Hiroaki Takadama

TEM‐EDX study of mechanism of bonelike apatite formation on bioactive titanium metal in simulated body fluid

Bioactive titanium metal, which forms a bonelike apatite layer on its surface in the body and bonds to the bone through the apatite layer, can be prepared by NaOH and heat treatments to form an amorphous sodium titanate layer on the metal. In the present study, the mechanism of apatite formation on the bioactive titanium metal has been investigated in vitro. The metal surface was examined using transmission electron microscopy and energy dispersive X-ray spectrometry as a function of the soaking time in a simulated body fluid (SBF) and complemented with atomic emission spectroscopy analysis of the fluid. It was found that, immediately after immersion in the SBF, the metal exchanged Na(+) ions from the surface sodium titanate with H(3)O(+) ions in the fluid to form Ti-OH groups on its surface. The Ti-OH groups, immediately after they were formed, incorporated the calcium ions in the fluid to form an amorphous calcium titanate. After a long soaking time, the amorphous calcium titanate incorporated the phosphate ions in the fluid to form an amorphous calcium phosphate with a low Ca/P atomic ratio of 1.40. The amorphous calcium phosphate thereafter converted into bonelike crystalline apatite with a Ca/P ratio of 1.65, which is equal to the value of bone mineral. The initial formation of the amorphous calcium titanate is proposed to be a consequence of the electrostatic interaction of negatively charged units of titania, which are dissociated from the Ti-OH groups, with the positively charged calcium ions in the fluid. The amorphous calcium titanate is speculated to gain a positive charge and to interact with the negatively charged phosphate ions in the fluid to form the amorphous calcium phosphate, which eventually stabilizes into bonelike crystalline apatite.

An X-ray photoelectron spectroscopy study of the process of apatite formation on bioactive titanium metal.

Bioactive titanium metal, prepared by treatment with NaOH followed by an annealing stage to form a sodium titanate layer with a graded structure on its surface, forms a biologically active bone-like apatite layer on its surface in the body, and bonds to bone through this apatite layer. In this study, process of apatite formation on the bioactive titanium metal in a simulated body fluid was investigated using X-ray photoelectron spectroscopy. The bioactive titanium metal formed Ti-OH groups soon after soaking in the simulated body fluid, via the exchange of the Na(+) ions in the sodium titanate on its surface with H(3)O(+) ions in the fluid. The Ti-OH groups on the metal combined with the calcium ions in the fluid immediately to form a calcium titanate. After a long period, the calcium titanate on the metal took the phosphate ions as well as the calcium ions in the fluid to form the apatite nuclei. The apatite nuclei then proceeded to grow by consuming the calcium and phosphate ions in the fluid. These results indicate that the Ti-OH groups formed on the metal induce the apatite nucleation indirectly, by forming a calcium titanate. The initial formation mechanism of the calcium titanate may be attributable to the electrostatic interaction of the negatively charged Ti-OH groups with the positively charged calcium ions.

XPS study of the process of apatite formation on bioactive Ti–6Al–4V alloy in simulated body fluid

Abstract Bioactive Ti—6Al—4V alloy, which spontaneously forms a bonelike apatite layer on its surface in the body and bonds to living bone through this apatite layer, can be prepared by producing an amorphous sodium titanate on its surface by NaOH and heat treatments. In this study, the process of apatite formation on the bioactive Ti—6Al—4V alloy was investigated in vitro, by analyzing its surface with X-ray photoelectron spectroscopy as a function of soaking time in a simulated body fluid 4SBF). Thin-film X-ray diffractometry of the alloy surface and atomic emission spectroscopy of the fluid were also performed complementarily. It was found that immediately after immersion in the SBF,the alloy exchanged Na1 ions from the surface sodium titanate with H3O1 ions in the fluid to form Ti-OH groups on its surface. The Ti-OH groups, immediately after their formation,incorporated the calcium ions in the fluid to form calcium titanate. The calcium titanate thereafter incorporated the phosphate ions in the fluid to form an amorphous calcium phosphate, which was later crystallized into bonelike apatite. This process of apatite formation on the alloy was the same as on the pure titanium metal, because the alloy formed the sodium titanate free of Al and V by the NaOH and heat treatments. The initial formation of the calcium titanate is proposed to be a consequence of the electrostatic interaction of negatively charged units of titania dissociated from the Ti-OH groups with the positively charged calcium ions in the fluid. The calcium titanate is postulated to gain a positive charge and interact with the negatively charged phosphate ions in the fluid to form amorphous calcium phosphate, which eventually stabilizes into crystalline apatite.

Formation of a bioactive graded surface structure on Ti-15Mo-5Zr-3Al alloy by chemical treatment

Simple NaOH and heat treatments provided a Ti-15Mo-5Zr-3Al alloy with a bioactive graded surface structure of an amorphous sodium titanate, where the sodium titanate on the top surface gradually changed into the alloy substrate through titanium oxide. The sodium titanate was free of alloying species of Mo, Zr and Al, since almost all of them were released from the surface of alloy during the first NaOH treatment. The sodium titanate transformed into a hydrated titania via Na+ ion release to induce a bone-like apatite formation on the alloy substrate in a simulated body fluid (SBF). The alloying species neither were released into the SBF nor affected the apatite formation. In the process of apatite formation, the graded surface structure developed into one where the apatite on the top surface gradually changed into the alloy composition through hydrated titania and titanium oxide. It is expected that this graded structure will lead to a strong interfacial bonding strength between the apatite layer and the alloy substrate, thereby providing a tight integration of the alloy with living bone through the apatite layer.

Formation of bioactive functionally graded structure on Ti-6Al-4V alloy by chemical surface treatment.

An Al- and V-free sodium titanate hydrogel layer with a graded structure where the sodium titanate gradually decreases toward the interior, was formed on the surface of Ti-6Al-4V alloy, when the alloy was exposed to 5M NaOH solution at 60 °C for 24 h. This gel layer was transformed into an amorphous sodium titanate layer without giving considerable change in the graded structure, except a little increase in the depth of the oxygen distribution by a heat treatment at 600 °C for 1 h. The sodium titanate layer formed Ti-OH groups on its surface by exchanging its Na+ ion with H3O+ ion in simulated body fluid when soaked in the fluid, and thus formed Ti-OH groups induced the apatite nucleation. The apatite layer also formed a graded structure toward the substrate. The strong bond of the apatite layer to the substrate was attributed to this graded structure.

Positively charged bioactive Ti metal prepared by simple chemical and heat treatments

A highly bioactive bone-bonding Ti metal was obtained when Ti metal was simply heat-treated after a common acid treatment. This bone-bonding property was ascribed to the formation of apatite on the Ti metal in a body environment. The formation of apatite on the Ti metal was induced neither by its surface roughness nor by the rutile phase precipitated on its surface, but by its positively charged surface. The surface of the Ti metal was positively charged because acid groups were adsorbed on titanium hydride formed on the Ti metal by the acid treatment, and remained even after the titanium hydride was transformed into titanium oxide by the subsequent heat treatment. These results provide a new principle based on a positively charged surface for obtaining bioactive materials.

Ceramic–metal and ceramic–polymer composites prepared by a biomimetic process

A biomimetic process was developed to prepare apatite–metal and apatite–polymer composites. A variety of metals and organic polymers incorporated surface functional groups such as Si–OH, Ti–OH or Ta–OH to induce formation of a biologically active bonelike apatite by chemical treatment or physical adsorption. Subsequent immersion in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma or 1.5 SBF led to the formation of a dense and uniform bonelike apatite layer on the surface. Apatite–metal and apatite–polymer composites prepared in this way are believed to be very useful as artificial bone substitutes.

Preparation of bioactive Ti metal surface enriched with calcium ions by chemical treatment

A calcium solution treatment was applied to a NaOH-treated titanium metal to give it bioactivity, scratch resistance and moisture resistance. The titanium metal was soaked in a 5 M NaOH solution and then a 100 mM CaCl(2) solution to incorporate Ca(2+) ions into the titanium metal surface by ion exchange. This treated titanium metal was subsequently heated at 600 degrees C and soaked in hot water at 80 degrees C. The NaOH treatment incorporated approximately 5 at.% Na(+) ions into the Ti metal surface. These Na(+) ions were completely replaced by Ca(2+) ions by the CaCl(2) treatment. The number of Ca(2+) ions remained even after subsequent heat and water treatments. Although the NaOH-CaCl(2)-treated titanium metal showed slightly higher apatite-forming ability in a simulated body fluid than the NaOH-treated titanium metal, it lost its apatite-forming ability during the heat treatment. However, subsequent water or autoclave treatment restored the apatite-forming ability of the NaOH-CaCl(2)-heat-treated titanium metal. Although the apatite-forming ability of the NaOH-heat-treated titanium metal decreased dramatically when it was kept at high humidity, that of NaOH-CaCl(2)-heat-water-treated titanium metal was maintained even in the humid environment. The heat treatment increased the critical scratch resistance of the surface layer of the NaOH-CaCl(2)-treated titanium metal remarkably, and it did not deteriorate on subsequent water treatment.

Transmission electron microscopic study of interface between bioactive bone cement and bone : Comparison of apatite and wollastonite containing glass-ceramic filler with hydroxyapatite and β-tricalcium phosphate fillers

We developed a bioactive bone cement that consists of apatite and wollastonite containing glass-ceramic (AW-GC) powder and bisphenol-a-glycidyl methacrylate (Bis-GMA) based resin. In this study, we made three types of cement (designated AWC, HAC, and TCPC) consisting of either AW-GC, hydroxyapatite (HA), or beta-tricalcium phosphate (beta-TCP) powder as the inorganic filler and Bis-GMA based resin as the organic matrix. These cements were implanted into rat tibiae and cured in situ. Specimens were prepared 1, 2, 4, and 8 weeks after the operation and observed using transmission electron microscopy. Each of the bone cements was in direct contact with the bone. In AWC-implanted tibiae, the uncured surface layer of Bis-GMA based resin was completely filled with newly formed bone-like tissue 2 weeks after implantation. The AW-GC particles were surrounded by bone and were in contact with bone through an apatite layer. No intervening soft tissue was seen. In HAC-implanted tibiae, it took 4 weeks for the uncured layer to completely fill with newly formed bonelike tissue. The HA particles were also in contact with bone through an apatite layer. In TCPC-implanted tibiae, it took 8 weeks for the uncured layer to fill with newly formed bone-like tissue. The new bone that formed on the TCPC was not as dense as that on the AWC or HAC, and an intervening apatite layer was not evident. Results indicated that AWC had higher bioactivity than either HAC or TCPC.

Preparation of bioactive Ti-15Zr-4Nb-4Ta alloy from HCl and heat treatments after an NaOH treatment

Ti-15Zr-4Nb-4Ta alloy does not contain any cytotoxic elements and has a high mechanical strength. Water or HCl and heat treatments were applied to this alloy after NaOH treatment to form a bioactive titanium oxide layer with a nanometer scale roughness on its surface. The nanometer scale roughness was formed on the surface after the first NaOH treatment and remained, even after a subsequent water or HCl and heat treatment. A layer that was mainly composed of anatase was formed on the surface after the heat treatment. Thus, the treated alloy showed a high apatite-forming ability in an SBF, as well as a high scratch resistance. Its high apatite-forming ability was attributed to its positive surface charge. The same alloy subjected to a heat treatment without a water or HCl treatment after the NaOH treatment did not show an apatite-forming ability. This was attributed to a too slow release rate of sodium ions from the surface in an SBF. Ti-15Zr-4Nb-4Ta alloy samples subjected to a water or HCl and heat treatment after the NaOH treatment are expected to be useful as orthopedic and dental implants, since they can form an apatite layer on their surface in a living body and bond to living bone through this apatite layer.