Effect of heat treatments on apatite-forming ability of NaOH- and HCl-treated titanium metal

Titanium (Ti) metal was soaked in HCl solution after NaOH treatment and then subjected to heat treatments at different temperatures. Their apatite-forming abilities in a simulated body fluid (SBF) were discussed in terms of their surface structures and properties. The nanometer scale roughness formed on Ti metal after NaOH treatment remained after the HCl treatment and a subsequent heat treatment below 700°C. Hydrogen titanate was formed on Ti metal from an HCl treatment after NaOH treatment, and this was converted into titanium oxide of anatase and rutile phases by a subsequent heat treatment above 500°C. The scratch resistance of the surface layer increased with the formation of the titanium oxide after a heat treatment up to 700°C, and then decreased with increasing temperature. The Ti metal with a titanium oxide layer formed on its surface showed a high apatite-forming ability in SBF when the heat treatment temperature was in the range 500–700°C. The high apatite-forming ability was attributed to the positive surface charge in an SBF. These positive surface charges were ascribed to the presence of chloride ions, which were adsorbed on the surfaces and dissociated in the SBF to give an acid environment.     

Effect of HCl concentrations on apatite-forming ability of NaOH–HCl- and heat-treated titanium metal

Titanium (Ti) metal was treated with water or HCl solutions after 5 M NaOH solution treatment and then subjected to heat treatment at 600°C. The apatite-forming abilities of the treated Ti metals were examined in simulated body fluid. The apatite-forming ability of the Ti metal subjected to NaOH, water and heat treatment was lower than that of just NaOH and heat treatments. Ti metals subjected to NaOH, HCl and heat treatment showed apatite-forming abilities, which increased with increasing HCl concentrations up to the same level as that of NaOH- and heat-treated Ti metal. The former did not show a decrease in its apatite-forming ability, even in a humid environment for a long period, whereas the latter decreased its ability. The increase in the apatite-forming ability with increasing HCl concentrations suggests a different mechanism of apatite formation from that previously proposed.     

Nanostructured positively charged bioactive TiO<Subscript>2</Subscript> layer formed on Ti metal by NaOH,acid and heat treatments

Nanometer-scale roughness was generated on the surface of titanium (Ti) metal by NaOH treatment and remained after subsequent acid treatment with HCl, HNO₃ or H₂SO₄ solution, as long as the acid concentration was not high. It also remained after heat treatment. Sodium hydrogen titanate produced by NaOH treatment was transformed into hydrogen titanate after subsequent acid treatment as long as the acid concentration was not high. The hydrogen titanate was then transformed into titanium oxide (TiO₂) of anatase and rutile by heat treatment. Treated Ti metals exhibited high apatite-forming abilities in a simulated body fluid especially when the acid concentration was greater than 10 mM, irrespective of the type of acid solutions used. This high apatite-forming ability was maintained in humid environments for long periods. The high apatite-forming ability was attributed to the positive surface charge that formed on the TiO₂ layer and not to the surface roughness or a specific crystalline phase. This positively charged TiO₂ induced apatite formation by first selectively adsorbing negatively charged phosphate ions followed by positively charged calcium ions. Apatite formation is expected on the surfaces of such treated Ti metals after short periods, even in living systems. The bonding of metal to living bone is also expected to take place through this apatite layer.     

Osteoconduction of porous Ti metal enhanced by acid and heat treatments

Bone ingrowth into porous Ti metal is important for stable fixation of Ti metal implants to surrounding bone. However, without surface treatment this is limited to only a thin region of the outer surface of the Ti metal. In the present study, a porous Ti metal with a porosity of ~60 % and interpore connections of 70–200 micrometers in diameter was investigated in terms of its chemical and heat treatments, by implanting it into rabbit femur for periods varying from 3 to 12 weeks. The porous Ti metal subjected to heat treatment at 600 °C after H₂SO₄/HCl mixed acid treatment showed the largest bone ingrowth in comparison with those subjected to no treatment, only acid treatment, and only heat treatment even at an early stage after implantation, and remained as such even 12 weeks after implantation. Their bone ingrowths were well interpreted in terms of apatite-forming abilities of the Ti metals in body environment. Their apatite-forming abilities did not depend upon their surface roughness nor type of crystalline phase, but upon the positive surface charge.     

Bone bonding ability of a chemically and thermally treated low elastic modulus Ti alloy: gum metal

The gum metal with composition Ti–36Nb–2Ta–3Zr–0.3O, is free from cytotoxic elements and exhibits a low elastic modulus as well as high mechanical strength. We have previously demonstrated that this gum metal, once subjected to a series of surface treatments—immersion in 1 M NaOH (alkali treatment) and then 100 mM CaCl₂, before heating at 700 °C (sample: ACaH-GM), with an optional final hot water immersion (sample: ACaHW-GM)—has apatite-forming ability in simulated body fluid. To confirm the in vivo bioactivity of these treated alloys, failure loads between implants and bone at 4, 8, 16, and 26 weeks after implantation in rabbits’ tibiae were measured for untreated gum metal (UT-GM), ACaH-GM and ACaHW-GM, as well as pure titanium plates after alkali and heat treatment (AH-Ti). The ACaH-GM and UT-GM plates showed almost no bonding, whereas ACaHW-GM and AH-Ti plates showed successful bonding by 4 weeks, and their failure loads subsequently increased with time. The histological findings showed a large amount of new bone in contact with the surface of ACaHW-GM and AH-Ti plates, suggesting that the ACaHW treatment could impart bone-bonding bioactivity to a gum metal in vivo. Thus, with this improved bioactive treatment, these advantageous gum metals become useful candidates for orthopedic and dental devices.     

Effect of Ca contamination on apatite formation in a Ti metal subjected to NaOH and heat treatments

It has long been known that titanium (Ti) metal bonds to living bone through an apatite layer formed on its surface in the living body after it had previously been subjected to NaOH and heat treatments and as a result had formed sodium titanate on its surface. These treatments were applied to a porous Ti metal layer on a total hip joint and the resultant joint has been in clinical use since 2007. It has been also demonstrated that the apatite formation on the treated Ti metal in the living body also occurred in an acelullar simulated body fluid (SBF) with ion concentrations nearly equal to those of the human blood plasma, and hence bone-bonding ability of the treated Ti metal can be evaluated using SBF in vitro. However, it was recently found that certain Ti metals subjected to the same NaOH and heat treatments display apatite formation in SBF which is decreased with the increasing volume of the NaOH solution used in some cases. This indicates that bone-bonding ability of the treated Ti metal varies with the volume of the NaOH solution used. In the present study, this phenomenon was systematically investigated using commercial NaOH reagents and is considered in terms of the structure and composition of the surface layers of the treated Ti metals. It was found that a larger amount of the calcium contamination in the NaOH reagent is concentrated on the surface of the Ti metal during the NaOH treatment with an increasing volume of the NaOH solution, and that this inhibited apatite formation on the Ti metal in SBF by suppressing Na ion release from the sodium titanate into the surrounding fluid. Even a Ca contamination level of 0.0005 % of the NaOH reagent was sufficient to inhibit apatite formation. On the other hand, another NaOH reagent with a nominal purity of just 97 % did not exhibit any such inhibition, since it contained almost no Ca contamination. This indicates that NaOH reagent must be carefully selected for obtaining reliable bone-bonding implants of Ti metal by the NaOH and heat treatments.     

Effect of hot water and heat treatment on the apatite-forming ability of titania films formed on titanium metal via anodic oxidation in acetic acid solutions

Titanium and its alloys have been widely used for orthopedic implants because of their good biocompatibility. We have previously shown that the crystalline titania layers formed on the surface of titanium metal via anodic oxidation can induce apatite formation in simulated body fluid, whereas amorphous titania layers do not possess apatite-forming ability. In this study, hot water and heat treatments were applied to transform the titania layers from an amorphous structure into a crystalline structure after titanium metal had been anodized in acetic acid solution. The apatite-forming ability of titania layers subjected to the above treatments in simulated body fluid was investigated. The XRD and SEM results indicated hot water and/or heat treatment could greatly transform the crystal structure of titania layers from an amorphous structure into anatase, or a mixture of anatase and rutile. The abundance of Ti–OH groups formed by hot water treatment could contribute to apatite formation on the surface of titanium metals, and subsequent heat treatment would enhance the bond strength between the apatite layers and the titanium substrates. Thus, bioactive titanium metals could be prepared via anodic oxidation and subsequent hot water and heat treatment that would be suitable for applications under load-bearing conditions.     

Formation of a bioactive calcium titanate layer on gum metal by chemical treatment

The so-called gum metal with the composition Ti–36Nb–2Ta–3Zr–0.3O is free from cytotoxic elements and exhibits a low elastic modulus as well as high mechanical strength. In the present study, it was shown that this alloy exhibited a high capacity for apatite formation in a simulated body fluid when subjected to 1 M NaOH treatment, 100 mM CaCl₂ treatment, heat treatment at 700°C, and then hot water treatment. The high apatite formation was attributed to the CaTi₂O₅ which was precipitated on its surface, and found to be maintained even in a humid environment over a long period. The treated surface exhibited high scratch resistance, which is likely to be useful in clinical applications. The surface treatment had little effect on the unique mechanical properties described above. These results show that gum metal subjected to the present surface treatments exhibits a high potential for bone-bonding, which will be useful in orthopedic and dental implants.     

Apatite-forming ability of Ti–15Zr–4Nb–4Ta alloy induced by calcium solution treatment

Ti–15Zr–4Nb–4Ta alloy free from cytotoxic elements shows high mechanical strength and high corrosion resistance. However, simple NaOH and heat treatments cannot induce its ability to form apatite in the body environment. In the present study, this alloy was found to exhibit high apatite-forming ability when it was treated with NaOH and CaCl₂ solutions, and then subjected to heat and hot water treatments to form calcium titanate, rutile, and anatase on its surface. Its high apatite-forming ability was maintained even in 95% relative humidity at 80°C after 1 week. The surface layer of the treated alloy had scratch resistance high enough for handling hard surgical devices. Thus, the treated alloy is believed to be useful for orthopedic and dental implants.     

Influence of post-weld heat treatment on microstructure and adhesion of Ti/Cu composite

It is well known that brittle interfacial intermetallics are typically present in laminated composites and degrade their mechanical performance. To investigate the relationship between intermetallics and mechanical properties, Ti/Cu-laminated composite plates prepared by explosive welding were studied with different heat treatments. A transition layer containing four zones of intermetallic phases, Cu₄Ti, Cu₄Ti₃, CuTi, and CuTi₂, was found at the interface between Ti and Cu after explosive welding. The intermetallics became thicker as the temperature of the heat treatment increased, especially for the CuTi phase. The sample treated at 473?K exhibited the highest bonding strength of 199.7?MPa, which was attributed to the elimination of residual stress during the welding and the diffusion of copper to improve interfacial bonding.     

Effects of oxygen content of porous titanium metal on its apatite-forming ability and compressive strength

Porous titanium metal subjected to NaOH and heat treatments is useful as a bone substitute as it shows high mechanical strength as well as osteoconductivity and osteoinductivity. However, the porous metal is liable to be contaminated with oxygen gas during the fabrication process and this incorporated oxygen could lead to adverse effects on the bioactivity and mechanical properties of the prepared porous body. In this study, oxygen contamination during fabrication of bioactive porous bodies was measured. It was found that the oxygen content of the titanium metal was increased from 0.08 to 0.23 mass% when the porous body was prepared from bar stock, and it further increased up to 0.51 mass% when it was subjected to NaOH and heat treatments. Despite this, the porous bodies subjected to NaOH and heat treatments formed apatite on their pore walls within 1 day in a simulated body fluid. This result was consistent with the apatite-forming ability of NaOH- and heat-treated titanium plates with different oxygen contents in the range of 0.05 to 0.30 mass%. The compressive strength of the porous body was increased about 10% by the NaOH and heat treatments.     

In situ composite coating of titania-hydroxyapatite on titanium substrate by micro-arc oxidation coupled with electrophoretic deposition processing

In situ composite coating of hydroxyapatite (HA)/TiO₂ were produced on titanium (Ti) substrate by micro-arc oxidation coupled with electrophoretic deposition (MAO&EPD) technique with different concentrations of HA particles in the 0.2 M NaOH electrolyte solution. The surface morphology and chemical composition of the hybrid coating were effected by HA concentration. The amount of HA particles incorporated into coating layer increased with increasing HA concentration used in the electrolyte solution. The corrosion behavior of the coating layer in simulated body fluids (SBF) was evaluated using a potentiodynamic polarization test. The corrosion resistance of the coated sample was increased compared to the untreated Ti sample. The in vitro bioactivity assessment showed that the MAO&EPD treated Ti substrate possessed higher apatite-forming ability than the untreated Ti. Moreover, the apatite-forming ability had a positive correlation with HA concentration. In addition, the cell behavior was also examined using cell proliferation assay and alkaline phosphatase ability. The coating formed at HA concentration of 5 g/L exhibited the highest cell ability.     

Effect of thermal treatment on apatite-forming ability of NaOH-treated tantalum metal

The prerequisite for an artificial material to bond to living bone is the formation of bonelike apatite on its surface in the body. This apatite can be reproduced on its surface even in an acellular simulated body fluid with ion concentrations nearly equal to those of the human blood plasma. The present authors previously showed that the tantalum metal subjected to a NaOH treatment to form a sodium tantalate hydrogel layer on its surface forms the bonelike apatite on its surface in SBF in a short period. The gel layer as-formed on the metal is, however, not resistant against abrasion, and hence thus-treated metal is not useful for clinical applications. In the present study, effects of thermal treatment on the mechanical properties and apatite-forming ability of the NaOH-treated tantalum metal were investigated. The sodium tantalate gel on the NaOH-treated tantalum was dehydrated to convert into amorphous sodium tantalate by a thermal treatment at 300 °C in air environment and into crystalline sodium tantalates by the thermal treatment at 500 °C. Resistivity of the gel layer against both peeling-off and scratching was significantly improved by the thermal treatment at 300 °C. The high apatite-forming ability of the sodium tantalate hydrogel was a little decreased by the thermal treatment at 300 °C, but appreciably decreased by the thermal treatment at 500 °C. It is believed that the tantalum metal subjected to the 0.5 M-NaOH treatment and the subsequent thermal treatment at 300 °C is useful as implants in dental and orthopaedic fields, since it shows high bioactivity as well as high fracture toughness. © 2001 Kluwer Academic Publishers     

Enhanced apatite formation on Ti metal heated in PO2-controlled nitrogen atmosphere

The oxynitridation of biomedical titanium metal under a precisely regulated oxygen partial pressure (P_O2) of 10^? 14 Pa in nitrogen atmosphere at 973 K for 1 h strongly enhanced apatite formation compared with that on Ti heated in air. The factors governing the high apatite-forming ability are discussed from the viewpoint of the surface properties of Ti heated under a P_O2 of 10^? 14 Pa in nitrogen atmosphere determined from X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and zeta potential measurements. Nitrogen (N)-doped TiO₂ (interstitial N) was formed on pure Ti heated under a P_O2 of 10^? 14 Pa in nitrogen atmosphere at 973 K. The XPS O1s main peak shifted toward a lower binding energy upon heating under a P_O2 of 10^? 14 Pa. This shift may be due to the formation of oxygen vacancies. This Ti surface had a positive zeta potential of approximately 20 mV. According to time-of-flight secondary ion mass spectroscopy results, PO₄^3 ? ions were predominantly adsorbed on Ti soaked in simulated body fluid (SBF) after heat treatment, followed by calcium ions. It was concluded that the apatite formation kinetics can be described using the Avrami–Erofeev equation with an Avrami index of n = 2, which implies the instantaneous nucleation of apatite on the surface of Ti soaked in SBF after heat treatment at 973 K under a P_O2 of 10^? 14 Pa.     

Effect of heat treatment on bioactivity and mechanical properties of PDMS-modified CaO-SiO2-TiO2 hybrids via sol-gel process

Crack- and pore-free transparent monolithic disks of polydimethylsiloxane (PDMS)- modified CaO-SiO₂-TiO₂ hybrids were obtained by hydrolysis and polycondensation of PDMS, tetraethoxysilane, tetraisopropyltitanate and calcium nitrate. The product as-dried at 60 °C formed an apatite on its surface in a simulated body fluid (SBF) within only one day, indicating its high bioactivity. The apatite-forming ability decreased slightly by a heat treatment below 250 °C. The bending strength of the product was about 11 MPa, independent of the heat treatment. This average strength value is comparable to that of the human cancellous bone. Young's modulus of the products increased from 100 to 500 MPa with increasing heat treatment temperature from 60 to 250 °C, but its values were within the range of those of the human cancellous bone. The strain at failure of the products decreased with increasing heat treatment temperature. Failure strains went down to the magnitudes exhibited by the human cancellous bone, when the products were heat treated in the temperature range from 150 to 250 °C. Thus, highly bioactive hybrids with mechanical properties analogous to those of the human cancellous bone were obtained. This new kind of bioactive hybrid may be useful as a bone-repairing material. © 2001 Kluwer Academic Publishers     

Apatite mineralization abilities and mechanical properties of covalently cross-linked pectin hydrogels

Natural bone has features such as high fracture toughness and bone-bonding bioactivity, and is organic–inorganic hybrid composed of collagen and apatite crystals. Therefore, apatite-polymer hybrids designed to mimic the structure of bone represent candidates for high-performance bone substitutes. In this study, we prepared pectin hydrogels through covalent cross-linking using divinylsulfone (DVS) and investigated their apatite-forming abilities of the gels in simulated body fluid (SBF) and mechanical properties by tensile test. The obtained results were interpreted in terms of surface charge of the gels and chemical reaction with SBF. The apple- and citrus-derived gels formed the apatite on their surfaces in SBF within 3 days. These gels showed tensile strength around 30 MPa.     

Mechanical modulation and bioactive surface modification of porous Ti–10Mo alloy for bone implants

The objective of this work was to fabricate a suitable porous Ti–10Mo alloy as the human bone replacement implants. The porous Ti–10Mo alloy was fabricated by mechanical alloying and then consolidated by powder metallurgy technique. NH₄HCO₃ powder was used as space-holder. It was indicated that the mean pore size, porosity, compressive strength, and elastic modulus of porous Ti–10Mo alloy could be tailored by the amount of NH₄HCO₃, and then could be matched with those of human bones. Furthermore, porous Ti–10Mo alloy was treated by alkali heat treatment and soaked in the 1.5 times simulated body fluid (1.5SBF). It was observed that the surface and the inside pore wall of porous Ti–10Mo alloy with 25 wt.% NH₄HCO₃ covered with the apatite layer after soaked in 1.5SBF for 28 days. These phenomena indicated that the surface modified porous Ti–10Mo alloy exhibited a high potential for bone-bonding, which was expected to be used as bone tissue implant.     

Effects of nano-surface properties on initial osteoblast adhesion and Ca/P adsorption ability for titanium alloys

Titanium alloys (Ti6Al4V), while subjected to high temperature surface treatment, experience altered nano-surface characteristics. The effects of such surface treatments are examined, including the initial adhesion force experienced by osteoblasts, the Ca/P adsorption capability, and the nano-surface properties, including the amounts of amphoteric Ti-OH groups, surface topography, and surface roughness. The initial adhesion force is considered a quantitative indicator of cyto-compatibility in?vitro. Previously, a cyto-detacher was applied in a pioneer attempt measuring the initial adhesion force of fibroblasts on a metal surface. Presently, the cyto-detacher is further applied to evaluate the initial adhesion force of osteoblasts. Results reveal that (1)?titanium alloys subjected to heat treatment could promote the adsorption capability of Ca and P; (2)?titanium alloys subjected to heat treatment could have higher initial osteoblast adhesion forces; (3)?the adhesion strength of osteoblasts, ranging from 38.5 to 58.9?nN (nanonewtons), appears stronger for rougher surfaces. It is concluded that the heat treatment could have impacted the biocompatibility in terms of the initial osteoblast adhesion force and Ca/P adsorption capability.     

Enhancement of bonding strength by graded structure at interface between apatite layer and bioactive tantalum metal

Tantalum metal is a candidate for use as an implant material in high load-bearing bony defects, due to its attractive features such as high fracture toughness and high workability. This metal, however, does not have bone-bonding ability, i.e. bioactivity, and therefore the development of bioactive tantalum metal is highly desirable. It is known that the essential prerequisite for an artificial material to show bioactivity is to form a bonelike apatite layer on its surface in the body environment. The same type of apatite layer is formed in a simulated body fluid (SBF) with inorganic ion concentrations nearly equal to those of human blood plasma. The present authors previously showed that the apatite formation on tantalum metal in SBF was remarkably accelerated by treatment with 0.5 M-NaOH aqueous solution and subsequent firing at 300 °C, while untreated tantalum metal spontaneously formed the same apatite after a long soaking period. In the present study, the bonding strength of the apatite layer to the substrate was quantitatively evaluated in comparison with that to the untreated tantalum metal. Adhesive strength was measured as an estimation of bonding strength, and the surface microstructure of both the substrates was characterized in order to discuss the difference in the bonding strength in terms of surface structure. The apatite layer formed on the NaOH- and heat-treated tantalum metal shows higher adhesive strength than that formed on the untreated metal. The amorphous sodium tantalate layer formed on the tantalum metal by NaOH and heat treatments, has a smooth graded structure where its concentration gradually changes from the surface into the interior metal. Smooth graded structure with complex of apatite is constructed after soaking in SBF. The higher bonding strength of the apatite layer formed on the treated metal is attributed to its smooth graded structure.     

Precipitation of iron-rich intermetallics and mechanical properties of Al–Si–Mg–Fe alloys with Al–5Ti–B

Effects of added Al–5Ti–B master alloys on precipitation of iron-rich intermetallics and mechanical properties of A356 cast alloys with high Fe content (1.5?wt-%) were investigated using image analysis, scanning electron microscopy, and tensile testing. Results show that added Al–5Ti–B has apparent refinement on α (Al) grain size of A356 alloys that have high Fe content. 12?wt-% Al–5Ti–B is beneficial for improving mechanical properties of A356 cast alloys with high Fe content. Improved mechanical properties can be attributed to refined microstructure, the proper amounts of TiB₂ and Ti(AlSi)₃, and decreased porosity. An excessive amount of Al–5Ti–B deteriorates mechanical properties of alloys because it leads to the formation of large secondary intermetallics and increased porosity.