Metallic biomaterials are the oldest group of materials that have been used in the human body since BC. The use of gold as the most historical metal biomaterial has been used for at least 2500 years. For example, the use of gold dental crowns and dental bridges developed in Rome in 500 or 700 BC.
Figure 1. The use of gold plating in the era before Christ
Although in today’s world, with the advancement of materials science and the expansion of ceramics, polymers and composites, the position of metal biomaterials has become weaker than in the past, due to special properties such as strength, fatigue, resistance to tensile forces, and generally suitable mechanical properties, they are still considered as biomaterials. They have wide applications. In the following, the most common metals used in the medical industry are examined.
stainless steel
Among various steels, stainless steels are considered suitable for medical applications. Compared to other metals used in the body, this metal has a higher yield strength and its yield strength reaches 2800 MPa. Stainless steel is used in total hip implants, orthodontic wires, prefabricated crowns or veneers, orthopedic plates, internal fracture fixation devices, and the like. On the other hand, alloy 304 and its counterparts are used in medical tools such as endodontic tools, needle files, surgical blades, surgical forceps, and the like.
Figure 2. Set of steel surgical instruments
The first steels used as implants were 302 steels, which were used in fracture plates. But due to low corrosion resistance, the life of this alloy was shortened and it was replaced by 316 steel and later by 316L steel. This alloy was also replaced with 316LVM steel due to sensitivity to pitting corrosion and problems such as high infection. Of course, due to its bio-neutral properties and bacteria accumulation, this alloy requires surface modification. Among the surface modification methods, we can mention electrophoretic, electropolishing, ion implantation and the like.
Cobalt chromium alloys
Cobalt-based alloys used in medicine are mostly known as cobalt-chromium alloys. These alloys are worked in two categories and hot forging and with cobalt-chromium-molybdenum and cobalt-chromium-nickel-molybdenum compounds have found a place in the medical industry. The brand names of these alloys include Satellite, Vitalim and Vintria. One of the main applications of these alloys is the use in artificial knee and thigh joints, which is due to the wear resistance and corrosion resistance properties of this alloy. Other uses of this metal include orthodontic wire, dental veneers, and the base of fixed and movable artificial teeth.
Figure 3. Application of chromium cobalt in body joints
Although this alloy has higher corrosion resistance than stainless steel, albumin has negative effects on the corrosion resistance of this alloy. Also, long-term corrosion tests show the release of C ions such as chromium from this alloy. According to research, this alloy leads to carcinogenesis in the long term. On the other hand, the high weight of this alloy has reduced its use today. Surface modification with bioceramics with plasma surface modification methods is considered as a relatively successful effort in this field.
Titanium and its alloys
Titanium is one of the most widely used metal implants due to properties such as relatively low elastic modulus with excellent corrosion resistance, good physical-mechanical behavior at high temperature and high strength-to-weight ratio. Titanium has many applications in the human body as an implant, and its applications include screws, nails and stabilizing plates, artificial bone, dental implants and stents.
Titanium alloys are suitable materials for implants under load, but their corrosion resistance is weak. On the other hand, biocompatibility is only due to the presence of pores on the surface of the metal, and it is considered bioneutral in terms of biocompatibility. This means that it will not react in relation to the body, and because of this, a fibrous capsule will be formed around it and will cause the implant to loosen or loosen.
For this reason, to improve titanium implants, proper surface modification operations are required. Among the types of surface operations, we can mention oxidizing the surface, sandblasting the surface, laser surface modification, ion implantation, thermal spraying, and coating. The use of appropriate coating improves mechanical properties and also promotes biocompatibility and integration with bone, this application of metallic biomaterials is used in various industries.
Figure 1. Titanium and its various applications
magnesium
Magnesium is a suitable choice among metal alloys for use in orthopedic applications in absorbable screws and cardiac stents due to its elastic modulus close to bone, low density, compatibility with MRI images, and degradability properties. Magnesium and its alloys, in spite of favorable characteristics such as degradability and modulus close to bone and the absence of the stress shield phenomenon, have problems such as insufficient mechanical properties for the period of time required for implantable support, the rate of destruction is higher than the time required for The implant remains in position and the rate of hydrogen release is high.
Figure 2. Image of magnesium orthopedic screw used in animal body and Brunton’s test
There are various methods to modify the surface of magnesium alloys, which, in addition to maintaining the degradability and other advantages of magnesium implants, will improve properties such as increasing corrosion resistance and reducing hydrogen release. The modification methods reduce the magnesium corrosion rate by separating magnesium from the environment. Surface modification methods include conversion coatings, chemical vapor deposition (PVD), electrophoresis and plasma electrolytic oxidation (PEO). These methods help to improve corrosion properties and, on the other hand, properties such as antibacterial and bioactivity.
Zirconium
Zirconium and its alloys, like other metals, are mainly used in orthopedic and dental applications. These metals have good properties such as low specific gravity, high corrosion resistance and biocompatibility. Titanium-zirconium alloys with about 17% zirconium offer higher fatigue resistance and elongation properties than titanium. The evidence of in vivo tests shows that zirconium implants show good osteogenic properties.
Figure 3. Titanium-zirconium dental implant
The biocompatibility of pure zirconium and its alloys depends on its surface properties. On the other hand, due to the mineral placement of this metal next to radioactive substances, it causes problems in its purification. Also, resource limitations and the strategic nature of this product are obstacles to the development of this metallic biomaterial. One of the common methods to improve the properties of zirconium is the PEO method, which puts a coating of zirconium on zirconium with suitable wear and corrosion properties.