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Introduction to medical metal materials

Metal medical materials are one of the earliest medical materials used by humans. Their application can be traced back to 400 to 300 BC. The Phoenicians used metal wires to repair missing teeth. Subsequently, after a long period of development, until the late 19th century, humans successfully used precious metal silver to suture the patient’s kneecap (1880). Systematic research on metal medical materials began only after humans used nickel-plated steel screws for fracture treatment (1896). In the 1930s, with the successful development of cobalt-chromium alloys, stainless steel, titanium and alloys and their wide application in dentistry and orthopedics, metal medical materials gradually established their important position in biomedical materials. In the 1970s, the successful application of Ni-Ti shape memory alloys in clinical medicine and the development of biomedical coating materials on metal surfaces led to great development of biomedical metal materials.

1. Definition and application areas
Medical metal materials, also known as surgical implant metal materials, are mainly used for diagnosis, treatment, and replacement of tissues in the human body or improving their functions. In the past 20 years, although metal medical materials have developed slowly compared to biomedical materials such as polymer materials, composite materials, hybrid materials, and derivative materials, they have high strength, good toughness, bending fatigue resistance, and excellent processing properties. It has irreplaceable excellent properties, such as many other types of medical materials, and is the most widely used load-bearing implant material in clinical applications. Especially with the development of metal 3D printing technology, metal medical materials have become more widely used. The most important applications are: fracture internal fixation plates, screws, artificial joints, dental root implants, etc.

2. Commonly used metal medical materials
Medical metal materials used in clinical applications mainly include stainless steel, cobalt alloys, titanium alloys, shape memory alloys, precious metals, and pure metals such as tantalum, niobium, and zirconium.
1. Stainless steel
Stainless Steel as a biomedical material is an iron-based corrosion-resistant alloy and one of the earliest biomedical alloys developed. It is characterized by easy processing and low price. The corrosion resistance and yield strength can be improved through cold working to avoid fatigue fracture. Stainless steel can be divided into: austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, precipitation hardened stainless steel, etc. according to the microstructure. It is used to make medical devices: knives, scissors, hemostats (Figure 1), needles, It is also used to make artificial joints, fracture fixators, dental orthopedics, artificial heart valves and other devices. Among them, austenitic ultra-low carbon stainless steels 316L and 317L are most widely used in medical applications. In 1987, two alloys, 316L and 317L, were included in the international standards ISO 5832 and ISO 7153. In 1990, my country formulated the corresponding national standard, GB 12417, which was implemented in 1991.
The biocompatibility and related issues of medical stainless steel mainly involve tissue reactions caused by metal ions dissolving due to corrosion or wear after stainless steel is implanted in the human body. A large amount of clinical data shows that corrosion of medical stainless steel results in poor long-term implant stability. In addition, its density and elastic modulus are far different from those of human hard tissue, resulting in poor mechanical compatibility. Since corrosion will cause metal ions or other compounds to enter the surrounding tissues or the entire body, it can cause certain adverse histological reactions in the body, such as edema, infection, tissue necrosis, etc., leading to pain and allergic reactions. In particular, serious lesions are caused by the extraction of nickel ions in stainless steel (commonly used austenitic medical stainless steel contains about 10% nickel). In recent years, low-nickel and nickel-free medical stainless steel has been gradually being developed and applied.

2. Cobalt Alloy Medical cobalt alloy (Co-based Alloy as Biomedical Material) is also a commonly used metal medical material in medical treatment. Compared with stainless steel, medical cobalt alloy is more suitable for manufacturing long-term implants with harsh load-bearing conditions in the body, and it is corrosion-resistant. 40 times more durable than stainless steel. The earliest medical cobalt alloy developed was cobalt-chromium-molybdenum (Co-Cr-Mo) alloy, whose structure is extremely austenitic. In the 1970s, forged cobalt-nickel-chromium-aluminum-tungsten-iron (Co-Ni-Cr-Mo-W-Fe) alloy with good fatigue properties and MP35N cobalt-nickel-chromium aluminum alloys with multi-phase structure were developed. Cobalt alloys are mainly used to make artificial hip joints, knee joints, joint pins, bone plates, bone nails and bone pins, as shown in Figure 2. Currently, the most widely used casting is cobalt-chromium aluminum alloy, which has been incorporated into the ISO5582/4 standard. In 1990, my country included it in the national standard, GB12417.
Cobalt alloys mostly remain in a passivated state in the human body, and corrosion is rare. Compared with stainless steel, its passivation film is more stable and has better corrosion resistance. In terms of wear resistance, it is also the best among all medical metal materials. It is generally believed that there will be no obvious histological reaction after implantation in the human body. However, because cobalt alloys are more expensive, and artificial hip joints made of cobalt alloys cause Co and Ni plasma to dissolve due to metal wear and corrosion, the loosening rate in the body is high, and the released Co and Ni elements are seriously sensitizing. Biological problems can easily cause cell and tissue necrosis in the body, leading to patient pain and joint looseness and subsidence, and its application is subject to certain limitations. In recent years, surface modification technology has been used to improve the surface properties of cobalt alloys, effectively improving their clinical effects.

3. Titanium alloy
Medical titanium alloy (Ti-based- Alloy as Biomedical Material) is one of the metals with the best biocompatibility currently known. Since the 1940s, titanium and titanium alloys have gradually been used in clinical medicine. In 1951, humans began to use pure titanium to make bone plates and bone screws. In the mid-1970s, titanium and titanium alloys began to gain widespread medical applications and became one of the most promising medical materials. At present, titanium and titanium alloys are mainly used in orthopedics, especially limb and skull reconstruction. They are used to make various fracture internal fixation devices, artificial joints, skulls and dura maters (Figure 3), artificial heart valves, teeth, Gums, brackets and crowns. Among them, the titanium alloy with the most medical applications is TC4 (Ti-6A1-4V). This alloy has an α-β two-phase mixed structure at room temperature. Through solid solution treatment and aging treatment, its strength and other mechanical properties can be significantly improved.
The density of titanium and titanium alloys is around 4.5g/cm3, which is almost half that of stainless steel and cobalt alloys. The density is close to that of human hard tissue, and its biocompatibility, corrosion resistance and fatigue resistance are better than stainless steel and cobalt alloys. , is currently the best metal medical material. The incompatibility of titanium and titanium alloys with the human body is due to the dense titanium oxide (TiO2) passivation film on its surface after implantation, which has the ability to induce the deposition of calcium and phosphorus ions in body fluids to form apatite, showing certain biological activity. And osseointegration ability, especially suitable for intraosseous implantation. The disadvantages of titanium and titanium alloys are low hardness and poor wear resistance. If wear occurs, it will first cause the destruction of the oxide film, and then the corrosion products of the wear particles will enter the human tissue. In particular, the toxic vanadium (V) contained in the Ti-6A1-4V alloy can cause the implant to fail. In order to improve the wear resistance of titanium and titanium alloys, the surfaces of titanium and titanium alloy products can be treated with high-temperature ion ammoniation or ion implantation technology to enhance their surface wear resistance. In recent years, some new titanium alloys (mainly β-type alloys) have focused on reducing elements that are harmful to the human body, such as V and Al, effectively improving the biocompatibility of titanium alloys.

4. Shape memory alloy
Research on medical shape memory alloys (Shape Memory Alloy as Biomedical Material) began in the 1970s and soon became widely used. The most widely used shape memory alloys in clinical practice are nickel-titanium shape memory alloys. The shape memory recovery temperature of medical nickel-titanium shape memory alloy is 36±2°C, which is consistent with human body temperature and clinically shows comparable biocompatibility to titanium alloys. However, because the nickel-titanium memory alloy contains a large amount of nickel, if the surface is not treated properly, the nickel ions may diffuse and penetrate into the surrounding tissue, causing cell and tissue necrosis. Medical-shaped memory alloys are mainly used in orthopedics and dentistry. The best example of nickel-titanium memory alloy applications is self-expanding stents, especially cardiovascular stents (Figure 4).
5. Precious metals and pure metals: tantalum, niobium and zirconium
Medical precious metals refer to the general name of gold, silver, platinum and their alloys used as biomedical materials. Precious metals have good biocompatibility, strong antioxidant and corrosion resistance, unique physical and chemical stability, excellent processing characteristics, and no toxic or side effects on human tissues. It is used for orthodontic repair (Figure 5), skull repair, implantable electronic devices, nerve repair devices, auricular vortex nerve stimulation devices, diaphragm nerve stimulation devices, optic nerve devices and pacemaker electrodes, etc.
Tantalum for dental restoration has good chemical stability and resistance to physiological corrosion. Tantalum oxides are basically not absorbed and do not exhibit toxic reactions. Tantalum can be used in combination with other metals without damaging the oxide film on its surface. Clinically, metals are used in combination without damaging the oxide film on their surface. Tantalum, niobium, zirconium, and titanium all have very similar tissue structures and chemical properties. They have also been used in biomedicine and are used as bone grafts, dental implant roots, dentures, cardiovascular stents, and artificial hearts. But in general, medical precious metals and metals such as tantalum, niobium, and zirconium are relatively expensive, and their widespread application is limited.

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