Transmission electron microscopy of coatings formed by plasma electrolytic oxidation of titanium

Wear Resistance of Plasma Electrolytic Oxidation Coatings on Ti-6Al-4V Eli Alloy Processed by Additive Manufacturing

The additive manufacturing (AM) technique can produce Ti-6Al-4V ELI (extra low interstitial) alloy for personalized biomedical devices. However, the Ti-6Al-4V ELI alloy presents poor tribological behavior. Regarding this, coatings are a feasible approach to improve the wear resistance of this alloy. In the literature, the tribological behavior of TiO2 coatings incorporated with Ca and P formed by one-step plasma electrolytic oxidation (PEO) on Ti-6Al-4V ELI alloy processed by AM has not been investigated. Thus, in the present work, it was studied the influence of Ti-6Al-4V ELI alloy processed by AM on the wear resistance and morphologic of the coating obtained by PEO (plasma electrolytic oxidation). In this way, three different voltages (200, 250, and 300 V) were employed for the PEO process and the voltage effect on the properties of the coatings. The coatings were characterized by contact profilometry, scanning electron microscopy, energy-dispersive spectroscopy, the sessile drop method, grazing-incidence X-ray diffraction, and wear tests, on a ball-on-plate tribometer. The increase in applied voltage promoted an increase in roughness, pore area, and a decrease in the pore population of the coatings. In addition, the coatings, mainly composed of anatase and rutile, showed good adhesion to the metallic substrate, and the presence of bioactive elements Ca and P were detected. The thickness of the coatings obtained by PEO increases drastically for voltages higher than 250 V (from 4.50 ± 0.33 to 23.83 ± 1.5 µm). However, coatings obtained with lower voltages presented thin and dense layers, which promoted a superior wear resistance (increase in wear rate from 1.99 × 10−6 to 2.60 × 10−5 mm3/s). Finally, compared to the uncoated substrate, the PEO coatings increased the wear resistance of the titanium alloy obtained by AM, also showing a superior wear resistance compared to the commercial Ti-6Al-4V alloy previously evaluated, being such a positive and promising behavior for application in the area of metallic implants.

Rapid structural regulation, apatite-inducing mechanism and in vivo investigation of microwave-assisted hydrothermally treated titania coating

Owing to the poor bioactivity of microarc oxidation (MAO) coating and the rapid activation ability of the microwave hydrothermal (MH) technique, MH treatment was applied to optimize the in vivo interface status between MAO-treated titanium and bone. In this study, consequently, new outermost layers were prepared using hydroxyapatite (HA) nanorods, HA submicron pillars or sodium titanate nanosheets. The results revealed that the NaOH concentration significantly influenced the surface structure and phase constitution of the MAO samples. Moreover, on enhancing the NaOH concentration, the number of HA phases was decreased. Further, the influence of the NaOH concentration on the interfacial bonding strength was insignificant for concentrations ≤0.5 mol L⁻¹. Transmission electron microscopy (TEM) analysis showed that the induction of apatite was accompanied by the dissolution of the HA crystals and there was excellent crystallographic matching with the HA crystals. The in vitro and in vivo analyses revealed that the MH-treated MAO sample with the HA nanorods possessed superior apatite-formation ability and osseointegration, including a small amount of soft tissue and optimal bone–implant interfacial bonding force, thus signifying strong potential for the optimization of the bone–implant interfacial status.

In this work, the micro/nano scale structures of HA nanorods integrated on a titanium were prepared using MAO and MH treatment. The in vivo results indicate that HA crystals play a crucial role in the improvement of the osseointegration.

Atomic layer deposition on dental materials: Processing conditions and surface functionalization to improve physical, chemical, and clinical properties - A review

Surface functionalization is an effective approach to improve and enhance the properties of dental materials. A review of atomic layer deposition (ALD) in the field of dental materials is presented. ALD is a well-established thin film deposition technique. It is being used for surface functionalization in different technologies and biological related applications. With film thickness control down to Ångström length scale and uniform conformal thin films even on complex 3D substrates, high quality thin films and their reproducibility are noteworthy advantages of ALD over other thin film deposition methods. Low temperature ALD allows temperature sensitive substrates to be functionalized with high quality ultra-thin films too. In the current work, ALD is elaborated as a promising method for surface modification of dental materials. Different aspects of conventional dental materials that can be enhanced using ALD are discussed. Also, the influence of different ALD thin films and their microstructure on the surface properties, corrosion-resistance, antibacterial activity, biofilm formation, and osteoblast compatibility are addressed. Depending on the stage of advancement for the studied materials reported in the literature, these studies are then categorized into four stages: fabrication & characterization, in vitro studies, in vivo studies, and human tests. Materials coated with ALD thin films with potential dental applications are also presented here and they are categorized as stage 1. The purpose of this review is to organize and present the up to date ALD research on dental materials. The current study can serve as a guide for future work on using ALD for surface functionalization and resulting property tuning of materials in real world dental applications.

Effects of the sources of calcium and phosphorus on the structural and functional properties of ceramic coatings on titanium dental implants produced by plasma electrolytic oxidation

Plasma Electrolytic Oxidation (PEO) is as a promising technique to modify metal surfaces by application of oxide ceramic coatings with appropriate physical, chemical and biological characteristics. Therefore, objective of this research was to find the simplest settings, yet able to produce relevant bioactive implant surfaces layers on Ti implants by means of PEO. We show that an electrolyte containing potassium dihydrogen phosphate as a source of P and either calcium hydroxide or calcium formate as a source of Ca in combination with a chelating agent, ethylenediamine tetraacetic acid (EDTA), is suitable for PEO to deliver coatings with desired properties. We determined surface morphology, roughness, wettability, chemical and phase composition of titanium after the PEO process. To investigate biocompatibility and bacterial properties of the PEO oxide coatings we used microbial and cell culture tests. The electrolyte based on Ca(OH)₂ and EDTA promotes active crystallization of apatites after PEO processing of the Ti implants. The PEO layers can increase electrochemical corrosion resistance. The PEO can be potentially used for development of bioactive surfaces with increased support of eukaryotic cells while inhibiting attachment and growth of bacteria without use of antibacterial agents.

A review of nanostructured surfaces and materials for dental implants: surface coating, patterning and functionalization for improved performance

The emerging field of nanostructured implants has enormous scope in the areas of medical science and dental implants. Surface nanofeatures provide significant potential solutions to medical problems by the introduction of better biomaterials, improved implant design, and surface engineering techniques such as coating, patterning, functionalization and molecular grafting at the nanoscale. This review is of an interdisciplinary nature, addressing the history and development of dental implants and the emerging area of nanotechnology in dental implants. After a brief introduction to nanotechnology in dental implants and the main classes of dental implants, an overview of different types of nanomaterials (i.e. metals, metal oxides, ceramics, polymers and hydrides) used in dental implant together with their unique properties, the influence of elemental compositions, and surface morphologies and possible applications are presented from a chemical point of view. In the core of this review, the dental implant materials, physical and chemical fabrication techniques and the role of nanotechnology in achieving ideal dental implants have been discussed. Finally, the critical parameters in dental implant design and available data on the current dental implant surfaces that use nanotopography in clinical dentistry have been discussed.

Characteristics and in vitro response of thin hydroxyapatite-titania films produced by plasma electrolytic oxidation of Ti alloys in electrolytes with particle additions

Enhanced incorporation of hydroxyapatite nanoparticles in porous titania coating formed by plasma electrolytic oxidation significantly increases surface osteogenic activity.

The enhancement of the biological properties of Ti by surface doping with hydroxyapatite (HA) is of great significance, especially for orthodontic applications. This study addressed the effects of HA particle size in the electrolyte suspension on the characteristics and biological properties of thin titania-based coatings produced on Ti–6Al–4V alloy by plasma electrolytic oxidation (PEO). Detailed morphological investigation of the coatings formed by a single-stage PEO process with two-step control of the electrical parameters was performed using the Minkowski functionals approach. The surface chemistry was studied by glow discharge optical emission spectroscopy and Fourier transform infrared spectroscopy, whereas mechanical properties were evaluated using scratch tests. The biological assessment included in vitro evaluation of the coating bioactivity in simulated body fluid (SBF) as well as studies of spreading, proliferation and osteoblastic differentiation of MC3T3-E1 cells. The results demonstrated that both HA micro- and nanoparticles were successfully incorporated in the coatings but had different effects on their surface morphology and elemental distributions. The micro-particles formed an irregular surface morphology featuring interpenetrated networks of fine pores and coating material, whereas the nanoparticles penetrated deeper into the coating matrix which retained major morphological features of the porous TiO₂ coating. All coatings suffered cohesive failure in scratch tests, but no adhesive failure was observed; moreover doping with HA increased the coating scratch resistance. In vitro tests in SBF revealed enhanced bioactivity of both HA-doped PEO coatings; furthermore, the cell proliferation/morphometric tests showed their good biocompatibility. Fluorescence microscopy revealed a well-organised actin cytoskeleton and focal adhesions in MC3T3-E1 cells cultivated on these substrates. The cell alkaline phosphatase activity in the presence of ascorbic acid and β-glycerophosphate was significantly increased, especially in HA nanoparticle-doped coatings.

Deposition of hydroxyapatite–incorporated TiO2 coating on titanium using plasma electrolytic oxidation coupled with electrophoretic deposition

K₂[TiO(C₂O₄)₂] is decomposed by micro-arcs on the anode producing TiO₂ while hydroxyapatite particles are incorporated into TiO₂ coating during electrophoretic deposition.

Conformal coating containing Ca, P, Si and Na with double-level porous surface structure on titanium formed by a three-step microarc oxidation

Development of conformal MAO coating on a Ti plate with a double-level porous surface, followed by characterization and evaluation in SBF.

Lanthanum-containing hydroxyapatite coating on ultrafine-grained titanium by micro-arc oxidation: a promising strategy to enhance overall performance of titanium

Titanium is widely used in biomedical materials, particularly in dental implants, because of its excellent biocompatibility and mechanical characteristics. However, titanium implant failures still remain in some cases, varying with implantation sites and patients. Improving its overall performance is a major focus of dental implant research. Equal-channel angular pressing (ECAP) can result in ultrafine-grained titanium with superior mechanical properties and better biocompatibility, which significantly benefits dental implants, and without any harmful alloying elements. Lanthanum (La) can inhibit the acidogenicity of dental plaque and La-containing hydroxyapatite (La-HA) possesses a series of attractive properties, in contrast to La-free HA. Micro-arc oxidation (MAO) is a promising technology that can produce porous and firmly adherent hydroxyapatite (HA) coatings on titanium substrates. Therefore, we hypothesize that porous La-containing hydroxyapatite coatings with different La content (0.89%, 1.3% and 1.79%) can be prepared on ultrafine-grained (~200–400 nm) titanium by ECAP and MAO in electrolytic solution containing 0.2 mol/L calcium acetate, 0.02 mol/L β-glycerol phosphate disodium salt pentahydrate (β-GP), and lanthanum nitrate with different concentrations to further improve the overall performance of titanium, which are expected to have great potential in medical applications as a dental implant.

Stability of plasma electrolytic oxidation coating on titanium in artificial saliva

Bioactive PEO coating on titanium with high Ca/P ratio was fabricated and characterized with respect to its morphology, composition and microstructure. Long-term electrochemical stability of the coating and Ti(4+) ion release was evaluated in artificial saliva. Influence of the lactic acid and fluoride ions on corrosion protection mechanism of the coated titanium was assessed using AC and DC electrochemical tests. The PEO-treated titanium maintained high passivity in the broad range of potentials up to 2.5 V (Ag/AgCl) for up to 8 weeks of immersion in unmodified saliva and exhibited Ti(4+) ion release <0.002 µg cm(-2) days(-1). The high corrosion resistance of the coating is determined by diffusion of reacting species through the coating and resistance of the inner dense part of the coating adjacent to the substrate. Acidification of saliva in the absence of fluoride ions does not affect the surface passivity, but the presence of 0.1 % of fluoride ions at pH ≤4.0 causes loss of adhesion of the coating due to inwards migration of fluoride ions and their adsorption at the substrate/coating interface in the presence of polarisation.

Fourier transform infrared absorption spectroscopy characterization of gaseous atmospheric pressure plasmas with 2 mm spatial resolution

This paper describes an optical setup built to record Fourier transform infrared (FTIR) absorption spectra in an atmospheric pressure plasma with a spatial resolution of 2 mm. The overall system consisted of three basic parts: (1) optical components located within the FTIR sample compartment, making it possible to define the size of the infrared beam (2 mm × 2 mm over a path length of 50 mm) imaged at the site of the plasma by (2) an optical interface positioned between the spectrometer and the plasma reactor. Once through the plasma region, (3) a retro-reflector module, located behind the plasma reactor, redirected the infrared beam coincident to the incident path up to a 45° beamsplitter to reflect the beam toward a narrow-band mercury-cadmium-telluride detector. The antireflective plasma-coating experiments performed with ammonia and silane demonstrated that it was possible to quantify 42 and 2 ppm of these species in argon, respectively. In the case of ammonia, this was approximately three times less than this gas concentration typically used in plasma coating experiments while the silane limit of quantification was 35 times lower. Moreover, 70% of the incoming infrared radiation was focused within a 2 mm width at the site of the plasma, in reasonable agreement with the expected spatial resolution. The possibility of reaching this spatial resolution thus enabled us to measure the gaseous precursor consumption as a function of their residence time in the plasma.

An electron microscopical study on the growth of TiO2-Ag antibacterial coatings on Ti6Al7Nb biomedical alloy

This research was aimed at investigating the growth mechanism of TiO(2)-Ag antibacterial coatings during plasma electrolytic oxidation (PEO) of Ti6Al7Nb biomedical alloy in an electrolyte based on calcium acetate/calcium glycerophosphate bearing Ag nanoparticles. The focus was on the mechanism of incorporation of Ag nanoparticles, their distribution and chemical composition within the porous coatings using high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) imaging techniques combined with energy dispersive X-ray spectroscopy (EDX) for chemical analyses. The PEO coatings were grown using different oxidation times, 10, 30, 60, 90, 120, 180, 240 and 300 s. The electron microscopy results confirmed the formation of a porous coating with incorporated Ag nanoparticles from the initial stages of oxidation (i.e. 10 s), with further Ag incorporation as the PEO process was continued for longer durations. The Ag nanoparticles were embedded in the dense oxide layer, fused into the pore walls and on the surface of the coatings without any change in their morphology or chemistry as detected by HRTEM, SEM and EDX. Ag seems to be delivered to the sites of coating growth (where dielectric breakdown occurs) through different transport pathways, i.e. open pores, cracks and short-circuit channels.

Synthesis and properties of hydroxyapatite-containing porous titania coating on ultrafine-grained titanium by micro-arc oxidation

Equal channel angular pressing results in ultrafine-grained (approximately 200-500 nm) Ti with superior mechanical properties without harmful alloying elements, which benefits medical implants. To further improve the bioactivity of Ti surfaces, Ca/P-containing porous titania coatings were prepared on ultrafine-grained and coarse-grained Ti by micro-arc oxidation (MAO). The phase identification, composition, morphology and microstructure of the coatings and the thermal stability of ultrafine-grained Ti during MAO were investigated subsequently. The amounts of Ca, P and the Ca/P ratio of the coatings formed on ultrafine-grained Ti were greater than those on coarse-grained Ti. Nanocrystalline hydroxyapatite and alpha-Ca(3)(PO(4))(2) phases appeared in the MAO coating formed on ultrafine-grained Ti for 20 min (E20). Incubated in a simulated body fluid, bone-like apatite was completely formed on the surface of E20 after 2 days, thus evidencing preferable bioactivity. Compared with initial ultrafine-grained Ti, the microhardness of the E20 substrate was reduced by 8% to 2.9 GPa, which is considerably more than that of coarse-grained Ti (approximately 1.5 GPa).