Porous Apatite Coating on Various Titanium Metallic Materials via Low Temperature Processing
Nowadays, hydroxyapatite (HA) is widely used as bio ceramics in reconstructive surgery, in dentistry and as drug delivery materials due to the good biocompatibility and osteoconductivity. One of the limitations for the usage of these materials is their low mechanical strength. Thus, many researchers focus on the development of new biomaterials that combine the osteoconductivity characteristics of bioactive ceramics with sufficient strength and toughness for load-bearing applications. The combination of high strength of the metals with osteoconductivity properties of bioactive ceramics makes HA coated metallic implants, which titanium (Ti) or its alloys was mainly used, very attractive for the loaded-bearing applications in orthopedic and dental surgery. A plasma spraying method has been conventionally employed for the HA coating. However, this method has some problems (e.g. a poor coating-substrate adherence, lack of HA crystallinity) for the long-term performance and lifetime of the implants. Therefore, new HA coating methods have attracted great interests in recent years for replacing the high temperature techniques like plasma spraying. Hydrothermal hot-pressing (HHP) method is a possible processing route for producing a ceramic body at relatively low temperatures (under 300˚C).
The compression of samples under hydrothermal conditions accelerates densification of inorganic materials. It is known that the water of crystallization in calcium hydrogen phosphate dehydrate (CaHPO4・2H2O; DCPD) is slowly lost below 100˚C. If the released water can be utilized as a reaction solvent during the HHP treatment, it is to be expected that the joining HA to metal can be achieved simultaneously under the hydrothermal condition, in addition to the synthesis and solidification of HA through the chemical reaction as follows. Ti-based alloys are beneficial for biomedical applications due to their low density, excellent biocompatibility, and corrosion resistance. Combining the advantages of both bulk metallic glass and Ti-based alloy, Ti-based bulk metallic glasses are expected to be applied as a new type of biomaterial.
However, it is well-known that the surface of bulk metallic glasses, which are bioinert, must be bioactive to use as bone replacing medical/dental materials as well as Ti and its alloys. Recently, it was reported a concept called “Growing Integrated Layer” [GIL] that improves adhesion performance without cracking and peeling the ceramic coatings. In particular, if the metallic glass or alloy contains a very reactive component like Ti, it can grow on the bulk metallic materials with its “root” in the bulk. This was named the “Growing Integrated Layer” or “Graded Intermediate Layer [GIL]” and the “Growing Integration Process [GIP]” for its formation process. Multiple layered, laminated, integrated, graded, and diffused coatings have been investigated to decrease the stress accumulation, which however, it is not easy, particularly when the interface is sharp. Even widely diffused interface(s) of larger micron sizes are preferable for joining and coating bulk ceramics on metallic materials. Such a GIL of oxide films grown from the “seed,” i.e., the most reactive component in the bulk metallic materials is interesting as a novel process of oxide film formations, especially because the oxide film can be fabricated in a solution at such low temperatures as RT-200˚C when chemical and/or electrochemical potentials are added.
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Journal of Biochemistry & Biotechnology