Neide K. Kuromoto

Nanomechanical and nanotribological properties of bioactive titanium surfaces prepared by alkali treatment.

Alkali-heat treatment (AHT) is a simple and practical method to make titanium surfaces bioactive. Hydroxyapatite nucleates on Ti when in contact with body fluids due to the presence of a thin sodium titanate film produced by the AHT. This method was proposed more than a decade ago and it has been widely investigated at varied scopes. However, there is still little information about the mechanical properties of this film. In this work, the tribo-mechanical behavior of films produced by alkali treatment (AT) and AHT on Ti is investigated using instrumented indentation technique. The films were also characterized by TF-XRD, SEM, EDS and in vitro bioactivity tests. Analytical methods were employed to obtain the mechanical properties of the film from instrumented indentation data. The heat treatment subsequent to the alkaline processing increased the film elastic modulus from 1.7 GPa to 2.8 GPa, the hardness from 12 MPa to 20 MPa and the critical load for scratch test from 1.5 mN to 5.5 mN. Despite the overall improvement in the film bioactivity and tribo-mechanical behavior, the AHT elastic modulus is only 2% of the pristine Ti whereas hardness is less than 1%. This information must be considered for implant design purposes.

Effect of hydrogen on mechanical properties of nitrided austenitic steels

Hydrogen (H) embrittlement in austenitic stainless steels is restricted to the surface due to the low diffusion coefficient of H in face-centred cubic (fcc) structures. Depth-sensing indentation is a very important tool to investigate the mechanical properties of hydrogenated steels at near-surface regions. In the present work, the effect of glow discharge nitriding on the mechanical properties of steels submitted to cathodic hydrogenation was analyzed by instrumented indentation. Cracks are present at the surface after outgassing in the untreated sample. Hardness and elastic modulus are related to the nitriding process and charging time. Hardness of nitrided samples is higher at the near surface region. However, after hydrogenation, the hardness still increases, mainly due to the presence of martensitic phases. In nitrided samples, hydrogen and methane bubbles can be formed after hydrogenation, probably due to high hydrogen concentration under the nitrided layer. Load versus displacement curves in the nitrided and hydrogenated sample varies according to the indentation made on or outside a bubble region. Nitriding temperature affects the kinetics of bubble formation.

Treating orthopedic prosthesis with diamond-like carbon: minimizing debris in Ti6Al4V

Prostheses are subject to various forms of failing mechanisms, including wear from ordinary patient motion. Superficial treatments can improve tribological properties of the contact pair, minimizing wear and increasing prostheses lifetime. One possibility is the diamond-like carbon (DLC) coating, where Carbon is deposited with variable ratio of sp2/sp3 structures, leading to an increase in surface hardness. So in this research Ti6Al4V samples were coated with DLC using sputtering process to evaluate the debris release. The Ti6Al4V and Ti6Al4V plus DLC coating surfaces were analyzed using Raman spectroscopy and instrumented indentation (hardness). The wear behavior was tested using a reciprocating linear tribometer. The wear rate was smaller in the coated samples, producing less debris than the untreated Ti6Al4V alloy. Debris morphology was also evaluated, using scanning electronic microscopy, and it was observed that debris size from the coated samples were bigger than those observed from the uncoated Ti6Al4V alloy, above the size that generally triggers biological response from the host.

Apatite grown in niobium by two-step plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) of niobium plates were done electrochemically in two steps with electrolytes containing phosphorous and calcium being observed the formation of crystalline apatite. All samples were submitted to a first step of PEO using an electrolyte containing phosphate ions. The second oxidization step was made using three different electrolytes. Some samples were oxidized by PEO in electrolyte containing calcium, while in other samples it was used two mixtures of phosphoric acid and calcium acetate monohydrate solutions. Three different surface layers were obtained. The morphology and chemical composition of the films were analyzed by scanning electronic microscopy (SEM), and energy dispersive spectroscopy (EDS) respectively. It was observed that all samples submitted to two-step oxidation shown porous surface and a calcium and phosphorus rich layer. Average surface roughness (Ra) was measured by a profilometer remaining in the sub-micrometric range. The contact angle by sessile drop technique, using 1μL of distilled water was performed with an optical goniometer. It was verified a higher hydrophilicity in all surfaces compared to the polished niobium. Orthorhombic Nb2O5 was identified by XRD in the oxide layer. Crystalline apatite was identified by XRD in surfaces after the second oxidation made with the Ca-rich electrolyte and a mixture of an electrolyte richer in Ca compared to P. These results indicate that a two-step oxidized niobium surface present great features for applications in the osseointegration processes: favorable chemical composition that increase the biocompatibility, the formation of crystalline niobium pentoxide (orthorhombic), high hydrophilicity and formation of crystalline calcium phosphate (apatite) under adequate electrolyte composition.

Niobium treated by Plasma Electrolytic Oxidation with calcium and phosphorus electrolytes

Niobium plates were electrochemically treated by Plasma Electrolytic Oxidation (PEO) with electrolytes containing phosphorous and/or calcium. Three different electrolyte and experimental parameters were used forming three different surfaces. Film morphology, thickness, and chemical composition were analyzed by scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). A profilometer and the sessile drop technique measured the average surfaces roughness (Ra) and contact angles respectively. X-ray diffraction technique (XRD) analyzed the oxide crystallinity, and scratch tests evaluated the film adhesion. All oxidized surfaces presented pores, without observed cracks. Comparing the different experimental conditions, films obtained with phosphoric acid (P100) show superficial pores, phosphorus incorporation, high hydrophilicity, non-crystalline oxide formation, and good scratch resistance. Films treated with calcium acetate electrolyte (Ca100), compared to P100 exhibit smaller size pores and film thickness, smaller hydrophilicity, and lower scratch resistance. They also demonstrated higher oxide crystallinity, calcium incorporation, and pores interconnections. When the PEO was executed with a blended electrolyte containing calcium acetate and phosphoric acid (Ca50P50) the formed films presented the highest thickness, high phosphorus incorporation, and the lowest contact angle compared with other films. In addition, the pores size, the scratch resistance, calcium incorporation, and oxide crystallinity present intermediate values compared to P100 and Ca100 films. Film crystallinity seems to be influenced by calcium incorporation, whereas, hydrophilicity is phosphorus amount dependent. The pores amount and their interconnections reduced the scratch resistance. Surface features obtained in this work are largely mentioned as positive characteristics for osseointegration processes.

Bioactivity of self-organized TiO2 nanotubes used as surface treatment on Ti biomaterials

Titanium and its alloys are widely used as implants due to their excellent mechanical properties, corrosion resistance and biocompatibility. TiO2 nanotubes have been studied as surface treatment to increase the specific area and to improve osseointegration. However, the thermodynamic stability and bioactivity of these nanostructures must be evaluated. The objective of this research was to obtain nanotubes oxides on Ti6Al4V alloy and to analyze the electrochemical stability in physiological solution at 37 °C and the bioactive response of the biomaterial. The nanotubes were obtained by potentiostatic anodization. The morphology of the oxides was evaluated by scanning electron microscopy. The chemical characterization was analyzed by energy dispersive spectroscopy and x-ray photoelectron spectroscopy techniques. The electrochemical stability was analyzed by open circuit potential (OCP) and the bioactivity by biomimetic test in a simulated body fluid (SBF) solution. The OCP of the nanotubes oxides was shown to be more noble and stable than the compacted oxides. The biomaterial covered with theses oxides showed sealing by Ca and P after 30 d immersion in artificial blood. And after 15 d of immersion in SBF, the hydroxyapatite could be seen on the non-sealed nanotubes. TiO2 nanotube layers could improve the superficial chemical stability and also the osseointegration process.

Elastic modulus evaluation of Titania nanotubes obtained by anodic oxidation

The use of titania (TiO2) nanotubes is becoming one of the most attractive techniques as surface treatment for implants due its combination of morphology (that accelerates osteoblast adhesion and proliferation), bioactivity and possibility of being use as a drug vehicle. Anodic oxidation is one of the cheapest and simplest approaches to obtain highly ordered nanotubes. Parameters such as applied potential, reaction time and fluoride containing in the electrolyte define the nanotubes morphology. However, the mechanical properties of the nanotubes layer do not have been completely elucidated and they play a crucial role in the implant long term stability. The objective of this research was to obtain TiO2nanotubes using anodic oxidation and to determine their elastic modulus and hardness. The TiO2nanotubes layer was obtained in a fluoride containing electrolyte for 1 hour, one group at 15 V and another one at 25 V. The TiO2nanotubes morphology was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The elastic modulus and hardness were evaluated by nanoindentation experiments using a spherical tip. SEM images showed highly ordered nanotubes on all titanium surfaces and it was observed that the nanotubes diameters are directly related with the applied potential. Nanotubes diameters are 66 ± 9 nm and 131 ± 22 nm for nanotubes obtained at 15 V and 25 V, respectively. Nanoindentation test results showed a decrease in the elastic modulus comparing with titanium reference and these values approach to cortical bone elastic modulus. These results demonstrate that it was possible to obtain a homogeneous TiO2nanotubes layer that has mechanical properties adequate to improve implant long-term stability.

Mechanical properties of anodic titanium films containing ions of Ca and P submitted to heat and hydrothermal treatment

Anodic oxidation is a technique widely used to improve the bioactivity of Ti surface. In this study, micro-arc oxidation (MAO) was used to obtain an anodic film incorporating Ca and P ions to evaluate the effect of heat and hydrothermal treatment on the mechanical and in vitro bioactivity properties of these new layers. The MAO process was carried out using (CH3COO)2Ca·H2O and NaH2PO4·2H2O electrolytes under galvanostatic mode (150mA/cm(2)). The thermal treatments were made at 400°C and 600°C in air atmosphere while hydrothermal treatment was made in an alkaline water solution at 130°C. These surfaces presented desired mechanical properties for biomedical applications owing to the rutile and anatase phases in the anodic film that are more crystalline after thermal treatments; which provided an increase in hardness values and lower elastic modulus. The dry sliding wear resistance increased by performing thermal treatments on the surfaces with one condition still maintaining the film after the test. Bioactivity was investigated by immersion in simulated body fluid during 21 days and hydroxyapatite was formed on all samples. Finally, lower values of contact angle were obtained for heat treated samples.

Preliminary results of osteoblast adhesion on titanium anodic films

Osteoblast adhesion on metallic titanium coated with anodic films was evaluated. The anodic oxidation treatment was carried out on commercially pure-titanium (cp-Ti) substrate under the following conditions: 1.0M H2SO4/150V and 1.0M Na2SO4/100V. Osteoblast cells were cultured onto the samples for 4 hours. The morphologies of both Ti anodic films and cells were observed by scanning electron microscopy (SEM). The oxide films are rough with porous structure. Statistical differences in average surface roughness (Ra) values are found among both anodic films and substrate (abraded cp-Ti), measured by contact profilometry. The oxide films prepared in H2SO4 had greater contact angle, whereas there is no statistical significance between the values for the Ti anodic films produced in Na2SO4 and the substrate surface. Despite the some differences in morphology, roughness and contact angle between the treated and non-treated samples, cell morphologies were similar on all surfaces after 4h of culture. Further, was not clearly observed a correlation between the surface characteristics with cell behavior.

In Vitro Behavior of Two Distinct Titanium Surfaces Obtained by Anodic Oxidation

Two different Ti oxide films produced by anodic oxidation were submitted to in vitro bioactivity and cell culture tests. The oxide films were produced in 1.0M H2SO4/150V and 1.0M Na2SO4/100V. Surfaces were found to be homogeneous and rough, with the presence of pores. Both oxide films presented anatase and rutile phases. Ti oxide film produced in Na2SO4 was rougher than the film grown with H2SO4 and composed of a rutile-rich phase. Both films were constituted by TiO2 and Ti2O3 oxides. Despite the differences observed, after 7 days, a calcium phosphate layer was precipitated on both surfaces. Indeed, these two treatment conditions seem to be efficient to spread and attach osteoblast-like cells within 4h.