Adaptability of titanium alloy materials to the human body and types of titanium materials for medical use

In the medical field, the use of metal materials must be harmless to the human body. When the metal corrodes, it can dissolve metal ions, which affects the cellular tissues of living organisms (human body), so it is necessary to use metal materials that are not easy to corrode and are highly resistant to corrosion, titanium is a kind of highly corrosion-resistant material. For Medical metal materials from stainless steel series to cobalt-based and titanium-based alloy series, the titanium and titanium alloy ratio is increasing. The amount of titanium used in the medical industry worldwide is about 1,000t a year.

1. Adaptability of titanium to the human body (compatibility of titanium with living organisms)
To observe the response of metal ions to human adaptability, we conducted a cell adaptability evaluation method in a laboratory using cells sensitive to metal ions, such as mouse lung fibroblasts (V79 cells) and mouse fibroblast tissues (I929 cells), which are used for medical experiments in China, and an independent administrative body (the Technical Committee for the Standardization of Biological Evaluation of Medical Devices). We differentiate between the reactions of elemental monomer ions to the human body (organisms) provided by the Technical Base for Evaluation of Medical Device Biology (Standardised Technical Committee for Biological Evaluation of Medical Devices).
Vanadium (V), nickel (Ni), copper (Cu), etc., which are highly toxic, cause cell death within a short time at a limited level of parts per million (×10-6). In the case of vanadium (V) and nickel (N), for example, the results of the experiment on V79 cells are shown in the figure. 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 it 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 embedded in metal sheets for testing, these highly toxic metals certainly caused necrosis in the hard tissues (bones) and soft tissues (sinews) in the contact portion.

The other group for the indication of injurious, in the implantation of the attached state, in the fibrous tissue at the contact site, the formation of a kind of biological body to the body to discharge the reaction, iron, aluminum, gold, silver, and so on are so manifested. General metal materials such as SUS 304L stainless steel and SUS 36L stainless steel, as well as cobalt-chromium alloys, belong to this category. The metal piece embedded in the hard tissue does not fuse with the bone cells, and when the removal test is carried out a few weeks later, it is removed without resistance.
The third group is the least reactive 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, showing a body-like phenomenon.

As a result, titanium is a safe metal because it is less likely to cause injury to living organisms. When titanium alloys are used, depending on the alloying elements used, the corrosion resistance of titanium alloys is lower than that of pure titanium, and when corrosion occurs, the alloying elements can be leached out. It is necessary to select alloying elements that are corrosion-resistant and non-invasive. In titanium alloys, Ti-6AI-4V alloy has been used for a long time in aircraft 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 content of iron, oxygen, and hydrogen), have long been used. Recently, however, as part of the research and development of titanium alloys for implantation and placement, the Ti-13Nb-13Zr alloy has been standardized 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 an alloy that is actively discharging aluminum, which will be released soon.

2、Titanium material for medical use
The American ASTM standard (F-code) for medical use is equivalent to the world standard, and in Europe, the ISO standard and the ASTM standard are being sorted out and merged into the European standard. In Japan, we are in the process of consolidating domestic standards and have started to formulate standards based on ISO standards by consolidating the standards corresponding to ASTM and ISO standards.
The titanium materials specified in the ASTM standard for implants and attachments such as artificial knee joints and hip joints (including femoral heads) are listed according to their shapes. For a long time, pure titanium and Ti-6AI-4V alloys, including powdered materials, have been used to make various shapes of parts and components.

3、Medical titanium applications
Titanium is used in a large number of parts such as artificial femoral joints, artificial knee joints, and bone splints, and is also used in orthopedic surgery. By deforming joint inflammation Rheumatism [pronounced “Rumatism”, which means severe joint, and tendon pain, but also for an allergic disease – Translator’s note] and other causes of severe pain, resulting in walking difficulties, will be suffering from this disease patients Patients suffering from this condition are given artificial femoral joints and artificial knee replacements, which can eliminate the pain and allow them to walk. In Japan, 80,000 femoral joint replacements and 40,000 knee replacements are performed in a single year (2005 statistics). In the future, as society ages, it is expected that the demand for artificial joints will increase by a large percentage.
Titanium is not suitable for all artificial joint parts. In the joint part, where there is a lot of movement, titanium is not suitable because it wears out easily (ceramics and cobalt alloys are preferred), and titanium alloys are used for implanted parts. The surface of titanium alloys is uneven and coated with apatite and bone-sensitive materials such as bioglass to ensure early integration with biological bone. In addition, for fracture fixation, titanium alloy intramedullary nails and titanium alloy plates are used.

There is also an increasing trend in the field of dentistry, where implants and attachments are being used. Titanium is used in smaller quantities, but there are titanium alloys and pure titanium alloys in the form of plates, threads, sockets, and baskets as shown in the figure. These parts are driven directly into the jawbone and coated with apatite, which is representative of the composition of the bone, to be fixed in the gingival portion of the tooth. Titanium is suitable for metal implants in general dentistry. There are two methods, the precision casting method, and the superplastic forming method, and it is lighter in weight and does not taste bad to acidic foods compared to the previous cobalt and chromium alloys, but since the use of titanium is not covered by the health insurance diagnosis and treatment, the price is more expensive.

As an implantable accessory for internal medicine, a pacemaker can be implanted when a patient suffers from a low heart rate. An electrode wire is inserted from the subclavian vein to the heart, and this electrode inputs an electronic signal to the pacemaker, making it a pacemaker. Recently, pacemakers have been developed with a mass of 20g and a thickness of 6mm, which is small enough to be connected with an electrode wire and buried under the skin. The battery and control circuitry are contained in a small container (locket) made of pure titanium, which is non-invasive to living organisms. The battery has to last at least 6 years, so the container (locket) is required to be stable and safe for a long time. Currently, nearly 5,000 people in Japan have been benefited.

Titanium is also used in surgical instruments. Especially in the case of long brain and neurosurgery operations lasting more than 10 hours, forceps are required to be lightweight, and titanium products are used for hemostatic forceps and the like. Titanium is also used in many dental treatment instruments such as implants, surgical instruments for attachments, and vibrators for removing dental tartar. In addition to implantation and attachment, such as auxiliary equipment and wheelchairs, titanium is also being used. When a part of a limb is missing due to illness or an accident, a prosthesis is made to restore function, and since the main part of the prosthesis is made of metal, it is being applied in terms of lightness, durability (mainly corrosion and fatigue resistance), and compatibility with living organisms (Ni, Cr, etc.). In the case of wheelchairs, the main goal is to make the entire wheelchair lighter, so in some cases, titanium is used for almost all metal parts in the structure, such as the frame and wheels.