Introduction to Biomedical Metal Materials

 

Date：[2024/4/26]
 

Biomedical metal materials are metals or alloys used in biomedical materials, also known as surgical metal materials, and are a class of inert materials. This type of material has excellent properties such as high mechanical strength, fatigue resistance, and ease of processing, making it the most widely used load-bearing implant material in clinical practice. The application of such materials is very extensive, involving various aspects such as hard tissue, soft tissue, artificial organs, and surgical auxiliary equipment.

Biomedical metal materials are one of the earliest biomedical materials utilized by humans. Human beings started using gold to repair missing teeth very early on.
In 1546, pure gold flakes were used to repair skull defects
In 1588, a useful golden version was discovered for jawbone repair.
In 1775, there were records of using metal to fix internal fractures
In 1800, there were numerous reports of using metal plates to fix fractures;
In 1809, gold was used to make dental implants
In 1880, silver was used for knee bone suturing
In 1896, nickel plated steel nails were used for the treatment of fractures
In the 1930s, cobalt chromium alloys, stainless steel, titanium, and alloys were widely used
In the 1970s, the application of Ni Ti shape memory alloys and metal surface coatings
In the past 30 years, although the development has been slow, it still occupies an important position in clinical practice
Characteristics of Biomedical Metal Materials
1. Material toxicity
The toxicity of biomedical metal materials mainly comes from the corrosion or wear of metal surface ions or atoms into surrounding biological tissues, which act on cells, inhibit enzyme activity, organize enzyme diffusion, and destroy lysosomes. Specifically, it can manifest as the generation of toxic compounds with substances in the body. And metal ions entering the tissue fluid can cause edema, embolism, infection, and tumors. The commonly used detoxification methods include alloying, improving corrosion resistance, improving smoothness, surface coating, etc.
2. Physiological corrosiveness
The physiological corrosiveness of biomedical metal materials is the key to determining the success or failure of material implantation, and the impact of their products on the biological body determines the service life of implanted devices.
3. Mechanical properties
Biomedical metal materials need to have sufficient strength and plasticity. Generally speaking, the requirements for metal materials in artificial hip joints are: yield strength>450Mpa; Tensile strength>800Mpa; Fatigue strength>400Mpa; Elongation rate>8%. The elastic modulus of materials is usually greater than that of bones, which leads to different strains between materials and bones, and the relative displacement at the interface causes interface loosening; In addition, it generates stress shielding, causing functional degradation or absorption of bone tissue.
4. Wear resistance
Wear resistance affects the lifespan of implanted friction devices; And may produce harmful metal particles or debris, leading to inflammatory and toxic reactions in surrounding tissues. Improvements can be made through methods such as increasing hardness and surface treatment.
Common biomedical metal materials
1. Medical stainless steel
The main chemical composition of medical stainless steel is {316, 36L, 317L}; In applications, pitting corrosion and interface corrosion are prone to occur, resulting in poor long-term stability; Dissolved certain ions may induce tumor formation; Poor mechanical compatibility and no biological activity.
2. Medical cobalt based alloy
The main chemical components are Co Cr Mo, Co Cr W-Ni, etc. The corrosion resistance is several times higher than that of stainless steel, and there is generally no obvious tissue reaction. As the interface loosening rate of artificial hip joint is relatively high, the release of Co ions can cause cell and tissue necrosis, skin allergic reactions, and so on. It has excellent friction resistance and strong load-bearing capacity. Usually used as a vocal implant.
3. Medical titanium and its alloys
Titanium and titanium alloys are currently the most widely used implanted metal biomaterials, with low density, high specific strength, low elastic modulus, better corrosion resistance and fatigue resistance than stainless steel and cobalt alloys. They have good biocompatibility, but low hardness, poor friction resistance, less ideal fatigue and fracture toughness, still high elastic modulus, and contain toxic elements in the alloy. According to the properties of the alloy, titanium alloys are mainly divided into α、β、α-β Titanium alloy.
4. Medical magnesium alloy
Magnesium alloy, as a biodegradable medical material, is known as the third generation of biomedical materials. Magnesium is a mild element to the human body, with good absorbability and biocompatibility. In orthopedic implants, it has a density and elastic modulus similar to bone. Magnesium alloy also has a controllable corrosion rate, which has great application prospects in cardiovascular implantation and bone repair. The current research on magnesium alloys mainly includes WE43, AZ31, Mg Ca, MgZnCa, etc. Magnesium alloys mainly suffer from issues such as excessive corrosion rate and insufficient mechanical strength.
Other medical metal materials
Gold, silver, platinum and other precious metals, which were first widely used in clinical treatment, have good stability and processing performance. However, due to their high price, their wide application is limited. Cash is still widely used in dentistry, acupuncture and moxibustion, implantation in vivo and medical biosensors.
Tantalum, niobium, and zirconium have good chemical stability and physiological corrosion resistance. Oxides are basically not absorbed and do not exhibit toxic reactions. They can be used in combination with other metals without damaging their surface oxide film. And it exhibits good biocompatibility, but its application is also limited due to its high price.
Shape memory alloy is a new type of medical biomaterial. The main shape memory alloys used in clinical practice are nickel titanium shape memory alloys. Medical nickel titanium shape memory alloys have shape memory properties and superelasticity in the phase transition zone. They are relatively soft at low temperatures and can deform. When heated to human body temperature, they immediately return to their original shape, generating sustained and gentle restoring force. At this time, the material is harder and more elastic, which can play a corrective or supportive role. It has comparable biocompatibility to stainless steel and titanium alloys. Its excellent biocompatibility, corrosion resistance, wear resistance, and non toxicity are known as the new functional materials of the 21st century. However, nickel ions in nickel titanium memory alloys may diffuse and penetrate into surrounding tissues, causing adverse reactions. Medical shape memory alloys are mainly used in plastic surgery and dentistry. The best example of using nickel titanium memory alloys is self expanding stents, especially cardiovascular stents.
The main problems currently existing in medical metal materials
After years of clinical application, medical metal materials still face many problems. In addition to the common host reactions of medical materials, they are also directly or indirectly affected by metal corrosion and wear. Medical metal materials contain a significant amount of alloying elements, but their allowable concentrations in the human body are very low. These alloying elements often exhibit strong negative charges, which can change their electronic valence states and combine with organic or inorganic substances in living organisms to form complex chemical compounds (some of which contain strong toxicity). In addition, after metal materials are implanted into the human body, corrosion, wear, and other factors can lead to the dissolution of metal ions and metal; Ions entering tissue fluid can trigger certain biological reactions, such as tissue reactions, blood reactions, and systemic reactions, manifested as edema, thromboembolism, infection, and tumors.
In addition, in human blood, due to the negative charge of platelets, cells, and proteins, while metal ions usually have a positive charge, the precipitation of a large number of metal ions in the blood is also prone to the formation of blood clots. Among the essential trace elements for the human body, such as iron (Fe), chromium (Cr), nickel (Ni), molybdenum (Mo), and cobalt (Co), nickel, cobalt, and chromium ions have significant toxicity and sensitization reactions to the human body. Studies have reported the process of inflammation induced by metal ions released from implants, and it has been found that even submicromolar concentrations of zinc, nickel, and cobalt can induce the expression of E-selectin in endothelial cells. The long-standing issues of nickel allergy and nickel carcinogenesis in science have only received attention from various countries in recent decades. The restrictions on nickel content in daily and medical metal materials have become increasingly strict, and the maximum allowable nickel content in standard documents is also decreasing. This trend can be clearly seen from the European Parliament standards promulgated in 1967, 1988, and 1994. Therefore, in the development of new medical metal materials, it is necessary to strictly control the metal elements in them, and it is best to use fewer or no alloying elements that may cause toxicity and allergies to the human body.