Locus Encodes a Type I Toxin-Antitoxin System That Facilitates the Release of Lysogenic Sp5 Phage in Enterohemorrhagic Escherichia coli O157.

The toxin-antitoxin (TA) genetic modules control various bacterial events, such as plasmid maintenance, persister cell formation, and phage defense. They also exist in mobile genetic elements, including prophages; however, their physiological roles remain poorly understood. Here, we demonstrate that hokW-sokW, a putative TA locus encoded in Sakai prophage 5 (Sp5) in enterohemorrhagic Escherichia coli O157: H7 Sakai strain, functions as a type I TA system. Bacterial growth assays showed that the antitoxic activity of sokW RNA against HokW toxin partially requires an endoribonuclease, RNase III, and an RNA chaperone, Hfq. We also demonstrated that hokW-sokW assists Sp5-mediated lysis of E. coli cells when prophage induction is promoted by the DNA-damaging agent mitomycin C (MMC). We found that MMC treatment diminished sokW RNA and increased both the expression level and inner membrane localization of HokW in a RecA-dependent manner. Remarkably, the number of released Sp5 phages decreased by half in the absence of hokW-sokW. These results suggest that hokW-sokW plays a novel role as a TA system that facilitates the release of Sp5 phage progeny through E. coli lysis.

Higher-order topological Mott insulator on the pyrochlore lattice.

We provide the first unbiased evidence for a higher-order topological Mott insulator in three dimensions by numerically exact quantum Monte Carlo simulations. This insulating phase is adiabatically connected to a third-order topological insulator in the noninteracting limit, which features gapless modes around the corners of the pyrochlore lattice and is characterized by a [Formula: see text] spin-Berry phase. The difference between the correlated and non-correlated topological phases is that in the former phase the gapless corner modes emerge only in spin excitations being Mott-like. We also show that the topological phase transition from the third-order topological Mott insulator to the usual Mott insulator occurs when the bulk spin gap solely closes.

Manipulating Interactions between T4 Phage Long Tail Fibers and Escherichia coli Receptors.

Bacteriophages are the most abundant and diverse biological entities on Earth. Phages exhibit strict host specificity that is largely conferred by adsorption. However, the mechanism underlying this phage host specificity remains poorly understood. In this study, we examined the interaction between outer membrane protein C (OmpC), one of the Escherichia coli receptors, and the long tail fibers of bacteriophage T4. T4 phage uses OmpC of the K-12 strain, but not of the O157 strain, for adsorption, even though OmpCs from the two E. coli strains share 94% homology. We identified amino acids P177 and F182 in loop 4 of the K-12 OmpC as essential for T4 phage adsorption in the copresence of loops 1 and 5. Analyses of phage mutants capable of adsorbing to OmpC mutants demonstrated that amino acids at positions 937 and 942 of the gp37 protein, which is present in the distal tip (DT) region of the T4 long tail fibers, play an important role in adsorption. Furthermore, we created a T4 phage mutant library with artificial modifications in the DT region and isolated and characterized multiple phage mutants capable of adsorbing to OmpC of the O157 strain or lipopolysaccharide of the K-12 strain. These results shed light on the mechanism underlying the phage host specificity mediated by gp37 and OmpC and may be useful in the development of phage therapy via artificial modifications of the DT region of T4 phage. IMPORTANCE Understanding the host specificity of phages will lead to the development of phage therapy. The interaction between outer membrane protein C (OmpC), one of the Escherichia coli receptors, and the gp37 protein present in the distal tip (DT) region of the long tail fibers of T4 bacteriophages largely determines their host specificity. Here, we elucidated the amino acid residues important for the interaction between gp37 and OmpC. This result suggests that the shapes of both proteins at the binding interface play important roles in their interactions, which are likely mediated by multiple residues of both binding partners. Additionally, we successfully isolated multiple phage mutants capable of adsorbing to a variety of E. coli receptors using a mutant T4 phage library with artificial modifications in the DT region, providing a foundation for the alteration of the host specificity.

Effects of toothbrush abrasion on surface and antibacterial properties of hydroxyapatite-tryptophan complex with gray titania.

Photocatalysts have attracted attention in the medical field for their antibacterial effects. However, typical photocatalysts are activated by ultraviolet rays, which may have adverse effects. Therefore, we focused on a new photocatalyst that is activated by visible light, hydroxyapatite (HAp), and amino acid complex with gray titania, and evaluated its antibacterial effects against Porphyromonas gingivalis and effect by toothbrushing. The test sample was a titanium alloy substrate, and four surface treatments were applied: (1) substrate only, (2) substrate with HAp complex, (3) substrate with HAp complex with gray titania, and (4) HAp-tryptophan complex with gray titania (TR). These surface treatments were evaluated with or without toothbrushing (8 total groups). Surface roughness (Sa), fluorescent X-ray analysis (XRF), and scanning electron microscopy (SEM) were used to evaluate surface properties. To investigate antibacterial effects, each sample was seeded with P. gingivalis, irradiated with red light, and total viable bacterial count was determined. For Sa measurement, TR showed no significant difference after toothbrushing. However, in XRF and SEM observation, TR exhibited peeling of the applied coating after toothbrushing. In the antibacterial test, TR showed a decrease in P. gingivalis under no toothbrushing condition. Conversely, with toothbrushing, the TR coating appeared to peel. However, no significant difference in P. gingivalis count was observed among all groups. HAp-tryptophan complex with gray titania coating showed an antibacterial effect against P. gingivalis when irradiated with visible light. However, toothbrushing can result in coat peeling and consequently reduce the antibacterial effect.

Damage evaluation of HAp-coated porous titanium foam in simulated body fluid based on compression fatigue behavior.

Although pure Ti is nontoxic, alloying elements may be released into the surrounding tissue when Ti alloys are used, and this can cause cytotoxicity. Therefore, this study performed the damage evaluation of hydroxyapatite (HAp)-coated porous Ti components subjected to cyclic compression in a simulated body fluid (SBF). The HAp coating layer was deposited on the surface of porous Ti by electrophoresis, and a dense and homogeneous coating morphology was observed on the surface of the porous Ti. To specify damage types of HAp coating in situ, acoustic emission (AE) measurements and microscopic observations were simultaneously conducted during compressive fatigue loading tests to detect the specific failure mode. Compression tests revealed that the interfacial strength between the HAp coating and porous Ti was higher than the yield strength of the porous body (7-9 MPa). The AE signals were detected only in the plastic deformation stage of porous Ti, which indicated that they were generated because of plastic deformation/fractures in the porous body. Compressive fatigue tests revealed that no significant HAp coating damage occurred when the applied maximum stress was within the elastic limit of porous Ti in air. In contrast, the HAp coating exhibited delamination at the initial stage of cyclic loading at all stress levels in SBF, while the fatigue limit of the coated porous substrate, 2 MPa, was not affected by the SBF medium. Though the delamination of the HAp coating in SBF occurred during the early stages of fatigue loading, the amorphous calcium phosphate layer was recovered partly through re-precipitation from SBF. The AE signals from the delamination of the HAp coating or fracture in porous Ti could be identified using the peak voltage and frequencies. As microscopic observations were limited to certain parts of the porous body, AE signals were clustered according to the types of failure. The clustered AE signals were successfully correlated with the fatigue behavior of porous Ti. Corrosion fatigue was determined to be the primary mechanism for the delamination of the HAp coating on porous Ti in SBF.

The Antibacterial Activity of Hydroxyapatite-Tryptophan Complex with Gray Titania by Photocatalysis Using LED Diodes.

PURPOSE: Peri-implantitis is an important biologic complication that can lead to implant failure. Proper treatment should effectively kill bacteria, not harm the implant surface, and promote regeneration. Recently, photocatalytic coating without antibiotics or external agents was proposed to be an alternative to antibiotic therapy. The aim of this study was to evaluate the photocatalytic antibacterial effect of a new titanium implant coating made from hydroxyapatite-tryptophan complex and gray titania, which was activated by two visible lights. MATERIALS AND METHODS: Titanium alloy substrate was plasma sprayed with hydroxyapatite (80 wt%) and dititanium trioxide (20 wt%) and then pressed with tryptophan. Three bacteria related to peri-implantitis, Porphyromonas gingivalis, Tannerella forsythia, and Aggregatibacter actinomycetemcomitans, were used in this study. Six conditions were tested: (1) control group (only bacteria), (2) photocatalytic sample in darkness (bacteria and coated sample in darkness), (3) red laser for 15 minutes (bacteria irradiated with photoactivated disinfection [PAD] light, 650-nm wavelength), (4) broadband light-emitting diode (LED) for 15 minutes (bacteria irradiated with broadband LED, peak wavelength at 470 nm), (5) photocatalysis by red laser for 15 minutes (bacteria and coated sample irradiated with PAD light), and (6) photocatalysis by broadband LED for 15 minutes (bacteria and coated sample irradiated with broadband LED). After 15 minutes of irradiation, photocatalytic antibacterial effects were evaluated by total viable bacterial count, adenosine triphosphate (ATP) assay, and LIVE/DEAD assay. RESULTS: The number of all bacteria tested was significantly decreased by the photocatalytic effect of both visible light sources (P < .05). For P gingivalis, viable bacteria of lethal photosensitization groups were also significantly decreased (P < .05), especially when using the broadband LED. However, the coating material itself did not have antibacterial properties without light activation. There was no significant difference in ATP among groups (P > .05). LIVE/DEAD staining showed that red fluorescent bacterial cells were present in photocatalytic groups from the two light sources. CONCLUSION: Photoactivated hydroxyapatite-tryptophan complex and gray titania as a photocatalytic coating has antibacterial effects on bacteria associated with peri-implantitis.

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.

A Short Peptide Derived from the ZorO Toxin Functions as an Effective Antimicrobial.

Antimicrobial peptides are potential molecules for the development of novel antibiotic agents. The ZorO toxin of a type I toxin-antitoxin system in Escherichia coli O157:H7 is composed of 29 amino acids and its endogenous expression inhibits E. coli growth. However, little is known about its inhibitory mechanism. In this study, we demonstrate that the ZorO localized in the inner membrane affects the plasma membrane integrity and potential when expressed in E. coli cells, which triggers the production of cytotoxic hydroxyl radicals. We further show that five internal amino acids (Ala-Leu-Leu-Arg-Leu; ALLRL) of ZorO are necessary for its toxicity. This result prompted us to address the potential of the synthetic ALLRL peptide as an antimicrobial. Exogenously-added ALLRL peptide to Gram-positive bacteria, Staphylococcus aureus and Bacillus subtilis, and a fungus, Candida albicans, trigger cell membrane damage and exhibit growth defect, while having no effect on Gram-negative bacterium, E. coli. The ALLRL peptide retains its activity under the physiological salt concentrations, which is in contrast to natural antimicrobial peptides. Importantly, this peptide has no toxicity against mammalian cells. Taken together, an effective and short peptide, ALLRL, would be an attractive antimicrobial to Gram-positive bacteria and C. albicans.

Cytotoxicity of hydroxyapatite-tyrosine complex with gray titania coating on titanium alloy surface to L929 mouse fibroblasts.

Hydroxyapatite particles (HAp) have been widely used by many dental implant systems as an implant coating material because of their osteoconductive properties. This study aimed at improving the antibacterial effect of HAp as a substitute for antibiotic agents which can increase drug resistance. HAp/gray titania was selected as the coating material for on the titanium alloy substrate due to its antibacterial properties after photocatalytic reaction. When combined with amino acids, HAp can form a fluorescent complex which enhances this property. Before clinical application, this new coating should be examined for cytotoxic effects against biological cells or tissues. Therefore, L929 mouse fibroblasts were used to represent fibrous tissue surrounding dental implant. After performing a 6-day alamarBlue assay, the new coating method using hydroxyapatite-tyrosine complex with gray titania on titanium alloy surface can be said to have no influence on the growth of fibroblasts.

Visualization of rapid electron precipitation via chorus element wave-particle interactions.

Chorus waves, among the most intense electromagnetic emissions in the Earth's magnetosphere, magnetized planets, and laboratory plasmas, play an important role in the acceleration and loss of energetic electrons in the plasma universe through resonant interactions with electrons. However, the spatial evolution of the electron resonant interactions with electromagnetic waves remains poorly understood owing to imaging difficulties. Here we provide a compelling visualization of chorus element wave-particle interactions in the Earth's magnetosphere. Through in-situ measurements of chorus waveforms with the Arase satellite and transient auroral flashes from electron precipitation events as detected by 100-Hz video sampling from the ground, Earth's aurora becomes a display for the resonant interactions. Our observations capture an asymmetric spatial development, correlated strongly with the amplitude variation of discrete chorus elements. This finding is not theoretically predicted but helps in understanding the rapid scattering processes of energetic electrons near the Earth and other magnetized planets.

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.

RnlB Antitoxin of the Escherichia coli RnlA-RnlB Toxin-Antitoxin Module Requires RNase HI for Inhibition of RnlA Toxin Activity.

The Escherichia coli RnlA-RnlB toxin-antitoxin system is related to the anti-phage mechanism. Under normal growth conditions, an RnlA toxin with endoribonuclease activity is inhibited by binding of its cognate RnlB antitoxin. After bacteriophage T4 infection, RnlA is activated by the disappearance of RnlB, resulting in the rapid degradation of T4 mRNAs and consequently no T4 propagation when T4 dmd encoding a phage antitoxin against RnlA is defective. Intriguingly, E. coli RNase HI, which plays a key role in DNA replication, is required for the activation of RnlA and stimulates the RNA cleavage activity of RnlA. Here, we report an additional role of RNase HI in the regulation of RnlA-RnlB system. Both RNase HI and RnlB are associated with NRD (one of three domains of RnlA). The interaction between RnlB and NRD depends on RNase HI. Exogenous expression of RnlA in wild-type cells has no effect on cell growth because of endogenous RnlB and this inhibition of RnlA toxicity requires RNase HI and NRD. These results suggest that RNase HI recruits RnlB to RnlA through NRD for inhibiting RnlA toxicity and thus plays two contrary roles in the regulation of RnlA-RnlB system.

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.