The use of metal materials in the medical industry must be absolutely harmless to the human body. When the metal corrosion, it can dissolve metal ions, for the organism (human body) of the cellular organization of the impact, so it is necessary to choose not easy to corrode, high corrosion-resistant metal materials, titanium is a kind of high corrosion-resistant materials. Medical metal materials from stainless steel series to cobalt-based and titanium-based alloy series, titanium and titanium alloy ratio is increasing. The amount of titanium used in the medical industry worldwide is about 1,000 tons a year.
In order to observe the response of metal ions to human body adaptability, a cell adaptability evaluation method was conducted in a laboratory using cells sensitive to metal ions, such as mouse lung fibroblasts (V79 cells) and mouse (20-day-old mouse) fibroblast tissues (I929 cells), which are used for medical experiments in China, and by the Independent Administrative Institution of Product Evaluation Technology (IAIPET). We divided the elemental monomer ions provided by the Technical Committee for Standardization of Biological Evaluation of Medical Devices (TCSBMD) into three groups as shown in Fig. 1 by their reactions to the human body (living organisms).
Vanadium (V), nickel (Ni), and copper (Cu) are recognized as highly toxic, and cell death occurs within a short period of time at a concentration of parts per million (×10-6) of these elements. The results of the experiment with vanadium (V) and nickel (N) in V79 cells are shown in Figure 2. The results of the one-week immersion test showed that all cells died when nickel was around 10×10-6 (pm = parts per million), while vanadium (V) was two digits less, and all cells died when vanadium was around 0.6×10-6. Secondly, when the hard tissues (bones) and soft tissues (sinews) of small animals, such as rats and rabbits, were buried in metal sheets for testing, it is certain that these toxic metals caused necrosis in the hard tissues (bones) and soft tissues (sinews) in the contact portion.
a-V79 cell population formation energy in relation to nickel (Ni) ion concentration.
b-V79 cell population formation energy versus vanadium (V) ion concentration.
The other group is indicated to be injurious, in the state of implantation and attachment, in the fibrous tissue at the contact site, forming a kind of biomass reaction to be discharged to the outside of the body, and this is the case for iron, aluminum, gold, silver, and so on. General metal materials such as SUS 304L stainless steel and SUS 36L stainless steel as well as cobalt-chromium alloys belong to this type. The metal piece embedded in the hard tissue, which is not fused with the bone cells, is removed smoothly and without resistance when the removal test is carried out a few weeks later.
The third group has the least reaction with living organisms and is suitable for implantation and attachment of titanium, zirconium, niobium, tantalum, platinum and so on. When these metals are implanted in or attached to living organisms, they are closely bound to the hard and soft tissues, and become somatized.
As a result, titanium is a safe metal because it is less likely to cause injury to living organisms. When titanium alloys are used, the alloying elements used are less corrosion resistant than pure titanium, and when corrosion occurs, the alloying elements are dissolved. It is necessary to choose alloying elements that are corrosion-resistant and non-invasive. In titanium alloys, Ti-6AI-4V alloy has been used for a long time in airplane manufacturing and seawater-resistant engineering equipment, and has a large number of examples of use. In the medical field, ELI alloys, which have good corrosion resistance (low iron, oxygen, and hydrogen content), have been used for a long time. Recently, however, as part of the research and development of titanium alloys for implantation and placement, we have standardized the Ti-13Nb-13Zr alloy by replacing vanadium (V) with niobium (Nb), which is a non-injurious alloy, based on the report on the mutagenicity of the monomer (ASTM, ISO). There is also a well-received and aggressive aluminum-expelling alloy that will soon be available.
