Hierarchically porous surface of HA-sandblasted Ti implant screw using the plasma electrolytic oxidation: Physical characterization and biological responses

The surface topological features of bioimplants are among the key indicators for bone tissue replacement because they directly affect cell morphology, adhesion, proliferation, and differentiation. In this study, we investigated the physical, electrochemical, and biological responses of sandblasted titanium (SB-Ti) surfaces with pore geometries fabricated using a plasma electrolytic oxidation (PEO) process. The PEO treatment was conducted at an applied voltage of 280 V in a solution bath consisting of 0.15 mol L-1 calcium acetate monohydrate and 0.02 mol L-1 calcium glycerophosphate for 3 min. The surface chemistry, wettability, mechanical properties and corrosion behavior of PEO-treated sandblasted Ti implants using hydroxyapatite particles (PEO-SB-Ti) were improved with the distribution of calcium phosphorous porous oxide layers, and showed a homogeneous and hierarchically porous surface with clusters of nanopores in a bath containing calcium acetate monohydrate and calcium glycerophosphate. To demonstrate the efficacy of PEO-SB-Ti, we investigated whether the implant affects biological responses. The proposed PEO-SB-Ti were evaluated with the aim of obtaining a multifunctional bone replacement model that could efficiently induce osteogenic differentiation as well as antibacterial activities. These physical and biological responses suggest that the PEO-SB-Ti may have a great potential for use an artificial bone replacement compared to that of the controls.

Influence of Ultrasound on the Characteristics of CaP Coatings Generated Via the Micro-arc Oxidation Process in Relation to Biomedical Engineering

Over the past decade, bone tissue engineering has been at the core of attention because of an increasing number of implant surgeries. The purpose of this study was to obtain coatings on titanium (Ti) implants with improved properties in terms of biomedical applications and to investigate the effect of ultrasound (US) on these properties during the micro-arc oxidation (MAO) process. The influence of various process parameters, such as time and current density, as well as US mode, on the properties of such coatings was evaluated. Novel porous calcium-phosphate-based coatings were obtained on commercially pure Ti. Their microstructure, chemical composition, topography, wettability, nanomechanical properties, thickness, adhesion to the substrate, and corrosion resistance were analyzed. In addition, cytocompatibility evaluation was checked with the human osteoblasts. The properties of the coatings varied significantly, depending on applied process parameters. The US application during the MAO process contributes to the increase of coating thickness, porosity, roughness, and skewness, as well as augmented calcium incorporation. The most advantageous coating was obtained at a current of 136 mA, time 450 s, and unipolar rectangular US, as it exhibits high porosity, adequate wettability, and beneficial skewness, which enabled increased adhesion and proliferation of osteoblasts during in vitro studies. Finally, the conducted research demonstrated the influence of various UMAO process parameters, which allowed for the selection of appropriate Ti implant modification for specific biomedical utilization.

Rapid Plasma Electrolytic Oxidation Synthesis of Intermetallic PtBi/MgO/Mg Monolithic Catalyst for Efficient Removal of Organic Pollutants

The intermetallic PtBi/MgO/Mg monolithic catalyst was first prepared using non-equilibrium plasma electrolytic oxidation (PEO) technology. Spherical aberration-corrected transmission electron microscope (ACTEM) observation confirms the successful synthesis of the PtBi intermetallic structure. The efficiency of PtBi/Mg/MgO catalysts in catalyzing the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4 was demonstrated. The activity factor for the catalyst is 31.8 s-1 g-1, which is much higher than reported values. In addition, the resultant catalyst also exhibits excellent catalytic activity in the organic pollutant reaction of p-nitrobenzoic acid (p-NBA) and methyl orange (MO). Moreover, benefiting from ordered atomic structures and the half-embedded PtBi nanoparticles (NPs), the catalyst demonstrates excellent stability and reproducibility in the degradation of 4-NP. This study provides an example of a simple method for the preparation of intermetallic structures as catalysts for organic pollutant degradation.

Electrochemical Deposition of Copper on Bioactive Porous Titanium Dioxide Layer: Antibacterial and Pro-Osteogenic Activities

Ti surfaces must exhibit antibacterial activity without cytotoxicity to promote bone reconstruction and prevent infection simultaneously. In this study, we employed a two-step electrochemical treatment process, namely, microarc oxidation (MAO) and cathodic electrochemical deposition (CED), to modify Ti surfaces. During the MAO step, a porous TiO2 (pTiO2) layer with a surface roughness of approximately 2.0 μm was generated on the Ti surface, and in the CED step, Cu was deposited onto the pTiO2 layer on the Ti surface, forming Cu@pTiO2. Cu@pTiO2 exhibited a similar structure, adhesion strength, and crystal phase to pTiO2. Moreover, X-ray photoelectron spectroscopy (XPS) confirmed the presence of Cu in Cu@pTiO2 at an approximate concentration of 1.0 atom %. Cu@pTiO2 demonstrated a sustained release of Cu ions for a minimum of 28 days in a simulated in vivo environment. In vitro experiments revealed that Cu@pTiO2 effectively eradicated approximately 99% of Staphylococcus aureus and Escherichia coli and inhibited biofilm formation, in contrast to the Ti and pTiO2 surfaces. Moreover, Cu@pTiO2 supported the proliferation of osteoblast-like cells at a rate comparable to that observed on the Ti and pTiO2 surfaces. Similar to pTiO2, Cu@pTiO2 promoted the calcification of osteoblast-like cells compared with Ti. In summary, we successfully conferred antibacterial and pro-osteogenic activities to Ti surfaces without inducing cytotoxic effects or structural and mechanical alterations in pTiO2 through the application of MAO and CED processes. Moreover, we found that the pTiO2 layer promoted bacterial growth and biofilm formation more effectively than the Ti surface, highlighting the potential drawbacks of rough and porous surfaces. Our findings provide fundamental insights into the surface design of Ti-based medical devices for bone reconstruction and infection prevention.

Hierarchical Hybrid Coatings with Drug-Eluting Capacity for Mg Alloy Biomaterials

A hierarchical hybrid coating (HHC) comprising a ceramic oxide layer and two biodegradable polymeric (polycaprolactone, PCL) layers has been developed on Mg3Zn0.4Ca cast alloy in order to provide a controlled degradation rate and functionality by creating a favorable porous surface topography for cell adhesion. The inner, ceramic layer formed by plasma electrolytic oxidation (PEO) has been enriched in bioactive elements (Ca, P, Si). The intermediate PCL layer sealed the defect in the PEO layer and the outer microporous PCL layer loaded with the appropriate active molecule, thus providing drug-eluting capacity. Morphological, chemical, and biological characterizations of the manufactured coatings loaded with ciprofloxacin (CIP) and paracetamol (PAR) have been carried out. In vitro assays with cell lines relevant for cardiovascular implants and bone prosthesis (endothelial cells and premyoblasts) showed that the drug-loaded coating allows for cell proliferation and viability. The study of CIP and PAR cytotoxicity and release rate indicated that the porous PCL layer does not release concentrations detrimental to the cells. However, complete system assays revealed that corrosion behavior and increase of the pH negatively affects cell viability. H2 evolution during corrosion of Mg alloy substrate generates blisters in PCL layer that accelerate the corrosion locally in crevice microenvironment. A detailed mechanism of the system degradation is disclosed. The accelerated degradation of the developed system may present interest for its further adaptation to new cancer therapy strategies.

Frequency Response Evaluation as Diagnostic and Optimization Tool for Pulsed Unipolar Plasma Electrolytic Oxidation Process and Resultant Coatings on Zirconium

This study aims to bridge various diagnostic tools for the development of smart plasma electrolytic oxidation (PEO) technologies. PEO treatments of commercially pure Zr were carried out using the pulsed unipolar polarisation (PUP) regime with frequency sweep in an alkaline phosphate-silicate electrolyte. Methods of in situ impedance spectroscopy and electrical transient analysis were used for the process diagnostics under the video imaging of the PEO. Two cutoff frequencies, 170-190 Hz and 620-650 Hz, were identified for the PEO-assisted charge transfer process. An equivalent circuit for the metal-oxide-electrolyte system under PUP PEO conditions was developed; from the capacitance values, two geometrical dielectric barriers were evaluated: a thinner 0.5-1 µm inner layer of the coating and a thicker 4-6 µm outer layer. These estimates were in agreement with the coating cross-sectional morphology. Based on comparing the results obtained using different techniques, the frequencies at which the uniform coatings with the best protective properties were formed were identified. For the selected electrolyte system and polarisation regime, these frequencies ranged from 2 to 5 kHz where the overall circuit reactance was minimal; therefore, the power factor was as close to one as possible. This opens the possibilities for the optimization of the pulsed PEO process and online control of unobservable surface characteristics, e.g., the thickness of the coating layers, thus contributing towards the development of smart PEO technologies.

The Influence of Potassium Hexafluorophosphate on the Morphology and Anticorrosive Properties of Conversion Coatings Formed on the AM50 Magnesium Alloy by Plasma Electrolytic Oxidation

In this study, conversion coatings were produced on the AM50 magnesium alloy by a plasma electrolytic oxidation (PEO) process in alkaline-silicate electrolyte with the addition of potassium hexafluorophosphate, using a unipolar pulse power source. The coating microstructure and its composition were determined using scanning electron microscopy (SEM) and an X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the conversion coatings was evaluated by means of potentiodynamic polarization tests (PDP) and electrochemical impedance spectroscopy (EIS) in a dilute Harrison solution (DHS). It has been found that the properties (microstructure, composition, and coating thickness) of the obtained layer and, therefore, their anticorrosive resistance strongly depend on the electrolyte composition. The best anticorrosive properties were observed in the layers obtained in the presence of 2.5 g/L KPF6. It was found that the conversion coating produced with the addition of hexafluorophosphate is characterized by a different morphology (sponge-like) and better anticorrosion properties, in comparison to the coating obtained with the addition of fluoride and orthophosphate salts commonly used in PEO synthesis. The sponge-like structure, which is similar to bone structure in combination with the presence of phosphates in the layer, can increase the biocompatibility and the possibility of self-healing of this coating. However, neither Mg(PF6)2, nor any other compounds containing PF6-, have been found in the layers produced.

Effects of Zn-ZnO Core-Shell Nanoparticles on Antimicrobial Mechanisms and Immune Cell Activation

The deposition of zinc-zinc oxide nanoparticles (Zn-ZnO NPs) onto porous Ta2O5 surfaces enriched with calcium phosphate by DC magnetron sputtering was investigated to improve the surface antimicrobial activity without triggering an inflammatory response. Different sizes and amounts of Zn NPs obtained by two optimized different depositions and an additional thin carbon (C) layer deposited over the NPs were explored. The deposition of the Zn NPs and the C layer mitigates the surface porosity, increasing the surface hydrophobicity and decreasing the surface roughness. The possible antimicrobial effect and immune system activation of Zn-ZnO NPs were investigated in Candida albicans and macrophage cells, respectively. It was found that the developed surfaces displayed a fungistatic behavior, as they impair the growth of C. albicans between 5 and 24 h of culture. This behavior was more evident on the surfaces with bigger NPs and the highest amounts of Zn. The same trend was observed in both reactive oxygen species (ROS) generation and loss of C. albicans' membrane integrity. After 24 h of culture, cell toxicity was also dependent on the amount of the NPs. Cell toxicity was observed in surfaces with the highest amount of Zn NPs and with the C layer, while cells were able to grow without any signs of cytotoxicity in the porous surfaces with the lowest amount of NPs. The same Zn-dose-dependent behavior was noticed in the TNF-α production. The Zn-containing surfaces show a vastly inferior cytokine secretion than the lipopolysaccharide (LPS)-stimulated cells, indicating that the modified surfaces do not induce an inflammatory response from macrophage cells. This study provides insights for understanding the Zn amount threshold that allows a simultaneous inhibition of the fungi growth with no toxic effect and the main antimicrobial mechanisms of Zn-ZnO NPs, contributing to future clinical applications.

Antibacterial Calcium Phosphate Coatings for Biomedical Applications Fabricated via Micro-Arc Oxidation

A promising method for improving the functional properties of calcium-phosphate coatings is the incorporation of various antibacterial additives into their structure. The microbial contamination of a superficial wound is inevitable, even if the rules of asepsis and antisepsis are optimally applied. One of the main problems is that bacteria often become resistant to antibiotics over time. However, this does not apply to certain elements, chemical compounds and drugs with antimicrobial properties. In this study, the fabrication and properties of zinc-containing calcium-phosphate coatings that were formed via micro-arc oxidation from three different electrolyte solutions are investigated. The first electrolyte is based on calcium oxide, the second on hydroxyapatite and the third on calcium acetate. By adding zinc oxide to the three electrolyte solutions, antibacterial properties of the coatings are achieved. Although the same amount of zinc oxide has been added to each electrolyte solution, the zinc concentration in the coatings obtained vary greatly. Furthermore, this study investigates the morphology, structure and chemical composition of the coatings. The antibacterial properties of the zinc-containing coatings were tested toward three strains of bacteria-Staphylococcus aureus, methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa. Coatings of calcium acetate and zinc oxide contained the highest amount of zinc and displayed the highest zinc release. Moreover, coatings containing hydroxyapatite and zinc oxide show the highest antibacterial activity toward Pseudomonas aeruginosa, and coatings containing calcium acetate and zinc oxide show the highest antibacterial activities toward Staphylococcus aureus and methicillin-resistant Staphylococcus aureus.

Nanodiamond Decorated PEO Oxide Coatings on NiTi Alloy

Cardiovascular diseases (CVDs) remain a leading cause of death in the European population, primarily attributed to atherosclerosis and subsequent complications. Although statin drugs effectively prevent atherosclerosis, they fail to reduce plaque size and vascular stenosis. Bare metal stents (BMS) have shown promise in acute coronary disease treatment but are associated with restenosis in the stent. Drug-eluting stents (DES) have improved restenosis rates but present long-term complications. To overcome these limitations, nanomaterial-based modifications of the stent surfaces have been explored. This study focuses on the incorporation of detonation nanodiamonds (NDs) into a plasma electrolytic oxidation (PEO) coating on nitinol stents to enhance their performance. The functionalized ND showed a high surface-to-volume ratio and was incorporated into the oxide layer to mimic high-density lipoproteins (HDL) for reverse cholesterol transport (RCT). We provide substantial characterization of DND, including stability in two media (acetone and water), Fourier transmission infrared spectroscopy, and nanoparticle tracking analysis. The characterization of the modified ND revealed successful functionalization and adequate suspension stability. Scanning electron microscopy with EDX demonstrated successful incorporation of DND into the ceramic layer, but the formation of a porous surface is possible only in the high-voltage PEO. The biological assessment demonstrated the biocompatibility of the decorated nitinol surface with enhanced cell adhesion and proliferation. This study presents a novel approach to improving the performance of nitinol stents using ND-based surface modifications, providing a promising avenue for cardiovascular disease.

Healable Anti-Corrosive and Wear-Resistant Silicone-Oil-Impregnated Porous Oxide Layer of Aluminum Alloy by Plasma Electrolytic Oxidation

Lubricant (or oil)-impregnated porous surface has been considered as a promising surface treatment to realize multifunctionality. In this study, silicone oil was impregnated into a hard porous oxide layer created by the plasma electrolytic oxidation (PEO) of aluminum (Al) alloys. The monolayer of polydimethylsiloxane (PDMS) from silicone oil is formed on a porous oxide layer; thus, a water-repellent slippery oil-impregnated surface is realized on Al alloy, showing a low contact angle hysteresis of less than 5°. This water repellency significantly enhanced the corrosion resistance by more than four orders of magnitude compared to that of the PEO-treated Al alloy without silicone oil impregnation. The silicone oil within the porous oxide layer also provides a lubricating effect to improve wear resistance by reducing friction coefficients from ~0.6 to ~0.1. In addition, because the PDMS monolayer can be restored by frictional heat, the water-repellent surface is tolerant to physical damage to the oxide surface. Hence, the results of this fundamental study provide a new approach for the post-treatment of PEO for Al alloys.

The Effects of the Pre-Anodized Film Thickness on Growth Mechanism of Plasma Electrolytic Oxidation Coatings on the 1060 Al Substrate

To increase the density of the micro-arc oxide coating, AA 1060 samples were pretreated with an anodic oxide film in an oxalic acid solution. Plasma electrolytic oxidation (PEO) was performed to investigate the effect of the thickness of the pre-anodic oxide film on the soft-sparking mechanism. The experimental results revealed that the PEO coating phases with different thicknesses of the pre-anodized films contained both Al and gamma-alumina (γ-Al2O3). The pre-anodized film changes the final morphology of the coating, accelerating the soft sparking transition and retaining the soft sparking. At a pre-anodized film thickness of ≤7.7 μm, the anodized films thickened before being broken through. When the pre-anodized film thickness was ≥13.1 μm, partial dissolution of the anodized films occurred before they were struck through. Two growth mechanisms for PEO coatings with different pre-anodized film thicknesses were proposed.

In Vivo Safety of New Coating for Biodegradable Magnesium Implants

Biodegradable Magnesium (Mg) implants are promising alternatives to permanent metallic prosthesis. To improve the biocompatibility and with the aim of degradation control, we provided Plasma Electrolytic Oxidation (PEO) of pure Mg implant in silicate-based solution with NaOH (S1 250 V) and Ca(OH)2 (S2 300 V). Despite the well-structured surface, S1 250 V implants induced enormous innate immunity reaction with the prevalence of neutrophils (MPO+) and M1-macrophages (CD68+), causing secondary alteration and massive necrosis in the peri-implant area in a week. This reaction was also accompanied by systemic changes in visceral organs affecting animals' survival after seven days of the experiment. In contrast, S2 300 V implantation was associated with focal lymphohistiocytic infiltration and granulation tissue formation, defining a more favorable outcome. This reaction was associated with the prevalence of M2-macrophages (CD163+) and high density of αSMA+ myofibroblasts, implying a resolution of inflammation and effective tissue repair at the site of the implantation. At 30 days, no remnants of S2 300 V implants were found, suggesting complete resorption with minor histological changes in peri-implant tissues. In conclusion, Ca(OH)2-contained silicate-based solution allows generating biocompatible coating reducing toxicity and immunogenicity with appropriate degradation properties that make it a promising candidate for medical applications.

The Effect of Mechanical Pretreatment on the Electrochemical Characteristics of PEO Coatings Prepared on Magnesium Alloy AZ80

The main objective of this article is to provide new information on the effects of mechanical pretreatment of AZ80 magnesium alloy ground with SiC emery papers of different grain sizes on the plasma electrolytic oxidation (PEO) process and corrosion properties of AZ80 in 0.1 M NaCl solution. Then, the roughness of the coated samples was measured by confocal microscopy. The corrosion properties of the ground and coated surfaces were determined by potentiodynamic polarization (PDP) within 1 h of exposure, and electrochemical impedance spectroscopy (EIS) was performed during 168 h of exposure at laboratory temperature. Consequently, the obtained results of the PDP measurements were evaluated by the Tafel analysis and the EIS evaluation was performed by the equivalent circuit analysis through Nyquist diagrams. The morphology and structure of PEO coatings were observed by scanning electron microscopy (SEM) through the secondary imaging technology, and the presence of certain elements in PEO coatings was analyzed by EDS analysis.

Osteoconductive, Osteogenic, and Antipathogenic Plasma Electrolytic Oxidation Coatings on Titanium Implants with BMP-2

We report a one-pot plasma electrolytic oxidation (PEO) strategy for forming a multi-element oxide layer on the titanium surface using complex electrolytes containing Na2HPO4, Ca(OH)2, (NH2)2CO, Na2SiO3, CuSO4, and KOH compounds. For even better bone implant ingrowth, PEO coatings were additionally loaded with bone morphogenetic protein-2 (BMP-2). The samples were tested in vivo in a mouse craniotomy model. Tests for bactericidal and fungicidal activity were carried out using clinically isolated multi-drug-resistant Escherichia coli (E. coli) K261, E. coli U20, methicillin-resistant Staphylococcus aureus (S. aureus) CSA154 bacterial strains, and Neurospora crassa (N. crassa) and Candida albicans (C. albicans) D2528/20 fungi. The PEO-Cu coating effectively inactivated both Gram-positive and Gram-negative bacteria at low concentrations of Cu2+ ions: minimal bactericidal concentration for E. coli and N. crassa (99.9999%) and minimal inhibitory concentration (99.0%) for S. aureus were 5 ppm. For all studied bacterial and fungal strains, PEO-Cu coating completely prevented the formation of bacterial and fungal biofilms. PEO and PEO-Cu coatings demonstrated bone remodeling and moderate osteoconductivity in vivo, while BMP-2 significantly enhanced osteoconduction and osteogenesis. The obtained results are encouraging and indicate that Ti-based materials with PEO coatings loaded with BMP-2 can be widely used in customized medicine as implants for orthopedics and cranio-maxillofacial surgery.

Effect of Ultrasonic Frequency on Structure and Corrosion Properties of Coating Formed on Magnesium Alloy via Plasma Electrolytic Oxidation

This study explores the application of ultrasonic vibration during plasma electrolytic oxidation (PEO) to enhance the corrosion resistance of magnesium (Mg) alloy. To this end, three different ultrasonic frequencies of 0, 40, and 135 kHz were utilized during PEO. In the presence of ultrasonic waves, the formation of a uniform and dense oxide layer on Mg alloys is facilitated. This is achieved through plasma softening, acoustic streaming, and improved mass transport for successful deposition and continuous reforming of the oxide layer. The oxide layer exhibits superior protective properties against corrosive environments due to the increase in compactness. Increasing ultrasonic frequency from 40 to 135 kHz, however, suppresses the optimum growth of the oxide layer due to the occurrence of super-soft plasma swarms, which results in a low coating thickness. The integration of ultrasonic vibration with PEO presents a promising avenue for practical implementation in industries seeking to enhance the corrosion protection of Mg alloys, manipulating microstructures and composition.

Cleaning Strategies of Synthesized Bioactive Coatings by PEO on Ti-6Al-4V Alloys of Organic Contaminations

The effect of various cleaning methods on coating morphology and their effectiveness in removing organic contaminants has been studied in this research. Bioactive coatings containing titanium oxides and hydroxyapatite (HAP) were obtained through plasma electrolytic oxidation in aqueous electrolytes and molten salts. The cleaning procedure for the coated surface was performed using autoclave (A), ultraviolet light (UV), radio frequency (RF), air plasma (P), and UV-ozone cleaner (O). The samples were characterized using scanning electron microscopy (SEM) with an EDS detector, X-ray photoelectron spectroscopy (XPS), X-ray phase analysis (XRD), and contact angle (CA) measurements. The conducted studies revealed that the samples obtained from molten salt exhibited a finer crystalline structure morphology (275 nm) compared to those obtained from aqueous electrolytes (350 nm). After applying surface cleaning methods, the carbon content decreased from 5.21 at.% to 0.11 at.% (XPS), which directly corresponds to a reduction in organic contaminations and a decrease in the contact angle as follows: A > UV > P > O. This holds true for both coatings obtained in molten salt (25.3° > 19.5° > 10.5° > 7.5°) and coatings obtained in aqueous electrolytes (35.2° > 28.3° > 26.1° > 16.6°). The most effective and moderate cleaning method is ozone treatment.

Plasma Electrolytic Oxidation Coatings of a 6061 Al Alloy in an Electrolyte with the Addition of K2ZrF6

In this study, white thermal control coatings were produced on a 6061 Al alloy using plasma electrolytic oxidation (PEO). The coatings were mainly formed by incorporating K₂ZrF₆. The phase composition, microstructure, thickness, and roughness of the coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), a surface roughness tester, and an eddy current thickness meter, respectively. The solar absorbance and infrared emissivity of the PEO coatings were measured using a UV-Vis-NIR spectrophotometer and FTIR spectrometer, respectively. The addition of K₂ZrF₆ to the trisodium phosphate electrolyte was found to significantly enhance the thickness of the white PEO coating on the Al alloy, with the coating thickness increasing in proportion to the concentration of K₂ZrF₆. Meanwhile, the surface roughness was observed to stabilize at a certain level as the K₂ZrF₆ concentration increased. At the same time, the addition of K₂ZrF₆ altered the growth mechanism of the coating. In the absence of K₂ZrF₆ in the electrolyte, the PEO coating on the Al alloy surface predominantly developed outwards. However, with the introduction of K₂ZrF₆, the coating's growth mode transitioned to a combination of outward and inward growth, with the proportion of inward growth progressively increasing in proportion to the concentration of K₂ZrF₆. The addition of K₂ZrF₆ substantially enhanced the adhesion of the coating to the substrate and endowed it with exceptional thermal shock resistance, as the inward growth of the coating was facilitated by the presence of K₂ZrF₆. In addition, the phase composition of the aluminum alloy PEO coating in the electrolyte containing K₂ZrF₆ was dominated by tetragonal zirconia (t-ZrO₂) and monoclinic zirconia (m-ZrO₂). With the increase in K₂ZrF₆ concentration, the L* value of the coating increased from 71.69 to 90.53. Moreover, the coating absorbance α decreased, while the emissivity ε increased. Notably, at a K₂ZrF₆ concentration of 15 g/L, the coating exhibited the lowest absorbance (0.16) and the highest emissivity (0.72), which are attributed to the enhanced roughness resulting from the substantial increase in coating thickness caused by the addition of K₂ZrF₆, as well as the presence of ZrO₂ with higher emissivity within the coating.

Plasma Electrolytic Oxidation for Dental Implant Surface Treatment

Current technologies of plasma electrolytic oxidation (PEO) for modifying the surfaces of dental implants made of the Grade IV titan alloy provide predictable long-term results in implant dentistry. The aim of the study is to evaluate the efficacy of PEO technology comparing two types of surface modification of dental implants made of VT1-0 medical titanium alloy.

Materials and Methods

50 IRIS dental implants (Scientific Production Company LICOSTOM, Russia), 10-mm long and 4 mm in diameter, were manufactured from the VT1-0 alloy. The implant surface was treated by two PEO methods: 1) in the aqueous solution of alkaline electrolyte without any additional modifiers (PEO-Ti); 2) in the aqueous solution of orthophosphoric acid-based electrolyte containing calcium carbonate (PEO-Ca). Implants made of VT1-0 alloy after milling and without additional treatment served as control samples. The implant surfaces were studied by electron microscopy and energy dispersive X-ray spectrometry. Some of the implants were installed in sheep, samples were obtained at 2, 4, and 8 weeks and studied by microcomputer tomography.

Results

Regardless of the electrolyte composition, a highly developed porous surface was formed in the samples with PEO-modified surfaces. The surface of the PEO-Ti samples in a simple unmodified electrolyte was characterized by a large number of open pores with a wide range of size distribution from 200 nm to 3 μm. The pore size distribution was of a monomodal character, with a maximum near 0.23 μm. The PEO samples in the Ca-containing electrolyte had pores also in a wide range from ~80 nm to ~7 μm. The pore distribution, in contrast to PEO-Ti, was bimodal in nature, with the main maximum in the region of 1.05 μm and the concomitant maximum near 2.45 μm.The obtained surfaces of both types (PEO with Ca and Ti) possessed high purity and optimal microroughness for osseointegration. Both types of PEO treatment (PEO with Ca and Ti) have demonstrated a similar osseointegrative potential, nevertheless, the surface of the PEO-Ca showed a better contact with the implant surface (49.8%) than PEO-Ti (42.4%) obviously due to the presence of calcium in its composition.

Conclusion

The PEO-formed implant surfaces demonstrate high osseointegrative properties after any variants of treatment and show the potential for application in osteoporosis.

Investigation of Tribological Characteristics of PEO Coatings Formed on Ti6Al4V Titanium Alloy in Electrolytes with Graphene Oxide Additives

Coatings with a thickness from ~40 to ~50 µm on Ti6Al4V titanium alloys were formed by plasma electrolytic oxidation (PEO) in a silicate-hypophosphite electrolyte with the addition of graphene oxide. The PEO treatment was carried out in the anode-cathode mode (50 Hz) at a ratio of anode and cathode currents of 1:1; their sum density was 20 A/dm², and the treatment's duration was 30 min. The effect of the graphene oxide's concentration in the electrolyte on the thickness, roughness, hardness, surface morphology, structure, composition, and tribological characteristics of the PEO coatings was studied. Wear experiments, under dry conditions, were carried out in a ball-on-disk tribotester with an applied load of 5 N, a sliding speed of 0.1 m·s^-1, and a sliding distance of 1000 m. According to the obtained results, the addition of graphene oxide (GO) into the base silicate-hypophosphite electrolyte leads to a slight decrease in the coefficient of friction (from 0.73 to 0.69) and a reduction in the wear rate by more than 1.5 times (from 8.04 to 5.2 mm³/N·m), with an increase in the GO's concentration from 0 to 0.5 kg/m³, respectively. This occurs due to the formation of a GO-containing lubricating tribolayer upon contact with the coating of the counter-body in the friction pair. Delamination of the coatings during wear occurs due to contact fatigue; with an increase in the concentration of GO in the electrolyte from 0 to 0.5 kg/m³, this process slows down by more than four times.

Nitrilotriacetic Acid Improves Plasma Electrolytic Oxidation of Titanium for Biomedical Applications

Dental implants have become a routine, affordable, and highly reliable technology to replace tooth loss. In this regard, titanium and its alloys are the metals of choice for the manufacture of dental implants because they are chemically inert and biocompatible. However, for special cohorts of patients, there is still a need for improvements, specifically to increase the ability of implants to integrate into the bone and gum tissues and to prevent bacterial infections that can subsequently lead to peri-implantitis and implant failures. Therefore, titanium implants require sophisticated approaches to improve their postoperative healing and long-term stability. Such treatments range from sandblasting to calcium phosphate coating, fluoride application, ultraviolet irradiation, and anodization to increase the bioactivity of the surface. Plasma electrolytic oxidation (PEO) has gained popularity as a method for modifying metal surfaces and delivering the desired mechanical and chemical properties. The outcome of PEO treatment depends on the electrochemical parameters and composition of the bath electrolyte. In this study, we investigated how complexing agents affect the PEO surfaces and found that nitrilotriacetic acid (NTA) can be used to develop efficient PEO protocols. The PEO surfaces generated with NTA in combination with sources of calcium and phosphorus were shown to increase the corrosion resistance of the titanium substrate. They also support cell proliferation and reduce bacterial colonization and, hence, lead to a reduction in failed implants and repeated surgeries. Moreover, NTA is an ecologically favorable chelating agent. These features are necessary for the biomedical industry to be able to contribute to the sustainability of the public healthcare system. Therefore, NTA is proposed to be used as a component of the PEO bath electrolyte to obtain bioactive surface layers with properties desired for next-generation dental implants.

Influence of PEO Electrolyzer Geometry on Current Density Distribution and Resultant Coating Properties on Zr-1Nb Alloy

This paper is devoted to the study of the current density distribution effect on plasma electrolytic oxidation process and resultant coatings on a Zr-1Nb alloy. The influence of the distance between the plates simultaneously placed into an electrolyzer was evaluated to assess the throwing power of the PEO process. The current density on the facing surfaces of the plates decreases when the distance between them shrinks. This current density has a notable impact on the resultant PEO coating in terms of the surface morphology parameters and electrochemically evaluated corrosion resistance. The influence of this effect is low on the stages of anodizing and spark discharges (60-120 s of the PEO), and significantly increases on the stage of microarc discharges (120-360 s of the PEO). The coating obtained with a smaller distance between the plates, while having the same coating thickness as the others, exhibits higher wear resistance. New correlations between the current density, diffusion coefficient, time constant of nucleation and the coating thickness in the middle of the facing samples were established; in addition, a correlation of the coating morphology in this area with the roughness parameters RPc, RSm was shown. This study contributes to the development of optimized PEO processes for the simultaneously coated several devices of complex shape, e.g., orthopedic implants.

Application of Micro-Arc Discharges during Anodization of Tantalum for Synthesis of Photocatalytic Active Ta2O5 Coatings

Ta₂O₅ coatings were created using micro-arc discharges (MDs) during anodization on a tantalum substrate in a sodium phosphate electrolyte (10 g/L Na₃PO₄·10H₂O). During the process, the size of MDs increases while the number of MDs decreases. The elements and their ionization states present in MDs were identified using optical emission spectroscopy. The hydrogen Balmer line H_β shape analysis revealed the presence of two types of MDs, with estimated electron number densities of around 1.1 × 10²¹ m^-3 and 7.3 × 10²¹ m^-3. The effect of MDs duration on surface morphology, phase and chemical composition, optical absorption, and photoluminescent, properties of Ta₂O₅ coatings, as well as their applications in photocatalytic degradation of methyl orange, were investigated. The created coatings were crystalline and were primarily composed of Ta₂O₅ orthorhombic phase. Since Ta₂O₅ coatings feature strong absorption in the ultraviolet light region below 320 nm, their photocatalytic activity is very high and increases with the time of the MDs process. This was associated with an increase of oxygen vacancy defects in coatings formed during the MDs, which was confirmed by photoluminescent measurements. The photocatalytic activity after 8 h of irradiation was around 69%, 74%, 80%, and 88% for Ta₂O₅ coatings created after 3 min, 5 min, 10 min, and 15 min, respectively.

Predicting localised corrosion and mechanical performance of a PEO surface modified rare earth magnesium alloy for implant use through in-silico modelling

In this study, the influence of a plasma electrolytic oxidation (PEO) surface treatment on a medical-grade WE43-based magnesium alloy is examined through an experimental and computational framework that considers the effects of localised corrosion features and mechanical properties throughout the corrosion process. First, a comprehensive in-vitro immersion study was performed on WE43-based tensile specimens with and without PEO surface modification, which included fully automated spatial reconstruction of the phenomenological features of corrosion through micro-CT scanning, followed by uniaxial tensile testing. Then the experimental data of both unmodified and PEO-modified groups were used to calibrate parameters of a finite element-based surface corrosion model. In-vitro, it was found that the WE43-PEO modified group had a significantly lower corrosion rate and maintained significantly higher mechanical properties than the unmodified. While corrosion rates were ∼50% lower in the WE43-PEO modified specimens, the local geometric features of corroding surfaces remained similar to the unmodified WE43 group, however evolving after almost the double amount of time. We were also able to quantitatively demonstrate that the PEO surface treatment on magnesium continued to protect samples from corrosion throughout the entire period tested, and not just in the early stages of corrosion. Using the results from the testing framework, the model parameters of the surface-based corrosion model were identified for both groups. This enabled, for the first time, in-silico prediction of the physical features of corrosion and the mechanical performance of both unmodified and PEO modified magnesium specimens. This simulation framework can enable future in-silico design and optimisation of bioabsorbable magnesium devices for load-bearing medical applications.

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Examination of corrosion morphology and mechanics of PEO modified WE43.

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Automated phenomenological tracking of corrosion features by PitScan.

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Corrosion model of unmodified WE43 and WE43 PEO modified.

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Calibration through geometrical features and mechanical parameters followed.

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PEO treatment does not influence the severity of localised corrosion.

DMP1 and IFITM5 Regulate Osteogenic Differentiation of MC3T3-E1 on PEO-Treated Ti-6Al-4V-Ca2+/Pi surface

Despite numerous studies on various surface modifications on titanium and its alloys, it remains unclear what kind of titanium-based surface modifications are capable of controlling cell activity. This study aimed to understand the mechanism at the cellular and molecular levels and investigate the in vitro response of osteoblastic MC3T3-E1 cultured on the Ti-6Al-4V surface modified by plasma electrolytic oxidation (PEO) treatment. A Ti-6Al-4V surface was prepared by PEO at 180, 280, and 380 V for 3 or 10 min in an electrolyte containing Ca²⁺/P_i ions. Our results showed that PEO-treated Ti-6Al-4V-Ca²⁺/P_i surfaces enhanced the cell attachment and differentiation of MC3T3-E1 compared to the untreated Ti-6Al-4V control but did not affect cytotoxicity as shown by cell proliferation and cell death. Interestingly, on the Ti-6Al-4V-Ca²⁺/P_i surface treated by PEO at 280 V for 3 or 10 min, MC3T3-E1 showed a higher initial adhesion and mineralization. In addition, the alkaline phosphatase (ALP) activity significantly increased in MC3T3-E1 on the PEO-treated Ti-6Al-4V-Ca²⁺/P_i (280 V for 3 or 10 min). In RNA-seq analysis, the expression of dentin matrix protein 1 (DMP1), sortilin 1 (Sort1), signal-induced proliferation-associated 1 like 2 (SIPA1L2), and interferon-induced transmembrane protein 5 (IFITM5) was induced during the osteogenic differentiation of MC3T3-E1 on the PEO-treated Ti-6Al-4V-Ca²⁺/P_i. DMP1 and IFITM5 silencing decreased the expression of bone differentiation-related mRNAs and proteins and ALP activity in MC3T3-E1. These results suggest that the PEO-treated Ti-6Al-4V-Ca²⁺/P_i surface induces osteoblast differentiation by regulating the expression of DMP1 and IFITM5. Therefore, surface microstructure modification through PEO coatings with Ca²⁺/P_i ions could be used as a valuable method to improve biocompatibility properties of titanium alloys.

Plasma Electrolytic Oxidation of Titanium in Ni and Cu Hydroxide Suspensions towards Preparation of Electrocatalysts for Urea Oxidation

The increasing climate crisis requires an improvement in renewable energy technologies. One of them are fuel cells, devices that are capable of generating electricity directly from the chemical reaction that is taking place inside of them. Despite the advantages of these solutions, a lack of the appropriate materials is holding them back from commercialization. This research shows preliminary results from a simple way to prepare black TiO₂ coatings, doped with Cu or Ni using the plasma electrolytic oxidation process, which can be used as anodes in urea-fueled fuel cells. They show activity toward urea oxidation, with a maximum current density of 130 μA cm^-2 (@1 V vs. Hg|HgO) observed for Cu-enhanced TiO₂ and low potential of only 0.742 V (Vs Hg|HgO) required for 50 μA cm^-2 for Ni-enhanced TiO₂. These results demonstrate how the PEO process can be used for the preparation of TiO₂-based doped materials with electrocatalytic properties toward urea electrooxidation.

An Experimental Anodized and Low-Pressure Oxygen Plasma-Treated Titanium Dental Implant Surface-Preliminary Report

Chemical composition and physical parameters of the implant surface, such as roughness, regulate the cellular response leading to implant bone osseointegration. Possible implant surface modifications include anodization or the plasma electrolytic oxidation (PEO) treatment process that produces a thick and dense oxide coating superior to normal anodic oxidation. Experimental modifications with Plasma Electrolytic Oxidation (PEO) titanium and titanium alloy Ti6Al4V plates and PEO additionally treated with low-pressure oxygen plasma (PEO-S) were used in this study to evaluate their physical and chemical properties. Cytotoxicity of experimental titanium samples as well as cell adhesion to their surface were assessed using normal human dermal fibroblasts (NHDF) or L929 cell line. Moreover, the surface roughness, fractal dimension analysis, and texture analysis were calculated. Samples after surface treatment have substantially improved properties compared to the reference SLA (sandblasted and acid-etched) surface. The surface roughness (Sa) was 0.59-2.38 µm, and none of the tested surfaces had cytotoxic effect on NHDF and L929 cell lines. A greater cell growth of NHDF was observed on the tested PEO and PEO-S samples compared to reference SLA sample titanium.

Characteristics of Hydroxyapatite-Modified Coatings Based on TiO2 Obtained by Plasma Electrolytic Oxidation and Electrophoretic Deposition

In order to modify the surface of light metals and alloys, plasma electrolytic oxidation (PEO) is a useful electrochemical technique. During the oxidation process, by applying a positive high voltage greater than the dielectric breakdown value of the oxide layer, the formation of a ceramic film onto the substrate material is enabled. The resulting surface presents hardness, chemical stability, biocompatibility, and increased corrosion wear resistance. The current study aims to investigate the corrosion resistance and tribological properties of PEO-modified coatings on titanium substrates produced by applying either direct or pulsed current in a silicate-alkaline electrolyte. In this way, a uniform TiO₂ layer is formed, and subsequently, electrophoretic deposition of hydroxyapatite particles (HAP) is performed. The morpho-structural characteristics and chemical composition of the resulting coatings are investigated using scanning electron microscopy combined with energy dispersive spectroscopy analysis and X-ray diffraction. Dry sliding wear testing of the TiO₂ and HAP-modified TiO₂ coatings were carried out using a ball-on-disc configuration, while the corrosion resistance was electrochemically evaluated at 37 °C in a Ringer's solution. The corrosion rates of the investigated samples decreased significantly, up to two orders of magnitude, when the PEO treatment was applied, while the wear rate was 50% lower compared to the untreated titanium substrate.

Ciprofloxacin Release and Corrosion Behaviour of a Hybrid PEO/PCL Coating on Mg3Zn0.4Ca Alloy

In the present work, a hybrid hierarchical coating (HHC) system comprising a plasma electrolytic oxidation (PEO) coating and a homogeneously porous structured polycaprolactone (PCL) top-coat layer, loaded with ciprofloxacin (CIP), was developed on Mg3Zn0.4Ca alloy. According to the findings, the HHC system avoided burst release and ensured gradual drug elution (64% over 240 h). The multi-level protection of the magnesium alloy is achieved through sealing of the PEO coating pores by the polymer layer and the inhibiting effect of CIP (up to 74%). The corrosion inhibition effect of HHC and the eluted drug is associated with the formation of insoluble CIP-Me (Mg/Ca) chelates that repair the defects in the HHC and impede the access of corrosive species as corroborated by FTIR spectra, EIS and SEM images after 24 h of immersion. Therefore, CIP participates in an active protection mechanism by interacting with cations coming through the damaged coating.

Composite Coatings of AMg3 Alloy Formed by a Combination of Plasma Electrolytic Oxidation and Fluoropolymer Spraying

This paper presents the results of an investigation of the changes in the corrosion, wear resistance, and wettability of composite coatings formed on the AMg3 alloy through plasma electrolytic oxidation (PEO) and subsequent spraying with an organofluorine polymer. The evaluation of the electrochemical properties of the composite layers revealed a decrease in the corrosion current density compared with the PEO coating (from 3.8 × 10^-8 to 3.1 × 10^-11 A/cm²). The analysis of the wear resistance of composite coatings established that the application of this type of coating reduced the wear of the samples by two orders of magnitude when compared with the PEO layer. Using the contact-angle measurement, it was found that with an increase in the number of polymer spray applications, the wettability of coatings decreased, so the contact angle for the composite coating with triple fluoropolymer application increased by 134.3° compared to the base PEO coating.

New Polycaprolactone-Containing Self-Healing Coating Design for Enhance Corrosion Resistance of the Magnesium and Its Alloys

The method of hybrid coating formation on the surface of a bioresorbable wrought magnesium alloy and magnesium obtained by additive technology was proposed. Plasma electrolytic oxidation (PEO) with subsequent treatment of the material using an organic biocompatible corrosion inhibitor and a bioresorbable polymer material was used to obtain the protective layers. The optimal method of surface treatment was suggested. Using SEM/EDX analysis, XRD, XPS, and confocal Raman microspectroscopy, the composition of the formed surface layers was determined. The corrosion protection performance of the formed coatings was studied by potentiodynamic polarization and electrochemical impedance spectroscopy techniques in 0.9 wt.% NaCl and HBSS. Hydrogen evolution and mass loss tests were performed to study the corrosion rate of samples with different types of protective coatings. Sealing the pores of PEO coating with a polymeric material contributes to a significant reduction in the amount of the inhibitor diffusing into a corrosive medium. The best barrier properties were established for the hybrid coating formed with a one-stage application of benzotriazole and polycaprolactone. Such layers reduce the rate of alloy degradation due to active protection.

Behaviors of Micro-Arcs, Bubbles, and Coating Growth during Plasma Electrolytic Oxidation of β-Titanium Alloy

The plasma electrolytic oxidation (PEO) of a titanium alloy, Ti-15V-3Cr-3Sn-3Al, was performed to develop mechanical applications by improving the tribological characteristics. The behaviors of micro-arcs, bubbles, and coating growth during the PEO process were investigated under three different operating conditions, constant voltage (CV) operation, constant current operation (CC), and short treatment time (ST) operation, to control the surface structure and function by the PEO process. A low friction coefficient was achieved by CV operation at 500 V and by CC operation at 3.0 kA/m². The maximum coating thickness of 6.88 μm was achieved by CV operation at 500 V and 60 s. From the observation of micro-arcs, bubbles, and discharge craters by ST operation, the minimum discharge diameter of the micro-arc was 8 μm, and the discharge craters had a discharge pore size of 0.3 μm in diameter in the center with a petal-shaped burr around the discharge pore. During the PEO process, no bubble bursts around the micro-arcs and no backfilling of the discharge pores by the ejected materials were observed. Thus, the discharge pores remain a porous structure in the PEO coating for Ti. The utilization efficiency of the total charge density by CV operation above 300 V was lower than that by the conventional anodization process. The utilization efficiency of total charge density by CC operation was higher than that by the conventional anodization process.

PEO Coatings Modified with Halloysite Nanotubes: Composition, Properties, and Release Performance

In this work, the properties of the coatings formed on the Mg-Mn-Ce alloy by plasma electrolytic oxidation (PEO) in electrolytes containing halloysite nanotubes (HNTs) were investigated. The incorporation of halloysite nanotubes into the PEO coatings improved their mechanical characteristics, increased thickness, and corrosion resistance. The studied layers reduced corrosion current density by more than two times in comparison with the base PEO layer without HNTs (from 1.1 × 10^-7 A/cm² to 4.9 × 10^-8 A/cm²). The presence of halloysite nanotubes and products of their dihydroxylation that were formed under the PEO conditions had a positive impact on the microhardness of the obtained layers (this parameter increased from 4.5 ± 0.4 GPa to 7.3 ± 0.5 GPa). In comparison with the base PEO layer, coatings containing halloysite nanotubes exhibited sustained release and higher adsorption capacity regarding caffeine.

A SiO2 layer on PEO-treated Mg for enhanced corrosion resistance and bone regeneration

Magnesium (Mg) is a promising biodegradable metal for orthopedic applications, and plasma electrolytic oxidation (PEO) has been widely studied as a corrosion protection coating on Mg-based implants. However, the porous structures and easily formed cracks in fluid are disadvantageous for long-term corrosion protection. In this study, a SiO₂ layer was deposited on PEO-treated Mg to inhibit the formation of cracks on the PEO layer and prevent the permeation of corrosive fluid. The SiO₂ layer did not alter the surface morphology of the PEO layer but considerably enhanced its corrosion resistance. The in vitro culture of MC3T3-E1 cells demonstrated the good cytocompatibility and osteogenic induction ability of SiO₂-coated PEO-treated Mg, which could be attributed to Mg and Si ions released from the coating. The coating also favored the angiogenesis behaviors of HUVEC. Furthermore, with the continuous release of Mg and Si ions, the as-prepared implant showed a superior osseointegration ability in a rat bone implantation model. In summary, this newly designed Mg-based implant shows promising potential for orthopedic applications.

Modeling of Biological Activity of PEO-Coated Titanium Implants with Conjugates of Cyclic RGD Peptide with Amino Acid Bisphosphonates

Titanium is considered to be the most essential metal in the field of implantology. The main factors determining metal biocompatibility, among others, include the morphology and chemical composition of the titanium surface. Therefore, the aim of this work was to develop approaches to control the biological activity of the titanium surface by creating coatings that combine both an inorganic phase with a given morphology and organic molecules containing an integrin-selective peptide that regulate cell adhesion and proliferation. As such, we synthesized new c(RGDfC) derivatives of amino acid bisphosphonates (four examples) with different bisphosphonate anchors and maleimide linkers. These molecules were deposited on a highly developed porous surface obtained via the plasma electrolytic oxidation (PEO) of coarse-grained and nanostructured titanium. In vitro studies demonstrated the increase in the viability degree of mesenchymal stem cells and fibroblasts on the surface of coarse-grained or nanostructured titanium modified with PEO and a c(RGDfC) derivative of ε-aminocaproic acid bisphophonate with an SMCC linker. As a result, the use of conjugates of amino acid bisphosphonates with a cyclic RGD peptide for the modification of PEO-coated titanium opens the ways for the effective control of the biological activity of the metal implant surface.

Features of Composite Layers Created Using an Aqueous Suspension of a Fluoropolymer

This paper presents a method for the formation of composite-polymer-containing coatings on MA8 Mg alloy by plasma electrolytic oxidation (PEO), followed by the deposition of a fluoropolymer from an aqueous suspension of superdispersed polytetrafluoroethylene. The Scanning Electron Microscope(SEM), Energy Dispersive Spectroscopy(EDS), and X-ray Diffraction(XRD) analyses established morphological features as well as elemental and phase composition of composite coatings. The fact that the pores are filled with a fluoropolymer has been experimentally confirmed. An assessment of the corrosion properties of formed composite coatings revealed a decrease in the corrosion current density by more than four orders of magnitude in comparison with the base PEO layer. The highest resistance to the damaging effects of a corrosive environment, according to the results of long-term exposure tests, was demonstrated by coatings after three treatments with polytetrafluoroethylene. The obtained polymer-containing coatings have antifriction properties, reducing the wear of the coatings by more than 27-fold in comparison with the base PEO layer. It was revealed that composite coatings have superhydrophobic properties: the value of the contact angle reaches 154°, and the hysteresis of the contact angle is less than 10°.

Hydroxyapatite Coating on Ti-6Al-7Nb Alloy by Plasma Electrolytic Oxidation in Salt-Based Electrolyte

Titanium alloys have good biocompatibility and good mechanical properties, making them particularly suitable for dental and orthopedic implants. Improving their osseointegration with human bones is one of the most essential tasks. This can be achieved by developing hydroxyapatite (HA) on the treating surface using the plasma electrolytic oxidation (PEO) method in molten salt. In this study, a coating of titanium oxide-containing HA nanoparticles was formed on Ti-6Al-7Nb alloy by PEO in molten salt. Then, samples were subjected to hydrothermal treatment (HTT) to form HA crystals sized 0.5 to 1 μm. The effect of the current and voltage frequency for the creation of the coating on the morphology, chemical, and phase composition was studied. The anti-corrosion properties of the samples were studied using the potentiodynamic polarization test (PPT) and electrochemical impedance spectroscopy (EIS). An assessment of the morphology of the sample formed at a frequency of 100 Hz shows that the structure of this coating has a uniform submicron porosity, and its surface shows high hydrophilicity and anti-corrosion properties (4.90 × 10⁶ Ohm·cm²). In this work, for the first time, the process of formation of a bioactive coating consisting of titanium oxides and HA was studied by the PEO method in molten salts.

Improving Corrosion and Photocatalytic Properties of Composite Oxide Layer Fabricated by Plasma Electrolytic Oxidation with NaAlO2

The present work dealt with the development of a protective and functional oxide layer via one-step plasma electrolytic oxidation (PEO) on pure titanium by employing highly concentrated aluminate solution in a short processing time. A compositional analysis showed that Al₂TiO₅ active compound was formed successfully by means of Al₂O₃ incorporation when TiO₂ was spontaneously developed with the aid of plasma swarms. The electrochemical performance showed the protective and functional capabilities of the layer, which was attributed to the respective amounts of Al₂O₃ and Al₂TiO₅. Such capabilities were achieved in a short processing time, thus reducing the total production cost.

Comparison of 2D and 3D Plasma Electrolytic Oxidation (PEO)-Based Coating Porosity Data Obtained by X-ray Tomography Rendering and a Classical Metallographic Approach

In this work, the porosity of plasma electrolytic oxidation (PEO)-based coatings on Al- and Mg-based substrates was studied by two imaging techniques-namely, SEM and computer microtomography. Two approaches for porosity determination were chosen; relatively simple and fast SEM surface and cross-sectional imaging was compared with X-ray micro computed tomography (microCT) rendering. Differences between 2D and 3D porosity were demonstrated and explained. A more compact PEO coating was found on the Al substrate, with a lower porosity compared to Mg substrates under the same processing parameters. Furthermore, huge pore clusters were detected with microCT. Overall, 2D surface porosity calculations did not show sufficient accuracy for them to become the recommended method for the exact evaluation of the porosity of PEO coatings; microCT is a more appropriate method for porosity evaluation compared to SEM imaging. Moreover, the advantage of 3D microCT images clearly lies in the detection of closed and open porosity, which are important for coating properties.

Electrochemical Behavior of SiC-Coated AA2014 Alloy through Plasma Electrolytic Oxidation

In this study, the corrosion performance of AA2014 aluminum alloy was enhanced by coating the alloy with a layer containing silica (SiC) that was formed by the plasma electrolytic oxidation (PEO) process. The PEO process was performed with different electrical parameters (frequency, current mode, and duty ratio) and both with and without SiC to investigate the microstructural and electrochemical differences in the coated samples produced from the process. The microstructure and composition of the PEO coatings were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). A potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of the AA2014-PEO-coated samples. The potentiodynamic polarization showed that the SiC-PEO-coated samples had a significantly decreased corrosion rate (99.8%) compared with the uncoated AA2014 Al alloy. Our results showed that the coats containing SiC possessed a much higher corrosion resistance than both the uncoated AA2014 Al alloy (8,344,673%) and the SiC-free coatings, which possess low corrosion resistance, because of their higher chemical stability and more compact microstructure.

Plasma Electrolytic Oxidation of Zr-1%Nb Alloy: Effect of Sodium Silicate and Boric Acid Addition to Calcium Acetate-Based Electrolyte

This work aimed at the development of wear and corrosion resistant oxide coatings for medical implants made of zirconium alloy, by plasma electrolytic oxidation (PEO). The effect of sodium silicate and boric acid addition to calcium acetate electrolyte on the coating properties was studied. Different aspects of the PEO coating were investigated: microstructure, electrochemical and wear behavior, wettability and apatite-forming ability. The resultant coatings consist of a dense inner layer 1.4–2.2 µm thick and a porous outer layer. The total thickness of the coating is 12–20 µm. It was found that the coating contains the tetragonal zirconia (70–95%). The obtained coatings show high corrosion resistance and reduce the surface corrosion current by 1–3 orders of magnitude, depending on the electrolyte additive, compared to the uncoated surface. The addition of boric acid to the electrolyte significantly increases the wear resistance of the coating and reduces the coefficient of friction. In terms of the combination of the coating characteristics, the electrolyte with the addition of the alkali and boric acid is recommended as the most effective.

Investigation of Biocompatible PEO Coating Growth on cp-Ti with In Situ Spectroscopic Methods

The problem of the optimization of properties for biocompatible coatings as functional materials requires in-depth understanding of the coating formation processes; this allows for precise manufacturing of new generation implantable devices. Plasma electrolytic oxidation (PEO) opens the possibility for the design of biomimetic surfaces for better biocompatibility of titanium materials. The pulsed bipolar PEO process of cp-Ti under voltage control was investigated using joint analysis of the surface characterization and by in situ methods of impedance spectroscopy and optical emission spectroscopy. Scanning electron microscopy, X-ray diffractometry, coating thickness, and roughness measurements were used to characterize the surface morphology evolution during the treatment for 5 min. In situ impedance spectroscopy facilitated the evaluation of the PEO process frequency response and proposed the underlying equivalent circuit where parameters were correlated with the coating layer properties. In situ optical emission spectroscopy helped to analyze the spectral line evolutions for the substrate material and electrolyte species and to justify a method to estimate the coating thickness via the relation of the spectral line intensities. As a result, the optimal treatment time was established as 2 min; this provides a 9-11 µm thick PEO coating with Ra 1 µm, 3-5% porosity, and containing 75% of anatase. The methods for in-situ spectral diagnostics of the coating thickness and roughness were justified so that the treatment time can be corrected online when the coating achieves the required properties.

Nanodesigned coatings obtained by plasma electrolytic oxidation of titanium implant and their cytotoxicity

The titanium implant was treated with plasma electrolytic oxidation and subsequent ionic exchange and thermal treatment in order to obtain bioactive layer consisting of titanium oxide, calcium and sodium titanates and hydroxyapatite, as confirmed by X-ray diffraction (XRD). Scanning electron microscopy (SEM) revealed that the given method, besides corresponding phase composition, enables suitable nanotopology for cell attachment and proliferation. Cytotoxicity investigations by MTT, LDH and propidium iodide assays and light microscopy showed that these coatings were not toxic to L929 cells.

Anticorrosion and Cytocompatibility Assessment of Graphene-Doped Hybrid Silica and Plasma Electrolytic Oxidation Coatings for Biomedical Applications

Magnesium AZ31 alloy substrates were coated with different coatings, including sol–gel silica-reinforced with graphene nanoplatelets, sol–gel silica, plasma electrolytic oxidation (PEO), and combinations of them, to improve cytocompatibility and control the corrosion rate. Electrochemical corrosion tests, as well as hydrogen evolution tests, were carried out using Hanks’ solution as the electrolyte to assess the anticorrosion behavior of the different coating systems in a simulated body fluid. Preliminary cytocompatibility assessment of the different coating systems was carried out by measuring the metabolic activity, deoxyribonucleic acid quantification, and the cell growth of premyoblastic C2C12-GFP cell cultures on the surface of the different coating systems. Anticorrosion behavior and cytocompatibility were improved with the application of the different coating systems. The use of combined PEO + SG and PEO + SG/GNP coatings significantly decreased the degradation of the specimens. The monolayer sol–gel coatings, with and without GNPs, presented the best cytocompatibility improvement.

Icephobic Performance of Combined Fluorine-Containing Composite Layers on Al-Mg-Mn-Si Alloy Surface

This paper presents the results of an evaluation of anti-icing properties of samples obtained by plasma electrolytic oxidation (PEO) with a subsequent application of superdispersed polytetrafluoroethylene (SPTFE) and polyvinylidenefluoride (PVDF). A combined treatment of the samples with SPTFE and PVDF is also presented. It is revealed that impregnation of a PEO layer with fluoropolymer materials leads to a significant increase in surface relief uniformity. Combined PVDF–SPFTE layers with a ratio of PVDF to SPTFE of 1:4 reveal the best electrochemical characteristics, hydrophobicity and icephobic properties among all of the studied samples. It is shown that the decrease in corrosion current density Ic for PVDF–SPFTE coatings is higher by more than five orders of magnitude in comparison with uncoated aluminum alloy. The contact angle for PVDF–SPFTE coatings attain 160.5°, which allows us to classify the coating as superhydrophobic with promising anti-icing performance. A treatment of a PEO layer with PVDF–SPFTE leads to a decrease in ice adhesion strength by 22.1 times compared to an untreated PEO coating.

Influence of Process Parameters on the Tribological Behavior of PEO Coatings on CP-Titanium 4+ Alloys for Biomedical Applications

Wear resistant ceramic coatings were generated on novel commercially pure titanium grade 4+ alloys by the plasma electrolytic oxidation technique (PEO) in an aluminate and zirconia containing electrolyte. The coatings were obtained adopting a full regular two-level factorial design of experiments (DoE) varying the PEO process parameters current density, repetition rate and duty cycle. The generated coatings were characterized with respect to its wear resistance and mechanical properties by reciprocal ball-on-flat tests and nanoindentation measurements. Thickness, morphology and phase formation of the PEO coatings was analyzed by scanning electron microscopy (SEM/EDS) and X-ray diffraction. XRD results indicate the formation of crystalline aluminium titanate (TiAl2O5) as well as t-ZrO2 and alumina leading to an increase in hardness and wear resistance of the PEO coatings. Evaluation of the DoE’s parameter interaction shows that the main effects for generating wear resistant coatings are current density and repetition rate. In particular, the formation of mechanically stable and adhesive corundum and zirconia containing coatings with increasing current density and frequency turned out to be responsible for the improvement of the tribological properties. Overall, the PEO processing significantly improves the wear resistance of the CP titanium base alloy.

Wear and Corrosion Resistance of Plasma Electrolytic Oxidation Coatings on 6061 Al Alloy in Electrolytes with Aluminate and Phosphate

Phosphate and aluminate electrolytes were used to prepare plasma electrolytic oxidation (PEO) coatings on 6061 aluminum alloy. The surface and cross-section microstructure, element distribution, and phase composition of the PEO coatings were characterized by SEM, EDS, XPS, and XRD. The friction and wear properties were evaluated by pin-on-disk sliding tests under dry conditions. The corrosion resistance of PEO coatings was investigated by electrochemical corrosion and salt spray tests in acidic environments. It was found that the PEO coatings prepared from both phosphate and aluminate electrolytes were mainly composed of α-Al₂O₃ and γ-Al₂O₃. The results demonstrate that a bi-layer coating is formed in the phosphate electrolyte, and a single-layered dense alumina coating with a hardness of 1300 HV is realizable in the aluminate electrolyte. The aluminate PEO coating had a lower wear rate than the phosphate PEO coating. However, the phosphate PEO coating showed a better corrosion resistance in acidic environment, which is mainly attributed to the presence of an amorphous P element at the substrate/coating interface.

Plasma Electrolytic Oxidation (PEO) Process-Processing, Properties, and Applications

Plasma electrolytic oxidation (PEO) is a novel surface treatment process to produce thick, dense metal oxide coatings, especially on light metals, primarily to improve their wear and corrosion resistance. The coating manufactured from the PEO process is relatively superior to normal anodic oxidation. It is widely employed in the fields of mechanical, petrochemical, and biomedical industries, to name a few. Several investigations have been carried out to study the coating performance developed through the PEO process in the past. This review attempts to summarize and explain some of the fundamental aspects of the PEO process, mechanism of coating formation, the processing conditions that impact the process, the main characteristics of the process, the microstructures evolved in the coating, the mechanical and tribological properties of the coating, and the influence of environmental conditions on the coating process. Recently, the PEO process has also been employed to produce nanocomposite coatings by incorporating nanoparticles in the electrolyte. This review also narrates some of the recent developments in the field of nanocomposite coatings with examples and their applications. Additionally, some of the applications of the PEO coatings have been demonstrated. Moreover, the significance of the PEO process, its current trends, and its scope of future work are highlighted.

Oxygen Sensing of Pt/PEO-TiO2 in Humid Atmospheres at Moderate Temperatures

Here, we show that the presence of adsorbed water improves the oxygen-sensing properties of Pt/TiO2 at moderate temperatures. The studied interface is based on porous plasma electrolytic oxidized titanium (PEO-TiO2) covered with platinum clusters. The electrical resistance across Pt/PEO-TiO2 is explained by an electronic depletion layer. Oxygen adsorbates further increase the depletion by inducing extrinsic interface states, which are occupied by TiO2 conduction band electrons. The high oxygen partial pressure in ambient air substantially limits the electron transport across the interface. Our DC measurements at defined levels of humidity at 30 ∘C show that adsorbed water counteracts this shortcoming, allowing oxygen sensing at room conditions. In addition, response and recovery times from temporal oxygen exposure decrease with humidity. We attribute the effects to competing adsorption processes and reactions of water with adsorbed oxygen species and/or lattice oxygen, which involve electron re-injection to the TiO2 conduction band. Elevated temperatures up to 170 ∘C attenuate the effects, presumably due to the lower binding strength to the surface of molecular water compared with oxygen adsorbates.

Antibacterial Titanium Implants Biofunctionalized by Plasma Electrolytic Oxidation with Silver, Zinc, and Copper: A Systematic Review

Patients receiving orthopedic implants are at risk of implant-associated infections (IAI). A growing number of antibiotic-resistant bacteria threaten to hamper the treatment of IAI. The focus has, therefore, shifted towards the development of implants with intrinsic antibacterial activity to prevent the occurrence of infection. The use of Ag, Cu, and Zn has gained momentum as these elements display strong antibacterial behavior and target a wide spectrum of bacteria. In order to incorporate these elements into the surface of titanium-based bone implants, plasma electrolytic oxidation (PEO) has been widely investigated as a single-step process that can biofunctionalize these (highly porous) implant surfaces. Here, we present a systematic review of the studies published between 2009 until 2020 on the biomaterial properties, antibacterial behavior, and biocompatibility of titanium implants biofunctionalized by PEO using Ag, Cu, and Zn. We observed that 100% of surfaces bearing Ag (Ag-surfaces), 93% of surfaces bearing Cu (Cu-surfaces), 73% of surfaces bearing Zn (Zn-surfaces), and 100% of surfaces combining Ag, Cu, and Zn resulted in a significant (i.e., >50%) reduction of bacterial load, while 13% of Ag-surfaces, 10% of Cu-surfaces, and none of Zn or combined Ag, Cu, and Zn surfaces reported cytotoxicity against osteoblasts, stem cells, and immune cells. A majority of the studies investigated the antibacterial activity against S. aureus. Important areas for future research include the biofunctionalization of additively manufactured porous implants and surfaces combining Ag, Cu, and Zn. Furthermore, the antibacterial activity of such implants should be determined in assays focused on prevention, rather than the treatment of IAIs. These implants should be tested using appropriate in vivo bone infection models capable of assessing whether titanium implants biofunctionalized by PEO with Ag, Cu, and Zn can contribute to protect patients against IAI.